Growth and Properties of Nanocrystalline Ge and Ge1-xCx Films and Photovoltaic Devices
Niu, Xuejun (2005) Growth and Properties of Nanocrystalline Ge and Ge1-xCx Films and Photovoltaic Devices. PhD thesis, Iowa State University.
Full text available as:
Germanium, with its almost direct gap band structure and high carrier mobility, is an attractive material for photovoltaic applications. To overcome the disadvantage of its low band gap, carbon is added to the germanium lattice. 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 alloy 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 controlling the amount of hydrogen dilution during growth, with higher dilutions leading to a smaller grain size. Grain size varied between 15 nm and 74 nm. Hall measurements were used to characterize the electrical properties of the films, and they showed that as-grown films were always n type, with carrier concentrations in the 10^16/cm3 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+-n-n+ 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 1017 cm-3 range. Diffusion length was measured using QE vs voltage techniques and was estimated to be ~0.5 um. 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.
Archive Staff Only: edit this record