Date of Award
Doctor of Philosophy
Electrical and Computer Engineering
Vikram L. Dalal
Germanium, with its almost direct gap band structure, is an attractive material for photovoltaic applications. Carbon is added to the germanium lattice to increase the band gap. Since germanium and carbon are not soluble in equilibrium, only a metastable process can be used for growth.;In this research, we used a remote, low pressure electron cyclotron resonance plasma enhanced chemical vapor deposition system (ECR-PECVD) to deposit nanocrystalline Ge and Ge1-xCx films. Nanocrystalline germanium:H films were grown on glass and stainless steel substrates from a mixture of germane and hydrogen. X-ray diffraction spectra revealed a predominant <220> orientation in the films. Raman spectra showed a sharp peak at 300 cm -1. The grain size in the films could be controlled by hydrogen dilution during growth, with higher dilutions leading to a smaller grain size. Grain size varied between 15 nm and 74 nm. Hall measurements showed that as-grown films were always n type, with carrier concentrations in the 1016/cm 3 range. The mobility of electrons was shown to increase with increasing grain size, with the highest mobility being 5.4 cm2/V-sec at 300 K. Mobility and carrier concentration both increased with increasing temperature, the latter observation implying that there is a distribution of deep states in the material. p+nn+ devices were made with nanocrystalline germanium base layer and they showed significant increase in short-circuit current compared to nanocrystalline silicon devices. This is attributed to the higher absorption of the material down to the infrared region as shown by the quantum efficiency (QE) measurement. Defect density measured by C-V measurement was in the 10 17 cm-3 range. Diffusion length was measured using QE vs. voltage techniques and was estimated to be ~0.5 mum.;To add C into the lattice CH4 or C2H4 gas was added to the stock gas mixture. Absorption curve shifts to higher energy with higher C content. Incorporation of C atoms into the Ge lattice was shown to noticeably degrade the material on both mobility and crystallinity. However device performance is still reasonable and the QE curve shifts to higher energy, thus showing the photovoltaic potential of the material.
Digital Repository @ Iowa State University, http://lib.dr.iastate.edu
Niu, Xuejun, "Growth and properties of nanocrystalline Ge and Ge[subscript 1-x]C[subscript x] films and photovoltaic devices " (2005). Retrospective Theses and Dissertations. 1252.