Note: Descriptions are shown in the official language in which they were submitted.
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V-GROOVE SEMICONDUCTOR
LIGHT EMITTING DEVICES
BACKGROUND OF THE INVENTION
This invention relates to light emitting
semiconductor devices, such as lasers and LEDs, and more
particularly to the confinement of current flow in these
devices.
One of the earliest structures for confining
current to flow in a relatively narrow channel through the
active region of a light emitting device was the stripe
geometry contact first proposed for semiconductor lasers
~y R. A. Furnanage and D. K. Wilson (U.S. Patent 3,363,1g5
issued on January 9, 1968). The stripe geometry reduces
the threshold current for lasing ~compared to broad area
lasers) and limits the spatial width of the output beam.
Since that early proposal, numerous laser configurations
have been devised to implement the stripe geometry concept:
(1) the oxide stripe laser; ~2~ the proton bombarded laser;
(3) the mesa stripe laser; t4) the reverse-blased p-n
junction isolation laser; (5) rib~waveguide lasers; and
(6) buried heterostructures of various types.
The most commonly used configuration for the past
eleven years, however, has been the proton bombarded,
GaAs-AlGaAs double heterostructure (DH) laser described,
for example, by H. C. Casey, Jr. and M. B. Panish in
Heterostructure Lasers, Part B, pp. 207-210, Academic
Press, Inc., N.Y., N.Y. (1978). Despite its various
shortcomings, lasers of this type have regularly exhibited
lifetimes in excess of 100,000 hours and a number have
exceeded 1,000,000 hours (based on accelerated aging
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tests). Long lifetimes have also been observed in DH LEDs
employing similar proton bombardment to d~lineate the
current channel.
~ everal of the shortcomings of proton bor,t1oarded
DH lasers are discussed by R. ~l. Dixon et al in The Bell
System Technical Journal, Vol. 59, No. ~, pp. 975-9~5
(1980). They explored experimentally the optical
nonlinearity (presence of "kinksl3 in the light-current
(L-I) characteristics) and the threshold-current
distribution of AlGaAs, proton-bombardment-delineated,
stripe geometry DH lasers as a function of stripe width (5,
8, and 12 ~m) in cases in which the protons did and did not
penetrate the active layer. They demonstrated that shallow
proton bombardment with adequately narrow stripes (e y.,
5 ~m) can result in satisfactory optical linearity ~kinks
are driven to non-obtrusivel high current levels) without
the threshold penalty that has been associated with
narrow-stripe lasers in which the protons penetrate the
active layer. On th0 other hand, lasers with such narrow
stripes have exhibited a statistically meaningful~ although
not demonstrably fundamental, decrease in lifetime. In
addition, failure of the protons to penetrate the active
layer increases device capacitance and hence reduces speed
of response and, moreover, increases lateral current
spreading and hence increases spontaneous emission. In
digital systems, the latter implies a higher modulation
current to achieve a predetermined extinction ratio or a
lower extinction ratio for a predetermined modulation
current. G~ c~ v~ ~ 3'?~,~3~-
The concurrently filed~application~of R. ~. Dixon
et al, supr~, describes stripe geometry, proton
bombardment~deIineated DH lasers in which satisfactorily
high optical linearity, low capacitance, and low
spontaneous emission levels are achieved by means of a
current confinement scheme in which the current channel is
narrower at the top near the p-side contact and wider at
the bottom near the active layer. More generally, the
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Dixon et al application describes light emitting semi-
conductor devices (lasers or LEDs) having a semiconductor
body, an active region within the body, and constraining
means through which current flows from a major surface of
the body to the active region, thereby causing radiative
recombination of holes and electrons in the active region.
The constraining means includes first means forming a
relatively narrow first channel ~hich extends from approx-
imately the major surface into the body to a depth short
of the active region, and second means forming a relatively
wider second channel ~hich extends from approximately that
depth into or through the active region. Illustratively,
the first and second means comprise high resistivity
regions which bound the channel. These regions can be
formed by a number of techniques including proton
bombardment, oxygen bombardment, or suitable etching and
regrowth of high resistivity material.
SUM~IARY OF THE INVENTION
In accordance with an aspect of the invention
there is provided in a semiconductor light emitting
device, a semiconductor body comprising an active region
in which optical radiation is generated when current flows
therethrough, and means for constraining said current to
flow from a major surface of said body in a channel
through said active region, said constraining means
comprising elongated groove first means for causing said
current to flow in relatively narrow upper channel which
extends from said surface to a depth short of said active
region, and second means for causing said current flow in
relatively wider lower channel which extends from at least
said depth to said active region.
In accordance with an illustrative embodiment of
our invention, a semiconductor light emitting device
includes a semiconductor body having a major surface, an
active region within the body, and constraining means
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through which current flows from the major surface to the
active region, thereby causing radiative recombination of
holes and electrons in the active region. The constraining
means includes first means comprising an elongated groove
S (e.g., a V-groove) in the major surface forming a
relatively narrow first channel which extends into the
body to a depth short of the active region, and second
means ~e.g., proton bombarded zones) forming a relatively
wider second channel which extends from at least that
depth into or through the active region.
In an alternative embodiment, the first means
includes high resistivity regions adjacent the major
surface which bound at least a portion of the oblique sides
of the V-groove; i.e., the V groove penetrates through
these regions. In yet another embodiment the V-groove is
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refilled with semiconductor material.
srief Descriptinn of the Drawing
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These and other objects of our invention,
together with its various features and advantages, can be
readily understood from the following more detailed
description taken in conjunction with the accompanyiny
drawingO In the interests of clarity, the figures have not
been drawn to scale. ~here appropriate, alements cornmon to
the various figures have been given identical reference
numbers.
FIG. 1 is an isometric view of a semiconduc-tor
laser or LED in accordance with one embodiment of our
invention;
FIG. 2 is a cross-sectional view of a laser or
15 LED in accordance with another embodiment of our invention
in which the V-groove penetrates a high resistivity region;
and
FIGo 3 is a cross-sectional view of a laser or
LED in accordance with yet another embodiment of our
invention in which the V-groove is refilled with
semiconductor materialu
- Detailed Description
General Considerations
With reference now to FIG. 1, there is shown a
semiconductor light emitting device (laser or LED)
comprising a semiconductor body 11 which includes an
intermediate region 14. Region 14, which may include one
or more layers, includes an active region which emits
predominantly s-timulated radiation 22 in the case of a
laser or spontaneous radiation in the case of an LED~ when
pumping current is applied thereto. Electrode means,
illustratively contacts 16 and 18 on body ll, is provided
along with a voltage source 20, to supply the pumping
current. In addition, body 11 includes constraining
means 32-34 which cause the pumping current to flow in a
relatively narrow channel 36-38 from the top contact 16
through the active region after which the current may
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spread out to bottom contact 1~.
Before discussing our invention in detail, it
will be helpful to discuss first the general configuration
of a preferred configuration of a semiconductor light
emitting device known as a double heterostructure (~H). As
shown in FIG. 1, a DH comprises first and second relatively
wide bandgap, opposite conductivity type, semiconductor
claddin~ layers 10 and 12, respectively, and, essentially
latticed matched thereto, intermediate region 14 which is
between and contiguous with the cladding layers. The
intermediate reyion 14 includes a narrower bandgap active
layer, here shown to be coextensive with the region 14,
capable of emitting radiation when the cladding layers are
forward biased. From the standpoint of quantum efficiency,
the active layer is preferably a direct bandgap
semiconductor~ Layers 10, 12, and 14 can be made of
materials selected from a number of systems; for example,
GaAs-AlGaAs or GaAsSb-AlGaAs, for operation at short
wavelengths in the 0.7-0.9 ~,m range approximately, and
InP-InGaAsP or InP-AlGaInAs for operation at wavelengths
longer than about 1 ~m (e.g., 1.1-1.6 ~m).
Voltage source 20 forward biases the cladding
layers and thereby injects carriers into the active layer.
These carriers recombine to generate spontaneous radiation
in the case of an LED and predominantly stimulated
radiation in the case of a laser. In either case, however,
the radiation has a wavelength corresponding to the bandgap
of the active layer material. Moreover, in the case of a
laser the radiation 22 is emitted in the form of a
collimated beam along a resonator axis 23 perpendicular to
a pair of mirrors 24 and 26 formed illustratively by
cleaved crystal facets or etched surfaces. These mirrors
constitute optical feedback means for generating stimulated
radiation. In other applications, for example inteyrated
optics, diffraction gratings may be employ0d as a
substitute for one or both of the mirrors.
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Although the electrode means depicted in the
device of FIG. 1 includes broad area contacts 16 and 18
suitable for use in a laser or edge emitting LED, it is
well known in the art that these contacts can be patterned
to form various geometrical shapes Thus, in an LED
contact 16 may be a broad area contact, but contact 18 may
be an annular ring (not shown) which accommodates an etched
hole (not shown) in one side of body 11. Where the bottom
portions (e.gO, substrate 28) of body 11 is absorbing, this
etched hole can be used to couple radiation propagating
perpendicular to the layers into an optical fiber ~not
shown) positioned in the hole.
The conductivity type of the active layer is
not critical. It may be n-type, p-type, intrinsic or
compensated since in typical modes of operation under
forward bias the number o~ injected carriers may exceed
the doping level of the active layer. In addition, the
intermediate region 14 may include multiple layers which
constitute an active region, e.g. contiguous p- and n-type
layers of the same bandgap forming a p-n homojunction or of
different bandgaps forming a p-n heterojunction. Further~
more, the heterostructure may take on configurations other
than the simple double heterostructure including, by way
of example but without limitation, separate confinement
heterostructures as described by I~ Hayashi in U.S. Patent
3,691,476, and strip buried heterostructures of the type
described by R. A. Logan and W. T. Tsang in U.S. Patent
4,190,813. In the latter case of a DH isotype laser, the
cladding layers are of the same conductivity type, and the
p-n junction i5 located external to, but within a diffusion
length of, the active region. One embodiment of our
invention, described infra with respect to FIG. 3, is a
V-groove isotype DH laser or LED.
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For CW laser operation at roorn temperature, the
thickness of the active layer is preferably between
approximately ~/2 and 1.0 ~m, where ~ is the wavelength of
the radiation as measured in the semiconductor~ For low
threshold operation, the thickness is typically 0.12 to
0.20 ~m. ~owever, for LED operation a thicker active
layer, typically 2 to 3 ~-m, is suitable. In either case,
for rOom temperature operation the laser or LED is
typically bonded to a suitable heat sink, not shown.
I~ practice, the layers of a double
heterostructure are typically grown by an epitaxial process
such as liquid phase epitaxy (LPE), molecular beam epitaxy
(MBE), or metallo-organic chemical vapor deposition (MO-
CVD). Epitaxial growth takes place on a single crystal
substrate 28 which may include a buffer layer (not shown)
between the substrate 28 and the first cladding layer 10.
Also, a contact facilitating layer 30 is optionally
included between the second cladding 12 and the top
contact 16. The opposite contact 18 is formed on the
bottom of substrate 28.
As mentioned previously, in order to constrain
the pumping current generated by source 20 to flow in a
relatively narrow channel 36-33 through the active region,
constraining means 32-34 is provided in body 11.
Basic V-Groove Structures
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In accordance with an illustrative embodiment of
our invention as shown in FIG. 1, current constraining
means 32-34 includes first means 34 defining a relatively
narrow upper channel 36 and second means 32 defining a
relatively wider lower channel 38. Illustratively, the
constraining means comprises V-groove first means 34 which
extends from major surface 44 to a depth dl short of the
active region, thereby defining relatively narrow upper
channel 36; and further comprises laterally separate, high
resistivity regions 32 which bound lower wider channel 3~,
and which extend from at least depth dl approximately, to
the active region (i.e., into or throllgh the active
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region). ~s shown, separated regions 32 illustratively
extend from surface 4~ and preferably through the active
region. V-groove 3~ is positioned within the space between
regions 32. However, it is not essential that the high
resistivity regions 32 actually reach all the way to the
major surface 4~. In fact, for contacting purposes it may
be advantageous to have a high conductivity layer
interposed between regions 32 and contact 16 as described
by R. W. Dixon et al in U. S. Patent ~,124,826.
With reference to the DM of FIG. 1, the V-
groove 3~ has a width Sl at major surface 44 and a depth d
where it penetrates second cladding layer 12, thereby
defining upper channel 36 as having essentially the same
width~ In contrast, the high resistivity regions 32 are
separated by a wider distance S2 > Sl and extend from
surface 44 to a depth d2 > dl into and preferably through
the active region, thereby defining the wider lower
channel 38 of width S2.
Alternatively, as shown in FIG. 2, the upper
channel 36 can be further restric-ted by additional high
resistivity regions 32.1 which bound a portion of the
oblique sides 34.1 of V-groove 34, thus defining the upper
channel width Si of FIG. 2 as being less than Sl of FIG. 1.
In practice, the regions 32 and 3201 can be fabricated
(e.g., by proton bombardment) to depths of d2 and d3,
respectively (d2 > d3); and then the V-groove 3~ can be
etched to a depth dl so as to penetrate the regions 32.1
(d3 < dl < d2).
These V-groove configurations are expected to
exhibit several advantages. First, the narrow upper
channel 36 increases the current density in the active
region and thereby causes kinks in lasers to be shifted to
higher current levels out of the range of typical
operation. Second, this feature should also result in more
uniformly distributed lasing thresholds and lower lasing
thresholds, providing higher device yields. Third, ~ecause
the wider lower channel 38 reduces lateral current
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diffusion and spreading, less spontaneous radiation is
emitted outside the resonator of the laser~ thereby
allowing for a lower modulation current for a predeter~ined
ex~inction ratio in digital applications. Fourth, the
latter feature results in reduced device capacitance for
both lasers and LEDs, thereby permitting higher speed of
operation (i.eO, higher pulse repetition rates in digital
applications).
It is to be understood that -the above-described
arrangements are merely illustrative of the many possible
specific embodiments which can be devised to represent
application of the principles of the invention. ~umerous
and varied other arrangements can be devised in accordance
with these principles by those skilled in the art without
departing from the spirit and scope of the invention. In
particular, the V-groove 34 of FIG. 1 or FIG. 2 may be
refilled with semiconductor material, resulting in devices
(especially lasers) with several useful characteristics as
discussed below. Moreover, although the groove has been
described as a V-groove, its precise geometrical shape is
not critical. A V-groove results when III-V semiconductors
are subject to certain etchants which preferentially etch
crystallographic planes, but V-groove or rectangular
grooves might result from other etchants or other processes
(e.g., ion beam milling or plasma etching).
Refilled V-Groove Structures
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As shown in FIG. 3, the V-groove has been filled
with semiconductor material 34' and, depending on the
procedure used to effect refilling, may or may not result
in ~ormation of layers 34.2 which are adjacent the V-groove
and on major surface 44. Moreover, depending on the
material of cladding layer 1~ and the type of procedures
used, material 34' may or may not be epitaxial (i.e.
monocrystalline).
Several embodiments result depending on the
relative size of the bandgaps EcJ oE the DH layers relative
to that o~ V-groove material 34'. Case I:
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Eg (34') > Eg (14); that is, the V-groove material 34' has
a larger bandgap than the active layer 14. As a
consequence, laser radiation penetrating V-groove
material 34' experiences reduced absorption as compared
with FIG~ 1. Case II: Eg (341) > Eg (12) > Eg (14); in
addition, claddiny layers 10 and 12 and the active layer 14
all have the same conductivity type, and V-groove
material 34' and cladding layer 12 have opposite
conductivity types. This configuration is a form of
isotype laser in which the p-n junction is located along
oblique surfaces 34.1. In this case, the V-groove
material 34' is preferably monocrystalline. Case III:
Eg (12) > Eg (34') ~ Eg (14); that is, the V-groove
material 34' has a lower bandgap than cladding layer 12 but
lS a higher bandgap than active layer 14. As a consequence
the refractive indices n have the relationship
n (14) > n (34l) > n (12) so that the laser radiation would
be refractive index guided along the V-groove.
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