This review systematically summarizes the latest research advances of AlN crystals grown by the PVT method and their applications.

The effects of porous structures on the performance of a GaN UV photodetector were investigated for the first time.

Porous template has the function of relieving the stress of epitaxially grown GaN crystals and blocking dislocations. In this study, a 2-inch self-standing porous GaN crystal film was fabricated for...

Metal-organic chemical vapor deposition (MOCVD)-grown GaN on sapphire substrate was etched by hot phosphoric acids. Pyramid structures were obtained in the N-polar face of the MOCVD-GaN. Details of the formation process and morphology of the structures were discussed. The crystallographic plane index of the pyramid facet was calculated dependent on the symmetry of the wurtzite crystal structure and the tilt angle. The substrates with pyramid structures were utilized in subsequent hydride vapor phase epitaxy (HVPE) growth of GaN. Free-standing crystals were obtained, while HVPE-grown GaN achieved a certain thickness. Raman spectroscopy was employed to obtain the stress conditions of the HVPE-GaN without and with sapphire substrate. The mechanism of the self-separation process was discussed. This facile wet etching method may provide a simple way to acquire free-standing GaN by HVPE growth.

Based on the actual hot zone structure of an AlN crystal growth resistance furnace, the global numerical simulation on the heat transfer process in the AlN crystal growth was performed. The influence of different heater structures on the growth of AlN crystals was investigated. It was found that the top heater can effectively reduce the axial temperature gradient, and the side heater 2 has a similar effect on the axial gradient, but the effect feedback is slightly weaker. The axial temperature gradient tends to increase when the bottom heater is added to the furnace, and the adjustable range of the axial temperature gradient of the side 1 heater + bottom heater mode is the largest. Our work will provide important reference values for AlN crystal growth by the resistance method.

In the process of PVT growth of AlN crystals, there is difficult to maintain ideal thermodynamic equilibrium conditions, causing crystal defects being inevitably generated. High temperature annealing technology has received much attention due to their effectiveness in improving crystal integrity. In this paper, AlN samples grown by PVT method were annealed at high temperature in N 2 atmosphere. In order to evaluate the crystalline quality and structural perfection of AlN before and after thermal annealing, high-resolution X-ray diffraction (HRXRD) and Raman spectrum were carried out. In addition, the impurity related band gap changes in the optical properties of AlN crystals were characterized by room temperature photoluminescence (PL) and absorption spectra. The crystal quality of these AlN crystals was significantly improved after annealing at 1400-1800 ℃. The full width at half maximum (FWHM) of the ( 1012) plane X-ray rocking curve decreased from 104.04 to 79.92 arcsec (1 arcsec=0.01592°) after annealing at 1400 ℃ . As the annealing temperature increases, the absorption was significantly enhanced and the band gap became larger, indicating that the annealing process was beneficial to improve the quality of AlN crystals. The results of secondary ion mass spectrometry (SIMS) demonstrate that the annealing process reduces the C impurity, resulting in an increase in band gap of AlN crystal, which is consistent with the results of optical absorption.