Room-temperature Optically-pumped (Al)GaSb Vertical Cavity Surface

Emitting Laser Monolithically Grown on a Si (100) Substrate

G. Balakrishnan, A. Jallipalli, P. Rotella, S.H. Huang, A. Khoshakhlagh, A. Amtout,

S. Krishna, L.R. Dawson, C.P. Hains and D.L. Huffaker

Center for High Technology Materials, University of New Mexico

1313 Goddard SE, Albuquerque, NM 87106

Tel. (505) 272-7845, Fax. (505) 272-7801, e-mail: huffaker@chtm.unm.edu

We report monolithic vertical cavity surface emitting lasers (VCSELs) on a Si

substrate operating under room-temperature optically-pumped conditions. The GaSb

multi-quantum well active region in an Al(Ga)Sb half-wave cavity spacer layer is

embedded in AlSb/AlGaSb distributed Bragg reflectors. The 13% lattice mismatch is

accommodated by a spontaneously formed 2-D array of 90° misfit dislocations at the

AlSb/Si interface. This growth mode produces relaxed (98%), very low defect-density

(~8x105/cm2) material indicated in x-ray diffraction, transmission electron microscopy

and etch-pitch density measurements. The VCSEL characterization includes lasing

spectra and light-in versus light-out curves. A peak threshold excitation density of Ith =

0.1 mJ/cm2 and a multimode lasing spectrum peaked at 1.62 µm, results from a 3 mm

pump-spot size.

I. INTRODUCTION:

Monolithic growth of III-V materials on Si has been pursued for over two

decades. The primary objective is the integration of III-V light emitters with Si CMOS

device technology.1-6 The III-V/Si integration has also been attempted through a variety

of methods including conventional wafer bonding,7,8 novel methods like recess mounting

of devices and newer wafer bonding techniques that incorporate an intermediate layer

such as polymers or spin-on glass to bond the III-Vs to the Si.9,10 While these methods

allow independent optimization of both device and circuitry, monolithic growth offers

better utilization of the integrating platform, lack of complex assembly and better heat

dissipation. The monolithic approach utilizing GaAs/AlGaAs has resulted in room-

temperature (RT) edge emitting lasers2 and even vertical cavity lasers3 (VCSELs) on Si

(100). While these early results were encouraging, the device characteristics are marginal

due to micro-cracks that result from the large GaAs/Si thermal mismatch and high

dislocation density in the GaAs buffer.4

Our approach to monolithic III-V growth on Si is fundamentally different from

the previously reported work due to the unique growth mode of AlSb on Si compared to

GaAs on Si.11,12 We utilize a very thin AlSb layer (50 Å) nucleated on Si, which relieves

almost the entire strain caused by the 13% lattice mismatch via a 2-D array of 90° misfit

dislocations. These dislocations form at the III-V epi/Si interface and propagate within

that plane. They do not thread vertically into the material. We have fully characterized

this growth mode,12 which extends to GaSb on GaAs and other highly mismatched

systems, and we have developed a theoretical model to support our experimental data.11

In contrast, GaAs on Si predominantly forms 60º misfits and results in extensive

threading dislocations.13,14 Apart from an optimized growth mode, the growth of AlSb on

Si has another advantage over growth of GaAs on Si in that it has a significantly better

agreement of the substrate’s and epi-layer’s thermal expansion coefficients.15 At 300K,

AlSb has a thermal expansion coefficient of 2.55 x 10-6/K which is very close to that of

silicon which is 2.59 x 10-6/K. In comparison, the expansion coefficient of GaAs is 6.93 x

10-6/K, which is a much higher than that of the silicon substrate, resulting in tensile strain

build up as the material cools down from growth temperature to room temperature. We

haven’t observed any microcracks or wafer bending in our Si wafers even after the

growth of 10 µm of AlSb.

Low defect AlSb buffers on Si have been previously reported16,17,18 to result in

optically smooth surfaces, viewed by nomarski microscopy, and very few threading

dislocations according to transmission electron microscopy (TEM) analysis. The growth

of AlSb on Si was first explored in the mid-1980’s by Van der Ziel and co-workers.17

This work led to double heterostructure lasers (Jth=13 kA/cm2) and photodetectors.

However, the growth mechanisms of the highly lattice mismatched epitaxy were not

discussed. Other studies of AlSb on Si were limited to x-ray diffraction studies and basic

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

or growth mode. Our group has previously demonstrated RT, photopumped (PP)

operation of a monolithically grown edge-emitting laser on Si.19 In this paper, we

overview the highly mismatched growth mode and describe the RT PP lasing of a GaSb

quantum well (QW)-based VCSEL monolithically grown on a Si (100) substrate.

II. GROWTH MODE AND INTERFACE CHARACTERIZATION

The VCSEL epitaxial structure is grown in a V80H molecular beam epitaxy

reactor. Prior to growth, the Si substrate surface is hydrogen-passivated by immersing

the wafer in a HF bath. The loosely bonded hydrogen is removed by heating the

substrate to 500 °C in vacuum. A thermal cycle at 800 °C ensures the removal of oxide

remnants. This is verified by reflection high-energy electron diffraction (RHEED),

which shows a (2 x 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 the Si results in an interconnected chevron pattern.

Superimposed on this pattern is also a 3 x 3 pattern. This implies that the initial growth of

AlSb results in the formation of islands with {111} facets and truncated top with (100)

plane. After deposition of ~150 Å GaSb, the RHEED pattern becomes a pure (3 x 3)

pattern indicating a planar growth mode has been achieved.

The interface and initial bulk growth has been analyzed using atomic force

microscopy (AFM). Figures 1 (a)-(c) show AFM data after 3, 18 and 54 ML of AlSb

deposition. At 3 MLs, the QD density is 1011 QDs/cm2 with dot height and diameter of

1-3 nm and 20 nm, respectively.14 Figure 1(b) shows the growth at 18 MLs. The effect of

this continued deposition causes the individual islands to coalesce but remain

crystallographic in contrast to InAs/GaAs QD growth where island coalescence leads to

large defective islands.14 Figures 1(c) shows continued coalescence towards planar

growth with 54 MLs deposition. The insets show corresponding RHEED patterns at each

stage of the nucleation layer growth. At 3 MLs, the RHEED pattern is spotty with

overlaid chevrons characteristic of QD growth. After 54 MLs deposition, the

spotty/chevron character has transformed to a streaky (3 x 3) pattern associated with

planar growth after 54 ML deposition.

The misfit array and resulting bulk material has been studied carefully using low

and high-resolution TEM bright-field images. Figures 2 (a) shows the cross section of

AlSb grown on Si, the thickness of the epi-layer is 0.5 µm and it has a very low defect

density. Figure 2 (b) show a 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 interface. The bright spots

in the image correspond to misfit dislocation sites.20 The misfits are arranged in a highly

periodic 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 separation, measured to be ~ 34.6 Å, corresponds to exactly 8 AlSb

lattice sites and 9 Si lattice sites. Thus, every 9th Si atom has a pair of dangling bonds

(one going into and out of the image plane) to accommodate the larger Sb atom in the

next (001) plane.

Careful examination of the atomic lattice surrounding the misfits using very high

resolution HR-TEM, as in Fig.2(b), allows the identification of misfits and analysis of

strain relief. Completing a Burger’s circuit around one misfit dislocation indicates that

the Burger’s vector lies along the interface and identifies the misfit as 90 ˚ type.

Measurement of the Si substrate and AlSb bulk lattice constants within 4 MLs of the

interface yield ao= 0.3840 nm and ao= 0.4338 nm, respectively, which are equivalent to

the published values along [1-10] and indicate 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 density Eε, with the energy density dissipated from a two-

dimensional misfit array, Ed1. The strain energy density, Eε, is found using

2

2

//29.0m

JBhE

c== !

! where f

fs

a

aa !=

//"

and )1(

)1(2

!

!µ

"

+= fB

. In these equations,

ε// is the in-plane strain, B is a constant, h = 0.25 nm is thickness of the strained material

measured from TEM, as = 0.543095 nm, is the in-plane lattice constant of the GaAs

substrate, af = 0.61355 nm is the lattice constant of the relaxed AlSb film, µ = 2.215 x

1010 N/m2 is the AlSb shear modulus, ν= 0.33 is AlSb Poisson’s ratio, 13.0=!

=s

fs

a

aaf is

the AlSb/Si lattice mismatch, nma

bf

4338.0

2

== is burger’s vector along [1-10]

direction in GaAs substrate. The dislocation energy per unit area dissipated by a 2D

misfit array is calculated 2

12876.0

2

m

J

s

EE

d

d== where

( )

N

mm

Nb

Ef

d

10

29

2

102

10*4213.4

)31.01(4

10*39975.0*10*4.2

)1(4

!

!

=

!=

!"

#$#

µ

is energy per unit

length of single edge dislocation. The misfit spacing, S, can be derived theoretically by

nmf

bS 34.3== which agrees very well with S = 3.46 nm measured from TEM images. A

comparison of values for 229.0m

JE =! and Ed

1 = 0.2876 J/m2 for a film thickness of h

= 0.25 nm indicates that the misfit dislocations relieve 98.5 % of the strain energy

generated by the AlSb/Si lattice mismatch at the growth temperature and allow fully

relaxed bulk AlSb growth.

III. MONOLITHIC PHOTOPUMPED VCSEL

The VCSEL structure, shown in Fig.3, is designed for PP operation at 1650 nm.

The lower distributed Bragg reflector (DBR) includes 30 pairs of AlSb/Al0.15Ga0.85Sb

quarter wave layers (1197 Å and 1013 Å thick, respectively). The half-wave AlSb

cavity spacer includes 6 x 100 Å GaSb QWs separated by 100 Å AlSb barriers. The

upper DBR is the output coupler and includes 25 pairs AlSb/Al0.15Ga0.85Sb quarter-wave

layers, capped with a quarter-wave layer of GaSb (d=975 Å) to prevent native oxidation

of the Al-bearing layer. The VCSEL growth is initiated at 420 °C with a 50 Å AlSb

nucleation layer, and then the temperature is ramped from 420 to 500 °C for the device

growth. We note that excellent material quality is achieved at growth temperatures

ranging from 420 °C to 500 °C.

The quality of the epi-material is indicated by defect density estimated by etch-pit

density tests. The etch-pit density decoration count offers a ceiling for the defect-density

count and indicates the presence of 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 tabulated in Table I. The

table shows the etch pit density at three etch depths of 1000, 7000 and 14000 nm within

the VCSEL structure corresponding to regions in the upper DBR, within the QWs and

very close to the nucleation layer, respectively. The defect density is fairly constant

throughout the structure with a maximum value of 2 x 106/cm2 and an average value of 8

x 105/cm2.

The VCSEL structure is analyzed under RT, PP conditions. The pump source is a

TOPAS optical parametric amplifier (λp=1.475 µm) 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 µJ or 0.28 mJ/cm2 within the 3 mm circular pump spot obtained on

the sample. The emission from the VCSEL is detected using an InSb broad-area detector.

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

The LL curve in Fig.4(a) shows peak threshold for the device is Ith = 0.1 mJ/cm2. With

increasing pump intensity, the output continues to increase from threshold to 1.6 x Ith

above threshold. At this point, the output intensity rolls over very rapidly due to the red-

shift in the gain caused by heating. The spectra in Fig.4(b) change in intensity and shape

from sub-threshold to lasing at 0.4 x Ith, 1.0 x Ith and 1.1 x Ith. The lasing spectrum is

highly multimode (FWHM = 20 nm) due to the very large pump spot size.

IV. CONCLUSION

We have demonstrated a RT-PP III-Sb VCSEL monolithically-grown on Si (001).

Very high quality material with defect density < 8x105/cm2 is indicated by etch-pit

density studies. Both spectra and LL curves indicate a threshold excitation density of Ith

= 0.24 mJ/cm2. The lasing spectra, peaked at λ=1.65 µm, is highly multi-mode just

above threshold due to the large pump-spot diameter. Furthermore, we have

demonstrated that a periodic array of 90 ˚ misfit dislocations can be formed under

specific growth parameters to fully relieve 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 strain energy due to the 13% lattice mismatch. The growth mode

after only ~ 50 MLs of deposition appears planar from observation of RHEED. Finally,

the defect-free, strain-relieved bulk material enabled by this growth mode will lead to

new devices, especially in the infrared regime, along with novel integration schemes.

This collection of data indicates a promising technology for monolithic integration of III-

V emitter on Si.

References: [1] T.H.Windhorn, G.M.Metze, B.Y.Tsaur, and J.C.Fan, “AlGaAs double- heterostructure diode lasers fabricated on a monolithic GaAs/Si substrate”, Appl. Phys. Lett. vol. 45, no.4, pp. 309-311, August 1984. [2] D.G. Deppe, N.Holonyak Jr., D.W.Nam, K.C.Hsieh, G.S.Jackson, R.J.Matyi, H.Shichiujo, J.E. Epler , and H.F.Chung , “Room-temperature continuous operation of p-n AlxGa1–xAs-GaAs quantum well heterostructure lasers grown on Si”, Appl. Phys. Lett. vol. 51, no.9, pp.637-639, August 1987. [3] D.G.Deppe , Naresh Chand, J.P.Van der Ziel, and G.J.Zydzik , “AlxGa1–xAs-GaAs vertical-cavity surface-emitting laser grown on Si substrate”, Appl. Phys. Lett., vol. 56, no.8, pp.740-742, February 1990. [4] D.G. Deppe , N. Holonyak Jr., K.C. Hsieh, D.W.Nam, and W.E.Plano, “Dislocation reduction by impurity diffusion in epitaxial GaAs grown on Si”, Appl.Phys.Lett.,vol. 52, no.21, pp.1812-1814, May 1988. [5] Naresh Chand, F. Ren , A.T.Macrander, J.P.van der Ziel , A.M.Sergent, R.Hull, S.N.G.Chu, Y.K.Chen , and D.V.Lang, “GaAs-on-Si: Improved growth conditions, properties of undoped GaAs, high mobility, and fabrication of high-performance AlGaAs/GaAs selectively doped heterostructure transistors and ring oscillators”, J. Appl. Phys. vol. 67, no. 5, pp. 2343-2353, March 1990. [6] K. K. Linder, J. Phillips, O. Qasaimeh, X.F.Liu , S.Krishna, P. Bhattacharya, and J.C.Jiang , “Self-organized In0.4Ga0.6As quantum-dot lasers grown on Si substrates”,Appl. Phys. Lett. vol. 74, no. 10, pp. 1355-1357, March 1999. [7] H.Wada, T.Kamijoh , “1.3 µm InP-InGaAsP lasers fabricated on Si substrates by wafer bonding”, IEEE J. Sel. Top. Quantum Electron., vol. 3,no.3, pp.937-942, June 1997. [8] H.Wada, T.Kamijoh, “Wafer bonding of InP to Si and its application to optical devices”, Japanese J. of Applied Physics, Part 1 (Regular Papers, Short Notes & Review Papers), vol. 37, no 3B, pp 1383-1390, March 1998. [9] E.Atmaca, V.Lei, M.Teo, N.Drego, D.Boning, C.G.Fonstad, Loke Wan Khai, Yoon Soon Fatt, “RM integration of InP based 1.55 µm P-i-N photodetectors with silicon CMOS optical clock distribution circuits”, 2003 International Symp. on Compound Semiconductors: Post-Conference Proceedings, (IEEE Cat. No.03TH8767), 204, 2004

[10] H.C.Lin, K.L.Chang, G.W.Pickrell, K.C.Hsieh, K.Y.Cheng, “Low temperature wafer bonding by spin on glass”, J. Vac. Sci. Technol., B: Microelectron.Nanometer Struct. Process. Meas. Phenom., vol. 20, no.2, pp.752-754, March 2002.

[11] A. Jallipalli, G. Balakrishnan, A. Khoshakhlagh, , L.R. Dawson and D.L. Huffaker,

“Modeling Misfit Dislocation Arrays for the Growth of Low-Defect Density AlSb on Si”, under preparation.

[12] S.H. Huang, G. Balakrishnan, A. Khoshakhlagh, A. Jallipalli, L.R. Dawson, Y.-C.Xin, L.F.Lester and D.L. Huffaker, “Strain Relief by Periodic Misfit Arrays for Low Defect Density GaSb on GaAs”, submitted to applied physics letters.

[13] G.Balakrishnan, S.Huang, L.R. Dawson, Y.-C. Xin , P.Conlin, and D.L. Huffaker, “Growth mechanisms of highly mismatched AlSb on a Si substrate”, Appl. Phys. Lett., vol.86, pp.034105-1-034105-3, January 2005. [14] G.Balakrishnan , S.Huang , A. Khoshakhlagh , L.R.Dawson , Y.-C.Xin , P.Conlin , and D.L.Huffaker , “High quality AlSb bulk material on Si substrates using a monolithic self-assembled quantum dot nucleation layer”, J. Vac. Sci. Technol., B: Microelectron. Nanometer Struct. Process. Meas. Phenom., vol. 23, no.3, pp.1010- 1012, May 2005. [15] V. Kumar, B.S.R. Sastry, “Thermal expansion cofficient of binary semiconductors”,

Cryst. Res. Technol., vol.36, no.6, pp.565-569, June 2001 . [16] Chin-An Chang, H. Takaoka, L. L. Chang, and L. Esaki, Appl. Phys. Lett.,vol. 40, no.11, pp.983-985, June 1982. [17] J.P.Van der Ziel , R.J.Malik , J.F.Walker, and R.M.Mikulyak , “Optically pumped laser oscillation in the 1.6–1.8 µm region from Al0.4Ga0.6Sb/GaSb/Al0.4Ga0.6Sb double heterostructures grown by molecular beam heteroepitaxy on Si”, Appl. Phys. Lett. vol.48, no.7, pp.454-456, February 1986. [18] K.Akahane , N.Yamamoto , S.Gozu , N.Ohtani , “Heteroepitaxial growth of GaSb on Si(001)”, J. Crys. Growth., vol. 264, no.1-3, pp. 21-25, March 2004. [19] G.Balakrishnan, S.H.Huang, A.Khoshakhlagh , P.Hill , A.Amtout , S.Krishna , G.P. Donati , L.R.Dawson and D.L.Huffaker, “Room-temperature optically-pumped InGaSb quantum well lasers monolithically grown on Si(100) substrate”, Electron. Lett., vol.41, no.9, pp. 531-532, April 2005. [20] David B. Williams and C. Barry Carter, “Transmission Electron Microscopy”, (Kluwer Academic/ Plenum Publishers, New York, NY, 1996).

Table Captions Table I: The etch-pit density for different sections of the VCSEL.

Enchant 1000 nm (Lower DBR)

7000 nm (Active region)

14000 nm (Top DBR)

20% KOH solution

9 x 105/cm2 8.5 x 105/cm2 8.5 x 105/cm2

H2O2:H2SO4 (2:1)

1.4 x 106/cm2 9 x 105/cm2 9 x 105/cm2

Etch-pit decoration density in different sections of the VCSEL

Figure Captions

Figure 1. AFM images showing surface structure after (a) 3 ML, (b) 18 MLs and (c) 54 MLs of AlSb deposition on Si. Figure (a) and (c) also show the RHEED image for the corresponding growths. Figure 2. (a) Cross-sectional TEM image of the (110) plane showing defect free AlSb on Si. (b) HR-TEM of high quality GaSb grown on AlSb/Si, with periodic misfit dislocations along the AlSb-Si interface. Figure 3. VCSEL structure for 1.65 µm emission grown on Si. Figure 4. RT-PP lasing results at 1.65 µm from VCSEL grown on Si. (a) LL curve showing peak threshold intensity Ith = 0.1 mJ/cm2, (b) spectra at pump intensities - 0.4 x Ith, 1.0 x Ith and 1.1 x Ith.

Figure 1, Balakrishnan et. al.

(a) 3 ML AlSb on Si

(b) 18 ML AlSb on Si

(c) 54 ML AlSb on Si

300 nm [110]

Figure 2, Balakrishnan et. al.

Si

AlSb buffer

Si

a) TEM of AlSb/Si interface.

100 nm [1-10]

AlSb 110

100

b) HR-TEM of GaSb/AlSb/Si.

[1-10]

3 nm

Figure 3, Balakrishnan et. al.

Figure 4, Balakrishnan et. al.

b) RT spectra from VCSEL on Si. Ith(peak) = 0.1 mJ/cm2

IP= 1 x Ith

IP= 0.4 x Ith

IP= 1.1 x Ith

a) RT L-L curve for VCSEL on Si. Ith(peak) = 0.1 mJ/cm2

λ emission = 1.65 µm.

Excitation Energy (mJ/cm2 per pulse)

0.0 0.1 0.2 0.3

Inte

nsity

(L

inea

r Sc

ale)

In

tens

ity

(Lin

ear

Scal

e)