Type of presentation: Oral

MS-14-O-2233 Strain relaxation of high In-content InGaN epilayers grown by PAMBE

Bazioti C.1, Kehagias T.1, Walther T.2, Papadomanolaki E.3, Iliopoulos E.3, 4, Dimitrakopulos G. P.1
1Physics Department, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece, 2Department of Electronic & Electrical Engineering, University of Sheffield, Sheffield S1 3JD, UK, 3Microelectronics Research Group, Physics Department, University of Crete, P.O. Box 2208, 71003 Heraklion-Crete, Greece, 4IESL, FORTH, P.O. Box 1385, 71110 Heraklion-Crete
kbazio@physics.auth.gr

High indium content InGaN epilayers are particularly interesting for high-efficiency photovoltaic applications. However, defect reduction is crucial in order to increase the internal quantum efficiency of device structures. Such alloys generally exhibit complex microstructural behavior and strain relaxation that are very sensitive to the growth conditions, due to their intrinsic metastable character that leads to the phenomena of indium phase separation and composition pulling.
We have applied a combination of transmission electron microscopy (TEM) characterization techniques, including electron diffraction, HRTEM, geometrical phase analysis (GPA), z-contrast STEM, and EDX, together with high resolution x-ray diffraction (HRXRD), in order to elucidate the influence of strain relaxation on the defect content and indium compositional variations of high alloy concentration InGaN epilayers. Thin films of up to ~500 nm and 10-60% indium content were grown on (0001) GaN templates using plasma-assisted molecular beam epitaxy (PAMBE).
Strain relaxation was found to promote the introduction of a-type threading dislocations, as shown in Fig. 1. The emanation level of TDs was found to differ depending on the growth conditions. At higher growth temperatures or low indium fluxes, phase separation was observed leading to the formation of a strained InGaN interfacial interlayer of lower indium concentration as measured by EDX and GPA, which is shown in Fig. 2(a). Strain relaxation, manifested by the emanation of TDs, took place at and above this strained interlayer, as illustrated in Fig. 2(b). In addition to the discontinuous composition pulling, another characteristic feature of such phase separated epilayers was the appearance of multiple basal stacking faults (SFs) above the internal InGaN interface. Such SFs also acted as TD sources leading to increase of the defect content.
On the other hand, at lower growth temperatures or high indium incident fluxes, TD emanation commenced from the InGaN/GaN interface, showing that strain relaxation took place there (Fig. 3). This was further verified by the observation of regular arrays of misfit dislocations. The epilayer surface morphologies were correlated to the growth modes. Good quality epilayers with a ~40% indium content, showing no mesoscale phase separation, with smooth surfaces, were achieved.

This research has been co-financed by the European Union (European Social Fund – ESF) and Greek national funds through the Operational Program "Education and Lifelong Learning" of the National Strategic Reference Framework (NSRF) - Research Funding Program: THALES.

Fig. 1: Cross sectional TEM (XTEM) images of a 300 nm thick InGaN epilayer with 43% indium content grown under low indium flux, showing emanation of a-type TDs from a strained InGaN interfacial interlayer. (a) Dark field (DF) image with g 1-100. (b) Bright field (BF) image with g 0002.

Fig. 2: (a) Annular DF image of a 200 nm thick InGaN epilayer containing 18% In, grown at high temperature under stoichiometric flux. A self-formed interfacial InGaN layer (s-InGaN) is indicated. (b) HRTEM image showing emanation of TD half loops from the s-InGaN interface (arrows). The GaN/s-InGaN and s-InGaN/GaN interfaces are also indicated.

Fig. 3: XTEM image showing a 450 nm thick InGaN epilayer with 42% indium content showing no phase separation due to the lower growth temperature. (a) BF image with g 1-100. (b) BF image with g 0002.