High quality synthesis of polycrystalline Ge films as infrared light absorbing layers for multi-junction solar cell applications

Synthesizing infrared light absorbing materials such as Ge on inexpensive substrates is the key to lowering the cost of multi-junction solar cells. However, in Ge thin films on insulators, there are no demonstrated examples of photoresponsivity due to grain boundaries and defect-induced acceptors. We have achieved dramatically larger grain size and updated the highest carrier mobility of polycrystalline Ge thin films by heat deposition[1], interfacial layer insertion[2, 3], elemental doping[4] and nucleation control[5] in the solid-phase crystallization (SPC) method. In this study, we combined these results to achieve even lower grain boundary density and defects, as well as to demonstrate photoresponsivity. Amorphous (a-)Si underlayer (~1 nm) and a-Ge (50 nm) were deposited on TiN coated glass substrate. Note that the a-Ge layer was deposited at 125 °C and TiN is the bottom electrode. The samples were then annealed to induce SPC and post-annealed. In addition, by epitaxial growth, light-absorption layers (500 nm in total) were formed. The grain size is still larger than that of the conventional method by heat deposition of a-Ge layer[1] and insertion of a-Si layer[2], leading to significant reduction in grain boundary density. In the case of this sample, polycrystalline with large grain size are synthesized in thinner film and then epitaxially grown into thicker film for light absorption. As a result, a spectrum of photoresponsivity unique to Ge, which absorbs infrared light, is obtained. This is the first demonstration of photoresponsivity caused by polycrystalline Ge films. [1] K. Toko et al., Sci. Rep. 7, 16981 (2017). [2] C. Xu et al., Appl. Phys. Lett. 115, 042101 (2019). [3] T. Imajo et al., ACS Appl. Electron. Mater. 4, 269 (2022). [4] S. Maeda et al., ACS Appl. Mater. Int. 14, 54848 (2022). [5] S. Maeda et al., Cryst. Growth Des. 23, 5535 (2023).