Liquid phase crystallization of Ge over direct Si substrate as a template for GaAs applications

The direct growth of Ge over Si substrates provides a complementary metal-oxide-semiconductor compatible low-cost way that can be used as a template for GaAs based solar cell and other applications. In this work, the previously reported liquid phase crystallization (LPC) process from our group is used to grow crystalline Ge directly over the Si substrate. No buffer layer is used to relax the lattice mismatch induced strain. The results show a crystalline growth of Ge that is confirmed from x-ray diffraction measurement. The surface morphology is investigated using scanning electron microscope, showing large grain growth in the range from 2-10 μm. The transmission electron microscope investigations show that the threading dislocation densities extend up to ~250 nm from the Si/Ge interface. After ~250 nm from the Si/Ge interface, the Ge film becomes relaxed and hence, can be used as a template for GaAs based solar cell devices.

I. Introduction

The solar cell devices made up of III-V semiconductors report highest efficiency values [1]. However, the cost of producing these cells is high and hence, limiting their use in concentrated photovoltaic (CPV) and space applications. In this high cost; the cell cost is acritical; however, the substrate cost is the main contributing factor [2], [3]. On the other hand, the Si-based photovoltaics is inexpensive due to the mature Si-technology. Therefore, an alternative to achieve the low-cost III-V semiconductor-based photovoltaics devices, is to use Si as a substrate [4]– [6]. However, the lattice mismatch between Si substrate and III-V semiconductors like GaAs introduces defects and dislocations densities. These defects and dislocations cause a degradation in the device performance [7], [8]. A potential solution to overcome this problem is to use Ge-on-Si substrate. The lattice constant of Ge (5.658 Å) perfectly matches with the GaAs. It enables the fabrication and integration of GaAs based solar cell devices on low-cost Si platform. However, the growth of defect-free Ge directly over Si-substrate is difficult due to a lattice mismatch of ~ 4% [9]. The high strain values associated with ~ 4% lattice mismatch leads to dislocation densities as high as 10¹⁰-10¹² cm⁻² [10]. These dislocation densities lie to few nanometers from the Si/Ge interface and extend to the surface acting as recombination centers for carriers, and hence killing the device efficiency [11]. The use of a buffer layer between Si and Ge is a potential solution to reduce the dislocation densities as low as 10⁶ cm⁻² [12]. However, this approach requires a thicker buffer layer (10 μm) [13] and multiple-steps for its deposition; which makes it a costly and extensive affair.