The Electrical Transport Properties Of In₂O₃ And Al-doped ZnO Films

The transparent conductive oxide?TCO?is a kind of wide band-gap semiconductors with high conductivity and high visible light transmittance.In addition,the carrier concentration of TCO is 2 to 3 orders of magnitude lower than that of typical metals.Thus,the transparent conductive oxide is an excellent material to study basic physics.In this paper,TCO is studied as the carrier and the study involved is as follows:the influence of the electron-electron?e-e?interaction on the electrical conductivity???and the Hall coefficient?R_H?in granular metals,the influence of the e-e scattering on the dephasing mechanism in three-dimensional?3D?disordered conductors,the influence of variable-range-hopping?VRH?conduction on thermopower?S?and resistivity???,and the influence of film thickness on the electrical transport properties.All samples were deposited by the standard rf sputtering method in this paper.For the first question:When the films become thinner enough,the grains form a system of weak connection.Thus,the films form granular arrays.According to the theory concerning the electron-electron interaction in the presence of granularity,the change of?and R_H are more likely to be observed in the systems with lower carrier concentrations.Here,we have prepared ultrathin In₂O₃ and Al doped ZnO?AZO,the doping content of Al is 4%?films for research.We found that the grains of AZO in ultrathin AZO films form a system of weak connection,so ultrathin AZO films form granular arrays.And the Hall coefficient and the conductivity are proportional to lnT?T is temperature?at low temperatures,respectively.These lnT laws originate mainly from the e-e interaction effect in the presence of granularity.Our results provide strong experimental supports for the recent theory concerning the e-e interaction in the presence of granularity.However,ultrathin In₂O₃ films different from ultrathin AZO films,and do not form granular arrays with decreasing thickness.At low temperatures,?and R_H in ultrathin In₂O₃ films are influenced mainly by traditional two-dimensional e-e interaction.For the second question:the electron-phonon?e-ph?scattering is often the dominant dephasing mechanism in three-dimensional?3D?disordered conductors.According to the theories of the e-e scattering and the e-ph scattering in 3D disordered conductors,the e-e scattering is more likely to be observed in 3D systems with lower carrier concentrations.Thus,we have prepared 3D In₂O₃ and heavily doped AZO?the doping content of Al is 10%?films for study.It is found that the dominant dephasing mechanism of 3D In₂O₃ films is the small-and the large-energy-transfer e-e scattering rather than the e-ph scattering.However,the only small-energy-transfer e-e scattering is the dominant dephasing mechanism in 3D heavily doped AZO films.Though the electron concentrations of 3D heavily doped AZO films are nearly equal to that of 3D In₂O₃ films,the values of k_Fl?k_F is the Fermi wave number,l is the electron mean free path?of AZO films are less than that of 3D In₂O₃ films.Thus,for 3D heavily doped AZO films,the large-energy-transfer e-e scattering strength is less than the small-energy-transfer e-e scattering strength.Therefore,the small-energy-transfer e-e scattering governs the dephasing processes of 3D heavily doped AZO films.These characteristics of In₂O₃ films with relative large k_Fl and low carrier concentrations give us the opportunity to simultaneously observe small-and large-energy-transfer e-e scattering.In a word,our results for the first time fully validate the theory of small-and large-energy-transfer e-e scattering suggested 40 years ago.For the third question:For In₂O₃ films,it has been reported that the temperature behaviors of resistivity show a crossover from Mott to Efros-Shklovskii?ES?VRH conduction by tuning oxygen contents.And the thermopower of In₂O₃ is?25?ten times larger than that of Sn doped In₂O₃ and F doped SnO₂.Thus,for the In₂O₃,the variation of thermopower can be more easily observed in the experiment.Based on the above mention,we deposited 3D In₂O₃ films with different oxygen partial pressures.In the resistivity and thermopower data of the most resistive films,we observe a crossover from Mott to ES VRH conduction with decreasing temperature.Thus,for the first time,our results quantitatively demonstrate the validity of the theoretical predictions of Mott VRH thermopower in experiment.For the fourth question:The electrical transport properties may be changed with decreasing thickness.Especially,when the films are thin enough,they may possess mesoscopic characteristics in physical properties,which will in turn influence the applications of films.Considering the temperature behaviors of Hall coefficient and resistivity can reveal the transport properties of films,we systematically investigated the temperature dependences of Hall coefficient and resistivity in heavily doped AZO films?the doping content of Al is 10%?with different thicknesses?12.57-971.18 nm?.The study found that all samples are homogeneous films.With decreasing film thickness,the metal-insulator transition occurs.Below?25?100 K,the VRH conduction is the dominant transport mechanism of the insulating films.With decreasing temperature,the temperature dependences of resistivity presentsln??T^-1/4and ln??T^-1/2laws,respectively.Thus it can be seen that a crossover from Mott to ES VRH conduction is also observed in the insulating films.