| As one of the third-generation wide band-gap semiconductor materials, aluminum nitride(Al N) displays outstanding properties, such as wide band-gap, high melting point, high critical breakdown field strength, thermal stability and resistance to chemical corrosion. Recently, Al N is mainly applied to light emission diodes(LED), laser diodes(LD), second harmonic generation and surface acoustic wave devices. As an effective method for the growth of Al N crystals, Physical Vapor Transport(PVT) method is widely used in various research institutions. Currently, 2 inch Al N single crystals have been prepared by PVT method internationally. However, the domestic is still in its infancy in terms of preparation of Al N crystals, especially in the aspects of growth mechanism, luminescence mechanism of complex-defect, growth habit and preparation technics. In this study, we explore the effects of technological parameter on crystal growth, and research on the growth mechanism, luminescence mechanism of complex-defects and nucleation habit of Al N crystal to provide theoretical and technical guidance for the growth of high quality Al N crystal.The main contents of this dissertation include: designing the shape and component of heater by theoretical calculations, simulations and experiments to explore suitable process parameters; by spontaneous nucleation growth of Al N crystal to investigate the growth mechanism of low dimensional structures and bulk crystal; by growing Al N on tungsten(W), tantalum(Ta) substrate to research nucleation habit; by exploring the process parameters to prepare Al N crystal and characterize the crystalline quality of the Al N crystals.The shape of heater, growth temperature and growth pressure(N2) of Al N crystal growth was explored. The results showed that: by tuning the position of small steps of the complex-heater(i.e., configurated a small step in the interior of cylindrical heater), the temperature distribution can be adjusted to obtain excellent temperature gradient of crystal growth. Within the growth temperature increased from 1800℃ to 2200℃, the morphologies of Al N crystals gradually transform from nanowire to crystal. The growth rate of Al N crystal changes with growth pressure, and it reach a fastest growth rate when the pressure was 6.0×104Pa. The Al N crystal growth optimum parameters are determined as: the growth temperature was 2200℃, growing atmosphere was N2 with growth pressure of 6.0×104Pa, temperature gradient set as 3-5℃/mm.The growth mechanism of Al N crystal growth and optical property of Al N nanohelices were investigated. The study results of growth mechanism showed that: the nature factors driving Al N growth which following periodic stacking model is spontaneous polar of Al N crystal. Within the driven of spontaneous polarity,the growth mechanism can be proposed as periodic stacking of oblique hexagonal prism, i.e., the basic unit of Al N crystal growth is oblique hexagonal prism which formed by derangement of Al N atomic; Al N crystal growth process can be regarded as periodic stacking of oblique hexagonal prisms; Crystal morphology is determined by the thickness of the oblique hexagonal prism and its rotation angle along axial direction; With temperature-increasing, Al N crystal morphologies gradually transition from whiskers to grains, which twist or tilt along the [0001] direction with the co-effect of stacking model and wurtzite crystallographic characteristics; With extension of growth period at 2200℃ temperature, a large number of grain boundaries and micro-tube like defects formed in crystal. The study results of optical property showed that: there are three types of intrinsic defects in a the nanohelices, which are nitrogen vacancy(VN), aluminum interstitial(Ali), and complex-defact(VNAli), respectively. The complex-defact(VNAli) is the origin of long afterglow properties, and its lifetime can reach more than ten minutes. First-principles calculation shows that VNAli can lead to 595 nm luminescence, which may led to long afterglow properties, and the long afterglow effect of 600 nm in the Al N is verified experimentally. Complex-defect VNAli in Al N crystals leads to a decline of energy level from conduction bond, which generat a trap level with 0.35 e V, meanwhile, VNAli in Al N crystals also leads to a formation of defect levels in band gap. The difference between the energy level and the trap energy level is 2.07 e V(600nm), which result the 600 nm long afterglow effect of Al N nanohelix.The nucleation habit on W and Ta substrate and continuous growth process of Al N crystal growth were studied,furthermore, large Al N polycrystalline ingots was prepared successfully. The study results of nucleation habit showed that: as intrinsic properties of material, surface potential energy(SPE) and heterogeneous nucleation energy(HNE) have significant impact on the nucleation number and site of Al N crystal, which can be tuned via morphology modifications. The initial nucleation stage of Al N was summarized as three steps:(i) formation process of schottky contact: the work function of substrate determines the adsorption capacity of Al N vapor on the substrate which effect of crystal nucleation density;(ii) migration of adsorbed Al N atoms on substrate surface: the adsorbed atoms are constrained by the periodic SPE in the atomic migration process, and a larger SPE difference provide a stable location for the atom;(iii) nucleation of Al N: atoms at a stable site should overcome heterogeneous nuclear energy to nucleate. The study results of continuous growth process showed that: it is easily grow up for grains with(0001),(10ˉ11) and(10ˉ10) nucleation plane on Ta C substrate, which can grow up to a large grain finally. The size of obtained polycrystalline Al N ingot was: Φ30mm×22mm; the diameter of single crystal was: 8mm×6mm. The quality of Al N crystal gradually increased with the extension of growth time, full width at half maximum of E2(high) was 7cm-1, and full width at half maximum of(0002) rocking curve was 0.15 o detected on crystal growth with 120 h. |
PDF Full Text Request