Strategies for controlled electronic doping of colloidal quantum dots

Over the last several years tremendous progressed progress has been made in incorporating Colloidal Quantum Dots (CQDs) as photoactive components in optoelectronic devices. A significant part of that progress is associated with significant advancements made in achievingon controlled electronic doping of the CQDs and thus improving the electronic properties of CQDs solids. Today, a variety of strategies exists towards that purpose and this minireview aims at surveying major published works in this subject. Additional attention is given to the many challenges associated with the task of doping CQDs as well as to the optoelectronic functionalities and applications being realized when successfully achieving light and heavy electronic doping of CQDs.Peer ReviewedPostprint (author's final draft

Characterization and Fabrication of Active Matrix Thin Film Transistors for an Addressable Microfluidic Electrowetting Channel Device

The characterization and fabrication of active matrix thin film transistors (TFTs) has been studied for an addressable microfluidic electrowetting channel device as application. A new transparent semiconductor material, Amorphous Indium Gallium Zinc Oxide (a-IGZO), is used for TFT, which shows high electrical performance rather than amorphous silicon based TFT; higher mobility and even higher transparency. The purpose of this dissertation is to optimize each TFT process including the optimization of a-IGZO properties to achieve robust device for application. To minimize hysteresis of TFT curves, the gate dielectric is discussed extensively in this dissertation. By optimizing gas ratio of NH3SiH4, it is found that the TFT with NH3 rich SiNx gate dielectric deposited with NH3/SiH4 =5.1 and stoichiometric SiO2 demonstrates best condition to reduce hysteresis. a-IGZO films is investigated as a function of power and substrate bias effect which affects to electrical performance; the higher power and substrate bias increase the carrier density in the film and mainly cause threshold voltage(VT) to shift in the negative gate voltage direction and mobility to increase, respectively. In addition, the powerful method to estimate the electrical properties of a-IGZO is proposed by calculating O2 and IGZO flux during sputtering in which the incorporation ratio with O2/IGZO ≈1 demonstrates the optimized a-IGZO film for TFT. It is confirmed that both physical and chemical adsorption affects the electrical property of a-IGZO channel by studying TFT-IV characteristics with different pressure and analyzing X-ray photoelectron spectroscopy (XPS), which mainly affects the VT instability. The sputtered SiO2 passivation shows better electrical performance. To achieve electrically compatible (lower back channel current) a-IGZO film to SiO2 sputter passivated device, a-IGZO TFTs require oxygen rich a-IGZO back channel by employing two step a-IGZO deposition process (2nd 10nm a-IGZO with PO2 = 1.5mTorr on 1st 40nm a-IGZO with PO2=1mTor). Electrowetting microfluidic channel device as application using a-IGZO TFTs is studied by doing preliminary test. The electrowetting channel test using polymer post device platform is candidate for addressable electrowetting microfluidic channel device driven by active matrix type a-IGZO TFT

Interpretation and Regulation of Electronic Defects in IGZO TFTs Through Materials & Processes

The recent rise in the market for consumer electronics has fueled extensive research in the field of display. Thin-Film Transistors (TFTs) are used as active matrix switching devices for flat panel displays such as LCD and OLED. The following investigation involves an amorphous metal-oxide semiconductor that has the potential for improved performance over current technology, while maintaining high manufacturability. Indium-Gallium-Zinc-Oxide (IGZO) is a semiconductor material which is at the onset of commercialization. The low-temperature large-area deposition compatibility of IGZO makes it an attractive technology from a manufacturing standpoint, with an electron mobility that is 10 times higher than current amorphous silicon technology. The stability of IGZO TFTs continues to be a challenge due to the presence of defect states and problems associated with interface passivation. The goal of this dissertation is to further the understanding of the role of defect states in IGZO, and investigate materials and processes needed to regulate defects to the level at which the associated influence on device operation is controlled. The relationships between processes associated with IGZO TFT operation including IGZO sputter deposition, annealing conditions and back-channel passivation are established through process experimentation, materials analysis, electrical characterization, and modeling of electronic properties and transistor behavior. Each of these components has been essential in formulating and testing several hypotheses on the mechanisms involved, and directing efforts towards achieving the goal. Key accomplishments and quantified results are summarized as follows: • XPS analysis identified differences in oxygen vacancies in samples before and after oxidizing ambient annealing at 400 °C, showing a drop in relative integrated area of the O 1s peak from 32% to 19%, which experimentally translates to over a thousand fold decrease in the channel free electron concentration. • Transport behavior at cryogenic temperatures identified variable range hopping as the electron transport mechanism at temperature below 130 K, whereas at temperature greater than 130 K, the current vs temperature response followed an Arrhenius relationship consistent with extended state transport. • Refinement of an IGZO material model for TCAD simulation, which consists of oxygen vacancy donors providing an integrated space charge concentration NVO = +5e15 cm-3, and acceptor-like band-tail states with a total integrated ionized concentration of NTA = -2e18 cm-3. An intrinsic electron mobility was established to be Un = 12.7 cm2/V∙s. • A SPICE-compatible 2D on-state operation model for IGZO TFTs has been developed which includes the integration of drain-impressed deionization of band-tail states and results in a 2D modification of free channel charge. The model provides an exceptional match to measured data and TCAD simulation, with model parameters for channel mobility (Uch = 12 cm2/V∙s) and threshold voltage (VT = 0.14 V) having a close match to TCAD analogs. • TCAD material and device models for bottom-gate and double-gate TFT configurations have been developed which depict the role of defect states on device operation, as well as provide insight and support of a presented hypothesis on DIBL like device behavior associated with back-channel interface trap inhomogeneity. This phenomenon has been named Trap Associated Barrier Lowering (TABL). • A process integration scheme has been developed that includes IGZO back-channel passivation with PECVD SiO2, furnace annealing in O2 at 400 °C, and a thin capping layer of alumina deposited via atomic layer deposition. This process supports device stability when subjected to negative and positive bias stress conditions, and thermal stability up to 140 °C. It also enables TFT operation at short channel lengths (Leff ~ 3 µm) with steep subthreshold characteristics (SS ~ 120 mV/dec). The details of these contributions in the interpretation and regulation of electronic defect states in IGZO TFTs is presented, along with the support of device characteristics that are among the best reported in the literature. Additional material on a complementary technology which utilizes flash-lamp annealing of amorphous silicon will also be described. Flash-Lamp Annealed Polycrystalline Silicon (FLAPS) has realized n-channel and p-channel TFTs with promising results, and may provide an option for future applications with the highest performance demands. IGZO is rapidly emerging as the candidate to replace a-Si:H and address the performance needs of display products produced by large panel manufacturing

Tuning Nanocrystal Surface Depletion by Controlling Dopant Distribution as a Route Toward Enhanced Film Conductivity

Electron conduction through bare metal oxide nanocrystal (NC) films is hindered by surface depletion regions resulting from the presence of surface states. We control the radial dopant distribution in tin-doped indium oxide (ITO) NCs as a means to manipulate the NC depletion width. We find in films of ITO NCs of equal overall dopant concentration that those with dopant-enriched surfaces show decreased depletion width and increased conductivity. Variable temperature conductivity data shows electron localization length increases and associated depletion width decreases monotonically with increased density of dopants near the NC surface. We calculate band profiles for NCs of differing radial dopant distributions and, in agreement with variable temperature conductivity fits, find NCs with dopant-enriched surfaces have narrower depletion widths and longer localization lengths than those with dopant-enriched cores. Following amelioration of NC surface depletion by atomic layer deposition of alumina, all films of equal overall dopant concentration have similar conductivity. Variable temperature conductivity measurements on alumina-capped films indicate all films behave as granular metals. Herein, we conclude that dopant-enriched surfaces decrease the near-surface depletion region, which directly increases the electron localization length and conductivity of NC films

Fluorescent Silicon Clusters and Nanoparticles

The fluorescence of silicon clusters is reviewed. Atomic clusters of silicon have been at the focus of research for several decades because of the relevance of size effects for material properties, the importance of silicon in electronics and the potential applications in bio-medicine. To date numerous examples of nanostructured forms of fluorescent silicon have been reported. This article introduces the principles and underlying concepts relevant for fluorescence of nanostructured silicon such as excitation, energy relaxation, radiative and non-radiative decay pathways and surface passivation. Experimental methods for the production of silicon clusters are presented. The geometric and electronic properties are reviewed and the implications for the ability to emit fluorescence are discussed. Free and pure silicon clusters produced in molecular beams appear to have properties that are unfavourable for light emission. However, when passivated or embedded in a suitable host, they may emit fluorescence. The current available data show that both quantum confinement and localised transitions, often at the surface, are responsible for fluorescence. By building silicon clusters atom by atom, and by embedding them in shells atom by atom, new insights into the microscopic origins of fluorescence from nanoscale silicon can be expected.Comment: 5 figures, chapter in "Silicon Nanomaterials Sourcebook", editor Klaus D. Sattler, CRC Press, August 201

Improving solar cell performance through surface modification of silicon

This project sets out to improve the efficiency of thin crystalline silicon solar cells by enhancing the photoexcitation through light harvesting, and by developing a novel surface passivation technique via the covalent attachment of organic molecules to the surface. To aid the characterisation of these novel structures, we have developed a new measurement technique for surface recombination using the Kelvin prove.A passivation method through the attachment of alkyl monolayers to the silicon surface has been developed. These layers were shown to have good passivation properties whilst retaining excellent resilience to oxidation. The passivation effect was determined to be caused by the generation of surface charge, as measured by the Kelvin probe. Further functionalisation of the organic monolayers was undertaken to attach fluorescent chromophores. A novel method for measurement of the surface recombination velocity was developed utilising the Kelvin probe. Changing the incident photon flux, and thus the photogenerated current, allows measurement of the surface recombination current through the change in surface photovoltage. This dependence can then be used to extract the value of the surface recombination velocity. Furthermore, we have shown that this method can be developed further into a mapping technique for surface recombination lifetime, of potentially significant industrial interest.The effect of the silicon surface charge on the passivation observed has been investigated through the attachment of charged monolayers. It was found that through the attachment of a positive charge, the observed recombination lifetime in n-type silicon decreased whilst a negative charge (through the attachment of carboxylic acid groups to the surface) was found to improve the surface passivation. The carboxylic acid functional groups were charged through immersion in triethylamine (base) and returned to the neutral, starting state through immersion in acetic acid. We have found that the recombination lifetime decreases linearly with decreasing charge-surface distance. This technique allows an in-depth study of surface passivation to be carried out by separating the two principal causes for passivation – the removal of surface states and charge attachment to the surface.Fluorescent chromophores were attached to the silicon surface by two different techniques – through the reaction of an alcohol-terminated monolayer with an acyl chloride porphyrin and by palladium-catalysed cross-coupling of an allyl-terminated surface with a cyanine dye. The fluorescence quenching was investigated at various chromophore-silicon distances by varying the length of the alkyl chain spacer. We find that the fluorescence lifetime decreases with decreasing chromophore-silicon distance, and follows a logarithmic trend. Further work is required, however, to combine sensitisation with surface passivation as incorporation of a sensitisation layer by the palladium cross coupling of an allyl-terminated surface results in metal contamination to the surface, reducing recombination lifetime

量子ドット太陽電池とペロブスカイト太陽電池における界面エンジニアリングと界面電荷移動ダイナミクス

電気通信大学201

Investigation of Thermal Stress Degradation in Indium-Gallium-Zinc-Oxide TFTs

The performance of IGZO TFTs has improved significantly in recent years, however device stability still remains a significant issue. Thermal stability of IGZO TFTs be- comes very crucial to ensure desired performance of end-product. Both bottom-gate (BG) and double-gate (DG) TFTs were observed to degrade with hotplate treatments under 200◦C. Such events are rarely reported in the literature, and thus became the primary focus of this work. The mechanism causing the instability is not completely understood, however experimental results indicate the instability occurs either di- rectly or indirectly due to the influence of H2O within the passivation oxide above the IGZO channel region. DG TFTs saw more pronounced degradation, which led to the hypothesis that there may be a reaction of the top gate metal with H2O molecules in the passivation oxide, liberating monatomic hydrogen. Both H2O and hydrogen behave as donor states in IGZO, thus rendering the channel more conduc- tive. The thermal stability also demonstrated a dependence on channel length, with shorter channel devices showing greater stability. This may be due to the metalized source/drain regions acting as effective getter to water during a 400◦C passivation anneal which is performed prior to top-gate metal deposition. This hypothesis led to an investigation on atomic layer deposition (ALD) of capping layers over the passiva- tion oxide of IGZO TFTs to act as an effective barrier to water/hydrogen migrating to the underlying IGZO channel

High Performance Gallium Tin Zinc Oxide Thin Film Transistors By Rf Magnetron Sputtering

With the growing need for large area display technology and the push for a faster and cheaper alternative to the current amorphous indium gallium zinc oxide (a-IGZO) as the active channel layer for pixel-driven thin film transistors (TFTs) display applications, gallium tin zinc oxide (GSZO) has shown to be a promising candidate due to the similar electronic configuration of Sn4+ and In3+. Post deposition annealing at 450 °C of the films in air was found to lead to a high atomic concentration of Sn4+ in the films as ascertained by x-ray photoelectron spectroscopy, which is one of the prerequisites for improved performance of the device. In this work a systematic and detailed study of GSZO TFTs with the channel annealed at 450°C has been carried out, and different effects have been investigated, including: oxygen flow, deposition contacts, further annealing in different ambients and presence of passivation layer on the TFT performance. The electrical and optical stability of the GSZO TFTs have also been the subject of study. These studies provided a more insight into the role of surface and interface states on the TFT performance and its degradation mechanism under stress. Improved device performance with VON of -3.5 V, ION/IOFF of 108, μFE = 4.36 V-1 s-1, and sub-threshold swing (SS) of 0.38 V/dec has been achieved, which is close to those of industrial standard IGZO TFTs. Thus, this work demonstrates GSZO based TFTs as a promising and viable option to the IGZO TFTs