Semiconductor Nanoelectronic Devices Based on Ballistic and Quantum Effects

Sammanfattning: 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.
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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.