Modelling CdTe thin film growth over realistic time scales

2015-06-24T09:08:43Z (GMT) by Miao Yu
Cadmium Telluride (CdTe) is an excellent material for low-cost, high efficiency thin-film solar cells and holds the record for watts/cost performance. The laboratory record efficiency of CdTe solar cells lags significantly behind the theoretical maximum for the material. This discrepancy is often attributed to defects such as grain boundaries and dislocations. Thus it is important to do research on how these defects are formed during the growth process. Atomistic simulations, such as Molecular Dynamics (MD) and on-the-fly Kinetic Monte Carlo (OTF-KMC), are widely used in partnership with experiments in addressing problems in materials science. In this work we use computer simulation to predict the growth of the sputter deposited CdTe thin film. At the first stage, MD studies of small cluster energetic impacts were carried out by repeatedly depositing CdxTey (x, y = 0, 1) clusters onto different CdTe surfaces with different energies at random positions. The impacts were simulated on Cd- and Te-terminated (100) surfaces and Cd- and Te-terminated (111) surfaces with typical industrial energies varies from 1 to 40 eV at a temperature of 350 K. More than 1,000 simulations have been preformed for each of these cases so as to sample the possible deposition positions and to collect sufficient statistics. The behaviour of deposited clusters under different conditions are studies. To simulate the process of thin film growth is the next stage in this work. We use different techniques to simulate the growth process on different surfaces. OTF-KMC simulations are performed to simulate the thin film growth process on the (111) CdTe surfaces. Starting with several ad-atoms deposited on the surfaces, in each step, the OTF-KMC method searches for all possible atomic movements (transitions) and randomly selects a transition or deposition to execute based on their corresponding rates. The thin film grows with more and more clusters to be deposited onto the surface with numerous ad-atom diffusions. The growth process on the dimerised Te-terminated (100) surface is very interesting. Knowledge of how the Te dimers on the surface split during the growth is gained in the simulations. MD is used to simulate the growth process with an accelerated deposition rate. Several simulations with different deposition energies are performed to see the differences of dissociation of the surface Te dimers. Post-annealing at different temperatures are applied after the growth simulations to find the optimal annealing temperature.