DEFECT-RELATED OPTICAL-TRANSITIONS IN GAN

Sub-band-gap absorption of GaN grown by metal-organic chemical vapor d eposition on sapphire was investigated by photothermal deflection spec troscopy (PDS), transmission measurements, and the constant photocurre nt method (CPM). We determine acceptor binding energies in undoped GaN at 220 and about 720 meV. A comparison between absorption and CPM spe ctra yields the dependence of the quantum efficiency-mobility-lifetime -product (eta mu tau) versus energy and gives relevant information abo ut the excitation mechanisms. CPM spectra show a significantly smaller absorption (up to a factor of 1/10) in the range between 3.0 and 3.3 eV as compared to PDS. This indicates that the majority of carriers ex cited with these photon energies have a relatively small eta mu tau pr oduct and thus do not contribute to the externally detected photocurre nt. We propose that in this energy range the spectrum is dominated by interband absorption in isolated cubic-phase crystallites in the hexag onal matrix and by excitation of electrons from occupied accepters int o the conduction band of the main hexagonal crystal modification (h-Ga N). Temperature-dependent photoluminescence measurements, excited with energies below and above the direct band gap of hexagonal GaN, confir m this interpretation and can be correlated with the-subgap absorption detected by PDS. Transient photocurrent measurements show a persisten t photoconductivity, which can also be explained by the existence of i solated cubic-phase inclusions.