The Mechanism of Photovoltaic Effect of Ge Film Obliquely Deposited in Vacuum

The investigations were carried out on photovoltaic effects of Ge films vacuum-deposited on glass substrate at an oblique angle of incidence of 45°. By introducing air slowly to the vacuum system the change in photovoltage of the film with the pressure was measured. By illumination through a band-pass filter (0.3µ≤λ≤0.4µ), the photovoltage reversed its sign as the pressure was increased. The sign was changed from negative to positive in case of the illumination from frontside of the substrate, while in case of the illumination from backside, it was changed from positive to negative. The magnitude of the photovoltage increased remarkably in both cases as the pressure approached atmospheric one. On the other hand, when a short-cut filter (0.69µ≤λ) was used, the photovoltages increased with the pressure but their polarities were always positive regardless of the direction of the illumination. Spectral sensitivity of photovoltage in air was also measured. Only in case of the illumination from backside of the substrate, the photovoltage showed negative polarity in the region of wavelength shorter than 0.5µ. These changes in photovoltages with the pressure are ascribed to those of surface recombination velocities of the film due to gas adsorption. The origin of these photovoltages is discussed on the basis of the Dember effect which arises from the difference between electron and hole mobilities.