MOCVD growth and optical properties of non-polar (1 1–2 0) a-plane GaN on (1 0–1 2) r-plane sapphire substrate Hongbo Yu a,n , Mustafa Ozturk a , Pakize Demirel a , Huseyin Cakmak a , Ekmel Ozbay a,b,c a Nanotechnology Research Center, Bilkent University, Bilkent, 06800 Ankara, Turkey b Department of Physics, Bilkent University, Bilkent, 06800 Ankara, Turkey c Department of Electrical and Electronics Engineering, Bilkent University, Bilkent, 06800 Ankara, Turkey article info Article history: Received 28 May 2010 Received in revised form 7 August 2010 Accepted 27 August 2010 Communicated by R. Fornari Available online 28 September 2010 Keywords: A1. Atomic force microscopy A1. Crystal structure A1. X-ray diffraction A3. Metalorganic vapor phase epitaxy B1. Nitrides B2. Semiconducting gallium compounds abstract Non-polar a-plane GaN film with crystalline quality and anisotropy improvement is grown by use of high temperature AlN/AlGaN buffer, which is directly deposited on r-plane sapphire by pulse flows. Compared to the a-plane GaN grown on AlN buffer, X-ray rocking curve analysis reveals a remarkable reduction in the full width at half maximum, both on-axis and off-axis. Atomic force microscopy image exhibits a fully coalesced pit-free surface morphology with low root-mean-square roughness ( 1.5 nm). Photoluminescence is carried out on the a-plane GaN grown on r-plane sapphire. It is found that, at low temperature, the dominant emission at 3.42 eV is composed of two separate peaks with different characteristics, which provide explanations for the controversial attributions of this peak in previous studies. & 2010 Elsevier B.V. All rights reserved. 1. Introduction In recent years, non-polar and semi-polar planes of III-nitride have attracted much attention because of the benefits of completely or partially avoiding the polarization-induced electric fields along the c-axis [1–5]. Specially, a-plane (1 1–2 0) GaN film and heterostructures grown on r-plane (1 0–1 2) sapphire have great potential due to the high thermal stability, low price, and technological maturity of the sapphire substrate. However, planar a-plane GaN film grown on r-plane sapphire still suffers from very high density of crystal defects, such as threading dislocations (TDs) and basal-plane stacking faults (BSFs) [6,7], which deterio- rate device performance and reduce the light emitting efficiency. In order to improve the crystal quality of the a-plane GaN film, AlN and GaN nucleation or buffer layers were used and optimized [2,8–10], similar to the c-plane GaN growth [11,12]. Recently, Dai et al. [13] reported that the low temperature AlGaN nucleation layer showed superior properties than those of the AlN and GaN nucleation layer due to the reduction in lattice mismatch with the r-plane sapphire and a-plane GaN. However, little work has been reported on the growth of high temperature AlN/AlGaN on r-plane sapphire substrates as a buffer for subsequent GaN growth. In the present study, we demonstrate the growth of high temperature AlN/AlGaN structure by novel pulse flows, and comparatively investigate the effects of AlN/AlGaN and AlN buffer layers on the crystalline quality of a-plane GaN films. The optical properties of a-plane GaN were also studied by photolumines- cence (PL) characterizations. 2. Experimental details The samples were all grown on double-polished (1–1 0 2) r-plane sapphire by a low-pressure metalorganic chemical vapor deposition (MOCVD) technique. Trimethylgallium (TMGa), trimethylaluminium (TMAl), and NH 3 were used as the precursors for Ga, Al, and N, respectively. Initially, the sapphire substrate was annealed at 1050 1C for 10 min in order to remove the surface contaminants. Then, 250 nm-thick (1 1–2 0) a-plane AlN and AlN/ AlGaN films were directly grown on r-plane sapphire as a buffer for subsequent GaN growth. Fig. 1 shows the gas flow sequences that were used for the AlN and AlN/AlGaN structure growths. For the AlN film (sample A), TMAl flow was continuous during the NH 3 pulse-flow sequence, similar to the previous report [14]. For sample B, the NH 3 and TMGa sources were simultaneously supplied by pulse flows while the TMAl flow was continuous, which results in an AlN/AlGaN multilayer structure. The MOCVD growth pressure, temperature, and V/III ratio were controlled to 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.2010.08.052 n Corresponding author. Tel.: + 90 312 290 1024; fax: + 90 312 290 1015. E-mail addresses: yuhongbows@gmail.com, hongboyu@bilkent.edu.tr (H. Yu). Journal of Crystal Growth 312 (2010) 3438–3442