Optical and valence band studies of ZnP 2 thin films
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Conditions have been developed for the preparation of ZnP2 and deposition of its stoichiometric thin films, using the flash evaporation technique. Structural properties of the ZnP2 obtained have been studied using x-ray diffraction, and chemical composition has been established by the polarography technique. Optical absorption of thin films of j8-ZnP2 has been investigated over the range 1.2-3.2 eV. Analysis of thin film data showed that j8-ZnP2 is a direct band gap material. The XPS and UPS of /3-ZnP2 show a shift in binding energy (BE), which is due to transfer of electrons from zinc to phosphorus.
I. INTRODUCTION The II-V compounds, particularly Zn3P2'~10 and ZnP2 (a and /3),11'12 have become of ever-increasing importance in the last few years due to their applications for solar energy conversion. ZnP2 exists as two distinct modifications: a red tetragonal (a) form and a black monoclinic (0) form. Although some information is available in the literature, much of the previously published data on these two polymorphs is on single crystals. However, in the literature, there are no reliable data for the optical properties of thin films of ZnP2 — in particular, the determination of the band gap of these compounds in thin film form. This communication investigates the optical properties of the vacuum evaporated ZnP 2 (/3) thin films and also obtains further information on the valence band structure of these compounds by x-ray photoelectron spectroscopic studies. II. EXPERIMENTAL The starting materials were metallic 99.9999% zinc and 99.9999% red phosphorus. Zn3P2 was directly syna) Address
correspondence to this author; NCL communication number 4409.
thesized from these elements by reaction in an evacuated silica ampoule, as described earlier. 10 Stoichiometric quantities of Zn and red P were slowly heated to 850 °C for 24 h. The polycrystalline Zn3P2 ingot thus obtained was used as the starting material for preparation of ZnP2. Stackelberg and Paulus13 have synthesized only the red tetragonal modification of ZnP2 apart from Zn3P2. Hegyi et al.14 have prepared and characterized the black monoclinic /3-ZnP2. Rubenstein and Dean15 prepared both a and /3-ZnP 2 by placing zinc and phosphorus in silica ampoules separately and heating in a two-zone furnace. In the present work, both a and /3-ZnP 2 have been synthesized in a two-step process, as described by Kanel et al.12 First, the Zn3P2 ingot obtained by subliming Zn metal and red P has been purified by further sublimation, and the purified compound is transformed into ZnP2 through a subsequent reaction with excess phosphorus at a temperature of 800—850 °C, in an evacuated silica ampoule. The pressure in the ampoule depends on the temperature and the amount of excess phosphorus. Both red (a) and black (/3) modifications are obtained in various proportions by this method. The colors red and black indicated forma-
FIG. 1. XRD pattern of ZnP2 powder.
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2 6 (DEGREES) J. Mater. Res., Vol. 4, No. 1, Jan/Feb 1989
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