Disorder and localization in bandtail hopping transport: experiments and concepts

作者：Godet C^*; Kleider JP

来源：Journal of Materials Science: Materials in Electronics , 2006, 17(6): 413-426.

DOI：10.1007/s10854-006-8088-5

This review describes the main transport experiments (including Ohmic and nonlinear transport in both dc and ac regimes) and related concepts which can be used to derive quantitative information on the localized electronic states above the Fermi level and in the bandtails, in low carrier mobility materials governed by hopping conductivity. For bandtail hopping, a model based on the filling rate of electronic states near the preferred transport path shows a sharp decrease of the hopping mobility with increasing bandtail disorder. While the slope T-0(1/4), derived from ln sigma vs T-1/4 plots, is weakly sensitive to the shape of the DOS distribution, the prefactor sigma(circle circle) values are extremely sensitive to bandtail disorder; uniform and exponential distributions can be discriminated experimentally (sigma(circle circle) >> 10(2) S.cm(-1) for bandtail hopping).
As an illustration, the localized pi states distribution in amorphous carbon nitride a-C1-xNx:H (x=0.23) is investigated in sandwich devices. Using a wide range of temperatures, electric fields and ac frequencies, several methods are proposed to derive some DOS parameters: density of states at the Fermi level, localization radius and disorder energy of the exponential bandtail distribution. In the field-enhanced bandtail hopping and in the activationless regimes, both apparent slope T-0(1/4) and prefactor sigma(circle circle) values are also properly described. An analytical model, based on the filling rate of localized states near the preferred transport energy, shows that the (F-S/T) scaling properties of nonlinear conductivity sigma(F, T) (with S similar to 2/3 in a-C1-xNx:H) are related to the field-dependent effective temperature describing the non-equilibrium distribution of pi(*) electrons in the bandtail. In ac transport, the characteristic response time tau(F, T) shows a linear ln tau vs T-1/4 behaviour, with a smaller T-dependence as the field strength increases, closely related to the filling rate obtained from the dc hopping transport.

• 出版日期2006-6