ZnGep 2 : Optical Transparency and Melt Composition
- PDF / 1,776,863 Bytes
- 7 Pages / 415.8 x 637.2 pts Page_size
- 82 Downloads / 226 Views
457
Mat. Res. Soc. Symp. Proc. Vol. 607 @2000 Materials Research Society
EXPERIMENTAL TECHNIQUE ZnGeP 2 was synthesised from high purity elements using a modified two-temperature technique, yielding over 500 gms of polycrystalline ZGP material from each process run [7]. Three ZnGeP 2 single crystals were grown from each polycrystalline ingot. The material was melted in pyrolytic boron nitride (PBN) crucibles in a vertical Bridgman furnace. The hot zone temperature was 1060'C. The cold zone temperature was 990*C. The temperature gradient near the melting point was 3°C.cm- and the pull rate was 0.5mm-hour-. Variation of the melt composition was achieved by altering the composition of the vapour phase above the melt by varying the quantity of excess zinc and phosphorus added to the melt to generate the equilibrium vapour phase in the sealed growth ampoule. The required excesses were calculated from the ideal gas law. The first Zn-rich crystal was grown under increased zinc pressure of 1.1 atm; the phosphorus partial pressure was 6.9 atm. The second crystal was grown with additional charge of P4 only, creating the equilibrium pressure of 7.2 atm at the melt temperature of 1060"C. This melt is calculated to be close to stoichiometry since, as discussed above, the required equilibrium zinc partial pressure is negligible. The third crystal was grown without any added excess of the volatile components: the vapour phase above the melt was generated by loss of these components from the melt itself, so it became Ge-rich. The crystals were then sliced for assessment. The assessment included annealing studies (600'C, 300 hours, in vacuo), e-beam irradiation treatment, examination by transmission electron microscopy (TEM), and optical transparency measurements. The defect structure of the crystals was investigated at a magnification x65000x85000 using an 125kv electron microscope. Slices for this study were cut from the upper part of the ingots and were then cleaved to produce small fragments suitable for mounting on a carbon film and examination by TEM. Analysis of the micro-precipitates was performed in situ using electron diffraction. Optical transmission was measured on an SDL-3 spectrophotometer (LOMO St. Petersburg, Russia) with a wavelength range of 0.7-2.5 mkm. For the range from 2.5-10 mkm a two-beam spectrophotometer (SPECORD M80, Karl Zeiss Yena, Germany) was used. RESULTS AND DISCUSSION Crystal growth After crystallization, the top section of the crystal grown under increased zinc pressure (Pznl.latm, Pp4=6.9atm) shows some surface porosity, which indicates that the total pressure in the vapour space above the melt was higher than the equilibrium pressure for the stoichimetric composition. The melt should therefore be enriched in the volatile components. The second crystal, of nominally stoichiometric composition (Pp4=7.2atm) showed no porosity and the top of the ingot appeared to be single phase. The third crystal had an eutectic in the last-to-freeze section. X-Ray powder analysis indicated a eutectic composed of
Data Loading...
