Thin CdTe Layers Deposited by a Chamberless Inline Process using MOCVD, Simulation and Experiment

Abstract

The deposition of thin Cadmium Telluride (CdTe) layers was performed by a chamberless metalorganic chemical vapour deposition process, and trends in growth rates were compared with computational fluid dynamics numerical modelling. Dimethylcadmium and diisopropyltelluride were used as the reactants, released from a recently developed coating head orientated above the glass substrate (of area 15 × 15 cm 2 ). Depositions were performed in static mode and dynamic mode (i.e., over a moving substrate). The deposited CdTe film weights were compared against the calculated theoretical value of the molar supply of the precursors, in order to estimate material utilisation. The numerical simulation gave insight into the effect that the exhaust's restricted flow orifice configuration had on the deposition uniformity observed in the static experiments. It was shown that > 59% of material utilisation could be achieved under favourable deposition conditions. The activation energy determined from the Arrhenius plot of growth rate was~60 kJ/mol and was in good agreement with previously reported CdTe growth using metalorganic chemical vapour deposition (MOCVD). Process requirements for using a chamberless environment for the inline deposition of compound semiconductor layers were presented.First Solar has achieved a low production cost of~$0.49 per watt using VTD for commercial CdTe PV modules [10], whereas the efficiency has been increased to 21%, as reported by Green et al. [11]. Since PVD techniques are generally associated with vacuum processing, an alternative non-vacuum and scalable deposition processes, such as chamberless MOCVD, are desirable. Material utilisation of 40% for MOCVD of CdTe and conversion efficiency exceeding 11% for the fabricated CdTe solar cell devices have already been demonstrated using an inline reactor design [12,13].Gas flow behaviour has a significant impact on the uniformity and reproducibility of coatings produced by MOCVD. In a previous paper, we reported the modelling of a vertical low-pressure MOCVD reactor geometry [14], providing an understanding of the film growth mechanism and complex chemical reactions.Our group used computational fluid dynamics (CFD) modelling in the development of the coating head [15] for the chamberless coating setup described in the present paper. In a precursor to the work reported in this paper, Yang et al. [16] conducted three-dimensional (3D) simulations using CFD code -Fluent, investigating the effects of various conditions on CdTe film growth in an inline MOCVD reactor, where precursor gas flows were delivered normally to the direction of substrate translation. There, the sequential reactions involved in the CdTe deposition process were significantly simplified by the adoption of a proposed global surface reaction.This paper focused on two aspects of our chamberless inline MOCVD process: (1) the influence and optimisation of the exhaust restricted flow orifice (RFO) configuration, related to the deposition profile in static mode (i.e., on a stationary substrate),...

Numerical Analysis of an Inline Metal-Organic Chemical Vapour Deposition Process Based on Sliding-Mesh Modelling

“…Some discrepancies can be observed for both grid types within the temperature region from 658 to 688 K. This is mainly because (1) the low temperature region (<668 K) is limited by the reactor kinetics, and the predicted growth rate is very sensitive to the deposition temperature, following an exponential form ∝ exp(−1/T). The differences arising in this region are mainly attributed to the consideration of the overall surface chemical reaction on the substrate [22,23,30]; (2) the high temperature region (>688 K) is mainly controlled by mass transfer, and a fast surface reaction is occurred, with the deposition rate being proportional to T 3/2 ; (3) between the two regions (668 to 688 K), there exists a transition zone where reaction kinetics and mass transfer both affect the deposition process significantly. The joint effect and Coatings 2020, 10, 1198 6 of 16 the consideration of the overall reaction both lead to the discrepancies between simulation results and the experimental data.…”

“…Ramadan and Im [16] investigated the film thickness uniformity in a planetary reactor by optimising multiple parameters. Among these commercial reactors, the inline MOCVD reactor has been successfully employed to deposit CdTe thin film by a group of researchers [17][18][19][20][21]; as such, the inline deposition process with a moving substrate is found to be beneficial to thin film uniformity and productivity [22,23]. However, current simulations on the inline process of CdTe growth are mainly conducted in a steady state, while the flow behaviour with the influence of susceptor moving speed still lacks a systematic investigation.…”