Current Research

Researchers within the QED group are investigating the electrical and optical properties of nanometer scale semiconductor devices. At these small length scales the device properties are no longer governed by semi-classical physics, but are instead determined by quantum mechanical effects. The group makes its own quantum semiconductor devices here at UNSW, and uses a variety of electronic and optical probes, at milliKelvin temperatures and in strong magnetic fields, to further the understanding of quantum electronics.

A common theme of our work is to understand the properties of valence band holes in semiconductor nanostructures. It has been known for over 100 years that electricity is semiconductors is carried by negatively charged electrons and positively charged holes However while the properties of electrons are very well understood, the understanding of holes (particularly when confined to move in nanostructures) is much less well understood. For example, you have probably been taught that valence band holes are essentially just heavy electrons, with a positive charge and a positive effective mass. This is incorrect. Holes are spin-3/2 particles whereas electrons are spin-1/2. It is only in the past few years that we are beginning to understand just how different holes are from electrons. For instance, whereas electrons have a well defined dipole moment that couples their spin to an externally applied magnetic field, holes also have quadrupole and octopole moments that have no equivalent in electrons. These unique spin properties have led to proposals for novel spin based hole transistors, that could run much faster and with lower power than conventional devices that rely on the charge of electrons, as well as new types of quantum bits for quantum information applications. But much work remains to be done before we even have a basic understanding of the spin properties of holes. In particular we have active projects in:

  Shallow hole quantum dots

Hole spins in GaAs semiconductors have a strong spin–orbit interaction and a weak hyperfine coupling to the surrounding nuclear spins, and therefore, are very interesting for the study of spin control and coherence times.
However, the investigation of hole spins in conventional GaAs /AlGaAs heterostructures has been limited due to the difficulties in fabrication. We are interested in developing new fabrication methods to make stable small hole quantum dot based on shallow undoped GaAs /AlGaAs heterostructures in which a single hole spin can be isolated and studied.

Capture1.JPGCapture.JPG Holes in silicon quantum dots

Although there has been much work studying electrons in silicon quantum dots, there has been much less work on holes in silicon quantum dots. Unlike electrons in silicon, holes have a strong spin–orbit interaction and are very interesting for the study of spin control and manipulation using only electric fields. In collaboration with Andrew Dzurak's group in ANFF we are studying the properties of holes in silicon MOS quantum dots.

R. Li, F. E. Hudson, A. S. Dzurak, and A. R. Hamilton, Single hole transport in a silicon metal-oxide-semiconductor quantum dot, Appl. Phys. Lett. 103, 163508 (2013).

Abstract Image

The electronic and magnetic properties of two-dimensional holes in semiconductor quantum wells

We perform electrical measurements of hole based devices at ultra-low temperatures and in intense magnetic fields to understand their complex spin properties.
We study holes in a number of materials, including gallium arsenide, silicon, and diamond. It is difficult to directly probe the spin properties of hole devices through purely electrical measurements, so we have developed a number of interference techniques to allow the spin properties to be studied.

A. Srinivasan et al, Using a tuneable quantum wire to measure the large out-of-plane spin splitting of quasi two-dimensional holes in a GaAs nanostructure, Nano Letters 13, 148 (2013).


Hole Quantum Wires

In a very low disorder short one-dimensional quantum wire, electrons or holes can travel without scattering at low temperatures - they behave as waves rather than particles. While the spin properties of electrons in quantum wires are mostly well understood, there is not even a basic understanding of the spin properties of holes in quantum wires. This is a bit surprising given that the average computer chip contains billions of transistors based on hole quantun wires!

For example, whereas the spin-splitting of electrons in a magnetic field is simply given by ΔE=gμBB, where g is a constant known as the Lande g-factor, for holes the g-factor is a 3x3 tensor. This complexity results from the coupling of hole spins to electric fields in the device – such as the electric fields used to confine the holes into quantum wires and quantum dots. Several projects in the QED group use hole quantum wires to directly probe the spin properties of quantum confined holes, providing the key data against which new theories can be tested.

J.C.H. Chen et al, Observation of orientation- and k-dependent Zeeman spin-splitting in hole quantum wires on (100)-oriented AlGaAs /GaAs Heterostructures, New Journal of Physics 12, 033043 (2010).


Electron-electron interactions and the forbidden metal-insulator transition in two dimensional systems

Most transistors, such as the building block of modern computers, the MOSFET, use a thin (two-dimensional) sheet of highly mobile electrons to carry the electric current. These 2D systems are not only of immense technological significance, but have led to profound new fundamental phenomena, with discoveries such as the Quantum Hall Effect, the Fractional Quantum Hall Effect, and the High Electron Mobility Transistor (HEMT) have each leading to separate Nobel Prizes (1985, 1998, and 2000 respectively). However, despite being created over 40 years ago, there is still no universally accepted theoretical understanding of the fundamental electronic properties of high quality two-dimensional systems. Even the nature of the electronic state is not completely known over the complete range of carrier density and disorder (pictured left).


Undoped AlGaAs/GaAs Semiconductor Billiards

Disorder increasingly affects performance as electronic devices are reduced in size. The ionized dopants used to populate a device with electrons lead to unpredictable changes in the behaviour of devices such as quantum dots each time they are cooled for use. We show that a quantum dot can be used as a highly sensitive probe of changes in disorder potential and that, by removing the ionized dopants and populating the dot electrostatically, its electronic properties become reproducible with high fidelity after thermal cycling to room temperature.

Impact of Small-Angle Scattering on Ballistic Transport in Quantum Dots” – A. M. See, I. Pilgrim, B. C. Scannell, R. Montgomery, O. Klochan, A. M. Burke, M. Aagesen, P. E. Lindelof, I. Farrer, D. A. Ritchie, R. P. Taylor, A. R. Hamilton and A. P. Micolich, Phys. Rev. Lett., 108, 196807 (2012).


Single electron/hole transistors and quantum dots

In a single electron transitor it is possible to observe effects associated with the motion of individual electrons. We have developed a novel technique for making highly stable nanoscale single electron and single hole transistors, and are investigating the coherent properties of electrons and holes in quantum dots and chaotic billiards.

For further details see O. Klochan et al, Applied Physics Letters 96, 092103 (2010) and A.M. See et al, Applied Physics Letters 96, 112104 (2010).


Quantum and scattering lifetimes in two dimensional systems

We are examining the low-temperature properties of high-quality heterostructures, in which scattering is dominated by two types of disorder: remote ionised impurities (RII) and homogeneous background (BG) impurities. However it turns out calculations involving the homogeneous background impurities, which affect all quantum devices, is non-trivial and hinders direct comparison between theory and experiment. We are working to make comparison between theory and experiment easier, and to related these scattering lifetimes to observed transport phenomena.


Inter-device interactions in strongly coupled quantum devices

As the dimensions of individual components in a "chip" shrink, and we pack these components ever closer together, it will no longer be possible to ignore interactions between devices. So as well as understanding how electron-electron interactions within a device affect its electrical properties, it is also important to understand the interactions between devices in order to design the next generation of nano-electronic devices.

Topic revision: r7 - 10 Dec 2014, AlexHamilton
Quantum Electronic Devices