ACCEPTOR BINDING-ENERGY IN GAN AND RELATED ALLOYS

The recent achievement of p-type doping in GaN epitaxial films emphasi zes the importance of the acceptor ionization energy with relevance to the application of group III nitrides in visible-light-emitting devic es. Measured values of approximately 200 meV suggest that doping effic iencies at room temperature may be no more than 1%, resulting in undes irable series resistance in LEDS and lasers. Consideration of the effe ctive mass hydrogen model of the acceptor ground state in GaAs, GaP an d GaN suggests that this large value of GaN results from the strong el ectronegativity of the nitrogen atom and the corresponding reduction i n dielectric constant, compared with the other compounds. This is furt her emphasized by the need to use the high-frequency dielectric consta nt rather than the static value when the ionization energy is greater than the TO phonon energy. Using epsilon(infinity) = 5.35 and an effec tive mass of 0.4m provides good agreement with the measured ionization energy of GaN, this value of m(h) being consistent with estimates bas ed on recent hole mobility measurements. It is also consistent with sm oothly increasing hole mass in the sequence GaAs (0.33m), GaP (0.37m), GaN (0.40m). Finally, the opportunity is taken to speculate about lik ely values of acceptor ionization energies in AlGaN and AlGaAsN alloys .