First principles simulations of liquid semiconductors: Electronic, structural and dynamic properties.

摘要

We develop ab initio molecular dynamics simulation technique to examine liquid semiconductors. Our methods use quantum interatomic forces, computed within the pseudopotential-density functional method (PDFM). In our work, we study typical representatives of IV, III-V and II-VI materials: Si, Ge, GaAs and CdTe. We show that, upon melting, IV and III-V semiconductors experience semiconductor → metal transition, while more ionic II-VI compounds remain semiconductors in the melt. Metallic type conductivity of liquid IV and III-V materials results from the structural changes of the systems in the melt. In our simulations, “open” zinc-blende (diamond for Si and Ge) structures transform into a more close-packed configuration during solid → liquid transition. Their coordination number, equal to 4 in the crystalline phase, changes to ∼6 in the liquid. We demonstrate that this leads to the breaking of covalent bonds and delocalization of electrons. According to our results, the density of states function of liquid IV and III-V semiconductors has a well defined “free electron” character. For these materials, the electrical conductivity jumps by one to two orders of magnitude during melting. This is opposite to the behavior of the majority of II-VI compounds. In our work, we examine CdTe, typical II-VI semiconductor. Although the dc conductivity of CdTe increases by a factor of 40 as it melts, this material remains a semiconductor in the liquid: its electrical conductivity increases with the temperature. At variance with IV and III-V semiconductors, liquid CdTe retains its tetrahedral environment with the coordination number of ∼4. We discover that a significant number of anion-cation bonds are conserved in liquid CdTe as opposed to IV and III-V materials. This is in agreement with the small entropy change observed in the melting process of CdTe. In our simulations, we find that further heating of molten CdTe results in significant structural changes with a transformation to a more close-packed atomic structure. The coordination number of the superheated phase is ∼6 and the dc electrical conductivity is an order of magnitude larger than the melting temperature value. This, along with the disappearing of the finite band gap, suggests a gradual semiconductor → metal transition in CdTe system at a temperature higher than the melting point. We find in liquid CdTe, near the melting temperature, atoms of Te form helicon chains. We suggested that this can be a partial explanation of difficulties encountered in growing pure bulk CdTe from the melt. In our work we demonstrate an effective alternative to the Car-Parrinello method for ab initio molecular dynamics simulations. We also show a practical procedure of ensemble preparation. Good agreement of the electronic, structural and dynamic properties with experiment data validate the usage of our method. We describe an efficient algorithm of diagonalizing the non-local pseudopotential term of Hamiltonian in real space instead of Fourier space. This allows us to accelerate iterative diagonalization schemes for the systems with a large number of atoms. We present a discussion of the implementation of this approach on different computer platforms.