- Email: [email protected]
GaAs on silicon heterostructures for application to photovoltaic–photoelectrolysis cells
Journal of Crystal Growth 225 (2001) 359–365
Selectively-grown InGaP/GaAs on silicon heterostructures for application to photovoltaic–photoelectrolysis cells Michael G. Mauk*, Anthony N. Tata, Bryan W. Feyock AstroPower Inc., Solar Park, Newark, DE 19716-2000, USA
Abstract Photovoltaic–photoelectrochemical (PV-PEC) cells based on InGaP/GaAs show excellent prospects for efficient production of hydrogen by electrolysis of water using solar energy. We describe a combined close-spaced vapor transport (CSVT)/liquid-phase epitaxy (LPE) process to produce arrays of selectively-grown mesas of InGaP/GaAs on silicon substrates. Unlike other semiconductor devices, the PV-PEC cell is well suited for such selectively-grown, discontinuous heteroepitaxial films. Thus, this device application affords exploiting the potential advantages of selective epitaxy, namely, the substantial reduction of stress and defects caused by thermal expansion and lattice mismatch between the silicon substrate and III–V epilayers. # 2001 Published by Elsevier Science B.V. PACS: 81.05 Ea; 81.15 Lm; 84.60 Dn Keywords: A3. Chemical vapor deposition processes; A3. Liquid phase epitaxy; A3. Selective epitaxy; B2. Semiconducting III–V materials; B3. Solar cells
1. Introduction and overview A monolithic InGaP/GaAs photovoltaic–photoelectrochemical (PV-PEC) cell developed at the National Renewable Energy Laboratory (NREL) has achieved record efficiencies in generating hydrogen from sunlight [1]. This device is a relatively sophisticated design comprised of a multilayer, epitaxially-grown structure of InGaP and GaAs made by metal organic chemical vapor deposition (MOCVD) on a GaAs substrate. The purpose of the work reported here is to explore and assess crystal growth techniques to make a simpler and less expensive version of the NREL *Corresponding author. Tel.: +1-302-366-0400, ext: 133; fax: +1-302-368-6474. E-mail address: [email protected] (M.G. Mauk).
PV-PEC device without paying too steep a penalty in efficiency, stability, or functionality. Substantial cost reductions in the NREL cell could be anticipated if (1) the single-crystal GaAs substrate is replaced by a single-crystal or polycrystalline silicon substrate, and (2) cheaper methods of epitaxy are used to form the InGaP/GaAs structure. Here, we use both means of cost reduction by developing a simplified InGaP/GaAs PV-PEC cell on a silicon substrate using a combined close-spaced vapor transport (CSVT)/ liquid-phase epitaxy (LPE) process instead of MOCVD. There is also the incentive to make the PV-PEC fabrication technology compatible with the existing processes and infrastructure of the silicon solar cell industry. The main issues in InGaP/GaAs-on-silicon heteroepitaxy are related to stress and high defect densities caused by the
0022-0248/01/$ - see front matter # 2001 Published by Elsevier Science B.V. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 0 8 7 0 - 3
360
M.G. Mauk et al. / Journal of Crystal Growth 225 (2001) 359–365
relatively large lattice mismatches and the unequal thermal contraction of the silicon substrate and III–V epilayer on cooling from growth temperatures. The most significant aspect of our approach is the concept of using selective growth to reduce stress and defects in heteroepitaxy of III–V compounds on silicon. In selective epitaxy, islands of the compound semiconductor are grown on the silicon substrate in areas of the silicon surface defined by photolithography. The key advantage is that the stress and defects can be greatly reduced in comparison to non-selectively-grown heteroepitaxial films. For instance, Yamaguchi [2] has indicated a nearly 100-fold reduction in stress and defects for GaAs-on-silicon films, provided the lateral dimensions of the islands are less than about 30 mm. This phenomenon has been consistently demonstrated in a number of semiconductor materials [3]. Selective epitaxy is difficult to exploit with conventional semiconductor devices since a discrete, discontinuous epitaxial layer is generally not useful for most devices. However, in the PVPEC cell the junction is formed between the electrolyte solution and the InGaP and there is no need to connect individual mesas. We emphasize this aspect since this critical feature permits the utilization of discontinuous or discrete epitaxial layers (e.g., a selectively-grown array of mesas). Further, unlike other devices such as solar cells, the PV-PEC cell is probably not critically sensitive to the relatively high surface areas engendered by a mesa array design. In short, the basis of this approach can be summarized as follows: (1) a selective mode of growth solves many of the fundamental problems of heteroepitaxy (thermal stress and defects); and (2) the operation of the PV-PEC cell, whereby the junction is formed by a solid semiconductor electrode and the liquid electrolyte in which the electrode is immersed}thus avoiding the need for either forming a junction or applying frontside contact metallization}is a near ideal device application for such selective heteroepitaxial structures. Two similar variations of the fabrication process are shown in Fig. 1. The utilization of such InGaP/ GaAs/Si structures (with additional backside metallization) as a PV-PEC cell is shown in
Fig. 2. To make the device structures, we use a two-step process starting with the chemical vapor deposition (CVD) of a GaAs ‘‘buffer’’ layer on silicon, followed by metallic solution growth, i.e., liquid-phase epitaxy, of InGaP on the GaAs buffer layer. The function of the buffer is to provide a seeding layer for subsequent growth of InGaP. The rationale of this ‘‘hybrid’’ CVD/LPE technique is as follows. We ultimately grow the InGaP by liquid-phase epitaxy because LPE can produce high quality InGaP layers in a relatively simple process. Further, LPE exhibits excellent selectivity wherein growth is restricted to openings patterned in the masked substrate. Selective growth is critical for stress and defect reduction. Similar selective epitaxy is nearly impossible to achieve with MOCVD or MBE. However, it is extremely difficult to grow InGaP directly on silicon by LPE because of the relatively large (4%) lattice mismatch between silicon and InGaP. In general, the LPE process is severely hindered when the lattice mismatch between epitaxial layer and substrate exceeds about 1%. Related hybrid CVD/ LPE processes have produced excellent GaAs-onsilicon [4–8] and InP-on-silicon material [9]. In the first CVD step, GaAs epitaxial films are grown on single-crystal silicon substrates using a simple CSVT process described below. In option A (Fig. 1), the silicon substrate (A.1) is first masked with a thermally-grown silicon dioxide layer with a thickness of 150–200 nm (A.2). Using photolithography and selective etching with buffered HF, the oxide mask is then patterned with openings to expose the underlying silicon (A.3). The mask openings serve as sites for preferential nucleation of GaAs in a vapor-phase epitaxy step (A.4). In a subsequent and separate LPE step, In0.5Ga0.5P (which is closely lattice matched to the relaxed GaAs-on-silicon layer) is grown selectively on the GaAs mesas (A.5). Option B (Fig. 1) is similar to option A, except that a (non-selective) GaAs film is grown on an unmasked silicon substrate by CSVT (B.2), and is then patterned into GaAs-onsilicon mesas by photolithography and selective etching (B.3). In this case, the LPE step is selective by virtue of the preferential nucleation of In0.5Ga0.5P on GaAs mesas rather than directly on the exposed silicon. Nucleation of InGaP directly
M.G. Mauk et al. / Journal of Crystal Growth 225 (2001) 359–365
361
Fig. 1. Fabrication sequence options for InGaP/GaAs/Si PV-PEC Cells made by a combined selective CSVT/LPE process.
on silicon is unfavorable due the lattice mismatch. We demonstrated both processes, but since option B is slightly simpler, we used this approach to make the samples that are characterized below, unless otherwise noted.
and of GaAs on Ge has been previously reported [10–13]. For this application, we have adapted the CSVT process for selective GaAs-on-silicon epitaxy. The GaAs CSVT process is based on a reversible transport reaction that uses water vapor as a transport agent.
2. GaAs-on-silicon selective heteroepitaxy by CSVT
2GaAsðsÞ þ H2 OðvÞ Ð Ga2 OðvÞ þ H2 ðgÞ
We use a simple CSVT process for heteroepitaxy of GaAs on silicon. CSVT of GaAs on GaAs
A GaAs source and the silicon seed are separated by distance d (here=1 mm). The source (at a
T2 T1
þAs2 ðvÞfor þ 12As4 ðvÞg:
362
M.G. Mauk et al. / Journal of Crystal Growth 225 (2001) 359–365
Fig. 2. InGaP/GaAs/Si structure as a photovoltaic/photoelectrochemical hydrogen generator.
Fig. 3. Mask geometry for selective CSVT.
Fig. 4. Top-view photomicrograph of selective CSVT GaAson-oxide-masked (1 1 1) silicon patterned with 35 mm 35 mm windows. This structure corresponds to A.4 in Fig. 1.
temperature T2 ) and seed (at a temperature T1 ) are heated individually in an infrared light-based fused silica tube furnace. The mask geometry is shown in Fig. 3. We currently use a mask with w=35 mm and d=50 mm. We have had best results (for seeding the InGaP in the subsequent LPE step) with a 0.2 mm thick GaAs-on-silicon
layer grown with T2=8508C and T1=8008C. Fig. 4 shows a selectively-grown CSVT GaAs on an oxide-masked silicon substrate and Fig. 5 shows a non-selectively-grown GaAs-on-silicon layer that was defined by post-growth patterning. These structures are used as substrates in the LPE step.
M.G. Mauk et al. / Journal of Crystal Growth 225 (2001) 359–365
363
Fig. 6. Top-view photomicrograph of 35 mm 35 mm selective LPE InGaP mesa on CSVT GaAs/Si.
Fig. 5. Scanning electron micrograph of CSVT GaAs-on(1 1 1) silicon patterned into 35 mm 35 mm mesas by postgrowth photolithography and etching. This structure corresponds to B.3 in Fig. 1.
3. Selective InGaP liquid-phase epitaxy For InGaP LPE we use a standard horizontal slideboat technique as is commonly employed for research in and production of various compound semiconductor optoelectronics devices such as light-emitting diodes, semiconductor lasers, detectors, and solar cells. For the LPE step, the substrate is a silicon wafer with an array of CSVT-grown heteroepitaxial GaAs-on-silicon mesas as described above. There is substantial prior work [14–16] on the LPE of InGaP alloys on GaAs substrates and much of this previous work is relevant to our efforts here. The atomic fractions of indium, gallium, and phosphorus are XIn=0.962; XGa=0.011; and XP=0.027. The melts were comprised of 5 g indium, 51 mg GaP, and 107 mg InP. In what is essentially a step cooling technique, the substrate was contacted with the supersaturated melt at a temperature T G for a time y. Growth temperatures were varied in steps of 18C between 7608C and 7908C. The smoothest InGaP surfaces were achieved at a growth temperature of 7818C. Figs. 6 and 7 show
Fig. 7. Scanning electron micrograph of 35 mm 35 mm selective LPE InGaP mesa on CSVT GaAs/Si.
two views of the InGaP selective LPE. The InGaP thickness as a function of growth time is shown in Fig. 8. The square root dependence of epilayer thickness (measured by a surface profiler) on growth time is consistent with that normally observed with non-selective, lattice-matched LPE growth using step cooling [17]. Fig. 9 is a highresolution double crystal X-ray diffraction
364
M.G. Mauk et al. / Journal of Crystal Growth 225 (2001) 359–365
4. Discussion and conclusion
Fig. 8. Growth rate of selective LPE InGaP on CSVT GaAson-silicon.
Fig. 9. High-resolution double-crystal X-ray diffraction rocking curve of 3 mm thick selectively-grown In0.5Ga0.5P LPE layer on 0.2 mm thick CSVT GaAs-on-silicon mesa array. GaAs-onsilicon CSVT layer is 0.2 mm thick. Selectively-grown mesas are 35 mm 35 mm squares with centers spaced approx. 50 mm apart.
rocking curve of the InGaP/GaAs/silicon composite structure. Samples are metallized and tested in a 3 molar sulfuric acid solution. They show rectifying current–voltage characteristics and electrolysis of water to generate hydrogen is observed at a forward voltage of 1.9 V and a current density of 8 mA/cm2. There were no apparent signs of electrolytic corrosion of the InGaP/GaAs/Si electrode.
We have demonstrated a simple method to produce selectively-grown InGaP/GaAs structures for application to PV-PEC cells. The operation of such PV-PEC cells is compatible with selectivelygrown heterostructures, and this feature is exploited in reducing thermal stress and lattice mismatch effects in growing InGaP/GaAs on a silicon substrate. Without using selective epitaxy, the InGaP/GaAs-on-silicon films are highly prone to cracking and peeling, and have an extremely rough surface morphology. This work suggests an alternative realization of the MOCVD-grown device on a GaAs substrate reported by NREL. The ultimate aim for a monolithic device is to form a silicon p–n junction ‘‘bottom’’ solar cell and optimize the short-wavelength transmission of the InGaP/GaAs so that the silicon bottom cell generates about 0.3 V and a photocurrent matched to the electrolysis current of the InGaP cell. A useful feature of the GaAs-on-silicon CSVT process, as observed in this work, is the diffusion of arsenic (an n-type dopant) into the p-type silicon substrate to form the emitter of a silicon solar cell which can serve to forward bias the InGaP electrolysis cell. It is clear that there is much latitude for a global optimization of the device design and epitaxial growth methods in pursuit of high-efficiency, low-cost PV-PEC cells described here.
Acknowledgements This work was supported by the US National Science Foundation under SBIR Grant DMI9906490.
References [1] O. Khaselev, J.A. Turner, Science 280 (1998) 425. [2] M. Yamaguchi, J. Mater. Res. 6 (2) (1991) 376. [3] E.A. Fitzgerald, P.D. Kirchner, R. Proano, G.D. Pettit, J.M. Woodall, D.G. Ast, Appl. Phys. Lett. 52 (18) (1988) 1496. [4] J.P. van der Ziel, R.A. Logan, N. Chand, J. Appl. Phys. 64 (6) (1988) 3201.
M.G. Mauk et al. / Journal of Crystal Growth 225 (2001) 359–365 [5] S. Sakai, R.J. Matyi, H. Shichijo, J. Appl. Phys. 63 (4) (1988) 1075. [6] S. Nakamura, S. Sakai, S.S. Chang, R. Ramaswamy, J.-H. Kim, G. Radhakrishnan, J.K. Liu, J. Katz, J. Crystal Growth 97 (1989) 303. [7] Y. Yazawa, T. Minemura, T. Unno, J. Appl. Phys. 69 (1) (1991) 273. [8] S. Saravanan, K. Jeganathan, K. Baskar, T. Jimbo, T. Soga, M. Umeno, J. Crystal Growth 192 (1998) 23. [9] R. Schnabel, M. Grundmann, R. Engelhardt, J. Oertel, A. Krost, D. Bimberg, R. Opitz, M. Schmidbauer, R. Ko¨hler, J. Crystal Growth 156 (1995) 337. [10] F.H. Nicoll, J. Electrochem. Soc. 110 (11) (1963) 1165.
365
[11] P.H. Robinson, RCA Rev. (1963) 585. [12] G. Perrier, R. Phillipe, J.P. Dodelet, J. Mater. Res. 3 (5) (1988) 1031. [13] D. Cote, J.P. Dodelet, B.A. Lombos, J.I. Dickson, J. Electrochem. Soc. 133 (9) (1986) 1925. [14] H. Asai, K. Oe, J. Crystal Growth 62 (1983) 67. [15] M.C. Wu, Y.K. Su, K.Y. Cheng, C.Y. Chang, J. Appl. Phys. 58 (4) (1985) 1537. [16] Y.K. Su, M.C. Wu, C.Y. Chang, K.Y. Cheng, J. Crystal Growth 76 (1986) 299. [17] J.J. Hsieh, Liquid-phase epitaxy, in: T.S. Moss, S.P. Keller (Eds.), Handbook on Semiconductors, Vol. 3, NorthHolland, Amsterdam, 1980.
Selectively-grown InGaP/GaAs on silicon heterostructures for application to photovoltaic–photoelectrolysis cells Michael G. Mauk*, Anthony N. Tata, Bryan W. Feyock AstroPower Inc., Solar Park, Newark, DE 19716-2000, USA
Abstract Photovoltaic–photoelectrochemical (PV-PEC) cells based on InGaP/GaAs show excellent prospects for efficient production of hydrogen by electrolysis of water using solar energy. We describe a combined close-spaced vapor transport (CSVT)/liquid-phase epitaxy (LPE) process to produce arrays of selectively-grown mesas of InGaP/GaAs on silicon substrates. Unlike other semiconductor devices, the PV-PEC cell is well suited for such selectively-grown, discontinuous heteroepitaxial films. Thus, this device application affords exploiting the potential advantages of selective epitaxy, namely, the substantial reduction of stress and defects caused by thermal expansion and lattice mismatch between the silicon substrate and III–V epilayers. # 2001 Published by Elsevier Science B.V. PACS: 81.05 Ea; 81.15 Lm; 84.60 Dn Keywords: A3. Chemical vapor deposition processes; A3. Liquid phase epitaxy; A3. Selective epitaxy; B2. Semiconducting III–V materials; B3. Solar cells
1. Introduction and overview A monolithic InGaP/GaAs photovoltaic–photoelectrochemical (PV-PEC) cell developed at the National Renewable Energy Laboratory (NREL) has achieved record efficiencies in generating hydrogen from sunlight [1]. This device is a relatively sophisticated design comprised of a multilayer, epitaxially-grown structure of InGaP and GaAs made by metal organic chemical vapor deposition (MOCVD) on a GaAs substrate. The purpose of the work reported here is to explore and assess crystal growth techniques to make a simpler and less expensive version of the NREL *Corresponding author. Tel.: +1-302-366-0400, ext: 133; fax: +1-302-368-6474. E-mail address: [email protected] (M.G. Mauk).
PV-PEC device without paying too steep a penalty in efficiency, stability, or functionality. Substantial cost reductions in the NREL cell could be anticipated if (1) the single-crystal GaAs substrate is replaced by a single-crystal or polycrystalline silicon substrate, and (2) cheaper methods of epitaxy are used to form the InGaP/GaAs structure. Here, we use both means of cost reduction by developing a simplified InGaP/GaAs PV-PEC cell on a silicon substrate using a combined close-spaced vapor transport (CSVT)/ liquid-phase epitaxy (LPE) process instead of MOCVD. There is also the incentive to make the PV-PEC fabrication technology compatible with the existing processes and infrastructure of the silicon solar cell industry. The main issues in InGaP/GaAs-on-silicon heteroepitaxy are related to stress and high defect densities caused by the
0022-0248/01/$ - see front matter # 2001 Published by Elsevier Science B.V. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 0 8 7 0 - 3
360
M.G. Mauk et al. / Journal of Crystal Growth 225 (2001) 359–365
relatively large lattice mismatches and the unequal thermal contraction of the silicon substrate and III–V epilayer on cooling from growth temperatures. The most significant aspect of our approach is the concept of using selective growth to reduce stress and defects in heteroepitaxy of III–V compounds on silicon. In selective epitaxy, islands of the compound semiconductor are grown on the silicon substrate in areas of the silicon surface defined by photolithography. The key advantage is that the stress and defects can be greatly reduced in comparison to non-selectively-grown heteroepitaxial films. For instance, Yamaguchi [2] has indicated a nearly 100-fold reduction in stress and defects for GaAs-on-silicon films, provided the lateral dimensions of the islands are less than about 30 mm. This phenomenon has been consistently demonstrated in a number of semiconductor materials [3]. Selective epitaxy is difficult to exploit with conventional semiconductor devices since a discrete, discontinuous epitaxial layer is generally not useful for most devices. However, in the PVPEC cell the junction is formed between the electrolyte solution and the InGaP and there is no need to connect individual mesas. We emphasize this aspect since this critical feature permits the utilization of discontinuous or discrete epitaxial layers (e.g., a selectively-grown array of mesas). Further, unlike other devices such as solar cells, the PV-PEC cell is probably not critically sensitive to the relatively high surface areas engendered by a mesa array design. In short, the basis of this approach can be summarized as follows: (1) a selective mode of growth solves many of the fundamental problems of heteroepitaxy (thermal stress and defects); and (2) the operation of the PV-PEC cell, whereby the junction is formed by a solid semiconductor electrode and the liquid electrolyte in which the electrode is immersed}thus avoiding the need for either forming a junction or applying frontside contact metallization}is a near ideal device application for such selective heteroepitaxial structures. Two similar variations of the fabrication process are shown in Fig. 1. The utilization of such InGaP/ GaAs/Si structures (with additional backside metallization) as a PV-PEC cell is shown in
Fig. 2. To make the device structures, we use a two-step process starting with the chemical vapor deposition (CVD) of a GaAs ‘‘buffer’’ layer on silicon, followed by metallic solution growth, i.e., liquid-phase epitaxy, of InGaP on the GaAs buffer layer. The function of the buffer is to provide a seeding layer for subsequent growth of InGaP. The rationale of this ‘‘hybrid’’ CVD/LPE technique is as follows. We ultimately grow the InGaP by liquid-phase epitaxy because LPE can produce high quality InGaP layers in a relatively simple process. Further, LPE exhibits excellent selectivity wherein growth is restricted to openings patterned in the masked substrate. Selective growth is critical for stress and defect reduction. Similar selective epitaxy is nearly impossible to achieve with MOCVD or MBE. However, it is extremely difficult to grow InGaP directly on silicon by LPE because of the relatively large (4%) lattice mismatch between silicon and InGaP. In general, the LPE process is severely hindered when the lattice mismatch between epitaxial layer and substrate exceeds about 1%. Related hybrid CVD/ LPE processes have produced excellent GaAs-onsilicon [4–8] and InP-on-silicon material [9]. In the first CVD step, GaAs epitaxial films are grown on single-crystal silicon substrates using a simple CSVT process described below. In option A (Fig. 1), the silicon substrate (A.1) is first masked with a thermally-grown silicon dioxide layer with a thickness of 150–200 nm (A.2). Using photolithography and selective etching with buffered HF, the oxide mask is then patterned with openings to expose the underlying silicon (A.3). The mask openings serve as sites for preferential nucleation of GaAs in a vapor-phase epitaxy step (A.4). In a subsequent and separate LPE step, In0.5Ga0.5P (which is closely lattice matched to the relaxed GaAs-on-silicon layer) is grown selectively on the GaAs mesas (A.5). Option B (Fig. 1) is similar to option A, except that a (non-selective) GaAs film is grown on an unmasked silicon substrate by CSVT (B.2), and is then patterned into GaAs-onsilicon mesas by photolithography and selective etching (B.3). In this case, the LPE step is selective by virtue of the preferential nucleation of In0.5Ga0.5P on GaAs mesas rather than directly on the exposed silicon. Nucleation of InGaP directly
M.G. Mauk et al. / Journal of Crystal Growth 225 (2001) 359–365
361
Fig. 1. Fabrication sequence options for InGaP/GaAs/Si PV-PEC Cells made by a combined selective CSVT/LPE process.
on silicon is unfavorable due the lattice mismatch. We demonstrated both processes, but since option B is slightly simpler, we used this approach to make the samples that are characterized below, unless otherwise noted.
and of GaAs on Ge has been previously reported [10–13]. For this application, we have adapted the CSVT process for selective GaAs-on-silicon epitaxy. The GaAs CSVT process is based on a reversible transport reaction that uses water vapor as a transport agent.
2. GaAs-on-silicon selective heteroepitaxy by CSVT
2GaAsðsÞ þ H2 OðvÞ Ð Ga2 OðvÞ þ H2 ðgÞ
We use a simple CSVT process for heteroepitaxy of GaAs on silicon. CSVT of GaAs on GaAs
A GaAs source and the silicon seed are separated by distance d (here=1 mm). The source (at a
T2 T1
þAs2 ðvÞfor þ 12As4 ðvÞg:
362
M.G. Mauk et al. / Journal of Crystal Growth 225 (2001) 359–365
Fig. 2. InGaP/GaAs/Si structure as a photovoltaic/photoelectrochemical hydrogen generator.
Fig. 3. Mask geometry for selective CSVT.
Fig. 4. Top-view photomicrograph of selective CSVT GaAson-oxide-masked (1 1 1) silicon patterned with 35 mm 35 mm windows. This structure corresponds to A.4 in Fig. 1.
temperature T2 ) and seed (at a temperature T1 ) are heated individually in an infrared light-based fused silica tube furnace. The mask geometry is shown in Fig. 3. We currently use a mask with w=35 mm and d=50 mm. We have had best results (for seeding the InGaP in the subsequent LPE step) with a 0.2 mm thick GaAs-on-silicon
layer grown with T2=8508C and T1=8008C. Fig. 4 shows a selectively-grown CSVT GaAs on an oxide-masked silicon substrate and Fig. 5 shows a non-selectively-grown GaAs-on-silicon layer that was defined by post-growth patterning. These structures are used as substrates in the LPE step.
M.G. Mauk et al. / Journal of Crystal Growth 225 (2001) 359–365
363
Fig. 6. Top-view photomicrograph of 35 mm 35 mm selective LPE InGaP mesa on CSVT GaAs/Si.
Fig. 5. Scanning electron micrograph of CSVT GaAs-on(1 1 1) silicon patterned into 35 mm 35 mm mesas by postgrowth photolithography and etching. This structure corresponds to B.3 in Fig. 1.
3. Selective InGaP liquid-phase epitaxy For InGaP LPE we use a standard horizontal slideboat technique as is commonly employed for research in and production of various compound semiconductor optoelectronics devices such as light-emitting diodes, semiconductor lasers, detectors, and solar cells. For the LPE step, the substrate is a silicon wafer with an array of CSVT-grown heteroepitaxial GaAs-on-silicon mesas as described above. There is substantial prior work [14–16] on the LPE of InGaP alloys on GaAs substrates and much of this previous work is relevant to our efforts here. The atomic fractions of indium, gallium, and phosphorus are XIn=0.962; XGa=0.011; and XP=0.027. The melts were comprised of 5 g indium, 51 mg GaP, and 107 mg InP. In what is essentially a step cooling technique, the substrate was contacted with the supersaturated melt at a temperature T G for a time y. Growth temperatures were varied in steps of 18C between 7608C and 7908C. The smoothest InGaP surfaces were achieved at a growth temperature of 7818C. Figs. 6 and 7 show
Fig. 7. Scanning electron micrograph of 35 mm 35 mm selective LPE InGaP mesa on CSVT GaAs/Si.
two views of the InGaP selective LPE. The InGaP thickness as a function of growth time is shown in Fig. 8. The square root dependence of epilayer thickness (measured by a surface profiler) on growth time is consistent with that normally observed with non-selective, lattice-matched LPE growth using step cooling [17]. Fig. 9 is a highresolution double crystal X-ray diffraction
364
M.G. Mauk et al. / Journal of Crystal Growth 225 (2001) 359–365
4. Discussion and conclusion
Fig. 8. Growth rate of selective LPE InGaP on CSVT GaAson-silicon.
Fig. 9. High-resolution double-crystal X-ray diffraction rocking curve of 3 mm thick selectively-grown In0.5Ga0.5P LPE layer on 0.2 mm thick CSVT GaAs-on-silicon mesa array. GaAs-onsilicon CSVT layer is 0.2 mm thick. Selectively-grown mesas are 35 mm 35 mm squares with centers spaced approx. 50 mm apart.
rocking curve of the InGaP/GaAs/silicon composite structure. Samples are metallized and tested in a 3 molar sulfuric acid solution. They show rectifying current–voltage characteristics and electrolysis of water to generate hydrogen is observed at a forward voltage of 1.9 V and a current density of 8 mA/cm2. There were no apparent signs of electrolytic corrosion of the InGaP/GaAs/Si electrode.
We have demonstrated a simple method to produce selectively-grown InGaP/GaAs structures for application to PV-PEC cells. The operation of such PV-PEC cells is compatible with selectivelygrown heterostructures, and this feature is exploited in reducing thermal stress and lattice mismatch effects in growing InGaP/GaAs on a silicon substrate. Without using selective epitaxy, the InGaP/GaAs-on-silicon films are highly prone to cracking and peeling, and have an extremely rough surface morphology. This work suggests an alternative realization of the MOCVD-grown device on a GaAs substrate reported by NREL. The ultimate aim for a monolithic device is to form a silicon p–n junction ‘‘bottom’’ solar cell and optimize the short-wavelength transmission of the InGaP/GaAs so that the silicon bottom cell generates about 0.3 V and a photocurrent matched to the electrolysis current of the InGaP cell. A useful feature of the GaAs-on-silicon CSVT process, as observed in this work, is the diffusion of arsenic (an n-type dopant) into the p-type silicon substrate to form the emitter of a silicon solar cell which can serve to forward bias the InGaP electrolysis cell. It is clear that there is much latitude for a global optimization of the device design and epitaxial growth methods in pursuit of high-efficiency, low-cost PV-PEC cells described here.
Acknowledgements This work was supported by the US National Science Foundation under SBIR Grant DMI9906490.
References [1] O. Khaselev, J.A. Turner, Science 280 (1998) 425. [2] M. Yamaguchi, J. Mater. Res. 6 (2) (1991) 376. [3] E.A. Fitzgerald, P.D. Kirchner, R. Proano, G.D. Pettit, J.M. Woodall, D.G. Ast, Appl. Phys. Lett. 52 (18) (1988) 1496. [4] J.P. van der Ziel, R.A. Logan, N. Chand, J. Appl. Phys. 64 (6) (1988) 3201.
M.G. Mauk et al. / Journal of Crystal Growth 225 (2001) 359–365 [5] S. Sakai, R.J. Matyi, H. Shichijo, J. Appl. Phys. 63 (4) (1988) 1075. [6] S. Nakamura, S. Sakai, S.S. Chang, R. Ramaswamy, J.-H. Kim, G. Radhakrishnan, J.K. Liu, J. Katz, J. Crystal Growth 97 (1989) 303. [7] Y. Yazawa, T. Minemura, T. Unno, J. Appl. Phys. 69 (1) (1991) 273. [8] S. Saravanan, K. Jeganathan, K. Baskar, T. Jimbo, T. Soga, M. Umeno, J. Crystal Growth 192 (1998) 23. [9] R. Schnabel, M. Grundmann, R. Engelhardt, J. Oertel, A. Krost, D. Bimberg, R. Opitz, M. Schmidbauer, R. Ko¨hler, J. Crystal Growth 156 (1995) 337. [10] F.H. Nicoll, J. Electrochem. Soc. 110 (11) (1963) 1165.
365
[11] P.H. Robinson, RCA Rev. (1963) 585. [12] G. Perrier, R. Phillipe, J.P. Dodelet, J. Mater. Res. 3 (5) (1988) 1031. [13] D. Cote, J.P. Dodelet, B.A. Lombos, J.I. Dickson, J. Electrochem. Soc. 133 (9) (1986) 1925. [14] H. Asai, K. Oe, J. Crystal Growth 62 (1983) 67. [15] M.C. Wu, Y.K. Su, K.Y. Cheng, C.Y. Chang, J. Appl. Phys. 58 (4) (1985) 1537. [16] Y.K. Su, M.C. Wu, C.Y. Chang, K.Y. Cheng, J. Crystal Growth 76 (1986) 299. [17] J.J. Hsieh, Liquid-phase epitaxy, in: T.S. Moss, S.P. Keller (Eds.), Handbook on Semiconductors, Vol. 3, NorthHolland, Amsterdam, 1980.