As a small sprinkling of spice can drastically change the color and flavor of cuisine, point defects play crucial roles in tuning the electronic properties of semiconductor devices. For example, even at the parts-per-million level, an impurity species can increase the conductivity of an insulating host by many orders of magnitude. This relies on the introduction of impurity energy states that happen to lie near the conduction or valence band edges of the host. In practice, it is the ability to tune the conductivity by impurities that distinguishes semiconductors from insulators, more so than the magnitude of the energy band gap. The discrete nature of these point defects and their random placement in materials is becoming increasingly important for classical devices such as transistors at the nanoscale. Beyond the classical, the quantized electronic and spin states of point defects have drawn considerable interest for quantum science and technology, with the long-term potential for better scalability and room temperature operation.
The group’s work in this area has focused on how the local environment influences the properties of defects in semiconductors, and also how this influence can be tuned with atomic-scale manipulation of position, charge and spin states. The III-V family of semiconductors (e.g. GaAs, InSb) is an ideal platform for such studies, as commercial substrates exist with controlled doping levels, and it happens that the crystals are easy to cleave in vacuum, revealing a pristine surface without any additional preparation.
A ‘crater’ formed in an atomically flat InSb surface by ionization of a single atom.
Single dopant control in GaAs