Interface of GaN grown on Ge(1 1 1) by plasma assisted molecular beam epitaxy R.R. Lieten a,b,n , O. Richard b , S. Degroote b , M. Leys b , H. Bender b , G. Borghs b a Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA b IMEC, Kapeldreef 75, 3001 Leuven, Belgium article info Article history: Received 22 January 2010 Received in revised form 8 October 2010 Accepted 24 October 2010 Communicated by: E. Calleja Available online 28 October 2010 Keywords: A1. Diffusion A1. Interfaces A3. Molecular beam epitaxy B1. GaN B1. Germanium B1. Ge3N4 abstract Early efforts to grow GaN layers on germanium substrates by plasma assisted molecular beam epitaxy led to GaN domains, rotated by 81 relative to each other. Increased insight in the growth of GaN on germanium resulted in the suppression of these domain and consequently high quality layers. In this study the interface of these improved layers is investigated with transmission electron microscopy. The GaN layers show high crystal quality and an atomically abrupt interface with the Ge substrate. A thin, single crystalline Ge 3 N 4 layer is observed in between the GaN layer and Ge substrate. This Ge 3 N 4 layer remains present even at growth temperatures (850 1C) far above the decomposition temperature of Ge 3 N 4 in vacuum (600 1C). Triangular voids in the Ge substrate are observed after growth. Reducing the Ga flux at the onset of GaN growth helps to reduce the triangular defect size. This indicates that the formation of voids in the Ge substrate strongly depends on the presence of Ga atoms at the onset of growth. However complete elimination was not achieved. The formation of voids in the germanium substrate leads to diffusion of Ge into the GaN layer. Therefore we examined the diffusion of Ge atoms into the GaN layer and Ga atoms into the Ge substrate. It was found that the diffusion of Ge into the GaN layer and Ga into the Ge substrate can be influenced by the growth temperature but cannot be completely suppressed. Our results suggest that Ga atoms diffuse through small imperfections in the Ge 3 N 4 interlayer and locally etch the Ge substrate, leading to the diffusion of Ga and Ge atoms. & 2010 Elsevier B.V. All rights reserved. 1. Introduction Group III-Nitrides show physical properties interesting to many electronic and optoelectronic devices. Nitride materials are pre- dominantly grown by heteroepitaxy on foreign substrates [1]. Metal organic vapor phase epitaxy and molecular beam epitaxy (MBE) are the most important growth techniques. Sapphire, SiC, and Si substrates are most commonly used for heteroepitaxial growth of nitrides. We have shown that heteroepitaxial growth of GaN on Ge(1 1 1) by plasma assisted MBE (PAMBE) can lead to GaN layers of high crystal quality [2]. The use of Ge substrates for III-Nitrides growth, and in particular InGaN, could be advantageous for vertical conduction. The commonly used substrates for InGaN epitaxy, namely sapphire, SiC and Si, require high bandgap or low- temperature and therefore low-quality buffer layers, and further- more sapphire is an insulator. These substrates are therefore not well suited for vertical current conduction between the III-Nitride surface and the backside of the substrate. A direct photo electro- lysis cell, using photocurrent to split H 2 O into H 2 and O 2 , is an example of a device, which would benefit from vertical conduction, as back contacts can be applied in such a design. Another promising application is a high-efficiency solar cell, with InGaN absorbing the UV and visible part and germanium the infrared part of the solar spectrum. Previous structural characterization revealed that the GaN layers grown on germanium consisted of misoriented domains [3]. These domains are GaN grains rotated relative to each other around the GaN- (0 0 0 1) zone axis by about 41 clockwise and counterclockwise with respect to the Ge substrate [3]. Recently we showed that the formation of rotated domains can be suppressed by enhancing step flow growth with respect to 2D nucleation [4]. This can be achieved by increasing the growth temperature, decreasing the N flux or increasing the step density [4]. Character- ization by high resolution X-ray diffraction (HRXRD) of a 47 nm thick layer of GaN revealed a (0 0 0 2) and (1 0 1 ¯ 2) o full width at half maximum (FWHM) value of respectively 408 and 935 arc sec. This is comparable to the best GaN grown on Si(1 1 1) by metal organic chemical vapor phase deposition, obtained by growing 500 nm GaN and using AlN and AlGaN buffer layers in between the GaN and Si substrate [5]. Our GaN layers show comparable quality for a thickness of only 47 nm GaN and without using any buffer layer, which is, as mentioned before, useful for vertical devices. In this study we present images obtained by transmission electron microscopy (TEM) and scanning TEM (STEM) on the interface Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jcrysgro Journal of Crystal Growth 0022-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2010.10.146 n Corresponding author at: IMEC, Kapeldreef 75, 3001 Leuven, Belgium. E-mail addresses: Ruben.Lieten@gmail.com, Ruben.Lieten@imec.be (R.R. Lieten). Journal of Crystal Growth 314 (2011) 71–75