Sodium Induced Microstructural Changes in MOCVD-Grown CdTe Thin Films

The properties of CdTe thin films deposited by metal-organic vapour deposition (MOCVD) on Mo-coated glass substrates are studied with particular interest in the effect of Na. It is shown that for radio frequency sputtered Mo layers on soda-lime glass substrates the Na content in the metal layer is regulated by the deposition pressure of the Mo layer itself due to increased porosity of the metal layer. Na diffusion from the glass into the CdTe layer results in grain enlargements to such extend that film continuity is broken following CdCl2 treatment for both low and high Na content. When Na is added post-Mo deposition on quartz substrates, the growth of large faceted grains is observed in the as-deposited stage, increasing by an order of magnitude compared with Na-free layers. However, following the CdCl2 activation step the average grain size is reduced but still observed to be larger than in the Na-free case. The CdTe crystal structure is analysed for the different sample combinations and initial device data suggest a small improvement in open circuit voltage when Na is present.