Marcel Schmeits

Thermal admittance spectroscopy of Mg-doped GaN Schottky diodes

Thermal admittance spectroscopy measurements at temperatures ranging from room temperature to 90 K are performed on Schottky structures based on Mg-doped GaN layers grown by metalorganic vapor phase epitaxy on sapphire. The analysis of the experimental data is made by a detailed theoretical study of the steady-state and small-signal electrical characteristics of the structures. Numerical simulations are based on the solution of the basic semiconductor equations for the structure consisting of two Schottky diodes connected back to back by a conduction channel formed by the GaN layer. The description explicitly includes the Mg-related acceptor level, with its temperature- and position-dependent incomplete occupation state, leading to a dynamic exchange with the valence band. It fully reproduces the variations with temperature of the capacitance–frequency and conductance over frequency curves, allowing to give for all temperature ranges the origin of the various contributions to the junction capacitance and of the microscopic mechanisms responsible for the capacitance–frequency cutoff. Series resistance effects are shown to be dominant at temperatures above 230 K, whereas the Mg-related acceptor level governs the electrical behavior below 230 K. The existence of a second acceptor level with an activation energy of several tens of meV is revealed from the analysis of the characteristics at low temperature. An optimized fitting procedure based on the comparison of the electrical characteristics obtained from the numerical simulations to the experimental data allows one to determine the microscopic parameters describing the structure, among which the acceptor activation energies, thermal capture cross sections, concentrations, and the Schottky contact barrier heights are the most important ones. The obtained activation energy of the Mg-acceptor level of 210 meV is by a factor of 2 larger than that obtained from a classical Arrhenius plot, showing that a complete description of Mg-doped GaN junctions requires the correct treatment of the Mg level, acting as a dopant and as deep impurity, as well as the inclusion of series resistance effects.

Electron and hole mobility determination in organic layers by analysis of admittance spectroscopy

We present results of a theoretical study on the simultaneous determination of the electron and hole mobilities in organic layers by admittance spectroscopy. By solving the basic semiconductor equations under steady-state and small-signal conditions for a typical system composed of an organic layer sandwiched between two electrodes, we show that it is effectively possible to obtain by admittance measurements on the same organic device, both the electron and hole mobilities. The analysis of the effect of the various parameters characterizing the structure allows determining the conditions for the simultaneous appearance of two resonance peaks in the curve representing the negative differential susceptance −ΔB a as function of the modulation frequency f. The frequency position of the two peaks allows us to obtain the transit time of both types of carriers and to deduce the mobility of the electrons and holes. Three major conditions have to be realized: injection of both electrons and holes at the respective...

Electrical conduction by interface states in semiconductor heterojunctions

Electrical conduction in semiconductor heterojunctions containing defect states in the interface region is studied. As the classical drift-diffusion mechanism cannot in any case explain electrical conduction in semiconductor heterojunctions, tunnelling involving interface states is often considered as a possible conduction path. A theoretical treatment is made where defect states in the interface region with a continuous energy distribution are included. Electrical conduction through this defect band then allows the transit of electrons from the conduction band of one semiconductor to the valence band of the second component. The analysis is initiated by electrical measurements on n-CdS/p-CdTe heterojunctions obtained by chemical vapour deposition of CdS on (111) oriented CdTe single crystals, for which current-voltage and capacitance-frequency results are shown. The theoretical analysis is based on the numerical resolution of Poissons equation and the continuity equations of electrons, holes and defect states, where a current component corresponding to the defect band conduction is explicitly included. Comparison with the experimental curves shows that this formalism yields an efficient tool to model the conduction process through the interface region. It also allows us to determine critical values of the physical parameters when a particular step in the conduction mechanism becomes dominant.

Competition between deep impurity and dopant behavior of Mg in GaN Schottky diodes

The effect of the deep acceptor Mg on the electrical characteristics of p-doped GaN Schottky diodes is analyzed. The theoretical study is based on the numerical resolution of the basic semiconductor equations, including the continuity equation for the Mg-related acceptor level. It gives the steady-state and small-signal analysis of p-doped GaN:Mg Schottky diodes, yielding as final result the frequency dependent capacitance and conductance of the structure. It is shown that the low-frequency characteristics are determined by the carrier exchange between the Mg related impurity level and the valence band, whereas above the impurity transition frequency, the hole modulation of the depletion layer edge governs the electrical response. Detailed results are shown on the effect of temperature, applied steady-state voltage and series resistance. The study of two back-to-back connected GaN Schottky diodes reveals the appearance of typical features in the electrical characteristics, depending on the respective Scho...

On the electronic structure of the Si(100)−2×1 surface☆

Abstract We have investigated the electronic structure of the asymmetric dimer model for the Si(100)−2×1 surface using our tight binding scattering theoretical method for semiinfinite crystal on the basis of a realistic bulk Si description. Our results are in good agreement with all available UPS and ARUPS data resolving outstanding discrepancies between previous theoretical results and experiment and they lend strong further support to the asymmetric dimer model proposed by Chadi.

Small-signal analysis of semiconductor heterojunctions with interacting interface states

A small-signal analysis of electrical conduction is given for semiconductor heterojunctions with a continuous density of states distribution inside the gap. The defect states are supposed to fully interact, such that their occupation can be described by a Fermi function with a quasi-Fermi energy for non-equilibrium situations. After numerical integration of Poissons equation and the continuity equations for electrons, holes and defect states one obtains the capacitance - frequency and conductance - frequency characteristics, depending on defect concentration, density of states function, temperature and applied voltage. Application to a InGaAs/InP heterojunction shows characteristic features due to the presence of interface states. The analysis illustrates the differences with the case of discrete interface states, continuous distributions of non-interacting states, and of bulk defects.

Investigation of defect levels in Mg-doped GaN Schottky structures by thermal admittance spectroscopy

Schottky structures based on Mg-doped GaN layers grown by metalorganic chemical vapor deposition (MOCVD) on sapphire substrate are studied by thermal admittance spectroscopy from 90 K to room temperature. Evidence of two impurity levels results from the analysis of the observed peaks in the conductance curves, whose positions and strengths are temperature dependent. The experimental results are analyzed within a detailed theoretical study of the steady-state and small-signal electrical characteristics of the structure. Numerical simulations are based on the solution of the basic semiconductor equations for the structure consisting of two Schottky diodes connected back-to-back by a conduction channel formed by the GaN layer.

Electromagnetic interaction between spherical cavities in solids

Abstract The dispersion energy between two spherical cavities in a dielectric is determined. Distant voids are found to interact mainly through ground state fluctuations of surface dipolar modes, just as full spheres do.