Basic research on optical communication wavelength inter-subband transition of III-nitride semiconductors

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The characteristics of intersubband transition in III-nitride quantum wells (QWs) are promising for ultra-high speed optical communication devices such as detectors, modulators and all-optical switches.In this research project, clear optical absorption and ultra-fast absorption relaxation were demonstrated by GaN/AlN MQW crystals which grown by rf-plasma assisted molecular beam epitaxy (RF-MBE).First of all, we have optimized a growth condition of AlN buffer layer by RF-MBE and atomically flat AlN was obtained on sapphire substrate then growth condition of GaN/AlN MQW was optimized, consequently high quality GaN/AlN ISBT crystal could be obtained. We also carried out systematic investigation of FWHM of ISBT absorption spectra dependence on in-plan distribution of GaN well layer thickness and surface roughness RMS value.The carrier relaxation dynamics was investigated by two-color pump-probe technique in a wide energy range around 800 meV (1.55μm). The observed relaxation time was composed by ultra-fast 140 fs component and slow 1.3 ps one. Carrier relaxation model was constructed considering a phase space filling of the upper subband and a carrier cooling process in the lower subband.All-optical modulation using 1.55 pm ISBT resonant induced by UV interband (IBT) resonant light (325 nm or 213 nm) was demonstrated by GaN/AlN MQW waveguide device.The growth condition of GaN based nanocolumns including GaN/AlN MQW was developed and optical properties were investigated. The ISBT absorption at 1.55μm was observed for nanocolumns for the first time.In-rich InGaN is an attractive candidate for ISBT material at optical communication wavelength due to large conduction band offset between GaN and AlN. We have demonstrated the growth of high quality In-rich InGaN by RF-MBE.