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GaSb heterostructures for wafer-bonded thermophotovoltaic devices
ARTICLE IN PRESS
Journal of Crystal Growth 261 (2004) 372–378
Growth and characterization of InAsSb/GaInAsAb/ AlGaAsAb/GaSb heterostructures for wafer-bonded thermophotovoltaic devices C.A. Wang*, D.A. Shiau, D.R. Calawa Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, MA 02420-9108, USA
Abstract InAsSb/GaInAsSb/AlGaAsSb/GaSb heterostructures were grown by organometallic vapor phase epitaxy for wafer bonding and epitaxial transfer. The InAsSb epilayer, which is used as an etch-stop layer, is the template for subsequent growth of GaInAsSb/AlGaAsSb/GaSb thermophotovoltaic device structures. Atomic force microscopy images of asgrown InAsSb epilayers exhibit extreme roughness with features that are aligned with the substrate miscut orientation. Interior InAsSb surfaces, which were prepared by selectively etching the cap layers, are atomically smooth. Thus, the InAsSb surface undergoes extensive roughening during cool down. X-ray characterization of InAsSb/GaSb/GaInAsSb heterostructures that were grown without interruption between successive layers shows that GaInAsSb material quality is maintained. GaInAsSb/AlGaAsSb heterostructures wafer-bonded to GaAs substrates with an internal reflector have enhanced optical quality compared to unbonded heterostructures. r 2003 Elsevier B.V. All rights reserved. PACS: 61.16.Ch; 68.35.Ct; 68.55.Jk; 81.15.Kk Keywords: A1. Roughening; A1. Surfaces; A3. Organometallic vapor phase epitaxy; B2. GaInAsSb; B2. InAsSb; B3. Solar cells
1. Introduction Lattice-matched GaSb-based III–V epitaxial layers are being developed for thermophotovoltaic (TPV) energy conversion systems that operate with thermal sources heated to B1000 C [1]. Recently, there has been interest in monolithically seriesinterconnected TPV cells to build open-circuit voltage Voc ; reduce parasitic losses; and simplify module assembly [2,3]. Since these interconnec*Corresponding author. Tel.: +1-781-981-4466; fax: +1781-981-0122. E-mail address: [email protected] (C.A. Wang).
tions require electrical isolation of devices, and semi-insulating (SI) GaSb substrates are not available, GaInAsSb/GaSb epilayers were grown on a lattice-matched AlGaAsSb cell-isolation diode [2,3]. A monolithically interconnected module (MIM) that consisted of 15 cells was fabricated, and a Voc value of 0.42 V was reported [3]. An alternative and versatile approach for achieving electrical isolation of monolithic devices is to wafer bond GaSb-based epilayers to a handle wafer, and then remove the substrate [4–8]. Although both growth and subsequent wafer processing can be more involved, additional benefits might be gained to improve TPV
0022-0248/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2003.11.030
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Fig. 1. Schematic structure of GaInAsSb/AlGaAsSb/GaSb TPV cells wafer bonded to GaAs handle wafer with SiOx/Ti/Au internal reflector for those with monolithic series interconnections.
device performance [6]. For example, by utilizing a dielectric/metal layer as an adhesive layer, GaInAsSb/AlGaAsSb/GaSb TPV devices were wafer bonded to SI GaAs with an internal reflector, as shown in Fig. 1 [8]. It was reported that the photoluminescence (PL) intensity increased about three times. Furthermore, linear voltage building was achieved, and Voc of 1.8 V at a short-circuit current density of 0.4 A/cm2 was measured for a 10-cell MIM device compared to 0.2 V for a single cell. To implement the wafer bonding and epitaxial transfer process, an etch-stop layer is grown between the substrate and device layers to facilitate removal of the substrate. For GaSb-based materials, InAsSb is used as this layer [5–8]. Although the InAsSb is a sacrificial layer and is ultimately removed, it must be of high quality to insure that the quality of the subsequently grown device layers are not compromised by the InAsSb epitaxy. This paper discusses the growth and characterization of InAsSb/GaInAsSb/AlGaAsSb/GaSb heterostructures by organometallic vapor phase epitaxy (OMVPE) for wafer bonding and epitaxial transfer to SI GaAs. The main emphasis is on growth of the InAsSb etch-stop layer, since OMVPE growth of GaInAsSb, AlGaAsSb, and TPV device structures has previously been reported [9,10].
2. Experimental procedure 2.1. OMVPE growth The TPV structure used for wafer bonding and epitaxial transfer is shown in Fig. 2. The epitaxial
n-GaSb lateral conduction layer n-GaInAsSb p-GaInAsSb p-AlGaAsSb window p-GaSb contact u-InAsSb etch stop u-GaSb buffer GaSb substrate Fig. 2. Schematic of InAsSb/GaSb/AlGaAsSb/GaInAsSb TPV device structure for wafer bonding.
layers are grown in a reverse sequence compared to conventional GaSb-based TPV structures [9], and the structure consists of the following layers on (0 0 1) n-GaSb substrate miscut 6 toward (1–11)B: u-GaSb buffer layer, u-InAsSb, p-GaSb, p-AlGaAsSb, p-GaInAsSb, n-GaInAsSb, and n-GaSb. The InAsSb/GaInAsSb/AlGaAsSb/GaSb heterostructures were grown lattice matched to 5-cm-diam GaSb substrates in a vertical rotating-disk OMVPE reactor, which has previously been described [10]. Solution trimethylindium (TMIn), triethylgallium, tritertiarybutylaluminum, tertiarybutylarsine (TBAs), and trimethylantimony (TMSb) were used as precursors, with diethyltellurium and dimethylzinc as n- and p-type dopants, respectively. The H2 carrier gas flow rate was 10 slpm; the reactor pressure was 150 Torr; and the susceptor rotation rate was 250 rpm. Growth studies were performed to establish conditions for the InAsSb etch-stop layer. These
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layers were grown at a temperature of 525 C with pTMSb =ðpTMSb þ pTBAs Þ; where pTMSb and pTBAs are the partial pressure of TMSb and TBAs, respectively, ranging from 0.08 to 1.2. The V/III ratio was kept constant at 2. These ratios were calculated using vapor pressures values of 1.7, 40.96, and 5.27 Torr, for TMIn, TBAs, and TMSb, respectively [11]. The InAsSb growth rate was 2.6 mm/h as determined from in situ reflectance [12]. The 0.54 eV GaInAsSb active layers were grown at 525 C [9], while the GaSb buffer and AlGaAsSb window and contact layers were grown at 550 C [10], as previously reported. To assess material quality of heterostructures that incorporate the InAsSb etch-stop layer, structures both with and without this layer were grown for comparison.
about 50 mm of the substrate. The remaining substrate and GaSb buffer layer were then selectively removed using a CrO3:HF:H2O based etch to expose the InAsSb layer. Finally, the InAsSb etch-stop layer was selectively removed with a citric acid:H2O2 etchant [13]. 2.3. Characterization Epitaxial structures were characterized by highresolution X-ray diffraction (HRXRD); PL at 4 and 300 K using a PbS detector; atomic force microscopy (AFM) operated in tapping mode with etched Si cantilevers that have a nominal tip radius of 5–10 nm; and time-resolved PL using a HgCdTe detector [14]. The Sb composition in InAsSb layers was determined from HRXRD measurements.
2.2. Wafer bonding and epitaxial transfer 3. Results and discussion Details of the wafer bonding process were described previously [6,8], and are summarized here. SI GaAs was used as the handle wafer since its thermal expansion coefficient (a ¼ 5:7 106 K1) is closely matched to that of GaSb (a ¼ 6:9 106 K1). After megasonically cleaning in solvents and chemically etching the epitaxial and handle SI GaAs wafers to remove native oxides, the epitaxial wafer was sputter-coated with SiOx/Ti/Au while the GaAs wafer was sputtercoated with Ti/Au. The SiOx dielectric layer was incorporated to provide electrical isolation and was designed to have higher reflectivity than would normally be obtained from a single metal layer [6]. The SiOx thickness was 200 nm, while the Au thickness was 2 mm, which are thicknesses that were found to result in low residual stress. To bond the wafers, the two Au surfaces were placed in contact; the wafers were heated under vacuum to a temperature of 250 C; and a mechanical pressure of 250 psi was applied. Epitaxial transfer was accomplished by removal of the GaSb substrate and GaSb and InAsSb etchstop layers. The 5-cm-diam bonded wafers were first cut into four quarters for ease of handling and processing. The bulk of the 500-mm-thick GaSb substrate was removed by spin-etching with H2O2:H2O:NaK tartrate tetrahydrate, leaving
3.1. InAsSb epitaxy Since GaInAsSb layers are grown at 525 C [9], InAsSb growth conditions were optimized for this same temperature to avoid growth interruption. A low V/III ratio of 2 was sufficient to prevent metalrich growth, and a mirror-smooth morphology was observed by Nomarski contrast microscopy. This V/III ratio is similar to the value reported in Ref. [15] for InAsSb grown by OMVPE at 500 C with TMIn, TBAs, and triethylantimony. Nominally lattice-matched InAsSb epilayers exhibit excellent structural quality as evidenced by the narrow full-width at half-maximum (FWHM) of the HRXRD rocking curve, shown as the lower curve in Fig. 3. The 0.5-mm-thick layer has FWHM of 37 arcsec, which is nominally the same value that was determined from X-ray simulations. The upper curve shown in Fig. 3 is the HRXRD curve for an InAsSb layer in which the TMSb flow had not stabilized before switching into the reactor. The broad peak at about 1600 arcsec is associated with InAs. Thickness fringes indicate that the thickness of this InAs layer is about 56 nm. Fig. 4 shows the dependence of lattice-matching and Sb incorporation in InAs1xSbx epilayers as a function of the
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Diffraction Angle (arc s) Fig. 3. X-ray rocking curves of InAs0.906Sb0.094 grown on GaSb at 525 C. The lower curve is for InAsSb grown with sufficient TMSb flow stabilization, while the upper curve (offset) is for a layer without TMSb flow stabilization. The peak at B1600 arcsec corresponds to InAs.
X-ray Splitting (arc s)
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pTMSb/(pTMSb + pTBAs) Fig. 4. Lattice mismatch of InAs1xSbx (left axis) and Sb concentration in InAs1xSbx (right axis) as a function of pTMSb =ðpTMSb þ pTBAs Þ:
pTMSb =ðpTMSb þ pTBAs Þ: The data indicate that the Sb incorporation is readily controlled. Fig. 5 summarizes the PL peak energy measured at 4 K of InAs1xSbx epilayers with various alloy composition. The layers are all about 0.5 mm thick. The FWHM values of these layers are typically between 11 and 15 meV. Also shown are the energy gap dependence on composition as given by Ref. [16] (solid curve) and energy values reported in Ref. [17]. The results obtained in this study are only slightly lower by about 0.002 eV than the solid curve while those from Ref. [17] are more than 0.02 eV lower. Those lower values were attributed to Cu–Pt ordering [18,19]. Therefore, InAsSb alloys grown in this study are probably atomically disordered.
Energy Gap or PL Peak Energy at 4 K (eV)
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0.45 0.40 0.35 0.30 0.25 0.20 0
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Fig. 5. Energy gap (solid line) or PL peak energy (open circles) at 4 K as a function the x (Sb) in InAs1xSbx. The solid line is from Ref. [16], and the triangles are from Ref. [17].
Since the TPV device layers are subsequently grown on the InAsSb epilayer, it is critical that this nominally lattice-matched layer be as atomically smooth as a GaSb buffer layer. For GaSb grown at 525 C or 550 C, AFM studies indicate that the GaSb surface is vicinal and typically exhibits a root-mean-square (rms) roughness of 0.2 nm or less. Fig. 6a shows an AFM image of an as-grown 0.5-mm-thick InAsSb layer. The surface has an rms roughness of 2.8 nm and exhibits an undulation with periodicity of about 0.25 mm. This undulation is mainly aligned perpendicular to the substrate miscut direction, i.e., the undulation is aligned with the step edges. A 1-mm-thick InAsSb epilayer has a similar morphology and slightly larger rms roughness of 3.4 nm. The InASb morphology is significantly rough, and clearly is undesirable as a surface for growth of GaInAsSb TPV layers. Nevertheless, a 1-mm-thick GaSb epilayer was grown on 0.25-mm-thick InAsSb, and the AFM image is shown in Fig. 6b. The surface is vicinal and has an rms roughness of only 0.15 nm. It is surprising that the morphology of the InAsSb/GaSb heterostructure would improve with successive GaSb growth. However, another possible explanation is that the InAsSb surface undergoes restructuring while cooling down after OMVPE growth. Such observations have been reported for InP and GaAs [20,21]. In those
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Fig. 6. AFM images of (a) as-grown InAsSb epilayer and (b) 1 mm-thick GaSb epilayer grown on a 0.25-mm-thick InAsSb epilayer. The rms roughness is 2.81 and 0.15 nm for the images shown in (a) and (b), respectively.
studies, InP/InGaAs and GaAs/AlAs heterostructures were grown with various interruption times between the successive layers. Capping layers were precisely removed by selective chemical etching to expose the underlying layer, and then AFM was used to examine this interior surface. Interior surfaces of GaAs grown without interruption were nearly vicinal, whereas macrosteps were observed for samples after cooling down [21]. To investigate the possibility of surface restructuring, InAsSb/GaInAsSb/GaSb heterostructures were grown on InAsSb epilayers of thicknesses that were varied from 0.1 to 0.5 mm. The GaSb and GaInAsSb layers were selectively removed with CrO3:HF:H2O. Selectivity of this etch was ideal: an InAsSb layer placed in the etch solution for as long as 30 min did not show evidence of InAsSb removal. Fig. 7 shows an AFM image of the interior surface of a 0.5-mm-thick InAsSb layer that was capped with GaSb. The surface has rms roughness of 0.2 nm and is significantly smoother than the sample shown in Fig. 6a. The rms roughness of a similar structure grown with 0.1 mm InAsSb is similar at 0.28 nm. Thus, the AFM results suggest that the InAsSb undergoes extensive roughening during cool down, and that surface roughness of the interior surface of the InAsSb etch-stop layer is similar to the GaSb buffer layer. Since growth interruption could result in surface roughening, GaSb layers were subsequently grown on InAsSb without interruption. To study material quality of heterostructures that incorporate the InAsSb etch-stop layer, GaInAsSb control and InAsSb/GaSb/GaInAsSb structures were grown. Figs. 8a and b show
Fig. 7. AFM image of interior InAsSb surface, which was exposed by selectively removing the GaSb capping layer. The rms roughness is 0.2 nm. The dramatic difference in surface morphology compared to that shown in Fig. 6a is indicative of extensive surface roughening that occurred during cool down.
reciprocal space maps of the control layer and InAsSb/GaSb/GaInAsSb heterostructure grown with a 0.1-mm-thick InAsSb layer, respectively. The similarity of the two maps indicates that the InAsSb etch-stop layer does not degrade the structural quality of subsequently grown GaInAsSb. The PL efficiency of these samples could not be assessed because the InAsSb is absorbing due to its lower energy gap. 3.2. Wafer-bonded GaInAsSb/AlGaAsSb/GaSb epitaxy The structural and optical quality of waferbonded GaInAsSb layers was previously reported
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Intensity (cps)
105 104 103 102 101 –1000
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(b)
Fig. 9. X-ray diffraction of 0.54-eV GaInAsSb/AlGaASb (a) control TPV structure on GaSb substrate and (b) wafer-bonded TPV structure on SI GaAs.
10-3
[6,8] and a few results are summarized here. Fig. 9 shows the HRXRD rocking curves for the latticematched GaInAsSb/AlGaAsSb/GaSb TPV control structure on a GaSb substrate (Fig. 9a) and the wafer-bonded GaInAsSb/AlGaAsSb/GaSb TPV device structure on a GaAs handle wafer (Fig. 9b). The FWHM of the diffraction peak of the control structure is B30 arcsec. The FWHM of the wafer-bonded epitaxy is only slightly broadened at 51 arcsec. This broadening is predominantly due to curvature in the wafer due to differences in the thermal expansion coefficients of the different materials. Both samples exhibit thickness fringes, which is indicative of the excellent structural quality. Optical quality was evaluated by 300 K PL and PL decay. Special lifetime structures were prepared for this comparison. The double heterostructures (DHs) consist of a 1.5-mm-thick p-GaInAsSb layer doped at 2 1017 cm3 and AlGaAsSb cap layers. The control structure was grown without the InAsSb etch-stop layer, while the sample for wafer bonding was grown with a 0.1-mm-thick InAsSb epilayer. The PL intensity from the wafer-bonded sample is more than 3 times greater than the
PL Intensity (arb. units)
Fig. 8. X-ray reciprocal space maps of (a) GaInAsSb control structure and (b) InAsSb/GaInAsSb structure.
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WaferBonded τ = 85 ns
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Control 36 ns
0
200 400 Time (ns)
600
Fig. 10. PL decay of control and wafer-bonded 0.54-eV GaInAsSb/AlGaAsb lifetime structures.
control structure, and time-resolved PL measurements [8,14] of the wafer-bonded and control GaInAsSb DHs, shown in Fig. 10, indicates that the wafer-bonded sample with the internal reflector has a PL lifetime tPL that is more than two times higher at 85 ns compared to the control sample with tPL ¼ 36 ns. These results suggest that the optical quality of InASb/GaInAsSb heterostructures are comparable to those grown without
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an InAsSb layer. Furthermore, photons that might normally be absorbed by the substrate are reflected back to the active layer and reabsorbed, and therefore the minority-carrier lifetime is increased.
4. Conclusions InAsSb epilayers were grown by OMVPE for incorporation as etch-stop layers in InAsSb/ GaInAsSb/AlGaAsSb/GaSb heterostructures for wafer bonding and epitaxial transfer. An Sb distribution coefficient close to one was obtained for nominally lattice-matched InAsSb epilayers. AFM images of as-grown InAsSb indicated that these surfaces have an rms roughness that is typically 3 nm, while the interior InAsSb surfaces, which are obtained by selectively etching GaSb/ GaInAsSb layers, have rms roughness of 0.2 nm. This suggests that InAsSb undergoes extensive roughening during cool down, and growth interruptions could roughen the surface. InAsSb/ GaSb/GaInAsSb heterostructures grown with an InAsSb etch-stop layer without interruption exhibit material quality that is similar to control structures grown without InAsSb.
Acknowledgements The authors gratefully acknowledge A. Lin and J.W. Chludzinski of MIT Lincoln Laboratory for technical assistance in materials characterization, P.G. Murphy and A.C. Anderson for wafer bonding, D. Donetsky and S. Anikeev of Stony Brook University for lifetime measurements, and P. Dutta and I. Bhat of Rensselaer Polytechnic Institute for guidance in selective etching. This work was sponsored by the Department of Energy under AF Contract No. F19628-00-C-0002.
References [1] G.W. Charache, J.L. Egley, D.M. DePoy, L.R. Danielson, M.J. Freeman, R.J. Dziendziel, J.F. Moynihan, P.F. Baldasaro, B.C. Campbell, C.A. Wang, H.K. Choi, G.W. Turner, S.J. Wojtczuk, P. Colter, P. Sharps, M. Timmons, R.E. Fahey, K. Zhang, J. Electron. Mater. 27 (1998) 1038. [2] J.G. Cederberg, R.M. Biefeld, M.J. Hafich, M. Palmisiano, J. Crystal Growth 248 (2003) 289. [3] M.N. Palmisiano, R.M. Biefeld, J.G. Cederberg, M.J. Hafich, G.M. Peake, AIP Conf. Proc. 653 (2003) 30. [4] Y. Zheng, P.D. Moran, Z.F. Guan, S.S. Lau, D.M. Hansen, T.F. Kuech, T.E. Haynes, T. Hoechbauer, M. Natasi, J. Electron. Mater. 29 (2000) 916. [5] P.D. Moran, D. Chow, A. Hunter, T.F. Kuech, Appl. Phys. Lett. 78 (2001) 2232. [6] C.A. Wang, P.G. Murphy, P.W. O’Brien, D.A. Shiau, A.C. Anderson, Z.L. Liau, D.M. DePoy, G. Nichols, AIP Conf. Proc. 653 (2003) 473. [7] C.A. Wang, Z.L. Liau, D.A. Shiau, P.M. Nitishin, Mat. Res. Soc. Sym. 768 (2003) 101. [8] C.A. Wang, R.K. Huang, D.A. Shiau, M.K. Connors, P.G. Murphy, P.W. O’Brien, A.C. Anderson, D.M. DePoy, G. Nichols, M. Palmisiano, Appl. Phys. Lett. 83 (2003) 1286. [9] C.A. Wang, H.K. Choi, G.W. Charache, IEE Proc.Optoelectron. 147 (2000) 193. [10] C.A. Wang, J. Crystal Growth 170 (1997) 725. [11] G.B. Stringfellow, Organometallic Vapor-Phase Epitaxy, Academic Press, New York, 1999. [12] C.J. Vineis, C.A. Wang, K.F. Jensen, J. Crystal Growth 225 (2001) 420. [13] G.C. DeSalvo, R. Kaspi, C.A. Bozada, J. Electrochem. Soc. 141 (1994) 3526. [14] D. Donetsky, S. Anikeev, G. Belenky, S. Luryi, C.A. Wang, G. Nichols, Appl. Phys. Lett. 81 (2002) 4769. [15] R.M. Biefeld, A.A. Allerman, S.R. Kurtz, K.C. Baucom, J. Crystal Growth 195 (1998) 356–364. [16] A. Rogalski, R. Ciupa, W. Larkowski, Solid State Electron. 39 (1996) 159. [17] R.M. Biefeld, K.C. Baucom, S.R. Kurtz, J. Crystal Growth 137 (1994) 231. [18] S.R. Kurtz, R.M. Biefeld, L.R. Dawson, K.C. Baucom, A.J. Howard, Appl. Phys. Lett. 64 (1994) 812. [19] D.M. Follstaedt, R.M. Biefeld, S.R. Kurtz, K.C. Baucom, J. Electron. Mater. 24 (1995) 819. [20] R. Rettig, W. Stolz, Physica E 2 (1998) 277. [21] G. Bernatz, S. Nau, R. Rettig, W. Stolz, J. Electron. Mater. 29 (2000) 129.
Journal of Crystal Growth 261 (2004) 372–378
Growth and characterization of InAsSb/GaInAsAb/ AlGaAsAb/GaSb heterostructures for wafer-bonded thermophotovoltaic devices C.A. Wang*, D.A. Shiau, D.R. Calawa Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, MA 02420-9108, USA
Abstract InAsSb/GaInAsSb/AlGaAsSb/GaSb heterostructures were grown by organometallic vapor phase epitaxy for wafer bonding and epitaxial transfer. The InAsSb epilayer, which is used as an etch-stop layer, is the template for subsequent growth of GaInAsSb/AlGaAsSb/GaSb thermophotovoltaic device structures. Atomic force microscopy images of asgrown InAsSb epilayers exhibit extreme roughness with features that are aligned with the substrate miscut orientation. Interior InAsSb surfaces, which were prepared by selectively etching the cap layers, are atomically smooth. Thus, the InAsSb surface undergoes extensive roughening during cool down. X-ray characterization of InAsSb/GaSb/GaInAsSb heterostructures that were grown without interruption between successive layers shows that GaInAsSb material quality is maintained. GaInAsSb/AlGaAsSb heterostructures wafer-bonded to GaAs substrates with an internal reflector have enhanced optical quality compared to unbonded heterostructures. r 2003 Elsevier B.V. All rights reserved. PACS: 61.16.Ch; 68.35.Ct; 68.55.Jk; 81.15.Kk Keywords: A1. Roughening; A1. Surfaces; A3. Organometallic vapor phase epitaxy; B2. GaInAsSb; B2. InAsSb; B3. Solar cells
1. Introduction Lattice-matched GaSb-based III–V epitaxial layers are being developed for thermophotovoltaic (TPV) energy conversion systems that operate with thermal sources heated to B1000 C [1]. Recently, there has been interest in monolithically seriesinterconnected TPV cells to build open-circuit voltage Voc ; reduce parasitic losses; and simplify module assembly [2,3]. Since these interconnec*Corresponding author. Tel.: +1-781-981-4466; fax: +1781-981-0122. E-mail address: [email protected] (C.A. Wang).
tions require electrical isolation of devices, and semi-insulating (SI) GaSb substrates are not available, GaInAsSb/GaSb epilayers were grown on a lattice-matched AlGaAsSb cell-isolation diode [2,3]. A monolithically interconnected module (MIM) that consisted of 15 cells was fabricated, and a Voc value of 0.42 V was reported [3]. An alternative and versatile approach for achieving electrical isolation of monolithic devices is to wafer bond GaSb-based epilayers to a handle wafer, and then remove the substrate [4–8]. Although both growth and subsequent wafer processing can be more involved, additional benefits might be gained to improve TPV
0022-0248/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2003.11.030
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Fig. 1. Schematic structure of GaInAsSb/AlGaAsSb/GaSb TPV cells wafer bonded to GaAs handle wafer with SiOx/Ti/Au internal reflector for those with monolithic series interconnections.
device performance [6]. For example, by utilizing a dielectric/metal layer as an adhesive layer, GaInAsSb/AlGaAsSb/GaSb TPV devices were wafer bonded to SI GaAs with an internal reflector, as shown in Fig. 1 [8]. It was reported that the photoluminescence (PL) intensity increased about three times. Furthermore, linear voltage building was achieved, and Voc of 1.8 V at a short-circuit current density of 0.4 A/cm2 was measured for a 10-cell MIM device compared to 0.2 V for a single cell. To implement the wafer bonding and epitaxial transfer process, an etch-stop layer is grown between the substrate and device layers to facilitate removal of the substrate. For GaSb-based materials, InAsSb is used as this layer [5–8]. Although the InAsSb is a sacrificial layer and is ultimately removed, it must be of high quality to insure that the quality of the subsequently grown device layers are not compromised by the InAsSb epitaxy. This paper discusses the growth and characterization of InAsSb/GaInAsSb/AlGaAsSb/GaSb heterostructures by organometallic vapor phase epitaxy (OMVPE) for wafer bonding and epitaxial transfer to SI GaAs. The main emphasis is on growth of the InAsSb etch-stop layer, since OMVPE growth of GaInAsSb, AlGaAsSb, and TPV device structures has previously been reported [9,10].
2. Experimental procedure 2.1. OMVPE growth The TPV structure used for wafer bonding and epitaxial transfer is shown in Fig. 2. The epitaxial
n-GaSb lateral conduction layer n-GaInAsSb p-GaInAsSb p-AlGaAsSb window p-GaSb contact u-InAsSb etch stop u-GaSb buffer GaSb substrate Fig. 2. Schematic of InAsSb/GaSb/AlGaAsSb/GaInAsSb TPV device structure for wafer bonding.
layers are grown in a reverse sequence compared to conventional GaSb-based TPV structures [9], and the structure consists of the following layers on (0 0 1) n-GaSb substrate miscut 6 toward (1–11)B: u-GaSb buffer layer, u-InAsSb, p-GaSb, p-AlGaAsSb, p-GaInAsSb, n-GaInAsSb, and n-GaSb. The InAsSb/GaInAsSb/AlGaAsSb/GaSb heterostructures were grown lattice matched to 5-cm-diam GaSb substrates in a vertical rotating-disk OMVPE reactor, which has previously been described [10]. Solution trimethylindium (TMIn), triethylgallium, tritertiarybutylaluminum, tertiarybutylarsine (TBAs), and trimethylantimony (TMSb) were used as precursors, with diethyltellurium and dimethylzinc as n- and p-type dopants, respectively. The H2 carrier gas flow rate was 10 slpm; the reactor pressure was 150 Torr; and the susceptor rotation rate was 250 rpm. Growth studies were performed to establish conditions for the InAsSb etch-stop layer. These
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layers were grown at a temperature of 525 C with pTMSb =ðpTMSb þ pTBAs Þ; where pTMSb and pTBAs are the partial pressure of TMSb and TBAs, respectively, ranging from 0.08 to 1.2. The V/III ratio was kept constant at 2. These ratios were calculated using vapor pressures values of 1.7, 40.96, and 5.27 Torr, for TMIn, TBAs, and TMSb, respectively [11]. The InAsSb growth rate was 2.6 mm/h as determined from in situ reflectance [12]. The 0.54 eV GaInAsSb active layers were grown at 525 C [9], while the GaSb buffer and AlGaAsSb window and contact layers were grown at 550 C [10], as previously reported. To assess material quality of heterostructures that incorporate the InAsSb etch-stop layer, structures both with and without this layer were grown for comparison.
about 50 mm of the substrate. The remaining substrate and GaSb buffer layer were then selectively removed using a CrO3:HF:H2O based etch to expose the InAsSb layer. Finally, the InAsSb etch-stop layer was selectively removed with a citric acid:H2O2 etchant [13]. 2.3. Characterization Epitaxial structures were characterized by highresolution X-ray diffraction (HRXRD); PL at 4 and 300 K using a PbS detector; atomic force microscopy (AFM) operated in tapping mode with etched Si cantilevers that have a nominal tip radius of 5–10 nm; and time-resolved PL using a HgCdTe detector [14]. The Sb composition in InAsSb layers was determined from HRXRD measurements.
2.2. Wafer bonding and epitaxial transfer 3. Results and discussion Details of the wafer bonding process were described previously [6,8], and are summarized here. SI GaAs was used as the handle wafer since its thermal expansion coefficient (a ¼ 5:7 106 K1) is closely matched to that of GaSb (a ¼ 6:9 106 K1). After megasonically cleaning in solvents and chemically etching the epitaxial and handle SI GaAs wafers to remove native oxides, the epitaxial wafer was sputter-coated with SiOx/Ti/Au while the GaAs wafer was sputtercoated with Ti/Au. The SiOx dielectric layer was incorporated to provide electrical isolation and was designed to have higher reflectivity than would normally be obtained from a single metal layer [6]. The SiOx thickness was 200 nm, while the Au thickness was 2 mm, which are thicknesses that were found to result in low residual stress. To bond the wafers, the two Au surfaces were placed in contact; the wafers were heated under vacuum to a temperature of 250 C; and a mechanical pressure of 250 psi was applied. Epitaxial transfer was accomplished by removal of the GaSb substrate and GaSb and InAsSb etchstop layers. The 5-cm-diam bonded wafers were first cut into four quarters for ease of handling and processing. The bulk of the 500-mm-thick GaSb substrate was removed by spin-etching with H2O2:H2O:NaK tartrate tetrahydrate, leaving
3.1. InAsSb epitaxy Since GaInAsSb layers are grown at 525 C [9], InAsSb growth conditions were optimized for this same temperature to avoid growth interruption. A low V/III ratio of 2 was sufficient to prevent metalrich growth, and a mirror-smooth morphology was observed by Nomarski contrast microscopy. This V/III ratio is similar to the value reported in Ref. [15] for InAsSb grown by OMVPE at 500 C with TMIn, TBAs, and triethylantimony. Nominally lattice-matched InAsSb epilayers exhibit excellent structural quality as evidenced by the narrow full-width at half-maximum (FWHM) of the HRXRD rocking curve, shown as the lower curve in Fig. 3. The 0.5-mm-thick layer has FWHM of 37 arcsec, which is nominally the same value that was determined from X-ray simulations. The upper curve shown in Fig. 3 is the HRXRD curve for an InAsSb layer in which the TMSb flow had not stabilized before switching into the reactor. The broad peak at about 1600 arcsec is associated with InAs. Thickness fringes indicate that the thickness of this InAs layer is about 56 nm. Fig. 4 shows the dependence of lattice-matching and Sb incorporation in InAs1xSbx epilayers as a function of the
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Diffraction Angle (arc s) Fig. 3. X-ray rocking curves of InAs0.906Sb0.094 grown on GaSb at 525 C. The lower curve is for InAsSb grown with sufficient TMSb flow stabilization, while the upper curve (offset) is for a layer without TMSb flow stabilization. The peak at B1600 arcsec corresponds to InAs.
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pTMSb/(pTMSb + pTBAs) Fig. 4. Lattice mismatch of InAs1xSbx (left axis) and Sb concentration in InAs1xSbx (right axis) as a function of pTMSb =ðpTMSb þ pTBAs Þ:
pTMSb =ðpTMSb þ pTBAs Þ: The data indicate that the Sb incorporation is readily controlled. Fig. 5 summarizes the PL peak energy measured at 4 K of InAs1xSbx epilayers with various alloy composition. The layers are all about 0.5 mm thick. The FWHM values of these layers are typically between 11 and 15 meV. Also shown are the energy gap dependence on composition as given by Ref. [16] (solid curve) and energy values reported in Ref. [17]. The results obtained in this study are only slightly lower by about 0.002 eV than the solid curve while those from Ref. [17] are more than 0.02 eV lower. Those lower values were attributed to Cu–Pt ordering [18,19]. Therefore, InAsSb alloys grown in this study are probably atomically disordered.
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Fig. 5. Energy gap (solid line) or PL peak energy (open circles) at 4 K as a function the x (Sb) in InAs1xSbx. The solid line is from Ref. [16], and the triangles are from Ref. [17].
Since the TPV device layers are subsequently grown on the InAsSb epilayer, it is critical that this nominally lattice-matched layer be as atomically smooth as a GaSb buffer layer. For GaSb grown at 525 C or 550 C, AFM studies indicate that the GaSb surface is vicinal and typically exhibits a root-mean-square (rms) roughness of 0.2 nm or less. Fig. 6a shows an AFM image of an as-grown 0.5-mm-thick InAsSb layer. The surface has an rms roughness of 2.8 nm and exhibits an undulation with periodicity of about 0.25 mm. This undulation is mainly aligned perpendicular to the substrate miscut direction, i.e., the undulation is aligned with the step edges. A 1-mm-thick InAsSb epilayer has a similar morphology and slightly larger rms roughness of 3.4 nm. The InASb morphology is significantly rough, and clearly is undesirable as a surface for growth of GaInAsSb TPV layers. Nevertheless, a 1-mm-thick GaSb epilayer was grown on 0.25-mm-thick InAsSb, and the AFM image is shown in Fig. 6b. The surface is vicinal and has an rms roughness of only 0.15 nm. It is surprising that the morphology of the InAsSb/GaSb heterostructure would improve with successive GaSb growth. However, another possible explanation is that the InAsSb surface undergoes restructuring while cooling down after OMVPE growth. Such observations have been reported for InP and GaAs [20,21]. In those
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Fig. 6. AFM images of (a) as-grown InAsSb epilayer and (b) 1 mm-thick GaSb epilayer grown on a 0.25-mm-thick InAsSb epilayer. The rms roughness is 2.81 and 0.15 nm for the images shown in (a) and (b), respectively.
studies, InP/InGaAs and GaAs/AlAs heterostructures were grown with various interruption times between the successive layers. Capping layers were precisely removed by selective chemical etching to expose the underlying layer, and then AFM was used to examine this interior surface. Interior surfaces of GaAs grown without interruption were nearly vicinal, whereas macrosteps were observed for samples after cooling down [21]. To investigate the possibility of surface restructuring, InAsSb/GaInAsSb/GaSb heterostructures were grown on InAsSb epilayers of thicknesses that were varied from 0.1 to 0.5 mm. The GaSb and GaInAsSb layers were selectively removed with CrO3:HF:H2O. Selectivity of this etch was ideal: an InAsSb layer placed in the etch solution for as long as 30 min did not show evidence of InAsSb removal. Fig. 7 shows an AFM image of the interior surface of a 0.5-mm-thick InAsSb layer that was capped with GaSb. The surface has rms roughness of 0.2 nm and is significantly smoother than the sample shown in Fig. 6a. The rms roughness of a similar structure grown with 0.1 mm InAsSb is similar at 0.28 nm. Thus, the AFM results suggest that the InAsSb undergoes extensive roughening during cool down, and that surface roughness of the interior surface of the InAsSb etch-stop layer is similar to the GaSb buffer layer. Since growth interruption could result in surface roughening, GaSb layers were subsequently grown on InAsSb without interruption. To study material quality of heterostructures that incorporate the InAsSb etch-stop layer, GaInAsSb control and InAsSb/GaSb/GaInAsSb structures were grown. Figs. 8a and b show
Fig. 7. AFM image of interior InAsSb surface, which was exposed by selectively removing the GaSb capping layer. The rms roughness is 0.2 nm. The dramatic difference in surface morphology compared to that shown in Fig. 6a is indicative of extensive surface roughening that occurred during cool down.
reciprocal space maps of the control layer and InAsSb/GaSb/GaInAsSb heterostructure grown with a 0.1-mm-thick InAsSb layer, respectively. The similarity of the two maps indicates that the InAsSb etch-stop layer does not degrade the structural quality of subsequently grown GaInAsSb. The PL efficiency of these samples could not be assessed because the InAsSb is absorbing due to its lower energy gap. 3.2. Wafer-bonded GaInAsSb/AlGaAsSb/GaSb epitaxy The structural and optical quality of waferbonded GaInAsSb layers was previously reported
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[6,8] and a few results are summarized here. Fig. 9 shows the HRXRD rocking curves for the latticematched GaInAsSb/AlGaAsSb/GaSb TPV control structure on a GaSb substrate (Fig. 9a) and the wafer-bonded GaInAsSb/AlGaAsSb/GaSb TPV device structure on a GaAs handle wafer (Fig. 9b). The FWHM of the diffraction peak of the control structure is B30 arcsec. The FWHM of the wafer-bonded epitaxy is only slightly broadened at 51 arcsec. This broadening is predominantly due to curvature in the wafer due to differences in the thermal expansion coefficients of the different materials. Both samples exhibit thickness fringes, which is indicative of the excellent structural quality. Optical quality was evaluated by 300 K PL and PL decay. Special lifetime structures were prepared for this comparison. The double heterostructures (DHs) consist of a 1.5-mm-thick p-GaInAsSb layer doped at 2 1017 cm3 and AlGaAsSb cap layers. The control structure was grown without the InAsSb etch-stop layer, while the sample for wafer bonding was grown with a 0.1-mm-thick InAsSb epilayer. The PL intensity from the wafer-bonded sample is more than 3 times greater than the
PL Intensity (arb. units)
Fig. 8. X-ray reciprocal space maps of (a) GaInAsSb control structure and (b) InAsSb/GaInAsSb structure.
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Fig. 10. PL decay of control and wafer-bonded 0.54-eV GaInAsSb/AlGaAsb lifetime structures.
control structure, and time-resolved PL measurements [8,14] of the wafer-bonded and control GaInAsSb DHs, shown in Fig. 10, indicates that the wafer-bonded sample with the internal reflector has a PL lifetime tPL that is more than two times higher at 85 ns compared to the control sample with tPL ¼ 36 ns. These results suggest that the optical quality of InASb/GaInAsSb heterostructures are comparable to those grown without
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an InAsSb layer. Furthermore, photons that might normally be absorbed by the substrate are reflected back to the active layer and reabsorbed, and therefore the minority-carrier lifetime is increased.
4. Conclusions InAsSb epilayers were grown by OMVPE for incorporation as etch-stop layers in InAsSb/ GaInAsSb/AlGaAsSb/GaSb heterostructures for wafer bonding and epitaxial transfer. An Sb distribution coefficient close to one was obtained for nominally lattice-matched InAsSb epilayers. AFM images of as-grown InAsSb indicated that these surfaces have an rms roughness that is typically 3 nm, while the interior InAsSb surfaces, which are obtained by selectively etching GaSb/ GaInAsSb layers, have rms roughness of 0.2 nm. This suggests that InAsSb undergoes extensive roughening during cool down, and growth interruptions could roughen the surface. InAsSb/ GaSb/GaInAsSb heterostructures grown with an InAsSb etch-stop layer without interruption exhibit material quality that is similar to control structures grown without InAsSb.
Acknowledgements The authors gratefully acknowledge A. Lin and J.W. Chludzinski of MIT Lincoln Laboratory for technical assistance in materials characterization, P.G. Murphy and A.C. Anderson for wafer bonding, D. Donetsky and S. Anikeev of Stony Brook University for lifetime measurements, and P. Dutta and I. Bhat of Rensselaer Polytechnic Institute for guidance in selective etching. This work was sponsored by the Department of Energy under AF Contract No. F19628-00-C-0002.
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