Electrical resistivity of nanocrystalline Al-doped zinc oxide films as a function of Al content and the degree of its segregation at the grain boundaries

Abstract

Highly transparent and conducting Al-doped ZnO (AZO) films are prepared via sol-gel method with a broad range of nominal Al-doping. The film porosity and morphology is determined by the rate of temperature ramping during the drying of the gel phase. The minimum resistivity is observed to occur around 1.5–2 at. % Al-doped films, irrespective of the morphology and microstructure. It is found by local chemical analysis that Al tends to segregate at the grain boundaries and above a critical concentration, the segregated Al starts to dominate the electronic transport in nanocrystalline AZO. The optical measurements corroborate these findings showing a systematic increase in carrier density only up to 1.5–2 at. % Al-doping. It is concluded that the presence of the resistivity minimum is not merely determined by a solubility limit but is a result of the interplay between the changing carrier concentration and carrier scattering at the segregated Al.

Gas sensing properties of <i>c</i>-axis-oriented Al-incorporated ZnO films epitaxially grown on (11-20) sapphire substrates using pulsed laser deposition

“…These images indicate that all of the films had densely packed uniform grains, and the grain size of the films decreased with increasing Al doping level. This Al content dependence of the grain size agreed with previously reported results, 25), 26) which indicated that the grains of Al-doped ZnO films grown by rf magnetron sputtering and solgel spin coating become smaller as the Al doping level increases. The decrease in grain size due to Al doping can be explained by the fact that the Al impurities in the ZnO films inhibit grain growth.…”

Effect of aluminum addition on the optical, morphology and electrical behavior of spin coated zinc oxide thin films

“…In fact, enhancement of the electron mobility with addition of extra zinc ions is caused by the impending increase in the average particle size and hence reduction in the grain boundary scattering. Lin et al, 37 Tsang et al 38 and Nasr et al 16 have also reported resistivity of as low as 0.6x10 -2 , 0.2x10 -2 and 2.3x10 -cm in spin coated thin films of composition 0.2-0.6, 2 and 1.5-2 at% Al-ZnO, respectively, but only after complex annealing process in forming gas, vacuum, and /or air or after subjecting to KrF excimer laser irradiation. In contrast, the present finding demonstrates reduction of resistivity to 2.3x10 -2 -cm in 1at% AlZnO thin films by simply increasing the zinc content slightly and annealing in vacuum at 500…”

“…Aluminum is one such dopant which gets incorporated at the zinc site. 16,34 ZnO thin films show high sheet resistance (R s ∼ 63M / ) and in fact behave like an insulator. But, on heating in vacuum ∼10 -3 mbar at 500…”

“…35 Obviously, about 2.4% of aluminum atoms seem to contribute to electron density while the rest create Zn 2+ vacancies, fill traps, form colour centres at anion vacancies, and/ or segregate as Al 2 O 3 along the grain boundaries. 16 The conductivity of ZnO thin films can be understood on the basis of Zn/O atomic ratio equal to, less or more than unity. When (Zn/O) atomic ratio is equal to one, the composition becomes ZnO and hence no free charges will be available for conduction.…”

Band gap modified Al-doped Zn1−x Mg x O and Zn1−y Cd y O transparent conducting thin films

“…[3][4][5] Al-doped ZnO (ZAO) film resistivity of less than 2 9 10 -4 X cm has been attained, which is comparable to those obtained in ITO films [3]. Furthermore, many investigators have examined the electrical and optical properties of Al-doped ZnO with a broad range of nominal Al-doping, and a critical Al content with the minimum resistivity is usually observed, depending on preparation methods and conditions [6][7][8][9]. Lu et al [7] reported that the optimal result for ZAO films synthesized by magnetron sputtering located at Al content of 4%.…”