Study On AlGaN/AlInN Distributed Bragg Reflectors In Solar-blind Ultraviolet Area

Ⅲ-nitride materials (AIN, GaN, InN and their ternary or quaternary alloy compounds) are direct-band gap semiconductors. Owing to their advantages of the band gaps from0.7eV to6.2eV by adjusting the component and excellent physical and chemical stabilities, Ⅲ-nitride semiconductors are the most favorite material system for the fabrication optoelectronic devices operable from infrared to ultraviolet spectrum region. Distributed Bragg reflectors (DBRs) based on AlGaN/GaN or AlInN/GaN have been carried out in microcavity devices, which work in visible and near-ultraviolet region. However, the research on DBRs structure used in the ultraviolet region stem from the difficulty for growth good qulity high-A1-content AlGaN and the big strain in the structure caused by the lattice mismatch and thermal mismatch.In this dissertation, the regularity of the refraction index of the high-Al content AlGaN and AlInN is simulated by the formula provided by Brunner. And two solar-blind AlGaN/AlInN DBRs structures are designed and fabricated on an AIN template by plasma-assisted molecular-beam epitaxy. The properties of the two DBRs are also investigated by experimental characterizations. The main conclusions we have obtained are listed as follows: 1. The peak reflectivity variation of Al0.98In0.02N/AlyGa1-yN DBR structure with central wavelength at246nm fabricated on LT-AIN layer is calculated by the transfer matrix method. The refractive index values of AlyGa1-yN at246nm decrease as the Al content increasing, which results in reduction the refractive index difference between two layers in DBRs. And this lead to decrease the peak reflectivity of DBRs with increasing Al component in AlyGa1-yN layer.2. Two solar-blind AlGaN/AlInN DBRs structures with different periods and Al concentration are successfully grown by MBE. The strains in the DBR samples are analyzed by high resolution x-ray diffraction (HRXRD) and reciprocal space mapping (RSM). The results show that the strains in the samples increase with the Al composition and the DBRs period increasing. And the contents of In, Al and Ga in the two DBR structures are measured by SIMS and simulated by HRXRD.3. The13.5-pair Al0.98In0.02N/Al0.77Ga0.23N DBR structure exhibited a peak reflectivity of83.9%with central wavelength at246nm and a stopband width of18nm. The average refractive index contrast extracted from experimental data was9.25%for the Al0.98In0.02N/Al0.77Ga0.23N DBR at246nm, which is unexpectedly high. Based on the experimental refractive indices, the reflectivity spectrum of the DBR structure is simulated using the transfer matrix method. The results show that there some discrepancy exists between the simulated and experimental reflectivity spectra. Our investigations indicate that the the discrepancy is ascribed to the nonuniformity of layer thickness and the blurry, rough interface between the AlInN and AlGaN layers.4. Simulation results based on the experimental data indicated that a25.5-period Al0.98In0.02N/Al0.8Ga0.2N DBR will exhibit reflectivity higher than99%and a stopband width of19nm centered in the solar-blind UV region.