Fabrication and Characteristics of GaN-Based Microcavity Light-Emitting Diodes with High Reflectivity AlN/GaN Distributed Bragg Reflectors Yu-Chun PENG, Chih-Chiang KAO, Hung-Wen HUANG, Jung-Tang CHU, Tien-Chang LU, Hao-Chung KUO, Shing-Chung WANG  and Chang-Chin YU 1 Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 300 Taiwan, R.O.C. 1 Highlink Corporation, Hsinchu 300 Taiwan, R.O.C. (Received September 26, 2005; accepted December 21, 2005; published online April 25, 2006) In this paper, we report a GaN-based microcavity light-emitting diode (MCLED) which is composed of 25 pairs of high- reflectivity GaN/AlN distributed Bragg reflector (DBR) and 6 pairs of ex-situ deposited SiO 2 /TiO 2 dielectric mirrors. The electroluminescence peak of this structure matched well with the high reflectance area of the top and bottom DBRs, and shows a narrow emission of approximately 6.7 nm. The fabricated device also shows a more excellent performance on the stability of the emission peak wavelength while varying injection current density and operating temperature than a regular LED. [DOI: 10.1143/JJAP.45.3446] KEYWORDS: GaN, microcavity, MCLED, light-emitting diode Over the past few years, GaN-based III–V compound semiconductors have attracted great attention for the applications of short-wavelength optoelectronic devices due to their wide band gap structure. GaN-based light emitters, which include both light emitting diodes (LEDs) and laser diodes (LDs), are now used widely in many applications such as illumination, exterior automotive light- ing, printers, full color display, traffic signals, the back light of liquid crystal display and high-density data storage. 1–4) With the development of optically pumped GaN-based vertical-cavity surface-emitting layer (VCSEL), 5–8) the real- ization of an electrically pumping GaN-based VCSEL has become possible. Recently, some groups have demonstrated the microcavity light-emitting diodes (MCLEDs), 9–16) a structure which is similar to VCSEL. Such device has several advantages over VCSEL, such as having a circular beam shape, capability for light emission in a vertical direction can be used for a fully monolithic test and having two dimensional arrays. An epitaxially grown nitride dis- tributed Bragg reflector (DBR) was mainly utilized as the bottom mirror and a dielectric DBR was utilized as the top mirror to form the high-reflectivity micro-cavity structure. Most of these groups chose GaN/Al x Ga 1x N DBRs as the bottom mirror of the cavity. Diagne et al. 9) used 60 pairs of GaN/Al 0:25 Ga 0:75 N DBR to form the bottom mirror with 99% reflectivity. Arita et al. 14) employed 26 pairs of GaN/ Al 0:40 Ga 0:60 N DBR to form the bottom mirror with 91% reflectivity. All these AlGaN/GaN DBR structures required large numbers of pairs due to the relatively low refractive index contrast between Al x Ga 1x N and GaN. The DBR structure using AlN/GaN has a higher refractive index contrast (n=n ¼ 0:16) 17) that can achieve high reflectivity with relatively less numbers of pairs. Therefore, the AlN/ GaN is attractive for application in nitride VCSEL. Recently, we have developed a high-reflectivity AlN/GaN DBR structure with a peak reflectance of 94% and a stop bandwidth of approximately 18 nm with relatively smooth surface morphology. 18) Moreover, we also developed the laser operation of GaN-based VCSEL incorporating AlN/ GaN DBR by optical pumping. 19) In this paper, we report the fabrication and performances of GaN-based MCLED with a hybrid structure, composed of high-reflectivity, crack-free, wide-stop-bandwidth in situ-grown AlN/GaN bottom DBRs and ex situ-deposited SiO 2 /TiO 2 DBRs. The current and temperature dependent electroluminescence (EL) of the MCLED was investigated. The fabricated MCLED shows a relatively more stable EL than a conventional LED while varying injected current density and operating temperature. The nitride heterostructure was grown by the metal– organic chemical vapor deposition (MOCVD) system (EM- CORE D-75) on a polished optical-grade c-face (0001) 2-in. diameter sapphire substrate. The 30 nm-thick GaN buffer layer was first grown on the sapphire substrate at 530  C, then 1-mm-thick undoped GaN was grown on the buffer layer at 1040  C. Thereafter, the 25 pairs of quarter- wavelength GaN/AlN stack were grown as the high- reflectivity bottom DBR. Finally, the 3 InGaN/GaN microcavity structure was grown on top of the GaN/AlN DBR, composed of ten pairs of InGaN/GaN MQW layers, surrounded by Si-doped n-type GaN and Mg-doped p-type GaN layers. The reflectivity spectrum of the AlN/GaN DBR and the photoluminescence (PL) of the as-grown sample are shown in Fig. 1. The peak reflectance of the 25 pairs of GaN/AlN DBR was 94% at 450 nm. The PL spectrum was located at 458.5 nm with 10.5 nm FWHM and matched well with the high-reflectance area. The schematic diagram of the GaN-based MCLED is shown in Fig. 2. The MCLED was fabricated by the following steps. Initially, the 0.6 mm SiO 2 was deposited by plasma-enhanced chemical vapor deposition (PE-CVD) and patterned by photolithography to define the mesa region. Mesa etching was then performed with Cl 2 /Ar as the etching gas using an ICP-RIE system (SAMCO ICP-RIE 101iPH) in which both the ICP source power and bias power operated at 13.56 MHz. Then, we deposited a 0.3 mm SiNx layer as the current confinement layer and patterned the layer to define the current-injected aperture. The metal contact layers, including the transparent contact and pad layers, were patterned by a lift-off procedure and deposited onto samples by electron beam evaporation. Ni/Au (5/5 nm) was used for the transparent electrode, Ti/Al/Ni/Au (20/150/20/ 200 nm) was used for the n-type electrode and Ni/Au (20/ 150 nm) was deposited onto the p-type electrode. Finally, the  Author to whom correspondence should be addressed. E-mail address: scwang@cc.nctu.edu.tw Japanese Journal of Applied Physics Vol. 45, No. 4B, 2006, pp. 3446–3448 #2006 The Japan Society of Applied Physics 3446 Brief Communication