III-Nitride Polymorphs: XN (X=Al, Ga, In) in the Pnma phase.

The structural, mechanical, elastic anisotropy, and electronic properties, together with the stability, effective mass of holes and electrons for XN (X=Al, Ga, In) in the Pnma phase are investigated by using density functional theory calculations. The elastic constants and the phonon spectra all manifest III-nitride polymorphs: XN (X=Al, Ga, In) in the Pnma phase in this work are mechanically and dynamically stable at ambient pressure. Al atoms, Ga atoms, and In atoms lead to new electrical and band-gap properties: XN (X=Al, Ga, In) in the Pnma phase are all semiconductor materials with direct band gaps of 4.76 eV, 2.80 eV, and 0.66 eV, respectively, which present great application potentials in the new generation electronic devices such as ultraviolet detectors, visible light detectors, infrared detectors, violet-light diodes, and light-emitting diodes.