Single-crystalline, colorless, GaN has been prepared by a vapor-phase growth technique previously used to prepare GaAs, GaP, and GaSb. These crystals are the first reported speciments of GaN suitable for good electrical and optical evaluation of this compound. It has been determined that GaN has a direct energy bandgap of 3.39 eV, and that undoped crystals prepared by this method have a very high inherent electron concentration, typically above 1019/cm3, which is probably related to a high density of nitrogen vacancies. Conducting p-type specimens have been prepared using Ge as the dopant; but this result has been difficult to reproduce, and the samples have been electrically inhomogeneous.

In all previous studies of the photoelectrolysis of water, very little attention has been paid to the carrier generation and transport properties of the optically active semiconductor electrode. To gather such information the present work theoretically analyzes the spectral response of TiO2 photoelectrolysis cells. Comparison with experimental results allows us to determine the diffusion length of minority carriers in TiO2 for the first time. It is the hole transport that governs the spectral response curve, not the electron transport. The quantum efficiency of carrier generation in TiO2 electrodes in the photolysis mode can be increased to 80% by doping the crystals with Al. The sunlight conversion efficiency has been raised to 1.3% from 0.4% reported earlier by others. The spectral response of the device has been extended into the visible portion of the spectrum through sensitization of the TiO2 with Cr dopant impurities, allowing hydrogen generation with visible light. The photoelectrolytic processes associated with the impurity doped crystals are discussed.

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