(646d) First-Principles Theoretical Analysis of Dopant Adsorption and Diffusion on Surfaces of II-VI Compound Semiconductor Nanocrystals

Nanocrystalline structures of compound semiconductors, such as the II-VI compounds ZnS, CdSe, and ZnSe, exhibit size-dependent optoelectronic properties and form the basis for a new generation of highly-integrated nanoelectronic and photovoltaic devices, as well as biological labels. Single-crystalline grains of these materials that are smaller than the corresponding exciton Bohr radius exhibit size-dependent luminescence due to quantum confinement. Doping in semiconductor nanocrystals could allow for precise control of their optical and electronic properties, thus enabling applications in solar cells and spintronics. However, in spite of its feasibility, doping of semiconductor nanocrystals has been an extremely difficult task. Self-purification of nanocrystals, an intrinsic mechanism of expulsion of impurities from the nanocrystal surface, makes the incorporation of dopants into nanocrystals even more difficult. In this context, we aim at obtaining a fundamental and quantitative understanding of dopant adsorption and diffusion on II-VI semiconductor nanocrystal surfaces, which can help elucidate the mechanisms of dopant incorporation into growing nanocrystals.

In this presentation, we report results on dopant adsorption and diffusion on surface facets of ZnSe nanocrystals based on first-principles density functional theory (DFT) calculations within the generalized gradient approximation. In our DFT calculations, we have employed slab supercells, plane-wave basis sets, and ultrasoft pseudopotentials. We have also implemented the nudged elastic band method including a climbing image to construct fully optimized dopant diffusion pathways and obtained accurate saddle-point configurations and activation energy barriers.

Specifically, we have focused on the incorporation of Mn dopant atoms into ZnSe nanocrystals through their surface facets. We found that the (001) facet is the dominant nanocrystal surface for Mn incorporation consistent with previously published findings [S. C. Erwin, et al., Nature 436, 91 (2005)]. Our DFT calculations indicate that the binding energy for Mn adsorption onto various sites of the ZnSe(001)-(2x1) surface increases with increasing dopant surface concentration. This low binding energy at low dopant surface concentration provides an explanation for doping difficulties during nanocrystal growth. In addition, we have analyzed several dopant migration pathways for Mn diffusion on the ZnSe(001)-(2x1) surface. The calculated activation barriers for migration along the Se dimer rows range from 0.17 eV to 0.43 eV, depending upon the dopant surface concentration. Due to the low activation barriers, dopant atoms can migrate fast along the Se dimer rows without substantial surface relaxation. However, migration of the Mn atom across the Se dimer rows from the dimer site (Mn adsorbed onto the Se dimer) to the trough site (Mn adsorbed in the trough between Se dimers), is governed by a high-barrier pathway. At higher dopant surface concentrations, the binding energy for Mn adsorption onto the trough site is greater than that for Mn adsorption onto the dimer site. In conjunction with the high activation barriers for Mn migration in the trough parallel to the Se dimer rows, this implies that migration to a trough site may lead to dopant incorporation into the ZnSe nanocrystal