Modeling and experimental study on the growth of silicon germanium film by plasma enhanced chemical vapor deposition

Hydrogenated microcrystalline silicon germanium microc-SiGe:H deposited by plasma enhanced chemical vapor deposition (PECVD) is of great interest to photovoltaic (PV) applications due to its low process temperature and good uniformity over large area. The nature of high optical absorption and low optical bandgap makes it promising as the bottom cell absorbing layer for tandem junction solar cells. However, the addition of germane (GeH4) gas changes deposited film properties and makes it rather complicated for the established silane (SiH4) based discharge process with hydrogen (H2) dilution. Despite existing experimental studies for SiH 4/GeH4/H2 3-gas mixture discharge and comprehensive numerical simulations for SiH4/H2 or SiH4/Ar plasma, to the author's best knowledge, a numerical model for both SiH 4 and GeH4 in a high pressure regime is yet to be developed. The plasma discharge, the film growth and their effects on film properties and the solar device performance need deep understanding.;In this dissertation, the growth of the microc-SiGe:H film by radio frequency (RF) PECVD is studied through modeling simulation as well as experiments. The first numerical model for the glow discharge of SiH4/GeH 4/H2 3-gas mixture in a high pressure regime is developed based on one dimensional fluid model. Transports of electrons, molecules, radicals and ions in the RF excitation are described by diffusion equations that are coupled with the Poisson's equation. The deposition is integrated as the boundary conditions for discharge equations through the sticking coefficient model. Neutral ionizations, radical dissociations and chemical reactions in the gas phase and surface kinetics such as the diffusive motion, chemical reactions and the hydrogen etching are included with interaction rate constants. Solved with an explicit central-difference discretization scheme, the model simulates mathematical features that reflect the plasma physics such as the plasma sheath and gas species distributions. The model predicts effects of process conditions on the deposition rate and the Ge chemical content which agree well with experimental results.;Tandem junction solar devices are fabricated with the developed microc-SiGe:H film as the bottom cell absorbing layer. Film properties are characterized by determining the Ge content with the Raman peak shift and estimating the optical bandgap with the spectral response measurement. The deposition process is investigated following the fractional factorial experiment design in the 5% Ge content window and then in the amorphous-to-microcrystalline phase transition regime. Gradient Ge content structure is also applied to improve the interface. The conversion efficiency is obtained at 10.62% for the device with 1.2microm thick microc-SiGe:H bottom cell, which is higher than that of the reference device with 1.95microm microc-Si:H.;This dissertation has demonstrated a powerful modeling tool to study the multi-gas discharge and deposition in the PECVD environment. The physics behind experimental trends is understood by analyzing temporal and spatial distributions of individual gas species and their interactions. It presents the comprehensive understanding of the growth of the microc-SiGe:H film which leads to the realization of high efficiency and high throughput solar cell devices.