An approach for growing high-quality AlGaN/AlN multiple quantum wells (MQW) emitting in deep UV region is proposed. The structures are deposited on bulk AlN substrates. Structural analysis by using x-ray diffraction confirms high crystalline quality of these structures. Photoluminescence dependences on excitation intensity and temperature under band-to-band excitation of AlN barrier layers and under selective excitation of the quantum wells are presented. Al0.5Ga0.5N/AlN MQW grown on bulk AlN demonstrate emission at 260 nm with high emission intensity. Stimulated emission of these structures at 258 nm was observed. The results prove great potential of growing structures with high-aluminum-content layers on bulk AlN substrates.

We report on observation of stimulated emission at 258 nm in AlN/AlGaN multiple quantum wells. The structures were grown over Al-face single crystal bulk AlN substrates. AlN/AlGaN structures with 50% of Al in the well material were grown using low-pressure metalorganic chemical vapour deposition. Characterization by using X-ray, AFM, SEM, and photoluminescence techniques indicated high structural quality of the structures. The stimulated emission was measured using the variable stripe length method under excitation by 4-ns-long pulses of the fifth harmonic of Nd:YAG laser radiation at 213 nm (5.82 eV). The stimulated emission exhibited a characteristic superlinear dependence of emission intensity on the pump intensity as well as an exponential increase of the sample-edge emission intensity with increasing stripe length up to ∼430 μm and the intensity saturation beyond this range. The observation of stimulated emission at 258 nm is very promising for the future development of III-nitride-based deep-UV laser diodes on bulk AlN substrates.

Photoluminescence of the GaN layers grown both on N-face and Al-face bulk AlN is studied under CW and pulsed laser excitation in the temperature range from 8 K to 300 K. We compare localization of excitons, residual strain, and activation energies for thermally activated transfer of carriers to nonradiative recombination. At high excitation intensities, conditions for carrier heating, which is important for the threshold of stimulated emission, are also investigated.

We report on high Al-content AlGaN-based deep UV emitter structures grown over single crystal, slightly off c-axis (5.8 degrees) bulk AlN substrates. AlN/AlGaN multiple quantum well (MQW) structures with up to 50% of Al in the well material were grown by using low-pressure MOCVD and characterized by using X-ray, AFM, SEM and photoluminescence techniques. Two light sources, one at 213 nm wavelength for selective excitation of quantum well layers and another one at 193 nm to excite both wells and barriers, were exploited. A weak temperature dependence (from 8 K to 300 K) of the luminescence intensity and the absence of blue-shift of the luminescence peak with increasing excitation intensity pointed to a low density of localized states, in a good agreement with the X-ray data, which indicated very high quality of these MQW structures.The most striking result was observation of stimulated emission at wavelength as short as 258 nm in Al0.5Ga0.5N/AlN MQWs grown on bulk AlN single crystals.