Ambipolar Phototransport (μτ e = μτ h ) Observed as an Intrinsic Property of a-SiGe:H

A study is presented of a series of high quality PECVD a-Si 0.33 Ge 0.67 films, produced by cathodic deposition, in which small concentrations of PH 3 , B 2 H 6 or air impurities were added during deposition. The quantum efficiency-mobility-lifetime product (ημτ) increases, and the ambipolar diffusion length (L amb ) decreases monotonically with dopant concentration for both PH 3 and B 2 H 6 . This result is strong evidence that for these films neither photocarrier is dominant (μτ e = μτ h ) at zero doping. This result is very different from what has been typically observed by other researchers, that the electron is the dominant photocarrier for undoped a-SiGe:H. Drive level capacitance (DLC) measurements of these alloys show an unusual behavior of being temperature-independent, and the dark conductivity activation energy is maximum for zero doping. It is proposed that all of these unusual properties are due to the unusually low impurity concentration of these films, and that these properties are, in fact, the intrinsic properties for a-SiGe:H alloys. To verify this, films were prepared with a calibrated and controlled air leak introduced during deposition. As the air leak was increased, the film properties changed to typical behavior. Even for air concentrations as low as 2 ppm (gas phase), the transport measurements showed changes consistent with a shift in the Fermi level toward the conduction band.