Note: Descriptions are shown in the official language in which they were submitted.
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TITLE OF THE INVENTION
OPTICAL DEVICES FEATURING TEXTURED SEMICONDUCTOR LAYERS
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims the priority of U.S.
Provisional Application No. 60/732,034 filed October 31,
2005 and entitled, OPTICAL DEVICES FEATURING TEXTURED
SEMMICONDUCTOR LAYERS. This application is also a
continuation-in-part of pending U.S. Application No.
11/107,150 filed April 15, 2005 aind entitled OPTICAL
DEVICES FEATURING TEXTURED SEMICONDUCTOR LAYERS, which
claims the priority of U.S. Provisional Application No.
60/562,489 filed April 15, 2004 and entitled, FORMATION
OF TEXTURED III-NITRIDE TEMPLATES FOR THE FABRICATION OF
EFFICIENT OPTICAL DEVICES, U.S. Provisional Application
No. 60/615,047 filed October 1, 2004 and entitled,
FORMATION OF TEXTURED III-NITRIDE TEMPLATES FOR THE
FABRICATION OF EFFICIENT OPTICAL DEVICES, and U.S.
Provisional Application No. 60/645,704 filed January 21,
2005 and entitled, NITRIDE LEDS BASED ON FLAT AND
WRINKLED QUANTUM WELLS. Further, this application is a
continuation-in-part of PCT/US/2005/012849 filed April
15, 2005 and entitled OPTICAL DEVICES FEATURING TEXTURED
SEMMICONDUCTOR LAYERS. Each of the above listed earlier
applications is hereby incorporated by reference herein.
STATEMENT REGARDING FEDERALLY SPONSORED
RESEARCH OR DEVELOPMENT
Part of the work leading to this invention was
carried out with United States Government support
provided under Contract No. DAAD19-00-2-0004 awarded by
United States Army Research Office and Grant No DE-FC26-
04NT42275 from the United States Department of Energy.
Thus, the United States Government has certain rights in
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this invention.
BACKGROUND OF THE INVENTION
A light emitting diode (LED) is a semiconductor
optical device capable of producing light in the
infrared, visible or ultraviolet (W) region. LEDs
emitting in the visible and ultraviolet are made using
gallium nitride (GaN) and its alloys with indium nitride
(InN) and aluminum nitride (AlN). These devices
generally consist of p and n-type semiconductor layers
arranged into a p-n junction. In a standard LED device,
semiconductor layers are evenly grown onto a polished
substrate such as GaAs or sapphire. A typical
semiconductor layer is composed of gallium nitride (GaN)
that has been doped to be a p or n-type layer.
Important figures of inerit, for an LED are its
internal quantum efficiency (IQE) and light extraction
efficiency. For a typical LED the IQE depends on many
factors, such as the concentration of point defect, Auger
processes and device design. In the case of nitride LEDs
grown along polar (0001) and (000-1) directions the
internal efficiency is also reduced due to the distortion
of the quantum wells between the n- and p-doped layers
caused by the internal electric fields. The light
extraction efficiency of standard LEDs based on GaN is
determined from Snell's law to be 4% per surface. An LED
commonly includes several quantum wells made of a small
energy gap semiconductor (well) and a wider bandgap
semiconductor (barrier). Visible LEDs employ indium
gallium nitride (InGaN) as the well and GaN as the
barrier. Ultraviolet LEDs employ AlGaN of different
compositions as both wells and barriers. The IQE of an
LED device based on nitride semiconductors grown along
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polar direction is reduced by electric fields across its
quantum wells. This phenomenon is referred to as the
quantum confined Stark effect (QCSE). The QCSE affects
LED light emission by red shifting the emission
wavelength and reducing photoluminescence intensity. The
rather small value of light extraction efficiency in the
standard LED is the result of the high refraction index
of the semiconductor layer at the exit interface.
A number of approaches have been proposed to enhance
the extraction of light from LEDs. For example, in GaAs
LEDs, the extraction of light is affected by the
absorption of the emitted light in the GaAs substrate. To
mitigate this problem, one can use epitaxial lift-off and
wafer bonding methods to transfer the GaAs LED structure
to transparent substrates. Another approach involving the
optimization of LED surface geometry (such as the
truncated inverted pyramid), combined with the use of
substrate mirrors, has pushed the extraction limit to
30%. Other approaches involve the use of a continuously
variable refracti n index transparent material to reduce
the back-reflection at the interface. Some of these
approaches have some manufacturing limitations and the
last one suffers from fast index-material degradation
with time.
An approach that is recently becoming increasingly
attractive is photon extraction from randomly micro-
textured thin film surfaces. It has significantly
improved extraction efficiency, with record external
quantum efficiencies of 44% demonstrated at room
temperature for GaAs based LEDs (Windish et al., 2000).
In this reference, the textured surface was formed after
the growth of the LED using lithographic methods. It
turns out that, even in that case, most of the photons
are still extracted from within the emission cone inside
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the critical angle corresponding to a flat surface.
Consequently there is still a wide room for improving
extraction well beyond the present values.
Visible and W LEDs based on GaN and other III-
nitride materials are used widely for full color
displays, automotive lighting, consumer electronics
backlighting, traffic lights, and white LEDs for solid
state lighting. A variety of approaches are used towards
formation of white LEDs. One approach is the utilization
of three-color LEDs (RGB) and an alternative approach
using hybrid methods such as W LEDs in combination with
a tri-color phosphor or blue and blue/red LEDs with two
or one color phosphor. Current white LED performance has
reached 30 lm/W, while efficiency more than 200 lm/W is
required for commercially attractive semiconductor
lighting.
The current IQE for electron-hole pair conversion to
photons of nitride LEDs is - 21 0(Tsao, 2002). Thus the
IQE needs to be increased to 600-70o for applications
related to solid-state lighting. To accomplish this, a
number of improvements in the current state of the art
are required. For example, band-gap engineering (quantum
wells, quantum dots) must be involved to optimize
carrier-to-photon conversion. Also, improvements in the
various layers of an LED structure are required to reduce
the defect density and thus improve carrier transport to
the active region. Such improvements reduce parasitic
heating and lead to device longevity, enhanced color
stability, and reduced consumer cost over lifetime.
SUMMARY OF THE INVENTION
The present invention provides a device for use as a
light emitter or sensor or as a solar cell. For an
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emitter of the invention based on polar semiconductors
such as III-nitrides, the IQE and light extraction
efficiency is improved over conventional devices. For a
sensor or solar cell, the efficiency of coupling light
into the device is also improved. In one embodiment, the
semiconductor material is deposited in layers, starting
with as grown textured initial semiconductor layer
deposited onto a substrate. In one embodiment, the layer
is randomly textured as grown on the substrate so as to
have a textured surface morphology. The substrate and
textured layer can be used as a template for the growth
of multiple semiconductor layers. For example, a device
may comprise a second layer deposited onto the first
textured layer. These layers can be deposited with p and
n dopants to form a p-n junction LED. The textured
emitting layer enhances light escape. The initial
semiconductor layer preferably serves as a barrier layer
onto which a quantum well layer is grown. Each of the
semiconductor layers conforms to the texture of the first
grown layer and thus the external surface of the LED from
where the light is extracted has approximately the same
texture as the initial semiconductor layer.
Preferably, multiple quantum wells comprising a
plurality of barrier and quantum well layers are
deposited on one another as alternating semiconductor
layers each replicating the original texture. The
texturing replicated through the barrier and well layers
repositions the quantum wells so that their surfaces are
not perpendicular to the [0001] polar direction. Thus
the quantum wells maintain almost their square well
shape, since they are not distorted by internal fields
due to polarization. As a result the hole and electron
wavefunctions overlap, leading to efficient recombination
and thus drastically improving the IQE.
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Devices of the invention can comprise substrates
such as silicon (Si), gallium arsenide (GaAs), gallium
nitride (GaN), aluminum nitride (A1N), indium nitride
(InN), aluminum gallium nitride (AlGaN), indium gallium
nitride, indium aluminum nitride, indium gallium aluminum
nitride (InA.1GaN), silicon carbide, zinc oxide, sapphire,
and glass. The sapphire substrate may also undergo
nitridation before a layer is deposited thereon.
Semiconductor layers grown on the GaN template, or
on another layer in the total growth process, can be
deposited by any suitable process. Examples of such
deposition processes include hydride vapor phase epitaxy
(HVPE), molecular beam epitaxy (MBE), metal-organic
chemical vapor deposition (MOCVD), liquid phase epitaxy
and laser ablation. A layer of a semiconductor device may
comprise III-nitride materials such as GaN, AlN InN or
any combination of these materials. The substrate may be
textured before layer growth or by choosing appropriate
conditions of growth such that the first semiconductor
layer on the substrate has a textured surface.
The semiconductor layer can comprise a dopant so
that the layer is p or n-type. Exemplary dopants include
beryllium, selenium, germanium, magnesium, zinc, calcium,
Si, sulfur, oxygen or a combination of these dopants. A
layer may also be a mono or poly crystalline layer. A
device of the invention also can include several p and n-
type layers and one or more buffer layers, which
generally aid layer growth. An exemplary buffer layer is
a GaN semiconductor layer. A buffer layer may be
deposited onto a substrate or between semiconductor
layers.
The semiconductor layer for a device of the
invention may be deposited to be from about 10 angstroms
(A) to 100 microns (pm) thick. The texturing of a GaN
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template and the deposited layers have an average peak-
to-valley distance of about 100 nanometers (nm) to 5pm.
The present invention also provides a method of
fabricating a semiconductor device of the invention. The
method comprises providing a substrate- and growing a
first semiconductor layer on the surface of the
substrate. The first layer can be randomly textured
spontaneously as grown or randomly textured by a textured
substrate surface. The substrate or first layer can then
be used as a template to deposit other semiconductor
layers having the same texture as the template. In a
preferred embodiment, a fabrication method includes
growing several quantum wells. The multiple quantum-wells
are textured by the first layer, substrate or a
combination thereof.
DESCRIPTION OF THE DRAWINGS
Other features and advantages of the present
invention will be apparent from the detailed description
of the invention that follows, taken in conjunction with
the accompanying drawings of which:
Figure 1 is a partial representation of a textured
template of the invention;
Figures 2a and 2b are partial representations of a
semiconductor layer deposited onto the textured template
of Figure 1 to form a p-n junction;
Figures 3a and 3b are partial representations of
multiple quantum wells and a semiconductor layer
deposited onto the textured template of Fig. 1;
Figures 4a and 4b are partial representations of a
substrate having a textured surface that textures
semiconductor layers including multiple quantum wells
deposited thereon;
Figures 5a and 5b are partial representations of a
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substrate having textured surfaces with textured
semiconductor layers including multiple quantum wells
deposited thereon; Figure 5c is a schematic
representation of a UV LED structure based on nitride
semiconductors.
Figure 6 is a transmission electron microscope (TEM)
view of textured GaN / AlGaN multiple quantum wells grown
on a textured GaN template;
Figure 7 is a radiating electrically pumped GaN
wafer level LED having InGaN MQWs;
Figure 8a is a scanning electron microscope (SEM)
image of a gallium nitride (GaN) textured template of the
invention;
Figure 8b is an SEM image of a conventional, smooth
GaN semiconductor layer;
Figure 8C shows surface morphology by AFM of the
smooth GaN template of Fig. 8b;
Figure 9 is a comparison of photoluminescence
between a conventional GaN layer and the textured
template of_Figure 8a;
Figure 10a is an atomic force microscope (AFM) image
of the textured template of Figure 8a; Figure 10b shows a
depth analysis plot of the imaged area;
Figures 11 and 12 show photoluminescence spectra of
conventional, smooth quantum wells (Fig. 11) and textured
quantum wells (Fig. 12) grown on the textured template of
. Figure 8a;
Figure 13 is an electroluminescence spectrum of a p-
n junction LED device comprising the textured template of
Figure 8a;
Figure 14a shows the emission spectrum of a
commercially available white LED (Lumileds LXHL-BW02;
Technical Data Sheet DS25); Figure 14b shows the
electroluminescence spectrum of an LED of the invention
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(textured InGaN/GaN MQWs grown on a textured GaN template
produced by HVPE), measured under a DC injection current
of 30 mA; Figure 14c shows the radiating white GaN LED of
Fig. 14b under a DC injection current of 25 mA;
Figures 15a-15c show the electroluminescence spectra
of LEDs similar to that used to obtain the data of Fig.
14b, using the indicated values of DC injection current;
Figure 16 is a photograph of an LED under conditions
described in Fig. 15b, showing that much of the wafer
emits green light, whereas certain parts emit blue light;
Figure 17 shows electroluminescence spectra of LED
structures taken from parts of a wafer having different
texture; The DC injection current is listed on the right
side of each graph in the same order as the corresponding
curves;
Figure 18 is an atomic force microscope (AFM) image
of a 50 micron thick atomically smooth GaN template grown
by HVPE; The visible striations are steps corresponding
to a change in thickness of approximately 2 A;
Figure 19 is a schematic of the cross-section of
certain LED embodiments;
Figure 20 is a schematic of an HVPE reactor;
Figure 21 is a reflectance spectrum of a randomly
textured GaN template grown via HVPE;
Figure 22a shows the photoluminescence efficiency of
several GaN templates with smooth and varying degrees of
textured surface; Figure 22b shows the photoluminescence
efficiency of two GaN templates with smooth or textured
surface, and having the same concentration of carrier;
Figure 23 shows the surface texture obtained in GaN
films grown on the R-plane of sapphire (1-102) by HVPE;
Figure 24 shows the reflectivity of the textured
surface described in Fig. 23;
Figure 25a shows the surface morphology of a
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textured GaN template (VH81) by AFM; Figure 25b is a
roughness analysis of the textured template in Fig. 25a;
Figure 26a shows depth analysis of the textured
template in Fig. 25a; Figure 26b shows spectral density
analysis of the textured template in Fig. 25a;
Figure 27a shows the surface morphology of a
textured GaN template (VH129) by AFM; Figure 27b is a
roughness analysis of the textured template in Fig. 27a;
Figure 28a shows depth analysis of the textured
template in Fig. 27a; Figure 28b shows spectral density
analysis of the textured template in Fig. 27a;
Figure 29a shows the surface morphology of a
textured GaN template (VH63) by AFM; Figure 29b is a
roughness analysis of the textured template in Fig. 29a;
Figure 30a shows depth analysis of the textured
template in Fig. 29a; Figure 30b shows spectral density
analysis of the textured template in Fig. 29a;
Figure 31a shows the surface morphology of a
textured GaN template (VH119) by AFM; Figure 31b is a
roughness analysis of the textured template in Fig. 31a;
Figure 32a shows depth analysis of the textured
template in Fig. 32a; Figure 32b shows spectral density
analysis of the textured template in Fig. 31a;
Figures 33a, 33b, 33c, and 33d show the
photoluminescence spectra for GaN templates having
different rms roughness as described in Figs. 25a, 27a,
29a, and 31a, respectively.
Figure 34 shows the peak intensity versus rms
roughness for the textured templates in Figs. 25a, 27a,
29a, and 31a;
Figure 35 shows the AFM surface morphology and
roughness analysis of GaN/AlGaN MQWs grown on a GaN
textured template (VH129);
Figure 36 shows the photoluminescence spectra for a
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GaN (7nm)/Al0.2Ga0.8N (8nm) MQW structure and the GaN
textured template (VH129, see Fig. 28) used to grow the
MQW structure.
Figure 37 shows photoluminescence spectra for
identical GaN/AlGaN MQWs grown by MBE on textured and
atomically smooth GaN templates;
Figure 38 shows schematically the types of surface
positions used for the cathodoluminescence analysis shown
in Fig. 39;
Figure 39 shows cathodoluminescence spectra taken at
points A to C indicated in Fig. 38;
Figure 40a shows the effect of quantum well
dis.tortion; Figure 40b shows photoluminescence peak
position of AlGaN/GaN MQWs grown along the non-polar (M-
plane) and the polar (C-plane) direction;
Figure 41a shows an AFM scan of a textured template
surface; Figure 41b shows depth analysis of the AFM data
from Fig. 41a;
Figure 42 shows a plot of electron mobility versus
electron concentration for textured GaN templates;
Figures 43a and 43b illustrate the analysis of
photon escape probability for smooth (Fig. 43a) and
textured (Fig. 43b) surfaces;
Figure 44 shows the x-ray diffraction pattern around
the (0002) Bragg peak for ten period GaN (7 nm)/
A10.2Ga0.8N (8 nm) MQWs grown on a GaN textured template;
Figures 45a and 45b are smooth (45a) and randomly
textured (45b) GaN templates prepared by MBE; The random
surface texturing of Fig. 45b was produced by growing the
GaN film under nitrogen-rich conditions;
Figures 46a and 46b show the photoluminescence
emission peak (46a) and luminescence intensity (46b) for
quantum well layers of different thickness;
Figure 47a shows polarization and internal electric
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field effects in a wrinkled quantum well layer; Figure
47b shows electron accumulation at the base of inclined
sections of a wrinkled quantum well layer;
Figure 48 is a schematic representation of a
variable color indicator embodiment;
Figure 49 is a schematic representation of a
variable color illumination device embodiment;
Figure 50 is a schematic representation of a color
display embodiment; and
Figure 51 is a schematic representation of a color
projector embodiment.
DETAILED DESCRIPTION OF THE INVENTION
An LED or photodetector of the present invention has
improvement in one or both of light external extraction
efficiency and IQE. Light extraction efficiency is
improved with a textured emitting surface which is
typically replicated through the process of applying
layers from an initial semiconductor substrate layer.
Further, an LED of the invention has a dichromatic
electroluminescence spectrum whose color is controlled by
the bias current through the LED.
Control over growth rate- and use of appropriate
deposition procedures will form a textured surface layer
on the initial substrate. This texture is replicated
through subsequent layers as they are applied resulting
in an emitting layer that has greatly improved light
extraction efficiency. Final surface texturing can also
be achieved by separately texturing the underlying
substrate or using an unpolished substrate which is
decorated with deep groves since the wafers are usually
cut from an ingot using a saw.
Improvement in IQE of an LED is achieved through the
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incorporation of multiple quantum wells (MQWs), in the p-
n junction. This results in better confinement of
injected electrons and holes from the n- and p-sides
respectively and thus more efficient recombination.
When a semiconductor device containing quantum wells
is grown on a polar orientation the quantum wells
resulting are distorted, resulting in separation of the
holes and electrons. This places the electron-hole
regions farther apart, reducing the efficiency of hole-
electron recombination for the generation of light. The
LED of the invention overcomes this deficiency by growing
the quantum wells on a textured surface. This way the
quantum wells are not distorted, and thus the electrons
and holes in the wells recombine more efficiently.
In one embodiment of the LED according to the
present invention the LED is formed on a substrate 2 with
a textured semiconductor layer 4 deposited onto the
substrate as shown in Fig. 1 and Figs. 2a and 2b, more
fully discussed below. The layer is textured as grown on
the substrate so as to have a textured surface topology
(or morphology) 10. The substrate and textured layer can
be used as a template for the growth of multiple
semiconductor layers to form the LED. Such textured A1N
templates may also be used to produce W LEDs. For
example, a device may comprise a second layer deposited
onto the first textured layer. These layers can be doped
to form a p-n junction for an LED. Appropriate dopants
can include selenium, germanium, zinc, magnesium,
beryllium, calcium, Si, sulfur, oxygen or any combination
thereof. Each of the semiconductor layers can be textured
by replication from the first grown layer and its
textured surface to have a textured emitting surface of
improved extraction efficiency.
In another embodiment, as shown in Figs. 3a, b,
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Figs. 4a, b, and Figs. 5a, b, more fully discussed below,
multiple quantum wells comprising a plurality of barrier
and quantum well layers are deposited on one another as
alternating semiconductor layers between the n- and p-
doped layers of the device. As used herein, the term
"quantum well" refers to a quantum well layer together
with an adjacent barrier layer. The multiple quantum
wells are textured by replication from the textured
surface of the first layer as they are grown thereon. .
In most cases a cladding layer of n-doped AlGaN of
variable thickness is grown between the textured layer
and the quantum wells.
Suitable substrates that can be used for growth of
the first layer are known in the art. Exemplary
substrates include sapphire, gallium arsenide (GaAs),
gallium nitride (GaN), aluminum nitride (AlN), silicon
carbide, zinc oxide silicon (Si) and glass. For example,
a preferred substrate can include (0001) zinc oxide,
(111) Si, (111) GaAs, (0001) GaN, (0001) A1N. (0001)
sapphire, (11-20) sapphire and (0001) silicon carbide.
A substrate for a device of the invention can be
prepared for semiconductor layer growth by chemically
cleaning a growth surface. Optionally, a growth surface
of the substrate may be polished. The substrate may also
be thermally out-gassed prior to layer growth. The
surface of the substrate can be optionally exposed to
nitridation such as disclosed in United States Patent No.
6,953,703, which is incorporated by reference herein.
Growth on an unpolished, raw, as cut substrate
facilitates growing a textured surface on it.
A semiconductor layer may be grown by processes such
as hydride vapor phase epitaxy (HVPE), an alternative
name for which is halide vapor phase epitaxy, MOCVD or
MBE, liquid phase epitaxy (LPE), laser ablation and
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variations of these methods. Typical growth processes
have been disclosed in United States Patent Nos.
5,725,674, 6,123,768, 5,847,397 and 5,385,862, which are
incorporated by reference herein. The semiconductor
layer can also be grown in the presence of nitrogen to
yield a nitride layer. Examples of a nitride layer are
GaN, InN, AlN and their alloys.
Fig. 1 shows a partial representation of a
semiconductor device of the invention. In a preferred
embodiment, the device is textured and comprises a
substrate 2 and first layer 4 textured as grown thereon.
The substrate 2 can be textured or polished smooth
initially. The first layer 4 is textured as grown on the
substrate 2 to have a textured surface topology 10.
Preferably, the first layer is grown by a modified HVPE
deposition process to create the textured surface 10.
The modified HVPE process yields a textured as grown
first layer in part by etching defective areas of the
layer with an increased hydrochloric acid (HCl)
concentration. The HC1 concentration of the modified
HVPE process is substantially higher than that of typical
deposition processes as exemplified below.
In one embodiment, the first layer 4 can be a
semiconductor layer comprising a group III nitride layer.
The layer 4 is preferably a p or n-type semiconductor
layer by suitable doping during deposition or it can be
an insulating layer as for example AlN or both as shown
below. A layer 4 can optionally be grown on a buffer
layer deposited onto the substrate such as described in
U.S. Patent No. 5,686,738, which is incorporated by
reference herein.
The thickness of the substrate 2 and layer 4 can
cover a broad range, although the thickness of the layer
4 may influence the extent of texturing replicated at its
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surface. For. example, a l00 pm thick layer can have a
peak-to-valley texture distance of about 100 nm to 5 pm.
The texturing of the semiconductor layer affects its
light extraction characteristics of LED layers grown
thereon that replicate the texture. The semiconductor
layer 4 is typically randomly textured as grown. Layer 4
may be single or poly crystalline material.
Fig. 2a shows a second layer 8 grown onto the device
of Fig. 1. The layer 8 can be grown by any suitable
deposition process. The second layer is grown on the
textured surface 10 of the first layer 4. The second
layer 8 is preferably not so thick so as to bury the
textured surface topology 10 of the first layer 4 as
shown in Fig. 2b. Preferably, the second layer 8 can have
an upper surface 9 that is textured by replication by the
layer 4 as shown in Fig. 2a.
Preferably, the layer 8 is a semiconductor layer
comprising a group III nitride. The second layer 8 is
typically a p or n-type semiconductor layer opposite to
the doping of layer 4: The second layer 8 may be a
single or poly crystalline semiconductor layer. In one
embodiment, the first and second layers 4 and 8 doping
forms a p-n junction 3 for use as a photosensor or
emitter. These devices can be used for electronic
displays, solid state lights, computers or solar panels.
Electrodes 11 and 13 connect to the layers 4 and 8 as is
know in the art for such use.
Figs. 3a and 3b are partial representations of an
LED having multiple quantum wells 6 grown onto the device
of Fig. 1. The quantum wells 6 are textured by the
surface topology of the first layer 4. As described
above, the first layer 4 can be textured as grown onto
the substrate 2. In one embodiment, the multiple quantum
wells 6 can comprise one or more barrier layers 5 and
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alternating quantum well layers 7.
Several barrier layers 5 and quantum well layers 7
can be grown as alternating semiconductor layers each
replicating the textured first layer 4. For example,
quantum wells can be formed by a barrier layer 5 grown on
the first layer 2. A quantum well layer 7 is then grown
onto the barrier layer 5. A second barrier layer 5 is
then grown on the quantum well layer 7 followed by a
second quantum well layer. In one embodiment, the
composition of quantum well layer 7 and first layer 4 are
matched in composition. A barrier layer 5 can have a
composition that differs from both the first 4 and
quantum well layer 7.
The barrier layer 5 may comprise one or more group
III-V nitride compounds. In one embodiment, one or more
barrier layers 5 are AlGaN. Similarly, one or more
quantum well layers 7 are a group III Nitride such as
GaN, or another III - V compound. The layers can also be
grown by any suitable deposition process. The layers may
be single or poly crystalline layers.
The thickness of each of the layers is typically
thin enough for texturing of the layer beneath to
replicate to the surface above. The extent of texturing
with the layers can affect IQE and light extraction
efficiency. Preferably, a device of the invention
comprises from one to twenty quantum wells that comprise
a plurality of barrier layers 5 and quantum well layers
7.
Figs. 3a and 3b also show an upper semiconductor
layer 8 grown on the multiple textured quantum wells G.
The layer 8 can be grown by a known deposition process
and may be a textured layer 9 (Fig. 3a) or be so thick so
as to bury the textured surface topology of the first
layer 4 (Fig. 3b) or have it polished off.
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Preferably, the layer 8 is a semiconductor layer
comprising a group III nitride. The upper layer 8 may
also be a p or n-type semiconductor layer, opposite to
the layer 4 so as to form a p-n junction. The p-n
junction allows functioning as a semiconductor device
such as an LED or photodetector. The upper layer 8 can be
a single or poly crystalline semiconductor layer. The
multiple quantum wells 6 can also comprise textured as
grown barrier layers 5 and quantum well layers 7. For
example, layers 5 and 7 may be grown by a deposition
process such as HVPE, MBE, or MOCVD.
The device structure shown in Fig. 3a can exhibit
internal quantum efficiencies and external light
extraction efficiencies that are significantly higher
than the efficiencies of a conventional device. The Fig.
3b device possesses IQE increases overall.
A device of the invention can have a light
extraction efficiency approaching one-hundred (100)
percent. Similarly, such a device may have an IQE in the
range of fifty to sixty percent or more.
Figs. 4a and 4b show a device with a substrate
having an initial textured surface. Subsequent layers
from the first layer 4 can be deposited on the textured
substrate 2 such that the upper surfaces are textured by
replication.
The device of Fig. 4a includes a textured surface 9
on layer 8 or in Fig. 4b, an untextured layer in that
embodiment.
In an alternative embodiment, the substrate can
comprise both upper and lower textured surfaces 9 and 15,
as shown, for example, in Fig. 5a using substantially the
same procedures as described above. In Fig. 5b, only
bottom layer 2, surface 15 is textured and can function
as an emitting surface.
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For example, Fig. 5c is an LED using a smooth AlN
template 4a on a sapphire substrate 2. Between the A1N
template and the quantum well and barrier layers 7 and 5
there is a thick AlGaN layer 4b known as cladding or
contact layer. This layer can be used with other forms
of the invention described herein. Over those -are p-
doped layers layers 8a and 8b of AlGaN and GaN
respectively. Layers 4b and 8b receive electrical
connections 11 and 13 with light extraction downward
through sapphire substrate 2. Layer 8a can be used with
other forms of the invention described herein and
functions as an electron blocking layer preventing the
loss of electrons. Layers 5 and 7 while shown smooth for
clarity are to be understood to be wrinkled as desired.
The present invention also provides a method of
fabricating a semiconductor device of the invention. The
method comprises providing a substrate and growing a
first semiconductor layer on the surface of the
substrate. The first layer can be randomly textured as
grown, textured lithographically post-growth, or randomly
textured by a textured substrate surface as described
below. The substrate or first layer can then be used as a
template to deposit and texture other semiconductor
layers. Such a template can be sold at this stage of
production, allowing others to complete the layering
replicating the texture up to the emitting layer.
in a preferred embodiment, a fabrication method
includes growing several quantum wells in which the wells
comprise both barrier and quantum well layers that can be
deposited as alternating semiconductor layers. The
multiple quantum wells are textured by the first layer,
substrate or a combination thereof.
This invention describes a method of forming on a
substrate thick GaN and other III-nitride films
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(templates) having a particular texture. Such
spontaneously formed textured nitride templates are used
as substrates for the growth of high efficiency devices
such as III-Nitride light emitting diodes (LEDs), solar
cells and photodetectors. The high efficiency of such
devices is due to two effects; (a) efficient light
extraction for LEDs and efficient coupling of light into
the material for the case of solar cells and
photodetectors and (b) improvements in IQE of LEDs based
on textured III-Nitride MQWs due to suppression of
polarization effects.
This invention relates to a method of preparing
textured Group III-nitride templates during growth of the
nitride films by HVPE, MOCVD, and MBE. Furthermore, such
textured nitride templates are used as substrates for the
growth and fabrication of LED structures with improved
IQE as well as more efficient extraction efficiency.
Besides LEDs, other devices such as solar cells and
photodetectors, fabricated on such textured templates are
going to have improved efficiency as well. Reference is
made to commonly owned US PATENTS: 5,385,862; 5,633,192;
5,686,738; 6,123,768; 5,725,674 incorporated herein by
reference.
While the internal efficiency of an LED is an
inherent material and device design property, the
external efficiency of such a device is a measure of
light extraction efficiency from the semiconductor. The
large contrast between the GaN index of refraction and
the surrounding material (usually air) causes total
internal reflection for most of the light produced inside
the active material. For the index of refraction of GaN
(n = 2.5), the escape cone for internal light is limited
by Snell's law within a critical angle of sin = 1/ n, or
8= 23.5 . That limits the total extracted radiation to a
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solid angle:
0 = 2rn (1 - cos8)
Thus the total fraction of light that can escape
from the semiconductor can be calculated by dividing the
previous expression by 4 n:
Q /4 n = 1/2 (1 - cosA)
According to this expression, only 40 of the incident
radiation is extracted in a GaN based LED. Thus, in LEDs,
most of the internally reflected radiation is re-
absorbed, since in an LED that operates below lasing
threshold, the per-pass stimulated gain is less than per-
pass absorption losses.
The formation of III-nitride templates and epitaxial
growth of nitride devices on such templates can be
developed, for example, using three different epitaxial
methods, which are described below.
The HVPE method is used for the development of GaN
or AlN quasi-substrates (templates). This deposition
method employs HC1 to transport the Ga to the substrate
in the form of GaCl. Growth of GaN in the presence of HC1
has also a number of additional advantages. HC1 etches
excess Ga from the surface of the growing film, and this
enables high growth rates (100-200-p/hr). It also etches
defective GaN occurring primarily at the boundaries of
the hexagonal domains due to incomplete coalescence of
such domains. Finally, another advantage is the leaching
of metallic impurities, which tend to contribute
recombination centers in most semiconductors. Thus this
method leads to very high quality GaN films.
A textured GaN template according to the invention
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is grown by a modified HVPE process. The GaN template can
be grown via a modified HVPE reactor. In the reactor, the
group III precursor can be GaCl gas, which is synthesized
upstream by flowing HC1 on a quartz-boat containing Ga at
temperatures from about 500 C to 1000 C. GaCl gas then
mixes with ammonia (NH3) downstream near the surface of
the substrate wafer to form GaN at temperatures between
about 900 C to 1200 C. A GaN or AlN or AlGaN template of
the invention can be grown along polar- and non-polar
directions. The templates can also grow in their cubic
structure by choosing a substrate having cubic symmetry
such as for example (100)Si (001)GaAs. In this case the
subsequent nitride layers grown on it will have cubic
symmetry as well.
The modified reactor is generally divided into four
zones in which each zone temperature can be individually
controlled. The reactor also has three separate delivery
tubes for the reactant gases and diluents. Nitrogen or
hydrogen is used as a diluent and carrier gases to NH3 and
HC1. Nitrogen is sent through the middle tube where it
acts as a downstream gas sheath to prevent the premixing
of the GaCl and NH3 before the gases contact the substrate
surface. The texturing of the GaN layer can be
attributed to the etching effects of HC1. For example,
texturing occurs as HC1 etches Ga from the surface of the
growing layer. HC1 also etches defective GaN at the
boundary domains of the first layer. The HC1
concentration of the modified HVPE process is
substantially higher than that of typical deposition
processes where texturing is avoided.
The textured GaN templates can be grown under high
growth rate conditions ranging from about 30 to 200 m
per hr that is controlled by the flow ratio of NH3 to the
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group III precursor. The flow ratio is typically about
300 to 10. The template's growth is performed by
pretreatment of the substrate with GaCl gas or by exposing
the sapphire surface to ammonia for a short time
(nitridation) at 1000 C followed by the growth of a thin
GaN buffer layer from 550 C to 650 C. The growth area can
then be ramped to about 1070 C for high temperature
epilayer growth of GaN. The substrate can also be
pretreated prior to growth with sputtered zinc oxide.
The usual thickness of the zinc oxide is from about 500 A.
to 1500 A. Growth of the template is then performed by
heating the chamber to the growth temperature and flowing
the reactant gases in order to initiate growth.
MOCVD is the method currently used by industry for
the growth of GaN-based LEDs. This method produces
nitrides by the reaction of Group III-alkyls (e.g. (CH3)
3Ga or (C2H5) 3Ga) with NH3. One problem with this method
is the cost associated with the high consumption of NH3.
Growth of GaN films at 1p/hr requires 5 to 10 lpm of
NH3.
The MBE method forms III-nitrides by the reaction of
Group III elements with molecular nitrogen activated by
various forms of RF or microwave plasmas. An alternative
approach is the reaction of Group III elements with
ammonia on a heated substrate. The Group III elements can
be either evaporated from effusion cells or provided in
the form of Group III alkyls. It is generally believed
that products produced by the MBE method are more
expensive due to throughput issues. However, in the
growth of nitrides, a significant part of the cost is
determined by the consumption of nitrogen precursors.
During MBE growth of nitride devices, one employs
approximately 1 to 50 sccm of nitrogen or ammonia, which
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is several orders of magnitude less than what is employed
during MOCVD growth. This together with the fact that MBE
production equipment employs multi-wafer deposition
systems makes the MBE method attractive for the
development of inexpensive nitride devices. InGaN-based
laser diodes have recently been produced by the MBE
method [Hooper et al., Electronics Letters, Vol. 40, 8
Jan. 20041.
In one aspect of the invention, the surface of a GaN
template is randomly textured. Appropriate random surface
texture can be produced by any suitable mechanical or
chemical techniques, including modified HVPE. In modified
HVPE, the surface texture of the GaN template can be
controlled by varying the group-III to group-V ratio.
For example, using a molar ratio of NH3 to HC1 of 5:1 to
10:1 yields randomly textured GaN templates by modified
HVPE, whereas conventional HVPE using higher ratios such
as 20:1 to 50:1 or higher yields smooth templates. Other
methods to produce randomly textured GaN templates
include incomplete nitridation of a substrate such as a
sapphire wafer, or using an extremely thin GaN buffer.
Growth of GaN at high temperatures under nitrogen-rich
conditions can also yield randomly textured GaN templates
by the MBE method. For example, using a molar ratio of
Ga/N of less than 1 produces randomly textured GaN
templates. Using a molar ratio of Ga/N of more than 1
results in smooth GaN templates.
Surface texture can be investigated using available
techniques such as atomic force microscopy (AFM) and
scanning electron microscopy (SEM). The degree of
randomness can be ascertained by evaluating the
distribution of surface depth; a randomly textured
surface shows an approximately Gaussian distribution of
surface depth. In order to obtain optimal light
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extraction from an emitter, the average surface depth is
preferably in the range of the wavelength of light
emitted. For example, for a visible light LED, an
average surface depth in the range of 200 nm to 1.5 pm is
preferred.
Textured III-nitride templates can be formed either
along polar or non-polar directions.
The majority of the work on III-nitrides reported in
the literature involves the heteroepitaxial growth of
these materials on either (0001) sapphire or 6H-SiC
substrates by various deposition methods. Materials and
devices grown on these substrates contain a high density
of threading defects (dislocations and inversion domain
boundaries). Furthermore, the [0001] orientation is a
polar direction in the non-centrosymmetric wurtzite
structure, which gives rise to internal electric fields
in heterostructures due to spontaneous and piezoelectric
polarizations. While such polarization effects may be
desirable in some type of devices (e.g. piezoelectric
doping in FETs), they may be undesirable for emitters
based on multiple quantum well (MQW) structures due to
the QCSE. This effect causes a red-shift in QW emission
due to the distortion of the quantum wells, and also
results in a reduced quantum efficiency because the
electron and hole wave functions are separated in space.
Recently it has been demonstrated that growth of
GaN/ AlGaN MQWs on the R-plane sapphire (10-12) leads to
films along the (11-20) direction (Iyer et al., 2003).
The (11-20) direction has the polarization vector in the
plane of the MQWs, and this eliminates internal fields
perpendicular to the quantum wells. Therefore, emission
from such quantum wells is not red-shifted and the
luminescence efficiency is not reduced.
Textured nitride templates along polar direction can
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be grown on (0001) sapphire, (11-20) sapphire, 6H-SiC,
(0001) ZnO, (111) Si, and (111) GaAs. Textured nitride
templates along non-polar directions can be grown on the
R-plane (10-12) and the M-plane (10-10) sapphire
substrates and corresponding planes of 6H-SiC and ZnO.
Such textured templates can be grown by the three
deposition methods as discussed previously.
The textured nitride templates can be used as
substrates for the growth of highly efficient LEDs. By
the virtue that the surface is spontaneously textured to
some degree during growth, the gradual changing of the
index of refraction from the bulk of the semiconductor to
air effectively increases the light escape cone and
reduces loss of light via internal reflections. Thus
light emitted from the semiconductor is extracted more
efficiently, thereby increasing the external quantum
efficiency of the device. In the same argument,
photodetectors and solar cells grown on such templates
would absorb the light more efficiently and they would
not require additional anti-reflection coatings.
Furthermore, textured nitride surfaces can also
increase the IQE of LEDs based III-Nitride semiconductor
MQWs due to partial suppression of the polarization
effects.
GaN templates can be grown by the HVPE method with
variable surface texture. These templates can be
characterized by studying their surface morphology,
reflectivity, transport and photoluminescence properties.
The luminescence extraction efficiency can be
approximately 1000. GaN/AlGaN MQWs can be grown on both
smooth and textured GaN templates. The photoluminescence
measurements on the "wrinkle" QWs indicate a significant
improvement in the IQE compared to that from the smooth
QWs, a result attributed to the reduction of the quantum
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confined Stark-effect (QCSE) since the QWs are not
perpendicular to the [0001] polar direction. Nitride-LED
structures incorporating "wrinkled" QWS have
significantly higher external quantum efficiency than
those employing smooth quantum wells.
The textured boundary between a GaN layer ("top
layer") and air (or other material) increases the
extraction efficiency with respect to photon trajectories
across the boundary by reducing the amount of total
internal reflection within the top layer. The surface
features of the textured surface can have feature
dimensions as small as about one wavelength; however
larger texture features are acceptable. The top layer can
be grown conformally over a lower layer, such as a
textured template. The top layer can, but need not, be as
much as several thousand A thick.
The boundary between the GaN layer and air (or other
material) can be textured by growing or depositing the
GaN layer directly on a textured template, such as an n-
type GaN layer, or on intervening layers, such as quantum
wells (QWs) or MQWs, that have been conformally grown or
deposited on the textured template. Alternatively, a
smooth GaN layer can be grown and subsequently its
surface can be roughened, such as by lithography, even if
the GaN layer is not grown on a textured surface. Such
post-growth roughening can damage the surface of the GaN
layer. For example, "point defects" can be created.
However, this damage can be remediated, such as by
annealing.
MQWs can be grown on the n-GaN layer before a p-GaN
layer is grown on the MQWs. For example, the MQW layers
can be grown by MBE or MOCVD. In one embodiment, ten
pairs of GaN wells and AlGaN barriers are grown, each
well and each barrier layer being about 78 A thick. In
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another embodiment, the layers are 50 A thick each.
However, a wide range (including less than 50 A and
greater than 78 A) of thicknesses of the well and of the
barrier layers is acceptable. The total thickness of the
MQWs can be as much as or more than 1,000 A. Furthermore,
the well and the barrier layers need not be of equal
thicknesses. For example, 70 A (each) well layers can be
combined with 80 A (each) barrier layers.
As noted, the textured MQWs between the n-type and
the p-type GaN layers increase the IQE of the P-N
junction, thereby increasing the amount of light produced
by the P-N junction (or the amount of external light that
is detected in the junction in the case of a
photodetector). Embodiments can include the textured
junction alone, the textured top layer alone or a
combination of the textured junction and the textured top
layer. In addition, any of these embodiments can include
or alternatively omit the textured QWs -or MQWs.
A textured P-N junction (with or without QWs or
MQWs) has more surface (contact) area in the junction
than a smooth P-N junction, given a constant diameter or
other outer dimension of an LED or other semiconductor
device. This increased surface area can increase the
efficiency of the device.
To register lithographic masks or the like with
integrated circuit wafers, such as for subsequent process
steps involving the wafer, an operator typically observes
the wafer through a microscope while a light source
illuminates the wafer through the microscope. The light
illuminates the top surface of the wafer, making
registration marks on the wafer visible to the operator.
However, little light is reflected from a device that
includes one or more of the characteristics described
above. Consequently, observing the registration marks on
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the surface of a wafer constructed according to one or
more aspects of the present invention using an optical
microscope can be difficult. This observational
difficulty can lead to difficulty registering
lithographic masks used in subsequent processing steps.
To overcome this difficulty, in accordance with another
embodiment of the present invention, a light source
illuminates the edge (side) of the wafer, thereby making
the registration marks and the like on the wafer visible
to the operator. Light is transmitted from the surface of
the wafer, through the microscope, to the operator,
rather than being reflected from the surface, as in the
prior art. The light source can, but need not, be
external to the microscope.
The invention also provides a novel type of white
LED. An LED based on textured InGaN/GaN MQWs grown on
textured GaN templates produced by HVPE produces
dichromatic electroluminescence, resulting in white
light. For example, the color temperature of a white LED
according to the invention can be in the range of about
2500 K to about 7500 K, and can be varied by altering the
DC injection current. A first peak of electroluminescence
is typically in the range of about 390 - 450 nm, and a
second peak is in the range of about 500 - 600 nm. The
color of the combined dichromatic emission depends on the
bias or injection current used to drive
electroluminescence. The overall color is blue-shifted
with increasing injection current, due to an increase in
the overall contribution from the peak in the 390 - 450
nm range. The dichromatic emission of LEDs according to
the invention is believed to result from the emission of
light from two or more distinct regions of randomly
textured MQWs. Quantum well layers in randomly textured
MQWs have at least two distinct thicknesses, since the
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deposition process results in somewhat thicker well
layers in flat regions and somewhat thinner well layers
on inclined regions. Thinner well layers emit at higher
energies and therefore produce an emission peak which is
blue-shifted compared to thicker well layers.
In one embodiment, an LED according to the invention
is combined with one or more. conventional LEDs to yield
an altered or full spectrum combination LED device. In
another embodiment, two or more LEDS according to the
invention, each having distinct electroluminescence
properties, such as color temperature, are combined to
yield an altered or full spectrum combination LED device.
In addition to the bias current effect on LED color,
the entire electroluminescence spectrum of LEDs of the
invention can be altered by varying the In content. In
can be present in amounts varying from at least 10% to
1000i of any given III-nitride layer of the device.
Increasing the In content results in a red-shift of the
electroluminescence spectrum.
The variable color feature of LEDs according to the
invention has numerous applications, including use to
fabricate variable color indicators and displays, color
image displays to show still pictures or photographs as
well as video images, and projection devices for both
still images and video. Techniques and devices for
arranging and controlling LEDs according to the invention
to produce color image displays are well known in the
art: For example, conventional and digital drivers for
LED image displays are disclosed in U.S. Patent No.
7,109,957, which is hereby incorporated by reference.
Such control devices can be modified to control the bias
current of the present LEDs so as to modify their color
to produce a color image. Similarly, technology
including control circuitry, software, and optics for
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producing projectors using LED arrays is well known, and
can be adapted for use with the LEDs according to the
invention. For example, U.S. Patent No. 6,224,216
describes such LED projectors, and is incorporated by
reference in its entirety..
The examples herein are provided to illustrate
advantages of the present invention. The examples can
include or incorporate any of the variations or
embodiments of the invention described above. The
embodiments described above may also each include or
incorporate the variations of any or all other
embodiments of the invention. The following examples are
not intended in any way to limit the scope of the
invention.
EXAMPLE I
Growth of Textured GaN Templates by HVPE
Textured GaN templates were fabricated by the
modified HVPE process described above. Fig. 7 shows an
electrically excited wafer level LED radiating at p
contact 20. This blue LED structure was made on an
unpolished (0001) sapphire substrate. On this substrate
was grown 3 microns of heavily doped n-type GaN, followed
by 10 MQWs consisting of InGaN with 13o indium as the
wells and GaN as the barriers. The growth of the MQWs is
followed by a thin (about 10nm) electron blocking layer
consisting of AlGaN with 30% Al doped p-type with
magnesium, and this is followed by 200nm of heavily p-
type doped GaN with magnesium. The free surface from
where the light is emitted has replicated the morphology
of the unpolished sapphire substrate.
Fig-. 8a shows a scanning electron microscope (SEM)
image of a GaN template randomly textured as grown via
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the modified HVPE process. The image was captured with
the sample tilted about thirty degrees with respect to
the electron beam. Growth of the GaN layer occurred on a
(0001) sapphire substrate. The growth was performed via
a process using 25 standard cubic centimeters per minute
(sccm) of HC1 during pretreatment at 1000 C. The process
also employed a ratio of ammonia to the group III
precursor of 150 during growth of the buffer layer at
about 590 C. The stage of high temperature growth at
1070 C then used an ammonia to group III ratio of 60. The
extent or degree of texturing of the template was
determined to be dependent upon the amount of GaCl
arriving at the growth front. Such an amount of GaCl can
also control the growth rate.
In comparison to Fig. 8a, Fig. 8b shows an SEM image
of a standard GaN layer that is atomically smooth. As
shown, the surface topology of the conventional GaN layer
is untextured despite a few surface defects. The image
was captured with the sample tilted about thirty degrees
with respect to the electron beam. Photoluminescence of
the conventional GaN layer having an atomically smooth
surface was compared to that of a randomly textured
gallium nitride template of the invention. Both layer
samples were measured at conditions that were identical
using a 10 milliwatt (mW) helium cadmium laser as the
excitation source.
The results of the comparison are shown by Fig. 9 in
which the photoluminescence intensity of the textured
template is more than fifty times greater than the
intensity of the smooth GaN layer. Enhanced light
extraction occurs through a surface that is textured
particularly with the high index of refraction of such
semiconductor layers. The textured surface provides an
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increase in the escape cone of a single photon compared
to the limited escape cone by a high index of refraction
change between a GaN layer and air.
The randomness of texturing of a group III layer
template of the invention is illustrated in Figs. 10a and
10b. Fig. l0a is an atomic force microscope image of a
GaN template of the invention with a depth analysis plot
of the imaged area in Fig. 10b. The plot shows the
Gaussian distribution of the surface topology for the
template, characteristic of randomness. The average peak-
to-valley surface topology is approximately 1.3 microns.
EXAMPLE II
Growth Of Multiple Wrinkled Quantum Wells On A Textured
Template
Fig. 6 is a transmission electron microscope image
showing multiple quantum wells on a textured surface
(wrinkled quantum wells). The quantum wells comprise ten
pairs of AlGaN and GaN layers. An individual GaN layer
may comprise a textured quantum well layer with the AlGaN
layer serving as the barrier layer. The composition of
the AlGaN layer, for example, is Alo,aGao,$N. Generally,
that is AlXGal_XN. The multiple quantum wells can also be
made by any combination of small gap III-V nitride films
(wells) and large gap III-V nitride films (barriers). The
composition of the MQW determines the emission energy of
light from about 0.7 eV of pure InN to 6eV from pure A1N.
The plurality of quantum well layers are grown by any
suitable deposition process. A MBE process involves the
reaction of a group III material with nitrogen that has
been activated by radio frequency or microwave plasma.
An alternative approach would be to react group III
materials with ammonia on a heated substrate.
The group III materials for semiconductor growth
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through a growth process can be evaporated from effusion
cells or may be provided in the form of group III alkyls.
During semiconductor growth in an MBE or plasma-assisted
MEE process, nitrogen or ammonia gas is typically used
from about 1 to 100 sccm. As the quantum wells are
grown, the layers of quantum wells replicate the texture
of the template. Such MBE processes are known in the
art. The invention also contemplates other typical
approaches for semiconductor layer growth that may be
employed by a person of ordinary skill within the art.
The ten pairs of AlGaN and GaN textured quantum
wells had a well thickness of about 7 nanometers (nm) and
a corresponding barrier layer thickness of about 8 nm.
The plurality of quantum wells were grown with the
substrate at a temperature of about 750 C. An AlGaN
barrier layer is first grown upon a group III-v textured
template of the invention. The barrier layer is then a
surface for deposition of a quantum well, GaN layer. The
GaN layer then serves as a growth surface for the next
barrier layer. This growth pattern can be continued
until multiple quantum well layers are formed. The wells
replicate the surface topology of the underlying textured
template. The thicknesses of the well and barrier layers
can, for example, also be from 10 A to more than 500 A.
Figs. 11 and 12 show photoluminescence spectra of
conventional quantum wells and textured quantum wells
grown on a textured template of the invention
respectively. The photoluminescence spectrum from the
quantum wells grown onto a conventional smooth GaN layer
exhibits a high intensity peak at 364 nm, which is due
primarily to the smooth bulk GaN layer underneath the
MQWs. The extremely low and broad luminescence peak at
about 396 nm was assumed to be due to the smooth wells.
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A cathodoluminescence spectrum of the smooth well sample
was used to verify the assumption. The spectrum was
performed using low acceleration voltage of about 4 kV in
order to probe the quantum wells. The results are shown
by the inset of Fig. 11. The results confirm that the
broad peak occurring at 396 nm corresponds to the
conventional quantum wells.
Thus, the luminescence observed from the smooth
quantum wells is shown to be greatly reduced in magnitude
and red-shifted with respect to the bulk. These results
are consistent with the QCSE.
In comparison to typical quantum wells, the
photoluminescence spectra of those wells that are
textured by a textured template of the invention are
blue-shifted with respect to the luminescence spectra of
the bulk GaN layer. The plurality of textured quantum
wells also exhibits substantially increased luminescence
as compared to the template on which the wells are grown.
These results indicate that wrinkled wells formed on
a textured group III-nitride template are not distorted
by the internal fields associated with polarization.
Fig. 12 also shows that the peak photoluminescence for
the textured quantum wells is more than about seven
hundred times higher than those grown on a conventional
smooth GaN layer. The difference is due to both enhanced
light extraction through the textured surface and the
enhanced spontaneous emission rate of the quantum wells
due to elimination of the QCSE.
EXAMPLE III
Textured Surface Made By Etching Of A Masked Template
In this.example, a textured substrate is created
with a textured surface on which additional layers are
grown, while replicating the textured features. The
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additional layers may be grown so as to form a textured
template, a p-n junction or an optical device of the
invention. The additional layer(s) may also comprise
multiple quantum wells formed by a plurality of well and
barrier layers. The surface of the substrate to be
textured may be smooth or previously textured. The
surface of the substrate can also be unaltered or
otherwise natural.
A mask structure comprising a monolayer of
monodisperse spherical colloidal particles is coated onto
the surface of the substrate. The substrate can include
silicon, silicon carbide, sapphire, gallium arsenide,
gallium nitride, aluminum nitride, zinc oxide, or glass.
Spherical monodisperse colloidal particles can be
commercially obtained in sizes ranging from 0.02 to 10
microns. The packing of the particles onto the surface
of the substrate may be either periodic or random
depending on the technique used for coating. Coating of
the mask structure over a one to five inch diameter
portion of a substrate requires several minutes. Such a
coated area can define 108 to 1012 submicron features on
the substrate.
The masked surface may then be etched by, for
example, ion beam etching. The etching forms the
individual particles into pillars on the substrate
surface. The aspect ratio and shape of the pillars is
determined by the relative mask etch rates and the
underlying substrate material. To minimize the aspect
ratio of the pillars, both physical and chemically
assisted ion beam etching can be employed. The surface
of the substrate can then be etched by a liquid or gas
such as hydrogen fluoride, chlorine, boron tri-chloride
or argon. The etching of the substrate due to the liquid
or gas is less significant in some areas than others as
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the pillars tend to retard or prevent portions of the
substrate surface from being etched.
After etching, the pillars on the surface of the
substrate can be removed by a solvent. The solvent
dissolves the pillars to yield the substrate with a
textured surface. The surface of the substrate can then
be used to grow additional layers that replicate the
textured features. This technique for etching and
texturing the surface of a substrate has also been
described in greater detail by Deckman et al.,
"Molecular-scale microporous superlattices," MRS
Bulletin, pp. 24-26 (1987).
EXAMPLE IV
Fabrication And Characterization Of LED Structures On
Textured GaN Templates
LED structures were fabricated on HVPE grown
templates having different textures. The device structure
is shown schematically in Fig. 19. 800 lZ X 800 p mesas
were formed by ICP etching. Metal contacts were deposited
by beam evaporation to n-GaN:
Ti(10nm)/Al(120nm)/Ni(20nm)/Au(80nm) and to p-GaN:
Ni(5nm)/Au(20nm). The Au metal on the top of the mesa was
quite thick and transmitted only a small fraction of the
light generated within the LED structure. The spectral
dependence of two devices having different surface
texture is shown as a function of injection current in
Fig. 17. These data indicate that by increasing the
injection current the emitted light filled the entire
region of the visible spectrum and thus produced white
light. By visual inspection of the emission, the blue and
the green emissions were observed to originate from
different parts of the mesa. This is evidence that the
broad green spectrum is not related to defects, but to
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emission from planar QWs which are approximately
perpendicular to the polarization direction. This
emission is red shifted due to the QCSE. This
interpretation is consistent with the second LED having a
flatter surface.
EXAMPLE V
Fabrication Of Thick n-GaN Templates By The HVPE Method
On C-Plane Sapphire Substrates
The growth conditions in this method were adjusted
to lead to n-type GaN templates with various degrees of
surface morphology from atomically smooth to completely
random texture. These GaN templates were characterized by
studying their reflectivity in the W and visible parts
of the spectrum as well as their photoluminescence (PL)
excited with a He-Cd laser. The reflectivity was
suppressed from approximately 20% for smooth surfaces to
approximately 1 o to 2% for the randomly texture surfaces
in the entire spectral region. The photoluminescence
intensity from the textured GaN templates was found to be
significantly higher compared to that from identically
produced and similarly doped GaN templates having
atomically smooth surfaces. Specifically, the ratio
between the integrated photoluminescence from the GaN
textured template and the GaN template with a smooth
surface, measured under identical conditions, was about
55. This significant enhancement of the photoluminescence
from the randomly textured GaN template is attributed
partly to enhanced light extraction through the textured
surface, which is expected to be only 4% from the smooth
surface, and partly to enhancement in spontaneous
emission rate due to exciton localization at the textured
surface.
Identical GaN/AlGaN MQWs, with well and barrier
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widths of 7 nm, were grown on both the textured and the
smooth GaN templates by plasma-assisted MBE and their
optical properties were evaluated by photoluminescence
(PL) and cathodoluminescence (CL) measurements. The
photoluminescence spectra of the smooth and "wrinkled"
QWs had significant differences. The photoluminescence
from the smooth quantum wells had a single peak at 396
nm, consistent with the expected red-shift from the
photoluminescence spectra of the bulk GaN films due to
the QCSE. The photoluminescence peak from the wrinkled
QWs occurred at 358 nm, which is blue-shifted with
respect to the photoluminescence spectra of the bulk GaN
films, a result consistent with QWs having a square
configuration. Furthermore, the integrated
photoluminescence intensity from the multiple "wrinkled"
quantum wells was about 700 times higher than that of the
smooth MQWs.
The significant enhancement of the photoluminescence
from the "wrinkled" QWs is attributed partly to
enhancement in light extraction through the textured
surface and partly to enhanced spontaneous emission rate.
The increase in the IQE is believed to be due to the
reduction of the QCSE, since the quantum wells are not
perpendicular to the polar [0001) direction. Further
enhancement in IQE is believed to be due to quantum
carrier confinement from "wedge" electronic eigen-modes.
The latter has its origin to the transition in the
carrier behavior from 2D to 1D due the V-shaped
intersecting planes of the quantum wells, and thus the
"wedges" behave as quantum wires, which cause
localization and trapping of excitons.
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EXAMPLE VI
Formation Of III-Nitride Textured Surfaces Along Polar
[0001] Directions
GaN textured templates were prepared by the HVPE
method. The GaN textured templates were grown on a custom
built HVPE reactor (see Fig. 20). In this.reactor, the
Group III precursor, GaCl(g), was synthesized upstream by
flowing hydrogen chloride (HC1) onto a quartz boat
containing Ga at temperatures between 500 C to 1000 C.
GaCl(g) then mixes with ammonia (NH3) downstream near the
surface of the sapphire wafer to form GaN at temperatures
between 900 C to 1200 C as shown in Fig. 20. The reactor
was divided into four zones, wherein each zone
temperature was controlled individually. It had three
separate delivery tubes for the reactant gasses and
diluents. Nitrogen and/or hydrogen were used as diluents
and carrier gasses to both NH3 and HC1. Nitrogen was also
sent through the middle tube where it acted as a gas
curtain or sheath downstream to prevent the premixing of
the GaCl and NH3 before they hit the substrate surface.
The GaN templates (both with smooth and randomly
textured surfaces) were grown under high growth rate
conditions ranging from 30 - 200 pm/hr that was
controlled by the NH3/Group III precursors flow ratios of
10 to 300. The templates were grown using a variety of
techniques. One of these was a three-step growth method
employing a substrate surface pretreatment with GaCl(g)
or nitridation of the sapphire substrate at 1000 C,
followed by a thin GaN buffer layer growth at 590 C. The
growth zone was then ramped-up to 1070 C for the high
temperature GaN growth. Another method employed an
external pretreatment of the sapphire surface prior to
growth with sputtered ZnO. The usual thickness of the ZnO
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was from 500 A to 1500 A. Growth of the GaN templates was
then carried out by directly heating up the chamber up to
the growth temperature and flowing the reactant gases to
initiate the growth.
Fig. 8a shows an SEM image of a GaN template with
random texture grown via the HVPE method. The growth was
carried out with a three-step growth technique using 25
sccm of HC1 during the pretreatment at 1000 C, an
NH3/Group III ratio of 150 during the buffer layer growth
at 590 C and an NH3/Group III ratio of 60 during the
high temperature growth at 1070 C. The degree of texture
was found to depend on the amount of GaCl arriving at the
growth front which also controls the growth rate.
The reflectivity of the textured surface, described
in Fig. 8a, is shown in Fig. 21. As can be seen from this
figure, the reflectivity was below 1 o between 325nm and
700 nm. This should be contrasted with the reflectivity
of a smooth film, which is about 180.
The room temperature photoluminescence (PL) from two
GaN films grown by the HVPE method, one with atomically
smooth surface and the other with a randomly textured
surface, is shown in Fig. 9. The two films were measured
under identical conditions using a 10mW HeCd laser. From
these data we see that the photoluminescence intensity of
the sample with the textured surface was 55 times larger
than the photoluminescence intensity of the smooth film.
EXAMPLE VII
Photoluminescence Of GaN Templates With Different Surface
Roughness
GaN templates with various surface textures were
grown and their photoluminescence spectra were measured
using an Argon-ion laser emitting at 244 nm and at output
power of 20 mW. These templates were characterized first
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by atomic force microscopy (AFM). Figs. 25 to 32 show the
AFM surface morphologies for the GaN textured templates
VH092403-81 (VH81), VH082504-129 (VH129), VH061603-63
(VH63) and VH080604-119 (VH119). The rms roughness of
these templates varies from 627 nm to 238 nm. Other
information from these data is listed in the figure
captions.
The luminescence spectra for the GaN textured
templates described in Figs. 25 to 32 are shown in Fig.
33. Listed in the inset are the rms roughness of the
various templates as well as the full width at half
maximum (FWHM).
The peak intensity versus the rms roughness is shown
in Fig. 34. It is apparent from these data that the
luminescence intensity increases with rms roughness.
To further test the efficiency of the GaN textured
template in light extraction, excitation intensity-
dependent photoluminescence measurements were done. Fig.
22(a) shows a comparison of the efficiency between
20. several templates with different degree of texturing,
including a GaN with a smooth surface, while Fig. 22(b)
shows measurements done on a smooth film and a textured
template with the same carrier concentration. From the
figure, it is evident that the high photoluminescence
intensity is not due to high n-doping concentration.
EXAMPLE VIII
Formation of III-Nitride Textured Surfaces along Non-
Polar Directions
Fig. 23 shows the type of surface texture obtained
in GaN films grown on the R-plane sapphire (1-102) by
HVPE. This template was also grown by the three-step
process as described in Example VI.
The reflectivity of the textured surface described
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in Fig. 23 is shown in Fig. 24. As can be seen from this
figure, the reflectivity was below 1 o between 325nm and
700 nm.
EXAMPLE IX
Formation Of GaN/AlGaN Multiple Quantum Wells (MQWs) On A
Textured GaN Template Grown Along The Polar Direction
To further test the ability to form LED structures
on the textured GaN templates, 10 pairs of
GaN/AlO.2GaO.8N MQWs were deposited by MBE on GaN
textured template VH129 (see Fig. 28). The MQWs were
formed using an RF plasma source to activate molecular
nitrogen and Knudsen effusion cells to evaporate the Ga
and Al. Various MQWs were formed and doped n-type with Si
introduced either in the quantum wells or the barriers or
both. Alternatively, the MQWs could have been grown using
NH3 as the nitrogen source. Similar MQW structures could
also have been grown by the MOCVD method. Similar methods
could also be used to grow InGaN/AlGaN MQWs with various
compositions for emission in the near UV and the visible
part of the electromagnetic spectrum. Fig. 35 shows the
AFM surface morphology of the GaN/AlGaN MQWs grown on the
GaN textured template VH129. The surface morphology and
texture did not change upon the deposition of the MQWs.
In other words, the MQWs coated the surface of the
template conformally.
EXAMPLE X
Photoluminescence Of Gan/Algan Multiple Quantum Wells On
A Textured GaN Template Grown Along The Polar Direction
The photoluminescence spectra for one GaN
(7nm)/A10.2Ga0.8N (8nm) MQW structure grown on GaN
texture template VH082504-129 (Fig. 28) is shown in Fig.
36. As can be seen from the data, the luminescence
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intensity from the MQWs is significantly higher than that
of the GaN textured template. Specifically, the ratio of
the peak intensities is 14. Furthermore, the emission
from the MQWs is blue shifted compared to the emission
from the GaN textured template.
In Fig. 37, photoluminescence spectra are shown for
identical GaN/AlGaN MQWs grown by MBE on textured and
atomically smooth GaN templates. Inset (a) shows in
larger scale the photoluminescence spectrum from the MQWs
grown on the smooth GaN template. The main peak in the
photoluminescence spectrum from the smooth template is
due to the photoluminescence from the template itself.
The photoluminescence from the MQWs has been quenched due
to the QCSE which is present in MQWs grown along the
polar [0001] direction. The luminescence spectra from the
MQWs on the smooth GaN template is shown in inset (b), in
which low voltage (4 kV) cathodoluminescence (CL) was
used to probe as near the surface as possi.ble.- From the
inset, the luminescence peak of the MQWs is centered at
396 nm. Thus, if the number of counts from the MQWs on
the smooth template is estimated at about 5000, and the
peak intensity of the photoluminescence from the MQWs on
the textured templates is 3.50 x 106, the ratio is around
700.
To understand this very significant increase in
photoluminescence. intensity from GaN (5nm)/A10.2Ga0.8N
(8nm) MQWs grown on GaN textured templates, spot
cathodoluminescence measurements were carried out from
such a sample. Specifically, the cathodoluminescence
spectra were measured by focusing the electron beam on
flat areas of the sample (approximately [0001]
orientation) and sloping areas as indicated in Fig. 38.
This figure depicts the cross section of the surface
along a certain direction. The cathodoluminescence
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spectra taken at points A to C indicated in Fig. 38 are
shown in Fig. 39. Fig. 39(a) shows the
cathodoluminescence spectra from a large illuminated area
(60 pm x 40 pm) . The two peaks at 356 nm and 375 nm are
attributed to luminescence from quantum wells which are
not perpendicular to the [0001] polar direction (356 nm)
and quantum wells which are almost perpendicular to the
[0001] polar direction (375 nm). The 356 nm peak is blue-
shifted with respect to the bulk GaN cathodoluminescence
peak (364 nm) while the 375 nm is red-shifted. The red-
shift of the 375 peak as well as its weak intensity can
be accounted for by the internal electric fields due to
polarization effects which distort the MQWs. This
phenomenon is the QCSE. Fig. 39(b) show the spectra from
a point illuminated area on a semi-flat area of the MQWs
as shown in Fig. 38. As expected, the luminescence at 382
nm is from the distorted quantum wells due to QCSE. The
smaller peak at 359 nm is attributed to miniature
roughness in the flat surfaces and thus a small fraction
of the quantum well surface is not perpendicular to the
[0001]. Fig. 39(c) shows the cathodoluminescence spectra
taken from point B in Fig. 38. In this case, the spectrum
can be deconvoluted into two peaks, one from the MQW
emission at 356 nm and another at 364 nm attributed to
emission from the GaN template. Again the data support
that MQWs whose surfaces are not perpendicular to the
[0001] direction have emissions which are blue-shifted
with respect to the bulk as well as intense luminescence
due to the significant reduction of the QCSE. Fig. 39(d)
shows the cathodoluminescence spectra from point C of
Fig. 38. Again, the luminescence occurs at 356 nm
consistent with QW emission not suffering from the QCSE.
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EXAMPLE XI
Formation Of GaN P-N Junction LED Structure On A Textured
GaN Template Grown Along The Polar Direction
A highly conductive Mg-doped p-GaN (hole
concentration - 1018 cm-3) of thickness -0.5 micrometers
was deposited by MBE on top of the n-type auto-doped
(electron concentration - 1019 cm-3 is typical) textured
GaN template. The p-type GaN film was formed using an RF
plasma source to activate molecular nitrogen and Knudsen
effusion cells to evaporate the Ga and Mg. Growth took
place at extreme Ga-rich conditions, which helps the
incorporation of Mg at relatively high substrate
temperatures (700 C - 800 C). Alternatively, the p-type
layer could have been grown using NH3 as the nitrogen
source. A similar p-type layer could also have been grown
by the MOCVD or the HVPE methods. Fig. 13 shows a wafer-
level electroluminescence spectrum of a GaN p-n junction
structure made on a textured GaN template. This spectrum
was taken at room temperature under current injection of
80 mA.
EXAMPLE XII
Growth And Characterization Of GaN/AlGaN MQW LEDs
The majority of the work on III-nitrides reported in
the literature involves the heteroepitaxial growth of
these materials on either (0001) sapphire or 6H-SiC
substrates by various deposition methods. Materials and
devices grown on these substrates contain a high density
of threading defects (dislocations and inversion domain
boundaries). Furthermore, the [0001] orientation is a
polar direction in the non-centrosymmetric wurtzite
structure, which gives rise to internal electric fields
in heterostructures due to spontaneous and piezoelectric
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polarizations. While such polarization effects may be
desirable in some type of devices (e.g. piezoelectric
doping in FETs), they may be undesirable for emitters
based on multiple quantum well (MQW) structures due to
the QCSE. This effect causes a red-shift in QW emission
due to the distortion of the quantum wells,' and also
results in reduced quantum efficiency because the
electron and hole wave functions are separated in space
(see Fig. 40a).
Homoepitaxial growth has been demonstrated for
GaN/AlGaN MQWs on free-standing (10-10) GaN substrates
(M-plane), as well as for similar MQWs grown on sapphire
(10-12) (R-plane) which leads to films along the [11-20]
direction. Both the [10-10] and the [11-20] directions
have the polarization vectors in the planes of the MQWs.
As indicated in Fig. 40b, photoluminescence peak position
of AlGaN/GaN MQWs follows square-well behavior for those
grown along the non-polar directions while similar MQWs
with the polar direction show a significant red-shift.
The luminescence emission efficiency was about 20 times
larger for the QWs grown along non-polar directions for
quantum wells more than 5nm thick.
Growth and Characterization of GaN Templates by HVPE
Growth and characterization of GaN templates by the
HVPE method was reported by Cabalu and co-workers, which
is hereby incorporated by reference in its entirety.
Further, the dissertation entitled "Development Of GaN-
Based Ultraviolet And Visible Lightemitting Diodes Using
Hydride Vapor-Phase Epitaxy And Molecular Beam Epitaxy"
by Jasper Sicat Cabalu, Boston University (2006) is also
incorporated by reference in its entirety.
GaN templates (both with smooth and randomly
textured surfaces) were grown under high growth rate
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conditions ranging from 30 - 200 pm/hr, which was
controlled by the NH3/Group III precursors flow ratios of
to 300. During the growth of the templates, a three-
step growth method was employed. This consisted of a GaCl
5 pretreatment step done at 1000 C, followed by growth of
a low temperature GaN buffer at temperatures between
550 C to 650 C, and finally growth of the high
temperature GaN epilayer.
The templates were characterized by scanning
10 electron microscopy (SEM), photoluminescence, reflectance
and Hall-effect measurements. Photoluminescence
measurements were done using a He-Cd laser as the
excitation source, while the reflectivity measurements
were done using a 150 W Xenon lamp-as a broadband light
source.
Fig. 8 shows a SEM image of a smooth (Fig. 8b) and a
textured (Fig. 8a) GaN template. These images were taken
with the sample tilted 30 with respect to the electron
beam. The degree of surface texture on the templates was
found to depend on the amount of GaCl arriving at the
growth front, which also controls the growth rate of the
film. The atomic force microscopy surface morphology of
the smooth GaN template is shown in Fig. 8c. As can be
seen from these results, the film was atomically smooth
and it was grown under the step-flow growth mode.
Fig. 41 (a) shows a 100 }Zm2 AFM scan of a textured
template surface. Depth analysis of the AFM data (Fig.
41(b)) shows a Gaussian distribution (random
distribution) of surface roughness. GaN templates with
various degrees of surface texture were produced with
average depths ranging from 800 nm to 3pm.
The reflectivity of the textured template described
in Fig. 8a was measured to be below 1o between 325 nm and
700 nm. That is, almost all the incident light from the
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broad band light source is coupled-in to the GaN textured
template. This should be contrasted with the reflectivity
of a smooth film, which is about 180.
The electron mobility versus electron concentration
for the investigated textured templates is shown in Fig.
42. As can be seen from the data, these GaN templates
were all heavily auto-doped n-type and thus suitable for
bottom contact layer in GaN LEDs.
The room temperature photoluminescence (PL) from two
GaN templates, grown by the HVPE method, one with
atomically smooth surface, and the other with a randomly
textured surface is shown in Fig. 9. The two samples were
measured under identical conditions using a 10 mW He-Cd
laser. The peak photoluminescence intensity of the sample
with the textured surface was approximately 55 times
larger than the photoluminescence intensity of the sample
with smooth surface.
The significant enhancement of the photoluminescence
intensity from the randomly textured GaN surface is
attributed partly to the enhanced light extraction
through the textured surface. Due to the random texturing
of the surface, there is an increase in the escape
probability of a single photon since the escape cone is
not limited by the one defined by the indices of
refraction of the semiconductor and air. This is because
the index of refraction in the textured template varies
gradually along the optical axis from the value of 2.5,
corresponding GaN, to 1.0, corresponding to air. In other
words, there are additional escape angles available for
each emitted photon due to the random texture at the
interface. This is analogous to the transmittance through
a diffraction grating, wherein the grating imparts a
phase shift to the incident wave and bends the wavefront
at specific angles depending on the wavelength of the
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incident light [9] In this case, the phase shift is
controlled by the periodic variation of thickness (or
periodic "surface texture") of the grating material at
the grating/air interface. In the case of the textured
GaN templates, the texture of the surface is not
periodic, but random, and this creates random phase
shifts across the interface. This leads to escape angle
randomization that effectively increases the photon
escape probability. Thus, the surface texture allows for
more escape angles that are not within the critical angle
as defined using a smooth interface as illustrated in
Fig. 43b).
If the extraction efficiency of the emitted
photoluminescence from the textured GaN template is 100o,
then under the assumption of equal IQE of the smooth and
texture templates, the ratio of the photoluminescence
intensity from the textured and smooth templates should
have been 25. However, the data shown here indicate that
this ratio is equal to 55. This implies the IQE of the
textured GaN template should be at least two times higher
than that of the smooth GaN template. The IQE from the
textured template actually should be more than a factor
of two greater than that of the smooth template because
it is unlikely that the extraction efficiency from the
textured template is exactly 1000-.. In theory, the
disorder associated with the textured surface leads to a
certain degree of potential fluctuations and thus-
excitons are trapped in local potential minima. This
leads to an enhanced spontaneous,emission probability due
to exciton localization.
Growth and Characterization of GaN/AlGaN MQWs by MBE on
GaN Templates with Variable Surface Texture.
The growth and characterization of GaN/AlGaN MQWs on
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GaN templates with variable surface texture are described
by Cabalu et al., which is hereby incorporated by
reference in its entirety.
Ten pairs of GaN/AlO.2GaO.8N MQWs were deposited by
MBE on both textured as well as smooth GaN templates with
thicknesses 7 nm for the wells and 8 nm for the barriers,
at a substrate temperature of 7500C. AFM studies of the
MQWs on the textured template indicate that the MQWs have
coated the textured GaN template conformally.
The x-ray diffraction pattern around the (0002)
Bragg peak for ten period GaN (7 nm) / A10 . 2Ga0 . 8N (8 nm)
MQWs grown on a GaN textured template is shown in Fig.
44. This figure shows further evidence that MQWs can be
formed on such randomly textured templates as shown by
the appearance of the primary and higher order
superlattice peaks. Furthermore, observation of these
peaks indicates an abrupt interface between the AlGaN
barriers and the GaN wells. Fig. 44 also shows the
simulation result using the kinematical scattering model.
Assuming that the AlGaN barriers and GaN wells have equal
growth rates, simulation results determined a period of
15.4 nm, corresponding to 8.2 nm barrier width and 7.2 nm
well width. From the position of the zeroth order
superlattice peak and assuming the validity of Vegard's
law in this material system, the Al composition in the
AlGaN barriers was determined to be -200. These values
are in agreement to the aimed thicknesses (8 nm barrier
and 7 nm well widths) and alloy composition (20o Al)
during the growth.
The photoluminescence spectra from the MQWs grown on
the smooth and textured GaN templates are shown in Figs.
11 and 12, respectively. The photoluminescence spectra
from the MQWs grown on the smooth GaN template ( Fig . 11)
showed primarily the photoluminescence from the GaN
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template at 364 nm and an extremely small and broad
luminescence peak at about 396 nm. Further verification
that this small peak is due to luminescence from the QWs
was produced by measuring the cathodoluminescence spectra
of the same sample using low acceleration voltage (4 kV)
in order to probe the QWs. These' data are shown in the
inset of Fig. 11. Indeed the data show, in addition to
luminescence from the GaN template at 364 nm, a broad
peak occurring at 396 nm which corresponds to
cathodoluminescence from the QWs. Thus the luminescence
from the QWs is red-shifted with respect to the bulk and
is significantly reduced in magnitude. Both of these
results are consistent -with the quantum confined Stark-
effect (QCSE) since these QWs are perpendicular to the
[0001] polar direction.
Fig. 12 shows the photoluminescence spectra from the
MQWs grown on the textured GaN template. For comparison,
the photoluminescence spectrum from the textured GaN
template is shown in the same figure. It is important to
note that the photoluminescence spectra from the MQWs
were blue--,shifted with respect to the bulk GaN
photoluminescence spectra and also the luminescence
intensity was significantly higher than that from the
textured GaN template. Both of these results are
consistent with square quantum wells. In other words,
because the quantum wells on the textured GaN templates
are not perpendicular to the [0001] direction, they are
not distorted by internal fields associated with
polarization.
A direct comparison of the peak photoluminescence of
the MQWs in Fig. 11 and Fig. 12 indicates that the
photoluminescence intensity from the "wrinkled" MQWs is -
700 times higher than that from the smooth MQWs. This
significant enhancement of the photoluminescence from the
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"wrinkled" QWs is attributable partly to enhancement in
light extraction through the textured surface and partly
to enhanced spontaneous emission rate. If the enhancement
due to light extraction from the textured surface is
assumed to be a factor of 25 higher than that from the
smooth surface, then there is an additional factor
approximately 30 which must be due to the enhancement of
the spontaneous emission rate. The evidence discussed
earlier suggests that the increase in the IQE is due to
the reduction of the QCSE, since the quantum wells are
not perpendicular to polar [0001] direction. Further
enhancement in IQE is also expected due to quantum
carrier confinement from "wedge" electronic eigen-modes.
The latter has its origin to the transition in the
carrier behavior from 2D to 1D due the V-shaped
intersecting planes of the quantum wells, and thus the
"wedges" behave as quantum wires, which cause
localization and trapping of excitons.
EXAMPLE XIII
Phosphorless White LEDs
This example describes a method of making GaN-base
white LEDs or LEDs of various colors without using an
emitter such as phosphorus.
Fig. 14a shows the spectrum of commercially
available white LEDs taken from LumiLeds Technical Data
Sheet DS25. This white LED is based on a nitride LED
structure emitting approximately at 430 nm and exciting a
YAG phosphor emitting a broad spectrum with a peak at 550
nm. Fabricated LED structures based on textured InGaN/GaN
MQWs were grown on textured GaN templates produced by
HVPE. These LEDs have similar spectra as the one shown in
Fig. 14a without the employment of phosphor.
Fig. 14b shows the electroluminescence spectra of
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such an LED. The spectra were measured under DC injection
current of 30 mA. These spectra have remarkable
similarity to that of the commercially available white
LEDs shown in Fig. 14a although no phosphor was used for
the generation of the broad emission at 537 nm. Fig. 14c
shows the LED whose spectrum is shown in Fig. 14b using a
DC injection current of 25 mA.
Relative intensity between these two peaks depended
on the level of current injection. The high-energy band
increased with the bias current. The same LED could
produce different colors, since the color depends on the
relative ratio of the two bands. In Fig. 15a-15c, the
spectra of other LED devices showing similar behavior are
presented.
Fig. 16 shows a photograph of an LED structure taken
under DC injection as described in Fig. 15b. As expected,
the LED has a greenish color since the green band is the
more dominant one. However, certain parts of the wafer
emitted blue light.
In Fig. 17, the dependence of electroluminescence
spectrum on the DC injection current is demonstrated for
two different LEDs. The LED on the right has a greater
proportion of flat surface.
EXAMPLE XIV
Fabrication Of Textured Templates Using MBE
GaN templates were made by plasma-assisted MBE, in
which gallium is reacted with atomic nitrogen obtained by
passing molecular nitrogen through a plasma source. Both
samples were grown at 825 C. The nucleation was
identical, except that growth under gallium-rich
conditions (flux of gallium much greater than flux of
active nitrogen) resulted in a smooth surface (Fig. 45a)
and growth under nitrogen-rich conditions (flux of active
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nitrogen much larger than the flux of gallium) resulted
in a randomly textured surface (Fig. 45b).
EXAMPLE XV
Dependence Of QCSE On Quantum Well Layer Thickness
Since the QCSE is expected to depend on the width of
the quantum well layers, the PL spectra of quantum well
layers with thicknesses of 5.5 and 7.0 nm were
investigated. Figures 46a and 46b show the dependence of
the emission peak and the luminescence intensity,
respectively, versus well width for both smooth and
textured GaN/A10.2Ga0.8N MQWs. As seen in Fig. 46a, the
PL spectra_ from the smooth MQWs were redshifted, while
those from the textured MQWs were slightly blue-shifted
with -respect to the bulk GaN emission. Correspondingly,
the PL intensity from the smooth MQWs increased as the
well width became narrower, while there was only a slight
increase from the textured MQWs, as shown in Fig. 46b.
These results are qualitatively consistent with the QCSE.
EXAMPLE XVI
Effects Of Internal Electric Fields and Polarization In
Quantum Well Layers
The enhancement in spontaneous emission at the
inclined sections of the quantum well layers can be
explained by the transition in carrier behavior from 2D
to 1D (and potentially OD) due the V-shaped intersecting
planes of the quantum wells. Thus, the inclined sections
can behave as quantum wires (or quantum dots), causing
localization and trapping of excitons. in addition, due
to polarization component parallel to the quantum well
layers, as shown in Fig. 47a, electron accumulation at
the intersecting quantum well planes can be expected, as
shown in Fig. 47b. Depending on the equilibrium charge
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density at these intersections, enhancement in
spontaneous emission may result from plasmonic effects.
While the present invention has been described
herein in conjunction with a preferred embodiment, a
person of ordinary skill in the art, after reading the
foregoing specification, will be able to effect changes,
substitutions of equivalents and other alterations to the
devices and methods that are set forth herein. Each
embodiment described above can also have included or
incorporated therewith such variations as disclosed with
regard to any or all of the other embodiments. It is
therefore intended that protection granted by Letter
Patent hereon be limited in breadth only by the
definitions that are contained in the appended claims and
any equivalents thereof.
REFERENCES
Cabalu et al., "Enhanced Light Extraction and Spontaneous
Emission From Textured GaN Templates Formed During
Growth by the HVPE Method", State of the Art Program on
Compound Semiconductors XLI and Nitride and Wide
Bandgap Semiconductors, Sensors and Electronics V,
Electrochemical Society Proceedings, Vol. 2004-06,
pp.351.
Cabalu et al., "Enhanced Light Extraction and Spontaneous
Emission From "Wrinkled" Quantum Wells Grown By Plasma-
Assisted Molecular Beam Epitaxy (PAMBE)", Presented at
the 22nd North America-Molecular Beam Epitaxy
Conference, Banff, Alberta, Canada, October 10-13,
2004, p. 110.
Iyer et al., "Growth and Characterization of Non-polar
(11-20) GaN and AlGaN/GaN MQWs on R-plane (10012
Sapphire", Mater. Res. Soc. Symp. Proc., Vol. 743, pp.
L3.20 (2003).
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Ryu et al., IEEE Journal of Selected Topics in
Quantum Electronics, Vol. 8, No. 231 (2002).
Tsao, J., "Light Emitting Diodes (LEDs) for General
Illumination", An OIDA Technology Roadmap Update
(2002).
Windish et. al., "40o efficient thin film surface
textured LEDs by optimization of natural lithography",
IEEE Trans. Electron Devices, Vol. 47, No. 1492 (2000).
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