Article pubs.acs.org/crystal
Indium Incorporation Induced Morphological Evolution and Strain Relaxation of High Indium Content InGaN Epilayers Grown by Metal−Organic Chemical Vapor Deposition Jianxun Liu,† Hongwei Liang,*,† Xiaochuan Xia,† Yang Liu,† Jun Liu,† Qasim Abbas,† Rensheng Shen,† Yingmin Luo,† Yuantao Zhang,‡ and Guotong Du†,‡ †
School of Physics, School of Microelectronics, Dalian University of Technology, Dalian 116024, P. R. China State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, P. R. China
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ABSTRACT: The indium (In) incorporation induced morphological evolution and strain relaxation of high In content InGaN epilayers grown by metal−organic chemical vapor deposition (MOCVD) were investigated. With the decrease of growth temperature from 753 to 627 °C, In incorporation increases from 0.10 to 0.42, and the surface morphology evolves from initially mound-like three-dimensional surface roughness to progressive smoothness. Such morphology evolution mechanism can be well accounted for by a selfregulating model of islands’ proportions considering both strain relaxation and In surfactant effect comprehensively. Additionally, the strain relaxation and microstructural defects of such InGaN epilayers were investigated by X-ray diffraction reciprocal space mapping and cross-sectional transmission electron microscopy, respectively. It is found that, with the increase of In content, plastic relaxation via generating random stacking faults along with the surface sawtooth roughening becomes a more important strain relaxation mechanism. The presented results contribute to better understanding of the microscopic nature and growth mechanisms of high In content InGaN epilayers grown by MOCVD.
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INTRODUCTION Considerable attention has been focused on the InGaN alloy during the past decades due to its fascinating applications in highly efficient solid-state lighting, photovoltaic cells, and optical telecommunication.1−5 Although tremendous progress has been made in the field of blue-green optoelectronic devices based on InGaN, there are numerous hurdles to achieve highquality InGaN films with high indium (In) content.6,7 The difficulty lies in weak In−N bond, strong misfit strain due to large lattice mismatch (∼11%) between InN and GaN, and associated low miscibility of the alloy composition.8,9 Up to now, relatively less effort has been devoted to high In content InGaN alloys (>30%), and the growth and strain relaxation process for high In content InGaN is still controversial. In order to dwell more into InGaN alloys, Pereira et al.10 carefully investigated the evolution of a three-dimensional (3D) surface morphology for InGaN layers grown increasingly in excess of the critical layer thickness at a fixed composition (22%). Their results clearly evidenced that the 3D growth mechanism results in elastically relaxed InGaN. Subsequently, Valdueza-Felip et al.11 reported the interplay between In incorporation and strain relaxation in high In content InxGa1−xN layers grown by molecular beam epitaxy (MBE). They found that the strain relaxation process is associated with an increase in the mosaicity of the layers and in the root-mean-square (RMS) surface © XXXX American Chemical Society
roughness. Moreover, for In content x = 0.13 to 0.48, best structural and morphological qualities were obtained under In excess conditions with 1−2 monolayer (ML) thick In coverage. However, the 3D growth involved strain relaxation is not invariably observed in the InGaN epilayer. Bazioti et al.12 systematically investigated the defects and strain relaxation of the InGaN epilayers grown by MBE, and found that no Stranski-Krastanov (S−K) related 2D-3D growth can be identified even in a high In content (x = 0.36) InGaN epilayer. They attributed this situation to the strain relaxation through introduction of misfit dislocations and multiple stacking faults (SFs) instead of 3D surface roughness. Despite all these efforts, the peculiarity of the In incorporation for InGaN alloys and their connections with the lattice relaxation and morphology evolution is still an issue needing further detailed study. Furthermore, previous literature mainly focused on MBE-grown high In content (x > 0.2) InGaN, which is surface kinetics-controlled and takes advantage of the low growth temperature (Tg) to avoid phase separation and decomposition.13 While for metal−organic chemical vapor deposition (MOCVD) grown high In content InGaN epilayers, Received: March 14, 2017 Revised: May 15, 2017 Published: May 18, 2017 A
DOI: 10.1021/acs.cgd.7b00365 Cryst. Growth Des. XXXX, XXX, XXX−XXX
Crystal Growth & Design
Article
The surface morphologies of the samples were studied by atomic force microscopy (AFM: Veeco D3100 system) operated in the tapping mode and field emission scanning electron microscopy (SEM: ZEISS, Germany). High-resolution XRD was performed in a Rigaku Ultima IV diffractometer using Cu Kα1 radiation. The strain states of the epilayers were characterized by XRD reciprocal space mapping (RSM) of (1015̅ ) diffraction, respectively. Cross-sectional TEM (XTEM) and high-resolution TEM (HRTEM) were performed using a 200 kV FEI Tecnai G2 F20 S-Twin scanning TEM.
which is governed by a diffusion process and differs from MBE,14 progress has often been limited by significantly degraded crystal quality due to the deficient ammonia cracking (
