Note: Descriptions are shown in the official language in which they were submitted.
Strip buried heterostructure laser
Background of the Invention
This invention relates to semiconductor junction
lasers.
The stripe geometry contact for junction lasers was
s proposed by R. A. Furnanage et al. (U.S. patent No.
3,363,195, granted January 9, 1968) more than a decade
ago and has been incorporated, in one form or another,
in various heterostructure laser configurations in use
and under study today. These lasers, which range from
10 the simple double heterostructure (DH) (I. Hayashi, U.S.
patent 3,758,875, granted September 11, 1973) to more com-
plicated buried heterostructure (BH) [T. Tsukada, Journal
of Applied Physics, Vol. 45, p. 4899 (1974)], each have
one or more advantageous operating characteristics.
The DH laser has the longest lifetime of all semi-
conductor lasers, exceeding 105 hours to date, and is
characterized by low thresholds and fundamental trans-
verse mode operation. On the other hand, it has a wide
beam divergence, a nonlinearity (known as a "kink") in
20 its light-current (L-I) characteristic, and incomplete
lateral current confinement.
The Tsukada BH laser, which includes a GaAs active
region completely surrounded by Alo 3Ga0 7As, has
effective transverse mode stabilization, but the
25 refractive index ~hange along the junction plane is so
large that stable fundamental mode lasing is possible
'
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-- 2 --
only for active layer widths of < l~m, resulting in low
output power (e.g., 1 mW) and large beam divergence in
both transverse directions. In BH lasers with wider
active layers, higher order modes are easily excited
near threshold.
Summary of the Invention
According to one aspect of the invention there is
provided a method of fabricating a semiconductor device
having an active region therein comprising the steps o:
epitaxially growing on a semiconductor body a first layer
from which said active region of a predetermined geometry
is to be formed, protecting the top surface of said first
layer from the introduction of defects therein during
subsequent processing by epitaxially growing a thin se
15 cond layer thereon which can be selectively etched in the
presence of said first layer, masking said se~ond layer to
define said geometry, selectively etching away said second
layer in the mask openings to expose the underlying first
layer, and removing at least a portion of said first layer
to effect said geometry.
Other aspects of this invention are claimed in our
patent application Serial No. 315,705 filed~on October 31,
1978 of which the present application is a division, and
in a second division thereof.
In accordance with one preferred embodiment of our
invention a strip buried heterostructure (SBH) laser com-
prises a pair of opposite-conductivity-type, wide bandgap,
semiconductor cladding layers separated by a narrower
bandgap, semiconductor active region characterized
_ 3 ~ 5B9~7
in that the active region includes a low-loss waveguide
layer and contiguous therewith a narrower bandgap active
layer in the form of a narrow strip which extends along the
longitudlnal (resonator) axis of the laser. Prefe~ably,
5 the bandgap difference between the waveguide and active
layers is sufficiently large to confine to ~he a~tive lay~
minority carriers injected therein when the cladding layers
are forward biased, yet small enough to allow a significant
portion of the stimulated radiation generated in the active
10 layer to be coupled into the waveguide layer, thereby
reducing the optical power density at the mirror facets.
In addition, it is also preferable that the bandgap
difference between the waveguide layer and the contiguous
opposite-conductivity-type cladding layer be large enough
15 to prevent significant leakage current between the two
layers under normal operating conditions of the laser.
Means are also preferably provided for
constraining pumping current to flow in a narrow channel
through the strip-shaped active layer. In one embodiment,
20 the constraining means includes a pair of laterally spaced
reverse~biased p-n junctions-near the top surface of the
laser. ~owever, other constraining means, such as proton
bombardment, are also suitable.
An illustrative embodiment of our SBH laser was
25 fabricated from the GaAs-AlGaAs materials system and
exhibited, over a wide operating range, high power output,
a linear L-I characteristic for all currents up to
catastrophic failure, stable fundamental transverse and
single longitudinal mode oscillation ana reduced beam
30 divergence, along with adequate lasing thresholds and
external quantum efficiencies.
In other embodiments of our invention, the strip
active layer is partially embedded in the low~loss
waveguide layer instead of being formed on top of a major
35 surface of the waveguide layer. In either case, however,
the portions of the major surface of the waveguide layer
adjacent the active layer can be provided with distributed
feedback gratings.
OGAi~s R. A. 31-3
Another aspect of our invention is a method for
defining the geometry of the strip active layer, or other
device active region, without the introduction of
debilitating defects therein during shaping operations.
thin epitaxial protective is grown on the active layer
before it is masked and shaped (e.g., by etching and/or
anodization), the protective layer is selectively etched
away in mas~ openings to expose the ~c~ive layeL, and thin
portions of the active layer are removed (e.g., by
anodization) to define the desired geo~etry. In the
fabrication of our SBH laser, these procedures a~e followed
by growing over the shaped structure a cladding layer
having a composition essentially identical to that of the
protective layer so that the latter is incorporated into
the former.
One additional aspect of our invention entails a
procedure for epitaxially growing a Group III-V compound
second layer (e.g., the AlGaAs cladding layer) from the
liquid phase on an Al~containing Group III-V compound first
20 layer (e.g., the ~lGaAs waveguide layer). After growth of
the first layer, ~ non-Al-containing Group III-V compound
epitaxial protective layer (e.g., GaAs) about several
hundred angstroms thick is formed on a major surface of the
first layer before exposing the first layer to an ambient
25 which would otherwise oxidize it. Processing, such as
etching and/or anodization, can be used to form the
protective layer from a much thic~er layer (e.g., that ~rom
which the active strip of an SBH laser is formed), or in
some applications the thin layer may be grown directly
(e.g., by deposition of the first layer and the protective
layer by molecular bea~ epitaxy). In either event, state
of the art technology has demonstrated that LPE yields
better quality layers for optical devices than ~lBE, but ~PE
growth on ambient-exposed Al-containing layers is difficult
35because Al tends to oxidize so readily. The protective
layer enables the use of LPE because the molten solution
used to grow the second layer dissolves the protective
layer so that the second layer grows directly on the first
L~AN, R. A. 31-3
.
` 5 ~ 5~'7
layer.
Brief Description of the Drawing
Our invention, togcther with its various f~atures
and advantages, can be readily understood from the
following more detailed description taken in conjunction
with the accompanying drawing, in which the fi~u~es are no~
drawn to scale for clarity of illustration.
FIG. 1 is a schematic isometric view of an S8H
laser in accordance with one embodiment of our invention in
which the strip active layer is formed on top of a major
surface of the waveguide layer;
FIG. 2 is an end view of an SBH laser in
accordance with another embodiment of our invention in
which the strip active layer is partially embedded in the
waveguide layer; and
FIG. 3 is a schematic isometric view of an SBH
laser in accordance with yet another embodiment of our
invention in which distributed feedback gratings flank the
strip active layer.
Detailed Description
SBH Laser Structure
~ ith reference now to FIG. 1, there is shown an
SBH laser 10 formed on a substrate 11 and comprising first
and second opposite-conductivity-type, wide handgap,
semiconductor cladding layers 12 and 14 separated by a
narrower bandgap, semiconductor active region 16
characterized in that active region 16 includes a low loss
waveguide layer 16.1 and contiguous therewith a narrower
bandgap active layer 16.2 in the form of a narrow strip
which extends along the longitudinal (resonator) axis 28 of
the laser.: The narrow strlp may be formed on top of a
major surface of waveguide-layer 16.1 as in FIG. 1 or, as
shown by layer 16.2' of FIG. 2, may be partially el~bedded
in haveguide layer 16.1'. In the latter case~ the major
surface of the waveguide layer is essentially coplanar with
a major surface of the active layer.
Means 18 is provided for constraining pumping
current to flow in a narrow channel through the active
~AN R A ~
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-- 6 --
layer 16.2 tor 16.2') when cladding layers 12 and 14 are
forward biased above the lasing threshold. Forward bias
voltage is applied by means of suitable ohmic contacts 20
and 22 formed on substrate 11 and means 18, respectively,
In the embodiment shown, constraining means 18 com-
prises a pair of laterally spaced p-n junctions 1~.1 and
18.2 which are reverse biased when cladding layers 12 and
1~ are forward biased. The junctions are formed by de-
positing on cladding layer 14 a layer 18.3 of the same
conductivity type and then forming a bifurcated, wider
bandgap, opposite-conductivity-type layer 18.~ on layer
18.3. This fabrication technique is described more
fully in our U.S. Patent Serial No. 4,169,997 issued on
October 2, 19790 The junctions 18.1 and 18.2 are thus
15 separated by a window which exposes a strip of layer
18.3. That strip is contacted by the central portion
22.1 of ohmic contact 22 so that pumping current flows
transversely through the layers in a narrow channel from
contact portion 22.1 in the window to active layer 16.2.
20 Current spreading can be further reduced by incorporating
an additional pair of spaced, reverse biased p-n junctions
at the substrate interface by the techniques described in
our aforementioned copending application or by using other
prior art schemes referenced in that application.
In addition, making the bandgap of cladding layer 14
sufficiently greater than that of active layer 16.2 pre-
vents any substantial amount of pumping current from
bypassing the active layer 16.2 by flowing directly
between the cladding and waveguide layers; i.e., the
30 turn-on voltage of p-n heterojunctions 16.3 between the
waveguide and cladding layers in larger (e.g., 1.6V) than
the turn-on voltage of p-n heterojunction 16.4 between
the waveguide and active layers (e.g., 1.4V).
The pumping current causes the injection of minority
35 carriers into active layer 16~2 where they un~ergo
radiative recombination to generate stimulated radia-
tion. A significant portion of the optical Eield of
this radiation preferably penetrates into the waveguide.
- -- 7 --
layer 16.1 so as to reduce the optical power density at the
mirror facets and thereby increase the threshold for
catastrophic damage. To this end the bandgap (o~ ,
refr2ctive index) difference between active layer 1~.2 and
5 wa~eguide layer 16.1 should be small enouyh to permit such
p~netration, yet large enough to con~ine injected minority
carriers to the active reCJion and thereby maintain
relatively high electronic gain. This laser configuration,
we have found, exhibits relatively high pulsed power
10 outputs (in the hundreds of milliwatts range) and,
suprisingly, a linear L-I characteristic-free of kinks - at
all power levels up to the catastrophic damage threshold.
In addition, this SBH laser exhibited stable fundamental
tranverse and single longitudinal mode operation.
In order to reduce the number of nonradiative
recombination defect centers at the heterojunction
interfaces between the various layers of our SBH laser, it
is preferred that essentially lattice matched materials be
utilize~. Fewer defect centers in general means lower
20 lasiny thresholds and longer lifetimes. In the Group III-V
compound system these materials include, for example,
GaAs-AlGaAs, GaAs-AlGaAsP, GaAsSb-AlGaAsSb and InP-InGaAsP.
Of these, GaAs-AlGaAs has the advantage that it is
substantially lattice matched over all solid solutions of
25 GaAs and AlAs. Using the latter system, the SBH laser of
FIG. 1 would typically comprise an n-GaAs substrate on
which the following layers would be epitaxially grown: an
n-AlxGal_xAs cladding layer 12 (0 < x < 1); an n-AlyGal_yAs
waveguide layer 16.1 (0 ~ y < 1; y < x); an n-, p- or
30 compensated active layer 16.2 of AlzGal_zAs [0 < z ~ 0.4;
z < y; and (y-z) adapted to confine injected carriers to
the active layer while at the same time permitting the
optical field to penetrate from the active layer into the
waveguide layer]; a p-AlqGal qAs cladding layer 14
[0 < q < 1; q > z and y; and (q-y) > (y-z) to prevent
significant pumping current from flowing across
heterojunctions 16.3]; a p-GaAs stop-etch and contacting
layer 18.3, and an n-AlrGal_rAs bifurcated layer 18.4
~Ar~ R. A 31-3
-
- 8 - ~.12~
.
(O < r C 1). Of course, it is obvious that the
conouctivity types of the various layers can be reversed.
For efficient operation at room temperature the
SBH laser is mounted on a heat sink (not shown) by means
5 well known in the art, and for continuous wave ope~ation at
room temperature, the thickness of the active layer 16.
should be less than 1.0 ~m and prefera~ly abou~
0.15-0.20 ~m.
An alternative embodiment of o~r SBH laser
10 incorporates a distributed feedback (DFB) grating which
provides frequency selectivity and in inteyrated optics
applications obviates the need ~or a discrete resonator
formed bY cleaved mirror facets. As shown in ~IG. 3, the
DFB grating comprises a plurality of parallel grooves 30
15 which are formed on the same major surface of waveguide
layer 16.1 as active layer 16.2, i.e., on heterojunctions
16.3. But, the grooves are formed on opposite sides o~ the
active layer 16.2 and e~tend perpendicular to the resonator
axis 28 (iOe., perpendicular to the elongated dimension of
20 strip active layer 16.2). As is well known in the art, to
provide feedback the periodicity of the grating should
preferably be equal to an odd integral number of half
wavelengths of the laser radiation as measured in the
semiconductor. This grating would typically be formed, for
25 example, by ion milling or chemically etching waveguide
layer 16.1 after depositing and suitably masking active
layer 16.2. Note that the interior ends of the grating
grooves should preferably be as close to the sides of the
active layer as possible to allo~ the optical field in the
30 active layer 16.2 to penetrate into the grating.
Illustratively, the grating ends should be within 1-2 ~m of
the active region. Although not depicted, the DFB
configuration of FIG. 3 could also be incorporated into the
embodiment of FIG. 2 by forming the grating, as before, on
35 the heterojunctions 16.3' on opposite sides of active layer
16.2'. In this case, the interior ends of the grating
grooves can be made right next to the sides of the embedded
active strip 16.2' by fabricating the grating first,
LOGAN, R. A. 31-3
' ' -
. :
39~7
uniformly everywhere, then etching the channel for the
embedded strip 16.2'.
In prior art buried heterostructure tBH) lasers,
effective transverse mode stabilization has been achieved
5 by introducing a built-in refractive index change along the
junction plane; for example, by embedding an active GaAs
core completely in Alo 3GaO 7AS cladding. f3Oweve~, the
index change along the junction plane is so large that
stable fundamental mode lasing is possible only for active
10 layer widths of < 1 ~m, resulting in low output power and
large beam divergence in the two transverse directions.
Yet, in lasers with wider active layers, higher order modes
are easily excited near threshold.
In our SB~ laser, the introduction of the
15 waveguide layer converts the core in a BH laser to a
strip-loaded waveguide having the thin active layer as the
strip and the thicker low-loss waveguide as the supporting
layer. This structure significantly reduces the effective
refractive index change along the junction plane. Hence,
20 fundamental transverse mode along the junction plane can be
easily obtained with much wider strip widths. As a result,
the output power is increased and the beam divergence is
reduced, while mode stabilization is maintained.
Furthermore, better device fabrication and performance
25 control can be achieved.
In the direction perpendicular to the junction
plane, the introduction of the waveguide layer greatly
increases the cavity thickness (e.g., from about 0.2 ~m to
about 1.6 ym) while still providing enouyh potential
30 barrier to confine the injected carriers in the active
strip. This thickening of the optical cavity does not
affect the threshold current but increases the output power
before catastrophic mirror failure and reduces the beam
divergence. Since the active strip is much thinner than
35 the waveguide layer, the fundamental transverse mode
(perpendicular to the junction plane) acquires more gain
than higher order modes.- This provides mode aiscrimination
against higher order modes even though they have slightly
L~AN ~ R . A. 31-3
hiqher mirror reflectivity. Finally, the waveyuide layer
is not ex~ected to decrease the quantum efficiency of the
laser because it is essentially lossless at the lasing ,
wavelength. Therefore, low current threshold, stable
fundamental transverse mode operation with linear light-
current characteristic and narrow beam divergenc~ in both
transverse directions up to substantially hiqh injection
current levels, and high output power should be obtainable
with our SB~I lasers. Indeed these properties have been
10 observed as discussed in the example which follows.
Example
- The following describes the fabrication of an SBH
laser from the GaAs-AlGaAs materials system. Dimensions,
materials, conductivity types and carrier concentrations
15 are intended to be illustrative only and should not be
construed as limitations on the scope of the invention.
Using a two-cycle liquid phase epitaxy (LPE)
technique, with suitable maskinq, etching and anodization
steps between the two cycles, we fabricated SBH lasers of
20 the type depicted in ~IG. 1 comprising: an (001) n-GaAs
substrate 11 doped with Si to about 1018 cm~3 and about
100 ~m thick; an n-A10 3GaO 7As cladding layer 12 doped
with Sn to about 2xlO17cm~3 and about 1.4 ~m thick; an n-
Alo lGaO gAs waveguide layer 16.1 doped with Sn to about
25 2xlO17cm 3; a p-GaAs active layer 16.2 doped with Ge to
about 3xlO17cm 3 and about 0.2 ~m thick and o~ various
widths - 2.5, 3.5, 5, 7.5 or 10 ~m; a p-Alo 3GaO 7As
cladding layer 14 doped with Ge to about 3xlO17cm 3 and
about 2.5 ~m thick; a p-GaAs contacting and stop-etch
30 layer 18.3 doped with Ge to about 5xlO17cm~3 and about
0.5 ~m thick, and an n-~10 45Gau 55As layer 1~.4 doped with
Sn to about 1017cm~3 and about 1 llm thick. The layer 18.4
had various window openings of comparable size to the
underlying active strips 16.2 and in substantial
35 registration there~ith. The substrate contact 20 comprised
a Au-Sn alloy whereas the top contact 22 comprised a Au-Zn
alloy.
~he fabrication of these SBH lasers proceeded as
~AN, R. A. 31-3
.
5~
follows. D-lrin~ the first LPE c3rowth cycle, layers 12 and
16.1 as described above were deposited on an n-Ga~ wafe~
(i.e., on the substrate 11) and then a p-Ga~s layer was,
deposited having a thic~ness equal to that desired for the
5 active layer 16.2. A thin (about 0.2 ~m) p-Alo 3GaO 7As
layer was then grown on the p-GaAs layer. Note, the last
layer was deposited to protect the top interface o~ the
active layer during subsequent processing steps and does
not yet correspond to tlle much thicker claddiny layer 14.
10 ~nis intermediate structure was removed from the LPE
chamber and the top surface of the thin ~lo 3GaO 7As layer
was anodized to form a native oxIde masking layer thereon~
Standard photolithographic techniques were then used to
form mask strips along the (110) direction in the oxide
15 layer and to expose the thin Alo 3GaO gAs layer between the
strips. The exposed Alo 3GaO 07As was selectively etched
in an iodine etchant ~113 g KI, 65 g I2, 100 cc El2O) to
expose the p-GaAs layer between the strips. Standard
anodization (which for~s a native oxide and consumes a
20 ~ortion of the semic~onductor) and st~ipping were then used
to relnove nearly all of the p-Ga~s layer between the
strips. It ~as important, however, to leave a thin (about
200 Angstrom thic~) layer of p-GaAs between the strips so
as not to expose the underlying n-A10 lGaO 9As to the
25 atmosphere. Such exposure makes subsequent LPE growth on
~l-containing Group III-V compounds very difficult.
After remcving the oxide strip masks and
subsequent chemical cleaning, the structure on the wa~er
comprised layers 12 and 16.1 with strip mesas of p-GaAs
30 (i.e., active layer 16.2) protected by the thin
Alo 3GaO 7As layer. The spaces between mesas were
protected with the thin ~about 200 Angstro.~ thick) p-GaAs
layer.
Next, the wafer was returned to the LPE chamber
3sand p~ 3GaO 7As layer 14 was grown thereon. During this
growth step the thin p-Alo 3GaO 7AS layers protecting the
tops of the active layers were incorporated into layer 14,
and the thin p GaAs layer between the strips was dissolved
G~N, R A. 31-3
.
- - 12 - ~ ~ ~5~'7
into the melt used to grow layer 14. ~rherefore~ layer 1~,
for all practical purposes, grew directly on the portions
of waveg~ide layer 16.1 between the strips as well as on
the strips themselves.
The contactinc3 and stop-etch p-GaAs layer 18.3
was then grown ~ollowed by an n~A10 g5Ga0 55As layer. l'he
latter was masked, usin~ the same photolithographic m~sk
used to define the st~ips, and then selectively e~ched,
usin~ the iodine etchant previously described~ down to the
10 p-GaAs layer 18.3, thereby bifurcatiny the n-A10 ~5Ga0 55As
layer as depicted by layer 18.4 of FIG. 1. Individual SBH
laser diodes were then formed by conventional
metallization, cleaving and heat-sinking procedures.
Light-current (L-I) characteristics of our SBH
15 lasers without anti-reflection mirror coatings were made
using standard measurement procedures. The measurements
with pulsed injection (150 ns pulse width, 1000 pulses/sec)
were made for active layer wiclths of about 5 ~M and 10 ~m
and lengths of 380 ~m. The top channel (window in
20 layer 18.4) widths of the lasers with 10 ym and 5 ~m wide
active strips were typically about 15 ~m and 10 ~m,
respectively. All lasers tested displayed excellent
linearity in ~-I characteristics. For lasers with 10 ~m
strips, this linearity continued, without catastrophic
25 failure, to about 10 times threshold current where a peak
power output of 400 m~ per face was measured. One laser
with a 5 ~m strip was tested to the catastrophic failure
limit. For that laser linearity continued up to about 15
times threshold at which a peak power output of 230 m~ per
30 face was measured. At this power catastrophic failure
occurred. Similarly, we measured the light-current
characteristics of other SBH lasers with 5 ~m wide active
layers pumped only to an output power of 100 mW per face to
avoid burnoùt. The uniformity and linearity of these
35 lasers was evident.
~ or SBH lasers with 10 ~m and 5 ~m wide active
layers, current thresholds were 150 mA-180 mA and
90 mA-150 mA, respectively, while the external quantum
LOGAN, R. A. 31-3
13 ~ 1~58~
,, ~ . .
efficiencies were 4~%-63~ and ~5~-35~. The lower external
quantum efficiency of the lasers with 5 ~m strips was due
to: (1) the larger top channel-to-strip width ratio, about
2, as co~pared to about 1.5 for lasers with 10 ~m strips,
5 and t2) the fact that as the top channel width decreases,
the amount of lateral current spreadiny in ~he p-GaAs and
p-~lo 3GaO 7As layers increases rapidly. B~ using mo~e
efficient lateral current confinement schemes, such as
laterally spaced, reverse-biased junctions at the substrate
10 interface in addition to those o~ FIG. 1, we believe that
much lower current thresholds can be obtained.
The far-field patterns, both along and
perpendicular to the junction plane, at various current
; levels above threshold were also measured for a typical SBH
15 laser with a nominal 5 ~m wide active layer. These
patterns were measured under pulsed operation U2 to 9 times
threshold. In the current region examined, the lasers
operated stably in the fundamental mode in both transverse
directions with no significant distortion of the field
20 patterns. In general, the beam divergences were about
8-10 degrees and 26-30 degrees in the directions parallel
and perpendicular to the junction plane, respectively. For
lasers with 10 ~m wide active layers, higher order modes
along the junction plane were excited near threshold and
25 successively changed into even higher order modes as the
current injection level was increased. We observed,
however, no "kink" or other nonlinearity associated with
mode transition. Lasers with 5 ~nl wide active layers,
under pulsed operation, exhibited single longitudinal mode
30 oscillation at injection currents as high as twice
threshold. In general, lasing occurred in several
longitudinal modes at slightly above the threshold current
Ith (~ 1.05 Ith), but the lasing power quickly concentrated
into a single longitudinal mode with a slight increase in
35 current. With increasing current, the longitudinal mode
shifted to an adjacent shorter wavelength mode, staying
predominantly a single mode over wide current inter~als
except during the brief mode transitions. Such current
LO~A~ R. A. 31-3
~ . ... . .
'
5~7
intervals shortened for high injection current levels.
It is to be understood that the above-described
arrangements are merely illustrative of the many specific
embodiments which can be devices to represent application
5 of the principles of our invention. Numerous and varied
other arrangements can be devices in acco~dance with these
principles by those skilled in the art without departing
from the spirit and scope of the invention. In particular,
in each of the embodiments of our SBl-i laser it is readily
10 possible to fabricate the strip active layer so that it is
shorter than the resonator ti.e., the active layer
terminates short of the mirror facets), thereby virtually
eliminating surface recombination of the facets. Thus, the
active layer would be entirely embedded in wider bandgap
15 material. Also note that with this modification to FIG. 3,
the D~B qratings near the facets can be made to extend
across the width of the laser.
LOGAN, R. A. 31-3
- .
