We discuss the generation mechanism of THz radiation and acoustic phonon pulse wave under ultra short pulse excitations in GaN-based light emitting diode (LED) structures containing InGaN/GaN multiple quantum wells. In order to understand the role of piezoelectricity in the THz radiation and acoustic phonon pulse wave generations, an external field was applied in these structures so that the piezoelectric field in the quantum wells was compensated under an external reverse bias. Coherent acoustic phonon pulse wave was found to be independent of the applied voltage, although the strain of the InGaN layers was crucial for the generation of the signals. The THz emission from these structures was found to increase with increasing reverse voltage and excitation energy, similar to the trend of the photocurrents in these structures. The bias and wavelength dependence of the THz generation suggests the carriers associated with the photocurrents are responsible for the THz radiation.
Band diagram of GaN-based p-i-n structures containing InGaN/GaN multiple quantum wells is discussed in the presence of an external electric field. Carrier lifetime, photocurrent, and photoluminescence intensity as a function of applied voltage can be understood based on the potential diagram. These structures effectively generate “spatially localized strain pulse,” which is not present in other materials such as in GaAs-based p-i-n structures. The mechanisms of the generation and propagation of the strain pulse and their relevance with the piezoelectricity of GaN are discussed. These structures with InGaN/GaN multiple quantum wells also produce electromagnetic radiation of 0.11 THz, which is absent in GaN-based double heterostructures.
Growth of II-VI and III-V nitrides by MBE and MOCVD is being studied. Using GaAs as a buffer and ZnMgSSe quaternary alloy as a cladding layer together with a ZnSe/ZnTe superlattice for ohmic contact, room temperature CW operation of II-VI lasers has been achieved. Focus topics include the (1) optical properties of ZnSe-based epilayers, (2) the effect of GaAs buffer surface construction and initial growth condition of ZnSe on the defect density and (3) etch-pit configuration of different type of defects in ZnSe-based epilayers. Multilayer p-GaN/MQW GaN-In<SUB>0.1</SUB>GaN/n-GaN were grown on sapphire substrate using MOVPE and plasma-assisted ionized source beam epitaxy. We investigated the (1) growth and properties of III-V nitride films by LP-MOVPE and PAMBE (Plasma Assisted Molecular Beam Epitaxy), (2) etch-pit and wet etching of GaN/(0001)Al<SUB>2</SUB>O<SUB>3</SUB>, (3) ohmic contacts of GaN and (4) GaN etching by CARIBE.