No description available.

This dissertation examines the carrier transport properties, diffusion length, effective carrier lifetime, and resistivity in two wide bandgap materials, GaN and diamond. A combination of two methods was used to obtain these transport properties. The two were optical beam induced current (OBIC) and electron beam induced current (EBIC) time of flight transient measurements. These techniques consist of measuring the current response to the drift and diffusion of generated electron-hole pair carriers created by a short-duration pulse of radiation. Under OBIC, a short duration pulsed optical source, with an electron beam excitation pulse time much less than the transit time of the material, was used to generate excess carriers within the absorption depth of the material. The second method of excitation, EBIC involved the use of a modified SEM with a photoemission source (L-EBIC) and a high speed pulsed thermionic electron source (T-EBIC) to generate an electron beam. This electron beam was used to create a large number of electron-hole pairs at various penetration depths within the materials. Measurements on GaN found the diffusion length was 7.84 µm with the L-EBIC and 7.78 µm with the T-EBIC. After annealing at 900°C for 30 min. the GaN diffusion length increased to 9.89 µm (L-EBIC). The dark resistivity was 1.79 x 10¹⁰Ω-cm, and the carrier lifetimes were 1.7 µs with L-EBIC and 3.36 & 3.9 ns with OBIC. The author believed that the L-EBIC result was a good representation of the carrier lifetime within the material, while the shorter OBIC results were due to the combine high surface and interface recombination processes. The diamond dark resistivity was found to be 6.14 x 10¹¹Ω-cm and the diffusion lengths were 94.1 µm and 97 µm from the L-EBIC and T-EBIC respectively. All measurements were within 10 % spread. The real value of this contribution lies in determining the diffusion lengths in GaN and diamond by the EBIC techniques, measuring the effective surface\interface and thin film carrier lifetime of GaN utilizing a combination of OBIC and L-EBIC techniques, and evaluating the dark resistivity in GaN and diamond materials. These measurements can lead to a better understanding and exploitation of the electrophysical behavior of these materials. / Ph. D.

Abstract Amorphous Selenium (a-Se) was first commercialized for use as a photoconductor in xerography during the middle of the twentieth century. Since then the hole transport properties of a-Se have been studied extensively, however the study of electron transport remains relatively limited. Flat panel digital X-ray detectors using a-Se as a photoconductor have been developed and are being used in mammographic screening. The charge transport properties of the photoconductor layer will in part determine the performance of the flat panel detector. X-ray absorption causes electron-hole pair generation in the bulk of the photoconductor, requiring both electrons and holes to drift across the sample and be collected. If these carriers are lost in the many localized trapping states as they cross the sample, they will not contribute to the image signal resulting in unnecessary radiation exposure to the patient. Eleven a-Se samples were deposited at the University of Saskatchewan varying in thickness from 13 μm to 501 μm. Pure a-Se was chosen to ensure uniformity across the thickness of the samples, that is, to ensure the composition of the film did not change across the thickness. Time of flight transient photoconductivity experiments (TOF) and interrupted field time of flight (IFTOF) measurements were performed to measure the electron drift mobility and lifetime respectively. The product of electron drift mobility μ and lifetime τ, hence the carrier range (μτ) at a given applied electric field. The electron range is an important parameter as this places limits on the practical thickness of the photoconducting layer in a detector. This study also includes an investigation into the effect of the definition of transit time on the calculated drift mobility and analysis of the dispersive transport properties of a-Se. It was observed that as sample thickness (L) increased, electron drift mobility (μ) decreased. In addition electron lifetime (τ) decreased dramatically in samples thinner than 50 μm. Electron range (μτ) was 2.26 × 〖10〗^(-6) cm^2/V in the 147μm sample and 5.46 × 〖10〗^(-8) cm^2/V in the 13 μm sample, a difference of almost two orders of magnitude. The comparison of the half current method and inflection point methods to calculate the transit time of the same TOF curve, shows that the calculated mobility can vary by as much as 24%. This illustrates clearly that it is important to use the same point on the TOF curve to define the transit time. Charge packet dispersion (spread) in the time domain in pure a-Se samples was proportional to L^m where L is the photoconductor thickness and m ~ 1.3, measured at both 1 V/μm and 4 V/μm.

The Cadmium Telluride thin film solar cell is one of the leading photovoltaic technologies. Efficiency improvements in the past decade made it a very attractive and practical source of renewable energy. Considering the theoretical limit, there is still room for improvement, especially the cell’s open circuit voltage (VOC). To improve VOC, the p-type carrier concentration and minority carrier lifetime of the CdTe absorber needs to be improved. Both these parameters are directly related to the point defect distribution of the semiconductor, which is a function of deposition stoichiometry, dopant incorporation and post-deposition treatments. CdTe films were deposited by the Elemental Vapor Transport (EVT) deposition method, which allowed in situ control of the vapor phase stoichiometry (Cd/Te ratio). Extrinsic doping of polycrystalline CdTe by in situ incorporation of antimony (Sb) and phosphorus (P) was investigated. The structural and electrical properties of CdTe thin films and solar cells were studied. Sb and P incorporation were found to increase the net p-doping concentration. Cl and Sb improved the minority carrier lifetime of polycrystalline CdTe, while lower lifetime with Cu and P doped films were indicated. Deep Level Transient Spectroscopy (DLTS) was performed on devices fabricated with different deposition stoichiometry, post-deposition treatments, and phosphorus dopant dose. Several majority and minority carrier traps were identified, and assigned to different point defects based on first principle studies in the literature and experimental conditions used for the deposition and processing of the thin films.

abstract: It has been a long-standing goal to epitaxially integrate III-V alloys with Si substrates which can enable low-cost microelectronic and optoelectronic systems. Among the III-V alloys, gallium phosphide (GaP) is a strong candidate, especially for solar cells applications. Gallium phosphide with small lattice mismatch (~0.4%) to Si enables coherent/pseudomorphic epitaxial growth with little crystalline defect creation. The band offset between Si and GaP suggests that GaP can function as an electron-selective contact, and it has been theoretically shown that GaP/Si integrated solar cells have the potential to overcome the limitations of common a-Si based heterojunction (SHJ) solar cells. Despite the promising potential of GaP/Si heterojunction solar cells, there are two main obstacles to realize high performance photovoltaic devices from this structure. First, the growth of the polar material (GaP) on the non-polar material (Si) is a challenge in how to suppress the formation of structural defects, such as anti-phase domains (APD). Further, it is widely observed that the minority-carrier lifetime of the Si substrates is significantly decreased during epitaxially growth of GaP on Si. In this dissertation, two different GaP growth methods were compared and analyzed, including migration-enhanced epitaxy (MEE) and traditional molecular beam epitaxy (MBE). High quality GaP can be realized on precisely oriented (001) Si substrates by MBE growth, and the investigation of structural defect creation in the GaP/Si epitaxial structures was conducted using high resolution X-ray diffraction (HRXRD) and high resolution transmission electron microscopy (HRTEM). The mechanisms responsible for lifetime degradation were further investigated, and it was found that external fast diffusors are the origin for the degradation. Two practical approaches including the use of both a SiNx diffusion barrier layer and P-diffused layers, to suppress the Si minority-carrier lifetime degradation during GaP epitaxial growth on Si by MBE were proposed. To achieve high performance of GaP/Si solar cells, different GaP/Si structures were designed, fabricated and compared, including GaP as a hetero-emitter, GaP as a heterojunction on the rear side, inserting passivation membrane layers at the GaP/Si interface, and GaP/wet-oxide functioning as a passivation contact. A designed of a-Si free carrier-selective contact MoOx/Si/GaP solar cells demonstrated 14.1% power conversion efficiency. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2017

No description available.

No description available.

Die hochempfindliche Methode der „Microwave Detected Photoconductivity“ (MDP) wird eingesetzt, um technologisch relevante Halbleiterparameter wie die Ladungsträgerlebensdauer, Photoleitfähigkeit und Defektkonzentrationen über viele Größenordnungen der optischen Anregung hinweg zu untersuchen. Durch die Entwicklung und die Anwendung eines neuartigen Modellierungssystems für die Ladungsträgerdynamik in Halbleitern können wichtige Defektparameter quantitativ aus MDP Messungen in Abhängigkeit der Anregungsintensität bestimmt werden. Ein Verfahren zur Charakterisierung von Haftstellen (Konzentration, Energielage, Einfangsquerschnitt) bei konstanter Temperatur wird vorgestellt. Das technologisch relevante Verfahren des quantitativen Eisennachweises in p-dotiertem Silizium wird für die MDP Methode angepasst und entsprechende Messergebnisse mit DLTS Resultaten verglichen. Ein detaillierter Vergleich der gängigsten kontaktlosen Messverfahren QSSPC und MW-PCD mit der MDP zeigt, dass entgegen gängiger Annahmen die unterschiedlichen Anregungsbedingungen zu drastischen Unterschieden in den gemessenen Werten der Ladungsträgerlebensdauer führen. Dies wird sowohl durch theoretische Berechnungen als auch durch praktische Messergebnisse belegt.

This thesis is dedicated to investigation of carrier dynamics in 3C-SiC crystals and heterostructures by using light-induced transient gratings and differential transmission techniques. The experimental studies in a wide range of excess carrier densities and temperatures revealed the influence of several particular growth conditions in different growth techniques on electronic properties of grown layers, and hence, a possibility to characterize indirectly the structural perfection of a layer by non-destructive optical means. Based on these results, we were able to evaluate and compare quality of 3C-SiC specimens manufactured by nearly all currently available growth techniques and to distinguish the most promising growth methods for 3C-SiC semiconductor. Moreover, we found a novel way, based on DT technique, to determine the carrier trap concentration in finite thickness layers, as well as to attribute them to specific impurity species. Finally, we investigated the influence of substrate surface features on carrier lifetime and mobility of the overgrown layers. / Disertacija yra skirta krūvininkų dinamikos tyrimams 3C-SiC kristaluose ir heterostruktūrose naudojant šviesa indukuotų dinaminių gardelių ir diferencinio pralaidumo metodikas. Atlikti tyrimai plačiame žadinimų ir temperatūros intervale atskleidė skirtingų auginimo technologijų ir jų tam tikrų parametrų įtaką opto-elektroninėms užaugintų sluoksnių savybėms, kas įgalino atlikti medžiagos neardančią, optinę kristalų struktūrinės kokybės charakterizaciją. Remiantis gautais rezultatais mes galėjome palyginti visomis plačiausiai naudojamomis auginimo technologijomis užaugintų 3C-SiC bandinių kokybę, taigi ir įvertinti perspektyviausią 3C-SiC puslaidininkio auginimo metodą. Šiame darbe atradome naują būdą, paremtą diferencinio pralaidumo metodika, krūvininkų gaudyklių koncentracijos ir šių priemaišinių atomų prigimties nustatymui pakankamai plonuose sluoksniuose. Taip pat, tyrėme padėklo paviršiaus įtaką krūvininkų gyvavimo laikui ir judriui epitaksiniuose sluoksniuose.

Disertacija yra skirta krūvininkų dinamikos tyrimams 3C-SiC kristaluose ir heterostruktūrose naudojant šviesa indukuotų dinaminių gardelių ir diferencinio pralaidumo metodikas. Atlikti tyrimai plačiame žadinimų ir temperatūros intervale atskleidė skirtingų auginimo technologijų ir jų tam tikrų parametrų įtaką opto-elektroninėms užaugintų sluoksnių savybėms, kas įgalino atlikti medžiagos neardančią, optinę kristalų struktūrinės kokybės charakterizaciją. Remiantis gautais rezultatais mes galėjome palyginti visomis plačiausiai naudojamomis auginimo technologijomis užaugintų 3C-SiC bandinių kokybę, taigi ir įvertinti perspektyviausią 3C-SiC puslaidininkio auginimo metodą. Šiame darbe atradome naują būdą, paremtą diferencinio pralaidumo metodika,krūvininkų gaudyklių koncentracijos ir šių priemaišinių atomų prigimties nustatymui pakankamai plonuose sluoksniuose. Taip pat, tyrėme padėklo paviršiaus įtaką krūvininkų gyvavimo laikui ir judriui epitaksiniuose sluoksniuose. / This thesis is dedicated to investigation of carrier dynamics in 3C-SiC crystals and heterostructures by using light-nduced transient gratings and differential transmission techniques. The experimental studies in a wide range of excess carrier densities and temperatures revealed the influence of several particular growth conditions in different growth techniques onelectronic properties of grown layers, and hence, a possibility to characterize indirectly the structural perfection of a layer by non-destructive optical means. Based on these results, we were able to evaluate and compare quality of 3C-SiC specimens manufactured by nearly all currently available growth techniques and to distinguish the most promising growth methods for 3C-SiC semiconductor. Moreover, we found a novel way, based on DT technique, to determine the carrier trap concentration in finite thickness layers, as well as to attribute them to specific impurity species. Finally, we investigated the influence of substratesurface features on carrier lifetime and mobility of the overgrown layers.