Melting versus Decomposition of GaN: Ab Initio Molecular Dynamics Study and Comparison to Experimental Data

The technology of GaN is very advanced due to excellent figures of merit relevant for key applications like LEDs or high-power-high-frequency transistors. Nevertheless, some fundamental physical properties, including the phase diagram, of this so important semiconductor, are still not determined. In this work, simulations of the GaN crystal behavior at heating to extremely high temperatures in a wide pressure range are reported. Ab initio density functional theory (DFT) molecular dynamics extensive calculations were used to obtain the temporal change of the system. Heating of an initially perfect wurtzite GaN crystal to a temperature above 3500 K resulted in the destruction of the crystalline lattice and transition of the system to a perfectly disordered fluid. At the pressure of 7.1 GPa and below, the formation of N-2 molecules inside the cluster was detected; therefore, the process was identified as the thermal decomposition of GaN. At pressures as high as 15 GPa, the formation of N-2 molecules was suppressed despite the total loss of the crystalline order. The system consisted of single gallium and nitrogen atoms, so the high-pressure process was identified as congruent melting of GaN. An excellent agreement of the simulation results with well-established experimental data on GaN decomposition has been found. The absence of N-2 molecules in the fluid at high pressures indicates that the intersection of the decomposition line and the melting line occurs in the pressure range between 7 and 15 GPa. At pressures above the intersection point, gallium nitride can be congruently melted without decomposition, in contrast to lower pressures where GaN decomposes before reaching the melting point. This is in agreement with the proposed melting model based on our earlier experiments, suggesting a reversal of the decomposition-melting sequence at heating at 12 GPa.