Study Of AlGaN/GaN Multi-quantum Wells Infrared Detection Based On Intersubband Transition

Due to the advantages of large conduction band offset, high electron and LO phonon interaction energy (～90meV) and ultrafast electronic relaxation rate in AlGaN/GaN multiple quantum wells, quantum well infrared detectors (QWIPs) based on the intersubband transitions (ISBT) in AlGaN/GaN multi-quantum wells (MQWs) have obvious advantages and potential applications in the fields of ultrafast, broad spectrum and high operating temperatures and so on. Moreover, AlGaN/GaN quantum wells is the only material that can realize the monolithical integration of ultraviolet and infrared dual-color detectors, making AlGaN/GaN multiple quantum wells and their detection devices are of great strategic significance in missile warning and guidance, etc..However, due to the strong polarization electrical field in AlGaN/GaN MQWs, design of QWIPs and rule of electron transition are more complicated. In addition, high density dislocations as well as severus interface diffusion in AlGaN heterostructures and their low-dimension quantum wells grown by MOCVD limit their performance improvement. For these problems, design of AlGaN/GaN quantum well structures, optimization of the AlGaN and MQWs growth process, ISBT infrared transitions, and device properties have been investigateed in this dissertation. The specific researches are as follows:(1) The effects of polarization and related structural parameters on the ISBT transition in AlGaN/GaN MQWs have been investigated by solving the Schrodinger and the Poisson equations self-consistently. The results show that the peak absorption coefficient is increased with the enhancement of polarization field while the ISBT transition wavelength is decreased, which is dissimilar to the case of interband transition. Moreover, the results show that the mechanisms responsible for the ISBT transition wavelength redshifting with the increase of barrier and well thicknesses are different. In addition, the influences of doping position and doping concentration on the ISBT transition wavelength have been studied. The results suggest that the transition wavelength is blue shifted with the increase of doping concentration, which can be attributed to the many-body effects. Based on the simulated results shown above, the appropriate structural parameters for AlGaN/GaN MQWs with ISBT absorption wavelength in3～5μm have been obtained, which will provide theoretical basis for the subsequent experiments. (2) The effects of low-temperature AlN interlayer on the growth mode of high temperature AlN grown directly on sapphire have been studied. The results show that the growth mode of high temperature AlN can be tuned by adjusting the AlN insertion layer growth temperature. As a result, two-dimensional growth with reduced dislocation density can be achieved and high-quality AlN films can be obtained after optimizing. Moreover, methods of pulsed atomic layer epitaxy (PALE) and continuous growth digitally alternated epitaxy and self-patterned technology combined with H2corrosion and SiNx burial have been innovatively proposed to optimize the growth of n-AlGaN. As a result, high quality n-AlGaN has been achieved with its (002) plane XRD FWHM of190arcsec,(102) plane of439arcsec, and RMS of0.98nm, which provides high quality templates for the subsequent quantum wells growth.(3) The MOCVD process parameters of AlGaN/GaN MQWs have been optimized, and the infrared ISBT transition around5.1μm has been successfully detected. Moreover, the ISBT transition wavelength has been successfully adjusted within the range of4.65～5.14μm as a result of stress control of MQWs by changing the Al mole composition of AlGaN templates from0to0.3. In addition, the influence of Si doping concentration on the ISBT infrared absorption peaks has been studied, and firstly found that with the quantum well doping dose increases, barrier thickness is decreased while the well thickness is increased. With the combined effects of quantum well structure parameters variation, stress relaxation and many-body effects, ISBT infrared absorption wavelength is firstly redshift and then blueshift.(4) The prototype device of AlGaN/GaN multi-quantum wells infrared detector has been prepared, and the infrared current signals at7.2μm and3.4μm have been detected. After analysis, these signals may be derived from the interaction of electrons and LO phonons or the defect energy levels. Moreover, the origin of dark current has been studied, and the results show that dark current is mainly derived from defects and dislocations.