Challenges and approaches of fabricating GaN-based green lasers

Abstract

During recent years, several research groups have demonstrated a steady progress in increasing InGaN quantum well (QW) laser emission wavelength [1][2][3][4][5], however the threshold current ( Fig. 1) and wall plug efficiency (WPE) are still worse than those for blue LDs. Reduction of operation current is needed to improve device reliability while keeping WPE comparable with alternative portable green laser technologies. In this paper we discuss the motivation, challenges, and progress in making group-III nitride lasers emitting in the green spectral range, and compare the devices created using epitaxial structures grown on GaN substrates with cplane (i.e. plane with (0001) crystallographic orientation) and semipolar-plane (i.e. with the plane tilted from c-plane by >0 and <90 degree) orientations. The different structure design choices are discussed, taking into account specific material properties and crystal growth requirements for these orientations. A major challenge is to grow InGaN quantum well (QW) in active region with sufficiently high In concentration (>25%) necessary for optical gain in green spectral range [1]. The problem is coming from mechanical stress, formation of defected areas and compositional fluctuations. In addition, the built-in electric fields and emission polarization anisotropy have to be taken into account. A comparison of optical gain between c-plane and semipolar plane QW is presented in Fig. 2. The optical gain for c-plane is reduced due to strong built-in electric fields that cause electron-hole separation in quantum wells therefore reducing recombination rate. A positive effect of built-in fields is however that it also reduces transition energy i.e. increases emission wavelength, which allows using less In concentration to achieve desired lasing wavelength [1]. But this benefit can only be useful if low optical gain is needed to reach threshold (Fig 1(a)), i.e. when optical losses are minimized. The optical gain of semipolar QW is much higher than in c-plane because of lower built-in electric fields and emission polarization anisotropy, if the stripe direction is properly chosen [6]. To choose the stripe orientation, one has to take into account that emission polarization depends not only on QW plane orientation, but even on pumping level, as we found it for (11-22) semipolar orientation [5]. A disadvantage of semipolar structures is that they require more In to achieve long emission wavelength and higher mechanical stress, while the stress-driven dislocation formation is much more prone for semipolar heterostructures.