Dynamic Response of Thin‐Film Semiconductors to AC Voltage Perturbations

Abstract

A theoretical treatment of a Schottky barrier dynamic response is developed on the basis of a general model of a semiconductor with thickness comparable in length to the space charge region width. It is shown that, when the space charge region approaches the metal/semiconductor interface, the electric field at this interface, induced by the charge accumulated on the metal, becomes significant with respect to the electric field induced by the charge accumulated on the semiconductor. Under this condition, the total capacitance of the Schottky barrier becomes independent of the polarization potential and tends to the value ε/L, like in a pure dielectric insulator. The term thin film is intended to be with respect to the screening length, which is a function of the volumetric charge density. In amorphous materials the transition potential at which the semiconducting to insulating behaviour is observed is dependent on the frequency. An approximated analytical solution for the capacitance of the junction is calculated. The model for finite thickness semiconductors is successfully applied to the study of anodic Nb(2)O(5), formed in phosphate buffer 0.5 M aqueous solution up to different formation potentials (namely 5, 10 and 20 V vs. Ag/AgCl). The finite thickness semiconductor model permits extrapolation to a general behaviour of the oxide in a wide range of frequencies, potentials and thicknesses, and identification of the electron transfer between adsorbed surface species and the conduction band of Nb(2)O(5) at potentials near to the flat band value.

Assessment on the use of the amorphous semiconductor theory for the analysis of oxide films

“…were simulated, one entitled as "thick" (L = 141 nm) and the other as "thin" (L = 28.2 nm). As discussed in a previous study [12], the finite thickness effect on the impedance data of amorphous semiconductor Schottky barriers is given by the ratio of the space charge region and the thickness of the semiconductor.…”

“…Lang et al [16,17], and later extended to the case of thin films by our group [12], the impedance response of the amorphous semiconductor can be described by two equations (eqs 2-3), which are related to the series capacitance and resistance, respectively:…”

“…where f(ω,ψ s ), given by the power law in eq (12), is employed to improve the fitting of C SC -1 by accounting for the energy variation in the DOS distribution, while N ̅ is a frequency-dependent empirical parameter, which is related to the average value of the DOS between E F and E ω . The value of a(ω) in dependence of the electronic structure of the amorphous semiconductor / electrolyte junction will be discussed by taking into account the electronic structure of the semiconductor.…”

“…empirical parameter used in eq (12) [-] f(ω,ψ S ) empirical correcting function in eq (11), defined by eq (12) [-] ψ * band bending equal to the minimum between ψ S and ψ g [V] ψ C band bending at the energy level E ω , defined by eq (5) [V]…”

“…In previous studies we have stressed several critical issues accompanying the use of such techniques without a correct and strict theoretical approach. The proper approach has to take into account the distribution of electronic states (DOS) inside the mobility gap of the oxides due to their not perfect stoichiometry, as well as to their possible amorphous structure [2,11,12]. In the latter cases, we have shown that the theory of amorphous semiconductors (a-SC) Schottky barriers, developed by R.A. Abram et al [13][14][15] and D.V.…”