Diffusion of Cd and Te adatoms on CdTe(111) surfaces: A computational study using density functional theory

Abstract

ARTICLES YOU MAY BE INTERESTED INA computational ab initio study of surface diffusion of sulfur on the CdTe (111) CdTe is one of the most promising semiconductor for thin-film based solar cells. Here we report a computational study of Cd and Te adatom diffusion on the CdTe (111) A-type (Cd terminated) and B-type (Te terminated) surfaces and their migration paths. The atomic and electronic structure calculations are performed under the DFT formalism and climbing Nudge Elastic Band (cNEB) method has been applied to evaluate the potential barrier of the Te and Cd diffusion. In general the minimum energy site on the surface is labeled as A a site. In case of Te and Cd on B-type surface, the sub-surface site (a site just below the top surface) is very close in energy to the A site. This is responsible for the subsurface accumulation of adatoms and therefore, expected to influence the defect formation during growth. The diffusion process of adatoms is considered from A a (occupied) to A a (empty) site at the nearest distance. We have explored three possible migration paths for the adatom diffusion. The adatom surface interaction is highly dependent on the type of the surface. Typically, Te interaction with both type (5.2 eV for A-type and 3.8 eV for B-type) is stronger than Cd interactions(2.4 eV for B-type and 0.39 eV for A-type). Cd interaction with the A-type surface is very weak. The distinct behavior of the A-type and B-type surfaces perceived in our study explain the need of maintaining the A-type surface during growth for smooth and stoichiometric growth. C

A computational ab initio study of surface diffusion of sulfur on the CdTe (111) surface

“…In our earlier study 24 we had considered various local minima on the CdTe (111) surface relevant to the diffusion mechanisms for Cd and Te. In the present case, we find that same set of sites provides local minima for atomic Sulfurs.…”

“…24 In this model we can change the size of the slab surface, number of the slab layers and lattice constant of crystalline slab. We have chosen generally six layer slabs for both, A-type and B-type, with three different surface cell sizes (2× 2, 3× 3 and 3× 4) for computing the interaction between adatom sulfur and surface of CdTe (111).…”

“…1 shows two views of the surface cells. The dangling bonds of the last layer are saturated with suitable pseudo-hydrogen (with charge 3/2 e for Cd and 1/2 e for Te) 24,26,27 in order to avoid (unphysical) charge transfer between the top and bottom layers. We have considered a 10-Å -thick vacuum separating region in order to curtail the interaction of the periodic slabs in the z-direction.…”

“…In our earlier work, we had found a relatively small occupancy of A a -site (below the surface). 24 The subsurface occupancy introduces stress along the surface and seems to be favorable for Te occupation, leading to Te clustering. 46 We have applied c-NEB method for a path from A a -site to A b -site.…”

“…This orientation is one of the dominantly observed one for CdTe. 23 By combining the kinetic behavior of Cd, 24 it is possible to predict conditions under which the epitaxy of CdS is possible. The type of the surface, A or B-type decides the epitaxial growth.…”