400-nm Band AlGalnN-Based High Power Laser Diodes
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400-nm Band AlGaInN-Based High Power Laser Diodes Takeharu Asano, Motonobu Takeya, Tsuyoshi Tojyo, Shinro Ikeda, Takashi Mizuno, Shinichi Ansai, Shu Goto, Satoru Kijima, Tomonori Hino, Shiro Uchida and Masao Ikeda Development Center, Sony Shiroishi Semiconductor Inc., 3-53-2 Shiratori, Shiroishi-shi, Miyagi-ken, 989-0734, Japan ABSTRACT High-power AlGaInN-based laser diodes (LDs) operating with high reliability in the 400-nm band have been successfully fabricated using a high-productivity process. Epitaxial lateral overgrowth (ELO) over a 10-µm region was employed to obtain a broad growth area with low dislocation density, and the thickness of the ELO-GaN layer was limited to approximately 5 µm in order to minimize wafer bending. These techniques allow for the easy and reproducible alignment of the laser stripe on the region of low dislocation density. The insertion of a GaInN interlayer between the active layer and the AlGaN electron blocking layer was effective for reducing the strain between these two layers, resulting in homogeneous luminescence from the active layer and lower operating current. A mean time to failure of 15000 h under 30-mW continuous-wave operation at 60 °C was realized as a direct result of the lower operating current. Productivity was remarkably improved by performing epitaxial growth on a 3-inch substrate. Highly uniform laser wafers were successfully fabricated by achieving minimal temperature variation (1000 ±7 °C) over the 3-inch substrate. The resultant laser structures varied in thickness by only ±5%, and the photoluminescence wavelength was consistent within ±2.5 nm over the entire 3-inch substrate. The average threshold current of 550 LDs selected from a fourth wafer was 32.7 mA, with small standard deviation of 3.2 mA. INTRODUCTION AlGaInN-based blue-violet laser diodes (BV-LDs) are promising light sources for high-density optical storage systems. Since the first demonstration of continuous-wave (CW) operation [1], remarkable progress has been achieved in the field of BV-LDs. In particular, epitaxial lateral overgrowth (ELO) has proven to be an effective means of reducing dislocation density, which is a major factor restricting device lifetimes [2-8]. Highly reliable devices with estimated lifetimes of 15000 h under 30-mW CW operation at 60 °C have been reported by Nagahama et al. using ELO-GaN [9]. However, the fabrication of these laser diodes involved the formation of ELO-GaN on a 200 µm-thick GaN substrate grown I4.1.1
by hydride vapor-phase epitaxy (HVPE). Such a process is too complex to allow commercially viable levels of productivity. Therefore, one of the remaining obstacles to the widespread adoption of BV-LDs as light sources in high-density optical storage systems is the realization of both high reliability and high productivity. The aim of the present study was to develop a simple ELO process that is capable of producing a lateral growth region of wider than 10 µm, and to minimize the thickness of the nitride layers. Reliability, or device lifetime, can be improved by
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