The Admittance Spectra Of Low-dimensional Semiconductor Systems

Admittance spectroscopy (AS) is a powerful tool for electronic characterization of nano semiconductor materials, which is used to determine energy levels structure, activation energy and band offset of quantum confinement system. But it is always blank in the systematic theory and detailed study at this area. This dissertation focuses on the theory modelling of AS for the quantum well (QW) and quantum dot (QD) structure. And deep discussion is taken for the case under different kinds of external field.Admittance spectroscopy is a measurement of the complex admittance of samples as a function of temperature. The conductance usually comes from a variety of scattering mechanisms during the transport process of carriers which are trapped in the confinement potential. In our experimental measurements, it is a. c. conductance. For the steady state, the carriers are emitted (or captured) out of (into) the trapping potential well simultaneously. But the whole system is still in the thermal equilibrium state. Considering a constant quantity of particles, a. c. conductance can be written as a function of the emission rate and the capture rate just by using the weak field approximation.We employ the semi-classical Boltzmann's equation to govern the transport process of carriers and do the calculations on the emission rate and the capture rate. Various scattering factors are taken as the perturbation on the transition from the confining state to the continuous state for the trapping carrier. With the application of Fermi Golden rule the matrix element of transition rate is obtained through the integral on all the final states. Further, considering all the original states and doing the integral on them, we can get emission rate and capture rate and then get the conductance. Because the Fermi and Bose statistic distribution are respectively taken to describe the occupation possibility of the carriers and phonons in different energy levels , it is natural that we can introduce temperature into our theory model. We can get the conductance corresponding to different temperature, which is obviously admittance spectroscopy.For the nano material, there must be a quantum confining effect because of its small size. Whatever nano structure (such as quantum well, quantum dot, nano wire, quantum ring) and whatever kind of confinement potential for different material the sample has, all of those different things indicate on the quantum state of carrier and the corresponding wave function in the end even under the different external field. For different systems, the only thing that had to be modulated for the model is considering different scattering mechanism and the different state of transport carriers due to different confining potential. Hence, the model is rather universal for the different cases in shape, dimensions or material.In this dissertation, we concern our effort on the GeSi system. Because IV semiconductor is a no-polar system and it has small interval energy levels space, the polar acoustic scattering is absent so nopolar phonon scattering, the oscillating potential produced by lattice vibrations, plays a role influence in the transport process. This type of perturbation potential leads to the transition rate. For quantum well system, we take finite square well model and take proper potential due to different composition of material in the well to do the further calculation and discussion. For quantum dot system, we mainly take the popular harmonic model and apply the corresponding wave function to the AS model. For the quantum well and quantum dot, a series of experiments were taken carefully and the experiment results were compared with the theory simulation.Finally, we give a new method to theoretically and experimentally determine the confinement strength of quantum dot just starting from our model, which is very difficult to obtain by other optic method for GeSi QD system. We checked the measurement results and the theory values through the relationship between the Coulomb charge energy space and the strength parameter and got good matched results.