Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVEN'I'ION
Field of the Invention
This invention relates to a method for fabr;cating thin film semiconductor
superlattice heterostructures and the resulting structures and clevices obtainable thereby
5 and more particularly to the production of quantum well structures on non-planar
substrates.
Description of the Prior AOrt
"Thick" (>500A) epitaxial layers have been grown on nonplanar substrates by
various growth techniques, e.g., liquid phase epitaxy (LPE), molecular beam epitaxy
10 (MBE), and organometallic chemical vapor deposition (OMCVD). In all cases, the
nonplanarity of the substrate gives rise to lateral thickness variations in the epitaxial
layers. Such laterally patterned structures have been useful for optical wave guiding
(essentially because the wavelength of light is comparable to the layer thicknesses
involved). O
Ultra-thin (<5~OA) epitaxial layers have been grown on planar substrates. For
such thin layers (i.e. Iayers whose thickness is comparable to the deBroglie wavelength
of charge carriers) quantum size effects in one dimension (along the growth direction)
modify the material properties (e.g., bandgap and refractive index). Hence, by tailoring
the thickness of the epitaxial layers, it has been possible to vary the resu]ting superlattice
20 (or quantum well) material properties. For example, selection of the superlattice (SL)
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periodicity results in selection of the material bandgap In addition, these superlattices
give rise to new features, e.g., enhanced nolllinear optical properties. Furthermore it has
been shown that the SL period (or layer thicknesses) in the direction of layer growth,
allows one to fabricate structured materials in which the physical properties in the
5 direction normal to the substrate plane differ based upon the SL period. Devices which
rely not only UpOIl the new properties of the SL materials, but also on quantum size
effects that occur in the individual layers, have also been demonstrated, e.g., quantum
well lasers, resonant tunneling devices, quantum-confined Stark effect modulators, etc.
The production of SL or quantum well devices having superlattice structures
10 which are more than one dimensional and/or which vary laterally in thickness is desirable
in order to obtain devices having different and/or enhanced physical properties and
having new and different capabilities than prior art quantum effect devices would
therefore be desirable.
Summary of the Invention
A method for the production of thin (<500 A) epitaxially grown semi-
conductor layers having a laterally varying SL periodicity is described. The method
comprises providing a nonplanar, e.g., a grooved, substrate on which the epitaxial
semiconductor layers are grown by the usual growth techniques e.g., LPE, MBE, orOMCVD.
_3_ 1 3 ~
The thin SL or quantum well (QW) semiconductor layers resulting from this
growth vary in thickness, and hence physical properties, laterally along the substrate
pl~ne. Such variations give rise not only to layers with varying properties but also to new
device structures employing such films. One characteristic of the resulting structured
5 materials is that they may be fablicated to tailor their properties in more than one
dimension.
Brief Description of the Drawings
FIG. 1 is a transrnission electron micrograph of a cross section of a superlattice
grown in an etched groove.
FIG. 2a-c illustrate carrier confinement induced by quantum well thickness
variations.
FIG. 3 is an elevational view of a semiconductor wafer having an array of
patterned etched depressions in one surface thereof.
FIG. 4 is a schematic illustration of the cross section taken across A-A or B-B
15 of the wafer shown in FIG. 3.
FIG. 5 is a schematic illustration of the cross section of a patterned quantum
well laser.
FIG. 6 is a graph showing the light output in terms of power versus the current
characteristic of a patterned, 280 micron long quantum well (QW) laser with pulsed
20 operation at room temperature.
FIG. 7 shows typical far field (a) and near field (b) distributions of the
patterned QW laser, measured in the junction plane.
FIG. 8 is a schematic illustration of a Mach-Zehnder type interferometer
employing a patterned QW wire.
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Detailed Description of the Invention
Generally, a method for patterning semiconductor superlattice heterostructures
comprises growing these superlattices on nonplanar patterned substrates to produce
variations in thickness and crystallographic orientations of the grown layers in order to
introduce lateral variations of the physical properties which depend on these parameters.
The lateral thickness variation results from the variation in the flux of the source beams
across surfaces with different angular orientations and because of the different effective
sticking coefficient associated with different crystal planes. If the patterned features are
fine enough, semiconductor materials with reduced carrier dimensionality can be
10 obtained. By proper selection of the features in the pattern, "artificial" materials useful in
high speed electronics and optical signal processing can be produced.
By way of example, GaAs substrates [(100) oriented] were patterned by using
conventional photolithography and wet chemical etching. Channels, ~3 llm wide and ~2
llm deep and aligned along the [011] direction, were etched through a photoresist mask
15 using H2SO4:H2O2:H2O (1:~:40) preferential chemical etching solution. The
photoresist mask was then removed, and the patterned substrates were cleaned by
degreasing, followed by etching for 45 seconds in H2SO4:H2O2:H2O (4:1:1) solution
and for 4 minutes in HCI. Finally, the samples were rinsed and blown dry. The
resulting grooves had rounded profiles because of the etching in the (4:1:1) solution.
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A superlattice consisting of five periods of alternate IOOA and IOOA
Alo 3GaO 7As layers, preceded by a thick GaAs buffer layer, was grown on the
patterned samples by MBE. (The layer thicknesses are the ones obtained for a planar
substrate.) The growth was carried out at 1 00C above the oxide desorption tempe3~ature,
5 typically 680C, and the As/Ga beam equivalent pressure ratio was -3. The substates
were rotated at 20 rpm during the growth. The superlattice growth features were studied
by using transmission electron microscopy (TEM) to examine [011] cross sections of the
grown samples.
FIG. 1 shows a TEM cross section of a superlattice that was grown in an etched
10 groove. The MBE growth resulted in the formation of facets Iying in specific crystal
planes, as indicated in this figure. It can be seen that the superlattice period decreased
significantly with increasing tilt angle of its growth plane relative to the horizontal (100)
plane. This period variation arises from the difference in the growth rate, measured
perpendicular to the crystal planes associated with the different planes. The difference in
15 the growth rate results both from the variation in the flux of the source beams across
planes with different orientation as well as from different sticking coefficients of the Ga
and the A1 atoms for different crystal planes. The asymmetry in the structure shown in
FIG. 1 is probably due to a slight misalignment of the etched grooves with respect to the
[01 1] direction.
Magnified TEM cross sections of the regions where superlattices of different
periods inters&ct, show the change in the superlattice period is mostly smooth and occurs
within <lOOA. The transition between the superlattices oriented
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along the (711) and the (554) planes, however, exhibits a "kink." This is
believed to be a result of migration of the source atoms, after reaching
the surface of the substrate, to more "favorable" crystal planes. Atoms
reaching the (554) plane, close to the intersection with the (711) plane,
S migrated to the neighboring (711) plane, which resulted in even smaller
growth rate along the (554) plane in that region. Note also that the GaAs
layers (the dark stripes in Figure 1) are thinner than the AlGaAs ones, in
the "kink" region, which indicates that this migration effect is stronger for
the Ga atoms than it is for the Al ones. The "kink" in the structure
10 shown in Fig. 2(c) extends over 200-300A. This dimension is comparable
to the diffusion length of the Ga and the Al atoms which migrate on the
growth surface.
Table I shown in a previously published article
entitled "Molecular Beam Epitaxy of GaAs/AlGaAs Superlattice
lS Heterostructures on Nonplanar Substrates", by E. Kapon et ~1,
Applied Phy~ics Letters 50(6), 9 February 1987, pages 347-349 lists the
crystal planes that were identified in the TEM cross section of Fig. 1,
along with the measured and the calculated tilt angles ~ between each
crystal plane and the horizontal (100) plane. Faces oriented along the
20 (111), (~11), and (811) planes were identified in other experiments oE
MBE growth on planar substrates as well. Table I also summarizes the
superlattice periods (measured normal to the crystal planes) obtained in
the present experiment. These periods should be compared to the period
obtained for a planar substrate, which was ~200A. It can be seen that
25 large variations in the periods of adjacent superlattice sections can be
achieved. The superlattice section Iying in the (554) plane has a period of
only 80A; this is less than half the period of the adjacent sections which
lies in the (711) plane. Table I also shows the values of ~/cos ~, which is
a measure for the relative effective sticking coefficient, for each of the
30 growth planes. The planes (411), (111~, (811~,
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and (711) have similar effective sticking coefficients and thus the growth rate on these
planes is determined mainly by their tilt angle. The effective sticking coefficient for the
(554) plane, on the other hand, is considerably smaller.
The use of the features of MBE growth on patterned substrates in the
5 fabrication of optoelectronic devices has already been pointed out. In particular, the
thickness variations exhibited by the epitaxial layers give rise to lateral variations in the
effective index of refraction, which can be used to define channel optical waveguides.
The demonstration of these thickness variations with epitaxial layers which are less than
a few hundred angstroms in thickness, however, is important because of the quantum size
10 effects exhibited by such ultrathin layer heterostructures. The strong dependence of the
confinement energy on the well width in quantum well heterostructures implies that
lateral variation in the quantum well width gives rise to lateral variations in physical
parameters which depend on this confinement energy. Therefore, it is expected that
superlattice heterostructures grown on nonplanar substrates and showing the lateral
15 period variations described above will exhibit lateral variations in physical properties
associated with the superlattice period.
One example of a physical parameter which depends on the superlattice period
is the refractive index. The lateral variations in the superlattice period can thus be used
to achieve lateral patterning of the index of refraction, which is useful for various
- 20 guided-wave optics applications.
Another example of an application of these patterned superlattices is the use ofthe difference in the effective band gap of quantum wells of different thickness to induce
carrier confinement in more than one dimension. Consider the quantum well
heterostructure whose cross section is described schematically in FIG. 2(a). TheS quantum well is thicker near the center of the structure. Consequently, the lowest Iying
bound states have a higher energy on both s;des of the structure [FIG. 2(b)]. Therefore,
the resulting steps in the carrier energies provide an effective potential well in the lateral
direction, which can serve to achieve lateral carrier confinement. FIG. 2(c) shows a
possible way to realize such a structure, by growing a quantum well heterostructllre in a
10 groove (see also FIG. 1). For a GaAs/A10 3gaO 7As quantum well heterostructure, a
decrease in the well thickness from 100 to 50A results in an increase in the confinement
energy by more than 50 meV. Such a potential step would be sufficient for achieving
carrier confinement, at least at low temperatures. Quantum well thickness variations of
this order can clearly be obtained by MBE growth on nonplanar surfaces, as is
15 demonstrated herein. However, the etched grooves should be made considerably
narrower (on the order of a few hundred angstroms) in order to observe quantum size
effects due to lateral carrier confinement. Such fine patterning can be achieved by using,
e.g., electron beam lithography. It should be noted that additional carrier confinement
effects can result in the structure described in FIG. 2(c) due to the tilt in the thinner
20 quantum well planes with respect to the thicker one. Three-dimensional confinement of
carriers might be obtained similarly by MBE growth on nonplanar substrates patterned
with two-dimensional features.
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The resulting structured materials now have tailored properties in more than
one dimension. For example, the variation in the bandgap can be utilized in order to trap
calTiers in more than one dimension. Prior techniques for carrier confinement insemiconductor heterostructures involved embedding a low bandgap material in high5 bandgap regions by using etch and regrowth steps. The method set forth herein requires
only a single growth step, and does not involve physical interfaces between the high-and
low- bandgap materials. Applications of 3-D bandgap tailoring include semiconductor
lasers, optical waveguides, diffraction gratings (e.g. gain modulation distributed feedback
lasers: the periodic corrugations in the substrates are made such that l~=m ~o/n, where
10 is the period, m is an integer, ~o is the vacuum wavelength, and n is the refractive index.
Then, in addition to the periodic variation in the real part of the refractive index, the
imaginary part is also modulated due to the QW thickness modulation. This leads to
single frequency emission.)
New advantages are obtained if the laterOI dimensions of the patterned QWs (or
15 the periods of the 3-D, 2-D SLs) are < 500-lOOOA. Then, the laterally confined carriers
exhibit 2-D or 3-D quantum size effects, and the resulting structured materials exhibit
quantum wire or quantum box properties (i.e., are characteriæed by 1-D or O-D carriers).
As such, they are expected to have new material properties; enhanced nonlinear optical
properties, for example. Semiconductor lasers made of such 3-D (or 2-D) SLs will have
20 lower thresholds, higher modulation band width and narrower line width. New electrical
transport properties of these materials are
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also expected.
In addition, the ability to create quantum wires and quantum boxes with this
technique opens the door for a new generation of electronic devices, relying on what may
be called "guided-wave electronics." Creating a single "wire" of ]ow bandgap material
S surrounded by high bandgap material and thinner QWs results in one-dimensionalelectrons guided by the wire. For example, PIGS. 3 and 4 demonstrate a physical
structure which gives rise to a two dimensional superlattice having varying thickness
trapping the carriers to result in zero-dimensional electrons. Lateral thickness variations
are obtained along the QW as defined by the ridge pattern. A one dimensional version of
10 such a patterned structure would result in a QW wire.
Referring to F1GS. 3 and 4, a structure is shown for a multi-dimensional (2-D)
embodiment of the invention. Here, a single crystal substrate wafter 31, e.g., GaAs, is
etched to form a plurality of spaced depressions 32 which may have circular, square or
other cross sections when viewed from the top of the substrate. In addition to the
15 depressions, the substrate 31 is etched to form a fine pattern as shown in FIG. 4. This
fine pattern is what would be observed from a cross-section across either plane A-A or
plane B-B of FIG. 3. Referring to FIG. 4, the fine pattern consists of a series of etched
grooves 33 having a plurality of epitaxially grown layers thereover. The first epitaxial
layer 34 and the tgp epitaxial layer 35 are high bandgap layers having thicknesses which
20 may exceed SOOA. A central epitaxial layer 36 of a low bandgap material is formed
between the two high bandgap layers 34 and 35, This central layer 36is less than
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500A1 and typically in the order of 100A, and varies laterally in thickness as previously
described. If the thickness of the quantum well is selected such that the width of the area
37 at the base of the groove (shown by a circle in the Figure) is less than the de Broglie
wavelength, carriers can be trapped in this area creating a QW wire along the base of the
S QW layer 36.
In FIG. 8 there is a representation of an interferometer which makes use of
such a QW wire. Here, a semiconductor device 80 includes an emitter 81 and a collector
82. A QW wire 83 is formed which extends from emitter 81 to collector 82 and has a
first fork 83 which diverges to form two arms 84 and 85 beyond the emitter 81 and
10 recombines at a second fork 86 prior to or at the collector 82. A base region 87 extends
in the area of one arm 84 of the QW wire for modulating the phase of the carrier wave
functions in that arm of the QW wire. In operation, carriers are injected from the emitter
into the QW wire in the region of the emitter, the carrier wave functions (initially of the
same phase) are split into the two arms 84 and 85 at the fork 83. By applying anlS appropriate signal to the base region, a modulated phase shift can be obtained in arm 84
relative to arm 84~ When the carrier wave functions recombine at the second fork 86, the
current in the QW wire will be modulated in accordance with the phase modulation in the
arm 84 (i.e. constructive or destructive recombination). It should be noted that ~his
configuration can be used for light modulation as well as current modulation where the
20 QW wire is in a laser structure and supports laser transmission.
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Quantum well (QW) heterostructure lasers offer a number of advantages over
conventional heterostructure diode lasers, including a lower threshold curTent density,
reduced temperature sensitivity and a potentially higher modulation bandwidth. Various
types of stripe geometry, such as ridge waveguide and buried heteIostructure
5 configurations have been used in the past for obtaining very low threshold QW lasers.
These stripe geometries introduce the lateral patterning in the diode laser structure which
is required for achieving efficient calTier and optical confinements parallel to the junction
plane.
The method described herein which utilizes the lateral variations in the
lO thickness of quantum wells grown on nonplanar substrates in order to achieve lateral
patterning of the energy bandgap and the index of refraction can be employed to make a
QW laser structure which relies on this QW patterning technique. By growing an
otherwise conventional GaAs/AlGaAs single QW laser heterostructure on a grooved
substrate, we obtain an effectively buried QW laser. The injected calTiers in this laser are
15 laterally confined to a ~ m wide QW stripe owing to the larger effective bandgap of
the thilmer QW layers on both sides of this stripe. Room temperature threshold curTents
as low as 6 mA (with uncoated facets) have been obtained.
The schematic cross section of the patterned QW laser is shown in FIG. 5.
Fabrication of the lasers began by etching V-shaped grooves Sl along the [Oll] crystal
20 d*ection Oll a
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(100) oriented n+-GaAs substrate 52. The grooves 51 were etched through a photoresist
mask by using H2SO4: H2O2(30%): H2O (1:8:40 by volume), and were 10 ~m wide
and 7 llm deep. Prior to the epitaxial growth, the grooved substrates 52 were cleaned by
degreasing, followed by etching first in HCl for 2 min., then in H2SO4:H2O2(30%):H2O
5 (4:1:1) for 2 min. and finally in HCl for an additional 2 min. The heterostructure layers
were then grown by molecular beam epitaxy (MBE) at 700C under an arsenic-rich
atmosphere.
The single quantum well, separate confinement laser heterostrucOture consistedO
of a 0.5 llm GaAs buffer layer 53 (Si, n=2 1018cm~3) 5 periods of a 150 A GaAs/150 A
10 Alo 5GaO 5As buffer superlattice 54, (Si, n=lOl~cm~3), a 1.25 llm Alo 5GaO 5As
cladding layer 55 (Si, n=1018cm~3),
a 0.2 ~m AlxGal xAs waveguide layer 56 with x linearly graded ~rom 0O.5 to 0.2,
performed by rapid switching of the Al shutter (Si, n=1 1017 cm~3), a 70 A undoped
GaAs active layer 57, a 0.2 llm AlxGal xAs waveguide ]ayer 58, x-0.2-0.5 (Be,
15 p=1 1017cm 3), a 1.25 llm Alo 5GaO 5As cladding layer 59 (Be, p=1 1018cm 3 and a 0.2
m GaAs contact layer 60 (Be, p=5 10 cm ). The layer thicknesses indicated above
are for the case of a planar, (100) oriented substrate. The actual grown layers exhibit
lateral thickness variations in the vicinity of the groove, as is evident from scanning
electron microscope (SEM) photographs (not shown) of a cleaved sample. Facets
20 oriented parallel to the ~111} and the {411} crystal planes form, and the growth rate
normal to these planes is about 50 and 80 percent, respectively, of the growth rate in the
(100) direction. Transmission electron microscopy studies show
14 ~ 3 ~
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that the thicker ( o70A), (l00) oriented GaAs QW at the center of the structure is bounded
by thinner (~40A), (lll) oriented QW layers on both sides. Tne width of the (l00)
oriented QW stripe is ~lllm. It is interesting to note the existence of lateral tone
variations along the Si doped .4105GaO5As layer visible in the SEM's probably
5 represent doping type variations.
The grown wafers were processed into diode lasers by evaporating AuBe/Au
contacts 6l on the p side, thinning the substrate and evaporating AuGe/Au n-contacts
(not shown). Lateral current confinement was obtained by defining a 27,1m wide
conductive stripe at the bottom of the groove using proton implantation (l00 keV energy,
l0 310l5 cm 2 dose; see FIG. 5). The lasers were tested at low duty cycle (200 nsec
pulses, lkHz repetition rate). Their optical field patterns were evaluated by employing a
video camera and a video analy~er. Below threshold, spontaneous emission emanated
mainly from the (l00) oriented QW stripe at the center of the structure. Weaker
spontaneous emission occurred at the (l00) oriented regions at the '7shoulders" of the
lS structure, probably due to current leakage. Above threshold, the device lased in a single
almost circular spot, with lateral full width at half maximum (FWHM) of ~2,1m. The
~hreshold current was as low as 6 mA (for uncoated devices) and the differentialefficiency was 35 percent. The light (power output~ versus current characteristics were
linear up to more than four times the threshold current (see FIG. 6). In this range of
20 curren~s the spectOrum of the laser exhibits a few (3-4) longitudinal modes and is centered
about ~ = ~8450A.
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Typical far field and near field distributions of the patterned QW laser,
measured in the junction plane, are given in FIG. 7 a ~c b, respectively. Stablefundamental mode operation was observed up to four times the threshold current. O
The patterned QW lasers lased in a single spatial spot at ~ = ~8450A up to
5 about four times the threshold currents. At higher currents, two additional lasing spots
appeared at the "shoulders" of the structure where the spontaneous emission due to
current leakagg had been observed. The wavelength for these additional lasing spots was
also at ~8450A which is consistent with the fact that the QW layers at the (100) stripe in
the center as well as in the (100) shoulders have a similar thickness. At still higher diode
10 currents, lasing occurred at theO~411} oriented regions (see FIG. 5), for which thOe
emission wavelength was ~8150A. This wavelength corresponds to the thinner (~5~A)
QW at the ~411} oriented sections of the active layer. Lasing from the thinnest, {111}
oriented QW's has not been observed at this time.
These observations indicate that carriers which are injected into the (100)
15 oriented QW stripe at the center of the laser are laterally confined to this stripe, which is
~1 ~m wide. This lateral dimension is considerably smaller than the carrier diffusion
length. The lateral carrier confinement is achieved probably due to the effective lateral
potential barriers which result from the reduced thickness (and possibly the different
orientation) of the ( 111~ oriented QW's.
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The absence of lasing from the ~ oriented QW's could be the result of the
peculiar doping distribution observed in the Si doped Alo 5GaO 5As. Si is amphoteric in
GaAs and AlGaAs and has been shown to give rise to n-doping on the (lOO) planes and
to p-doping Oll the ~ planes of layers grown on nonplanar substTates. Therefore, the
5 Si doped Aso 5GaO 5As cladding layer in our laser structure is expected to be n-type in
the (lOO) oriented sections and p-type in the ~lll} sections. This should result in the
relocation of the p-n junction to the substrate-AlO 5GaO 5As interface, and hence an
elimination of calTier injection into the QW, at the { l 1 l } oriented sections.
The far field distTibution of the patterned QW lasers (FIG. 7(a)) and their
lO spectTal characteristics show that their optical field is predominantly index guided. The
built-in lateral distribution of the refractive index results fTom the latera] variation in the
thickness of the epitaxial layers (including the QW layer), and their nonplanar
configuration. It should be noted that the higher bandgap of the thinner QW's
sulTounding the active, (lOO) oriented QW stripe makes these regions transparent at the
l~ laser wavelength. This reduces the threshold curTent and increases the differential
efficiency. Furthermore, the absence of substantial interband absorption in the ~lll}
oriented QW's results in real index guiding of the optical field which, in spite of its very
narrow near field distTibution, exhibits a single lobe far field distribution (see FIG. 7).
,
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The patterned QW lasers exhibit low threshold currents, as low as 6 mA for
280 llm long devices. This value is comparable to the best values (5 mA) achieved with
QW GaAs/AlGaAs lasers made by a single growth step, but is still larger than the lowest
value that has been achieved with buried-heterostructure (BH) QW lasers (2.5 mA for
5 uncoated lasers at room temperature). However, lower threshold currents and higher
differential efficiencies should be achievable with the patterned QW laser configuration
by reducing the current leakage and optimizing the laser structure. In particular, the
structure described here can be used to make lasers with substantially narrower active
regions using a sharper V groove. For sufficiently narrow active regions (a few hundred
lO Angstroms) such lasers are expected to exhibit one dimensional (quantum wire) carrier
characteristics which should lead to diode lasers with interesting and useful physical
properties. Similar patterned QW lasers grown on nonplanar substrates delineated by
two dimensional features might yield QW lasers with zero-dimensional carriers (quantum
box lasers).
In conclusion, we have demonstrated a patterned QW heterostructure injection
laser grown by MBE in which the lateral carrier confinement relies on thickness and
growth plane variations of the active QW layer. This results in an effectively buried
heterostructure laser than can be fabricated by a single crystal growth step. The
patterned QW GaAs/AlGaAs lasers are characterized by a low threshold current, as low
2~) as 6 mA at room temperature. Furthermore, the patterned QW laser structure is suitable
for obtaining semiconductor
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lasers with very small lateral dimensions, which should be useful in the fabrication of
quantum wire and quantum box laser heterostruc~ures. Furthermore, the low absorption
in the thinner QW regions in this laser structure makes it attractive for use in phase
locked arrays of semiconductor lasers.
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