GaN MOCVD on Si via single crystal rare-earth oxide buffer layer

GaN layers were grown on Si (111) by MOCVD using epitaxial single crystal rare-earth oxide buffer which helps to solve mechanical strain related problems arising due to lattice and thermal expansion mismatch between the III-N layer and the substrate. Chemical stability of the rare-earth oxide in contact with silicon as well with GaN is analyzed. Transformation of rare-earth oxide to rare earth nitride during GaN MOCVD process is studied. It is demonstrated that the oxide layer serves as Si diffusion barrier to GaN thus preventing back-etching effect. INTRODUCTION Efforts to grow GaN on large diameter Si wafers are driven by lower substrate price compared to native GaN or other foreign substrates like sapphire, SiC, as well as possibility to use depreciated Si fabs and thus reduction of cost of GaN wafer. The technology is already adopted by some major companies for fabrication of GaN power devices and, in some less extend, for light emitting diodes (LED) on 150 nm Si wafers and recently on 200mm wafers, as for example it was advertised by IMEC [1]. Generally, the GaNon-Si epitaxy requires AlN buffer layer which serves for strain management and prevention of Si diffusion to GaN. Still, AlN is not ideal material for either of the tasks. For example, Si diffusion which results in unintentional doping of a GaN layer still remains an issue because of Si solubility in AlN at eutectic point of 577°C. Application of other types of the buffer layers are considered [2] [3]. We introduce new material engineering solutions for the GaN-on-Si process by utilization of an epitaxialy grown single crystal rare-earth oxide buffer on Si. It offers flexibility to the epitaxy process by opening up possibility to use approaches typical for GaN epitaxy either on sapphire or Si: GaN or AlN first, respectively. In this work we study advantages of the oxide buffer as well as its chemical and structural properties that influence process at interface oxide/silicon and oxide/III-N. EXPERIMENT The erbium oxide was grown in a custom-made solid source epitaxy (SOE) system capable to handle up to 200 mm diameter wafers. HF dipped Si(111) wafers were loaded into the reactor and heated up there for 20 min at 750 °C in a low silicon flux in order to remove residual silicon dioxide from the surface and to obtain a flat 7x7 reconstructed silicon surface. The erbium oxide layers were grown at a growth rate of 1 μm/hour by the evaporation of the metal from an effusion cell, and molecular oxygen was delivered from a gas manifold. Oxygen partial pressure in the chamber was approximately 5.2 x10 mbar during oxide growth. The substrate temperature was 720 ̊C during growth of the oxides. Thomas Swan close shower head reactor capable to process up to 200 mm wafers was used for GaN metal organic chemical vapour deposition (MOCVD). Trimethylgallium (TMG) and NH3 were used as gallium and nitrogen sources, respectively. RESULTS AND DISCUSSION Under optimized SOE growth conditions as described in earlier works [4] [5], rare-earth oxide interface with silicon is abrupt with no silicon dioxide or silicide interlayer detectable (Fig 1a). However, during GaN MOCVD growth with temperatures above 1050oC, chemical reaction between silicon and the oxide takes place at the very interface between the layer and the substrate forming erbium silicate as can be identified from scanning transmission electron microscopy (STEM) (Fig. 1b) in combination with X-ray dispersion spectroscopy (EDX) (Fig. 2) and X-ray diffraction (XRD) peak at (Fig. 3). (a) (b) Figure 1. High resolution TEM image of Er2O3 layer on Si(111) before GaN MOCVD (a), and scanning TEM image of Er2O3 interface with Si(111) substrate after GaN MOCVD at 1150oC (b). Two peaks marked as (A) and (B) at Θ = 15.03o and Θ = 15.62o, respectively, could be attributed to polycrystalline erbium silicate as can be concluded from composition of the 5 nm 10 nm ErxSiyOz Er2O3