Browsing by Author "Tuomisto, Filip, Dr."

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    Influence of vacancy defects on the optical properties of natural diamond and aluminium nitride
    (Aalto University, 2012) Mäki, Jussi-Matti; Tuomisto, Filip, Dr.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of Science; Puska, Martti, Prof.
    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.
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    Positron annihilation studies on vacancy defects in group IV semiconductors
    (Aalto-yliopiston teknillinen korkeakoulu, 2010) Kuitunen, Katja; Tuomisto, Filip, Dr.; Teknillisen fysiikan laitos; Department of Applied Physics; Aalto-yliopiston teknillinen korkeakoulu
    Electrical properties of semiconductor materials are greatly influenced by point defects such as vacancies and interstitials. These defects are common and form during the growth and processing of the material. Positron annihilation spectroscopy is a method suitable for detecting and studying vacancy-type lattice defects. In this work, the formation, properties, and annealing of vacancy defects is studied in silicon, silicon-germanium, and germanium. Defects consisting of a vacancy and one or several donor atoms are one of the most common defects causing electron trapping and deactivation of n-type doping in silicon and silicon-germanium. In this work, the studies in silicon-germanium show that several germanium atoms accumulate around the vacancy-phosphorus (V-P) pair during the annealing of the samples. The increased Ge-decoration pulls the energy level (-/- -) down into the band-gap and makes the V-P pair decorated by several Ge atoms an especially effective trap for conduction electrons. The positron trapping in a vacancy surrounded by three As atoms (V-As3) is studied in highly As-doped Si. The positron detrapping from the V-As3 defect at high temperatures is observed and a binding energy of 0.27(3) eV of a positron to the V-As3 complex is determined. Defects can also be introduced deliberately by neutron-irradiation and ion-implantation. These techniques offer possibilities for studying the generation and annealing of vacancy defects. In this work, neutron-irradiated germanium is studied. Irradiation induced divacancy defects that are stable at room temperature are observed. A negative charge state of a divacancy is found to stabilize the defect even at 400°C. The divacancy is shown to form bigger clusters before the final recovery at 500°C. Finally, B-doping related problems are studied. The results show that He-implantation produces nanovoids that trap interstitials formed during the B-implantation, reducing the implantation related damage. The positron studies on the excimer laser annealed Si support theoretical calculations, which suggest vacancy formation at the maximum melt depth.