Note: Descriptions are shown in the official language in which they were submitted.
3SS
BACKGROUND OF THE INVE~TION
Recent developments in optical fibers and materials
with low transmission losses have enhanced the attractive-
ness of integrated optical circuits. Although these circuits
can be realized as discrete components, the advantage of
integrating devices in solid-state systems is widely
appreciated. Such solid-state systems will require a solid-
state,electrically excited, laser light source to produce
a high intensity, coherent light beam.
Solid-state laser devices having either planar or
cleaved end mirrors or other external mirrors are not
suitable for integrated structures. Generally, the mirrors
are displaced from each other by a very large multiple of half
wavelengths, for example, thousands of half wavelengths~
Thus, the mirrors can reinforce light of many different
wavelengths thereby making it very difficult to provide good
frequency selectivity and stability. Further, lasers having
cleaved or external end mirrors may require higher thres---
hold current densitities which act to decrease operating life.
Recently, interest has been focused upon distributed
feedback lasers. These structures do not utilize the
conventional cavity end mirrors, but instead provide feed-
back by means of Bragg Scattering from periodic perturbations
of the refractive index and/or gain of the laser medium
itself. Distributed feedback lasers are compact and provide
a high degree of frequency selectivity. However, as yet
no one has successfully applied this distributed feedback
concept to obtain oscillations in electrically pumped solid-
state lasers. If such a laser was capable of 300 degree
Kelvin (room temperature) operation, it could become the
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primary light source for prop~sed integrated optical circuits.
State-of-the-art distributed feedback solid-state and
solid-dye lasers utilize optical pumping and thus require an
independent light source. Such an independent pumping source,
which itself can be a laser device, and its associated focusing
optics, are hard to align properly for the desired impingement,
and, obviously, a separate laser pumying source is not compat-
ible with an integrated optical circuit.
~UMMARY OF THE IN~7ENTION ~.
In accordance with one aspect of this invention there
is provided a method of making an electrically pumped, dis-
tributed feedback, solid-state laser device comprising the steps
of: forming a periodic structure having a regular repetition -~
rate in a first layer of doped semiconductor material, forming a
second layer of semiconductor material on said first layer, said
first layer having a higher refractive index than said second
layer, forming a p-n junction in said first layer, said second
layer and said p-n junction defining a light guiding layer, and
providing electrical contacts for said laser device such that
when said device is forward biased carriers are injected across
said p-n junction into said light guiding layer to initiate the
production of laser light.
In accordance with another aspect of this invention
there is provided an electrically pumped, distributed feedback,
solid-state diode laser comprising: a semiconductor body
including a plurality of layers, pairs of said layers providing !,
a plurality of internal junctions, one of said layers being an
active region layer, said active region layer being bounded on
a first side by a first layer of material having a lower index
of refraction than said active region layer to provide a hetero-
junction between said active region layer and said first layer,
said active region layer being bounded on a second side by a
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la~3ss
second layer having a conductivity type different from the
conductivity type of said active region layer to provide a
rectifying junction between said active region layer and said
second layer, carriers injected under forward bias across said
rectifying junction undergoing radiative recombination in said
active region layer to generate stimulated coherent radiation, -: ~
one of said junctions being a non-planar heterojunction, said ~ -
heterojunction providing a periodic perturbation of refractive
index therealong, said perturbation interacting with at least
a portion of said radiation to cause said radiation to be
reflected by said periodic perturbation, said perturbation
having a period such that said reflected radiation is in phase.
In accordance with another aspect of this invention
there is provided an electrically pumped, distributed feedback,
solid-state diode laser for producing coherent radiation com-
prising: a semiconductor body including a plurality of layers, .
pairs of said layers providing a plurality of internal junctions,
one of said layers being an active region layer, said active
region layer being bounded on a first side by a first layer of
material having a lower index of refraction than said active
region layer to provide a heterojunction between said active
region layer and said first layer, said active region layer
being bounded on a second side by a second layer having a ~
conductivity type different from the conductivity type of said :
active region layer to provide a rectifying junction between
said active region layer and said second layer, carriers
injected under forward bias across said rectifying junction
undergoing radiative recombination in said active region layer
to ~enerate stimulated coherent radiation, one of said
junctions being a non-planar heterojunction, said heterojunction
providing a periodic perturbation of refractive inde~ therealong,
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said perturbation interacting with at least a portion of said -~
radiation to cause said radiation to be reflected by said
periodic perturbation, said perturbation having a period such
that said reflected radiation is in phase, said perturbation
being the only feedback mechanism producing said coherent
radiation.
In accordance with the invention in one of its aspects ~-
there is provided an electrically pumped, solid-state laser
device having a grating or physical periodic structure in a
region, or adjacent to a region, of semiconductor material that
acts as a light wave guide. The spacing of the perturbations
of the periodic structure are selected to be an integer number
of half wavelengths of the desired lasing wavelength in the
laser. The perturbations significantly interact with a portion
of the guided light beam. The improved electrically pumped, ~ -
, ., .:
solid-state laser device can be a single heterojunction device ;~
or a double heterojunction device. In both the single and
double heterojunction devices, perturbations in the concentra-
tion of the doping material may possibly act with the physical
perturbations to enhance the desirable interaction with the
guided light beam, thereby increasing the Bragg Scattering.
The improved electrically pumped single heterojunction
solid-state laser device is produced by a method which provides -
for formation of the periodic structure prior to formation of
the light guiding layer or region. The light guiding layer is
formed by a diffusion process. This process is carried out
after the formation of the periodic structure and the growth of
a GaA~ As layer. The growth is carried out at a low temperature
such that there is not applicable meltback or dissolution of
the periodic structure. Thus, a periodic structure having a
substantial depth is produced, thereby providing a significant
interaction with the laser light beam.
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The improved electrically pumped double hetero-
junction laser device allows a reduction in the thickness of
r l a ~
the light guid~ng layer due to the ~g~ refractive indexes
of materials on both sides of the light guiding layer! and
hence, a greater interaction between the periodic structure
and the laser light in that layer. The interaction may be
increased by a periodicity in the doping material which
periodicity exists in the light guiding layer in addition
to the mechanically formed periodic structure. If present,
this increased interaction causes a lower threshold pumping
current density to be required, which is believed to increase
the device life. Sufficient interaction may be obtainable
even with structures which are physically removed from the
active region.
DESCRIPTIO~ OF THE DRAWINGS
Figure 1 is a cross-sectional view of a single
heterojunction device according to the invention.
Figures 2a through 2g are cross-sectional and
perspective views of the device of Figure 1 during steps
of its manufacture.
Figure 3 shows a cross-sectional view of the device
of Figure 1 with light distribution characterized.
Figure 4 is a cross-sectional view of a double
heterojunction device according to the invention.
Figure 5 is a cross-sectional view of the device
of Figure 4 showing equi-level dopant lines.
Figure 6 is a cross-sectional view of a further
embodiment of a double heterojunction device according to
the invention.
Figures la and lb show the light output of the
light guiding layer of the device of Figure 1.
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Figure 7 is a cross-sectional view of a further
em~odiment of a dual heterojunction device according to
the invention.
DE~AILED DESCRIPTIO~ OF THE I~VENTION
There will be disclosed hereinafter an electrically
pumped, single heterojunction, distributed feedback diode
laser and an electrically pumped, double heterojunction,
distributed feedback diode laser and their methods of
manufacture. It should be noted that the ultimate use-
fulness of the devices to be discussed may not be in the
individual devices but rather in an integrated optical or
electro-optical circuit including these devices.
Referring now to Figure 1, there is shown an
electrically pumped, single heterojunction, distributed
J~OJ~
A feedbacX~laser device according to the invention. sasica
the device consists of an n-type Ga As layer 1, a p-type
Ga As layer 2, a p-type Ga Al As layer 3, and a p-type Ga As
layer 4. Layer 4 is not essential to the operation of the
device but is usèd for ease in fabrication of electrical
contacts. A periodic structure 5, shown in the form of a
grating having downwaraly extending teeth or channels 5a,
exists between the layers 2 and 3. The periodic structure
is produced on the top surface of an n-type Ga As substrate
(the bulk material of which at that time included the bulk
material of layers l and 2, as will be described in detail
hereinafter) by one of several methods, preferably surface
or ion milling, prior to the Ga Al As layer 3 being grown.
After growth of layers 3 and 4 and cleaning thereof, the
device is placed in a diffusion chamber and a diffusion of
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the p-type dopant such as Zn through layers 4 and 3 into
the n-type Ga As occurs to produce the p-type Ga As layer 2
and the p-n junction 6 between layers 1 and 2. Metal
contacts 7 and 8 are affixed to layers 1 and 4, respectively.
Due to its base material (Ga As) and doping
concentration, the layer 2 has a band gap of ~1.4 eV,
which is slightly lower than the ~ 1.41 eV band gap of the
layer 1 and substantially lower than the ~ 1.8 eV band gap
of layer 3. Also, the ~3.6 refractive index of the layer
2 is slightly greater than the ~3.58 refractive index of
layer 1 and substantially greater than the r~3.4 refractive
index of the layer 3. As is well known, when a forward
bias is applied to the p-n junction electrons are injected
from layer 1 into layer 2 and are confined to the layer 2
by the potential difference produced by the heterojunction
layer 3. With sufficient pump current population inversion
is achieved and gain is obtained. Light is produced by
radiative recombination of the carriers in this region 2.
This light is guided in layer 2 due to the higher refractive
index of the region relative to its adjacent layers 1 and 3.
Thus, the lasing light is substantially restricted to
propagation through layer 2. As will be discussed in
detail hereinafter, the inter-action between the light
in layer 2 and the periodic structure 5 produces Bragg
Scattering which reinforces and couples light travelling
through the layer 2 in both directions in a coherent manner
such that the reflections are in phase, thus allowing
laser operation in the absence of discrete end reflectors.
The method of fabricating the electrically
d I ~ d ,:
30 A pumped, solid-state distributed feedback~laser device of
Figure 1 will now be described in relation to Figure 2.
Fabrication is initiated from a substrate la of n-type
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Ga As having an impurity concentration ~D f about 1018
cm~3, as shown in Figure 2a. The impurity can be silicon,
although other n-type impurities can be used. The
lattice orientation of the Ga As substrate la is not critical,
good results have been achieved using a (100) oriented Ga As
substrate, but other orientations, for example, (110) or
(111) oriented Ga As should produce good results.
After having been cleaned, such as by an organic
solvent and/or by an acid, the periodic structure 5 is
formed on the top surface of the substrate la. The spacing
of the teeth of the periodic structure depends upon the
lasing wavelength in the Ga As light guiding layer 2, with
such relationship being approximately given by the
equation:
~ __ m
~ 2n
where ~ is the spacing of the periodic structure, m is
Bragg diffraction order which is a small integer, lambda
( A) is the lasing wavelength in the Ga As layer 2, and
n equals the refractive index of the Ga As layer 2. For
Ga As, lambda ( A ) equalsr_8~500A and n is approximately
3.6 such that ~ e~uals ~ 1,200A for m equals 1, ~ 2,400A
for m equals 2,~3,600A for ~ equals 3, ~ 4,800A for
equals 4, ~ 6,000A for ~ equals 5, and so on.
The production of grating spacings as small as
those indicated require either interferometric exposure of
a photo resist material, such as, Shiply AZ 1350 resist,
which is laid down on the top surface of substrate la in a
conventional manner to a depth of approximately 3,600A~or
by conventional electron resist techniques. The inter-
ferometric exposure technique is conventional and utilizes
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a beam splitter and two mirrors to direct two laser light
beams of equal intensity and equal polarization upon the
photo resist with equal angles of incidence. The relation-
ship between the grating spacing and the interferometric
exposure is given by the equation:
= laser
~ 2 sin 0 n
where
~ = grating spacing
~ = laser wavelength (not to be confused with the
diode laser wavelength ~ .)
= the angle between the normal to the
substrate surface and the laser beam,
and
n = refractive index of the medium surrounding
the substrate (air _ 1).
Since an Argon laser is generally used for the
interferometric pattern formation and the wavelength of
available Argon lasers is approximately 4579A, the grating
spacing ~ of 3600~ can be achieved by interfering the
laser beam at an angle of about 40. It is also possible
to fabricate first and second order gratings by
conventional techniques. For third order operation, the
interferometric pattern would be adjusted, as is well
known in the art, to expose thin strips of the photo resist
7 spaced about 3,600A apart, with each exposed strip about
1800A wide, as depicted in Figure 2b, wherein the darkened
areas of resist layer 7 represent the exposed areas. once
the grating pattern is formed on the resist 7, the resist
is developed to leave only the unexposed regions still on
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the top surface of the substrate la, as shown in Figure 2c.
Several possible methods are available for producing
the periodic structure in the substrate la now partly
protected by the resist 7. These methods include ion
implantation, ion milling, diffusion, etching or surface
milling. It is believed that ion milling produces the
best periodic structure. With ion millins the top surface
of substrate la is bombarded by Argon ions, or other inert
ions, with the ion beam potential being about 4,000 volts
and the beam current being about 100 microamps. The resist
7 protects the covered portions of substrate la, allowing the
ion beam to form the teeth or channels 5a in the un-
protected areas. It is important that the ion milling
process stop when the ion beam has substantially removed most
of the resist. If milling persists for too long a period,
the grating teeth become narrow and are affected by subsequent
process steps. It is also important that the milling be
performed at a shallow angle of incidence since pitting of
the surface results when the Ar~ beam impinges normal to the
plane of the substrate la. After the grating has been formed,
the remaining photo resist is removed by conventional means,
such as by an acetone bath. The depth of the teeth 5a of
the grating, which depth is important to the distributed
A feedback action to be described hereinafter3are about 1250A,
but laser operation can be obtained with both shallower
and deeper teeth. Figure 2d is an end view of the substrate
la with teeth 5a, and Figure 2e is a perspective view of
the device of Figure 2d.
Following cleaning of the substrate, the substrate
is placed in a liquid phase epitaxial growth furnace and a -
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p-type Gal_x Alx As layer 3 is grown on the corrugated
surface, as shown in Figure 2f. ~ext, a p-type Ga As
layer 4 is grown to facilitate the fabrication of
electrical contacts on the device. The thickness of layer
3 is preferably a few microns, having a dopant level of about
1018 cm~3. The formation of layers 3 and 4 are achieved
in a conventional manner and with conventional apparatus.
It is, however, important that the growth be done at a low
temperature, about 800C, since high temperatures will cause
a meltback or dissolution of the periodic structure 5. The
concentration of aluminum (x) in the Gal_x Alx As is
controlled such that the refractive index of the layer 3
is about 3.4 and the band gap is about 1.8 electron
volts. As previously explained, these values provide both the
required light wave guide structure and the electron
confinement. x is 0.3 at the substrate la-grown layer 3
interface, but it may vary between nearly 0 to .8. It is
noted in relation to Figure 2f that during the growth of
A layer 3, the p-type dopant diffuses slightly into the Ca
6 Q As ~5
~D substrate la ~D depicted by junction 6. It is believed
that the diffusion extends about .5 microns into
substrate la.
After further cleaning the device of Figure 2f
is placed in an evacuated diffusion ampoule along with zinc
arsenide and diffusion is accomplished for approximately 10
minutes at approximately 700C. This diffusion drives
p-type material further into substrate la thereby driving
the p-n junction 6 further into the substrate la to thereby
define layer 2, the light guiding layer, as shown in Figure
2g. The thickness of layer 2 is preferably about 2 microns,
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although a lesser or greater thickness is possible depending
upon the band gap and refractive indicies of layers 1 and 3.
Following removal from the diffusion ampoule, the
lower surface of substrate 1 (the part of substrate 12
below the ~unction 6) is polished to remove any zinc diffused
therein, to once again expose n-iype Ga As material. In some
cases it may be desirable to anneal this wafer to optimize
the device performance subsequent to the diffusion. The
Ga As layer 4 makes it easier to bond a contact to the top
layer of the laser device since, as is well known, it is
difficult to bond to an aluminum containing material. Tin
and gold are now sputtered onto the n-type substrate 1 and
titanium, platinum and gold are sputtered onto the upper
surface of layer 4 and then annealed to make n-type and p-type
ohmic contacts, respectively, thereby completing the
fabrication of the device of Figure 1. The formation of
the contacts is by conventional methods which are not part
of applicants' invention.
As noted, the depth of layer 2 is about 2 microns
and the depth of teeth 5a is about 1250A. Due to the
refractive index of layers l and 3, most of the laser
light which is initiated by electrical pumping is
confined within layer 2. Figure 3 shows a symbolic
representation of the laser light intensity in layers
1, 2 and 3. As illustrated, the increased depth of teeth
5a and the controlled thickness of layer 2, cause a
substantial portion of the light (shown lined) to interact
with the teeth or periodic structure. This increased
interaction produces increased Bragg Scattering and thereby
allows the device to lase at lower threshold current
,, , , , , . . ~ ~ . . .
lQ~43SS
densities. In the case of one diode when a pumping source
was coupled to electrode 8, with electrode 7 grounded, which
source delivered .5 microsecond pulses of r~7 amps and 15
volts, with a pulse delivered every 10-3 seconds, lasing
was observed at threshold current densities as low as 775
~mps/cm2, compared to more than 8400 Amps/cm2 needed to
initiate lasing in an otherwise identical single hetero-
junction semiconductor device without periodic structure or
corrugation feedback but with saw cut and/or angle polished
end faces.
The light emission spectrum of the distributed
feedback laser of the device of Figure 1 shifts very little
in wavelength as a function of current density, even up to
five times threshold current. This is in contrast to cleaved
mirror lasers. Also, due to the components (teeth) of the
periodic structure being only a half wavelength or small
multiple thereof apart, the distributed feedback laser of the
present invention has good frequency stability which is not
greatly affected by the length of the laser.
Referring now to Figure 4, there is shown a double
heterojunction diode laser in accordance with the invention.
This device differs from the device of Figure 1, by the
addition of an n-type Gal_x Alx As layer 10 between layers
1 and 2. The layer 10 has a lower refractive index,
about 3.4 and a higher band gap, about 1.8 eV than layer 2,
the former of which can be varied in accordance with
concentration (x) of the aluminum. This provides a better
light guiding and current confineing for layer 2 since it
is sandwiched between two layers that have both a sub-
3Q stantially higher band gap, there now being a difference in
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re~ractive index between the light guide layer 2 and the
adjacent layers 3 and 10 of .2, and a difference in band
gap of about .4 electron volts as opposed to a difference
I c~ n d
~ of .02 electron volts between layers land 2 of the device
of Figure 1. Because of the greater difference in
refractive index and band gap, layer 2 can be made very
thin, for example, about .5 microns thick or less as
opposed to 2 microns thick for the single heterojunction
device of Figure 1. With the layer 2 thinner, and with the
teeth of the periodic structure 5 extending significantly -
downward, that is about 1250A, there is increased interaction
between the guided laser beam in layer 2 and the periodic
structure and hence increased Bragg Scattering. This increase
in Bragg Scattering allows for lower threshold pumping current
densities which may increase device usefulness and lifetime
even more than the device of Figure 1.
In addition to the interaction between the
physically formed periodic structure 5 and the light in the
guiding layer 2, there is also believed to be interaction
between the guided light and a dopant produced periodic
structure. This can best be explained in relation to Figure 5
wherein the dotted lines 11 in layer 2 represent equi-level
dopant concentrations. As shown, the equi-level dopant
concentrations produce a periodic dopant structure which
extends further into the layer 2 than the mechanical
periodic structure 5. The equi-level dopant concentration
lines 11 have greater amplitude near granting 5, that is,
- vary greater and appear more like the grating 5 nearer tothe grating 5, becoming substantially flat near the p-n
junction 6. It is believed that the periodic structure
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of the equi-level dopant lines causes further Bragg Scattering
of the guided laser light ~eam in layer 2 and hence lasing
at even lower threshold pumping current densities.
Formation of the periodic structure in layer 2
may cause some damage to the Ga As material of layer 2.
Since the layer 2 is very thin in the double heterojunction --
laser device, the light amplitude in the device of Figure 5
extends outwardly from layer 2. Since a significant portion
of the light traverses areas of layers 3 and 10, it is
possible to have the periodic structure in these layers as
shown by periodic structures 12 and 14 in Figure 6. The
periodic structures 12 and 14 could have the same spacing
as the periodic structure 5 or a different spacing if a
different Bragg order is to be utilized or a different laser
material is used.
The device of Figure 4 is formed by growing on
(n-type GaAs) layer 1 an n-type Ga Al As layer 10, with a
doping concentration ND of~10 8/cm3. Thereafter, layer 2
is grown atop layer 10 and is doped p-type with Ge or is
left undoped. In the case of an undoped layer 2 there would
be some impurity left from the growing of-layer 10 and
layer 2 would have some impurity therein, approximately
1017/cm3. The grating 5 would then be formed atop layer 2
as previously described in relation to Figure 1. P-type
Ga Al As layer 3 is then grown on layer 2 with a diffusable
dopant such as zinc or cadmium being used if a larger coupling ~ ~ -
is necessary. The contacts 7 and 8 are formed in a
conventional manner as previously described in relation to
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Figure 1. Layer 10 is preferably l~thick, layer 2 is
preferably 0.1 to 0.5,~thick, and layer 3 is preferably ' '
1 thick.
In the case of various geometries déscribed
in Figure 6, fabrication would follow the logical sequence
outlined for the two previously described geometries. Other
geometries are also possible. For example, it may be
advantageQus to add an additional layer 18 of GaAlAs as
done in "large optical cavity (LOC)" lasers. Figure 7 shows
such a structure with aluminum concentrations indicated.
The grating may be placed in several positions in this
device. Such a structure may provide increased coupling or
ease of fabrication over a regular double heterostructure.
Figure la depicts the light output for the device
of Figure 1, with the light output of the double heterojunc-
tion device being similar. In this figure, the conventional
laser output is emitted from the end light guiding layer 2.
This output is produced by 3rd order Bragg scattering of the
grating. Due to the interaction of the grating with the
2'0 lasting light, light is scattered by lower order Bragg reflec-
tions when the spacing of the grating is chosen to principally
operate with 3rd order Bragg scattering. This lower order
light passes through the top and bottom surfaces of the
device in a collimated beam as also shown in Fig. la.
The lower order light is highly collimated and thus the lower
order light has superior coupling characteristics with fiber
optics, superior utility for point-to-point communications,
and is superior for other applications for which highly
collimated light is desired.
The lower order light output will have a different
angle to the normal to the top and bottom surfaces due to
the different refractive indicies of the materials on either
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side of the ligllt guiding layer. It may be necessary in
certain instances to coat the surface of the device
with an antireflection coating to allow these beams to
escape from the device and not be totally internally reflected.
Figure lb depicts all the multiplicities of parallel light
beams that comprise the low order light beam 1' shown in
Figure la. The other lower order light beams are also
comprised of a multiplicity of parallel light beams as shown
for beam 1'.
It should be noted also that a 2nd order Bragg -
grating will produce 1st order output in this fashion and
higher order grating will produce other lower order Bragg
laser output.
- Although the descriptions above are given of devices
without cleaved end faces or other external reflectors, all ~;
realization of the device described herein can be fabricated
with cleaves and/or reflectors. ~ ;
., ,' ,
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