High Mobility Single-Crystalline-Like Si and Ge Thin Films on Flexible Substrates by Roll-to-Roll Vapor Deposition Processes

The widespread use of high efficiency III–V multi-junction solar cells is limited by the cost and brittleness of Ge wafers that are used as templates for epitaxial growth. In order to provide an affordable and scalable bottom template, this work aims to achieve epitaxial growth of Ge and Si films on inexpensive and flexible substrates by roll-to-roll continuous deposition. These single-crystalline-like Si and Ge thin films on metal substrates can also be utilized in high mobility thin film transistors (TFTs) well beyond the realm of present-day TFTs based on amorphous Si and organic materials. The strategy of “seed and epitaxy” was employed for epitaxial growth of Ge and Si films. It consists of an initial growth of biaxially-textured seed layer on a flexible non-crystalline substrate by ion beam assisted deposition (IBAD) followed by a deposition of lattice and thermally-matched epitaxial layers. In epi-Ge growth, the epitaxial buffer stack of “CeO2/LaMnO3/MgO” was grown on IBAD MgO seed template by roll-to-roll magnetron sputtering. Single-crystalline-like Ge films was epitaxially grown on the CeO2-buffered templates by medium frequency magnetron sputtering or plasma enhanced chemical vapor deposition with carrier mobility values as high as 1100 cm2/V·s, which is about 1000 times higher than that of amorphous Ge. These epi-Ge templates were successfully utilized to grown n- or p-type single-crystalline GaAs thin films by metal organic chemical vapor deposition which have been used to fabricate solar cells. Moreover, the epi-Si thin films were also grown on this Ge template. The resulting n-type Si film is highly oriented along (004) direction with an electron mobility of 230 cm2/V-s. High performance TFTs fabricated on both single-crystalline-like n-Si channel on flexible metal substrate, and p-Ge on flexible glass substrate confirm the superior electronic quality of the grown films. The flexible TFT of n-Si (p-Ge) exhibits an on/off ratio of ~10E6 (10E6), a field-effect mobility of ~200 (105) cm2/V-s, and a threshold voltage of -0.7 (1.0) V. These devices with superior performance open up a new era toward the next-generation flexible electronics and optoelectronics.