Analysis of Compositionally and Structurally Graded Si:H and Si_1-xGe_x:H Thin Films by Real Time Spectroscopic Ellipsometry

Abstract

Silicon-germanium (Si_1−xGe_x:H) thin films have been prepared by plasma enhanced chemical vapor deposition of SiH₄ and GeH₄ and measured during growth using real time spectroscopic ellipsometry. A two-layer virtual interface analysis has been applied to study the structural evolution of Si:H films prepared in multistep processes utilizing alternating intermediate and low H₂-dilution material layers, which have been designed to produce predominately amorphous films with a controlled distribution of microcrystalline particles. The compositional evolution of alloy-graded a-Si_1−xGe_x:H has been studied as well using similar methods. In each case, depth profiles of microcrystalline content, f_μc, or Ge content, x, have been extracted. Additionally, real time spectroscopic ellipsometry has been used to monitor post-deposition exposure of a-Si:H, a-Si_1−xGe_x:H, and a-Ge:H films to a hydrogen plasma in situ in order to determine sub-surface amorphous film modification similar to that which would occur when a highly H₂-diluted layer is deposited on a layer prepared with lower dilution. These analyses provide guidance for enhanced performance of Si:H based solar cells, through controlled bandgap grading using compositionally graded amorphous binary alloys (a-Si_1−xGe_x:H) or the incorporation of controlled fractions of microcrystallites into bulk amorphous i-layer materials, and by providing a fundamental understanding of the modification of component layers during the deposition of subsequent layers in multilayer stacks.