The team, from Huazhong University of Science and Technology (HUST) in China, King Abdullah University of Science and Technology (KAUST) in Saudi Arabia and Ningbo Institute of Materials Technology and Engineering in China, have presented a deep ultraviolet (DUV) 249nm optically pumped III-nitride laser structure based on gallium nitride rather than the more usual wider-bandgap aluminium gallinitride (AlGaN) quantum wells (QWs) [Maocheng Shan et al, ACS Photonics, published online 20 September 2019]. Recently, the results were specifically reported in the News Features of Semiconductor Today which is an independent and not-for-profit international journal of the semiconductor industry. The journal focuses on important research developments and the latest industry developments in compound and advanced silicon semiconductors.
Among the problems for AlGaN DUV lasers are strong quantum-confined Stark effects (QCSEs) arising from electric fields, based in ionic charge polarization, that pull electrons away from holes, inhibiting recombination into photons. Also, high-Al-content AlGaN tends to emit radiation optically polarized in a transverse-magnetic mode, which is more difficult to use efficiently in light-emitting diode and edge-emitting laser structures.
Metal-organic chemical vapor deposition (MOCVD) on 2-inch c-plane sapphire resulted in an AlN/GaN MQW laser structure. The very short wavelength of 249nm was enabled by the extreme confinement of very thin GaN wells in thin AlN barriers. The corresponding photon energy was 5.0eV, a couple of eV higher than the 3.3eV band gap of bulk GaN. A somewhat lower growth temperature of 1040°C was chosen for the MQW to ensure high-quality wells, avoiding evaporation of the more volatile GaN material. The GaN wells were designed to be 4 monolayers (MLs) thick, while the AlN barriers were 6MLs. In metric measurements, according to x-ray analysis, the wells and barriers were 1.0nm and 1.5nm thick, respectively.
The similarity of the quantum well (QW) and barrier (QB) thicknesses was expected to lead to a higher refractive index, compared with the usual situation with significantly thicker barriers. The use of 40 wells was due to comprehensive considerations of the lateral optical confinement, penetration depth of the excitation laser beam, gain medium volume, strain relaxation, and material and interface quality.
Figure 1: (a) Schematic diagram of AlN/GaN MQW DUV laser grown on sapphire substrate; (b) simulated optical mode profile and refractive index distribution; (c) Laser emission spectra and inset of peak intensity and line width of spectra as a function of pumping power density.
With good material and facet quality, and large optical confinement factor, a low threshold of 190 kW/cm2 was achieved under optically pumped, which is comparable to state-of-the-art AlGaN MQW lasers at similar wavelengths on AlN and sapphire substrates. The TE-dominant optical polarization was measured for the stimulated and spontaneous emissions above and below the threshold, respectively. These results demonstrate that the outstanding potentials of the AlN/GaN MQWs for DUV LEDs and lasers.
This work is supported by the National Key Research and Development Program of China (Grant No. 2016YFB0400901), the Key Laboratory of infrared imaging materials and detectors, Shanghai Institute of Technical Physics, Chinese Academy of Sciences (Grant No. IIMDKFJJ-17-09), the National Natural Science Foundation of China (Grant No.61704062, 61774065), the China Postdoctoral Science Foundation (Grant No. 2016M602287) and the Director Fund of WNLO.
Thesis link:：
https://pubs.acs.org/doi/abs/10.1021/acsphotonics.9b00882
News Features link：
http://www.semiconductor-today.com/news_items/2019/oct/kaust-301019.shtml
