Yoshiro Hirayama

One- and Zero-Dimensional Tunneling Diodes Fabricated by Focused Ion Beam Implantation

One-dimensional and zero-dimensional tunneling diodes were fabricated using focused Ga ion beam implantation. The current vs voltage (I-V) curves of one-dimensional diodes showed broad steps near the onset of the tunneling current, followed by a series of small current peaks at higher voltages. The I–V curves of the zero-dimensional diodes, however, showed a series of peaks at the onset of the tunneling current. These differences are well reproduced by calculating the tunneling of a three-dimensional electron through the one-dimensional or zero-dimensional well levels. In the zero-dimensional diode, the amplitude difference between neighboring current peaks was constant because of the degeneracy of the zero-dimensional well levels confined by a harmonic lateral potential. One-dimensional diodes with strong lateral confinement in the contact region were fabricated by implanting a low dose of ions. This tunneling current is generated by the mixing of one-dimensional (emitter) and two-dimensional (well) subbands whose eigenfunctions have the same parity.

Single-electron charge qubit in a double quantum dot

A semiconductor double quantum dot provides a simple artificial two-level system (qubit) that can be manipulated by electronic signals. Full one-qubit operation is demonstrated with a high-speed voltage pulse. In addition, strength of decoherence can be controlled to allow efficient initialization of the qubit. Remaining uncontrolled decoherence is discussed with background charge fluctuations, cotunneling, and electron-phonon coupling. Moreover, charge detection of a double dot is demonstrated with a quantum-point-contact charge detector.

Dimensionality Control in GaAs Quantum Wells Dynamically‐Modulated by Surface Acoustic Waves

We demonstrate the dimensionality control in GaAs quantum wells induced by the dynamic modulation by surface acoustic waves (SAWs). The rapid changes in spatial carrier distribution is due to the strong piezoelectric fields induced by SAWs.

Imaging of Local Tunneling Barrier Height of InAs Nanostructures Using Low‐Temperature Scanning Tunneling Microscopy

The local tunneling barrier height (LTBH) has been measured using low‐temperature scanning tunneling microscopy to examine the local potential profile of an InAs nanostructure. The nanostructure is a faultily‐stacked nanocrystal in epitaxial InAs thin film grown on GaAs(111)A substrate. It is found that averaged LTBH is consistent with the workfunction or electron affinity of InAs. The nanostructure boundary is found to have a higher LTBH than the surroundings. The resonance peak calculated using this potential wall is comparable to that measured by spectroscopy of local density of states (LDOS) in the nanostructure. A gradual LTBH decrease is additionally observed at negative sample bias voltage near the boundary indicating downward band bending, which is consistent with LDOS there.

Direct probing of local-density-of-states in semiconductor nanostructures

The electronic features of semiconductor nanostructures, such as zero-dimensional states, are usually inferred from macroscopic optical and transport experiments. Although, direct probing of electrical features in semiconductor nanostructures looks very attractive, it is very difficult for a conventional semiconductor structure. However, direct probing becomes possible through a combination of low-temperature scanning tunneling microscopy and InAs(111)A surface in an ultra-high vacuum, where conductive electrons automatically accumulate near the clean surface. The clear observation of a Friedel oscillation pattern around a dislocation demonstrates successful mapping of the local-density-of-states (LDOS) of the conductive electrons. Inverted pyramidal defects are naturally formed during molecular beam epitaxial growth of InAs thin films on GaAs(111)A substrates and they operate as well-defined quantum dots. The measured LDOS pattern inside the quantum dots clearly changes as a function of energy, i.e. a sample bias, reflecting the LDOS pattern of each zero-dimensional state. A resonant concentration of the LDOS to the zero-dimensional energy levels is also demonstrated in these experiments. The LDOS measurements of a series of inverted pyramidal quantum dots with different side lengths and their comparison with theoretical calculations suggest a unique feature of the quantum dot system examined in this study.