Vacancy defects affect the optical properties of semiconductors in many ways. The defects form deep energy levels in the band gap acting either as a charge carrier recombination or generation centres. This is seen as absorption of light. In this thesis, the vacancy defects in diamond and aluminium nitride (AlN) and their influence on the optical properties have been studied using positron annihilation spectroscopy. A new method, optical transient positron spectroscopy, was developed for this purpose. A large fraction of the purest natural diamonds have a smoky brown tint. The brown colour can be removed in high pressure, high temperature (HPHT) treatments. The positron measurements presented in this thesis show that brown natural diamond contains a large concentration of clusters of 40-60 missing atoms. Optical wavelength photons excite electrons to the vacancy clusters, which causes the visible absorption and the smoky-brown colour of the diamond. Further, it was detected that vacancy cluster concentration decreases in correlation with the removal of the brown colour during the HPHT treatments. The decay of the excitation after switching off the illumination is very slow, taking several minutes. By combining flux-dependent measurements with the decay rates of the photoexcitation effects, the optical cross section of the clusters causing the brown colouration can be determined and the vacancy cluster concentration estimated self-consistently. Vacancy defects were studied in single-crystal bulk AlN substrates. Al vacancy-related positron signals (lifetime, Doppler broadending) are identified and the vacancy charge state was confirmed to be negative. By combining coincidence-Doppler broadening measurements with ab-initio theoretical calculations it is shown that in-grown Al vacancies are present as complexes with oxygen impurities, while high-energy particle irradiation produces predominantly isolate Al vacancies. Optical absorption measurements show that the latter cause absorption at ultraviolet wavelengths, important from the point of view of employing AlN as substrate material for opto-electronic devices.
|Translated title of the contribution||Vakanssivirheet ja niiden vaikutus optisiin ominaisuuksiin luonnontimantissa ja alumiininitridissä|
|Publication status||Published - 2012|
|MoE publication type||G5 Doctoral dissertation (article)|
- optical properties
- aluminium nitride