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Quantum tunneling, field induced injecting contact, and excitons

Citation

Liu, Yixin (1995) Quantum tunneling, field induced injecting contact, and excitons. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechETD:etd-10232007-143854

Abstract

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This thesis consists of three parts: Quantum tunneling simulation, Schottky barrier induced injecting contact on wide band gap II-VI materials, and excitons in semiconductor heterostructures.

Part I (chapter 2, 3) deals with quantum transport and electronic band structure in semiconductor heterostructures. In chapter 2, we present a new method for quantum transport calculations in tunnel structures employing multiband k.p theory. This method circumvents the numerical instability problems that arise in the standard transfer-matrix method. In addition to being numerically stable, efficient, and easy to implement, this method can also be easily generalized to include the magnetic field and strain effects. The development of this technique mainly consists of two parts, the discretization of effective-mass Schrodinger's equation using finite-difference method, and the formulation of boundary conditions. The treatment of boundary condition in quantum transport is similar to the Multiband Quantum Transmitting Boundary Method (MQTBM) for use with multiband tight-binding models. The calculations of transmission coefficients reduce to a set of linear equations, which can be solved very easily. With appropriate formulation of boundary conditions, this technique can be readily extended to the calculations of electronic band structures in quantum confinement and superlattice structures. We have applied this new technique to magnetotunneling in interband tunnel structures in chapter 3, and studied two prototypical device structures: Resonant Interband Tunneling (RIT) devices and Barrierless Resonant Interband Tunneling (BRIT) devices. Effects of transverse magnetic field on the band structures, transmission spectrum, and I - V characteristics are investigated. Evidence of heavy-hole resonance contribution can be identified in the change of I - V characteristics under applied magnetic field. The technique has also been illustrated for hole tunneling in p-type GaAs/AlAs double barrier tunnel structures, and calculations of electronic band structures in lattice-matched InAs/GaSb superlattices, and strained InAs/Ga[subscript I-x]In[subscript x]Sb superlattices.

Part II describes a novel approach to achieve ohmic injecting contact on wide bandgap II-VI semiconductors. The problem of making good ohmic contact to wide bandgap II-VI materials has been a major challenge in the effort of making visible light emitting diodes. The method we propose consists of forming the device structure in an electric field at elevated temperatures in the Schottky barrier region, to spatially separate the ionized dopants from the compensating centers. In this way, the ratio of dopants to compensating centers can be greatly increased at the semiconductor surface. Upon cooling, the dopant concentrations are frozen to retain a large net concentration of dopants in a thin surface layer, resulting in a depletion layer that is sufficiently thin to allow tunneling injection. Calculations of band profiles, distributions of dopant concentrations, and current-voltage characteristics were performed. We have selected the case of Al doped ZnTe in our study, in which two Al donors complex with a doubly negatively ionized Zn vacancy to produce total compensation. The results show that the bulk doping concentration and the total band bending during the forming process are the crucial factors for achieving injecting contacts. For Schottky barrier heights above 1 eV, doping concentrations as high as 10[superscript 20]cm[superscript -3] are needed.

In part III, we studied excitons in semiconductor heterostructures, consisting of two subjects: excitons in II-VI heterostructures, and exciton coherent transfer process in quantum structures. Calculations of exciton binding energies and oscillator strengths are performed in both Type-I strained CdTe/ZnTe superlattices with very small valence-band offset and Type-II strained ZnTe/ZnSe superlattices. A special variational approach was employed to take into account the effects of unusual band alignment, strain, and image charges at the heterojunction interface. It is found that the large enhancements of exciton binding energy and oscillator strength in the CdTe/ZnTe system are similar to what one finds in systems with a much larger valence band offset. For small CdTe layer thickness, however, the confinement of holes in the CdTe layer is weak, resulting in a lowering of the exciton binding energy. The oscillator strength in CdTe/ZnTe superlattice system shows the expected enhancement over the oscillator strengths in the bulk.

For the ZnTe/ZnSe system, the Type-II character of the heterojunction results in the confinement of the electrons and holes in different layers. It is found that strong confinement of electrons and holes by the large band offsets can give rise to a fairly large exciton binding energy for thin heterojunction layers. Also, the mismatch in dielectric constants induces an image charge at the interface, which modifies significantly the exciton Hamiltonian in an asymmetric superlattice structure and plays an important role in determining the degree of localization of the electron and hole at the interface.

We have investigated exciton coherent transfer in semiconductor quantum structures. In systems where the typical dimensions of the semiconductor quantum structures and the spacings between them are significantly smaller than the photon wavelength, the resonant transfer of excitons between two identical quantum structures is accomplished through the interaction of near field dipole-dipole transitions (exchange of virtual photons). The transfer matrix elements are calculated for three different geometries: quantum wells, quantum wires and quantum dots, respectively. The results show that the exciton transfer matrix element is proportional to exciton oscillator strength, and depends on exciton polarization. The transfer matrix element between quantum wells depends on the exciton wave vector in the plane of the wells, k[...], and vanishes when k[...] = 0. For quantum wire and quantum dot structures, the transfer matrix elements between two units separated by R vary as R[superscript -2] and R[superscript -3], respectively. For quantum structures with typical characteristic size of 50[...] with separation about 100 [...], the transfer matrix element is on the order of 10[superscript -3] meV. It corresponds to a resonant transfer time of 1 ns, comparable with the exciton lifetime. However, it is significantly smaller than the inhomogeneous broadening due to phonons and structural imperfection in most synthesized semiconductor quantum structures achievable today, which is typically on the order of a few meV, making the realization of experimental observation difficult. The study is to explore new ideas and potential technological applications based on excitonic devices.

Item Type:Thesis (Dissertation (Ph.D.))
Degree Grantor:California Institute of Technology
Division:Engineering and Applied Science
Major Option:Applied Physics
Thesis Availability:Restricted to Caltech community only
Research Advisor(s):
  • McGill, Thomas C.
Thesis Committee:
  • McGill, Thomas C. (chair)
  • McCaldin, James Oeland
Defense Date:9 March 1995
Record Number:CaltechETD:etd-10232007-143854
Persistent URL:http://resolver.caltech.edu/CaltechETD:etd-10232007-143854
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:4225
Collection:CaltechTHESIS
Deposited By: Imported from ETD-db
Deposited On:23 Oct 2007
Last Modified:26 Dec 2012 03:06

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