Optical absorption measurements were performed on a series of thin GaN epilayers. Sharp spectral features were observed due to the 1s A and B exciton transitions. Using polarization dependent absorption, the C exciton transition was identified. A broad absorption feature was observed at ϳ3.6 eV, which is attributed to indirect exciton-phonon absorption. The excitonic structure was found to persist well above room temperature. A fit to the Varshni formula yielded a temperature dependence of E(T)ϭE(Tϭ0)Ϫ11.8ϫ10 Ϫ4 T 2 (1414ϩT) eV for the A and B excitons. The exciton absorption linewidth was studied as a function of temperature, indicating that GaN exhibits very large exciton-phonon coupling.

We propose a general method to obtain high conductivity of either type in wide gap semiconductors where compensation normally limits conductivity of one or both types. We suggest that the successes of Amano et al. and of Nakamura et al. in obtaining more than 1018 cm-3 holes in GaN are particular examples of the general process that we propose.

Femtosecond degenerate four-wave-mixing ͑FWM͒ is used to study coherent dynamics of excitons in GaN epilayers. Spectrally resolved ͑SR͒ FWM data are dominated by the A and B intrinsic excitonic resonances. SR FWM combined with time-integrated ͑TI͒ FWM demonstrates that the excitonic resonances are nearly homogeneously broadened even at low temperature. The temperature-dependent dephasing rate is used to deduce exciton-phonon interaction rates. TI FWM shows a strong beating between the A and B excitons and the beats are shown to be true quantum beats. In addition, a 180°phase shift in the quantum beating was observed when the polarization geometry was changed from collinear to cross linear.

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