High Optical Performance of Pseudo-Single-Crystal Semiconductors Formed on Glass Using Layer-Exchanged Ge Seed Layer
Authors: Nishida Takeshi , Suemasu Takashi , Toko Kaoru 
Published: December 2020

Introduction

The multi-junction solar cell has continued to update the highest conversion efficiency of solar cells. However, the bulk Ge substrate, has been used as a bottom cell material for multi-junction solar cells, is too expensive for consumer application. One promising approach to reducing the fabrication cost is substituting the bulk Ge substrate with a Ge thin ﬁlm on an inexpensive glass substrate.
We formed a large-grained (> 100 µm) Ge layer on glass by using Al-induced layer exchange (ALILE) at a low temperature of 325 °C [1]. In addition, the resistivity of ALILE-Ge is low enough (< 103 cm-3) to be used as a bottom electrode due to highly Al doping [2]. Here, we adopted the Ge layer formed by ALILE as an epitaxial seed and grow Ge and GaAs as bottom and middle gap absorbing layers, respectively. We demonstrate the highest optical performance among the polycrystalline semiconductors formed on glass without transferring.

Experimental Procedure

First, the 50-nm-thick Ge seed layer on the glass substrate was prepared using ALILE (Fig. 1(a)). The upper Al layer is removed with HF (1.5%) treatment for 1 min. The resulting ALILE-Ge seed layer is highly (111)-oriented and large-grained (Figs. 1(b),(c)). Then, the Ge or GaAs layers were formed on the ALILE-Ge seed layer by solid phase epitaxy (SPE) or molecular beam epitaxy (MBE), respectively. For the SPE, amorphous-Ge layers (100-500 nm thick) were deposited at room temperature and then annealed at 350 °C in a N2 ambient (Fig. 2(a)). For the MBE, the 500-nm-thick GaAs layers were grown with heating the substrate at T
g = 500‒570 °C (Fig. 3(a)).

Results and Discussion

Figure 2(b) shows that the SPE-Ge layers appear larger minority carrier lifetime, τ
eff, than ALILE-Ge. In addition, τ
eff increases with the increasing thickness of SPE-Ge, w. The SPE-Ge for w = 500 nm achieves τ
eff of more than 1 µs. We calculated the bulk minority carrier lifetime of SPE-Ge, τ
bulk, as expressed the equation in Fig. 2(c). From the intercept of the least-squares line in Fig. 2(c), τ
bulk is determined to be 5.6 µs. Figure 2(d) shows that this value is close to that of a single-crystal Ge with hole concentration in the latter half of 1017 cm-3 [3]. SIMS measurement revealed that SPE-Ge has 6 × 1017 cm-3 of Al, which is diffused from ALILE-Ge. Assuming that the Al atoms in the SPE-Ge are fully activated as acceptors, τ
bulk of the SPE-Ge is mostly limited by impurity scattering.
Figure 3(b) shows that the EBSD image of the GaAs sample for T
g = 520 °C was very similar to that of ALILE-Ge. Figures 3(c) and 3(d) show the stacked structure of ITO/GaAs/Ge/glass as intended. The SAED pattern in Fig. 3(e) indicates that the GaAs film is epitaxially (111) oriented and single crystalline in this area (800 nm in diameter).
The composition of GaAs and the FWHMs of the TO and LO mode peaks were calculated from the EDX and Raman spectra, respectively. Figure 4(a) shows that the FWHMs initially decrease with increasing T
g, indicating that the higher T
g improves the crystallinity of the GaAs films. Conversely, the FWHMs increase for T
g > 550 °C, due to the reduction of the As atomic ratio in the GaAs. Figure 4(b) shows clear photoresponse spectra rising near a wavelength of 900 nm, which corresponds to the bandgap of GaAs. Figure 4(c) shows that internal quantum efficiency (IQE) is maximum at wavelengths of 700–800 nm for all samples. Figure 4(d) shows that the photoresponsivity and IQE initially increase with increasing T
g and decrease for T
g > 550 °C. This behavior is consistent with the FWHM results. We note that the photoresponsivity and IQE of the ALILE-Ge samples are comparable to those of the single-crystal GaAs which was simultaneously grown on Ge(111) substrate. This is because GaAs on ALILE-Ge is large-grained and behaves like a pseudo-single crystal. Thus, the maximum photoresponsivity (470 mA/W) and IQE (90%) were obtained at T
g = 550 °C.
In conclusion, we demonstrate the great potential of ALILE-Ge as an epitaxial seed for the Ge and GaAs layers. Their optical performance is highest among the polycrystalline ones formed on glass. This achievement will directly lead to the fabrication of novel high-eﬃciency multi-junction solar cells based on inexpensive substrates.

References

[1] K. Toko et al., Appl. Phys. Lett. 104, 022106 (2014).
[2] K. Kusano et al., ACS Appl. Energy Mater. 1, 5280 (2018).
[3] E. Gaubas et al., Appl. Phys. Lett. 89, 142106 (2006).

Figure 1