First-principles Study On Thermodynamic Properties And Phase Diagram Of GaN

The III-V compound gallium nitride (GaN) as semiconductors has a series ofexcellent properties, such as a wide band gaps, a high thermal conductivity, a strongradioresistance and a good chemical stability. The study and application of GaN is thehot issue in the current global research field about semiconductor. It has a broadapplication prospect in photoelectron, high power devices and high frequencymicrowave devices.GaN is an extremely stable compound and it has high melting point to about2500K. Under atmospheric pressure, GaN possesses wurtzite structure and thearrangement of atoms is incompact in this structure. With the temperature and pressureincreasing, however, the structure of GaN changes from the incompact wurtzite to themore compact rocksalt. We recognize that the atoms are the same but the arrangementof atoms is different in the two structures. So there are obvious differences in allaspects of the properties between the two structures.The structure parameters and the phase transition pressure between the twostructures are observed by the means of XRD. Meanwhile, these results are also verifiedby the theoretical simulation. But there is an obvious issue in both experiment andtheory. The phase transition pressure between the two structures are verified, however,how it will vary with the changing of temperature is unknown for us. Therefore, in orderto ascertain this key issue and measure the thermodynamic properties about the twostructures, the zero temperature must be extended to the finite temperature, in otherword, the effect of temperature should be taken into account. In fact, if the contributionof thermal vibration is considered and phonon spectrum based on lattice dynamics arecalculated, the influence of temperature on the phase transition pressure will be clearedfor us.In this article, the structure parameters and the phase transition pressure arecalculated by first principle method. Then we introduce the effect of temperature bycomputing the phonon spectrum and as a result we can obtain a series ofthermodynamic properties of both phases, such as thermal expansion coefficients, bulkmodulus and heat capacities. But, above all, we present the temperature dependence ofthe phase transition pressure for both wurtzite and rocksalt phases of GaN. As a result,our study shows that the temperature dependence of the phase transition pressure is more obvious above1600K than that below1600K. Therefore,1600K is the criticaltemperature which affects the change rate of the phase transition pressure. Ourcalculation also indicates that rocksalt phase is more stable than wurtzite phase onlyunder high pressure and temperature.