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Excitation-intensity dependent photoluminescence (PL) as well as effective recombination time of a series of c-plane GaN layers grown on sapphire substrates were measured at room temperature. It was shown that at low excitation level a high background free-electron concentration causes an increase of radiative recombination rate in the near band edge region of GaN that results in an increase of the PL efficiency. In the investigated series, the GaN layers with lower crystal quality but having higher free-electron concentration showed higher PL efficiency at excitation levels under ∼50 kW/cm2. At elevated excitation levels the impact of free-carrier concentration decreases and the PL efficiency is mainly determined by nonradiative recombination time. From simulations of excitation-intensity dependent PL efficiency, the Auger recombination coefficient in the investigated GaN samples was estimated to be much less then 10–30 cm6s–1. A drop of PL efficiency was observed at excitation levels higher than 2500 kW/cm2 when PL was registered from sample's surface. The reason of the drop was stimulated emission propagating along the in-plane direction and causing the saturation of photogenerated carrier density. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

ZnSe/ZnMgSSe and InGaN/GaN heterostructure based lasers under optical transverse pumping by pulsed N 2 -laser radiation were investigated in a wide spectral, temperature and excitation intensity range for various types of heterostructures which differed in the epitaxial layer composition, layer sequence and thickness. The spectral- angular distribution of the laser emission of the ZnSe/ZnMgSSe separate confinement heterostructures and the influence of the excitaion intensity and the cavity length on the laser mode structure were investigated. It was found that the main degradation mechanism of the ZnSe/ZnMgSSe multiple quantum well heterostructures at temperatures higher than 400 K is the diffusion of S atoms from the barriers into the quantum wells which leads to increasing point defect concentration in the active layers of the laser. The recovery of the laser threshold of the ZnSe/ZnMgSSe MQW-SCHs degraded during thermal annealing after the action of the inherent laser radiation is attributed to a significant decrease of the point defects in the active layers. Laser action of InGaN/GaN multiple quantum well heterostructures in the blue spectral region has been obtained for the first time and investigated. Temperature tuning of InGaN/GaN MQW laser emission from the violet to blue has been realized. The influence of the photoluminescence characteristics on the laser parameters of the InGaN/GaN MQW optically pumped lasers operating in the blue spectral region is investigated.

The influence of layer thickness, heterostructure design, optical confinement factor and spontaneous emission efficiency on laser parameters of GaN based quantum well optically pumped lasers is studied in wide spectral (373 - 470 nm), temperature (77 - 600 K) and excitation intensity (10 2 - 3 10 6 W/cm 2 ) regions. The laser threshold enhancement from 70 kW/cm 2 for the 421 nm operating laser to 900 kW/cm 2 for the 469.5 nm laser leads to the reduction of highest operation temperature of the laser from 585 K for the 421 nm laser to 295 K for the 469.5 nm laser with increasing operating wavelength. As a rule the far field pattern of the laser emission consists of two light spots localized at positive and negative angles of 30 - 50 degree(s). The laser spectra structure in the far-field of the SQWs and MQWs with low thickness of the active layers depended on the registration angle. The spatial distribution of the laser light in the far-field consisting of transverse and leaky modes was calculated and compared with the experimental results. Calculations of the optical confinement factor and the electromagnetic field distribution inside and outside of the heterostructures showed that the MQW lasers operate in the high order transverse mode regime. The spectral-angular distribution of the emission of the SQW and MQW lasers with low active layer thickness is due to the leaky mode formation.