A free charge carrier injected into an otherwise perfect crystal lattice can get localized at interruptions in the periodicity that are key both in providing and in limiting the functionality of the device. These interruptions in periodicity include surfaces of materials, interfaces between materials, extended defects such as stacking faults or dislocations, and point defects such as dilute impurities (e.g., dopants) or vacancy/interstitial defects.
Vacancy-type point defects, i.e., defects where one or several atoms are missing from a lattice site, are often present in rather high concentrations in semiconductors and insulators due to their relatively low formation enthalpies combined with relatively high stabilities (compared to other intrinsic point defects). In addition, thin film synthesis that usually combines far-from-thermodynamic equilibrium conditions with high growth speeds favors the formation of vacancy defects. Our interests are focused – but not limited to – on the defect-dopant interactions and more generally defect-functionality relationship in modern semiconductors for micro-electronics (Si, Ge and their group IV alloys), semiconductors for opto-electronics (“traditional” III-V compounds, their dilute nitrides and bismides, and the family of III-N semiconductors and their alloys), novel semiconductors for power electronics (e.g., Ga2O3 and related materials, other wide band gap oxides and nitrides), and deep color centers in wide-gap semiconductors for quantum technologies. We specialize in the development and applications of spectroscopic tools based on positron annihilation, and of theory of positrons in solids and associated computational methods. Using a combination of positron experiments and supporting theory we are often able to identify dominant positron traps at the atomic level. See Tuomisto F. & Makkonen I., Rev. Mod. Phys. for a detailed account on the experimental and theoretical methods along with illustrative examples of defect studies in semiconductors.
Senior personnel involved in this research line: