Abstract
For the limitation of experimental methods in crystal characterization, in this study, the mechanical, thermodynamic and electronic properties of wurtzite and zinc-blende GaN crystals were investigated by first-principles calculations based on density functional theory. Firstly, bulk moduli, shear moduli, elastic moduli and Poisson’s ratios of the two GaN polycrystals were calculated using Voigt and Hill approximations, and the results show wurtzite GaN has larger shear and elastic moduli and exhibits more obvious brittleness. Moreover, both wurtzite and zinc-blende GaN monocrystals present obvious mechanical anisotropic behavior. For wurtzite GaN monocrystal, the maximum and minimum elastic moduli are located at orientations [001] and <111>, respectively, while they are in the orientations <111> and <100> for zinc-blende GaN monocrystal, respectively. Compared to the elastic modulus, the shear moduli of the two GaN monocrystals have completely opposite direction dependences. However, different from elastic and shear moduli, the bulk moduli of the two monocrystals are nearly isotropic, especially for the zinc-blende GaN. Besides, in the wurtzite GaN, Poisson’s ratios at the planes containing [001] axis are anisotropic, and the maximum value is 0.31 which is located at the directions vertical to [001] axis. For zinc-blende GaN, Poisson’s ratios at planes (100) and (111) are isotropic, while the Poisson’s ratio at plane (110) exhibits dramatically anisotropic phenomenon. Additionally, the calculated Debye temperatures of wurtzite and zinc-blende GaN are 641.8 and 620.2 K, respectively. At 300 K, the calculated heat capacities of wurtzite and zinc-blende are 33.6 and 33.5 J mol−1 K−1, respectively. Finally, the band gap is located at the G point for the two crystals, and the band gaps of wurtzite and zinc-blende GaN are 3.62 eV and 3.06 eV, respectively. At the G point, the lowest energy of conduction band in the wurtzite GaN is larger, resulting in a wider band gap. Densities of states in the orbital hybridization between Ga and N atoms of wurtzite GaN are much higher, indicating more electrons participate in forming Ga-N ionic bonds in the wurtzite GaN.
Highlights
Group-III nitride semiconductors have attracted considerable attention in recent years because of their great potential for technological applications such as short-wavelength light-emitting diodes (LEDs), photocatalysts, optoelectronic nanodevices as well as high-temperature, high-power and high-frequency electronic devices [1,2,3,4]
With the rapid development of numerical methods, calculations and predictions on many mechanical and physical properties of compounds and alloys based on density functional theory (DFT) have been successfully performed and their validity has been verified [30,31,32,33]
First principle calculations based on DFT calculations were performed using the Cambridge Sequential Total Energy Package (CASTEP) program [34], which has already been employed successfully to calculate the physical properties of numerous alloys and compounds
Summary
Group-III nitride semiconductors have attracted considerable attention in recent years because of their great potential for technological applications such as short-wavelength light-emitting diodes (LEDs), photocatalysts, optoelectronic nanodevices as well as high-temperature, high-power and high-frequency electronic devices [1,2,3,4]. Most of the previous studies focused on the effect of surface adsorption [17,18,19,20], defects [21,22,23] and doping [24,25,26,27] on the chemical adsorption performance and their effect on electronic and optoelectronic properties of wurtzite GaN. For the limitation of experimental methods, systematic and in-depth studies on the mechanical and thermodynamic of wurtzite and zinc-blende GaN crystals remain very limited, especially for the anisotropic behavior of mechanical properties. With the rapid development of numerical methods, calculations and predictions on many mechanical and physical properties of compounds and alloys based on density functional theory (DFT) have been successfully performed and their validity has been verified [30,31,32,33]
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