Note: Descriptions are shown in the official language in which they were submitted.
lO~ iO
~3AC~GROUND OF THE INVENTION
In order to lower the threshold current density of
a double heterojunction diode laser below the critical limit
for CW operation, it is necessary to have the thickness of
the active layer on the order of 0.5 mic-ons or less. The
usual cleaved and sawed double heterojunction lasers have
cross-sectional areas which are typically on the order of 0.2
microns x 20 microns. To make the laser beam produced by
heterojunction diode lasers compatible with optical systems
utilizing round lenses, or other symmetric optical elements,
it is desirable to reduce the large dimensional unbalance
between active area width and thickness from the order of
several hundred to one to as close to a one to one ratio as
possible, with a five to one ratio being satisfactory.
Recently, a double heterojunction diode laser was
disclosed in which the width of the filamentary area of the
active layer was substantially reduced. The disclosed diode
laser is called a buried-heterostructure (BH) injecticn
laser since the filamentary active laser region is completely
surrounded by ~ region of lower index of refraction material,
that is, surrounded by GaAlAs when the active region is
GaAs. The typical fabrication process for the disclosed BH
laser is composed of four main steps: (i) a liquid phase
epitaxial growth step to produce on a GaAs substrate a first
GaAlAs layer, an active layer of GaAs, and a second GaAlAs
layer, (ii) a mesa etching step which removes part of the
two GaALAs layers and part of the GaAs layer to define the
active filamentary area, (iii) a second liquid phase epitaxial
growth step to provide a GaALAs layer around the mesa to
thereby completely surround the active filamentary area with
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material having a lower index of refraction than that of the
active filamentary area, that is, burying or surrounding the
active filamentary area with GaAlAs when the active region
material is GaAs, and finally (iiii) a selective diffusion of
a p-type dopent (zinc) to provide a p-type channel from the
non-substrate end of the device to the GaAlAs layer adjacent
the active filamentary area. The last step requires that an
apertured masking layer be formed on the non-substrate end of
the device with the aperture in precise alignment with the
top of the mesa.
The described process has the readily apparent
disadvantage of requiring two separate epitaxial growth steps.
Another disadvantage is that several layers of varying com-
position and thickness must be etched, or otherwise removed,
and these variances make the etching or removal difficult to
control. A further disadvantage is that subsequent to the
etching step and prior to thè second epitaxial growth, the
exposed surfaces of the mesa can get oxidized because of the
problem of aluminum contamination with such contamination
creating defects in the active filamentary area. Also, the
second epitaxial growth can cause melt back of the regions
formed by the first epitaxial growth with the likelihood of
further defects in the active region.
As noted, the zinc diffusion of the described process
to form a non-rectifying channel to the GaAlAs layer on the
non-substrate end of the active filamentary area, must be
through a masking aperture which is precisely aligned with
the mesa top. Such alignment is difficult to maintain because
the top of the mesa is hidden by the second epitaxial growth
layers and because tolerances of better than a micron must
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be maintained. In further reference to a zinc diffusion,
the diffused region must extend through a relatively thick
(5 micron) GaAlAs layer and terminate in a relatively thin
(1 micron) GaAl~s layer. If the diffusion is not deep
enough a rectifying barrier will be created that will prevent
pump-current flow and if the diffusion is too deep the
active region ~.5 microns thick) could be penetrated. The
zinc diffusion is hard to control due to the different
thicknesses ~etween layers, as discussed, and also due to the
varying thickness of each layer. Thus, the zinc diffusion
must be controlled extremely accurately. Another difficulty
with the mesa producing process is that the mesa is a very
long, thin plateau which is easily disturbed, that is, chipped
or broken off during the subsequent epitaxial growth and zinc
diffusion steps.
OBJECTS OF THE INVENTION
It is an object of an aspect of the invention to
provide an improved laser.
It is an object of an aspect of the invention to
provide an improved buried-heterojunction diode laser.
- It is an object of an aspect of the invention to
provide an improved buried heterojunction diode laser
capable of operating at low room temperature thresholdsand
in the lowest order TE, TM or TEM modes.
It is an object of an aspect of the invention
to provide an improved laser having an output beam compatible
with symmetrical optical elements.
It is an object of an aspect of the invention to
provide an improved method of making a buried-heterojunction
diode laser.
It is an object of an aspect of the present
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invention to provide an improved method of making a double
heterojunction diode laser that requires few process steps.
It is an object of an aspect of the present invention
to provide an improved method of making a buried heterojunction
diode laser that requires only one epitaxial growth.
It is an object of an aspect of the present invention
to provide an improved method of making a buried-heterojunction
diode laser wherein diffusion is easily controlled.
SUMMARY OF THE INVENTION
In accordance with one aspect of this invention there
is provided a heterojunction diode laser comprising: a sub~
strate body having a surface with an elongated groove formed
in said surface, a first layer of a first material deposited
on said surface, said first layer having a surface configurat-
ion remote from said surface of said substrate with a portion
which is concave toward said substrate within a substantial
portion of said groove, a second layer of a second material
deposited on said first layer, said second material having an
index of refraction greater than said first material, said
second layer having substantially planar portions on both
sides of said groove and a substantially thicker cross-
sectional portion substantially within said groove and in
contact with said concave portion of said surface of said
first layer remote from said substrate, a third layer of a
third material deposited on said second layer, said third
material having a lower index of refraction than said second
material, said second layer having a conductivity type
different from either said first layer or said third layer
such that a first rectifying junction exists at the interface
between said second layer and either said first layer or said
third layer, and means located on both sides of said groove
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formed in said surface of said substrate for restricting the
flow of pump current to a path through said groove and through
said thicker cross-sectional portion of said second layer when
said rectifying junction is forward biased, whereby radiation
recombination will occur in said thicker cross-sectional
portion of said second layer.
In accordance with another aspect of this invention
there is provided a heterojunction diode laser for producing
a substantially symmetrically output light beam comprising:
a substrate body of a semiconductor material having a surface
with an elongated groove formed in said surface, a first
layer of a semiconductor material in integral contact with
said surface of said substrate, said first layer having a sub-
stantially planar portion on each side of said groove and a
portion within said groove with a surface of said portion
within said groove inwardly arched toward said substrate body,
a second layer of a semiconductor material in integral contact
with said first layer, said second layer having a central
portion substantially within said groove and substantially
planar portions on each side of said central portion, said
central portion having a surface conforming to the shape of
said inwardly arched surface of said first layer and a sub-
stantially flat surface adjacent said inwardly arched surface, :
a third layer of semiconductor material in integral contact
with said second layer, said material of said second layer
having an index of refraction greater than the index of re-
fraction of both the material of said first layer and the
material of said third layer to thereby provide a hetero-
geneous laser, at least two of said first, second and third
layers being of a different conductivity type such that a
rectifying junction exists between two of said first, second
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and third layers, and means located on both sides of said
groove formed in said surface of said substrate for restricting
the flow of pump current to a path through said central portion
of said second layer when said rectifying junction is forward
biased, whereby radiation recombination will occur in said.
central portion of said second layer.
In accordance with another aspect of this invention
there is provided a method of making a heterojunction diode
laser comprising the steps of: forming in a semiconductor
material substrate of one conductivity type a surface layer of
second conductivity type, removing both a portion of said
: ~ surface layer and an additional part of said substrate material
adjacent thereto to provide an elongated groove in said sub-
strate, said groove dividing said surface layer into distinct
. parts, forming on said grooved surface of said substrate a
. first layer of semiconductor material of said one conductivity
type, with formation of said first layer being concluded while
the portion of the exposed surface of said first layer in
said groove is concave, forming on said first layer a second
. 20 layer of a semiconductor material of higher index of refraction
`~ than that of said material of said first layer and of either
conductivity type, with formation of said second layer being
concluded when the exposed surface of said second layer
directly over the concave surface of said first layer is sub-
stantially flat, and forming on said second layer a third layer
of a semiconductor material of lower index of refraction
; than that of said second layer and of a second conductivity ~ .
` type.
In accordance with another aspect of this invention
there is provided the method of making a heterojunction diode
laser comprising the steps of: forming in a semiconductor
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material substrate of one conductivity type a non-electrically
conducting surface layer, removing both a portion of said
surface layer and an additional part of said substrate adjacent
thereto to provide an eiongated groove in said substrate, said
groove dividing said surface layer into distinct parts, form-
ing on said grooved surface of said substrate a first layer
of semiconductor material of said one conductivity type, with
formation of said first layer being concluded while the
portion of the exposed surface of said first layer in said
groove is concave, forming on said first layer a second layer
of a semiconductor material of higher index of refraction than
. that of said material of said first layer and of either con-
ductivity type, with formation of said second layer being con-
cluded when the portion of said second layer within said
groove is substantially bowl-shaped, and forming on said second
layer a third layer of a semiconductor material of lower index
of refraction than that of said second layer and of a second :~ .
conductivity type.
: In accordance with another aspect of this invention
there is provided a method of making a heterojunction diode
laser comprising the steps of: forming in a semiconductor `
material substrate of one conductivity type surface layer of
the opposite conductivity type, removing both a portion of
said surface layer and an additional part of said substrate ~ :
material adjacent thereto to provide an elongated groove in ~ :
said substrate, said groove dividing said surface layer into : :
: distinct parts, forming on said grooved surface of said sub-
strate a first layer of semiconductor material of said one
conductivity type, with formation of said first layer being
concluded while the portion of said exposed surface of said
: first layer in said groove is curved toward said substrate,
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said first layer forming secondary rectifying junctions with
each of said distinct parts of said surface layer, forming on
said first layer a second layer of a material of higher index
of refraction than that of said material of said first layer,
with formation of said second layer being concluded when the
portion of said second layer within said groove has a thicker
cross-sectional extent in the central region of said groove
and a very shallow cross-sectional extent above the upper
extremities of said groove, and forming on said second layer
a third layer of a semiconductor material of lower index of
refraction than that of said material of said second layer,
at least one of said second and third layers being of the
opposite conductivity type than that of said first layer such
that a primary rectifying junction exists between these layers.
In an embodiment of this invention, a buried-
heterojunction (BH) diode laser is provided which is capable
of operating at room temperature thresholds and at the lowest
order TE, TM, or TEN mode. The BH diode laser is characterized
by an active region which is completely surrounded (buried)
by material of lower index of refraction and higher band gap.
The filamentary area of the active region is substantially
bowl-shaped, that is, thicker in the middle than at the
ends and is substantially completely within an elongated
channel of the laser substrate. This filamentary area con-
figuration and placement is effective to favour emitted light
in the central portion of the active region which permits the
; width of the light beam produced to be small (1-2~) and thus,
provides a laser having a fairly symmetrical light output
beam with lowest order transverse modes in both directions
and threshold currents as low as 10 milliamps.
The BH diode laser is made by a process in which
~ - 5d -
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the filamentary area of the active layer is formed substantially
within a groove etched in the diode substrate. First, a p-
type surface layer is provided in the n-type substrate to pro-
vide, subsequently, a current blocking junction. Next, an
elongated groove is etched in the substrate to a depth
_ 5e - '
,~ .
,
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deeper than the surface layer thickness. Following formation
of the groove, a layer of a light guiding and carrier confin-
ing material, a layer of active region material and a second
layer of light guiding and carrier confining material are
grown successively on the grooved surface of the substrate
by conventional liquid phase epitaxial growth or molecular
beam epitaxy, with the active region being doped such that
it orms a rectifying junction with one of the light guiding
layers~ Due to the shape of the groove, nucleation sites
for the first growth layer are more prevalent near the bottom
extremities of the groove than at other portions of the groove
and thus the first growth layer has a depressed central area
which is filled in by the growth of the active material layer
to provide an active region having a bowl-shaped filamentary
area.
DESCRIPTIO~ OF THE DR~WI~GS
Figure 1 is an end view of a BH laser is accordance
with the invention.
Figure 2 is a symbolic representation of light
wave energy distribution in the laser of Figure 1.
Figure 3A-3E shows various process steps in the
production of the laser of Figure 1.
DESCRIPTIO~ OF THE PREFERRED EMBODIME~T
Referring now to Figure 1, there is shown an end
view of a BH diode laser 2 in accordance with the invention.
Laser 2 includes a substrate 4, a diffused layer 6, a first
light wave guiding and carrier confining layer 8, an active
material layer having central portion 10 and end portions
10', a second light wave guiding and carrier confining layer
12, and a contact-facilitating layer 14. The central portion
10~4~()
of the layer 8 and the central portion 10 of the active
material layer are within a groove 16 formed in the substrate
4 and extending through the diffused layer 6. Groove 16 is
defined by lower extremities 1 and upper extremities 2.
Layer 8 and active material layer 10'-10-10' are
of different conductivity type to provide a rectifying junction
20 therebetween. Contacts 16 and 18 are provided in contact
with substrate 4 and layer 14, respectively, to provide means
for forward biasing rectifying junction 20 at the interface
of layer 8 and active material layer 10'-10-10'. Layers 4
and 8 are of different conductivity type than layer 6 such
that second and third rectifying junctions 22 and 23 exist
at the interface between layers 4 and 6 and 6 and 8, respectively.
When junction 20 is forward biased, junction 22 is also forward
biased and junction 23 is back biased. More specifically,
substrate 4 can be n-type GaAs, layer 6 can be p-type GaAs,
light wave guiding and carrier confining layer 8 can be n-type
GaAlAs, active layer 10'-10-10' can be p-type GaAs, light wave
guiding and carrier confining layer 12 can be p-type GaAlAs,
and contact-facilitating layer 14 can be p-type GaAs. Layer
10'-10-10' can be n-type GaAs in which case a rectifying
junction 20' would exist between the layer of active material
10'-10-10' and layer 12, and layer 10'-10-10' can be undoped
to provide a rectifying junction somewhere intermediate layers
8 and 12.
As discussed in detail hereinafter, the bowl-shape
of the central portion 10 of the active material layer is
controlled, in part, by the shape of the first light wave
guiding and carrier confining layer 8 which has a central
trough or elongated depression 8' which results from the
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groove 16 and the tendancy for nucleating atoms to attach
themselves more readily at places that require less energy
for bonding, which, in fact, are those places which have the
highest density of neighboring atoms. From Figure 1, it can
be seen that the groove angles at lower extremities 1 are
about 125 degrees whereas the groove angles at upper extremi-
ties 2 are about 235 degrees. Thus, there is a higher density
of neighboring atoms at lower extremities 1 than at upper
extremities and hence nucleation and incorporation of growth
material into the substrate lattice can occur more easily at
lower extremities 1 than at upper extremities 2. Other
nucleation control factors will be discussed when the method
of making the diode 2 is described.
As noted, the central portion 10 of the active
material layer has a bowl-shaped cross-section, being deeper
in the central region and very shallow adjacent the upper
extremities 2. Since the light beam rays generated as a
result of the recombination of carriers when junction 20 is
forward biased are guided by material having a high index of
reraction than adjacen t layers 8 and 12, the filamentary
area of the laser, defined symbolically as existing between
lines lOa and 10b, is confined to the central portion 10 of
the active material layer. Referring to Figure 2, the
emitted light energy distribution pattern is illustrated
symbolically by light energy distribution patterns 25 and
25' for the center and ends, respectively, of the bowl-
shaped, central portion 10 of the active material layer. A
majority of the laser light emitted is concentrated in the
middle of central portion 10, that is, within the filamentary
area defined by lines 10a and 10bo Since the filamentary
lO~S4~iO
area has a maximum depth on the order of 1 micron and a
width of about 1-2 microns, the laser output beam produced
by laser 2 has an approximately round shape which makes it
compatible with external round lenses thereby eliminating
the need for complicated lens arrangement which are required
with lasers having a mesa structure, as previously described.
The laser 2 of Figure 1 is made by a process which
requires only a single epitaxial growth step, which can be a
liquid phase epitaxial growth or a molecular beam epitaxial
growth. The process is initiated by placing in a diffusion
ampoule the substrate 4 which as previously noted can be n-
type GaAs and a p-type dopent, such as zinc, and diffusing
the dopent into the substrate 4 to form p-type region 6, as
shown in Figure 3A. Substrate 4 preferably has a dopent
level of 1-5 x 10 /cm and layer 6 preferably has a doping level
slightly greater than the doping level of substrate 4. Next
a layer of a conventional photoresist, such as the ultraviolet
sensitive photoresist Shipley AZ 1350 is deposited over the
layer 6 followed by exposure of the resist, as shown in
Figure 3B, wherein the dotted portions of resist layer 30
has been exposed making those portions insensitive to a reagent,
such æ Shipley developer when the photoresist is Shipley AZ
1350. The unexposed portion of the photoresist, which can be
1-2 microns in width or still smaller, is then removed such
as by immersion of the substrate wafer of Figure 3B in a bath
of a suitable photoresist developer reagent to provide the
structure of Figure 3C. A groove or channel 16 is then formed
in the substrate 4 in the area not protected by resist 30 to
provide the substrate wafer configuration of Figure 3D. The
depth of groove 16 is not critical but it is necessary that
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the groove 16 extend through the layer 6 in the substrate 4.
For example, layer 6 can be 0.6 microns thick in which case
groove 16 would be about 1.5 microns deep. The depth of layer
6 is not critical and the depth of groove 16 is also not
critical, for example, layer 6 can have a thickness range from
0.1 to 2 microns or more and groove 16 can be .2 to 2.5 microns
or more deep, provided that groove 16 extends through layer 6 in
substrate 4. Groove 16 can be formed by conventional chemical
etching, ion milling or a combination of these techniques or
other known techniques for removing the substrate material. When
the substrate material is as previously specifically specified,
that is, GaAs, an etch bath composed of 20 parts ethylene glycol,
5 parts phosphoric acid, and 1 part hydrogen peroxide is
satisfactory, with the etch bath maintained at room temperature
and with the etch bath being stirred during the etching process.
Since p-type material (the material of layer 6)
etches faster than n-type material (the material of substrate
4), the etched groove 16 takes on the sloping sidewall con-
figuration of Figure 3D, with the sloping walls having a
(111) A or GaAs plane atomic surface and the top surface of
substrate 4 having a (100) crystalographic orientation. The
upper width of the groove 16 is controlled by the opening in
the resist 30, and due to undercutting of the resist 30 by
the etch bath, the upper width of groove 16 can be slightly
wider than the opening of resist 30. The width of the bottom
of the groove 16 will depend greatly upon groove depth but is
generally on the order of the width of the opening in the
resist 30. It is noted again that the angle ~ formed between
each of the sidewalls and the bottom of the groove 16, that is,
at lower extremities 1 is less than 180 degrees whereas the
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angle ~ formed between the sidewalls and the top of the layer
6, that is, at the upper extremities 2 is greater than 180 .
These angles are important for nucleation site purposes.
Following formation of groove 16, the remaining
photoresist 30 is removed from the substrate wafer configuration
of 3D and the layers 10'-10-10', 12 and 14 are grown consecutively
by means of conventional liquid phase epitaxial growth to produce
the device of Figure 3E. Other method of layer growth could
also be used such as mulecular beam epitaxy. The layer 8 has
a depressed region in channel 16 due, in part, to the prevalence
of nucleation sites at extremities 1 which cause a greater growth
rate at those extremities. Layer 8 could have a thickness tl
adjacent to layer 6 of about 1 micron, although it could have
a thickness range of 0.5 to several microns. It is important,
however, that the growth of layer 8 be terminated before the
depressed central area is smoothed over. The portions 10' of
the active material layers are substantially planar with a
width t2 of about 0.2 microns (although a range of t2 from 0.1
to 1 micron or more is acceptable) and the central portion 10
of the active material layer is bowl-shaped and within the
groove 16. When thickness t2 is 0.2 microns, the thickness
t3 of the bowl-shaped central portion 10 would be about 0.4-
0.8 microns. Once again, the configuration of the bowl-
shaped central portion 10 of the active material layer is
controlled by nucleation sites on layer 8 with such sites
being greater in the depressed central area of layer 8.
Actually, the reasons layers 8 and 10'-10-10' grow as they
do is dependent upon other factors besides nucleation sites.
Some of these factors, which are well known to those versed
in the semiconductor fabrication art, are depth of groove
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16, width of groove 16, depth of diffusion of layer 6,
crystallographic orientation of substrate 4, growth times,
growth temperatures, cooling rates, degree of clean wiping
melts, and whether the melts are saturated or super saturated.
the Growth of the active material layer 10'-10-10' can be
continued until the entire top surface is substantially smooth,
however, this additional growth will substantially increase
the width of the active material that will provide guiding of
the light beam produced by carrier recombination, that is,
increase the width of the filamentary area, and thus the width
of the output laser beam will be substantially greater than
the height of the output laser beam thereby making symmetric
optical elements uncompatible.
The GaAlAs layer 12 is typically 1.5 microns thick
although the thickness range may be from 1 to 3 microns or
more. Layer 14 is typically 1-4 microns thick and substrate
4 is typically 100 microns thick. The concentration of
aluminum in layers 8 and 12 is typically 0.4, although a
range from 0.15 to 0.7 is acceptable. The doping levels of
layers 10, 12 and 14 are typically 1016-1017/cm3, 1017-1018/cm3,
and 1017-1019/cm3, respectively. The process is csmpleted by
applying electrodes 16 and 18 in a conventional manner. The
method is also directly applicable to the fabrication of optical
waveguides, modulators, directional couplers and other integrated
optical components.
In operation, when the diode 2 is forward biased,
that is, by applying a voltage to electrode 18 of approxi-
mately 1.4 volts greater than the potential applied to
electrode 16, the junction 20 (or the junction 20' when the
active material is n-type) is forward biased and electrons are
-12-
injected from the centra~ port~on of layer 8 into the
bowl-shaped central por~ion 10 of the active layer and are
confined there by the surrounding heterojunction layers 8
and 12. With sufficient pump current, population inversion
is achieved and gain is obtained with light produced by
radiative recombination of the carriers in layer 10. This
light is guided by layers 8 and 12 due to the lower index of
refraction of these layers relative to that of layer 10.
Pump current flow is restricted to a path through
channel 16 due to the back bias on junction 23 when the
junction 20 is forward biased. This pump current path confine-
ment could be achieved by other than diffusion techniques. For
example, substrate 4 can be provided with an intrinsic layer
instead of layer 6, or protron implantation could be used to
create insulating regions in place of layer 6. Also, by
selective growth, layer 6 could be grown instead of diffused.
The laser disclosed is capable of operating at low
room temperature thresholds (approximately 10 milliamps) and
operating in the lowest order TE, TM or TEM modes. What
makes this operation possible is that the central portion 10
of the active layer is completely or almost completely
surrounded (buried) in the groove 16 by layers 8 and 10 of
lower index of refraction materiai. The pump current is
substantially restricted to a flow path through the central
portion 10 of the active layer within the groove 16 because
regions 6 provide p-n junctions 23 which are back biased
when diode laser is forward biased for pumping mainly the
"buried" portion of the active region of the diode laser.
The major advantage of this structure is that most of the
carriers for radiation recombination are injected into the
"buried", bowl-shaped region 10 of the active layer and
substantially all of the light waves produced can be confined
to the bowl-shaped region 10 of the active layer, and more
particularly to the filamentary area of the active layer
because the effective index of refraction of the active
layer decreases as the active layer thickness decreases.
Thus, the light waves produced tend to be focused in the
center of the "buried" active layer because that is where
the active layer is thickest and thus has the highest index
of refraction. Accordingly, by controlling active region
geometry, it is possible to provide CW room temperature
lasers with lowest order transverse modes in both directions
and threshold currents as low as 10 milliamps.
