Investigation of 1µm-Thick InGaN Films Grown on O-face ZnO by Plasma-Assisted Molecular Beam Epitaxy

Abstract

Abstract Body: The (In,Ga)N alloy system is attractive for optoelectronic applications as it enables direct bandgap tuning within a large range (0.7 eV–3.4 eV). While highly efficient GaN-based blue light emitting diodes (LEDs) have been commercially available for a decade, GaN red LEDs suffer from low efficiency. Red LEDs require high In content InGaN (>35%) QWs. Growth of high In-content InGaN on the GaN substrate is challenging due to high vapor pressure of InN. InN starts decomposing at 500 °C which is well below optimum GaN growth temperature of ~700 °C in molecular beam epitaxy (MBE). There is a large lattice mismatch between InN and GaN (∼10%). Therefore, there is a great deal of interest in developing relaxed InGaN buffer as pseudo-substrate for optoelectronics applications. Various techniques have been employed in the past to obtain relaxed InGaN buffer layers including graded InGaN transition film and growth of InGaN on micrometer-size tiles of porousified GaN. Growth on oxide substrates, such as ScAlMgO4 and ZnO have also been explored as an alternative. ZnO has the same crystal structure as (In,Ga)N (wurtzite) and is in-plane lattice-matched with In0.2Ga0.8N [1]. Recently, we demonstrated growth of high-quality ∼300 nm-thick (In,Ga)N films with the In content varying from 11% to 23% on ZnO [2]. For this purpose, we developed low-temperature atomically smooth GaN thin films (~2 monolayers (ML)) to suppress the interfacial reaction between nitrides and the ZnO substrate at elevated temperatures using metal-enhanced epitaxy (MEE). In this talk, we will present successful growth of high-quality 1µm-thick InGaN with In content ranging from 19.5% to 30.5% on ZnO substrate by plasma-assisted molecular beam epitaxy. PL analysis showed a relatively uniform InGaN composition on samples grown at 600 °C and 580 °C. Lowering the growth temperature to 560 °C led to clusters with higher In content. The uniformity of low temperature InGaN growth was improved by periodic desorption of In from the surface. Although, STEM image showed a high threading dislocation density generated at the substrate interface, TDD reduced significantly through annihilation of dislocations as InGaN thickness was increased. A self-assembled superlattice structure of the InGaN on ZnO was confirmed by scanning transmission electron microscopy and atom probe tomography. We will show the relaxation of InGaN on ZnO by XRD reciprocal space map measurement. Reference: 1. A. Kobayashi et al. Pulsed sputtering epitaxial growth on m-plane InGaN lattice matched to ZnO, Sci. Rep. 7 1-6 (2017). 2. K Khan, Growth of high quality (In,Ga)N films on O-face ZnO substrates by plasma-assisted molecular beam epitaxy, AIP Adv. 10 075120 (2020)