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Indium Gallium Nitride on Germanium by Molecular Beam Epitaxy R.R. Lieten1,2 , W.-J. Tseng1,2, M. Leys2, J.-P. Locquet1, J. Dekoster2 1 Department of Physics and Astronomy, K.U. Leuven, 3001 Leuven, Belgium 2 IMEC, 3001 Leuven, Belgium

ABSTRACT Indium containing III-Nitride layers are predominantly grown by heteroepitaxy on foreign substrates, most often Al2O3, SiC and Si. We have investigated the epitaxial growth of InxGa1-xN (InGaN) alloys on Ge substrates. First we looked at the influence of buffer layers between the InGaN and Ge substrate. When applying a high temperature (850 ÛC) GaN buffer, the InGaN showed superior crystal quality. Furthermore the influence of growth parameters on the structural quality and composition of InGaN layers has been looked into. For a fixed gallium and nitrogen supply, the indium beam flux was increased incrementally. For both nitrogen- as well as for metal (Ga + In) rich growth conditions, the In incorporation increases for increasing In flux. However, for metal rich growth conditions, segregation of metallic In is observed. An optimum in crystal quality is obtained for a metal:nitrogen flux ratio close to unity. The XRD FWHM of the GaN (0002) reflection increases significantly after InGaN growth. Apparently the presence of indium deteriorates the GaN buffer during InGaN growth. The mechanism of the effect is not known yet. INTRODUCTION Single crystalline indium containing III-Nitrides show interesting electrical and optical properties. The ability to tune the direct band gap by changing the alloy composition is promising for solar matched photovoltaics. Indium containing III-Nitride layers are predominantly grown by heteroepitaxy on foreign substrates, most often Al2O3, SiC and Si [1,2,3]. This material system is promising for photovoltaic applications [2,3,4,5]. Better understanding of the growth processes is needed to improve the structural and optoelectronic properties. In this work we investigate the heteroepitaxial growth of Indium Gallium Nitride layers on germanium substrates with molecular beam epitaxy (MBE). Previously we have demonstrated heteroepitaxial growth of GaN on Ge(111) by plasma assisted molecular beam epitaxy (PAMBE) [6]. The use of Ge substrates for III-Nitrides growth, and in particular InGaN, could be advantageous for devices in which vertical conduction is required [7]. A direct photo electrolysis cell, using photocurrent to split H2O into H2 and O2, is an example of a device, which would benefit from vertical conduction. Using a back contact can much simplify such a design. Another promising application of InGaN on conducting Ge substrates is a high-efficiency solar cell. InGaN can absorb the UV and visible part of the solar spectrum and germanium the infrared part. In this work we report on the influence of buffer layers and growth parameters on the structural quality and composition of InGaN layers grown on Germanium (111) substrates. X-ray

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diffraction (XRD) has been used to determine the indium content, crystal quality, the homogeneity and the p