Sub-band-gap absorption of GaN grown by metal-organic chemical vapor deposition on sapphire was investigated by photothermal deflection spectroscopy ͑PDS͒, transmission measurements, and the constant photocurrent method ͑CPM͒. We determine acceptor binding energies in undoped GaN at 220 and about 720 meV. A comparison between absorption and CPM spectra yields the dependence of the quantum efficiency-mobilitylifetime-product ͑͒ versus energy and gives relevant information about the excitation mechanisms. CPM spectra show a significantly smaller absorption ͑up to a factor of 1/10͒ in the range between 3.0 and 3.3 eV as compared to PDS. This indicates that the majority of carriers excited with these photon energies have a relatively small product and thus do not contribute to the externally detected photocurrent. We propose that in this energy range the spectrum is dominated by interband absorption in isolated cubic-phase crystallites in the hexagonal matrix and by excitation of electrons from occupied acceptors into the conduction band of the main hexagonal crystal modification ͑h-GaN͒. Temperature-dependent photoluminescence measurements, excited with energies below and above the direct band gap of hexagonal GaN, confirm this interpretation and can be correlated with the subgap absorption detected by PDS. Transient photocurrent measurements show a persistent photoconductivity, which can also be explained by the existence of isolated cubic-phase inclusions.

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Subbandgap absorption of chemical-vapor-deposition diamond films, with nitrogen contents varying from 10 to 132 ppm has been explored by the constant-photoconductivity method ͑CPM͒, photothermal-deflection spectroscopy ͑PDS͒ and electron spin resonance ͑ESR͒. The spectra measured by PDS increase monotonically and are structureless with increasing photon energies indicating absorption due to amorphous carbon and graphite. The CPM data show distinct features, with absorption bands at hϭ1.6, 4.0, and 4.7 eV in the nominally undoped film, and 2.4 and 4.7 eV in nitrogen-rich layers respectively. The CPM spectra of the doped films are comparable to photoconductivity data of synthetic Ib diamond. The defect densities involved increase with increasing nitrogen content. From ESR, a vacancy-related defect density (gϭ2.0028) is deduced. Paramagnetic nitrogen (gϭ2.0024) can be detected in the high-quality CVD layer or by illuminating the nitrogenrich samples with photon energies larger than the band gap.