Beyond modulation doping

When an impurity atom in a semiconductor crystal has more (or fewer) valence electrons than the atom it replaces, it can donate one or more electrons to (or accept them from) the crystal lattice. The deliberate addition of such impurities, called dopants, is the traditional means of generating mobile charge carriers (negatively-charged electrons or positively-charged holes) within semiconductor devices, including the silicon-based metal-oxide-semiconductor field-effect transistors (MOSFETs) and compound semiconductor high-electron-mobility transistors (HEMTs) ubiquitous in modern electronics.

High-mobility, gallium-arsenide-based HEMTs in particular, which can be made from ultrahigh-purity wafers grown by molecular beam epitaxy (MBE), have also been instrumental in the discoveries of new physics, including the fractional quantum Hall (FQH) effect, microwave-induced resistance oscillations, Wigner solid phases in magnetic field, ballistic transport and conductance quantisation in one-dimensional channels, single-electron quantum dots, Kondo physics, spin-based solid-state qubits, possible excitonic superfluidity in double-quantum-well structures, and possible non-Abelian statistics in certain novel FQH states.

Even with the technique of modulation doping, where dopants are placed far away from the conducting channel, disorder due to the ionised dopants can still be felt by the carriers in a high-purity wafer, and this disorder can interfere with phenomena being studied. However, these intentional dopants are not necessary if one uses instead an external electric field to electrostatically induce a two-dimensional electron gas (2DEG) or hole gas (2DHG) at the semiconductor heterointerface. This electric field can be applied with electrostatic gates on the front and/or back side of devices. Although the proof-of-principle demonstration of undoped devices (which required only one working device) was reported more than eighteen years ago by Bell Labs (USA), the complex cleanroom fabrication process and the ensuing very low yield of working devices have prevented the use of undoped devices from becoming mainstream. Over the last three years, our group has made a number of technological breakthroughs which allow a 90+% yield of working devices, including Hall bars and nanostructures (e.g., quantum dots). This yield is now high enough to have research projects depend on a steady, reliable supply of high-quality samples.

To capitalise on this success, we propose to combine our ability to fabricate such devices on demand with our expertise in MBE semiconductor wafer growth and millikelvin temperature measurements to further progress on two of the topics listed above, the fractional quantum hall effect and spin-based solid-state qubits. Many "exotic" FQH states present in the second Landau level do not fit the Laughlin/Jain theory which describes "conventional" FQH states, and are particularly sensitive to dopant-induced disorder. Our experimental programme will shed light on the nature of these states, particularly the famous state at filling factor 5/2 and its possible non-Abelian properties. Gate-defined electron spin qubits in GaAs were once amongst the forerunner systems for the realisation of a quantum computer. However, this system suffers from the presence of hyperfine interactions and charge noise, both of which cause spin decoherence on timescales too short for a practical quantum computer. Our experimental programme will demonstrate how both hyperfine interactions and charge noise are significantly reduced when gate-defined double quantum dots are fabricated from undoped 2DHGs.

Our proposed work will yield fundamental insights into physical phenomena not easily accessible using even the highest quality doped heterostructures.

Historically, advances in the understanding of the physics of electronic systems have found implementation in practical devices. More and more device innovations rely on quantum effects, especially as miniaturisation of electronics surpasses the limitations of classical physics. Progress in our proposed research into quantum effects in electronic systems has the potential to have significant impact on consumer electronic devices in the long term, benefiting both consumers and device manufacturers.

The technology to fabricate undoped GaAs-based semiconductor heterostructures could lead to significant improvement in the performance of high-speed electronic devices such as those used in mobile phones and tablet computers. Another advantage of undoped devices is their unparalleled batch-to-batch reproducibility compared to their doped counterparts. Undoped devices could therefore also suit electronics manufacturers seeking reproducibility over maximum bandwidth. Another attractive property of undoped heterostructures is the possibility of operating them in both so-called "n-type" and "p-type" configurations in the same electronic device (reversing its polarity). This is not possible in inorganic doped electronic devices. The possibility of this new functionality has spurred the design of new circuit architectures by electrical engineers and computer scientists which, if implemented, may radically alter the power losses and speed of many existing electronic components.

Manufacturers of scientific equipment, in particular molecular beam epitaxy reactors for high-quality material growth and cryogenic measurement systems, also stand to benefit from our research programme. The performance of such systems used in our laboratory can be (and has been) improved, either through unconventional operation or in-house modification, and manufacturers have been keen to receive feedback.

The successful implementation of a practical quantum computer (which is likely to occur well after the end of this grant) could have a wide-ranging impact on the health and wealth of the UK. The most useful applications of quantum computing lie in simulating real quantum systems with many particles, which are almost impossible to simulate with conventional classical computers due to the time and resources required. Academic theorists in many fields, from fundamental physics to medical biology, would benefit. Direct beneficiaries in industry would include pharmaceutical, biotechnology and chemical companies to whom the ability to efficiently simulate molecular systems would be invaluable in developing new products. Other applications include breaking many of the cryptographic systems currently in use (e.g., the RSA public key) and searching through large databases. Government security agencies and commercial information technology entities are very interested in these types of application. For example, Microsoft and IBM are investing heavily in fundamental research for the realisation of a quantum computer. Microsoft in particular is interested in topological quantum computers: they fund a dedicated theoretical institute ("Station Q" in California) and provide funding to various experimental physics labs. Progress achieved in our research group could attract such funding to the UK.

Training of research staff in a wide range of interdisciplinary techniques is an important part of this project. These skills are highly valued in many technology-based industries, and therefore will contribute to the knowledge capacity of the UK.

There is considerable public interest in the implications of quantum mechanics to everyday life. Any breakthroughs in the understanding of the fundamental workings of our universe obviously have cultural significance as contributions to the body of human knowledge. The discovery of the effects sought in our experimental programme would most likely receive coverage in the mainstream press as a matter of popular interest.