Optical properties of ZnGeP2 and CdGeP2.

Abstract

Single crystals of the ternary chalcopyrite semiconductors, ZnGeP2 and CdGeP2, space group I42d, were grown by slow cooling in a 20 mol% lead solution. Room temperature lattice parameters were determined for both compounds, and the variations of the lattice parameters with temperature up to the melting point of ZnGeP2 were studied. Hall effect and resistivity measurements showed ZnGeP2 to be p-type while CdGeP2 changed from p-type to n-type conduction at 11°C. Polarised infrared reflectivity measurements were made in the range 40 to 700 cm-1. The spectra were analysed using both classical dispersion theory and Kramers-Kronig integration, yielding four modes in ZnGeP2 and five in CdGeP2 with E irreducible representation and two modes in both materials with B2 irreducible representation. Raman measurements in the backscattering configuration using a Krypton laser determined all thirteen group theoretically predicted modes in CdGeP2 while two were unobserved in ZnGeP2. These modes were interpreted in terms of the atomic motions of the III-V zinc blende analogues. The fundamental absorption edges of the two compounds were studied at room and liquid nitrogen temperatures, and the lowest direct transitions were determined at room temperature by electroreflectance. The polarisation modulation technique was also used to study the higher energy structure at 5K and 300K. It was deduced that CdGeP2 has a direct gap at 1.7eV, while ZnGeP2 is either indirect or "pseudo-direct" at about 2.0eV.

Date of Award	1974
Original language	English
Awarding Institution	University of Bath