Optical Emission Spectroscopy during Sputter Deposition of CdTe Solar Cells and CuTe-Based Back Contacts

In this dissertation sputtering processes are studied in detail through optical emission spectroscopy. In order to extract plasma parameters, experimental data and simulations were matched together. We could extract excitation temperatures, vibrational temperatures and rotational temperatures of the plasmas. To explain the simulations and to understand the different mechanisms involved in the sputtering plasmas, relevant aspects of atomic spectroscopy and molecular spectroscopy are reviewed here. A mixture of argon and nitrogen gas was used to sputter a Cu_xTe target by RF magnetron sputtering. The emission data were then studied as a function of deposition pressure and RF power. These data show many non equilibrium aspects of the plasma; however, in most cases the data are consistent with energy distributions of the rotational, vibrational, and electronic systems that can be characterized individually by distinct temperatures.

We have also used sputter deposition of Cu_xTe thin-film layers instead of our standard Cu/Au metal layers for back contacts to look for an improved back contact. We prepared three different compositions of Cu_xTe target material and studied the properties of sputtered films using X-Ray Diffraction (XRD), Energy Dispersive X-Ray Spectroscopy (EDS), Scanning Electron Microscopy (SEM) and Hall measurements. At optimized deposition conditions for Cu₂Te target sputtered films (2 nm thickness and 20 minutes annealing in vacuum) as determined from the thin-film properties, we sputtered this layer onto the back surface of the CdTe of the cell structure. We achieved efficiencies of 13.1% using Cu₂Te target sputtered films followed by Au which is very close to our best efficiency achieved with Cu/Au contacts.