Photoelectronic properties of zinc phosphide crystals, films and heterojunctions. Quarterly progress report No. 1, February 26-June 30, 1979

A closed tube method has been developed for the synthesis of Zn/sub 3/P/sub 2/ from the elements which eliminates the potential dangers associated with high P pressures. Subsequent materials synthesis will be done routinely by this method. Initial efforts at single crystal growth of Zn/sub 3/P/sub 2/ from this material yielded polycrystalline material because of nucleation along the walls of the tube. Techniques for improving the situation are being tested. Wafers of large-grain polycrystaline Zn/sub 3/P/sub 2/ have been received from Dr. A. Catalano at Delaware and will be used for exploratory research in the quarter ahead. Progress is reported on the development of a CVD apparatus for the deposition of wide bandgap oxides and sulfides on a variety of substrates including Zn/sub 3/P/sub 2/. Layer deposition by the close spaced vapor transport method to be applied to Zn/sub 3/P/sub 2/ was successfully tested using CdTe on both quartz and CdS substrates

Photoelectronic properties of zinc phosphide crystals, films and heterojunctions. Quarterly progress report No. 3, October 1-December 31, 1979

An increase in crystal growth rate for Zn/sub 3/P/sub 2/ was achieved by a vacuum baking step before crystal growth designed to reduce the pressure of excess gases in the ampoule. An analysis of the problem indicates that results are consistent with expectations. Samples have been submitted to SERI for mass spectroscopic evaluation and SIMS analysis. Construction of suitable equilibrium defect distribution diagrams for Zn/sub 3/P/sub 2/ was initiated. Typical diagrams with their interpretation will be included in the next report. Good rectifying diodes were prepared by vacuum evaporation of Mg onto etched surfaces of Zn/sub 3/P/sub 2/. A barrier height of 0.75 eV was measured from C-V data in good agreement with the published value of 0.80 eV. If the Zn/sub 3/P/sub 2/ surface was given a heat treatment in oxygen before Mg evaporation, the Mg made an ohmic contact indistinguishable from the normal ohmic Ag contact to Zn/sub 3/P/sub 2/. If the Zn/sub 3/P/sub 2/ surface was given a heat treatment in hydrogen before Mg evaporation, good diode characteristics were observed, with strong forward current saturation above 1V. The resistivity of Zn/sub 3/P/sub 2/ films deposited by CSVT in Ar is 1500 ohm-cm before laser annealing and 1200 ohm-cm after laser annealing. Laser annealing apparently produces a preferred orientation of these films. Microprobe analyses of the films deposited by CSVT and vacuum evaporation indicate that these films are almost always Zn-rich with an average 69 atomic % Zn and 31 atomic % P. Films deposited by vacuum evaporation, on the other hand, are strongly P-rich. Variations in stoichiometry with position on a single film are also observed

Photoelectronic properties of zinc phosphide crystals, films and heterojunctions. Quarterly progress report No. 2, July 1-September 30, 1979

The closed tube horizontal growth method has been pursued for the growth of single crystals of Zn/sub 3/P/sub 2/. The rate of material transport was increased by increasing the temperature difference between source and growth regions and by decreasing the distance involved. A boule with only 2 grains in a 12 mm diameter has been obtained. The as-grown resistivity of this single crystal Zn/sub 3/P/sub 2/ was 50 ohm-cm, which was reduced to 10 ohm-cm by subsequent annealing in hydrogen at 410/sup 0/C. Initial ZnO/Zn/sub 3/P/sub 2/ (CVD deposition of ZnO) and CdS/Zn/sub 3/P/sub 2/ (CdS by vacuum evaporation) heterostructures were fabricated using small samples of single crystal Zn/sub 3/P/sub 2/ sent from Tony Catalano at Delaware. Not surprisingly, only small photoresponse was obtained with these totally experimental cells. Zn/sub 3/P/sub 2/ films deposited on glass by CSVT were shown to be amorphous whereas those deposited on single crystal CdS were polycrystalline. Indeed several samples showed appreciable large-grain columnar growth at an angle to the substrate plane. Laser annealing was shown to have dramatic effects in crystallizing Zn/sub 3/P/sub 2/ films deposited on Si/sub 3/N/sub 4/ film substrates on sngle crystal Si. Thin films of Zn/sub 3/P/sub 2/ were deposited on glass substrates by vacuum evaporation. As-deposited film resistivities fell in the range of 10/sup 6/-10/sup 8/ ohm-cm; an apparently temporary decrease in resistivity by factors up to 50 could be obtained by annealing in hydrogen. Optical transmission and reflection spectra, as well as photoconductivity spectral response spectral response spectra were measured on a number of these films

Photoelectronic properties of zinc phosphide crystals, films, and heterojunctions. Quarterly progress report No. 11, October 1-December 31, 1981

Variations in crystal growth techniques are continuing with the goal of: (1) improving single crystal quality; and (2) producing variations in crystal properties by doping and post-growth variations of stoichiometry. DLTS measurements using Mg/Zn/sub 3/P/sub 2/ Schottky diodes gave information on three deep levels between 0.55 and 0.66 eV above the valence band in sublimation grown crystals with densities in the 10/sup 15/ to 10/sup 16/ cm/sup -3/ range, and a shallower level at 0.12 eV with a density of 10/sup 15/ cm/sup -3/ in an iodine-transport grown crystal. Investigation of surface properties of Zn/sub 3/P/sub 2/ indicate that a Br-MeOH etch leaves a Zn-rich surface for both sublimation-grown and iodine-transport grown crystals. Detailed measurements were made on thick Mg/Zn/sub 3/P/sub 2/ junctions on sublimation-grown and iodine-transport grown crystals of log J-V characteristics as a function of temperature, for crystal substrates as etched, and after heat treatment in hydrogen at several temperatures. A systematic change in the junction transport mechanism from tunneling at heat-treatment temperatures less than 300/sup 0/C to recombination/generation for heat-treatment temperatures between 300 and 500/sup 0/C was found for Zn/sub 3/P/sub 2/ crystals grown by both growth techniques. A simple model involving a depletion of free carrier density near the surface as a result of heat treatment in hydrogen is proposed. Thin film Mg/Zn/sub 3/P/sub 2/ cells showed log J-V characteristics that are strongly light dependent, indicating an increase in tunneling with illumination. Evidence that the barrier height of the Mg/Zn/sub 3/P/sub 2/ junction is actually of the order of 0.9 to 1.0 eV was obtained. Attempts to prepare ITO/Zn/sub 3/P/sub 2/ junctions by electron-beam evaporation of ITO on single crystal substrates yielded poor diodes and negligible photovoltaic behavior