Al-doped ZnO transparent conducting oxide with appealing electro-optical properties – Realization of indium free transparent conductors from sputtering targets with varying dopant concentrations

The objective of this work is to provide high quality indium free transparent and conducting oxide film in which ZnO is the primary component and Al is the only dopant. A range of Al-doped ZnO (AZO) transparent conductors has been realized from the developed AZO sputtering targets with varying dopant concentrations with content of the dopant material Al2O3 added in the range 0.5–5.0 wt.%. X-ray diffraction, Raman spectroscopy, AFM measurements, and elemental analyses reveal promising material properties. The sheet resistance of 2.3 Ω/sq, visible light transmittance over 90 %, and figure of merit of 75.9 mΩ–1 are the optimal electro-optical characteristics of AZO thin films. The resistivity of AZO films is dependent on the dopant concentrations with the lowest value estimated to be 1.66 × 10–4 Ω cm. A comparison of the figure of merit of AZO films with that of ITO suggests application potential of AZO as transparent conducting oxide. High transparency effect of AZO films in the near-UV spectral region is limited by the strong fundamental absorption. The hypsochromic shift of this onset of band-to-band absorption accentuates beyond 1.0 wt.% of Al2O3, which is primarily ascribed to the Moss-Burstein effect in this degenerate semiconductor. The bulk plasmon frequencies fall in the near infrared region where transmittance curves of AZO films completely differ from that of ZnO. A significantly high free carrier absorption in the near infrared suggests formation of free electron gas upon doping. With abundance of free electrons and low dielectric loss, AZO may serve as a promising infrared plasmonic material.