Physica I17B & 118B (1983) 561-563 North-HoUand Publishing Company 561 STATIC AND STRUCTURAL PROPERTIES OF III-V ZINCBLENDE SEMICONDUCTORS Sverre Froyen and Marvin L. Cohen Department of Physics, University of California, and Materials and Molecular Research Division, Lawrence Berkeley Laboratory, Berkeley, California 94720, U.S.A. The pseudopotential method within the local density approximation is used to investi- gate the static and structural properties of some III-V zincblende semiconductors. The results for lattice constants and bulk moduli are within a few percent of experi- ment. Comparisons of calculated total energies as a function of volume and structure yield information about solid-solid structural phase transformations. At high pressures, the results indicate that metallic rocksalt and B-Sn structures are lower in energy than zincblende. Other structures (e.g., CsCI and NiAs) are also investigated. i. INTRODUCTION Recent improvements in the diamond anvil techni- que have made x-ray experiments on III-V semi- conductors under hydrostatic pressure possible in situ. In some cases, these new data contra- dict earlier results where the samples had to be quenched and the pressure released before a dif- fraction pattern could be obtained. Unfortunate- ly, the new results are still not conclusive. Often they are only able to determine the cry- stal class and not the point group of the struc- ture. Moreover, earlier theoretical work has been unable to resolve these questions. The Phillips electronegativity scale I places these compounds on the border between the covalent compounds that transform to the B-Sn structure and the more ionic ones that go to rocksalt under pressure. Calculations based on this have not been able to distinguish between the various possible high pressure structures. It is now possible to do pseudopotential total energy calculations that are accurate enough to predict the relative stability of structures. 2 Using first principles pseudopotentials, we have performed total energy calculations for GaAs, AlAs, GaP, and AIP in various structures and as a function of volume. This yields information on the stability of the structures under pressure as well as transition pressures and volume changes. Part of this work has been published elsewhere3 and a more complete report will be published later. The calculational details can be found in Ref. 4. Exchange and correlation were treated in the lo- cal density formalism using the formulation of Hedin and Lundqvist. 5 The pseudopotentlals were generated by the method proposed by Hamann, Schluter, and Chiang. 6 2. STATIC PROPERTIES AT NORMAL PRESSURE Experimentally, all the compounds investigated are found in the zincblende structure at normal pressure, and our calculations yield this result. We are, however, not able to distinguish between the zincblende and the hexagonal wurtzite struc- ture which for GaAs differ in energy by less than i mRy. From the total energy versus volume curves, the lattice constants and the bulk moduli are compu- ted. The results for these quantities are given in Table I. Table I. a (~) B (GPa) Lattice constants and bulk moduli. GaAs AlAs GaP AlP calc 5.570 5.641 5.340 5.420 expa 5.653 5.662 5.451 5.451 -1.5% -0.4% -2.0% -0.6% calc 72.5 74.1 89.7 86.5 expb 74.8 88.7 -3.1% +1.1% ~ Ref. 7 Compiled by Ref. 8 3. HIGH PRESSURE PHASES Under pressure, the III-V semiconductors become metallic. This is generally believed to be a structural phase transformation where the atoms become more closely packed. The actual struc- ture is, in many cases, unknown, and it depends on the compound in question as well as tempera- ture. Below, we describe calculated results for a num- ber of structures we feel are reasonable candi- dates for high pressure phases. All are metal- lic. 3. I GaAs At a static pressure of approximately 170 kbar, 0 378-4363/83/0000-0000/$03.00 © 1983 North-Holland