Bulk and surface states analysis in a-Si:H by Schottky and MIS tunnel diodes capacitance and conductance measurements

Abstract

An experimental and theoretical study of both the capacitance and the conductance of Schottky and MIS tunnel diodes made on undoped a-SI:H, versus frequency, temperature, and voltage is presented in this paper. It is based on a general model using an equivalent electrical circuit of the devices, which permits an unambiguous distinction to be made between the contributions of bulk surface states, to determine their respective densities and to analyze their kinetic properties, It is shown that the bulk density of states (DOS) can be determined from capacitance and conductance measurements versus temperature and frequency on both Schottky and MIS devices, provided the modulation frequency is high enough to prevent any contribution from surface states. It is demonstrated that capacitance-voltage characteristics of Schottky diodes cannot provide reliable information about either bulk or surface as the diffusion theory and not the thermionic theory is appropriate to describe the transport of electrons across the junction. It is shown theoretically and experimentally that a MIS device is required to measure surface states, the insulator preventing the screening of their contribution by the metal. The measured surface DOS, derived by three different and independent methods, is very high (5×1012−2×1013 cm−2 eV−1) and essentially the same for all the investigated samples whose bulk DOS around the Fermi level ranges between 5×1015 and 4×1017 cm−3 eV−1. From conductance measurements, it is shown that the kinetics of the occupancy of both bulk and surface states is controlled by free electron transport in the conduction band and not by the states’ interaction kinetics with these electrons. The very high derived lower limit of the effective capture cross section sn of electrons by states around the Fermi level EF(sn ≳5×10{13 cm2) implies either that the interaction between states around EF and free electrons takes place via intermediate states of that the actual distance between the bulk Fermi level and the conduction level is smaller than the activation energy Es of the conductivity.