Possibilities of Increasing the Usability of Sputtered AZO Films as a Transparent Electrode

Abstract

Aluminum-doped zinc oxide (AZO) is a suitable material for use as transparent electrode. Due to its lower price it is applied in the production of silicon solar cells. However, it is difficult to obtain suitable electrical properties at low deposition temperatures. Moreover, the AZO films with low thickness exhibit significantly higher average resistivity, which limits their usability, especially if they are prepared on flexible substrates. A lot of effort is invested to replace indium tin oxide (ITO) to a much greater extent now, since Indium is rare and its price is rising, thus the preparation of lowthickness AZO films at low temperature is also subject of intense research. The presented paper summarizes the reported results, discusses the causes of the differences between ITO and AZO films, and suggests the direction of research and the potential solution that should lead to increased usability of AZO films and also possible replacement of ITO films.

Investigation on Transparent, Conductive ZnO:Al Films Deposited by Atomic Layer Deposition Process

“…Magnetron sputtering is a commonly used process to deposit AZO films. So far, many articles on sputtering-deposited AZO films and correspondingly application examples have been reported [ 11 , 12 , 13 , 14 ]. However, the sputtering process suffers from the distinct disadvantages of high-vacuum background pressure, surface damage of pre-deposited films, worse repeatability, and film composition limitations [ 15 , 16 ].…”

Growth Evolution of AZO thin Films Deposited by Magnetron Sputtering at Room Temperature

“…In the early stages of growth, the film begins to form as small crystals with different crystalline orientations that slowly increase in size until the grain boundaries touch, then growing vertically in a process described as evolutionary selection 33 , where the grains grow more rapidly in the direction normal to the substrate surface and coalesce with the other grains, forming a wormlike structure, increasing laterally and reducing the density of boundaries 34 .…”

“…For thicknesses above 80 nm, the carrier density and mean grain size become almost constant as a function of film thickness. Thus the contribution to scattering by grain boundaries and neutral impurities may be considered constant, and the degradation in mobility is more influenced by scattering by ionized impurities 3,38 and by phonons 34 . Figure 8 shows a plot of the contributions of the DZP model (ionized impurities and phonons) 37 and the Masetti model (ionized impurities) 34,38 in the lattice, together with the experimental data of samples.…”

“…Thus the contribution to scattering by grain boundaries and neutral impurities may be considered constant, and the degradation in mobility is more influenced by scattering by ionized impurities 3,38 and by phonons 34 . Figure 8 shows a plot of the contributions of the DZP model (ionized impurities and phonons) 37 and the Masetti model (ionized impurities) 34,38 in the lattice, together with the experimental data of samples. The dotted lines show the regions where the resistivity is 10 -4 , 10 -3 and 10 -2 Ωcm.…”

“…The dotted lines show the regions where the resistivity is 10 -4 , 10 -3 and 10 -2 Ωcm. The curves marked 8 nm and 16 nm (crystallite size) were produced considering the DZP + PS models 37 and the scattering by grain boundaries 34 , based on the experimental mobility Equation 3. The experimental points are limited above by scattering from ionized impurities and by grain boundaries, and to the right they are limited by the formation of neutral impurities, which limit the ionization efficiency and consequently reduce the carrier density.…”