Process-Property Relationships in Al-ZnO Films Grown by Atomic Layer Deposition

Zinc oxide (ZnO) and aluminum-doped zinc oxide (AZO) thin films were deposited onto amorphous silica substrates using an atomic layer deposition (ALD) process with diethyl zinc (DEZ), trimethyl aluminum (TMA), and deionized H20 in an Oxford Instruments OpAl ALD tool. The influence of different ALD process parameters on the structural, optical, and electrical properties of resulting films has been investigated using high resolution X-ray diffraction (XRD) methods, four-point resistivity measurements, and ultraviolet-visible (UV-vis) spectroscopy. The process parameters that are correlated with film properties include temperature, number of cycles, aluminum doping ratio, and aluminum doping sequence. A detailed high resolution XRD analysis of structure, grain size, and microstrain in the deposited films using symmetric Bragg-Brentano, grazing incidence, and x-ray reflectivity scans has been carried out, and a detailed correlation is presented between the XRD-determined structural properties and the electrical resistivity, optical band gap, and Urbach energy. A pseudo-Voigt XRD lineshape analysis method yields more accurate grain sizes that are smaller than those calculated using the more common Scherrer full-width-half-max or integral-breadth methods. A minimum electrical resistivity of 1.6 x 10-3 Ω-cm and minimum Urbach energy of 112 meV for AZO films deposited at 200°C and an 11:1 aluminum doping ratio was produced using a novel Al-doping dose sequence consisting of DEZ, TMA, DEZ, followed by H20 (ZAZW) ,interspersed between the DEZ-H20 ALD cycles. The first DEZ dose in the new doping sequence acts as a sacrificial layer that shields the underlying ZnO lattice from the next TMA and reduction in the the growth per cycle associated with its introduction into the deposition chamber. The surface bound DEZ is then replaced by the much larger TMA molecule and the final DEZ dose of the ZAZW binds to any vacant surface sites resulting in fewer defects as inferred by the smallest Urbach energy. The electrical resistivities for both ZnO and AZO films were found to be independent of film thickness, crystallographic texture, and grain size. Variations in crystallographic texture and anisotropic grain sizes have minimal influence on film resistivity, which suggests that factors other than film texture, such as intra-grain scattering, may be important in influencing film resistivity