Study on the anomalously high photovoltaic effect in germanium thin films

Abstract

The physical and electrical properties of obliquely vacuum-deposited germanium films are studied. The deposition angle, defined as the angle between the source flux and the substrate plane, ranged from 15 ° to 75 °. The magnitude of the photovoltage and the film resistivity decreases as the deposition angle increases. Various amounts of high-purity oxygen and nitrogen are introduced during the film deposition and the effects upon the film characteristics are observed. The photovoltage produced by these films upon illumination are examined. The photovoltages are typically reproducible within a factor of 4 or less. The maximum variation in photovoltage among supposedly identical films is less than a factor of 8. Both the magnitude and the polarity of the photovoltage can be controlled. The amount of nitrogen during deposition, from PN2≈1×10−6 to 1×10−4 Torr, has little effect upon either the photovoltage or the film resistivity. The amount of oxygen during deposition, from PO2≈1×10−6 to 1×10−4 Torr, alters the photovoltage and the resistivity drastically. The magnitude of the photovoltage and the film resistivity increases as the amount of oxygen during deposition increases. For 15 ° samples the average measured photovoltage magnitudes are 76.0, 27.6, and 12.3 V/cm for samples deposited in PO2≈1×10−4, 1×10−5, and 1×10−6 Torr, respectively; for 60 ° samples the average measured photovoltage magnitudes are 1.02, 0.58, and 0.125 V/cm. The average resistivity of the 15 ° samples is 2.10×108 and 9.18×106 Ω cm for samples deposited in oxygen partial pressure of 1×10−4 and 1×10−6 Torr, respectively. The corresponding average resistivities for 60 ° samples are 3.86×107 and 3.26×105 Ω cm. In addition, the polarity of the photovoltage reverses as the partial pressure of oxygen during deposition is increased from 1×10−5 to 1×10−4 Torr. The magnitude of the photovoltage is directly related to the resistivity of the films. The photovoltage increases as the thickness of the film increases. No relationship between the photovoltage and the film thickness gradient is observed. X-ray diffraction measurements indicate that the films are amorphous. ESCA investigation of the films indicates that the freshly deposited films consist of germanium and its associated oxides. ESCA study of aged films indicates that the films deposited in nitrogen atmosphere consist primarily of Ge and two oxides or hydroxides, while the films deposited in oxygen atmosphere consist primarily of Ge and one oxide or hydroxide. All films contain oxygen; however, no nitrogen is present in the films (even films deposited in high nitrogen partial pressures). A qualitative film structural model, based on nucleation and growth phenomena, is proposed to explain the observed effect.