Effects of very rapid thermal processing on the properties and bonding structure of Al-doped ZnO thin films

The combination of low-temperature deposition with very rapid thermal processing (vRTP) at air is a promising approach of synthesis of high-quality thin films of transparent conductive oxides (TCOs) at large areas. However, microscopic mechanisms of TCO film properties modification during such treatment are not understood properly which hampers application of the post-deposition vRTP. In order to resolve this problem, Al-doped ZnO (AZO) films grown onto unheated fused silica substrates by reactive pulsed magnetron sputtering were vRTP processed using a laser source with ~1 ms dwell time. The morphology, bonding structure, electrical and optical properties of as-deposited films were systematically compared with those of the vRTP processed ones. The electrical resistivity of the films drops from 1.4x10^-3 to 4.6x10^-4 Ohm cm with simultaneous substantial decrease of the film optical extinction in the visible at optimum processing conditions. The resistivity decrease is due to simultaneous increase of the free electron density (from 3.8x10²⁰ to 6.2x10²⁰ cm^-3) and mobility (from 12 to 22 cm²V^-1s^-1). It is also observed that vRTP leads to a substantial decrease of the A1(LO) and E1(LO) disorder-enhanced lines in Raman spectra. This is interpreted in terms of decreasing concentration of point defects because both x-ray diffraction and high-resolution TEM show no improvement of the film crystal quality which might have a similar effect on the Raman spectra. Substantial modification of the O K-edge x-ray absorption near edge structures (XANES) spectra of the vRTP processed AZO films (which are dense and void free already in as deposited state) suggests that these might be oxygen-related point defects or defect complexes. Decrease of concentration of these defects may explain increase of the free electron mobility and decrease of the optical extinction in the visible.
The authors gratefully acknowledge financial support of BMBF under the RainbowEnergy Project.