International Journal of Photoenergy
Volume 2016 (2016), Article ID 7218310, 5 pages
http://dx.doi.org/10.1155/2016/7218310

Research Article

High Quality GaAs Epilayers Grown on Si Substrate Using 100 nm Ge Buffer Layer

Wei-Cheng Kuo,¹ Hung-Chi Hsieh,¹ Wu Chih-Hung,² Huang Wen-Hsiang,² Chien-Chieh Lee,³ and Jenq-Yang Chang⁴

¹Institute of Materials Science and Engineering, National Central University, Taoyuan, Taiwan
²Institute of Nuclear Energy Research, Taoyuan, Taiwan
³Optical Science Center, National Central University, Taoyuan, Taiwan
⁴Department of Optics and Photonics, National Central University, Taoyuan, Taiwan

Received 20 April 2016; Revised 27 July 2016; Accepted 17 August 2016

Academic Editor: Bhushan Sopori

Copyright © 2016 Wei-Cheng Kuo et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

We present high quality GaAs epilayers that grow on virtual substrate with 100 nm Ge buffer layers. The thin Ge buffer layers were modulated by hydrogen flow rate from 60 to 90 sccm to improve crystal quality by electron cyclotron resonance chemical vapor deposition (ECR-CVD) at low growth temperature (180°C). The GaAs and Ge epilayers quality was verified by X-ray diffraction (XRD) and spectroscopy ellipsometry (SE). The full width at half maximum (FWHM) of the Ge and GaAs epilayers in XRD is 406 arcsec and 220 arcsec, respectively. In addition, the GaAs/Ge/Si interface is observed by transmission electron microscopy (TEM) to demonstrate the epitaxial growth. The defects at GaAs/Ge interface are localized within a few nanometers. It is clearly showed that the dislocation is well suppressed. The quality of the Ge buffer layer is the key of III–V/Si tandem cell. Therefore, the high quality GaAs epilayers that grow on virtual substrate with 100 nm Ge buffer layers is suitable to develop the low cost and high efficiency III–V/Si tandem solar cells.

1. Introduction

Photovoltaics is widely recognized as one of the most desirable options yet suggested for future sustainable energy supply, with on-going cost reduction key to fulfilling this potential. Crystalline silicon solar cell has continued dominating the solar energy market [1]. Yet, III–V solar cell has the highest conversion efficiency in the world [2]. However, this device has high cost, making it diffcult to command the solar cell market. Therefore, combining of III–V cells with Si cells does not offer only high performance but also cost reduction. This III–V/Si tandem cells efficiency can be over 40% [2].

Two different process technologies were investigated for the fabrication of III–V/Si tandem cells: direct epitaxial growth and wafer bonding. For example, Derendorf et al. [3] investigate GaInP/GaAs//Si solar cells with three active p-n junctions by surface activated direct wafer bonding between GaAs and Si. The highest efficiency is reached at 71 suns with an efficiency of 23.6%. Even so, direct epitaxial growth process has two advances compared with wafer bonding process, which can have large area production and easy integration with integrated circuit. Nevertheless, large lattice-mismatch and different thermal expansion coefficient between GaAs layer and Si substrate results in high threading dislocations, which influences open-circuit voltage () and carrier lifetime of the cells [4]. Thus, it requires a buffer region to solve the lattice-mismatch problem between GaAs and Si. Ge provides nearly lattice match with GaAs. GaAs growth on Ge/Si virtual substrates was investigated for potential applications of III–V solar cell technology [1, 5–9]. For example, Lueck et al. [6] developed GaInP/GaAs solar cells growth on Ge/Si virtual substrates with efficiencies of 16.8% under AM1.5-G illumination. Diaz et al. [1] presented a GaAsP/SiGe tandem solar cells growths on Si substrates. This tandem solar cells efficiency could reach 18.9% under AM1.5-G illumination.

The quality of the Ge buffer layer is the key of III–V/Si tandem cell. But the Ge layer thickness is also an important issue for the III–V/Si tandem cells. For example, Ichikawa et al. [8] proposed a four-junction III–V/Si tandem solar cell which uses thin Ge buffer layers. In this structure, Ge absorption is dependent on Ge layer thickness and Ge absorption is significantly affecting the bottom Si cell efficiency. However, it is difficult to obtain a Ge epilayer with high crystal quality and thin thickness. Therefore, we used the electron cyclotron resonance chemical vapor deposition (ECR-CVD) to grow the thin Ge epilayer (100 nm) on Si substrates and modulate the hydrogen flow ratio to improve the crystal quality at a low growth temperature of 180°C. Then, GaAs epilayers grow on Ge/Si by metal organic chemical vapor deposition (MOCVD).

2. Experimental

Ge epilayers on n-type offcut 6° towards the [110] CZ silicon wafers with resistivity of 1–10 Ω-cm at a low growth temperature of 180°C are obtained using ECR-CVD. Before deposition, the native oxide is removed in a 5% hydrofluoric acid solution. We modulated hydrogen flow rate from 60 to 90 sccm in an attempt to improve crystal quality to obtain epitaxial Ge on Si and the other deposition parameters are as follows: the source gas supplied via an inlet valve upon the ECR region includes Ar and 10% GeH₄ diluted with He; the working pressure is 25 mtorr; the microwave power is 0.14 W/cm². After the deposition, Ge is annealed in a nitrogen atmosphere at 700°C for 5 min to improve the crystallinity. The GaAs layers are grown on Ge/Si by MOCVD. Before GaAs films growth, the Ge/Si sample is baked at 700°C around AsH₃ environment to produce the precursor. The sources gas uses trimethylgallium (TMGa) and AsH₃; the working pressure is 50 mbar. A 20 nm GaAs seed layer is grown at 450°C on Ge/Si; 2 μm GaAs epilayers are grown at 620°C. The quality of GaAs and Ge films is checked by high resolution X-ray diffraction (XRD) measurement using the incident Cu Kα line as the X-ray source (wavelength is 0.154 nm) at 40 kV and 40 mA to obtain the intensity and the FWHM of the Ge (004) peaks. The deposition and crystallization rate of Ge films are identified by spectroscopy ellipsometry (SE). Transmission electron microscopy (TEM) was used to observe the GaAs/Ge/Si interface to demonstrate the epitaxial growth of Ge on Si.

3. Result

Figure 1 shows the XRD rocking curve of the Ge epilayers on Si grown at various H₂ flow rate and we fitted the FWHM of the Ge (004) peaks. We found that, with the increase of H₂ flow rate from 60 to 80 sccm, the FWHM decreases from 993 arcsec to 571 arcsec. Furthermore, the H₂ flow rate increases from 80 to 90 sccm, and the FWHM increases from 571 arcsec to 730 arcsec. The intensity trend is similar to FWHM of the Ge (004) peaks. It is known that the Ge (004) crystal, which is the crystal phase of the epilayer grown on Si, has a better quality at H₂ flow rate which equals 80 sccm. After annealing at 700°C, the FWHM of Ge decreases from 571 to 406 arcsec. Figure 2 shows the deposition and crystallization rate of Ge epilayers, measured by SE, as a function of H₂ flow rate. The thickness and crystallization rate of Ge films are fitted by Tauc–Lorentz model and Bruggeman effective medium approximation [10–13]. The Tauc–Lorentz model is given by

where is band gap energy, is energy of maximum absorption (peak transition energy), is the amplitude factor proportional to the density of the material, and is the broadening parameter that is inversely related to the short range order of the material. The deposition rate increases from 0.108 to 0.154 nm/sec as the H₂ flow rate increases from 60 to 90 sccm. Besides, crystallization rate of Ge films is the highest at H₂ flow rate 80 sccm,which is consistent with the Ge quality measurement result of XRD.

Figure 1: High resolution XRD rocking curve of the Ge (004) peak with various H₂ flow rate.

Figure 2: The deposition and crystallization rate as a function of H₂ flow rate.

Hydrogen adsorption and desorption reactions at Ge film surface are investigated [14]. In addition, the mechanism of passivation of germanium vacancies by atomic and molecular hydrogen has been reported [15]. Appropriate hydrogen can increase the crystallization of the Ge films [16–18]. Therefore, hydrogen passivates dangling bonds and helps Ge-Ge bonds to be stronger when H₂ flow rate is from 60 to 80 sccm. Moreover, an excess of hydrogen in the source gas can etch the film-growing surface and break Ge-H bond [14]. It forms dangling bonds, increasing the damage of the Ge surface. Thus, the Ge films quality tends to decrease when H₂ flow rate is from 80 to 90 sccm.

As the Ge epilayer is observed by SE and XRD, it demonstrates that when the H₂ flow rate is modulated, the crystal quality can be improved. We use TEM to identify the Ge/Si interface epitaxial quality. Figure 3 shows high resolution cross-sectional TEM image of the Ge films as deposited at H₂ flow rate of (a) 80 sccm and (b) 60 sccm. At the Ge/Si interface, Ge epilayers have some dislocation before annealing, the strain originating from the 4.2% lattice mismatch between Si and Ge leads to islanding accompanied by the formation of misfit dislocations. After that, the Ge films at H₂ flow rate of 80 sccm is annealed at 700°C. Figures 4(a) and 4(b) show high resolution cross-sectional TEM image of the Ge films annealing at 700°C and Ge/Si interface. The dislocation of Ge films is decreased, and it is clear that the Ge lattice on the c-Si substrate is epitaxial.

(a)

(b)

Figure 3: High resolution cross-sectional TEM image of the Ge films as deposited at H₂ flow rate of (a) 80 sccm and (b) 60 sccm.

(a)

(b)

Figure 4: (a) High resolution cross-sectional TEM image of the Ge films annealing at 700°C; (b) Ge/Si interface.

GaAs films quality is dependent on Ge buffer layer quality. After growth of thin Ge buffer layer, we deposit 2 μm GaAs epilayers on Ge/Si. The GaAs quality is studied by XRD and TEM. The XRD rocking curve of the Ge buffer layer on Si and GaAs epilayer on Ge/Si is shown in Figure 5. In XRD rocking curve, the FWHM of GaAs epilayer is 220 arcsec which is lower than Ge films. To further observe the interface details of the film, we used TEM to distinguish the epitaxial quality of the GaAs/Ge/Si structure of the film as shown in Figure 6. As can be seen, the defects at GaAs/Ge interface are localized within a few nanometers. It clearly shows that the dislocation is well suppressed.

Figure 5: High resolution XRD rocking curve of the Ge and GaAs peak.

Figure 6: High resolution cross-sectional TEM image of the GaAs/Ge/Si (a) interface and (b) lattice.

The quality and thin thickness of Ge films are principal for high efficiency III–V/Si tandem cells. But GaAs films grow on Si substrate for numerous investigation uses of thick Ge buffer layer. The XRD FWHM range of GaAs films on Ge/Si is 180–276 arcsec [19–21]. Thick Ge buffer layer can obtain high quality GaAs cells, but it reduces the optical loss of bottom Si cells seriously. In this study, we investigate high quality GaAs epilayers grown on Si substrate using 100 nm Ge buffer layer. The XRD FWHM of GaAs is 220 arcsec. This technology be applied to low cost and high efficiency III–V Si tandem solar cell.

4. Conclusion

For the high efficiency III–V/Si tandem cells, Ge quality and thickness are also important. However, it is difficult to obtain a Ge epilayer with high crystal quality and thin thickness. In this study, we used the ECR-CVD to grow the thin Ge epilayer (100 nm) on Si substrates and modulate the hydrogen flow ratio to improve the crystal quality at a low growth temperature of 180°C. The result indicates that the appropriate hydrogen rate can improve the Ge quality. The XRD FWHM of the Ge epilayers is 571 arcsec at H₂ = 80 sccm. Crystallization rate of Ge films is the highest at H₂ flow rate 80 sccm, which is consistent with the Ge quality measurement result of XRD. After annealing at 700°C, the FWHM of Ge was decreased from 571 to 406 arcsec. After growth of thin Ge buffer layer, 2 μm GaAs epilayers grow on Ge/Si by MOCVD. The FWHM of GaAs epilayer is 220 arcsec. An ordered GaAs/Ge/Si interface with epitaxial growth by TEM image can be identified. The quality of the Ge buffer layer is the key of III–V/Si tandem cell. Therefore, the high quality GaAs epilayers grow on virtual substrate with 100 nm Ge buffer layers for low cost and high efficiency III–V/Si tandem solar cells were developed and demonstrated.

Competing Interests

The authors declare that they have no competing interests.

Acknowledgments

This work was supported by Grant nos. NSC 104-2221-E-008-049-MY2 and 105-3113-E-008-007 received from the Ministry of Science and Technology of Taiwan. This work also acknowledged National Nano Device Laboratories for the constant support during the process.

References

M. Diaz, L. Wang, D. Li et al., “Tandem GaAsP/SiGe on Si solar cells,” Solar Energy Materials and Solar Cells, vol. 143, pp. 113–119, 2015.

K. J. Schmieder, A. Gerger, M. Diaz et al., “GaAsP on SiGe/Si material quality improvements with in-situ stress sensor and resulting tandem device performance,” Materials Science in Semiconductor Processing, vol. 39, pp. 614–620, 2015.

K. Derendorf, S. Essig, E. Oliva et al., “Fabrication of GaInP/GaAs//Si solar cells by surface activated direct wafer bonding,” IEEE Journal of Photovoltaics, vol. 3, no. 4, pp. 1423–1428, 2013.

C. L. Andre, J. J. Boeckl, D. M. Wilt et al., “Impact of dislocations on minority carrier electron and hole lifetimes in GaAs grown on metamorphic SiGe substrates,” Applied Physics Letters, vol. 84, no. 18, pp. 3447–3449, 2004.

C. L. Andre, J. A. Carlin, J. J. Boeckl et al., “Investigations of high-performance GaAs solar cells grown on Ge-Si1-xGex-Si substrates,” IEEE Transactions on Electron Devices, vol. 52, no. 6, pp. 1055–1060, 2005.

M. R. Lueck, C. L. Andre, A. J. Pitera, M. L. Lee, E. A. Fitzgerald, and S. A. Ringel, “Dual junction GaInP/GaAs solar cells grown on metamorphic SiGe/Si substrates with high open circuit voltage,” IEEE Electron Device Letters, vol. 27, no. 3, pp. 142–144, 2006.

M. Khizar and M. Yasin Akhtar Raja, “Design and development of high efficiency solar cells using thin film GaAs on nanostructured silicon,” in Proceedings of the 8th International Conference on High-Capacity Optical Networks and Emerging Technologies (HONET '11), pp. 42–45, IEEE, Riyadh, Saudi Arabia, December 2011.

R. Ichikawa, S. Takita, Y. Ishikawa, and K. Wada, “Germanium as a material to enable silicon photonics,” in Silicon Photonics II, D. J. Lockwood and L. Pavesi, Eds., pp. 131–141, 2011.

J. Faucher, A. Gerger, S. Tomasulo et al., “Single-junction GaAsP solar cells grown on SiGe graded buffers on Si,” Applied Physics Letters, vol. 103, no. 19, Article ID 191901, 2013.

G. E. Jellison Jr. and F. A. Modine, “Parameterization of the optical functions of amorphous materials in the interband region,” Applied Physics Letters, vol. 69, no. 3, pp. 371–373, 1996.

A. Fontcuberta I Morral, P. Roca I Cabarrocas, and C. Clerc, “Structure and hydrogen content of polymorphous silicon thin films studied by spectroscopic ellipsometry and nuclear measurements,” Physical Review B, vol. 69, no. 12, Article ID 125307, 2004.

B. von Blanckenhagen, D. Tonova, and J. Ullmann, “Structure and hydrogen content of polymorphous silicon thin films studied by spectroscopic ellipsometry and nuclear measurements,” Applied Optics, vol. 41, p. 16, 2002.

D. A. G. Bruggeman, “Berechnung verschiedener physikalischer Konstanten von heterogenen Substanzen. I. Dielektrizitätskonstanten und Leitfähigkeiten der Mischkörper aus isotropen Substanzen,” Annalen der Physik, vol. 416, no. 8, pp. 665–679, 1935.

A. S. Yanovsky and S. V. Kolomoets, “Hydrogen interaction with Si and Ge surface clusters,” Vacuum, vol. 54, no. 1–4, pp. 47–51, 1999.

F. C. Gozzo, M. N. Eberlin, and I. Chambouleyron, “Ab initio calculations of a vacancy in a Ge nano-cluster and its passivation with atomic and molecular hydrogen,” Journal of Non-Crystalline Solids, vol. 299–302, no. 1, pp. 174–178, 2002.

D. Bermejo and M. Cardona, “Infrared absorption in hydrogenated amorphous and crystallized germanium,” Journal of Non-Crystalline Solids, vol. 32, no. 1–3, pp. 421–430, 1979.

T. Sakata, K. Makihara, H. Deki, S. Higashi, and S. Miyazaki, “Low temperature high-rate growth of crystalline Ge films on quartz and crystalline Si substrates from VHF inductively-coupled plasma of GeH₄,” Thin Solid Films, vol. 517, no. 1, pp. 216–218, 2008.

Z. Tang, W. Wang, D. Wang, D. Liu, Q. Liu, and D. He, “The influence of H₂/Ar ratio on Ge content of the μc-SiGe: H films deposited by PECVD,” Journal of Alloys and Compounds, vol. 504, no. 2, pp. 403–406, 2010.

K. Ma, R. Urata, D. A. B. Miller, and J. S. Harris Jr., “Low-temperature growth of GaAs on Si used for ultrafast photoconductive switches,” IEEE Journal of Quantum Electronics, vol. 40, no. 6, pp. 800–804, 2004.

H. Tanoto, S. F. Yoon, W. K. Loke et al., “Heteroepitaxial growth of GaAs on (100) Ge/Si using migration enhanced epitaxy,” Journal of Applied Physics, vol. 103, no. 10, Article ID 104901, 2008.

D. Choi, J. S. Harris, E. Kim, P. C. McIntyre, J. Cagnon, and S. Stemmer, “High-quality III–V semiconductor MBE growth on Ge/Si virtual substrates for metal-oxide-semiconductor device fabrication,” Journal of Crystal Growth, vol. 311, no. 7, pp. 1962–1971, 2009.