Influence of buffer layers on the microstructure of MOVPE grown a-plane InN Masihhur R. Laskar a,Ã , Tapas Ganguli b , Abdul Kadir a , Nirupam Hatui a , A.A. Rahman a , A.P. Shah a , M.R. Gokhale a , Arnab Bhattacharya a a Department of Condensed Matter Physics and Materials Science, Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400005, India b Raja Ramanna Center for Advanced Technology, Indore 425013, India article info Available online 19 August 2010 Keywords: A1. High resolution X-ray diffraction A3. Metalorganic vapor phase epitaxy B1. Non-polar B2. Semiconducting III–V materials abstract We report a comparative study of the microstructure of a-plane ð11 20Þ InN epilayers grown on different buffer layers via metalorganic vapour phase epitaxy. Under optimized growth conditions, the crystalline quality of the InN epilayer is found to be best on an AlN buffer layer compared to that on GaN buffer layers or InN nucleation layers. All a-plane InN epilayers show an anisotropy in the in-plane mosaicity with the in-plane tilt value being different along the c- and m-directions. We also observe a Nagai-like tilt in all the epilayers. The magnitude and nature of this anisotropy and the Nagai-like tilt is also strongly influenced by the buffer layer. Our investigations show that InN grown on an AlN buffer has smaller in-plane tilt, basal plane stacking fault density, and background carrier concentration resulting in a higher mobility and a low band gap energy of  0:70 eV. & 2010 Elsevier B.V. All rights reserved. 1. Introduction There has been considerable research into improving the quality of InN epilayers in the past decade, motivated by the promise of optoelectronic device applications of InN that arise from its low ð 0:7 eVÞ bandgap, small effective mass, and high electron saturation velocity [1], with extensive work on optimiz- ing the growth of InN epilayers via metalorganic vapour phase epitaxy (MOVPE) being reported [2–4]. Detailed studies of the microstructure of InN epilayers grown in the typically used c-plane orientation have been carried out [5,6]. However, undesirable spontaneous polarization and piezoelectric fields in the c-plane oriented epilayers impair the performance of III-nitride devices, and there is a quest to grow nitride materials along the semi-polar or non-polar direction where the electric field is minimum or zero [7]. Among the III-nitrides, the MOVPE growth of InN is perhaps the most challenging due to the weak In–N bond, low decomposition temperature, poor ammonia decomposition at the low growth temperature. The epitaxial growth of InN in non-polar directions is much more challenging than growth of c-plane oriented films, because the different growth mechanisms are not well understood. There are few reports of growing a-plane ð11 20Þ InN by molecular beam epitaxy [8,9] and MOVPE [10,11]. In our previous work [11] we have explored the growth parameter space for a-plane InN in a close-coupled showerhead reactor and optimized the growth conditions for a-plane InN on GaN buffer layer. Under those optimized growth condition we have grown a-plane InN on different buffer layers. In this work we compare the structural, electrical, optical and morphological properties of a-plane InN grown on r-plane ð1 102Þ sapphire using GaN, AlN, and low- temperature InN buffer layers. We find that the best crystalline quality InN material is obtained for growth on an AlN buffer layer, that results in significantly improved crystalline quality, which is one of the best reported by MOVPE so far. 2. Experiment InN epilayers were grown via MOVPE in a 3  2 in. close- coupled showerhead reactor (Thomas Swan). Trimethylgallium (TMGa), trimethylaluminium (TMAl), trimethylindium (TMIn) and ammonia (NH 3 ) were used as precursors and getter-purified nitrogen (N 2 ) as carrier gas. Details of the growth procedure can be found in our earlier optimization study [11] from where the best conditions for the growth of a-plane InN were found to be temperature T ¼ 550 1C, pressure P ¼ 500 Torr, and V/III ¼ 11,000, at a TMIn flow of 200 sccm ð12:4 mmol=minÞ. To compare the effect of different buffer layers, about 0:2 mm thick InN epilayers were deposited on different buffer layers deposited on r-plane sapphire substrates. Apart from the standard 1 mm thick a-plane GaN buffer grown at 1040 1C temperature, 80 nm 3-stage AlN buffer layer (sequential deposition of 10 nm at 700 1C, 35 nm at 900 1C, 35 nm at 1040 1C) and 25 nm InN nucleation layer deposited at 500 1C were used. The thickness of the layers was estimated via in-situ diode laser reflectometry. The structural Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jcrysgro Journal of Crystal Growth 0022-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2010.08.019 Ã Corresponding author. E-mail address: laskar@tifr.res.in (M.R. Laskar). Journal of Crystal Growth 315 (2011) 233–237