Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Field of the Invention
This invention relates to high radiance, light emitting
devices for optical communications, and methods for manufacturing
them. More particularly, it relates to light emitting devices
that have a structure permitting highly efficient coupling with
an optical fiber, and methods for manufacturing them.
Brief Description of_the Drawings
Figure 1 is a vertical sectional view of the prior
art light emitting device,
Figure 2 is a vertical sectional view of an embodiment
of a light emitting device according to an embodiment of this
invention,
Figure 3 is a vertical sectional view of another
embodiment of light emitting device of this invention,
Figure 4 is a view for explaining the operation of
a device of this invention,
Figures 5a - 5e are process diagrams showing an
embodiment of a manufacturing method according to the invention,
Figure 6a ~is an exploded view showing components of
a device of this invention,
Figure 6b shows the finished product, and
Figure 7 is a view showing still another embodiment
of the invention.
Description of the Prior Art
A known light emitting device comprising a light
emitting diode for optical communication is described in
'Material of the Society for Researches in Light Quantum
Electronics, OQE 75-71' published by the Institute of Electrical
Communication in 1975 in Japan. More specifically, as illustrated
in Figure 1, on a semiconductor substrate 11 having a bandgap
wider than the energy range corresponding co the radiation
emitted by the device, there is gr~wn an
B-
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epitaxial layer 12 of opposite conductlvity type to that of the
substrate 11. Thereafter, a glass film layer 13 for current
confinement is provided with a hole in its central part. Ohmic
contacts 14 and 15 are formed on the bottom of the substrate
and on the layer 13, respectively. A p-n junction 16 is formed
between the substratè 11 and the epitaxial layer 12. Radiated
light produced in the p-n junction 16 is introduced into an
optical fiber (not shown) through a window 17 as-indicated by
the arrow L.
Another prior art device is disclosed in French patent
application 7416054 of D. Diguet et al., published May 12, 1975
under No. 2270753. It includes a semiconductor substrate which
is made of a material having a first bandgap, and an epitaxial
layer which is made of a semic~nductor material having a second
bandgap wider than the first one. The substrate and the
epitaxial layer are of the same conductivity type. A p-n
junction is formed by diffusing Zn from the outside surface of
the epitaxial layer to penetrate the semiconductor substrate
beyond the epitaxial layer.
~0 Of these`prior art devices, the first is disadvantageous
in that the area defined by the glass film layer for current
confinement and the area of the actual radiation region do not
agree, the radiation region becoming extended on account of the
"current spreading phenomenon". Since the p-n junction 16
extends to a side surface 18, it touches the external air and
causes non-radiative recombination due to a surface recombination
current. As a result the external efficiency is low. Further,
the semiconductor substrate 11 that exhibits the wider bandgap
has a low carrier concentration of the order of 10 cm
for the reason that in the preparation of a crystal its ohmlc
contact resistively with the electrode layers 14, 15 is
comparatively high, so that the energy efficiency in the case
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of coupling with the optical fiber is lowered.
In the second prior art device, the surface recombination
current is suppressed by the locali~ed p-n junction owing to the
Zn diffusion. In general, however, a diffused junction is
inferior to a grown junction by LPE in the degree of perfection
of the crystal at the radiation region. Consequently, the
external efficiency of this device is low. Another disadvantage
is that the life of this device is shorter than that of a device
with a grown junction.
Summary of the Invention
This invention has for its primary object to provide
a light emitting device having a structure for eliminating or
reducing the phenomenon by which the area of a radiation region
becomes larger than the area determined by a glass film for
current confinement, and also to provide a method of manufacturing
such device.
To this end the invention provides in a light emitting
device having a III-V compound semiconductor substrate whose
bandgap is wider than the energy range corresponding to the
radiation emitted by the device and which has a predetermined
conductivity type, a second III-V compound semiconductor layer--
deposited on an upper surface of said III-V compound semiconduc-
tor substrate and having the opposite conductivity type to that
of said substrate, a current control layer that covers an upper
surface of said second III-V compound semiconductor layer and
has a hole for current flow, a first electrode provided on said
current control layer and being in ohmic contact with said second
III-V compound semiconductor layer, and an ohmic contact elec-
trode provided on a bottom surface of said III-V compound semi-
conductor substrate and having a light extracting window at itscentral part, wherein said current control layer is made of a
layer of high carrier
-- 3 --
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concentration having the opposite conductivity type to that of
said second III-V compound semiconductor layer.
In the preferred embodiment, the junction for radiation
is not a diffused junction but is formed by liquid or vapor
phase epitaxial growth. Also the regional confinement for the
junction is achieved by the reverse-bias effect of the p-n
junction. The ohmic contact region of the III-V compound
semiconductor having the wider bandgap is given a sufficiently
high carrier concentration as to lower its contact resistivity
with the ohmic contact. The portion of a window for radiation
extraction is doped with no impurity and is left at a low
carrier concentration, thereby to reduce the internal absorption
of light.
Embodiments of this invention will now be described.
Description of the Preferred Embodiments
Embodiment 1:
Figure 2 shows an embodiment of light emitting
device according to this invention. Numeral 21 designates
a crystal layer for light transmission that is formed of a
p-conductivity type layer having a bandgap wider than the
energy range corresponding to the radiation emitted by the
device. Numerals 22 and 23 designate n-type and n -type-
crystal layers, respectively, which are successively
and continuously grown on the crystal layer 21. Numerals
24 and 25 indicate layers formed in the p-type crystal layer
21 and in the n-type crystal layer 22 and n -type crystal
layer 23, respectively, said layers being formed by diffusing
Zn thereinto. The Zn diffused layer 24 exhibits a low contact
resistivity to an electrode layer 26. The Zn diffused layer
25 exhibits a low contact resistivity to an electrode layer
27, and acts on a p-n junction 29 to confine the p-n junction
current. The electrode layers 26 and 27 are made of metal.
B
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.
Shown at 28 is a window for extracting radiation, indicated by
the arrow L, and an optical fiber (not shown) is attached to
this portion. The p-n junction 29 is formed by the liquid
phase epitaxial growth. By controlling the diffusion depth
of the Zn diffused, p -type layer 25 in the n-type crystal
layer 22 and the n -type crystal layer 23, the radiation region
can be formed in a size corresponding to the size of the
extracting window 28.
In this manner the Zn layer 25 is diffused in the ;
0 n-type crystal layer 22 and the n -type crystal layer 23, and
the diffusion depth is controlled, whereby the radiation region
is confined to a small area of the p-n junction 29, as explained
later, making it possible to attain light emission of very high `
radiance. The surface of the electrode layer for the n-type
ohmic contact 27 is so formed as to be flat without any
unevenness over the layer 23 and the layer 25 in order that
the electrode layer may efficiently radiate heat in close
contact with a heat sink (not shown).
The electrode 26 may be disposed directly on the
bottom of the p-type crystal layer 21, without providing the
Zn diffused layer 24, as shown in Figure 3.
With this structure the current flow reg-ion is
confined to a specific part only, the radiation being emitted ~-
from the small area of the p-n junction. Figure 4 shows
current paths when applying a voltage to the device shown in
Figure 2. Three cases can be considered; when electrons
starting from the electrode layer 27 travel along arrows
A, B and C. When electrons flow over a long distance in the
n-type crystal layer 22, as indicated by the arrow B, the
resistance is much higher than in the case where th~y flow
along the arrow A. Therefore, the number of electrons flowing
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as indicated by the arrow B i5 almost zero. Since the p-n
junction D between the n-type crystal layer 22 and the p-type
Zn diffused layer 25 is reverse-biased, the electrons cannot
flow as indicated by the arrow C. Therefore, the electrons
essentially flow as indicated by the arrow A without fail,
and in the p-n junction 29 currents are crowded into a portion
E indicated by a thick line, so that light of high radiance
is emitted from the portion E (arrow L).
It is also possible to omit the n crystal layer 23
provided on the n-type crystal layer 22 shown in Figure 2 and
Figure 3 and to make the corresponding portion n-type.
As apparent from this description~ this device
simultaneously solves the problems of the prior art, i.e., the
current spreading phenomenon of the radiation region, the
lowering of the external efficiency ascribable to the surface
recombination current, and the disadvantage of short life,
low reliability, etc.
Embodiment 2:
An examp~e of a manufacturing process will now be
described with referenee to ~igures 5a - Se.
As shown in Fig~re 5a, on a III-V eompo~nd semieonduetor
substrate doped with an impurity bestowing a predetermined
conductivity type, for example~ an n-type orp-type (1 0 0)
GaAs substrate 30 whose carrier concentration is in the order
of 10 em , a p-type Gal xAlxAs (0 < x < 1) layer 31 about
200 ~m thiek is grown by the liquid phase epitaxial growth.
By way of example, the value x may deerease eonti.nuously from
0.4 to 0.1 upwards from the substrate surfaee. Subsequently,
the grown layer is polished until the AlAs eomposition of
its upper surfaee beeomes above 15 % (above x = 0.15), and it
has aehieved a mirror surfaee. Aeeording to a eapaeitanee -
voltage measurement,
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the carrier concentration of the crystal layer 31 was
5 x 10 cm 3. In the next step, using the crystal layer 31
as a substrate and a sliding method employing a graphite jig,
a first layer 32 (p-type Gal xAl As layer, 0 ~ x _ 1), a second
layer 33 (n-type Gal Al As layer, 0 < x _ 1) and a third layer
34 (n -type Gal Al As layer, 0 < x _ 1) are successively and
continuously crystal-grown from a Ga solution (in which GaAs
or Al is used as a solute, Zn or Si representing a p-type
bestowing impurity or Te representing an n-type bestowing
impurity being used as dopant).
The thickness of the layers 32, 33 and 34 were,
for example, about 30 ~m, 2 ~m and 1 ~m, respectively. The
carrier concentrations of the respective layers were controlled
by the quantities of dopants Zn, Si and Te, and were, for
exa~ple, 2 3 x 10 cm , 1 x 10 8 cm 3 and 5 x 1018 cm 3.
Subsequently, as shown in Figure 5b, parts of the
substrate 30 and the crystal layer 31 are polished and removed
so that the total thickness may become 150 rum, and the exposed
surface of the crystal layer 31 is finished into a mirror
surface. Thereafter, an A1203 film 35 and a PSG (Phospho-
Silicate-Glass) film 36, which are 1000 R and 2000 ~ thick
respectively, are deposited on each of the front and rear
surfaces of the resultant structure. The outer peripheral
parts of the films 35 and 36 are then removed (when the device
of Figure 3 is to be produced, the films 35 and 36 are deposited
entirely on the bottom surface~, to form a diffusion mask of
a diameter of 40 ~m on the side of the third layer 34 and a
diffusion mask of a diameter of 150 ~m on the side of the
crystal layer 31.
3Q Thereafter, the resultant structure is vacuum-sealed
into a quartz ampoule together with a ZnAs2 source, and Zn
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diffused layers 37 and 38 about 2.5 ~m thick, as shown in
Figure 5c, are formed by a heat treatment at 650 C for 120
minutes (when the device of Figure 3 is to be fabricated,
the glass layer except the light extracting portion at the
bottom of the substrate is removed in advance). At this
time, the spacing between the diffusion surface A of the Zn
diffused layer 37 and the first layer 32, that is, the
thickness of the second layer 33 is about 0.5 ~m.
Subsequently, as shown in Figure 5d, using the films
35 and 36 as an evaporation mask, AuZn or AuSbZn forming an
ohmic contact electrode layer 39 on the p-side is evaporated
to a thickness of about 2 ~m.
Further, as shown in Figure 5e, that part of the
ohmic contact electrode layer 39 which corresponds to a light
extracting window 42 and the films 35 and 36 which have been
employed as the diffusion mask are respectively removed by
photo-lithography. At this time, the diffusion mask (films
35 and 36) on the n-side or on the upper side in the illustration
is covered with apiezon in advance. After completion of the
photo-lithographic treatment, the apiezon is removed with
trichloro-ethylene, and the films 35 and 36 that have been
employed as the diffusion mask on the n-side (upper side) are
successively removed. Subsequently, AuGe-Ni-Au 40 is evaporated
on the upper surface of the resultant structure as an n-type
ohmic contact electrode layer to a thickness of about 1 ~m.
Further, Au 41 being about 9 ~m thick is deposited on the
electrode layer 40 by the electrolytic plating.
Thereafter, the resultant structure in the form
of a wafer is cut by scribing into a chip of about 600 ~m x
600 ~m. Thus, a light emitting diode chip (abbreviated to
"LED chip") according to this invention is obtained.
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In a concrete example of this embodiment, a GaAs
substrate is used as the starting substrate, and the grown
substance is obtained by growing a layer of a mixed crystal
with the substrate material that has a bandgap wider than that
of the substrate material. The step of providing the n+-type
mixed crystal layer need not be carried out in some devices.
Figure 6a and Figure 6b are sectional, exploded and
assembled views respectively, showing components for assembling
a light emitting diode using the LED chip described above.
In these figures, numeral 61 designates a stem having an
insulating part 61a, numeral 62 is a submount, numeral 63 is the
LED chip, numeral 64 is a fiber connector, and numeral 65 is
an optical fiber.
The sequence of assembly is as follows. The submount
62 and the LED chip 63 are first bonded together into an
integral form. The submount 62 and the LED chip 63 in this
integral form are then bonded onto the lower surface of the
fiber connector 64. The resultant structure is then bonded
into the stem 61 by means of a layer 66 of a low fusing metal
such as indium, and the stem 61 and the fiber connector 64
are hermetically secured together with an epoxy resin 67.
Thereafter, the optical fiber 65 is passed through the fiber
connector 64 so that its lower end face is brought into
close contact with the light extracting window of the LED -'
chip 63. It is then fixed to the fiber connector 64 by epoxy
resin 68.
Measurements were taken and the characteristics
mentioned below were observed. The optical fiber 65 had a
numerical aperture of 0.16, a core diameter of 85 ~m, and a
length of 50 cm. When a d.c. current of 100 mA was passed,
the optical fiber output was 350 ~W on the average, the center
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wavelength of light emission was 8300 A, and the spectral
half width was 270 A. When the fiber was not attached, a much
larger value of 4 - 7 mW light output was obtained. The
thermal resistance was as low as 30 - 50 deg./W.
Since the thermal resistance was low, as mentioned,
and the heat radiation was favorable, saturation of the light
output versus increase of bias current was slight. When the
bias current was an average of 100 mA and the modulation depth
was 40 %, the modulation distortion of the light output was
as low as -50 dB. The current - voltage characteristics were
also inspected. No leakage current was found and such good
characteristics as a forward voltage of 1.65 V (IF = 100 mA, d.c.)
and a breakdown voltage of about 10 V were exhibited.
The radiation region was also measured. As a result,
the radiation diameter was extremely small, i.e. about 45 ~m,
and it was verified that the radiation region hardly spread
from the area confined by the selective Zn diffusion layer 25
in Figure 2. In this manner, a light emission of extraordinarily
high radiance can be obtained from a very small areaO
Embodiment 3:
Figure 7 shows a section through a light emitting
device according to another embodiment of this invention. A
light extracting window 51 is formed in such a way that a
portion corresponding to the light extracting window 28 in
Figure 2 is removed by mask etching with an etchant of H2S04 -
H202 - H20. In this case a p region 47 in a p-type portion
need not be formed by selective diffusion, but it may be
formed in such a way that, after diffusion over the entire
area of a wafer surface, removal by mask etching is carried
out to a depth slightly greater than the diffusion depth, i.e.,
the mask-etched portion becomes slightly deeper than the
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diffused layer 47. The other steps of manufacture may be similar
to those illustrated in Figures Sa - 5e.
An advantage in this case is that, by suitably
selecting the diameter of the light extracting window 51 to
be etched and removed, the coupling of the device with an
optical fiber is very effective and the difficult operation
of mask registration can be omitted. There is added the
advantage that, by such deep etching and removal, the light
output is enhanced to the amount of the light absorption
by the removed portion.
In Figure 7, numeral 43 indicates a p-conductivity
type layer, numeral 44 and n-conductivity type layer, numeral
45 an n -conductivity type layer, numerals 46 and 47 Zn
diffused layers formed simultaneously, numeral 48 a p-n
junction, numeral 49 an electrode layer for n-type ohmic
contact, and numeral 50 an electrode layer for p-type ohmic
contact.
Although, in the embodiments described, only the
use of Gal_xAlxAs (0 < x _ l) as the semiconductor material
has been stated, it is needless to say that similar effects
can be achieved with mixed crystals of other III-V compound
semiconductors, such as GaAsl xPX ( ' x _ 1), InxGal_xAs
(0 < x _ 1), GaAsl xSbx (0 _ x ' 1~ and Gal_xInxP (0 _ x < 1)
or with hetero-junctions employing III-V compound semiconductor
materials different from éach other. The process of crystal
growth is not restricted to liquid phase growth; a similar
method of manufacture is applicable and similar effects can be
achieved with vapor phase growth.
Further, although for simplicity the above description
relates to the fabrication of individual light emitting devices,
the invention is applicable to the fabrication of a function
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element in which a large number of light emitting diodes are
integrated on a single semiconductor substrate.
As set forth above, the radiation region of a p-n
~unction is confined to a very small area thereby to attain
light emission of high radiance and high efficiency, a diffused
layer of high carrier concentration is provided at a portion
of contact with an electrode layer thereby to lower the contact
resistivity, a portion of a light passage is left at a low
carrier concentration thereby to reduce the absorption of
light, and the coupling with an optical fiber can be easily
conducted, so that the device is very effective as a light
emitting device.
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