Al-doped ZnO ceramic sputtering targets based on nanocrystalline powders produced by emulsion detonation synthesis – deposition and application as a transparent conductive oxide material

Transparent conducting oxides (TCOs) have been largely used in the optoelectronic industry due to their singular combination of low electrical resistivity and high optical transmittance. They are usually deposited by magnetron sputtering systems being applied in several devices, specifically thin film solar cells (TFSCs). Sputtering targets are crucial components of the sputtering process, with many of the sputtered films properties dependent on the targets characteristics. The present thesis focuses on the development of high quality conductive Al-doped ZnO (AZO) ceramic sputtering targets based on nanostructured powders produced by emulsion detonation synthesis method (EDSM), and their application as a TCO. In this sense, the influence of several processing parameters was investigated from the targets raw-materials synthesis to the application of sputtered films in optoelectronic devices. The optimized manufactured AZO targets present a final density above 99 % with controlled grain size, an homogeneous microstructure with a well dispersed ZnAl2O4 spinel phase, and electrical resistivities of ~4 × 10-4 Ωcm independently on the Al-doping level among 0.5 and 2.0 wt. % Al2O3. Sintering conditions proved to have a great influence on the properties of the targets and their performance as a sputtering target. It was demonstrated that both deposition process and final properties of the films are related with the targets characteristics, which in turn depends on the initial powder properties. In parallel, the influence of several deposition parameters in the film´s properties sputtered from these targets was investigated. The sputtered AZO TCOs showed electrical properties at room temperature that are superior to simple oxides and comparable to a reference TCO – indium tin oxide (ITO), namely low electrical resistivity of 5.45 × 10-4 Ωcm, high carrier mobility (29.4 cm2V-1s-1), and high charge carrier concentration (3.97 × 1020 cm-3), and also average transmittance in the visible region > 80 %. These superior properties allowed their successful application in different optoelectronic devices

Development of p-type oxide semiconductors based on tin oxide and its alloys: application to thin film transistors

In spite of the recent p-type oxide TFTs developments based on SnOx and CuxO, the results achieved so far refer to devices processed at high temperatures and are limited by a low hole mobility and a low On-Off ratio and still there is no report on p-type oxide TFTs with performance similar to n-type, especially when comparing their field-effect mobility values, which are at least one order of magnitude higher on n-type oxide TFTs. Achieving high performance p-type oxide TFTs will definitely promote a new era for electronics in rigid and flexible substrates, away from silicon. None of the few reported p-channel oxide TFTs is suitable for practical applications, which demand significant improvements in the device engineering to meet the real-world electronic requirements, where low processing temperatures together with high mobility and high On-Off ratio are required for TFT and CMOS applications. The present thesis focuses on the study and optimization of p-type thin film transistors based on oxide semiconductors deposited by r.f. magnetron sputtering without intentional substrate heating. In this work several p-type oxide semiconductors were studied and optimized based on undoped tin oxide, Cu-doped SnOx and In-doped SnO2. The influence of the deposition parameters, such as the percentage of oxygen and the deposition pressure and post deposition annealing treatments (up to 200 °C) parameters was investigated in order to optimize the properties of the p-type thin films. The detailed study of the material was accomplished through various techniques of characterization of their electrical and optical properties, crystal structure, chemical composition, topology and morphology. The obtained undoped SnOx thin films showed p-type conduction for a narrow percentage of oxygen, between 2.5% and 4%, after an annealing treatment at 150 °C and 200 °C. The thin films have a mixture of both tetragonal β-Sn and α-SnO phases, mobilities between1.6 cm2/Vs and 2.6 cm2/Vs and a carrier concentration between 1016 and 1017 cm-3. TFTs produced with this material were optimized presenting very good electrical performances, with On-Off ratio ~104, µFE up to 3.5cm2/Vs and Vth between -0.41 V and 15 V. The influence of the dielectric was also studied and leading to new results. Depending on the gate dielectric used, n-, p-type and ambipolar devices were obtained for the same semiconductor deposition conditions. Doping SnOx with Cu also results in transparent p-type oxide semiconductors with mobilities between 1.6 cm2/Vs and 2.6 cm2/Vs and a carrier concentration between 1016 and 1017 cm-3. When applied as active layer, resulted in poor performance thin film transistors, with lower On-Off ratio and the higher Vth, despite µFE increased. When doping the SnO2 films with In, p-type conduction was achieved without the need of the annealing treatment. The obtained as deposited thin films are amorphous and show mobilities up to 27 cm2 /Vs and very low resistivities ~10-3 Ω cm, obtaining in this way the a p-type oxide transparent conductor with the lowest electrical resistivity so far reported in the literature

Sputtered Zn-Sn-O based thin-film transistors: Optimization and circuit simulation

The development of amorphous oxide semiconductors (AOS) has been accelerated with their application in thin-film transistors (TFTs) for transparent and flexible displays. Among the many AOS available, zinc tin oxide (ZTO) represents a promising material due to its enhanced chemical and physical properties and the abundance of its elements in nature results in low price, compared to IGZO, and favours its widespread use in mass technology production. In this work, ZTO thin films deposited by sputtering under different oxygen, hydrogen and RF power conditions were investigated. The study focus on their morphology, structure and optical behaviour and on their implementation as active channel layers in TFTs. Great device performance was obtained when deposited at a power of 160 W, in a 10% of oxygen partial pressure and 1% of hydrogen, at a 2.3 mTorr pressure. After an annealing temperature of 180 oC, mobility of 9.1 cm2 V-1s-1, subthreshold slope of 0.29 Vdec-1 and turn-on voltage of -2.0 V were achieved, using a sputtered multilayer dielectric based on Ta2O5-SiO2. The measured output and transfer a-ZTO TFT characteristics were modeled in an artificial neural network (ANNs) empirical model with very good accuracy. The model was used in the Cadence Spectre to simulate three logic gates at DC and transient analysis: inverter, NAND and NOR, with logic levels preserved up to 10 kHz

Novel techniques for the control of the properties of reactively-sputtered thin films

Precise control techniques are of fundamental importance in the accurate deposition of optical, mechanical, electrical and magnetic thin films. The objective of this work was twofold: to devise and evaluate novel control systems for reactive sputtering primarily oxide films, and investigate the effects of these processes on resultant film properties. [Continues.

Mechanically flexible, transparent conductors based on ultrathin metallic layers

Transparent Conductors are essential components in many opto-electronic devices. Ultrathin Metal Films (UTMFs) represent an effective alternative to the ITO state-of-art. Their potential was demonstrated in organic solar cells with efficiencies comparable to those with ITO

High rate deposition processes for thin film CdTe solar cells

This thesis describes the development of a fast rate method for the deposition of high quality CdS and CdTe thin films. The technique uses Pulsed DC Magnetron Sputtering (PDCMS). Surprisingly, the technique produces highly stable process conditions. CREST is the first laboratory worldwide to show that pulsed DC power may be used to deposit CdS and CdTe thin films. This is a very promising process technology with potential for eventual industrial deployment. The major advantage is that the process produces high deposition rates suitable for use in solar module manufacturing. These rates are over an order of magnitude faster than those obtained by RF sputtering. In common with other applications it has also been found that the energetics of the pulsed DC process produce excellent thin film properties and the power supply configuration avoids the need for complex matching circuits. Conventional deposition methodologies for CdS, Chemical Bath Deposition (CBD) and CdTe thin films, Electrodeposition (ED), have been chosen as baselines to compare film properties with Pulsed DC Magnetron Sputtering (PDCMS). One of the issues encountered with the deposition of CdS thin films (window layers) was the presence of pinholes. A Plasma cleaning process of FTO-coated glass prior to the deposition of the CdS/CdTe solar cell has been developed. It strongly modifies and activates the TCO surface, and improves the density and compactness of the deposited CdS thin film. This, in turn, improves the optical and morphological properties of the deposited CdS thin films, resulting in a higher refractive index. The pinhole removal and the increased density allows the use of a much thinner CdS layer, and this reduces absorption of blue spectrum photons and thereby increases the photocurrent and the efficiency of the thin film CdTe cell. Replacing the conventional magnetic stirrer with an ultrasonic probe in the chemical bath (sonoCBD) was found to result in CdS films with higher optical density, higher refractive index, pinhole and void-free, more compact and uniform along the surface and through the thickness of the deposited material. PDCMS at 150 kHz, 500 W, 2.5 μs, 2 s, results in a highly stable process with no plasma arcing. It allows close control of film thickness using time only. The CdS films exhibited a high level of texture in the direction. The grain size was typically ~50 nm. Pinholes and voids could be avoided by reducing the working gas pressure using gas flows ii below 20 sccm. The deposition rate was measured to be 1.33 nm/s on a rotating substrate holder. The equivalent deposition rate for a static substrate is 8.66 nm/s, which is high and much faster than can be achieved using a chemical bath deposition or RF magnetron sputtering. The transmission of CdS can be improved by engineering the band gap of the CdS layer. It has been shown that by adding oxygen to the working gas pressure in an RF sputtering deposition process it is possible to deposit an oxygenated CdS (CdS:O) layer with an improved band gap. In this thesis, oxygenated CdS films for CdTe TF-PV applications have been successfully deposited by using pulsed DC magnetron sputtering. The process is highly stable using a pulse frequency of 150 kHz and a 2.5 μs pulse reverse time. No plasma arcing was detected. A range of CdS:O films were deposited by using O2 flows from 1 sccm to 10 sccm during the deposition process. The deposition rates achieved using pulsed DC magnetron sputtering with only 500 W of power to the magnetron target were in the range ~1.49 nm/s ~2.44 nm/s, depending on the oxygen flow rate used. The properties of CdS thin films deposited by pulsed DC magnetron sputtering and chemical bath deposition have been studied and compared. The pulsed DC magnetron sputtering process produced CdS thin films with the preferred hexagonal oriented crystalline structure with a columnar grain growth, while sonoCBD deposited films were polycrystalline with a cubic structure and small grainy crystallites throughout the thickness of the films. Examination of the PDCMS deposited CdS films confirmed the increased grain size, increased density, and higher crystallinity compared to the sonoCBD CdS films. The deposition rate for CdS obtained using pulsed DC magnetron sputtering was 2.86 nm/s using only 500 W power on a six inch circular target compared to the much slower (0.027 nm/s) for the sonoChemical bath deposited layers. CdTe thin films were grown on CdS films prepared by sonoCBD and Pulsed DC magnetron sputtering. The results showed that the deposition technique used for the CdS layer affected the growth and properties of the CdTe film and also determined the deposition rate of CdTe, being 3 times faster on the sputtered CdS. PDCMS CdTe layers were deposited at ambient temperature, 500 W, 2.9 μs, 10 s, 150 kHz, with a thickness of approximately 2 μm on CdS/TEC10 coated glass. The layers appear iii uniform and smooth with a grain size less than 100 nm, highly compact with the morphology dominated by columnar grain growth. Stress analysis was performed on the CdTe layers deposited at room temperature using different gas flows. Magnetron sputtered thin films deposited under low gas pressure are often subject to compressive stress due to the high mobility of the atoms during the deposition process. A possible way to reduce the stress in the film is the post-deposition annealing treatment. As the lattice parameter increased; the stress in the film is relieved. Also, a changing the deposition substrate temperature had an effect on the microstructure of CdTe thin films. Increasing the deposition temperature increased the grain size, up to ~600 nm. CdTe thin films with low stress have been deposited on CdS/TEC10 coated glass by setting the deposition substrate temperature at ~200°C and using high argon flows ~ 70 sccm Ar. Finally, broadband multilayer ARCs using alternate high and low refractive index dielectric thin films have been developed to improve the light transmission into solar cell devices by reducing the reflection of the glass in the extended wavelength range utilised by thin-film CdTe devices. A four-layer multilayer stack has been designed and tested, which operates across the wavelength range used by thin-film CdTe PV devices (400 850 nm). Optical modelling predicts that the MAR coating reduces the WAR (400-850 nm) from the glass surface from 4.22% down to 1.22%. The application of the MAR coating on a thin-film CdTe solar cell increased the efficiency from 10.55% to 10.93% or by 0.38% in absolute terms. This is a useful 3.6% relative increase in efficiency. The increased light transmission leads to improvement of the short-circuit current density produced by the cell by 0.65 mA/cm2. The MAR sputtering process developed in this work is capable of scaling to an industrial level

Preparation and characterisation of transparent conducting oxides and thin films

Transparent conducting oxide (TCOs) thin films, including non-stoichiometric tin doped indium oxide (ITO) and aluminium doped zinc oxide (AZO), have found considerable applications in various displays, solar cells, and electrochromic devices, due to their unique combination of high electrical conductivity and optical transparency. TCO thin films are normally fabricated by sputtering, thermal vapour deposition and sol-gel method. Among them, sol-gel processing, which was employed in this project, is no doubt the simplest and cheapest processing method, The main objectives of this project were to produce indium tin oxides (ITO) and zinc aluminium oxides (AZO) nanoparticles with controlled particle size and morphology and to fabricate TCO thin films with high optical transmittance and electrical conductivity. In this research, hydrothermal method was used to synthesise ITO and AZO nanoparticles. Tin oxides, zinc oxides, ITO and AZO particles with the particle size ranging from 10 nm to several micrometers and different morphologies were synthesised through controlling the starting salts, alkaline solvents and hydrothermal treatment conditions. ITO and AZO thin films were fabricated via sol-gel technique through dip coating method. The effects of the starting salts, alkaline solvents, surfactant additives and coating and calcination conditions on the formation of thin films were investigated. XRD, TEM, FEG-SEM, DSC-TGA, UV-Vis spectrometer and four-point probe resistance meter were used to characterise the crystallinity, particle size, morphology, optical transmittance and sheet resistance of the particles and thin films. Crack-free thin films with high optical transmittance (>80% at 550 nm) and low sheet resistances (2.11 kΩ for ITO and 26.4 kΩ for AZO) were obtained in optimised processing conditions

Photo-engineered optoelectronic properties of transparent conductive oxides via reactive laser annealing (ReLA): the consequence of defects

The ongoing development of nanofabrication capabilities ever increases our ability to exploit the light-matter interactions occurring on the nanoscale, promising radical breakthroughs in many technological sectors. This research field, namely plasmonics, faces a major roadblock in respect to realistic applications. Specifically, to translate the potential of plasmonics into practical optical devices, the right set of materials are required; materials that can overcome the loss issue and expand the operational window towards the infrared and (IR) near-IR (NIR) spectral ranges. Transparent conductive oxides (TCOs) are appealing alternatives due to their transparency, refractory character, and maturity in electronic devices. Importantly, their non-stoichiometric character allows for the modulation of their optoelectronic properties through the manipulation of their defects. This research developed the knowledge of the optoelectronic properties of TCOs, via spectroscopic ellipsometry (SE) while examining high-throughput methods to tune their transport properties towards the requirements of IR plasmonics. Specifically, this research investigated, for the first time, the utility of reactive laser annealing (ReLA) to probe the crystal structure and donor state variations of sputtered tin-doped indium oxide (ITO) thin films. The demonstration of ReLA comprised an investigation of the role of the laser processing parameters (e.g., laser fluence, number of pulses and ambient composition) with an extensive characterisation methodology comprising: four-point probe, Hall Effect, X-ray diffractometry, energy-dispersive X-ray spectroscopy and X-ray photon spectroscopy. In this way, ReLA is established as a novel and versatile tool to tailor the properties of TCO films towards the requirements of potential plasmonic applications. Furthermore, SE in the wide spectral range between 0.034−6.5 eV was employed to get insights on the properties of seed TCO films and the laser-induced modifications, revealing how and why TCOs present challenges both for further elucidating the still not fully known mechanisms that govern their optoelectronic behaviour and for their integration into far-IR plasmonic devices