Studies on temperature dependent semiconductor to metal transitions in ZnO thin films sparsely doped with Al

Abstract

For a detailed study on the semiconductor to metal transition (SMT) in ZnO thin films doped with Al in the concentration range from 0.02 to 2%, we grew these films on (0001) sapphire substrates using sequential pulsed laser deposition. It was found that the Al concentration in the films increased monotonically with the ratio of ablation durations of the Alumina and ZnO targets used during the deposition. Using X-ray photo electron spectroscopy it was found that while most of the Al atoms occupy the Zn sites in the ZnO lattice, a small fraction of the Al also gets into the grain boundaries present in the films.The observed SMT temperature decreased from ~ 270 to ~ 50 K with increase in the Al concentration from 0.02 to 0.25 %. In the Al concentration range of ~ 0.5 to 2 % these doped ZnO films showed metallic behavior at all the temperatures without undergoing any SMT. A theoretical model based on thermal activation of electrons and electron 2 scatterings due to the grain boundaries, ionic impurities and phonons has been developed to explain the observed concentration and temperature dependent SMT.Keywords: Al doped ZnO, sequential PLD, metal to semiconductor transitions I. IntroductionStudies on the ZnO thin films doped with different elements is an area of contemporary research for the development of new functional materials mainly for photonic and photovoltaic applications 1 . One of the ramifications of this area of research is to obtain and study highly conductive and transparent films of ZnO using n-type dopants such as Al, In, B, Ga and Si 2-7 . Al has been extensively used for this purpose because of its easy availability, low cost, ease of doping and superior properties of the ensuing films. Studies on temperature dependent transport properties of such Al doped ZnO (AZO) films 8, 9 are essential to understand the conduction mechanisms under different conditions of the doping. An important effect that has been observed in the temperature dependent resistivity measurements of AZO thin films is the semiconductor to metal transition (SMT) 10-12 . SMT is an important phenomenon because it can be used to elicit the basic processes involved in the transport properties of AZO films. It is well known that as grown pure ZnO thin films are semiconducting in nature showing decrease of resistivity (ρ) with increase of temperature (i.e. it has negative temperature coefficient of resistivity) 2 . When doped with small amounts of (~ 2%) n-type dopant, the ZnO thin films become metallic showing increase of resistivity with increasing temperature (i.e. positive temperature coefficient of resistivity) 6, 10 . Hence it is expected that the 3 3 temperature dependent resistivity of sparsely doped AZO films with Al concentration < ~ 2% would show SMT. Although SMT in AZO thin films has been reported previously [10][11][12] , a systematic and detailed study of SMT in sparsely doped AZO thin films with controlled doping concentration is lacking. We studied the SMT in sparsely doped AZO thin films as a function of Al doping concen...

ITO and AZO films for low emissivity coatings in hybrid photovoltaic-thermal applications

“…For the AZO films grown at high temperature, the agreement between the resistivities obtained by the different techniques is very good, producing values in the 10 to 1 mΩ-cm for the 150 ºC to 350 ºC samples, respectively, carrier densities over 10 20 cm -3 , and Hall mobilities between 6 and 30 cm 2 V -1 s -1 . These resistivities are at the higher end of the values often found in the literature [21,26,39]. Conversely, the resistivities obtained for annealed AZO films show significant discrepancies between the electrical and the optical measurements derived from the Drude model.…”

“…Das et al [39] showed that below an aluminium concentration of 0.2%, there is a very strong dependence of the carrier density and resistivity of the AZO films with the dopant density. In this regime, small fluctuations in the active dopants results in a metal-insulator transition, changing abruptly the electrical and optical properties of the material.…”

Comparative Study of Annealed and High Temperature Grown ITO and AZO Films for Solar Energy Applications

“…These resistivities are at the higher end of the values often found in the literature [7]. Conversely, the resistivities obtained for annealed AZO films show significant discrepancies between the electrical and the optical measurements derived from the Drude model.…”

Effect of disorder on carrier transport in ZnO thin films grown by atomic layer deposition at different temperatures

“…The q(T) curves shown in Figures 4(a), 4(d), and 4(e), corresponding to the ALD-ZnO films grown at $150, 300, and 350 C, respectively, showed only semiconducting behaviour in the entire measurement temperature range, from room temperature down to liquid helium temperature. In contrast, a transition from metallic to semiconducting transport behaviour at lower temperatures 16,17,25,26 were observed in the q(T) curves shown in Figures 4(b) and 4(c) which correspond to the films grown at substrate temperatures of $200 and 250 C, respectively. The observed semiconducting behaviour in all the q(T) curves could not be fitted by the classical Arrhenius equation.…”