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1636 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 12, NO. 6, NOVEMBER/DECEMBER 2006

Room-Temperature Optically Pumped (Al)GaSbVertical-Cavity Surface-Emitting Laser

Monolithically Grown on an Si(1 0 0) SubstrateGanesh Balakrishnan, Anitha Jallipalli, Paul Rotella, Shenghong Huang, Arezou Khoshakhlagh, Abdenour Amtout,

Sanjay Krishna, Member, IEEE, L. Ralph Dawson, and Diana L. Huffaker, Senior Member, IEEE

AbstractWe report a monolithic vertical-cavity surface-emitting laser (VCSEL) on an Si(0 0 1) substrate operating underroom-temperature optically pumped conditions. The GaSb multi-quantum well active region in an Al(Ga)Sb half-wave cavity spacerlayer is embedded in AlSb/AlGaSb distributed Bragg reflectors.The 13% lattice mismatch is accommodated by a self-assembledtwo-dimensional array of 90 interfacial misfit dislocations re-sulting in spontaneously relaxed (98%) and very low defectdensity (106/cm2) epilayers. The material characterization isconducted through atomic force microscopy, transmission electronmicroscopy, and etch-pit density studies. The VCSEL characteri-zation includes lasing spectra and light-in versus light-out curves.A 3-mm pump spot size results in peak threshold excitation densityofIth = 0.1 mJ/cm

2 and a multimode lasing spectrum peak at1.62 m. The average output power measured from the device is25W at 1.6Ith.

Index TermsIntegrated optoelectronics, semiconductor lasers.

I. INTRODUCTION

T

he monolithic growth of IIIV materials on Si has been

pursued for over two decades [1][4]. The primary ob-

jective has been the integration of IIIV light emitters with SiCMOS device technology [5][10]. The IIIV/Si integration has

also been attempted through a variety of methods including con-

ventional wafer bonding [11], [12], recess mounting of devices,

and newer wafer bonding techniques that incorporate interme-

diate polymer layers or spin-on glass to bond the IIIVs to

Si [13], [14]. There has been a highly successful demonstration

of lasers on Si through evanescent mode coupling and Raman

effect by Fang et al. [15] and Jalali et al. [16], respectively.

While all the methods mentioned allow independent optimiza-

tion of device and circuitry, monolithic growth offers better

utilization of the integrating platform, lack of complex assem-

bly, and better heat dissipation. The monolithic approach utiliz-ing GaAs/AlGaAs can produce room-temperature (RT) edge-

emitting lasers [6] and even vertical-cavity surface-emitting

Manuscript received November 18, 2005; revised September 22, 2006.G. Balakrishnan, A. Jallipalli, P. Rotella, S. Huang, A. Khoshakhlagh,

S. Krishna, L. R. Dawson, and D. L. Huffaker are with the Center for HighTechnology Materials, University of New Mexico, Albuquerque, NM 87106USA (e-mail: [email protected]; [email protected]; [email protected];[email protected]; [email protected]; [email protected];[email protected]; [email protected]).

A. Amtout was with the Center for High Technology Materials, Universityof New Mexico, Albuquerque, NM 87106 USA. He is now with Emcore Cor-poration, Somerset, NJ 08873 USA (e-mail: [email protected]).

Digital Object Identifier 10.1109/JSTQE.2006.885342

lasers (VCSELs) [7] on Si(1 0 0). The recent demonstrations

of the monolithic III-As lasers on Si show the device perfor-

mance that parallel output powers and threshold currents of

lasers grown directly on GaAs substrates. Some of the promi-

nent results are by Kwon et al. [17] and Groenert et al. [18]

using SiGe metamorphic buffers, and by Mi et al. [19] using

GaAs metamorphic buffers on Si that achieve dislocation bend-

ing through InAs quantum dot (QD)-based strain fields [20].These results are very encouraging but the devices suffer from

reliability issues associated with the growth on metamorphic

buffers, GaAs/Si thermal mismatch, and high dislocation den-

sity in the GaAs buffer [1][4]. Furthermore, there may be appli-

cations in which the thick buffer layer may be incompatible with

the manufacturing processes, overall system, or chip design.

Our approach to monolithic IIIV growth on Si is funda-

mentally different from the previously reported work due to

the unique growth mode of AlSb on Si compared to GaAs on

Si [21][24]. We utilize a very thin AlSb layer (50 A) nucleated

on Si, which relieves almost the entire strain caused by the 13%

lattice mismatch via a self-assembled two-dimensional (2-D)

array of 90 interfacial misfit (IMF) dislocations. The IMF ar-ray forms at the IIIV/Si interface and remains localized within

that plane rather than propagating vertically into the material.

A detailed explanation of the IMF formation via atomic self-

assembly and energy minimization have been reported in [25].

Apart from the advantages of the IMF growth mode, the growth

of AlSb on Si offers significantly better agreement of the sub-

strate and epi-layer thermal expansion coefficients compared

to GaAs on Si [26]. At 300 K, AlSb has a thermal expansion

coefficient of 2.55 106/K, which is very close to that of Si,which is2.59106/K. In comparison, the expansion coeffi-cient of GaAs is 6.93

106/K, resulting in a significant tensile

strain during cool down from the growth temperature to RT. Thedeleterious effects include microcracks and wafer bowing [2].

A relatively small thermal expansion coefficient mismatch is

evidenced in our work by the absence of either the microcracks

or the wafer bending even in very thick (10 m) AlSb on a Siwafer.

The growth of AlSb on Si was first explored in the mid-

1980s by van der Ziel et al. [27]. This work led to optically

pumped (OP) double heterostructure lasers (Jth= 13kA/cm2)

and photodetectors. However, significant threading dislocations

were expected in their growth that were removed using strain

layer super-lattices. Other studies of AlSb on Si are limited to

X-ray diffraction (XRD) studies and basic photoluminescence

1089-7771/$20.00 2006 IEEE


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BALAKRISHNANet al.: ROOM-TEMPERATURE OPTICALLY PUMPED (Al)GaSb VERTICAL-CAVITY SURFACE-EMITTING LASER 1637

Fig. 1. AFM images showing surface structure after AlSb deposition on Si.(a) 3 ML. (b) 18 ML. (c) 54 ML. RHEED image for the corresponding growthsare also shown in (a) and (c).

(PL) characterization without the analysis of the strain-relief

mechanisms involved in the growth [28]. With the ability to

realize the IMF arrays at the AlSb/Si interface, our group has

previously demonstrated RT OP operation of a monolithically

grown III-Sb based edge-emitting lasers on Si [29]. In this paper,

we overview the highly mismatched growth mode and describe

the RT OP lasing of a GaSb quantum well (QW)-based VCSEL

monolithically grown on an Si(1 0 0) substrate.

II. GROWTH ANDINTERFACECHARACTERIZATION

The VCSEL epitaxial structure is grown in a V80H molecular

beam epitaxy reactor. Prior to thegrowth, the Si substratesurface

is hydrogen-passivated by immersing the wafer in an HF bath.

Heating the substrate to 500 C in vacuum removes the looselybondedhydrogen. A thermal cycle at 800 C ensures theremovalof the oxide remnants. This is verified by the reflection high-

energy electron diffraction (RHEED), which shows a (2 2)surface reconstruction with the removal of the oxide.

The RHEED pattern proceeds through two distinct phases

during the initial growth. The deposition of AlSb on Si results

in an interconnected chevron pattern. A 3 3 pattern is alsosuperimposed on this pattern. This implies that the initial growth

of AlSb results in the formation of islands with{1 1 1}facetsand truncated on top with (1 0 0) plane. After deposition of

150-A GaSb, the RHEED pattern becomes a pure (3 3)pattern indicating that a planar growth mode has been achieved.

We analyzed theinterfaceand initial bulk growthusingatomic

force microscopy (AFM). Fig. 1(a)(c) shows the AFM data

after 3, 18, and 54 monolayers (MLs) of AlSb deposition.

At 3 MLs, the island density is 1011 QDs/cm2 with a dot height

and diameter of 13 nm and 20 nm, respectively. Fig. 1(b) shows

the growth at 18 MLs. The effect of this continued deposition is

Fig. 2. (a)Cross-sectional TEMimageof the(1 1 0) plane showing defect-freeAlSb on Si. (b) HR-TEM of high-quality GaSb grown on AlSb/Si, with periodicmisfit dislocations along the AlSbSi interface.

that the individual islands coalesce but remain crystallographic.

Fig. 1(c) shows the continued coalescence toward the planar

growth with 54-ML deposition. The insets show the correspond-

ing RHEED patterns at each stage of thenucleation layer growth.

At 3 MLs, the RHEED pattern is spotty with overlaid chevrons

characteristic of the QD growth. After 54-ML deposition, thespotty/chevron character has transformed to a streaky (3 3)pattern associated with the planar growth after the 54-ML depo-

sition. The smoothest RHEED patterns were obtained when the

AlSb growth was terminated at50 A and GaSb was grown onthis surface. This effect has also been demonstrated by Akahane

et al.using PL and high-resolution XRD (HR-XRD) studies.

We carefully studied the IMF array and resulting bulk ma-

terial using low- and high-resolution TEM (HR-TEM) bright-

field images. Fig. 2(a) shows the cross section of AlSb (0.5m)grown with very low defect density on Si. Fig. 2(b) shows the

HR-TEM image of the strain-relaxed, defect-free GaSb (10 nm)

on an AlSb buffer (5 nm) nucleated on Si and the AlSb/Si in-

terface. The bright spots in the image correspond to the misfit

dislocation sites [30]. The misfits are arranged in a highly peri-

odic array and localized at the AlSb/Si interface. No threading

dislocations or dark-line defects are detectable in the bulk and

no misfit dislocations exist at any other location. The misfit sep-

aration, measured to be34.6 A, corresponds to exactly eightAlSb lattice sites and nine Si lattice sites. Thus, every ninth Si

atom has a pair of dangling bonds (one going into and the other

out of the image plane) to accommodate the larger Sb atom in

the next (0 0 1) plane.

Careful examination of the atomic lattice surrounding the

misfits using a very high resolution TEM, as in Fig. 2(b), al-

lows the identification of the misfits and analysis of the strain


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relief. Completing a Burgers circuit around one misfit disloca-

tion indicates that the Burgers vector lies along the interface

and identifies the misfit as 90 type. Measurement of the Sisubstrate and AlSb bulk lattice constants within 4 MLs of the

interface yielda0 = 0.3840and 0.4338 nm, respectively, whichare equivalent to the published values along[1 1 0]and indicates

a complete strain relaxation.Most of the strain energy generated by the AlSb/Si lattice

mismatch is dissipated by the misfit array at the interface. In the

following paragraph, we calculate and compare the strain energy

areal densityE , with the energy density dissipated from a 2-Dmisfit arrayEd [1]. The approximations that can be employedfor such calculations depend on the anisotropy factor of the ma-

terial system, which is expressed asA= 2C44/(C11C12),whereC11, C12, andC44 are referred to as the elastic stiffnesscoefficients [31]. When the anisotropy factor takes the value of

1, the given material is considered isotropic and approximations

associated with the isotropic materials can be made in the strain

calculations. However, for AlSb, the anisotropy factorA has a

value of 2.25, thus requiring tensor analysis for the strain energycalculations. The strain energy densityE is calculated as

E =2

//Bh = 0.6 7 J m2

where

// =asaf

afand B=

(C11+ 2C12)(C11C12)C11

for the (1 0 0) growth direction. In general, B for the growth on(1 0 0), (1 1 1), or (1 1 0) surfaces is given as

B = C11+ 2C122

3 C11+ 2C12C11+ 2(2C44C11+C12)(l21l22+l22l23+ l21l23)

where l1, l2, and l3 are the directional cosines that relate thedirection normal to the interface or cube axes [32]. For (1 0

0), the factorl21l22

+l22l23

+ l21l23

= 0, so, the second term in thedenominator inside the parenthesis vanishes. In these equations,

// is thein-plane strain,B is a constant,h= 0.58 nm (thicknessapproximated from TEM), as = 0.54310 nm is the in-planelattice constant of the Si substrate, af = 0.61355 nm is the

lattice constant of the relaxed AlSb film, = 4.976 1010

N/m2 is the shear modulus of AlSb,= 0.328is the Poissons ratiofor AlSb,f=|asaf|/as = 0.13 is the lattice mismatch forAlSb, and b= af/

2 = 0.4338 nm is Burgers vector along

[110] direction in Si substrate. The dislocation energy per unit

area dissipated by a 2-D misfit array is calculated as

Ed =2Eds

= 0.6647Jm2

for a film thickness of h= 0.58 nm. These results indicatethat the misfit dislocations relieve98.6% of the strain en-ergy generated by the AlSb/Si lattice mismatch at the growth

temperature, and allow almost fully relaxed bulk AlSb growth.

Fig. 3. VCSEL structure for 1.65-m emission grown on Si.

III. MONOLITHICOP VCSEL

The VCSEL structure, shown in Fig. 3, is designed for the

OP operation at 1650 nm. The lower distributed Bragg reflector

(DBR) includes 30 pairs of AlSb/Al0.15Ga0.85Sb quarter-wave

layers (1197- and 1013-A thick, respectively). The half-wave

AlSb cavity spacer includes 6 100 A GaSb QWs separatedby 100 A AlSb barriers. The upper DBR is the output coupler

and includes 25 pairs of AlSb/Al0.15 Ga0.85Sb quarter-wave

layers, capped with a quarter-wave layer of GaSb (d= 975 A)to prevent native oxidation of the Al-bearing layer. The VCSEL

growth is initiated at 420 C with a 50-A AlSb nucleation layer,

and then the temperature is ramped from 420 C to 500 C forthe device growth. We note that an excellent material quality is

achieved at growth temperatures ranging from 420 Cto500 C.

The quality of the epi-material is indicated by the defect

density estimated by the etch-pit density tests. The etch-pit

density decoration count offers a ceiling for the defect density

count and indicates the presence of the threading dislocations.

Two kinds of etches were used for this test, a 20% solution

of KOH and a mixture of H2O2 and H2SO4 (in a 2:1 ratio).

The two density tests produced almost identical results (see

Table I). The table shows the etch-pit density at etch depths of

1000, 7000, and 14 000 nm within the VCSEL structure cor-

responding to the regions in the upper DBR, within the QWs,

and very close to the nucleation layer, respectively. The de-

fect density is fairly constant throughout the structure, with

a maximum value of 2 106/cm2 and an average value of8 105/cm2.

We analyzed the VCSEL structure under RT OP conditions.

The pump source is a toxicity prescreening assay TOPAS opti-

cal parametric amplifier (p = 1.475m) pumped by a mode-locked Ti-sapphire laser. The 200-fs pulse width at a 1-kHz

repetition rate produces a maximum energy per pulse of 20 Jor 0.28 mJ/cm2 within the 3-mm circular pump spot obtained

on the sample. We detected the emission from the VCSEL using

an InSb broad-area detector. The light-in versus light-out (LL)


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Fig. 4. RT OP lasing results at 1.65m from VCSEL grown on Si. (a) LLcurve showing threshold intensity (Ith) at a pump power of 0.1 mJ/cm

2 perpulse. (b) Spectra at pump intensities: 0.4Ith, 1.0Ith, and 1.1Ith.

curve and spectral data are shown in Fig. 4(a) and (b). The LL

curve in Fig. 4(a) shows that the peak threshold for the device

isIth= 0.1 mJ/cm2. As the pump intensity increases, the output

continues to increase from the threshold to 1.6Ith. At this point,the output intensity rolls over very rapidly due to the red-shift in

the gain caused by heating. The average output power measured

from the device is 25 W at 1.6Ith. The spectra in Fig. 4(b)change in intensity and shape from sub-threshold to lasing at

0.4Ith, 1.0Ith, and 1.1Ith. The lasing spectrum is highly multi-mode (full-width at half-maximum (FWHM) = 20nm) due toa very large pump spot size.

IV. CONCLUSION

We have demonstrated an RT OP III-Sb VCSEL monolithi-

cally grown on Si(0 0 1). A very high quality material with de-

fect density< 8 105/cm2 is indicated by the etch-pit densitystudies. Both the spectra and the LL curves indicate a threshold

excitation density Ith= 0.1 mJ/cm2. The lasing spectra, peaked

at = 1.65m, is highly multimode just above the threshold

due to the large pump-spot diameter. The average output powermeasured from the device is 25W at 1.6Ith. We have demon-strated that a periodic IMF array can be formed under specific

growth parameters to fully relieve the strain energy in a highly

strained system such as AlSb on Si. Our calculations indicate

that the misfit dislocation array dissipates the majority (98.5%)

of the strain energy due to the 13% lattice mismatch. The growth

mode after only50 MLs of deposition appears planar from theobservation of RHEED. Finally, the defect-free, strain-relieved

bulk material enabled by this growth mode will lead to the new

devices, especially in the infrared regime, along with the novel

integration schemes. This collection of data indicates a promis-

ing technology for the monolithic integration of IIIV emitter

on Si.

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Ganesh Balakrishnan received the B.E. degree inelectronics and communications engineering fromthe University of Madras, Chennai, India, in 2000,the M.S. degree in communication engineeringfrom the University of Toledo, Toledo, OH, in 2001,andthe Ph.D. degree inoptical sciences from theUni-versity of New Mexico, Albuquerque, in 2006.

Currently, he is a Postdoctoral Researcher at theCenter for High Technology Materials, University ofNew Mexico. His research interests include semi-conductor laser design and growth on mismatched

platforms including III-Sb on silicon and GaAs for novel device development.He is the coauthor of six journal articles, and ten conference papers.

Anitha Jallipalli received the B.Sc. degree in elec-tronics and the M.C.A. degree from Nagarjuna Uni-versity, Guntur, Andhra Pradesh, India, in 1997 and2000,respectively, andthe M.S. degree in optical sci-ences in 2006 from the University of New Mexico,Albuquerque, where she is currently working towardthe Ph.D. degree in optical sciences.

From 2000 to 2003, she was a Lecturer at AndhraMahila Sabha School of Informatics, Osmania Uni-versity, India. Her current research interests includetheoretical modeling of misfit dislocations and elec-

trical and optical characterization of devices with interfacial states for IIIV

materials and Si.

PaulRotella received theB.S. degree in financefromVillanova University, Villanova, PA, in 1998, and theM.S. degree in electrical engineering, specializing inoptoelectronics.

He was a Test Engineer with Emcore Corpora-tion, where he worked with various device structuressuch as LEDs, photodiodes, HBTs, and MR sensors.Currently, he is a Research Engineer at the Centerfor High Technology Materials, University of NewMexico, Albuquerque. His research interests includematerial characterization of longwavelength emitting

materials.Mr. Rotella was the President of the Optical Society of America Student

Chapter from 2003 to 2004.

Shenghong Huangreceived the M.S. degree in elec-trical engineering, in 2003, from the University ofNew Mexico, Albuquerque, where he is currentlyworking toward the Ph.D. degree in electrical en-gineering.

His research interests include microscopy, growthoptimization, and characterization for novel quantumdot and highly mismatched IIIVand Si semiconduc-tor materials.

Arezou Khoshakhlagh received the B.S. degree inelectrical engineering, in 2004, from the Universityof New Mexico, Albuquerque, where she is currentlyworking toward the Ph.D. degree in optical sciencesand engineering.

Her research interestsincludemolecular beamepi-taxygrowthof novel mismatched materialsfor devicedevelopment.

Abdenour Amtout receivedthe Ph.D.degree in solidstate physics from the University of Montreal, Mon-treal, Canada, in 1994.

He was a Postdoctoral Fellow at the Universityof Montreal, where he studied the phonon replicas inrutile crystals and modeling band structures of nano-structures. During1999, he wasa Scientistat EmcoreCorporation, Somerset, NJ, where he was engaged inmodeling and characterization of GaN-based LEDsand IIIVbased vertical-cavitysurface-emitting laser(VCSELs). In 2003, he joined the Center for High

Technology Materials, University of New Mexico, Albuquerque, as a ResearchScientist, where he was engaged in the study of carrier dynamics of InAs/GaAsbased quantum dots and GaSb-based photodetectors using pump-probe spec-troscopy and time-resolved PL, modeling of band structures of quantum dots

and optical pumping of VCSELs. He recently rejoined Emcore Corporation,where he is engaged in the failure analysis on VCSELS.

Sanjay Krishna(S98M01) received the M.S. de-gree in physics from the Indian Institute of Technol-ogy Madras, Chennai, India, in 1996, and the M.S.degree in electrical engineering and Ph.D. degree inappliedphysicsfrom theUniversity of Michigan,AnnArbor, in 1999 and 2001, respectively.

In 2001, he joined the University of New Mex-ico, Albuquerque, as a tenure track Faculty Member,where he is currently an Associate Professor of elec-trical andcomputer engineering at theCenter forHighTechnology Materials. His current research interests

include growth, fabrication, and characterization of self-assembled quantum


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dots and type II InAs/InGaSb based strain layer superlattices for mid-infraredlasers and detectors, carrier dynamics and relaxation mechanisms in quasi-zerodimensional systems and the manipulation of these favorable relaxationtimes torealize high-temperature mid-infrared detectors. He is the author or coauthor ofmore than 40 peer-reviewed journal articles, over 40 conference presentations,and two book chapters. He holds four provisional patents.

Dr. Krishna is the recipient of the 1996 Gold Medal from the Indian Insti-tute of Technology Madras for the best academic performance in the Mastersprogram in physics. He is also the recipient of the Best Student Paper Award atthe 16th North American Molecular Beam Epitaxy Conference, Banff, Canada,in 1999, the 2002 Ralph E. Powe Junior Faculty Award from Oak Ridge As-sociated Universities, the 2003 Outstanding Engineering Award from the IEEEAlbuquerque Section, the 2004 Outstanding Researcher Award from the ECEDepartment, andthe 2005Schoolof Engineering Junior Faculty Teaching Excel-lence Award. He has also served as the Chair of the local IEEE/LEOS Chapter.

L. Ralph Dawson received the B.S. degree from California Institute of Tech-nology, Pasadena, in 1962, and the M.S. and Ph.D. degrees from the Universityof Southern California, Los Angeles, in 1965 and 1968, respectively, all in elec-trical engineering.

He was with Sandia National Laboratories, Albuquerque, NM, and Bell Lab-oratories, MurrayHill,NJ. Currently, he is a Research Professorof electrical andcomputer engineering at the Center for High Technology Materials, Universityof New Mexico, Albuquerque. His research interests include the MBE growthof IIIV materials, especially antimony-based narrow gap materials. He is theauthor or coauthor of more than 225 journal articles and conference papers.

Dr. Dawson has been an active member of the TMS Electronic MaterialsCommittee and has also twice chaired the Electronic Materials Conference.

Diana L. Huffaker (M96SM02) received the B.S.degree in engineering physics from the University ofArizona, Tucson, in 1986, and the M.S. degree inmaterials science and engineering and the Ph.D. de-gree in electrical engineering from the University ofTexas, Austin, in 1990 and 1994, respectively.

She was a Senior Research Scientist at PicolightIncorporated, Boulder, CO. Currently, she is an Asso-ciate Professor of electrical and computer engineer-ing at the University of New Mexico, Albuquerque.Her current research interests include directed and

self-assembled nanostructure solid state epitaxy, optoelectronic devices includ-ing solar cells and IIIV/Si photonics. She is the coauthor of more than 110refereed journal publications and two bookchapters. She holds two patents, withsix disclosures pending. She has given many invited presentations worldwide.

Dr. Huffaker is the recipient of the 2002 Compound Semiconductor In-ternational Symposium Young Scientist Award and the 2004 Alexander vonHumboldt Research Fellowship from the Technical University Berlin. She is amember of IEEE/LEOS, SPIE, WISE, MRS, OSA, and TMS. She is also anelected member of the IEEE/LEOS Board of Governors and the Chairman ofIEEE WIE Region 6.

balakrishnan 2006 room temperature opt

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