Properties of Nanocrystalline Germanium-Carbon Films and Devices

Abstract

Nanocrystalline Germanium-Carbon alloys, denoted by nc- (Ge,C):H, are a potentially useful new electronic material whose bandgap can be varied by changing the Ge:C ratios. We have shown previously that nanocrystalline (Ge,C):H films can be grown using remote ECR plasma deposition. In this paper, we report on the crystal structure, electron mobility and some device related properties of these materials. The materials were grown using mixtures of either Germane and methane, with significant hydrogen dilution, or from mixtures of ethylene and germane, also with significant hydrogen dilution. X-ray diffraction measurements indicated a predominantly <111> crystal structure. The grain size was in the range of 10 nm. Raman measurements clearly show the 300 cm-1 Ge peak in the films. Electron Hall mobilities were measured in these films and were found to be in the range of 2.5-3 cm2/V-sec. Proof-of-concept p+nn+ junction devices were fabricated and showed distinct photovoltaic properties. The open circuit voltage was found to be a strong function of the itnerfaces between n+ and n layers, and between p+ and n layers. The use of an amorphous n+/n interface at the back of the improved the device performance significantly. Capacitance measurements indicated that the device behaved according to standard p/n junction theory, with the n- doping in the base layer being in the 10¹⁷/cm³ range. Thus, the base layer was not intrinsic, but rather n type, as may be expected for a crystalline as opposed to amorphous material. Quantum efficiency data indicated that as C was added to the material, the edge of the QE curve shifted to higher photon energies, indicating larger bandgaps.