Nucleation and growth mode of the molecular beam epitaxy of GaN on 4H–SiC exploiting real time spectroscopic ellipsometry

Abstract

The heteroepitaxy of GaN directly grown directly on Si-face 4H–SiC(0 0 0 1) by molecular beam epitaxy is explored and characterized using in situ spectroscopic ellipsometry. Critical steps of the process, including SiC substrate cleaning, substrate termination through nitridation, GaN nucleation and growth, are monitored in real time. Some key relationships between growth mode, as observed by ellipsometry, and material properties are given.

Introduction

III-Nitrides on sapphire have seen great success for a range of applications, including UV-to-visible light-emitting diodes (LEDs), laser diodes (LDs) and photodetectors. III–N electronic devices operating at high power and high frequency require high resistivity and high thermal conductivity substrates. For this application, SiC is considered the most suitable choice because of its high thermal conductivity and relatively low lattice mismatch (3.4% with GaN and 0.9% with AlN) with III–Ns, which is expected to reduce interface defects resulting from the heteroepitaxial growth process, and yield improved structural quality of device layers. The heteroepitaxy of GaN on SiC often exploits an AlN buffer layer [1], [2] since GaN buffer layers yield high conductivity at the substrate-epitaxial layer interface. The growth of AlN and GaN on SiC is different; for example AlN wets SiC more effectively at the initiation of epitaxy [3]. Despite that, direct comparison of the structural quality of films on AlN buffers with GaN buffers shows the GaN buffers have higher structural perfection. Therefore, there is interest in characterizing the initial growth stages of GaN on SiC using a variety of buffers. Herein, we exploit in-situ spectroscopic ellipsometry (SE) to better understand GaN heteroepitaxy.

In order to obtain device-quality GaN, the preparation of the SiC substrate surface is crucial, as well as the choice of growth conditions. Previous research [4], [5] shows that GaN nucleation on SiC depends critically on the cleaning and termination of the SiC substrate prior to growth. However, those studies rely on interrupting growth and imaging the GaN nucleation layer on SiC with ex situ atomic force microscopy. In contrast, SE provides non-invasive and non-destructive real time monitoring of the heteroepitaxy growth mode yielding structural and mechanistic information.

In this paper, we discuss the direct growth of GaN on the Si-face 4H–SiC(0 0 0 1) substrates by molecular beam epitaxy (MBE). Spectroscopic ellipsometry is used for in situ real time monitoring of all the critical steps involved in the GaN growth process, from the substrate preparation to the epitaxial growth. It is shown that SE is sensitive to the removal of contaminants and the native oxide from the SiC surface, through the measurement of the residual absorption in the SiC pseudodielectric function. In addition, the extent of SiC nitridation, carried out intentionally or unintentionally prior to epitaxy can be monitored through the variation of the SiC dielectric function upon exposure of the surface to nitrogen. The impact of the SiC pre-treatment on GaN nucleation and epitaxial growth can be investigated exploiting ($\Psi ,\Delta$) trajectories, which can be modelled yielding information on the growth mode. An island nucleation model is used to interpret the initial GaN nucleation step, after which layer-by-layer growth of GaN on SiC occurs. The optical properties, such as the fundamental exciton transition, the dielectric function, refractive index, and extinction coefficient are determined. The spectroscopic ellipsometry data are complemented by reflection high energy electron diffraction (RHEED) and X-ray diffraction (XRD) measurements.