Preparation And Characterization Of Near Lattice-matched In_xAl(1-x)N Thin Films Grown On GaN Templates

III-V nitride alloys have special properties, such as high excitonic binding energy and direct band gap. These make them attractive for a wide range of technological applications in optoelectronics and high-power, high-frequency devices. However, the large lattice mismatch between the binaries (InN, GaN, and AlN) inevitably leads to the formation of defects at interfaces of the heterostructures that deteriorates the performance of devices and the operating lifetime. Interestingly, the ternary alloy InxAl1-xN can be perfectly grown on lattice-matched (LM) GaN for x?0.17. So there is no strain existing between the films of In0.17Al0.83N and GaN. At the same time, InxAl1-xN has a high refractive index and a high band gap contrast with respect to GaN. These advantages make InxAl1-xN attractive for high-reflectivity short-wavelength distributed Bragg reflectors (DBRs) and high-quality factor microcayities (MCs). In addition, due to large differences of the spontaneous polarization, the LM InxAl1-xN/GaN heterojunctions exhibit a high sheet carrier density (ns) of two-dimensional electron gas (2DEG).Nevertheless, the growth of high-quality InxAl1-xN has been challenging due to compromised surface and crystalline quality, anomalous ion channeling, phase segregation, and compositional variations. Furthermore, huge differences in optimized growth conditions of AlN and InN films pose additional technical challenges to achieve high-quality and accurately-controlled InxAl1-xN epitaxial films. Thus, meticulous optimization of the growth conditions of InxAl1-xN materials is crucial in order to grow high-quality epitaxial films.In this work, we present a systematic study on the effects of the key growth conditions such as growth temperature, Al cell temperature and In cell temperature on the quality of InxAl1-xN films grown by radio frequency molecular beam epitaxy (rf-MBE). The lattice-matched InxAl1-xN thin films with excellent crystalline quality and surface morphology have been grown on c-plane GaN templates by rf-MBE. The main results are presented below:1. We examined the effect of the growth temperatures on surface morphology and the quality of InxAl1-xN thin films grown on c-plane GaN templates by rf-MBE. A very narrow window of growth temperature for high-quality InxAl1-xN was identified around 525?. Two other growth parameters of Al cell and In cell temperatures were also investigated. It was found that the In mole fraction in InxAl1-xN can be controlled by varying the Al cell temperature. In addition, the crystalline quality and the surface morphologh of InxAl1-xN thin films were much sensitive to the In cell temperature. With the In cell temperature at above 760?, the growth temperature window of InxAl1-xN disappeared.2. We grew the the In0.17A10.83N thin films at the optimized growth conditions. It was found that these thin films were lattice-matched to the GaN templates and exhibited high crystalline quality. The value of FWHM of the In0.17A10.83N (0002) peak is 249.6 arcsec. The surface RMS of the In0.17A10.83N thin films is 0.32 nm. The surface of InxAl1-xN thin films shows a honeycomb-type structure, as found by SEM, with a diameter of?15 nm.3. Si- and Mg-doped InxAl1-xN has been investigated. It was found that the diffraction peak of Si doped InxAl1-xN is shifted compared to that of undoped InxAl1-xN. There are two possible explanations, either a smaller In content or a tensile strain existing due to the Si doping. In addition, Mg-doped InxAl1-xNthin films with different Mg cell temperatures were deposited on GaN templates by rf-MBE. Mg cell temperature did really affect the surface morphology and crystaline quality of the InxAl1-xN:Mg thin films. The most optimized Mg cell temperature was determined to be 325?, at which, the minimum RMS=0.30 nm and FWHM=350 arcsec were achieved.4. We studied the thermal stability of near lattice-matched InxAl1-xN thin films. The InxAl1-xN thin films were ex-situ annealed under N2 at temperatures from 550 to 1050?. It was found that the InxAl1-xN top layers undergo degradation during annealing at high temperatures?850? while annealing at the temperatures? 750? can improve the crystalline quality and the surface morphology. The in-situ thermal stablility of InxAl1-xN thin films has also been studied. The InxAl1-xN thin film is fully stable at temperature of 72? in MBE environments. The near lattice-matched GaN/InxAl1-xN thin film was also grown on the GaN template by MBE.