Structural and Optical Evaluation of Vacuum-Deposited GaP Films

Abstract

Optical absorption-edge shifts of up to 1.5 eV have been observed for GaP films deposited in textured polycrystalline states and in the amorphous state. The amorphous-crystalline transition occurs for a substrate temperature Ts of about 240°C. The films deposited at 240°C≤Ts≤270°C exhibiting the large optical absorption-edge shift are nontransparent, metallic-appearing, and exhibit a strong [110] texture. The amorphous films deposited at just below 240°C exhibit the same optical properties. With increasing Ts, the films exhibit decreasing optical absorption-edge shifts, becoming transparent-yellow for Ts≥425°C. However, a bulk optical behavior is achieved only by annealing the films at temperatures up to 600°C. The principal textures found by x-ray diffraction are [110] for 240°≤Ts≤270°C, and [111] for Ts>350°C. Reflection-electron-diffraction (RED) observations agree substantially with those obtained by x ray, but also indicate some mixed textures. A weak (101̄0) reflection is observed by both x rays and RED, indicating an incipient hcp phase; it is shown that this is not causally related to the optical-edge shift. X-ray lattice-constant measurements over the range of Ts from 250°–425°C indicate no observable systematic deviation from bulk values; these observations rule out any causal relationship between changes in lattice constant and the optical absorption-edge shift. Extinction bands or bounded-plasma resonance effects do not seem to be able to explain the experimental behavior; the shifts are most probably due to large concentrations of self-compensated natural defects. It is emphasized that these as-deposited and annealed GaP films cover the entire absorption spectrum (α vs E) of all III-V compounds (except InAs and InSb) plus Ge and Si.