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A new experimental method for studying defects limiting the optical properties of semiconductors and insulators – solving the mystery of the diamond only pretty in the dark

“Natural diamond” sounds like a brilliant and shiny gemstone. It is much less known that in fact the majority of the highest purity naturally occurring diamond has a brownish, smoky tint. This material looks much less nice and has a strongly reduced commercial value than its colorless counterpart – although diamond with blue or pink color is the most expensive (and hardest to find). The origin of the smoky brown color has been a mystery until now, while the blue color has for long been known to originate from boron impurities. Not all of the naturally occurring colors are yet understood – for instance the origin of pink is not known. Mr. Jussi-Matti Mäki (M.Sc. (Tech.)) and Docent Filip Tuomisto from the Department of Applied Physics have been studying natural diamond using antimatter already for a few years. Measurements of positron lifetimes in diamond have revealed that the crystal lattice of brown diamond contains relatively large voids, i.e., vacancy clusters formed by tens of missing atoms. By illuminating the samples with visible wavelengths of light during the positron experiments it was found that the positron trapping at these vacancy clusters is strongly enhanced. This is interpreted as electrons being excited from the occupied valence band states to deep level states generated by the vacancy clusters. The phenomenon is visible also with bare eyes as absorption that reduces all visible wavelengths from the light spectrum (more at the blue end), causing the smoky brown color. Mäki and Tuomisto developed a new experimental technique to study this effect. The method allows studying the optical and electronic properties of vacancy defects in more detail than with traditional methods. It is based on modulating the illumination conditions and measuring the time-dependent changes with positron annihilation spectroscopy. The same methodology can also be applied to studying materials that are more relevant from the opto-electronics industry point of view, such as gallium nitride based thin films that are used in light emitting diodes (LEDs). It is typical of vacancy defects to hinder the performance of LEDs, making the understanding of the properties of these defects an important goal to ease the development of future energy-efficient lighting solutions and other opto-electronics devices.

The research has been performed within the HIGHLIGHT project of the MIDE program in collaboration with the Department of Micro and Nanoscience and the DCT Research Centre (UK).