Optical studies on electron-beam-deposited Zn3P2 thin films

Abstract

After the report [1, 2] of the interesting results on the optoelectronic properties of Zn3P2, a member of the II3-V2 compound semiconductors, as a promising photovoltaic material, studies on the crystal growth and characterization of this compound have been actively pursued by many workers [3-5]. Although a large accumulation of experimental data on the electrical, optical and related properties of bulk ZngP2 (single/polycrystalline) exists in the literature, simitar results are very meagre on thin films of this material. Although thin films of Zn3P2 fabricated using techniques such as thermal evaporation [6, 7], HWE [8, 9], etc., suffer from the problems of high resistivity, deviation from stoichiometry and stability, good quality (stoichiometric and uniform), chemically stable and well-adherent polycrystalline thin films of Zn3P2 could be obtained by an electron-beam evaporation technique on glass substrate kept at elevated temperatures. Such films, found to be p-type, have exhibited resistivity values at 30°C (room temperature) in the range 0.5-1.0 Kf~ cm and the thickness of the films could be varied between 0.2 and 1.5/~m. In this letter some results on the optical properties of Zn3P2 thin films deposited on glass substrates by an electronbeam evaporation technique are reported. The source material used in the fabrication of thin films was prepared by direct reaction of specpure (JMC) materials Zn and P at 850 °C and by C/H2 reduction of specpure Zn3(PO4) 2 at 600 °C [3]. Films obtained by using both source materials exhibited nearly the same properties. The source-substrate distance was kept at 10cm and the substrate temperature (Ts) varied from 30 to 350 °C while depositing the films with the help of a six-position electron-beam source of an E306A Edward vacuum coating system, maintaining a 1.3 x 10 .4 Pa vacuum during evaporation. The films were found to be amorphous for temperatures (Ts) below 200 °C and polycrystalline for Ts above 200 °C for which crystallites were observed with preferential orientation (4 0 0, 2 2 4) corresponding to Zn3P 2 tetragonal phase. X-ray diffractograms taken with the help of a Philips PW 1840 X-ray diffractometer using CuK~ radiation for thin films deposited at some typical values of Ts are presented in Fig. 1. The average particle size estimated from these data was 0.7/~m. The d-values corresponding to the different peaks observed for polycrystalline films are indicated in Table I. The composition of the polycrystalline films, estimated using a LINK energy-dispersive X-ray microanalyser