Semiconductor Nanoelectronic Devices Based on Ballistic and Quantum Effects
Abstract: As current silicon-based microelectronic devices and circuits are approaching their fundamental limits, the research field of nanoelectronics is emerging worldwide. With this background, the present thesis focuses on semiconductor nanoelectronic devices based on ballistic and quantum effects. The main material studied was a modulation doped In0.75Ga0.25As/InP semiconductor two-dimensional electron gas grown by metal-organic vapor phase epitaxy. The thesis covers mainly three types of devices and their twofold integration: in-plane gate transistors, three-terminal ballistic junctions and quantum dots. Various advanced nanofabrication tools were used to realize the devices, such as electron beam lithography, focused ion beam lithography and atomic layer deposition. The theories behind the analysis of the experimental data include principles of field effect transistors, the Landauer-Büttiker formalism, the constant interaction model, etc. The principles of in-plane gate transistors can be explained by a classical theory. The source, drain, one-dimensional channel and two side gates were in the same plane; a setup that can be obtained by single step lithography. The gating efficiency of the two independent gates was voltage-dependent, which resulted in a simplified circuitry for implementing a logic function. At room temperature, an SR latch with a signal gain of ∼4 was realized by the integration of two in-plane gate transistors. Three-terminal ballistic junctions are nonlinear devices based on ballistic electron transport. When two terminals are applied with voltages, the third terminal will output a voltage close to the more negative voltage in the two inputs, as opposed to a simple average of the two. From numerical calculations, this ballistic effect persists up to room temperature. Three-terminal ballistic junctions are so robust that nonlinearity is observable in asymmetric devices and relatively large devices. They can be fabricated on several materials by assorted techniques. The junctions find their applications in analogue frequency mixers, phase detectors and digital SR latches and the circuits are simpler than conventional designs. The intrinsic speed of the devices is in the GHz or THz regime by virtue of the ballistic transport. It is believed that as-built junctions have a potential as building blocks in future nanoelectronics. Quantum dots are zero-dimensional boxes for electrons with a decent resemblance to natural atoms. Due to their nanoscale size, numerous interesting quantum effects can be observed. Gate-defined quantum dots were fabricated in InGaAs/InP by incorporating a high-k HfO2 (20-30 nm thick, grown by atomic layer deposition) as the gate dielectric. The gate leakage was suppressed and the gating efficiency improved. At 300 mK, charge stability diagrams of single and double quantum dots were measured and studied in detail. Zeeman splitting in a parallel magnetic field and charge sensing by nearby quantum point contacts were also investigated. The single and double quantum dots are expected to be useful in fields including single electron logic, stochastic resonance, spintronics, quantum computing, etc.
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