# Thin CdTe Layers Deposited by a Chamberless Inline Process using MOCVD, Simulation and Experiment
## 1 Introduction  
The growth rate of thin-film Cadmium Telluride (CdTe) using metalorganic chemical vapour deposition (MOCVD) has been found to be sensitive to both substrate temperature and reactant partial pressures [1], presenting a complex transport and kinetic process. Among the various deposition techniques used for CdTe growth, MOCVD is a favourable method, allowing control of the microstructure and stoichiometry of the films [2; 3; 4]. MOCVD is an attractive method for depositing CdTe and another group II-VI compound semiconductor thin films [5; 6]. It has been conventionally associated with a low-pressure batch process for the fabrication of optoelectronic devices [7], including multi-junction III-V photovoltaic cells for space applications [8]. Physical vapour deposition (PVD) techniques, such as closed-space sublimation (CSS) [9] or vapour transport deposition (VTD), are more commonly used for the deposition of thin-film CdTe for photovoltaic (PV) applications. For example,First Solar has achieved a low production cost of \(\sim\)50.49 per watt using VTD for commercial CdTe PV modules [10], whereas the efficiency has been increased to 21%, as reported by [PERSON] 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, [PERSON] 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), and (2) the effects of key deposition parameters on the pyrolysis of CdTe in the dynamic mode (i.e., on a moving substrate), including substrate temperature (\(T_{s}\)), precursor concentration (\(P_{con}\)) and total gas flow (\(F_{total}\)). The deposition profiles, measured film thicknesses and material (chemical precursor) utilisation were correlated to CFD simulations.