High reflectivity distributed Bragg reflectors for 1.55 lm VCSELs using InP/airgap J.Y. Tsai, T.C. Lu, S.C. Wang * Institute of Electro-Optical Engineering, National Chiao Tung University, 1001 Ta Hsueh Road, Hsinchu 30050, Taiwan, ROC Received 12 February 2003; received in revised form 21 March 2003; accepted 25 March 2003 Abstract A high reflecting InP/airgap distributed Bragg reflector (DBR) using InGaAs as sacrificial layers is demonstrated. The 3-pair InP/airgap DBR is formed by etching the InGaAs layers of the MOCVD grown InP/InGaAs structure using H 2 SO 4 solution. A rigid and stable InP/airgap DBR with a peak reflectivity of 99.9% at 1.54 lm and a stopband width of about 200 nm is achieved. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: DBR; VCSEL; Reflectivity 1. Introduction Long wavelength (1.3–1.5 lm) vertical cavity surface emitting lasers (VCSELs) are attractive for fiber-optic communication system. The excellent characteristics of VCSELs include single longitudinal mode output, small divergence circular emission beam profile, low power consumption and possible mass productions. However, the absence of high refractive index contrast in InP- lattice-matched materials impedes the progress of the development of 1.3–1.5 lm VCSELs compared to the short wavelength (0.78–0.98 lm) VCSELs. Recently, there are several reports using various ap- proaches to realize long-wavelength VCSELs. These approaches can be divided into three categories. First, the wafer fusion technique that bonds the InP-based active layers with the GaAs-based distributed Bragg reflectors (DBRs) was reported [1,2]. Second is the GaAs-based VCSELs using various types of active lay- ers for 1.3 lm, such as InGaAsN [3] or GaAsSb [4] quantum wells (QWs), or InAs quantum dots [5], and GaAs/AlAs DBR. However, it is rather difficult to ex- tend the emission wavelength to 1.55 lm range. Third category is the InP-based VCSELs utilizing high-quality InAlGaAs–InP [6] or InGaAsP–InP [7] as the active regions for 1.3–1.5 lm. But the availability of the lattice- matched material combinations with a large index of refraction difference for growth of highly reflecting DBRs is limited. For instance, the refraction index dif- ferences of InP-lattice-matched InP/InGaAsP [8] with Dn ¼ 0:27, InAlAs/InGaAlAs [9] with Dn ¼ 0:4, and Sb- based AlGaAsSb/GaAsSb [10] with Dn ¼ 0:44 DBR are only 0.27, 0.4 and 0.44 respectively. As a result, using these material combination DBRs require larger number of the pairs of about 48, 35 and 32 pairs respectively to obtain 99.9% reflectivity. In addition, using these DBRs, the field penetration depth tends to increase causing more absorption, and the heat dissipation could also be a problem. Recently, using InP/airgap structure as DBR for 1.55 lm VCSELs with using InGaAs as sacrificial layer was reported [11,12]. This structure has largest refractive index contrast of Dn ¼ 2:16 [13] and small optical loss than the conventional InP/InGaAsP and InAlAs/InGaAlAs material systems. The InP/airgap structure only requires 3-pairs to achieve high reflectivity of 99.9%. However in the reported InP/airgap DBR structures, the wet etching solution of FeCl 3 was used to etch the sacrificial InGaAs layer. The FeCl 3 solution has * Corresponding author. Tel.: +886-3-5712121x56320; fax: +886-3-5716631. E-mail address: scwang@cc.nctu.edu.tw (S.C. Wang). 0038-1101/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0038-1101(03)00144-8 Solid-State Electronics 47 (2003) 1825–1828 www.elsevier.com/locate/sse