Facet roughness analysis for InGaN/GaN lasers with cleaved facets

Abstract

Atomic force microscope images reveal a root-mean-square roughness ⌬dϭ16 nm for InGaN/GaN double-heterostructure laser structures with cleaved a-plane facets. The c-plane sapphire substrate cleaves cleanly along both the a and m planes. A theoretical model is developed which shows that the power reflectivity of the facets decreases with roughness by a factor of e Ϫ16 2 (n⌬d/ 0) 2 , where n is the refractive index of the semiconductor and 0 is the emission wavelength. Laser emission from the optically pumped cavities shows a TE/TM ratio of 100, an increase in differential quantum efficiency by a factor of 34 above threshold, and an emission line narrowing to 13.5 meV.

GaN-based lasers on SiC: influence of mirror reflectivity on L–I characteristics

“…3). Thus according to Stocker et al [3] we can assume the reflectance R of the uncoated facet to be 0.18. With the micro-mirror being in contact with the facet a reflectance of 0.92 is estimated (i.e., the reflectivity of the Al mirror at 410 nm).…”

“…At 410 nm wavelength, the refractive index of GaN is approximately 2.5 resulting in a reflectance of perfect GaN/air interfaces of merely 0.18 according to Fresnel equations. The facet reflectivity is further reduced by any roughness of the interface [3]. Whereas laser cavities on sapphire substrates are usually made by dry chemical etching [4], resonators on SiC substrates benefit from the feasibility of cleaving facets usually yielding superior (i.e., smoother) surfaces and thus higher reflectivity mirrors.…”

Fabrication of Smooth GaN-Based Laser Facets

“…This indicates that wet chemical etching may be a valuable tool for producing high-quality laser facets with reflectivities close to the ideal for a perfectly smooth surface. 16 For this etching method to be useful for fabricating pn-junction laser diodes, the dependence of etch rate and surface morphology on doping must be determined. A scanning electron microscope (SEM) image of p-type GaN after anisotropic wet etching in molten KOH at a temperature of 195°C is shown in Fig.…”

“…16 Crystallographic etching in molten KOH is used to smooth the PEC-etched surface to improve the reflectivity. The series of SEM images in Fig.…”

Final LDRD report : science-based solutions to achieve high-performance deep-UV laser diodes.

“…To date, several methods have been employed to fabricate facets for GaN-based near-UV lasers including cleaving, 29,30 milling with focused ion beams (FIB), 31,32 plasma etching, [33][34][35] chemically assisted ion beam etching, 36,37 and two-step processes combining plasma etching or photoenhanced electrochemical etching with wet chemical etching. [38][39][40][41][42] Of these methods, cleaving has produced the smoothest facet surfaces with root mean square (RMS) roughness values on the order of 1 nm for GaN-based laser diodes (LDs) grown on SiC.…”