1

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

4

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

6

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.

7

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.

8

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)

9

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

10

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.

References

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