Wide bandgap semiconductors - defects and dopants

A paramount challenge facing our common global future is access to sufficient supply of clean energy. Global energy demands are expected to double by year 2050 and the electricity demands will even triple. Hence, disruptive innovations are required for the penetration of fossil-free energy production, storage, conversion and distribution. Tailor-made materials and components, accomplished via controlling the interplay between bulk properties, nanoscale phenomena and atomistic processes, form the basis of future technologies. In this context, a decisive class of materials is semiconductors and especially, wide bandgap semiconductors (WGS) are considered as perhaps the most promising new concept. In the WEDD project, we address two of the most exciting and recognized WGS, namely zinc oxide (ZnO) and silicon carbide (SiC) which exhibit complementary properties. ZnO is a material of choice for optoelectronic devices, such as future generation solar cells, where it can act as an efficient emitter with high electrical conductivity and high transmittance of incident photons, and light emitting devices operating in the UV/blue wavelength regime. SiC is, on the other hand, a material of choice for power electronics required to convert and transport/distribute electrical energy, especially from renewable energy sources. Savings corresponding to 10-15 % of the energy produced by nuclear power plants world-wide are foreseen if todays Si-based power systems are replaced by SiC-based ones. An outstanding scientific highlight accomplished in WEDD is the identification of the carbon vacancy (Vc) in SiC and its role in limiting the lifetime of electrons/holes. This has been accomplished through close international collaboration with groups in Japan, Sweden and Hungary, and the results enable us to control the presence of Vc. Very recently, a new concept to tailor the Vc concentration has been demonstrated utilizing annealing treatments in carbon-rich ambient at moderate temperatures under thermodynamic equilibrium conditions. The experimental results are described by a quantitative simulation model including diffusivity and recombination of carbon vacancies and carbon interstitials as central ingredients. The experimental concept is straightforward and regarded as superior to the ones currently used to control the Vc concentration. Accordingly, SiC-devices with greatly reduced power losses can be expected in the future for energy conversion and transmission of electricity. Another spectacular highlight in WEDD is the demonstration of the role of hydrogen in the so-called E3 defect in ZnO. E3 is the most prevailing point defect in all types of ZnO materials and prevents realization of p-type doping. This new finding may pave the way to overcome the p-doping obstacle and enable realization of light emitting ZnO-bipolar devices. Further, through a study combining results from Density-Functional-Theory (DFT) calculations and electrical spectroscopy measurements, E3 is ascribed to a complex of one zinc vacancy with three trapped hydrogen atoms. Finally, the interaction between oxygen vacancies and interstitial hydrogen atoms has been eluciated showing a strong reactivity.

This proposal addresses fundamental issues regarding atomistic phenomena in mono-crystalline zinc oxide (ZnO) and silicon carbide (SiC) - two wide bandgap semiconductors with great promise for renewable energy technology, energy saving (solid state lighti ng and electrical power distribution) and information technology (sensors and optoelectronics). However, to benefit from the true potential of ZnO and SiC some critical scientific challenges exist and two of the most crucial ones are tackled here, i.e., " controllable and stable doping" and "electrically active defects". The origin of the commonly observed "inherent" n-type conductivity of as-grown ZnO will be studied in detail and the contributions from intrinsic defects and residual impurities are to be understood and determined quantitatively. An ultimate goal is to master the inherent n-type doping and realize uncompensated high-resistivity samples of device-worthy quality. Tunable and stable p-type doping is the most demanding challenge for a true bre akthrough of ZnO-technology in energy saving and optoelectronics. Non-equilibrium processes invoking ion implantation, diffusion and sample quenching are exploited for elements anticipated to have a suitable electronic structure as shallow p-type dopants. Further, the role of defect species, possibly Zn interstitials, occurring during thermal treatment is to be revealed. 4H-SiC is a prime material for low-loss power electronics and sensors operating in harsh environment but the performance is ultimately l imited by deep level defects. Especially, the dominant Z1/2 and EH6/7 centers will be studied regarding identification, suppression and diffusion mechanisms. The project is planned for 2 PhD students and 1 visiting scientist, and excellent facilities exis t at UiO-MiNaLab for sample growth, modification and characterization. Further, the project comprises extensive collaboration with leading international and national research partners.