Department of Physics, China University of Mining and Technology, Xuzhou 221116, People's Republic of China.
J Phys Condens Matter. 2013 Jan 30;25(4):045801. doi: 10.1088/0953-8984/25/4/045801. Epub 2012 Dec 17.
The structural stability, spontaneous polarization, piezoelectric response, and electronic structure of AlN and GaN under uniaxial strain along the [0001] direction are systematically investigated using HSE06 range-separated hybrid functionals. Our results exhibit interesting behavior. (i) AlN and GaN share the same structural transition from wurtzite to a graphite-like phase at very large compressive strains, similarly to other wurtzite semiconductors. Our calculations further reveal that this well-known phase transition is driven by the transverse-acoustic soft phonon mode associated with elastic instabilities. (ii) The applied tensile strain can either drastically suppress or strongly enhance the polarization and piezoelectricity, based on the value of the strain. Furthermore, large enhancements of polarization and piezoelectricity close to the phase-transition regions at large compressive strains are predicted, similar to those previously predicted in ferroelectric fields. Our calculations indicate that such colossal enhancements are strongly correlated to phase transitions when large atomic displacements are generated by external strains. (iii) Under the same strain, AlN and GaN have significantly different electronic properties: both wurtzite and graphite-like AlN always display direct band structures, while the the bandgap of wurtzite GaN is always direct and that of graphite-like GaN always indirect. Furthermore, the bandgap of graphite-like AlN is greatly enhanced by large compressive strain, but that of wurtzite GaN is not sensitive to compressive strain. Our results are drastically different from those for equibiaxial strain (Duan et al 2012 Appl. Phys. Lett. 100 022104).
采用 HSE06 范围分离混合泛函系统地研究了[0001]方向单轴应变下 AlN 和 GaN 的结构稳定性、自发极化、压电响应和电子结构。我们的结果表现出有趣的行为。（i）AlN 和 GaN 在很大的压缩应变下，与其他纤锌矿半导体一样，从纤锌矿结构转变为石墨相，这与其他纤锌矿半导体相同。我们的计算进一步表明，这种众所周知的相变是由与弹性不稳定性相关的横向声学软声子模式驱动的。（ii）根据应变的大小，施加的拉伸应变可以极大地抑制或强烈增强极化和压电性。此外，在大压缩应变的相变区域附近，预测了极化和压电性的大幅度增强，类似于铁电领域中先前预测的情况。我们的计算表明，当外部应变产生大的原子位移时，这种巨大的增强与相变强烈相关。（iii）在相同的应变下，AlN 和 GaN 具有显著不同的电子性质：纤锌矿和石墨相 AlN 总是显示直接带结构，而纤锌矿 GaN 的能带隙总是直接的，石墨相 GaN 的能带隙总是间接的。此外，大压缩应变极大地增强了石墨相 AlN 的带隙，但对纤锌矿 GaN 的带隙不敏感。我们的结果与等双轴应变（Duan 等人，2012 年 Appl. Phys. Lett. 100 022104）的结果有很大的不同。
