The Carrier Transport Mechanism And A Series Of Properties In AZO Transparent Conducting Thin Films

Transparent conducting oxide (TCO) films have been widely used as transparent electrode in the optoelectronic devices such as flat panel display and thin-film solar cells etc. In recent years, doped ZnO thin films are attracting great interest because of their advantages with high conductivity, good transparence on the visible light range, thermal stability, cost-effective production and non-toxicity and so on, and are regarded as potential substitutes for indium tin oxide (ITO). In order to improve their electrical and optical properties, one has done many theoretical and experimental studies. In this study we have investegated the carrier transport mechanism and a serial of properties in AZO films, and has resolved following problems.Reviewed the thermionic emission effect and tunneling effect at grain boundary, and more developed the theoris for that; In order to overcome a defect of previous theory on thermionic emission, applying the Fermi-Dirac statistics for the carrier distribution, developed this theory, and successfully derived the carrier mobility due to thermionic emission; In order to explain wonderfully the effect of tunneling effect on the carrier transport at grain boundary, transferred the integral expression for the mobility due to tunneling effect to more simple integral expression; estimated the effect of thermionic emission and tunneling effect on the carrier transport at grain boundary. As a result, at room temperature, for a potential barrier height Eb higher than 0.6Ef, the mobility due to tunneling effect is greater than that due to thermionic emission, especially, for the barrier height higher than Ef, is much greater, which shows that in such a case the tunneling effect dominates the carrier transport mechanism at grain boundary.Using the equations derived in this study, considered the carrier transport mechanism in AZO films under several conditions; Estimated the contribution of the ionized impurity scattering and grain boundary scattering on the carrier transport mechanism, the result of which shows that the considerable part of carrier transport mechanism is originate from the grain boundary scattering; Using the equations derived in this studies, explained carrier scattering mechanism at grain boundary, the result of which shows that Fermi level increasing due to increasing of carrier concentration is accompanied by increasing of grain barrier height, however activation energy is not varied in the main, but only fluctuating in the range of 180～240 meV.Considered the effect of the film thickness on the microstructure and electrical properties of films, and explained its mechanism in connection with film growth processes; The grian size and residual stress simultaneously increase as film thickness increasing, in both case of deposited at 100℃and 350℃, showing the existence of a some relationship between them; XPS analysis for the films shows that, as film thickness and substrate temperature increase, the chemisorbed oxygen and Al2O3 and ZnAl2O4 segregated at grain boundary decrease, especially the decrease of segregated Al oxide is more clear, however, the wurtzite ZnO content increases; The variation of carrier concentration with film thickness and substrate temperature, mainly, is due to formation of Al oxide segregated at grain boundary.Considered the effects of SiO2 barrier layer on the properties of AZO films deposited on the soda lime glass; For films deposited at 200℃, the effect of SiO2 barrier layer on the properties of AZO films deposited on the soda lime glass is not evident, however, for films deposited at 350℃, SiO2 barrier layer shows a evident influence. By introducing SiO2 barrier layer, the carrier concentration and mobility of films deposited on the soda lime glass increase evidently. SiO2 barrier layer formed at the temperature of 400℃more effectively suppress the diffusion of Na atoms into AZO film than that formed without intentional heating.Considered the effects of diluted hydrogen or oxygen ambient on the microstructure, electrical and optical properties of AZO films; The grain size decreases with increase of hydrogen flow, regardless substrate temperature, but for the films deposited at 400℃, the decreasing rate of grain size is more rapid than that deposited at 100℃; With increasing of oxygen flow, the grain size, at first, increases, but decreases after any flow; For the films deposited under diluted hydrogen ambient, in the case of 100℃, the carrier concentration and mobility increase with increasing of hydrogen flow up to any flow, however, excessive hydrogen flow leads to the deterioration of electrical properties; In the case of 400℃, both the carrier concentration and mobility decrease as hydrogen flow increase; Under diluted oxygen ambient, the electrical properties of AZO films are deteriorated, because the forming of Al oxide and increasing of defects are prompted.