Diode lasers emitting radiation of wavelenghths shorter than 500 nm were practically initiated in 1996 by Nakamura et al.[1] who reported the first blue-emitting diode lasers based on A^IIIB^V semiconductors. There have been, however, still some technological problems with manufacturing such nitride structures. The problems have been gradually overcome which has been strongly stimulated by extremely promising numerous application possibilities of blue and ultra-violet diode lasers in science, technology and various spheres of contemporary life. Hence a continuous progress in technology and optimisation of nitride devices is still accelerated by a strong demand for reliable, cheap, compact and high-performance lasing devices emitting radiation in this part of spectrum.

Structures of edge-emitting nitride diode lasers are usually equipped with electrodes placed on the same device side. It is a direct consequence of very high electrical resistivities of sapphire (Al₂O₃) used usually as a substrate material. Such structures require quite sophisticated and time-consuming processing which may additionally lead to a device damage and is definitely a reason of a considerable increase in a device cost. Besides, such a laser structure requires the p-side-up device configuration which is followed by heat-sinking problems deteriorating a device performance. The ridge-waveguide (RW) structure is usually used in these lasers creating an appropriate wave-guiding effect and, additionally, introducing better current-spreading confinement than the proton implantation, which is followed by lower lasing thresholds and much lower active-region temperature increases.

The main goal of the present work is to investigate an impact of diode-laser ridge sizes on its lasing performance with the aid of the comprehensive self-consistent optical-electrical-thermal-recombination model of nitride diode lasers [2]. The modern 405-nm InGaN/GaN RW laser structure reported by Kauer et al. [3] has been selected for this simulation. Contrary to usual structures of nitride diode lasers, the n-type GaN substrate material is used in this case instead of the sapphire one which enables application of the n-side bottom contact. This substrate is much more expensive than a sapphire one but it enables manufacturing nitride diode lasers of much better performance characteristics. Our simulation reveals that the lowest room-temperature lasing threshold may be expected for relatively narrow and deep ridges. For the structure under consideration, the lowest threshold current density of 5.75 kA/cm² has been determined for the 2.2-mm ridge width and the 400-nm etching depth. Then the active-region temperature increase was as low as only 24 K. For wider 5-mm ridge, this increase is twice higher. An impact of etching depth is more essential for narrower ridges. Quite a high values (between 120 and 140 K) of the characteristic parameter T₀ convince very good thermal properties of the above laser.

[1] S.Nakamura et al., J. Journal of Applied Physics, 35 (1996) L74

[2] R. P. Sarzała and W. Nakwaski, J. Phys.: Condens. Matter 16 (2004) S3121

[3] M. Kauer et al., Electronic Letters, 41 (2005) 739

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