Threshold currents of nitride vertical-cavity surface-emitting lasers with various active regions
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Internet Journal Nitride Semiconductor Research
Threshold currents of nitride vertical-cavity surfaceemitting lasers with various active regions Pawe‚Ma kowiak1 and W‚odzimierz Nakwaski1 1Institute
of Physics, Technical University of ód ,
(Received Thursday, June 18, 1998; accepted Wednesday, October 7, 1998)
A detailed threshold analysis of room-temperature pulsed operation of GaN/AlGaN/AlN verticalcavity surface-emitting lasers (VCSELs) is carried out. The model takes advantage of the latest results concerning gain in active regions, material absorption in the cladding layers, as well as cavity diffraction and scattering losses. The simulation showed that although VCSELs with single (S) or multiple (M) quantum-well (QW) active regions exhibit lower threshold currents, they are much more sensitive to any increase in optical losses than their bulk counterparts. In particular, decreasing the active region radius of gain-guided QW VCSELs below 5 µm (which increases diffraction losses) or increasing dislocation densities (which, in turn, raises scattering losses) gives an enormous rise to their threshold currents. Therefore small-size GaN VCSELs should have an index-guided structure. In the case of MQW VCSELs, the optimal number of quantum wells strongly depends on the reflectivities of resonator mirrors. According to our study, MQW GaN lasers usually require noticeably lower threshold currents compared to SQW lasers. The optimal number of QW active layers is lower in laser structures exhibiting lower optical losses. Although the best result occurred for an active region thickness of 4 nm, threshold currents for the various sizes differ insignificantly.
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Introduction
Nowadays, ultra-violet (UV) and blue light emitting nitride semiconductor lasers based on wide-gap GaN, AlN, and InN semiconductor materials and their solid solutions [1], are attracting considerable attention. This is due to possibly wide applications of these lasers, especially in high-density optical recording devices [2], and the printing and imaging industry. Recent technology advancements for nitride lasers, achieved primarily by Shuji Nakamura with his co-workers from Nichia Chemical Industries, Ltd., has resulted in immediate progress in laser performance. Only 12 months after the first announcement, at the beginning of 1996 [3], of pulsed operation of nitride lasers at room temperature (RT), the first RT continuous-wave (CW) operation was obtained [4]. Initially, the lifetime of these first RT CW nitride lasers was extremely short, but it was extended to 35 hours by the end of 1996 [5] and a potential 10,000 hours of RT CW operation was reported recently [6], another 12 months later. The latest Nichia achievements [6] are very promising but a high density of defects in their GaN devices [7] seems to be still an unsolved problem in edge-emitting lasers (EELs). These difficulties might be resolved
in nitride vertical-cavity surface-emitting lasers (VCSELs) due to the relatively small volumes of their active regions [8]. Also, the serious d
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