Abstract
The mechanical and electronic properties of two GaN crystals, wurtzite and zinc-blende GaN, under various hydrostatic pressures were investigated using first principles calculations. The results show that the lattice constants of the two GaN crystals calculated in this study are close to previous experimental results, and the two GaN crystals are stable under hydrostatic pressures up to 40 GPa. The pressure presents extremely similar trend effect on the volumes of unit cells and average Ga-N bond lengths of the two GaN crystals. The bulk modulus increases while the shear modulus decreases with the increase in pressure, resulting in the significant increase of the ratios of bulk moduli to shear moduli for the two GaN polycrystals. Different with the monotonic changes of bulk and shear moduli, the elastic moduli of the two GaN polycrystals may increase at first and then decrease with increasing pressure. The two GaN crystals are brittle materials at zero pressure, while they may exhibit ductile behaviour under high pressures. Moreover, the increase in pressure raises the elastic anisotropy of GaN crystals, and the anisotropy factors of the two GaN single crystals are quite different. Different with the obvious directional dependences of elastic modulus, shear modulus and Poisson’s ratio of the two GaN single crystals, there is no anisotropy for bulk modulus, especially for that of zinc-blende GaN. Furthermore, the band gaps of GaN crystals increase with increasing pressure, and zinc-blende GaN has a larger pressure coefficient. To further understand the pressure effect on the band gap, the band structure and density of states (DOSs) of GaN crystals were also analysed in this study.
Highlights
Because of superior performances, such as wide direct band gap, excellent luminous efficiency, corrosion resistance, low dielectric constant, high temperature resistance, electron mobility and outstanding mechanical strength [1,2,3,4,5], GaN is known as one of the third-generation semiconductor materials and can be applied potentially in LEDs, lasers, sensors, high power, spintronic devices, etc. [6,7]
generalised gradient approximation (GGA) overestimates the lattice parameters while local density approximation (LDA) underestimates them, and the lattice parameters obtained from LDA are much closer to experimental data
Our previous work revealed that, compared to GGA, LDA is more accurate in predicting elastic properties [38]
Summary
Because of superior performances, such as wide direct band gap, excellent luminous efficiency, corrosion resistance, low dielectric constant, high temperature resistance, electron mobility and outstanding mechanical strength [1,2,3,4,5], GaN is known as one of the third-generation semiconductor materials and can be applied potentially in LEDs, lasers, sensors, high power, spintronic devices, etc. [6,7]. It has been reported that, in a revised molecular-beam epitaxy system, cubic zinc-blende GaN can be obtained on β-SiC (100) substrates [8]. Compared with wurtzite GaN, zinc-blende GaN is supposed to be more flexible for molecular designs [9,10]. Crystals 2018, 8, 428 defects [15,16,17] and doping [18,19,20,21] on the electronic and optoelectronic performances of wurtzite GaN, while less research has been done about that of zinc-blende GaN. Because of deficiencies of experimental equipment and measurement methods, in-depth investigations on the mechanical and electronic performances of the two GaN crystals are still limited
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