High-reflectivity ultraviolet AlN/AlGaN distributed Bragg reflectors grown by metalorganic chemical vapor deposition Jun-Rong Chen, Shih-Chun Ling, Chin-Tsang Hung, Tsung-Shine Ko, Tien-Chang Lu à , Hao-Chung Kuo, Shing-Chung Wang Department of Photonics & Institute of Electro-Optical Engineering, National Chiao Tung University,1001 University Road, Hsinchu 30050, Taiwan article info Available online 20 August 2008 PACS: 78.55.Cr 61.66.Dk 68.55.Jk 78.66.Fd Keywords: A1. Crystal morphology A3. Metalorganic chemical vapor deposition B1. Nitrides B2. Semiconducting aluminum compounds abstract High-reflectivity ultraviolet distributed Bragg reflectors (DBRs), based on AlN/AlGaN quarter-wave layers, have been designed and grown on 2 in (0 0 0 1) sapphire substrates by metalorganic chemical vapor deposition. The growth of 20-pair AlN/Al 0.23 Ga 0.77 N DBR shows no observable cracks in the structure and achieves peak reflectivity of 90% at 367 nm together with a stop-band width of 24 nm. Furthermore, the growth of 34-pair AlN/Al 0.23 Ga 0.77 N DBR shows partial cracks from optical microscopy images. According to the room-temperature photoluminescence measurement, the emission spectrum of 34-pair AlN/Al 0.23 Ga 0.77 N DBR is broader than that of 20-pair AlN/Al 0.23 Ga 0.77 N DBR, which could be due to strain inhomogeneity generated by cracks. Despite the crystal quality problem, the peak reflectivity of 34-pair AlN/Al 0.23 Ga 0.77 N DBR could still achieve 97% at 358 nm and the stop-band width is 16 nm. & 2008 Elsevier B.V. All rights reserved. 1. Introduction High-reflectivity nitride-based distributed Bragg reflectors (DBRs) are important for the development of GaN-based optical devices such as resonant-cavity light-emitting diodes (RCLEDs) [1] and vertical-cavity surface-emitting lasers (VCSELs) [2]. Recently, nitride-based microcavity structures have attracted much atten- tion due to the investigation of fundamental phenomena includ- ing strong light-matter interaction [3–5], solid-state cavity quantum electrodynamics (CQED) [6], and dynamical Bose– Einstein condensates [7]. As far as nitride-based microcavities designed for the study of strong coupling phenomena are concerned, the active regions made of GaN bulk or GaN/AlGaN multiple quantum wells (MQWs) are preferred due to the relatively narrow photoluminescence (PL) linewidth as compared with InGaN/GaN MQWs. Therefore, high-quality nitride-based ultraviolet DBRs with a wavelength around 360nm at the center of the stop band are essential approach to fabricate nitride-based microcavities for the study of strong exciton–photon coupling. In general, ultraviolet nitride-based DBRs consist of AlInN/AlGaN or AlGaN/AlGaN material system. Especially, for devices containing pure GaN as an active medium, the ultraviolet DBRs cannot employ GaN as the layer materials because of the strong increases of optical absorption below 360 nm. Regarding the development of the ultraviolet DBRs for the microcavity structures made of active regions emitting in the 340–370nm range, several experi- mental results have been demonstrated. Crack-free lattice- matched ultraviolet AlInN/AlGaN DBRs have been achieved by Feltin et al. [8] in 2006. The DBRs exhibit a reflectivity higher than 99% at a wavelength as short as 340 nm and a stop-band width of 20 nm. Nevertheless, the growth of high-quality AlInN film is difficult due to the composition inhomogeneity and phase separation in AlInN, which results from large mismatch of covalent bond length and growth temperature between InN and AlN [9]. High-reflectivity ultraviolet AlGaN/AlGaN DBRs with peak reflectivity of 99% are mostly grown using molecular-beam epitaxy (MBE) system [10,11]. Although the MBE system can provide high-quality epitaxial film and sharp heterostructure, it is time-consuming for the growth of high-reflectivity nitride-based DBRs as a high number of pairs are required to overcome the relatively small refractive index contrast between bilayer con- stituents in the nitride material system. As for AlGaN-based ultraviolet DBRs grown by metalorganic chemical vapor deposi- tion (MOCVD) system, Wang et al. [12] have reported crack-free 25-pair AlGaN/AlGaN DBRs with a peak reflectivity of 91% at 353 nm and a stop-band width of 17 nm in 2004. Moreover, they demonstrated 25-pair AlGaN/AlGaN DBRs with peak reflectivity of 94% around 356nm and a stop-band width of 17 nm in 2005 [13]. Ji et al. [14] reported nearly crack-free 30-pair AlGaN/AlN DBRs with a peak reflectivity of 93% at a wavelength as short as 313nm in 2007. However, the peak reflectivity values of most AlGaN- based ultraviolet DBRs grown by MOCVD system are relatively ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jcrysgro Journal of Crystal Growth 0022-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2008.08.025 à Corresponding author. Tel.: +886 3 5712121x31234; fax: +886 5 5716631. E-mail address: timtclu@mail.nctu.edu.tw (T.-C. Lu). Journal of Crystal Growth 310 (2008) 4871–4875