Gallium oxide Ga2O3 has gained much attention because of its unique properties such as ultra-wide bandgap (4.56eV), and high breakdown field which makes it very promising for application in certain classes of Opto- and power electronics, solar-blind UV photodetectors, solar cells, etc. It is usually reported that there are five different polymorphs of Ga2O3, namely, the monoclinic (β-Ga2O3), rhombohedral (α), defective spinel (γ), cubic (δ), or orthorhombic (ε) structures. Because of these, the b-polymorph is the stable form under normal conditions and has been the most widely studied and utilized.
However, in gallium oxide, like other semiconductor oxides, obtaining low ohmic hole conductivity is still challenging. The difficulty is connected to low hole mobility caused by the big effective mass of holes, high ionization energies of native and impurity acceptors, compensation processes (appearance of hole killer defects) stipulated by high band-gap, and low lying valence band. After N impurity in β-Ga2O3, the N atom would substitute the site of O atom because it is closest to O in terms of the atomic size and electronic structure. And it is expected to lead to p-type conductivity.
We fulfilled the thermodynamic analysis of the concentration equilibrium for nitrogen doped gallium oxide using the Kroger method of quasi-chemical reactions, which gives one possibility to find the dependence of concentrations of defects and free carriers on growth/processing parameters and impurity concentration, and correspondingly to find their optimal values. Furthermore, the quasi–chemical solution model has a better treatment of configurational entropy which accounts for a non–random distribution of atoms. The analysis was carried out for the system Ga2O3 doped samples – vapor of components. According to the analysis, the hole concentration increases with increasing oxygen pressure, and at some critical value holes and substitutional nitrogen become dominant species (for T=900 °C, P=1 atm). Also further increase of oxygen pressure effects on hole concentration. We took into account the formation of the nitrogen molecule, which is expected to reduce the effect of doping. The formation energy of nitrogen molecules is quite high – 9.756 eV. In Gallium oxide taking into account dielectric screening with local field correction, dissociation energy turned out to be approximately 3 eV. Based on the results one can conclude, that for N doping of gallium oxide, the native donors - oxygen vacancy or Gallium interstitial are responsible for p-conductivity compensation; On the other hand, if we take into account the formation of the nitrogen molecule, then we will not get p-type conductivity.