MOVPE GROWTH OF HOMOEPITAXIAL GERMANIUM CELLS FOR PHOTOVOLTAIC AND THERMOPHOTOVOLTAIC APPLICATIONS USING ISO-BUTHYL GERMANE AS ORGANIC PRECURSOR

摘要

In the present work Ge layers were epitaxially deposited on Ge by Metal Organic Vapour Phase Epitaxy (MOVPE) using Iso-Butyl Germane (iBuGe) as organic precursor Samples were deposited using either only iBuGe or adding doses of AsH_3 as surfactant to obtain a better surface morphology. The nominally undoped layers were analysed by Atomic Force Microscopy (AFM), Transmission Electron Microscopy (TEM) and High Resolution X-Ray Diffraction (HRXRD). When iBuGe partial pressure was below 4×10~(-6), the Ge layers showed good morphology and crystallographic quality but for higher values of iBuGe partial pressure pits on the surface were evidenced by both AFM and TEM, with the pit density related to the growth rate. Better results in terms of surface morphology and FWHM x-ray diffraction profile were obtained on exactly oriented (001) Ge substrates. A good Ge/Ge interface with a low defect density was observed by TEM cross section micrographs. The use of AsH_3 as surfactant permitted to increase the iBuGe partial pressure used in the growth without degrading the layer properties. A decrease in the growth rate up to 4 times was observed with the use of surfactant. HR-XRD measurements evidenced a good crystal structure of the epitaxial germanium layer, while TEM revealed the presence of a high defect density at the interface. Using a p-type substrate as base and an n-type Ge layer as emitter, a p-n junction was realised by standard photolitographic techniques and Au as ohmic contacts. The I-V characterisation on these non passivated junctions showed a good rectifying behaviour and an open circuit voltage (VOC) comparable to the one reported in literature for diffused Ge cells.