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Design and growth of InAs Quantum Dash based MWIR VECSELs
Z. Mayes b ,V. Patel a, S. Reissmann a, T. J. Rotter a, P. Ahirwar a, S. P. R. Clark a, A. R. Albrecht a,
H. Xu a, C. P. Hains a, L. R. Dawson a, and G. Balakrishnan a
(REU program, student paper)
a Center for High Technology Materials, University of New Mexico, 1313 Goddard SE, Albuquerque, NM 87106
b Physics Department, Florida State University, 315 Keen Building, Tallahassee, FL, 32306
2. Abstract
We describe optical and structure characteristics of a proposed InAs quantum dash (QDash)
vertical external cavity surface-emitting laser (VECSEL) grown on InP by elemental source
molecular beam epitaxy (MBE) and operating at a wavelength of 2 µm. III-Sb based VECSELs
are the sole semiconductor disk lasers at 2 µm and only available in low-power top emitter
package because of lack of etch stop layer in antimonides. The existence of mature etch stop
recipes for InP allows the proposed design to be packaged as a bottom emitter, resulting in less
intra-cavity optical loss and possible order of magnitude increase in power output when
compared to current antimonide-based lasers. The gain region of the laser consists of a resonant
periodic gain structure (RPG) using a single QDash per antinode to optimize reduction of strain
accumulation and provide improved pump absorption. A lattice mismatch of 3.23% between InP-
matched growth matrix and InAs active region promotes formation of quantum dashes and
allows access to emission wavelengths >2.0 µm. Use of a AlAsSb/GaAsSb distributed Bragg
reflector (DBR) at 2.0 µm is explored.
3. Introduction
What is a VECSEL?
A Vertical External Cavity Surface Emitting Laser. A
VECSEL is a semiconductor disk laser that consists of a
Distributed Bragg Reflector (DBR) with the active region
grown on top of it. The optical cavity is completed by using
an external mirror as an output coupler, which also defines
the laser transverse mode.
VECSELs have many advantages over other semiconductor
lasers, such as: high beam quality, broad tunability,
wavelength versatility, and due to the external cavity design
they can incorporate a variety of intra-cavity elements.
Examples are: non linear crystals for frequency doubling and
quadrupling, birefringent filters for tuning, and using
Figure 1 (a) epitaxial structure and (b) schematic design of a VECSEL1
(a)
(b)
2
semiconductor saturable absorbers for mode-locking. The main disadvantage of the VECSEL
design is increased heat generation - and a limitation in its ability to dissipate heat. Therefore
chip packaging for thermal management is critical.
VECSELs are usually optically pumped and are therefore capable of easy power scalability. A
larger pump spot size leads to more power output. Continuous-Wave (CW) output in the tens of
watts has already been realized2.
Why is High Power Operation in 1.8 µm – 2.2 µm Range Important?
There are several applications in 1.8 µm – 2.2 µm range of the mid-infrared that are of
significant importance. Examples are: Light Detection And Ranging (LIDAR), atmospheric gas
detection for environmental monitoring and for chemical detection, and even surgical biological
tissue welding - due to the high absorption in water and minimal penetration depth of radiation at
this range . Given the advantages that VECSELs have over other lasers a 2.0 µm VECSEL
would be highly desirable.
Top Emitter VECSEL
There are two possible basic structural configurations for VECSELs: a top
emitter design which lases through a transparent heat spreader and a bottom
emitter design which lases through the optically transparent substrate. A top
emitter is grown with the DBR first and the active region on top of the
DBR; a heat spreader such as diamond is then placed on top of the active
region, inside the cavity. Top emitters have two disadvantages: diamond
only be capillary bonded, not soldered - making it difficult to attach, and it
the heat spreader may introduce introduce optical loss by absorption of the
stimulated photons.
Bottom Emitter VECSEL
A bottom emitter is grown with the active region
first and the DBR on top of the active region with a
heat spreader placed outside the cavity. A heat
spreader will not introduce optical losses in this
configuration because while the heat has to go through the DBR, it does
not interact with stimulated photons. As negligible light reaches the
outermost layers of the DBR and therefore the optical properties aren't of
concern, simpler soldering techniques and cheaper thermal grade (CV)
diamond can be used for thermal management. However the substrate has
to be removed from the optical cavity or the laser will not lase! A well
established etch stop layer is required for this. Bottom emitter designs
have resulted in greater than 65 Watts CW at 1020 nm3. This is
significantly higher power than is realizable with top emitters.
Figure 2: Top Emitter VECSEL
Figure 3 Bottom Emitter VECSEL
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State of the Art VECSEL technology at 2 µm
Currently III-Sb based VECSELs are the sole semiconductor disk lasers at 2 µm. These III-Sb
designs currently have a max power output of 6 Watts CW4. The active region is based on a well
established InGaSb/AlGaAsSb quantum well design and AlAs0.08Sb0.92/GaSb DBRs at this
wavelength are straightforward to grow4.
However, substrate removal is prevented by a lack of a
mature etch stop recipe for antimonide substrates. This
makes bottom emitter designs for current 2 µm
VECSELs impossible.
A possible alternative to III-Sb based designs is to use
an InAs-based [Quantum] Dash in a Well (DWELL)
active region design as demonstrated successfully at 2
µm by by Rotter, et al5., grown on an InP substrate,
with several DBR designs possible. InP has excellent
substrate removal chemistry, so a bottom emitter
VECSEL design on an InP material system and
operating at 2 µm should be possible. This would
almost an order of magnitude increase in VECSEL CW
output power at 2 µm!
Quantum Dashes(QDash)
The active region design and growth specifications will
be taken directly from work done by our group on
QDashes. A study on the formation of InAs quantum dashes on both GaAs and InP substrates for
an emission wavelength of 2 µm was undertaken by Balakrishnan, et al.7, and describes the
conditions necessary for the formation of InAs QDashes. On GaAs a metamorphic buffer (MB)
and smoothing layer were used to lattice match to InP for active region growth and for InP the
active layer was grown directly on the substrate - resulting in a simpler and more defect-free
epitaxial structure for InP versus an antimonide or arsenide substrate.
The QDashes were grown in InGaAs quantum wells
lattice-mached(LM) to InP: it was found that a lattice
strain ( 0 0/a a ) of 3.2% resulted in formation of
QDashes and strain > 4.5% forms Quantum Dots
(QD). Figure 5 shows the QDashes forming
preferentially along the 11-0 axis7.
QDs and QDashes form due to the lattice-mismatch
strain between InP and the underlying epilayer: only a
very thin layer can be grown before too much strain
accumulates.
Under certain growth conditions the strain is not relieved by forming dislocations (which is
Figure 2: (a) Epitaxial structure of a III-Sb (b) Band-diagram of a 2 µm III-Sb VECSEL6
(a)
(b)
Figure 3: AFM of QDash: 0.5 µm x 0.5 µm
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undesirable because it degrades the properties of the structure), but rather by entering a three-
dimensional growth regime8, referred to as Stranski-Krastanov
9 growth.
The InAs “islands” formed in this growth
mode are called self-assembled QDs with
dimensions on the order of a few
nanometers. What differentiates quantum
dots and quantum dashes is the degree of
strain present at the heteroepitaxial
interface: quantum dots forming under
conditions of high strain and quantum
dashes forming under conditions of less
strain.
See Figure 6 for an depiction of how
difference in strain magnitude affects the
preferential formation of QDots over
QDashes.
The growth and properties of an InAs quantum dash active region on InP for operation in the
1.44 µm - 2 µm range has been well studied by Rotter, et al. in our group. He comprehensively
investigated the growth aspects of quantum dashes and their application in the active region of
semiconductor laser diodes in his doctoral thesis10
. From his work comes specific information on
MBE growth conditions, structural and optical properties, as well as the performance parameters
of quantum dashes grown on InP-based alloys; all of these things will be used in design of the
proposed VECSEL's active region.
Why Use Quantum Dashes?
There are many benefits10
in using the QDash active region. Dashes exhibit broader gain
bandwidth compared to quantum wells: this allows for of tunability (> 300 nm) and makes the
laser suitable for mode locking. The highly coherent strain of dashes can help reduce cavity loss
through the suppression of Auger non-radiative recombination. The emission wavelength can
also be controlled by carefully adjusting several parameters during crystal growth: such as the
amount of InAs deposited - number of monolayers (ML) , the composition of the surrounding
material, and the growth temperature (see Figure 7). Lasing has been demonstrated at
wavelengths between 1.4 µm to 2.1 µm and may possibly be extended to longer wavelengths.
Figure 5: Tunability of QDashes10 - (a) 1440 nm structure, (b) 2020 nm structure
Figure 4: Formation of QDots vs QDashes
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It should be mentioned that the InAs QDash research that is being used is indebted to previous
InAs QDot research11
in by A. Albrecht. Many of the properties of dashes are similar to those of
dots and the study of dashes proceeded directly from work on quantum dots. The following
discussion will be about quantum dots, but all of the findings will be equally applicable to
quantum dashes.
Active Region Design
Figure 8 shows the epitaxial
structure of the active region. QDots
are grown in InGaAs quantum wells
and surrounded by InGaAlAs
barriers.
InGaAs was chosen as the quantum well
material because previous work by Liu,
et al12
. has shown that placing the QDs inside an GaInAs QW greatly increases performancedue
to improved carrier capture and reduced thermionic emission at higher temperatures.
Why are QDots so advantageous? A. Albrecht explains in his dissertation on quantum dots:
"The band-offset and the small dimensions of QDs lead to few, well separated energy
levels per dot, resulting in 3D confinement of carriers and a much reduced density of
states compared to bulk semiconductors or even quantum wells. This promises
advantages for QD-based lasers, like low transparency and threshold current densities as
well as reduced temperature sensitivity (pg. 7)."11
Quantum dot VECSELs have reached "new highs" in the quality of optical beam output and high
power, in part because of one key feature: the use of a one-dot-per-antinode Resonant Periodic
Gain (RPG) structure13
.
RPG Structures in the Active Region
An RPG is an optoelectronic structure that maximises the active medium gain by aligning the
Dot-in-a-Well (DWELL) structure that contains the quantum dots to the antinode regions of the
electric field of standing-wave mode of the cavity This improvement in gain is due to the
enhanced interaction between the electric field of the cavity's standing mode and charge carriers
that are confined in the DWELL structures14
.
Traditional VECSEL designs have clustered multiple DWELL layers at the antinodes of the
cavity's electric field to maximise the number of gain layers in the active region, but this kind of
design could have detrimental effects on the performance of the VECSEL from both strain
accumulation and thermal performance13
. Instead of using a clustered DWELL design this laser
will make use of a single DWELL layer per antinode (12 QD layers total) , resulting in a thicker
cavity structure. Doing so will both spatially spread out the strain caused by formation of the dot
structures to a greater degree - leading to superior strain management - and the thicker cavity
will enable increased pump absorption. This design drastically improves the performance of the
VECSELby allowing heat to flow out of the pumped region laterally; heat removal capacity
further augmented by the inclusion of a heat spreader soldered to the DBR.
Figure 6: Epitaxial structure of QD active region
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The marked difference in the
performance of the two designs was
demonstrated by A. Albrect in his
thesis11
. An index and mode profile
diagram of each design is shown in
Figure 9. The clustered design is
refered to as "4x3"- with four
groups of three closely spaced
DWELL layers at each antinode ;
the other has a single DWELL layer
at each antinode, repeated 12 times,
termed a "12x1".
In both structures the distance
between the clusters or single QD
layers corresponds to an optical
path length of λ/2.
The only component of the
proposed VECSEL that has not
been discussed is the DBR. While
the other components of the laser
have been tested at 2 µm, the DBR
that will be used has not!
4. Experimental Description
What is a DBR?
A Distributed Bragg Reflector is a type of reflective mirror that can be used in laser designs to
establish one end of the optical cavity. It is a structure formed from multiple layers of alternating
materials with varying refractive index, or by periodic variation of a characteristic such as
thickness of a dielectric waveguide, that results in a periodic variation in the effective refractive
index in the guide. Each layer boundary causes a partial reflection of an incoming light wave,
cumulatively increasing the overall reflection of incoming light with increasing number of
layers. A DBR works by building a constructive interference pattern as a result of the combined
reflections that occur across the width of the entire mirror (see Figure 10 ).
Figure 7: Index and mode profile diagrams for: (a) 4x3 structure, (b) 12x1 structure11
Figure 8: DBR - Principle of Operation. S.O. Kasap,”Optoelectronics and Photonics”, 2001.
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General DBR Design Considerations
A VECSEL using a quantum dash design for the active region is only likely to provide a small
percentage of gain per cavity round-trip and addional optical losses may occur if the substrate is
absorptive at the wavelength of operation. For this reason VECSEL semiconductor structures
usually contain a high-reflectivity mirror, with 98% or greater reflectivity. This is realized by a
stack of m periodic repeats of alternating quarter-wave thick layers of low (n1) and high (n2)
refractive indices, with the difference in refractive index of t he layers - e.g. n2 - n1, termed ∆n.
The reflectance for an ideal DBR at the design wavelength with m repeating pairs of layers is:
Equation 1: Ideal Reflectivity for a DBR1
This equation states that a large ∆n will provide near 100% reflectivity with fewer pairs of m
layers than if ∆n was small; a smaller structure is easier to grow and also has less thermal
impedance, so maximizing ∆n is important.
Where RDBR is the overall reflectivity of the mirror stack. The value of RDBR increases with
larger difference of index of refraction between the materials or with a larger number of DBR
periods. The spectral width of the stop band (∆λ) is the measured FWHM of the peak reflectance
and is given by:
Equation 2: Width of Stop Band; λ is free space wavelength1
This next equation implies that increasing ∆n will increase the size of the stop-band. It is
important to have a wide stop band so that the laser can be easily tuned, so maximizing ∆n is
again important.
Therefore, a high-quality DBR should have a large ∆n between its mirror layers. Since heat
flows through the DBR, another consideration is the selection of mirror materials that have good
thermal conductivity. The alloys chosen should be nonabsorbing at the laser wavelength in order
to avoid optical loss.
Issues Specific to the 2 µm DBR
There are two fundamental design issues that are specific to the DBR that will be grown for this
VECSEL:
1. Lattice-matching of selected material system to InP
2. Composition control of mixed Group Vs and/or. mixed Group IIIs during growth,
depending on the material system selected.
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Material System Selection: What Alternatives Exist?
For this VECSEL the DBR layers must both be lattice-matched (LM) to an InP substrate.
Referencing the bandgap energy vs. lattice constant chart in Figure 11, only binary or higher-
order alloy combinations that fall on the vertical line of InP's lattice constant can be used for the
DBR.
There are several alternatives for alloys; these are enumerated in Table 1.
Table 1: Alloy alternatives for DBR construction
This list of materials was used as a guide when looking through the literature for pre-existing
DBR designs that both were LM to InP and highly reflective at or near 2.0 µm. A summary of
possible DBR designs that were LM to InP was made by byBlum, et al.15,16
; the designs
described have successfully operated in the 1.3 µm to 1.74 µm range and it may be possible to
extend some of them to the 2.0 µm range.
Degree of Alloy Alloys Possible
Binaries none ... no binaries LM to InP except InP
Ternaries InGaAs, AlInAs, AlAsSb, GaAsSb
Quaternaries AlGaAsSb, InAlGaAs, AlInAsSb, InGaAsSb Quinaries AlInGaAsSb
Figure 9: Bandgap energy vs lattice constant chart for III-V semiconductor materials at room temperature
(Tien, 1988)
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Metamorphic AlGaAs/GaAs and metamorphic GaSb/AlSb17
have been used with success on
GaAs substrates but cannot be used for a InP lattice-matched design due to significant lattice
mismatch. Wafer bonding of a mirror system such as AlAs/GaAs18
was considered because it
would lift the constraint of lattice matching, but more complex fabrication methods would be
needed compared to monolithically-grown structures.
Blum, et al. have previously demonstrated a highly reflective GaAsSb/AlAsSb DBR lattice
matched to InP with a stop band centered at 1.74 µm with a maximum reflectivity exceeding
98%15
. This material system is to predicted at work well at 2.0 µm, with ∆n ~0.5, and is well
beyond the absorption band of GaAsSb. Lattice-matched InGaAs/InAlAs could be used, but it
has a smaller ∆n than the GaAsSb/AlAsSb system, so it would require more mirror layers. The
GaAsSb/AlAsSb looked the most promising so it was investigated further.
Parameters for GaAsSb/AlAsSb
A AlAsSb/GaAsSb DBR lattice-matched to InP will need to have no strain. Using the linear
interpolation technique described in Vurgaftman19
, et al. the composition of these compounds
required for lattice matching to InP can be computed:
GaAsySb(1-y):
AlAsySb(1-y):
Yields compositions of GaAs0.51Sb0.49 and AlAs0.56Sb0.44
No optical data on AlAs0.56Sb0.44 or GaAs0.51Sb0.49 at the specified composition was reliably or
readily available, so optical parameters from the DBR study by Blum, et al. at the 1.74 µm15
wavelength will be used for the purpose of demonstration. A determination of the actual indices
at 2.0 µm should be undertaken, perhaps using the novel methods outlined by Blum, et al 15
. for
determining the values at
1.74 µm.
A calculated reflectance
spectrum for the proposed
DBR was created using
Sandia Lab's "VERTICAL"
VECSEL simulation
software16
and is shown in
Fig.12. Modeling software
such as VERTICAL is used
to predict the properties of a
suggested VECSEL or
VECSEL component before
fabrication.
5.8690 5.6534 6.0960(1 )
5.8690 5.660 6.1357(1 )
y y
y y
Figure 10: Simulated DBR Reflectivity at 300k
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Growth Issues
The GaAsSb/AlAsSb material system does not have mixed Group III elements, but instead has
mixed Group V elements. Even though mixed Group IIIs are not an issue for growing this DBR,
it is useful to discuss the relatively simple growth of mixed Group IIIs so that the more complex
mechanism of mixed Group Vs growth can be more easily explained.
A crystal lattice grows by alternating layers of Group III and Group Vs ad-atoms; atoms of a
group compete to grow on the surface only within their sublattice, there is only competition for a
lattice position on the growth surface within a chemical group. Group III atoms have unity
sticking coefficients, which means that when they make contact with a surface they will adhere.
When a Group III ad-atom hits the surface, it sticks. Whatever comes out of a Group III MBE
source is what is deposited: the ratio of each elemental growth rate to overall growth rate
determines layer composition for Group IIIs. This makes it easy to determine the Group III
composition. However Group V atoms have non-unity sticking coefficients; the important of this
will be explained in following sections.
Epitaxial Growth by MBE
The VECSEL will be grown using
Molecular Beam Epitaxy (MBE). MBE
grows epitaxial films using advanced
evaporation sources. Each source directs a
“beam" at the heated substrate and the
simultaneous evaporation of different
elements allows compound
semiconductors such as GaAs, AlGaAs,
GaInAs, InP, etc. to be grown.
Growth rate is controlled by the beam flux;
layers are stopped and started by closing
and opening high speed shutters in front
sources. Figure 12: Pictograph of MBE Reactor
Figure 11: The Growth of a Crystal Lattice
11
During growth the substrate is rotated continuously to ensure uniformity in epitaxial
composition. Reflection High Energy Electron Diffraction (RHEED gun oscillation data
provides an "absolute" measurement of growth rate for Group III elements.
Group III Growth Rate - by RHEED Oscillation
Group III growth rates are measured by RHEED oscillation. The RHEED system consists of an
emitter that produces a beam of high energy electrons and aims it at the growth surface. When
the beam hits the surface it reflects from it and the reflected beams form a diffraction pattern on a
phosphor screen. In the diffraction pattern a stationary specular spot exists - by analyzing the
time-varying intensity of this spot growth rate for Group IIIs can be determined. A typical
example - in this case of GaAs - of a RHEED oscillation pattern is shown in Figure 15.
One period of oscillation corresponds to one monolayer of growth on a (001) oriented surface.
The amplitude of the intensity also provides information about the condition of the growth surface. A filled surface is smooth and specularly reflects the maximum amount of light, registering as maximum amplitude on the oscillation graph. A half-filled surface is as rough as it can possibly be and reflects very diffusely, this registers as a
minimum amplitude on the graph. Intermediate states have intermediate intensity, corresponding to the surface building up to a smooth layer or the gradual roughening of a layer as atoms begin to grow a new layer on top of a previously smooth surface.
The oscillation pattern is recorded for several periods and then a Fourier transform is applied to
get the group III growth rate.
X-Ray Diffraction
To verify the accuracy of RHEED oscillation data in determining Group III compositions, X-Ray
Diffraction (XRD) must be used. The angular distance between the substrate and epitaxial peaks
provides valuable diagnostic information on the structure of the crystal lattice of the epitaxial
layers. Symmetric and asymmetric ω-2θ scans provide information on the spacing of the
substrate and epitaxial peaks, which in turn yields information on the lattice constant and thus
composition of the epitaxial layers.
Figure 16 is an XRD diagram of a growth in which InGaAs was attemping to be lattice-matched
to InP. The more intense peak is the InP substrate peak and the less intense epitaxial peak is due
to InAlAs. The lattice constant for InAs is 6.058Å and for GaAs is 5.6534 Å; the goal of this
growth was to lattice match to InP (5.869 Å ). Because the epitaxial peak is to the right of the
substrate peak a tensile strain exists - that is, the lattice constant of the epitaxial layer is too
Figure 13: RHEED Oscillation for GaAs
12
small in comparison to the lattice constant of the substrate. This means that too much Al was
provided during growth and that the RHEED-determined growth rate was an underestimate (or
that there was drift in the flux of the Al source during growth, after the calibration was made).
The point is that RHEED data does not always accurately give absolute growth rate, it needs to
be checked against XRD. However, once we "correct" the growth rate with XRD analysis we
can determine the absolute growth rate for Group III elements because they have a unity sticking
coefficient and their growth rates are additive.
Figure 14: XRD diagram of InGaAs grown on InP.
For the mixed Group Vs that will be grown in the III-AsSb DBR mirror layers, growth rate is
much harder to determine. It was priorly mentioned that Group V elements have non-unity
sticking coefficients; this means that when a Group V ad-atom hits the growth surface, it may
either incorporate into the sub-lattice, or leave the surface. This means that the amount of
material leaving the elemental sources is not the amount of material that is being incorporated
onto the epitaxial surface; it also implies that an excess of Group V atoms will be required to
grow a sub-lattice: this excess is referred to as "overpressure". The actual proportion of
impinging Group V atoms that incorporate - the sticking coefficient - is a function of surface
temperature, surface coverage and structural details, as well as the kinetic energy of the
impinging particles. As a result, determining what ratio of fluxes is required to achieve a desired
composition is very difficult.
13
Growing mixed-Group V alloys is a trial and error process; it is possible to predictably grow a
given composition of Group Vs only if that particular composition has been grown before and all
the system and material parameter values can be replicated .
For the growth of a new structure an iterative trial-and-error adjustment process must be
employed to determine the flux ratio that will lead to a desired composition. XRD analysis
provides lattice constant and relaxation information information that can be used to determine
the composition ratio of As to Sb - simulation tools such as PeakSplitter can take this
information and provide accurate estimates of the elemental composition; this information can be
compared to the flux ratios that were used to grow the layer and then depending on whether the
epitaxial peak position is compressive or tensile, the flux ratio can adjusted to favor either As or
Sb to a greater degree. For DBR growth the goal is to find the flux ratio that will result in lattice-
match of each layer to InP; on an XRD diagram this will appear as a single peak - the substrate
and epitaxial peaks will overlap.
Fabrication Process
The laser structure will be grown by elemental source molecular beam epitaxy (MBE) in a VG Semicon V80H reactor. Elements used are In, Ga, Al, As, and Sb with an N+ doped InP substrate being used. The fluxes of will be defined such that all layers would grow lattice matched to InP, with the exception of the InAs quantum dashes, which will be compressively strained by 3.23%.
The VECSEL laser chip will be fabricated in a series of five steps. (1) the quantum dash active
region (Figure 17) will be epitaxially grown on the InP substrate; (2) the proposed DBR
structure of AlAsSb/GaAsSb will be grown next Before the substrate is removed the thin and
fragile epilayers will need to be mechanically supported so they do not disintegrate upon removal
of the substrate. To accomplish this the wafer is (3) metallized (see following explanation) for
the purpose of soldering it to the (4) CVD diamond, which acts as a heat spread. Now that the
epitaxial layers are supported (5) the substrate is removed via chemical etch. This process is
visually outlined in Figure 18.
Figure 15: Fabrication of VECSEL
14
The metallization process is essentially a soldering process to connect the epilayers to the
heatspreader. Using an electron beam at settings of 10kV and 100mA and under high vacuum,
(1) first titanium metal and then gold are evaporated onto the sample in layers 10nm thick. Using
a resistive metal evaporater at settings of 5V and 300A is thermally vaporized, depositing a (2) 5
µm layer of indium on the surface of the gold. (3) While low heat (~150 ˚C) is applied to the
CVD diamond heat spreader, the assembled stack is carefully pressed onto the heat spreader; the
surface of the indium metal softens and adheres to the heat spreader, mechanically bonding the
two.
Discussion
All of the components necessary for a bottom-emitter InP-based 2 µm VECSEL have been
discussed in detail. The novel aspects employed in the design of the InAs quantum dash active
reigon have been described, with the fabrication parameters and performance metrics being
provided by the research of Rotter, et al5,10,20
. and also by Balakrishnan, et al7. The DBR design
for operation at 2 µm was adapted from a 1.74 µm design by Blum & Dawson15
. If a refractive
index study of InP-LM GaAsSb/AlAsSb is carried out at 2 µm, then the thickness parameters of
the mirror layers can be fixed and a working DBR fabricated. At that point a working VECSEL
can be constructed and begin being characterized.
Figure 16: Bonding epitaxial stack to heat spreader.
15
Conclusion
We have identified a potential need for an 2 µm VECSEL based on a lattice-matched InP
material system, utilizing an InAs quantum dash based active region and a GaAsSb / InAsSb
DBR. This need is based on the current restriction of 2 µm VECSEL to III-Sb systems to top
emitter configurations; a bottom emitter configuration would be advantageous because of
increased power output, but this is not realizable for III-Sb VECSELs because there is no well
defined etch stop recipe for antimonides. Excellent substrate removal chemistry is available for
InP substrates, so a VECSEL based on InP would be able to harness the increased power output
of a bottom emitter configuration, enabling the application of advanced VECSEL technology to
existing 2 µm laser-based technologies
Acknowledgement
This research was done at the University of New Mexico for this publication and was funded by
AFOSR Optoelectronics Center Grant FA9550-09-1-0202. (PM: Dr. H. Schlossberg). My
participation in the REU program and involvement in this research was funded by National
Science Foundation. I would like to thank Dr. G. Balakrishnan for his invaluable assistance in
doing this research, as well as my listed mentors and collaborators, REU Program Director Dr.
M. Osinki, and REU Coordinator Linda Bugge.
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