Widespread use of organic light emitting diodes (OLEDs) in solid state lighting and display technologies require efficiency and lifetime improvements, as well as cost reductions, inclusive of the transparent conducting oxide (TCO). Indium tin oxide (ITO) is the standard TCO anode in OLEDs, but indium is expensive and the Earth's reserve of this element is limited. Zinc oxide (ZnO) and its variants such as aluminum-doped ZnO (AZO) exhibit comparable electrical conductivity and transmissivity to ITO, and are of interest for TCO applications. However, the workfunction of ZnO and AZO is smaller compared to ITO. The smaller workfunction of AZO results in a higher hole injection barrier at the anode/organic interface, and methods of tuning its workfunction are required. This dissertation tested the hypothesis that workfunction tuning of AZO films could be achieved by surface modification with electronegative oxygen and fluorine plasmas, or, via use of nanoscale transition metal oxide layers (MoOx, VOx and WOx). Extensive UPS, XPS and optical spectroscopy studies indicate that O2 and CFx plasma treatment results in an electronegative surface, surface charge redistribution, and a surface dipole moment which reinforces the original surface dipole leading to workfunction increases. Donor-like gap states associated with partially occupied d-bands due to non-stoichiometry determine the effective increased workfunction of the AZO/transition-metal oxide stacks. Reduced hole injection barriers were engineered by ensuring that the surface ad-layers were sufficiently thin to facilitate Fowler-Nordheim tunneling. Improved band alignments resulted in improved hole injection from the surface modified AZO anodes, as demonstrated by I-V characterization of hole only structures. Energy band alignments are proposed based on the aforementioned spectroscopies. Simple bilayer OLEDs employing the surface modified AZO anodes were fabricated and characterized to compare their performance with standard ITO. Anodes consisting of AZO with MoOx or VOx interfacial layers exhibited 50% and 71% improvement in power efficiency (PE) and external quantum efficiency (EQE), respectively, compared to ITO at a working voltage of 9 V. The efficiencies of dipole reinforced AZO (O2/CFx plasma treated) anodes were comparable to ITO. The improved performance of the surface modified AZO anodes compared to as-deposited AZO is ascribed to improved hole injection, improved charge balance, and improved radiative recombination kinetics. The results suggest that surface modified AZO anodes are a promising alternative to ITO, given the lower cost and Earth abundance of Al and Zn.

Cette thèse concerne l’étude des procédés de fabrication des grilles HKMG des technologies FDSOI 14 et 28 nm sur les performances électriques des transistors MOS. Elle a porté spécifiquement sur l'aspect fiabilité et la maîtrise du travail de sortie effectif (WFeff), au travers de la diffusion des additifs comme le lanthane (La) et l’aluminium (Al). Ce travail combine des techniques de caractérisation électriques et physico-chimiques et leur développement. L'effet de l'incorporation de ces additifs sur la fiabilité et la durée de vie du dispositif a été étudié. Le lanthane dégrade les performances de claquage TDDB et de dérives suite aux tests aux tensions négatives. L’introduction d’aluminium améliore le claquage TDDB, mais dégrade les dérives aux tensions positives. Ces comportements ont été reliés à des mécanismes physiques. Par ailleurs, la diffusion de ces additifs dans l’empilement de grille a été étudiée pour différents matériaux high-k en fonction de la température et de la durée de recuit de diffusion. Les doses d’additifs ont pu être ainsi mesurées, comparées et corrélées au décalage de travail de sortie effectif de grille. On a également étudié, les influences des paramètres du procédé de dépôt de grille TiN sur leur microstructure et les propriétés électriques du dispositif, identifiant certaines conditions à même de réduire la taille de grain ou la dispersion d’orientation cristalline. Toutefois, les modulations obtenues sur le travail de sortie effectif de grille dépendent plus du ratio Ti/N, suggérant un changement du dipôle à l'interface SiO2 / high-k. Enfin, une technique éprouvée de mesure de spectroscopie à rayon X sous tension a pu être mise en place grâce des dispositifs spécifiques et une méthodologie adaptée. Elle permet de mesurer les positions relatives des bandes d’énergie à l'intérieur de l’empilement de grille. Cette technique a démontré que le décalage du travail de sortie effectif induits par des additifs (La or Al) ou par des variations d'épaisseur de grille métallique TiN provient de modifications du dipôle à l'interface SiO2/ high-k. / This Ph.D. thesis is focused on the impact of the 14 and 28 nm FDSOI technologies HKMG stack processes on the electrical performance of MOS transistors. It concerns specifically the reliability aspect and the engineering of effective workfunction (WFeff ), through diffusion of lanthanum (La) and aluminum (Al) additives. This work combines electrical and physicochemical characterization techniques, and their development. The impact of La and Al incorporation, in the MOS gate stack, on reliability and device lifetime has been studied. La addition has a significant negative impact on device lifetime related to both NBTI and TDDB degradations. Addition of Al has a significant negative impact on lifetime related to PBTI, but on the contrary improves the lifetime for TDDB degradation. These impacts on device lifetime have been well correlated to the material changes inside the gate oxides. Moreover, diffusion of these additives into the HKMG stack with annealing temperature and time has been studied on different high-k materials. The diffused dose has been compared with the resulting shift in effective workfunction (WFeff), evidencing clear correlation. In addition, impact of TiN metal gate RF-PVD parameters on its crystal size and orientation, and device electrical properties has been studied. XRD technique has been used to obtain the crystal size and orientation information. These properties are significantly modulated by TiN process, with a low grain size and a unique crystal orientation obtained in some conditions. However, the WFeff modulations are rather correlated to the Ti/N ratio change, suggesting a change in the dipole at SiO2/high-k interface. Lastly, using specific test structures and a new test methodology, a robust and accurate XPS under bias technique has been developed to determine the relative band energy positions inside the HKMG stack of MOS devices. Using this technique, we demonstrated that WFeff shift induced by La and Al or by variations in gate thickness originates due to modifications of the dipole at SiO2/high-k interface.

Capacitance-voltage-frequency measurements on n+-GaN/AlxGa1−xN UV/IR dual-band detectors are reported. The presence of shallow Si-donor, deep Si-donor, and C-donor/N-vacancy defect states were found to significantly alter the electrical characteristics of the detectors. The barrier Al fraction was found to change the position of the interface defect states relative to the Fermi level. The sample with Al fraction of 0.1 shows a distinct capacitance-step and hysteresis, which is attributed to C-donor/N-vacancy electron trap states located above the Fermi level (200 meV) at the heterointerface; whereas, the sample with Al fraction of 0.026 shows negative capacitance and dispersion, indicating C-donor/N-vacancy and deep Si-donor defect states located below the Fermi level (88 meV). When an i-GaN buffer layer was added to the structure, an anomalous high-frequency capacitance peak was observed and attributed to resonance scattering due to hybridization of localized Si-donor states in the band gap with conduction band states at the i-GaN/n+-GaN interface.

In this work, homojunction interfacial workfunction internal photoemission (HIWIP) detectors based on GaAs, and heterojunction interfacial workfunction internal photoemission (HEIWIP) detectors based mainly on the Gallium Arsenide/Aluminum Gallium Arsenide material system are presented. Design principles of HIWIP and HEIWIP detectors, such as free carrier absorption, photocarrier generation, photoemission, and responsivity, are discussed in detail. Results of p-type HIWIPs based on GaAs material are presented. Homojunction detectors based on p-type GaAs were found to limit their operating wavelength range. This is mainly due to band depletion arising through carrier transitions from the heavy/light hole bands to the split off band. Designing n-type GaAs HIWIP detectors is difficult as it is strenuous to control their workfunction. Heterojunction detectors based on Gallium Arsenide/Aluminum Gallium Arsenide material system will allow tuning their threshold wavelength by adjusting the alloy composition of the Aluminum Gallium Arsenide/Gallium Arsenide barrier, while keeping a fixed doping density in the emitter. The detectors covered in this work operate from 1 to 128 micron (300 to 2.3 THz). Enhancement of detector response using resonance cavity architecture is demonstrated. Threshold wavelength extension of HEIWIPs by varying the Al composition of the barrier was investigated. The threshold limit of approximately 3.3 THz (92 micron), due to a practical Al fraction limit of approximately 0.005, can be overcome by replacing GaAs emitters in Gallium Arsenide/Aluminum Gallium Arsenide HEIWIPs with Aluminum Gallium Arsenide/Gallium Arsenide emitters. As the initial step, terahertz absorption for 1 micron-thick Be-doped Aluminum Gallium Arsenide epilayers (with different Al fraction and doping density) grown on GaAs substrates was measured. The absorption probability of the epilayers was derived from these absorption measurements. Based on the terahertz absorption results, an Aluminum Gallium Arsenide/Gallium Arsenide HEIWIP detector was designed and the extension of threshold frequency (f0) to 2.3 THz was successfully demonstrated. In a different study, switching in Gallium Arsenide/Aluminum Gallium Arsenide heterostructures from a tunneling dominated low conductance branch to a thermal emission dominated high conductance branch was investigated. This bistability leads to neuron-like voltage pulses observed in some heterostructure devices. The bias field that initiates the switching was determined from an iterative method that uses feedback information, such as carrier drift velocity and electron temperature, from hot carrier transport. The bias voltage needed to switch the device was found to decrease with the increasing device temperature.

This work describes infrared (IR) photon detector techniques based on novel semiconductor device concepts and detector designs. The aim of the investigation was to examine alternative IR detection concepts with a view to resolve some of the issues of existing IR detectors such as operating temperature and response range. Systems were fabricated to demonstrate the following IR detection concepts and determine detector parameters: (i) Near-infrared (NIR) detection based on dye-sensitization of nanostructured semiconductors, (ii) Displacement currents in semiconductor quantum dots (QDs) embedded dielectric media, (iii) Split-off band transitions in GaAs/AlGaAs heterojunction interfacial workfunction internal photoemission (HEIWIP) detectors. A far-infrared detector based on GaSb homojunction interfacial workfunction internal photoemission (HIWIP) structure is also discussed. Device concepts, detector structures, and experimental results discussed in the text are summarized below. Dye-sensitized (DS) detector structures consisting of n-TiO2/Dye/p-CuSCN heterostructures with several IR-sensitive dyes showed response peaks at 808, 812, 858, 866, 876, and 1056 nm at room temperature. The peak specific detectivity (D*) was 9.5E+10 Jones at 812 nm at room temperature. Radiation induced carrier generation alters the electronic polarizability of QDs provided the quenching of excitation is suppressed by separation of the QDs. A device constructed to illustrate this concept by embedding PbS QDs in paraffin wax showed a peak D* of 3E+8 Jones at ~540 nm at ambient temperature. A typical HEIWIP/HIWIP detector structures consist of single (or multiple) period(s) of doped emitter(s) and undoped barrier(s) which are sandwiched between two highly doped contact layers. A p-GaAs/AlGaAs HEIWIP structure showed enhanced absorption in NIR range due to heavy/light-hole band to split-off band transitions and leading to the development of GaAs based uncooled sensors for IR detection in the 2 5 μm wavelength range with a peak D* of 6.8E+5 Jones. A HIWIP detector based on p-GaSb/GaSb showed a free carrier response threshold wavelength at 97 µm (~3 THz)with a peak D* of 5.7E+11 Jones at 36 μm and 4.9 K. In this detector, a bolometric type response in the 97 - 200 µm (3-1.5 THz) range was also observed.

In this work, multi-band (multi-color) detector structures considering different semiconductor device concepts and architectures are presented. Results on detectors operating in ultraviolet-to-infrared regions (UV-to-IR) are discussed. Multi-band detectors are based on quantum dot (QD) structures; which include quantum-dots-in-a-well (DWELL), tunneling quantum dot infrared photodetectors (T-QDIPs), and bi-layer quantum dot infrared photodetectors (Bi-QDIPs); and homo-/heterojunction interfacial workfunction internal photoemission (HIWIP/HEIWIP) structures. QD-based detectors show multi-color characteristics in mid- and far-infrared (MIR/FIR) regions, where as HIWIP/HEIWIP detectors show responses in UV or near-infrared (NIR) regions, and MIR-to-FIR regions. In DWELL structures, InAs QDs are placed in an InGaAs/GaAs quantum well (QW) to introduce photon induced electronic transitions from energy states in the QD to that in QW, leading to multi-color response peaks. One of the DWELL detectors shows response peaks at ∼ 6.25, ∼ 10.5 and ∼ 23.3 µm. In T-QDIP structures, photoexcited carriers are selectively collected from InGaAs QDs through resonant tunneling, while the dark current is blocked using AlGaAs/InGaAsAlGaAs/ blocking barriers placed in the structure. A two-color T-QDIP with photoresponse peaks at 6 and 17 µm operating at room temperature and a 6 THz detector operating at 150 K are presented. Bi-QDIPs consist of two layers of InAs QDs with different QD sizes. The detector exhibits three distinct peaks at 5.6, 8.0, and 23.0 µm. A typical HIWIP/HEIWIP detector structure consists of a single (or series of) doped emitter(s) and undoped barrier(s), which are placed between two highly doped contact layers. The dual-band response arises from interband transitions of carriers in the undoped barrier and intraband transitions in the doped emitter. Two HIWIP detectors, p-GaAs/GaAs and p-Si/Si, showing interband responses with wavelength thresholds at 0.82 and 1.05 µm, and intraband responses with zero response thresholds at 70 and 32 µm, respectively, are presented. HEIWIP detectors based on n-GaN/AlGaN show an interband response in the UV region and intraband response in the 2-14 µm region. A GaN/AlGaN detector structure consisting of three electrical contacts for separate UV and IR active regions is proposed for simultaneous measurements of the two components of the photocurrent generated by UV and IR radiation.

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