Ever since discovery of semiconductors and their first application in transistors in the middle of the 20th century, academic and industrial researches were developing these wonderful materials. Following the requirement to enhance performance while reducing production costs, enthusiasts were able to pack high power in miniature sizes. Remember these big rooms full of glass and metal, filled with resistors and lamps making howling sounds just to perform a series of trivial calculus? You have a device several orders of magnitude more powerful in your hands nowadays, thanks to a silicon chip. An era of semiconductors, which nearly half a century ago, is still far from its end. Life proposes new problems that require adapted solutions, where nitride semiconductors take over. One can grow nitrides by combining a group 3 metal and a group 5 nitrogen (this is why this family of materials is sometimes called III-V semiconductors). During my research, I was studying electrical properties and defects formation in such AlN (aluminum nitride), GaN (gallium nitride) and a compound AlGaN semiconductors.

Why nitrides?

Nitrides (GaN/AlN) can be applied in opto- and power electronics, which requires devices to operate at high currents and high frequencies with improved stability and energy efficiency (for example, displays with high luminosity, lasers, microchips). Electrical properties of semiconductor can be further enhanced by doping – a process of adding electrically active impurities (for examples silicon which is a donor). Moreover, tuneable ultrawide bangap of AlGaN alloys allows designing deep ultraviolet (UV) light sources with enhanced quantum efficiency. Such UV light emitting devices can be applied in sanitizing systems against viruses and bacteria, which is relevant to fight current coronavirus pandemic or provide more people with drinking water. However, nitride semiconductors appear to suffer from limited electrical conductivity due to compensation of charge carriers despite doping.

How do you do this?

We do not fabricate semiconducting materials ourselves, but work in collaboration with crystal and alloy growing facilities from Poland, Germany and USA. Our collaborators develop growth or doping techniques in order to obtain desired electrical or optical properties, and send us samples for characterization and to study the fundamental processes happening within materials. Here at Helsinki Accelerator Laboratory we are able to study defects and their formation by a wide variety of techniques including positron annihilation methods and ion beams. Positron annihilation spectroscopy reveals information about physical and chemical properties of negatively and neutrally charged defects. Ion beams allow us produce defects in materials by irradiation or tune properties by doping. We can also complement the results obtained in our lab by conducting X-ray absorption spectroscopy that is done at European synchrotron facilities.

What’s cooking?

We have studied samples of highly Si-doped GaN that demonstrated low concentrations of free charge carriers and cation defects. These findings leave an open question on the mechanisms of Si compensation, which cannot be linked to formation of the acceptor-like defects or presence of acceptor-impurities, as their concentrations are relatively low. X-ray absorption experiments that the local environment of Si is different depending on whether the Si is compensated or not, as well as if the compensation is spatially correlated. Moreover, we studied in grown vacancy defects AlGaN alloys doped with Si and with 90% Al content. We showed, that vacancy defects caused by the increase in Fermi level due to the Si doping become important compensating centers at doping levels approaching 10^19 cm^(-3) in material with high carbon (C) content (grown at low V/III ratio). However, the cation vacancy defects that were present at relatively higher concentrations were not the key reason for the compensation in material with low C content (grown at high V/III ratio). Instead, at highest Si doping levels above 10^19 cm^(-3) Si DX center formation seemed to be the dominant compensation mechanism.

More about Igor's research:
Article on Origins of Electrical Compensation in Si-Doped HVPE GaN (https://doi.org/10.1002/pssb.202200568)
Article on Fabrication of GaN-air channels for embedded photonic structures (https://doi.org/10.1016/j.mssp.2022.107234)
Article on Europium diffusion in ammonothermal gallium nitride (https://doi.org/10.1016/j.apsusc.2023.157188)

Igor Prozeev is a PhD student in University of Helsinki. He recieved an encouragement grant from the Foundation in 2022.