Effect of residual gas on the optoelectronic properties of Mg-doped Ga0.75Al0.25N (0001) surface

In order to verify the effect of residual gas molecules (H-2, CH4, CO, H2O and CO2) adsorbing on the Mg-doped Ga1-xAlxN (0 0 0 1) surface, the photoelectronic properties of the adsorption systems are calculated. Through the first-principles calculation, the surface adsorption energy, work function, dipole moment, the Fermi and vacuum levels, the energy bands, density of states and absorption coefficients are analyzed. The calculation results demonstrate that the adsorption models of CO, H2O, and CO2 molecules are all physically stable structures. However, the adsorption of H-2 and CH4 may not be stable at room temperature. CO2 molecule is the most easily attached to the cathode surface and results in a significantly increasement of the work function. The residual gas captures part of the electrons generated by the cathode surface, and then produces a dipole moment directing from the cathode surface to the residual gas molecules. In addition, tiny alteration of energy band is observed on the residual gas adsorption system. Due to the orbital motion, some new energy levels are also generated in the deep energy band. Moreover, the adsorption of CO, H2O and CO2 molecules also exert a noticeable effect on the absorption coefficient.