Optical properties of amorphous thin films of the Zn-P system

The optical properties of thermally evaporated amorphous thin films with the compositions Zn57P43 (near Zn3P2) and Zn32P68 (near ZnP2) were studied. The absorption edges following the Tauc power law are attributed to the transitions between the localized states in the valence band tails and the delocalized states at the mobility edge of the conduction band. The deduced E(g) values fall into the 0.9-1.1 eV and 1.3-1.4 eV intervals for Zn57P43 and Zn32P68 amorphous films respectively. The transitions linking the localized states in the exponential tails of the valence and conduction bands are also discussed. The energies E(U) deduced from the Urbach tails are found to change from about 125 to 570 meV for Zn57P43 films and to be about 170 meV for Zn32P68. The exponential shoulder of the optical absorption results in E(T) energies ranging from about 570 to 1300 meV for Zn57P43 films and about 500 meV for Zn32P68. The feature at 5.4 eV in the reflectivity spectrum is attributed to the middle-range order found in these materials.