Thermal admittance spectroscopy of Mg-doped GaN Schottky diodes

Thermal admittance spectroscopy measurements at temperatures ranging from r oom temperature to 90 K are performed on Schottky structures based on Mg-do ped GaN layers grown by metalorganic vapor phase epitaxy on sapphire. The a nalysis of the experimental data is made by a detailed theoretical study of the steady-state and small-signal electrical characteristics of the struct ures. Numerical simulations are based on the solution of the basic semicond uctor equations for the structure consisting of two Schottky diodes connect ed back to back by a conduction channel formed by the GaN layer. The descri ption explicitly includes the Mg-related acceptor level, with its temperatu re- and position-dependent incomplete occupation state, leading to a dynami c exchange with the valence band. It fully reproduces the variations with t emperature of the capacitance-frequency and conductance over frequency curv es, allowing to give for all temperature ranges the origin of the various c ontributions to the junction capacitance and of the microscopic mechanisms responsible for the capacitance-frequency cutoff. Series resistance effects are shown to be dominant at temperatures above 230 K, whereas the Mg-relat ed acceptor level governs the electrical behavior below 230 K. The existenc e of a second acceptor level with an activation energy of several tens of m eV is revealed from the analysis of the characteristics at low temperature. An optimized fitting procedure based on the comparison of the electrical c haracteristics obtained from the numerical simulations to the experimental data allows one to determine the microscopic parameters describing the stru cture, among which the acceptor activation energies, thermal capture cross sections, concentrations, and the Schottky contact barrier heights are the most important ones. The obtained activation energy of the Mg-acceptor leve l of 210 meV is by a factor of 2 larger than that obtained from a classical Arrhenius plot, showing that a complete description of Mg-doped GaN juncti ons requires the correct treatment of the Mg level, acting as a dopant and as deep impurity, as well as the inclusion of series resistance effects. (C ) 2001 American Institute of Physics.