Quantum Transport in Nanoscale Semiconductor Devices

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Jones, Gregory Millington
Yang, Chia-Hung
Because of technological advancement, transistor dimensions are approaching the length scale of the electron Fermi wavelength, on the order of only nanometers. In this regime, quantum mechanical phenomena will dominate electron transport. Using InAs single quantum wells, we have fabricated Y-shaped electron waveguides whose lengths are smaller than the elastic mean free path. Electron transport in these waveguides is ballistic, a quantum mechanical phenomenon. Coupled to the electron waveguide are two gates used to coherently steer the electron wave. We demonstrate for the first time that gating modifies the electron's wave function, by changing its geometrical resonance in the waveguide. Evidence of this alteration is the observation of anti-correlated, oscillatory transconductances. Our data provides direct evidence of wavefunction steering in a transistor structure and has applications in high-speed, low-power electronics. Quantum computing, if realized, will have a significant impact in computer security. The development of quantum computers has been hindered by challenges in producing the basic building block, the qubit. Qubit approaches using semiconductors promise upscalability and can take the form of a single electron transistor. We have designed, fabricated, and characterized single electron transistors in InAs, and separately in silicon, for the application of quantum computing. With the InAs single electron transistor, we have demonstrated one-electron quantum dots using a single-top-gate transistor configuration on a composite quantum well. Electrical transport data indicates a 15meV charging energy and a 20meV orbital energy spacing, which implies a quantum dot of 20nm in diameter. InAs is attractive due to its large electron Landé g-factor. With the silicon-based single electron transistor, we have demonstrated a structure that is similar to conventional silicon-based metal-oxide-semiconductor field effect transistors. The substrate is undoped and becomes insulating at low temperatures. There are two layers of gates that when properly biased define the single electron transistor potential profile. The measured stability chart at 4.2K indicates a charging energy of 18meV. Our silicon-based single electron transistor is promising, because spin coherence times in silicon are orders of magnitude longer than those in GaAs.