Quantum confined states in cylindrical nanowire heterostructures

• Main Author: Smith, Ainsley H.
• Format: Others
• Published: DigitalCommons@Robert W. Woodruff Library, Atlanta University Center 2007
• Subjects: Physics
• Online Access: http://digitalcommons.auctr.edu/dissertations/2364; http://digitalcommons.auctr.edu/cgi/viewcontent.cgi?article=3686&context=dissertations

We present an investigation of quantum confinement effects in nanowire heterostructures through the use of an effective-mass model with a band-offset induced potential barrier. The characteristic size of microelectronics is rapidly approaching the nanometer scale and because of this, nanostructure based devices in the field of nanomaterial research is continually being emphasized. The quantum confinement effect exhibited by the nanowire is the most interesting in one-dimensional nanostructures. Potential applications for the nanowire include its use in the fabrication of high performance devices such as the p — n junction diode, the p-channel or n-channel coaxial gated field effect transistor, and the complimentary field effect transistors, to name a few. In the fabrication of such devices, a doping process is used in order to supply free carriers. This process involves introducing doped impurities which unfortunately causes difficulties. These difficulties are characterized by a marked decrease in the mobility of the aforementioned carriers and include the scattering of the free carriers. To remedy these problems a novel doping mechanism has been proposed. It involves the use of a radial heterojunction in a core-shell nanowire where it has been suggested that one can dope impurities in the shell and inject free carriers to the core or vice versa. This separation of free carriers reduces their scattering rate and improves their mobility, both preferred properties for high-speed devices. A better understanding of the heterojunction under strong cylindrical confinement is important to guide the future fabrication of nanowire-based high-speed devices. In order to achieve this, the question as to whether the band offset evolves with the size of the nanowire needs to be addressed. The inquiry into the relationship between band offset evolution and nanowire size led us to employ an effective-mass model with a band-offset induced potential barrier to study the band structure of carriers in cylindrical core-shell and core-multishell nanowires. Quantum confined states and band alignment effects are found to be dependent upon the height of the potential barrier, the core-shell radius ratio, and the diameter of the quantum wire. The subband charge densities are studied for clarifying the quantum confinement. By numerically solving the effective-mass model we were able to provide an interpretation of experimental observations on carrier accumulation and one-dimensional ballistic transport in Ge-Si core-shell nanowire heterostructures. The model serves as the continuum limit to the first-principles simulation approach.