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No description available.

The electrically active defects introduced in GaAs by electron beam deposition (EB) of Ta were characterised. The effect of electron beam deposition on the electrical properties of GaAs was evaluated by current-voltage (I-V), capacitance¬voltage (C- V) and deep level transient spectroscopy (DL TS). However, when electronic devices are formed by EB, defects may be introduced into the semiconductor material, depending on the properties of the metal being deposited. Depending on the application, these defects may have either advantages or detrimental effects on the performance of such a device. I-V measurements indicated that the EB induced damage results in an increase in ideality factor and decrease in the barrier height with increasing the applied substrate bias, while C- V measurements showed that EB deposition also caused a decrease in the barrier height. DL TS studies on the same material in the temperature range of 20 - 350 K showed that at least three electrically active defects are introduced during EB deposition, with energies (0.102 ± 0.004, 0.322 ± 0.014 and 0.637 ± 0.029 eV) within the band gap. DL TS data was used to construct concentration profiles of these defects as a function of depth below the surface. It was found that the defect concentration increases with increasing substrate bias during the deposition, irrespective of the direction of the applied bias. This may be related to the I-V characteristics of the SBDs. The SBDs investigated by IV measurements showed that GaAs yields SBDs with poorer characteristic. The influence of EB deposition on the device properties of SBDs fabricated on GaAs is presented. These device properties were monitored using a variable temperature I-V and C- V apparatus. In order to have an understanding of the change in electrical properties of these contacts after EB deposition, it is necessary to characterise the EB induced defects. DL TS was used to characterise the defects in terms of their D L TS signature and defect concentration. / Dissertation (MSc (Physics))--University of Pretoria, 2006. / Physics / unrestricted

Lighted push button switches and indicators serve many purposes in cockpits, shipboard applications and military ground vehicles. The quality of lighting produced by switches is vital to operators' understanding of the information displayed. Utilizing LED technology in lighted switches has challenges that can adversely affect lighting quality. Incomplete data exists to educate consumers about potential differences in LED switch performance between different manufacturers. LED switches from four different manufacturers were tested for six attributes of lighting quality: average luminance and power consumption at full voltage, sunlight readable contrast, luminance contrast under ambient sunlight, legend uniformity, and dual-color uniformity. Three of the four manufacturers have not developed LED push button switches that meet lighting quality standards established with incandescent technology.

Transient electroluminescence (EL) was used to measure the onset of emission delay in OLEDs based on transition metal, phosphorescent bis[3,5-bis(2-pyridyl)-1,2,4-triazolato] platinum(ΙΙ) and rare earth, phosphorescent Eu(hfa)3 with 4'-(p-tolyl)-2,2":6',2" terpyridine (ttrpy) doped into 4,4'-bis(carbazol-9-yl) triphenylamine (CBP), from which the carrier mobility was determined. For the Pt(ptp)2 doped CBP films in OLEDs with the structure: ITO/NPB (40nm)/mcp (10nm)/65% Pt(ptp)2:CBP (25nm)/TPBI (30nm)/Mg:Ag (100nm), where NPB=N, N'-bis(1-naphthyl)-N-N'-biphenyl-1, 1'-biphenyl-4, MCP= N, N'-dicarbazolyl-3,5-benzene, TPBI=1,3,5-tris(phenyl-2-benzimidazolyl)-benzene, delayed recombination was observed and based on its dependence on frequency and duty cycle, ascribed to trapping and de-trapping processes at the interface of the emissive layer and electron blocker. The result suggests that the exciton recombination zone is at, or close to the interface between the emissive layer and electron blocker. The lifetime of the thin films of phosphorescent emitter Pt(ptp)2 were studied for comparison with rare earth emitter Eu(hfa)3. The lifetime of 65% Pt(ptp)2:CBP co-film was around 638 nanoseconds at the emission peak of 572nm, and the lifetime of neat Eu(hfa)3 film was obtained around 1 millisecond at 616 nm, which supports the enhanced efficiency obtained from the Pt(ptp)2 devices. The long lifetime and narrow emission of the rare earth dopant Eu(hfa)3 is a fundamental factor limiting device performance. Red organic light emitting diodes (OLEDs) based on the rare earth emitter Eu(hfa)3 with 4'-(p-tolyl)-2,2":6',2" terpyridine (ttrpy) complex have been studied and improved with respect performance. The 4.5% Eu(hfa)3 doped into CBP device produced the best power efficiency of 0.53 lm/W, and current efficiency of 1.09 cd/A. The data suggests that the long lifetime of the f-f transition of the Eu ion is a principal limiting factor irrespective of how efficient the energy transfer from the host to the dopant and the antenna effect are.

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Defect passivation and surface modification of perovskite semiconductors play a key role in achieving highly efficient and stable perovskite solar cells (PSCs) and light-emitting diodes (LEDs). This dissertation describes three novel strategies for such defect passivation and surface modification. In the first strategy, we demonstrate a facile approach using inorganic perovskite quantum dots (QDs) to supply bulk- and surface-passivation agents to combine high power conversion efficiency (PCE) with high stability in CH3NH3PbI3 (MAPbI3) inverted PSCs. This strategy utilizes inorganic perovskite QDs to distribute elemental dopants uniformly across the MAPbI3 film and attach ligands to the film’s surface. Compared with pristine MAPbI3 films, MAPbI3 films processed with QDs show a reduction in tail states, smaller trap-state density, and an increase in carrier recombination lifetime. The strategy results in reduced voltage losses and an improvement in PCE from 18.3% to 21.5%, which is among the highest efficiencies for MAPbI3 devices. The devices maintain 80% of their initial PCE under 1-sun continuous illumination for 500 h and show improved thermal stability. In the second strategy, we reduce the efficiency gap between the inverted PSCs and regular PSCs using a trace amount of surface-anchoring, long-chain alkylamine ligands (AALs) as grain and interface modifiers. We show that long-chain AALs suppress nonradiative carrier recombination and improve the optoelectronic properties of mixed-cation mixed-halide perovskite films. These translate into a certified stabilized PCE of 22.3% (23.0% PCE for lab-measured champion devices). The devices operate for over 1000 hours at the maximum power point (MPP), under simulated AM1.5 illumination, without loss of efficiency. Finally, we report a strategy to passivate Cl vacancies in mixed halide perovskite (MHP) QDs using non-polar-solvent-soluble organic pseudohalide (n-dodecylammonium thiocyanate (DAT)), enabling blue MHP LEDs with enhanced efficiency. Density-function-theory calculations reveal that the thiocyanate (SCN-) groups fill in the Cl vacancies and remove deep electron traps within the bandgap. DAT-treated CsPb(BrxCl1-x)3 QDs exhibit near unity (~100%) photoluminescence quantum yields; and their blue (~470 nm) LEDs are spectrally stable with an external quantum efficiency (EQE) of 6.3% – a record for perovskite LEDs emitting at the 460-480 nm range relevant to Rec. 2020 display standards.

To date, group-III-nitride has undergone continuous improvements to provide a broader range of industrial applications, such as solid-state lighting (SSL), visible light communications (VLC), and light projection. Recently, VLC has attained substantial attention in the field of wireless communication because it offers ~ 370 THz of bandwidth of unregulated visible spectrum, which makes it a critical factor in the evolution of the 5G networks and beyond. GaN-based light-emitting diode (LED) and laser diode (LD) have become increasingly appealing in energy-sufficient SSL replacing conventional light sources. However, III- nitride LEDs suffer from efficiency-droop in their external quantum efficiency associated with high current densities, and their modulation bandwidth is limited to 10 ~ 100 MHz. Although LDs have shown gigabit-modulation bandwidth, unfavorable artifacts, such as speckles are observed, which may raise a concern about eye safety. This dissertation is devoted to the fabrication and electrical and optical characterization of a new class of III-nitride light-emitter known as superluminescent diode (SLD). SLD works in an amplified spontaneous emission (ASE) regime, and it combines several advantages from both LD and LED, such as droop-free, speckle-free, low-spatial coherence, broader emission, high-optical power, and directional beam. Here, SLDs were fabricated by a focused ion beam by tilting the front facet of the waveguide to suppress the lasing mode. They showed a high-power of 474 mW on c-plane GaN-substrate with a large spectral bandwidth of 6.5 nm at an optical power of 105 mW. To generate SLD- based white light, a YAG-phosphor-plate was integrated, and a CRI of 85.1 and CCT of 3392 K were measured. For the VLC link, SLD showed record high-data rates of 1.45 Gbps and 3.4 Gbps by OOK and DMT modulation schemes, respectively. Additionally, a widely single- and dual-wavelength tunability were designed using SLD-based external cavity (SLD-EC) configuration for a tunable blue laser source. These results underscore the practicality of c-plane SLDs in realizing high-power, high data rate, speckle-free, and droop-free SSL-VLC apparatus. Additionally, the SLD-EC configuration allows a wide range of applications, including biomedical applications, optical communication, and high-resolution spectroscopy.

Benzobisazoles are a class of molecules that initially found their use in high-performance materials as high tensile strength fibers. Recent modifications to the syntheses of benzobisazoles have allowed for the materials to be studied as an n-type material to be used in organic semiconductors, more specifically organic light-emitting diodes (OLEDs). The high molecular stability required to produce blue light gives an opportunity for benzobisazoles to fulfill the requirement. Prior work on benzobisazoles, more specifically, the oxygen analog benzobisoxazole, has been used to try to achieve blue (<450 nm) but fell short in terms of efficiency due to molecular design choices. The following describes new design strategies such as utilizing single-bond linkage between the electron rich and deficient molecules, as well as transitioning from polymer to small molecules to fine-tune the properties of the materials for semiconductor applications. Utilizing a new design strategy, we demonstrate the ability to blue-shift the emission on two benzobisoxazole-based polymers by adopting single bond linkage between the benzobisoxazole and electron rich moieties fluorene and carbazole and achieve a usable brightness (> 1000 Cd/m2) when incorporated into OLEDs. With further modification of the benzobisoxazole core piece by adding dual conjugation along both axes to produce small molecules, we were able to achieve a deeper blue emission at higher efficiencies due to the reduced conjugation and aggregation than our previous systems experienced. Development of the small molecules led us to adopt a modular synthetic strategy for the high-efficiency material design of benzobisoxazole-based materials. In combination with Density Functional Theory calculations, we show the viability of performing computer-backed molecular design to develop materials to be used in all types of semiconductor applications. From calculations, we synthesize benzobisoxazole cruciforms that have both electron rich and electron deficient moieties. These products we then compared to experimental data to confirm the validity of computer-based rational design of molecules for not only blue OLEDs but for all semiconductor applications. The extremely high number of possible combinations of electron rich and electron deficient moieties allows for extensive future studies for the most optimal substituents for proper energy leveling tuning.

Micro light-emitting diode (microLED) technologies have been rapidly developing in the past decade and stand to be the prominent display technology for high-brightness applications. MicroLED microdisplays are particularly well-suited for systems that compete with high ambient light, such as augmented reality headsets and smartwatches that reflect light from the sun. However, there are several technological issues to overcome before microLED cost can be driven to a point that enables widespread commercial use. This dissertation covers the current microLED technological landscape, key issues to overcome, and an in-depth discussion on microLED performance and applications using modeled and experimentally fabricated microLEDs. The first experiment focuses on microLED fabrication fidelity and methods to overcome the challenge of defect-free displays. Current ultra-high definition display resolution standards require approximately 25 million individual microLED emitters with an expected zero dead pixels. To better identify defect states at early stages of fabrication, this dissertation presents methods using photoluminescence and cathodoluminescence that can identify dry-etching related damage to GaN/InGaN microLEDs that result in dead pixels. Expanding on fabrication fidelity, the second study in this dissertation examines surface recombination losses in etched GaN/InGaN microLEDs from nitrogen vacancy trap states. As microLED emitter size decreases, the ratio of etched surface area to emitter area size increases and injected current recombining at surface trap states increases causing large efficiency losses. To combat this, this study examines pGaN contact geometry selections and the influence on surface recombination losses. In particular, the results show that there is a strong dependence on efficiency for a desired output power in relation to current density. Utilizing the fabrication knowledge from the first two studies, applications and implementations of microLED microdisplays as a structured illumination microscopy light source within miniaturized microscopes are presented. There is discussion on future miniaturization strategies and next steps to improve device performance. Finally, this dissertation includes a short discussion on a display-adjacent technology, organic field-effect transistors (OFETs). An investigation on the electrostatic discharge resilience of parylene in OFETs is presented for applications in flexible high-voltage thin-film transistors.