Please read the abstract in the section 00front of this document / Dissertation (M Eng (Mining Engineering))--University of Pretoria, 2007. / Mining Engineering / unrestricted

Title: Defects limiting charge collection in semiinsulated CdZnTe Author: Vít Zajac Department: Institute of Physics of Charles University Supervisor: doc. Ing. Jan Franc, DrSc., Institute of Physics of Charles University Abstract: We achieved to detect photoluminescence transitions deep in the band gap in 4 samples cut from 2 different crystals of semiinsulating Cd1-xZnxTe (x = 0,02 - 0,18) in a row of points along the growth axis. The spectral peaks give evidence of the presence of deep levels in the sample and the intensity of the photoluminescence peaks is to a certain extent proportional to the concentration of these levels. A comparison between resistivity and photoconductivity that were measured by a contact-less method showed that the change of photoluminescence intensity of deep levels does not bring about an unambigous change of neither resistivity nor photoconductivity. Correlation analysis of resistivity and photoconductivity of 6 samples from 4 different crystals confirmed the following model: A shift of the Fermi level within the band gap induced by a change of donor-acceptor compensation is accompanied by an unambigous change of resistivity and results in a change in occupation of the deep levels. This causes a change in the photoconductivity of the crystal because the occupation factor of...

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第19312号 / 工博第4109号 / 新制||工||1633(附属図書館) / 32314 / 京都大学大学院工学研究科電子工学専攻 / (主査)教授 木本 恒暢, 教授 引原 隆士, 准教授 船戸 充 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

The study of defects in semiconductors has been on-going for over 50 years. During this time, researchers have been studying the origins and identity of process induced defects, a task which has proved to be very demanding. While defects in silicon, the most widely used semiconductor, have been widely studied, there is more literature on n-type silicon than on p-type silicon. Compared to n-type silicon, p-type silicon is challenging to work with when it comes to making good Schottky diodes. A good rectifying device is essential for the performing of electrical characterisation techniques such as deep-level transient spectroscopy. In spite of this challenge p-silicon cannot be ignored. Many of the electronic devices are a combination of both n- and p-silicon therefore the need to understand the electronic properties of both materials. In this thesis, defects introduced in p-Si by electron beam deposition (EBD) were investigated. In order to understand these defects better, defects introduced by conditions of electron beam deposition (EBD) without metal deposition, were investigated. This process will be referred to as electron beam exposure (EBE). Finally, the defects were compared to defects induced by alpha-particle irradiation. EBD defects, introduced during electron beam deposition (EBD) of titanium (Ti) contacts on p-Si were investigated. The Schottky contacts were annealed within a temperature range of 200–400 oC. Current-voltage (I-V) measurements were conducted to monitor the change in electrical characteristics with every annealing step. Deep-level transient spectroscopy (DLTS) and Laplace-DLTS techniques were employed to identify the defects introduced after EBD and isochronal annealing of the Ti Schottky contacts. DLTS revealed that the main defects introduced during metallisation were hole traps with activation energy of 0.05 eV, 0.23 eV and 0.38 eV. Depth profiles of these defects showed that the formed close to the interface within a depth of 0.4 μm. Defects induced by EBE were studied by exposing samples for 50 minutes after which nickel (Ni) Schottky contacts were fabricated using resistive deposition. Only one defect with an activation energy of 0.55 eV was observed. This activation energy is similar to that of the I-defect. DLTS depth profiling revealed that the defect could be detected up to a depth of 0.8μm below the junction, which is significantly deeper than EBD defects. Defects induced when p-Si was irradiated by alpha particles from a 5.4 MeV americium (Am) 241 foil radioactive source with a fluence rate of 7×106 cm−2 s−1 at room temperature were investigated. After exposure at a fluence of 5.1×1010 cm−2, hole traps with the following activation energies were observed: 0.10 eV, identified as a tri-vacancy related defect, 0.33 eV, the interstitial carbon (Ci), 0.52 eV, a B-related defect and 0.16 eV. Low temperature irradiation experiments were also carried out using alpha- particles with the same fluence rate. Measurements were taken between 35 K and 120 K. The defect levels were at 0.10 eV, 0.14 eV and 0.18 eV. These levels were attributed to the boron-substitutional vacancy complex, the mono-vacancy and a vacancy-related defect, respectively. We conclude that EBD and EBE induced more complex defects than those induced by alpha-particle irradiation. / Thesis (PhD)--University of Pretoria, 2019. / Physics / PhD / Unrestricted

Virtual teams are being increasingly utilized in industry given their ability to bring together diverse knowledge and experience from individuals who are not geographically proximal. Having a diversity of knowledge within virtual teams is noted to benefit innovation outcomes; however, leveraging the benefits of diversity (both deep-level and functional level) is likely to require a capability to facilitate collaboration among team members. This dissertation examines collaboration capability and absorptive capacity at the virtual team level by evaluating the inter-relationships among the dimensions and their influence on team innovation. This research also tests the impact of team diversity on team innovation with an additional focus on understanding the moderating impact of collaboration capability and the mediating impact of absorptive capacity. Two dimensions of team diversity are examined. The first dimension, deep-level diversity, involves the individual characteristics, values, attitudes and preferences. The second dimension, functional-level diversity, which entails the diversity in functional and expertise backgrounds. Survey data was collected from 166 virtual team members and the validation process revealed satisfactory psychometric properties at the items and the constructs level. A confirmatory factor analysis (CFA) was carried out to determine the factor structure of the hypothesized models, as well as its reliability and validity.

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This work examines extended, point, and surface defects in GaN by means of electric force microscopy, photoluminescence and deep-level transient spectroscopy. Modeling of the surface band bending, its origin, and the effects of fabrication processing steps are discussed in the first part of the dissertation. Experimental results indicate that spontaneous polarization does not play a predominant role in GaN band bending. An increase of surface band bending due to annealing and etching was observed, while passivation did not produce changes. However, passivation did reduce reverse-bias leakage current by one to two orders of magnitude in GaN Schottky diodes. The optical properties of GaN were found to be sensitive to fabrication processing steps, most likely due to changes in the total density of surface states.The second part of this dissertation concerns the reduction of extended defects and associated deep levels in layers of GaN grown on different templates. Templates employing a low temperature GaN nucleation layer, epitaxial lateral overgrowth, and SiNx nanonetwork are compared in terms of deep level concentrations in the resulting GaN films. The concentrations of two types of traps, A (Ec-ET ~ 0.54-0.58 eV) and B (Ec-ET ~ 0.20-0.24 eV), were the highest for the sample with a low temperature nucleation layer and lowest for a sample with a 6 min SiNx deposition time. We surmise that the defects responsible for the dominant trap A are located along dislocation lines and form clusters.In the last part we investigate the piezoelectric and ferroelectric properties of PZT in Pb(Zr, Ti)O3(PZT)/GaN structures, and the effects of interface states. Sol-gel derived thin PZT films on GaN and Pt/Ti/SiO2/Si surfaces were studied by piezoresponse force microscopy (PFM), where quantitative characterization of piezoelectric properties of PZT films was performed. Superior piezoelectric properties of PZT/GaN/sapphire structures as compared to PZT/ Pt/Ti/SiO2/Si structures were observed and explained by a different preferred orientation of PZT. Despite the possible existence of a strong depolarization field at the PZT/GaN interface, we confirm with PFM the presence of a remanent polarization in PZT/GaN/sapphire structures.

A novel dislocation locking technique is used to study the behaviour of nitrogen and oxygen in silicon. Specimens containing well-defined arrays of dislocation half-loops are subjected to isothermal anneals of controlled duration, during which nitrogen or oxygen diffuses to the dislocations. The stress required to move the dislocations away from the impurities is then measured. Measurement of this unlocking stress as a function of annealing time and temperature allows information on the transport of nitrogen and oxygen to be deduced. Despite being present in a concentration of just 3E14cm-3 in some specimens, nitrogen is found to provide substantial benefits to the mechanical properties of float-zone silicon (FZ-Si). The segregation of nitrogen at dislocations is stable to at least 1200 degrees centigrade and the unlocking stress measured at 550 degrees centigrade is of similar magnitude to that found previously for oxygen in Czochralski silicon (Cz-Si). The unlocking stress initially rises linearly with annealing time, before it takes a constant value. The rate of the initial rise is dependent on temperature and the 1.5eV activation energy found agrees with that found previously. The rate of the initial rise also depends on nitrogen concentration. In the 500 to 700 degrees centigrade temperature range, the unlocking stress is found to decrease linearly as the temperature at which the unlocking process takes place increases. The results of a pre-annealing experiment confirm that oxygen monomers and dimers in Cz-Si exist in thermodynamic equilibrium at 550 degrees centigrade. Numerical simulation of oxygen diffusion to dislocations allows values of the effective diffusivity of oxygen in Cz-Si with four different oxygen concentrations to be deduced. At 500 degrees centigrade, the effective diffusivity depends upon oxygen concentration in a way which is consistent with oxygen dimers being responsible for transport. The transport of oxygen in Cz-Si at 550 to 600 degrees centigrade is found to be unaffected by nitrogen doping at a level of 2.1E15cm-3. The dislocation locking technique has also been used to study the effect of high concentrations of shallow dopants on oxygen transport in Cz-Si in the 350 to 550 degrees centigrade temperature range. Oxygen transport has been found to be unaffected by a high antimony concentration ~3E18cm-3, but is found to be enhanced by, on average, a factor of approximately 44 in Cz-Si with a high boron concentration ~5E18cm-3. Furthermore, deep-level transient spectroscopy (DLTS) and high-resolution DLTS (HR-DLTS) are used to study the electrical activity of defects in silicon. A deep-level with an enthalpy of 0.50eV and a concentration of order 10E11cm-3 is found in n-type nitrogen-doped FZ-Si and n-type nitrogen-doped neutron transmutation doped FZ-Si. No additional deep-levels are found in either material, for which the detection limit is 6E10cm-3. No deep-levels are found in p-type nitrogen-doped Cz-Si, for which the detection limit is approximately 10E12cm-3. DLTS and HR-DLTS are also used to investigate the electrical activity of oxygen-decorated dislocations in Cz-Si and states associated with oxygen at dislocation cores have been identified.

Das Halbleitermaterial Zinkselenid (ZnSe) wurde mit Deep Level Transient Spectroscopy (DLTS) untersucht. Fuer planar N-dotierte, MO-CVD-gewachsene ZnSe-Schichten auf p-GaAs wurden vorwiegend breite Zustandsverteilungen, aber auch tiefe Niveaus gefunden. In kristallin gezuechtetem, undotiertem ZnSe wurden tiefe Stoerstellen nachgewiesen.