A study on the law of oxidation rate in GaAs-based VCSELs

Optik 125 (2014) 5124–5127
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Optik
journal homepage: www.elsevier.de/ijleo
A study on the law of oxidation rate in GaAs-based VCSELs
∗ Corresponding author. E-mail address: fengyuan 1224@ (Y. Feng). /10.1016/j.ijleo.2014.04.078 0030-4026/© 2014 Elsevier GmbH. All rights reserved.
mainly by the diffusion reaction, the oxidation rate decreased to a given value and basically stabilized. 3. Results and analyses In the process of oxidation reaction, the chemical reactions are as follows [10]:
a r t i c l e
i n f o
a b s t r a c t
In order to accurately control the oxidation aperture in high power vertical cavity surface emitting lasers (VCSELs), the selective oxidation process is studied with experiments. Selective oxidation experiments are carried out upon the simulate wafer of VCSELs at different temperature. Oxidation products are examined at different oxidation depths of oxidation layer and each component content is analyzed. The results of the experiments are analyzed with the kinetics of thermal diffusion. The analyzed results show that the concentration of oxidant is e exponentially declined with increasing depth of oxidation in highpower VCSELs. The oxidation depth stability and precision can be improved by lowering the oxidation temperature and prolonging the oxidation time. © 2014 Elsevier GmbH. All rights reserved.
Y. Feng ∗ , G.J. Liu, C.L. Yan, Y.Q. Hao, Y. Wang, Z.J. Li, P. Lu
National Key Laboratory of High-Power Semiconductor Lasers, Changchun University of Science and Technology, Weixing Rd. 708t al. / Optik 125 (2014) 5124–5127
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Fig. 1. Cross-section SEM photograph of an oxided sample.
Fig. 4. Oxidation content vs. oxidation depth.
Article history: Received 5 September 2013 Accepted 26 April 2014 Keywords: VCSEL Selective oxidation Oxidation rate Kinetics of thermal diffusion e-Exponential law
1. Introduction High-power vertical-cavity surface-emitting lasers have widely been applied to data communication, data storage, optical sensors, printing industry, especially in the military applications [1–4], where using VCSEL array devices to pump solid state lasers has special tempting prospects [5]. In recent years, the development of high power VCSEL has become a research hotspot to domestic and international semiconductor lasers. Selective oxidation in the preparation of high-power VCSEL, the extensive use of technology to form a current injection window, to achieve electro-optical limit and the refractive index-guided. Selective oxidation process has become a key process of the preparation of the VCSEL [6,7]. Due to the fact that precise control of the oxidation depth and the thermal stability of the oxidation process, the VCSEL output power and electro-optical conversion efficiency are essential [8,9], so one of the difficulties in the selective oxidation is the controllability of the oxidation depth and accuracy. Thus, we adopted the oxidation of the same sample under different conditions and measured the content of each component in the reaction products. By using diffusion kinetics of the oxidation rate law of mathematical analysis and suggesting that oxidation time is shorter, the linear law of oxidant concentration is shown. When the oxidation time is longer where it is mainly controlled by diffusion, the oxidant concentration shows an e exponential attenuation.
2. Experiments The epitaxial wafer of VCSEL was grown on N-type GaAs substrate by using MBE and the active layer contains three GaAsAlGaAs quantum wells. The bottom Bragg reflector consists of 41 pairs of Al0.22 Ga0.78 As/GaAs and the top Bragg reflector consists of 22 pairs of Al0.22 Ga0.78 As/GaAs. There is a 35 nm thick Al0.98 Ga0.02 As layer located at the first pair in P-DBR in order to form buried AlGaAs/Alx Oy . The top layer is GaAs contact layer. The whole epitaxial wafer thickness is about 650 ␮m and the center wavelength of DBR reflection spectrum is at 800 nm. The photolithographic technique and wet chemical etching are used to form the oxidation window as Fig. 1. The lateral oxidation of Al0.98 Ga0.02 As in N2 /H2 O atmosphere is characterized with respect to oxidation temperature, N2 carrier gas flow, and the geometry of the mesa structures. The selective oxidation setup was shown in Fig. 2. The exposed Al0.98 Ga0.02 As layer was oxidized at 420 ◦ C, 440 ◦ C and 460 ◦ C in N2 /H2 O atmosphere. Dry N2 with a constant flow of 1 L/min was used as carrier gas for H2 O vapor. The water temperature was maintained at 90 ◦ C. The oxidation time was maintained in 40 min. The Cross-section of the oxided samples was SEM observed and the oxidation depth of samples was measured. In order to explore the oxidation law of the samples under different temperatures, we use SEM microanalysis of samples per 1 ␮ distance and measure the oxidation depth, corresponding to the various elements of the point oxidation layer X-ray energy spectrum (Fig. 3) and element composition analysis table (Table 1). According to X-ray energy spectra, we can qualitatively describe the resultant content of each point of the oxidation layer, and element