Synthesis and characterization of piezoelectric thin films as functional materials for sensing

This thesis reports about the sputter deposition and characterization of ZnO nanomaterials both in the form of dense and sponge-like thin films. It is shown that high-quality ZnO thin films can be successfully grown on both hard and flexible conductive substrates, with the final aim of proving that their piezoelectric and electrical properties can be successfully exploited in the fabrication of piezoelectric-based nanosensors and nanoactuators. To further state the versatility of ZnO thin films, both spin coated and sputtered dense ZnO thin films were used as seed layers for promoting the growth of well-aligned ZnO nanowires. A strong relationship between the kind of seed layer, i.e., sputtered or spin-coated, and the final NWs morphology, surface chemistry and thus wettability was noticed. In particular NWs grown on sputtered seed layers showed a superhydrophobic behavior, ideal for self-cleaning, anti-fogging or microfluidic devices. In contrast, on spin coated seed layers, highly hydrophilic NWs were obtained, being suitable for further surface functionalization with enhanced adsorption properties towards biological agents or dye for imaging, diagnostic, optical or photovoltaic applications. Finally, the sponge-like morphology is further exploited for the synthesis and characterization of Mn- and Sb- doped, sponge-like ZnO films. The presence of Mn dopant resulted in a high resistance contribution. On the contrary, typical ferroelectric switching phenomena were observed in the Sb-doped ZnO films, showing the presence of hysteretical polarization loops

Investigation of doped cuprous halides for photovoltaic and display applications

The thesis mainly focuses on the growth and optoelectronic characterisation of the doped cuprous halides (CuX) with high UV/blue emission properties and the light harvesting in the CuBr/Si based heterojunction photovoltaic (PV) cells. Since cuprous halides are short wavelength emitters with high excitonic binding energies, growth of lower resistivity, highly luminescent p and n-type films are essential for the development of the future excitonic based light emitting devices with these materials. We describe the deposition and characterisation of the Zn doped n-type CuCl and oxygen doped p-type CuBr with higher carrier concentration using pulsed dc magnetron sputtering and thermal evaporation followed by oxygen plasma exposure, respectively. The structural and morphological properties of the n-type Zn doped CuCl films are investigated using XRD, SEM and AFM measurements. Our findings show that, the crystallinity of CuCl increases with doping of Zn and the maximum is obtained for the 3 % doped film, beyond which crystallinity decreases. The Zn doping has no deleterious impact on the structural and luminescent properties of CuCl up to a doping percentage of 5. An order of magnitude reduction in the resistivity of the CuCl films is obtained as a result of Zn doping. The resistivity, n-type carrier concentration and carrier mobility corresponding to the 3 % Zn doped films were, respectively 6 Ωcm, ~ 9.8×1018 cm-3 and 0.1 cm2V-1s-1. The influence of Zn doping on the electronic structure of CuCl is investigated using photoemission spectroscopic studies. Our studies on the Cu and Cl core level spectra show the presence of trace amounts of Cu2+ species in the undoped CuCl sample along with the major Cu+ species. This is verified by the observation of the satellite and shoulder peaks in the Cu 2p core level and a higher binding energy tail in the Cl 2p core level spectra. The disappearance of the cupric species with doping of Zn (3 %) is also observed. Furthermore, the shifting of the valence band towards the higher binding energy confirms the filling of the conduction band owing to the Zn doping, which explains the improvement of the conductivity of the doped films. vii We have successfully deposited p-type CuBr films by doping of oxygen. Our findings show that, there is no significant influence on the structural properties of the CuBr up to an oxygen plasma exposure time of 5 min. The investigation of the optical properties confirms good luminescence of the CuBr films up to a plasma exposure time of 3 min. The electrical characterisations of the oxygen plasma exposed films reveal that, oxygen act as a good acceptor for CuBr. The resistivity of the 3 min oxygen plasma exposed samples reduces to the order of ~ 1 Ωcm. SIMS analysis shows that, the diffusion of oxygen is quite good in the CuBr films. The realization of the heterojunction PV cell based on p-CuBr/n-Si is investigated. The I-V characteristics confirm the rectification behaviour of the p-n diode with a turn on voltage of ~ 1 V. The photoresponse properties of the heterojunction are studied by measuring the I-V characteristics under illumination. The photogenerated carrier formation is confirmed by the increased reverse current under illumination of the heterojunction. The wavelength dependence of the photo current was also studied using LED illumination. Development of ultrathin transparent Cr contacts, useful for the electroluminescent device fabrication was also discussed. This can be used as an alternative to well-known Indium Tin Oxide films for the future fabrication of the CuX-based display devices

Zno thin films growth optimization for piezoelectric application

The piezoelectric response of ZnO thin films in heterostructure-based devices is strictly related to their structure and morphology. We optimize the fabrication of piezoelectric ZnO to reduce its surface roughness, improving the crystalline quality, taking into consideration the role of the metal electrode underneath. The role of thermal treatments, as well as sputtering gas composition, is investigated by means of atomic force microscopy and x-ray diffraction. The results show an optimal reduction in surface roughness and at the same time a good crystalline quality when 75% O2 is introduced in the sputtering gas and deposition is performed between room temperature and 573 K. Subsequent annealing at 773 K further improves the film quality. The introduction of Ti or Pt as bottom electrode maintains a good surface and crystalline quality. By means of piezoelectric force microscope, we prove a piezoelectric response of the film in accordance with the literature, in spite of the low ZnO thickness and the reduced grain size, with a unipolar orientation and homogenous displacement when deposited on Ti electrode

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

Design of a photodiode based on NiO/ZnO heterojunction

The recent interest in applications of ultraviolet photodiodes based on Nickel Oxide/ Zinc Oxide (NiO/ZnO) heterojunction heightens the need for more developments. One of the greatest challenges was to indicate which factors control the performance of these devices. Therefore, this research is based on three studies. In the first study, we managed to get the effect of the source solution quantity on structural and optical characteristics of ZnO and NiO thin films grown by spray pyrolysis for the design of NiO/ ZnO photodiodes. We assumed that increasing the film thickness reduces the defects and results in less recombination through higher crystallinity which enhances the photodetection. In the second study, we fabricated a NiO/ZnO junction by spray pyrolysis with optimum conditions. We performed a simulation to clarify the effects of heterojunction behavior and interface trap on the performance of NiO/ZnO photodiodes. The origin of current has been attributed to the tunneling, thermionic emission in the interface and due interface traps SRH generation and recombination controlling the carrier transport at the heterojunction. As a third study, the radio frequency sputtering technique was used to deposit NiO and ZnO thin films to form NiO/ZnO heterojunction in oxygen flow absence conditions. We found that the NiO/ZnO heterojunction has a semi-transparency in the visible range which makes this heterojunction suitable for broadband photodetection applications. This study demonstrates that NiO/ZnO heterojunction could play an important role in many applications such as broadband photodetection (ultraviolet and visible ranges), partial transparent optoelectronic devices and solar cells. These studies imply that using the crystal structure as desired has always been the key to designing and targeting high-quality heterojunctions. These results have allowed us to identify key parameters useful for the optimization of NiO/ZnO photodiodes, as well as to give realistic estimates of the performances of such UV device

Nanocomposite Films for Gas Sensing

Nanocomposite films are thin films formed by mixing two or more dissimilar materials having nano-dimensional phase(s) in order to control and develop new and improved structures and properties. The properties of nanocomposite films depend not only on the individual components used but also on the morphology and the interfacial characteristics. Nanocomposite films that combine materials with synergetic or complementary behaviours possess unique physical, chemical, optical, mechanical, magnetic and electrical properties unavailable from that of the component materials and have attracted much attention for a wide range of device applications such as gas sensors.NRC publication: Ye

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

Transparent conducting oxides and other functional thin films grown via aerosol assisted chemical vapour deposition

This thesis describes the preparation and characterisation of different functional thin films, with the main focus being transparent conducting oxide (TCO) thin films including both n-type and p-type, but also other functional thin films (such as catalytic thin films used in the oxygen evolution reaction (OER)) grown via aerosol assisted chemical vapour deposition (AACVD). The main aim to this work was to discover and investigate more suitable functional thin films (TCOs and catalytic materials) via the film preparation method, AACVD. The synthetic route of all the functional thin films used in this thesis is AACVD, which is one specialized form of CVD that is cost effective and easily scalable, operating at ambient pressure. In this thesis, Mo-, P- and B-doped zinc oxide (ZnO) thin films have been investigated as n-type TCOs, with all films showing low resistivities of 2.6 × 10-3 Ω.cm, 6.0 × 10-3 Ω.cm and 5.10 × 10-3 Ω.cm, respectively. The optical properties also reached over 75% as transmittance in the visible area for all doped thin films. Boron phosphide (BP) thin films were synthesized and investigated as a p-type TCO, although the results indicated that BP may not be an ideal p-type material and further doping investigations were not investigated. P-doped molybdenum disulfide (MoS2) thin films have been considered as potential catalytic materials in the OER area. The MoS2 thin films with 1 mol.% P doping displayed superior catalytic performance of OER process with lowest overpotential at 319 mV for 10 mA cm-2 as current density and 173 mV for 10 mA mg-1 as current density, respectively, in 1M KOH medium. Moreover, the mass activity was also high at 1000 A g-1 with small overpotential at 450 mV, which suggested the doping method could improve the properties of thin films through AACVD

An investigation of the performance and stability of zinc oxide thin-film transistors and the role of high-k dielectrics.

Transparent oxide semiconducting films have continued to receive considerable attention, from a fundamental and application-based point of view, primarily because of their useful fundamental properties. Of particular interest is zinc oxide (ZnO), an n-type semiconductor that exhibits excellent optical, electrical, catalytic and gas-sensing properties, and has many applications in various fields. In this work, thin film transistor (TFT) arrays based on ZnO have been prepared by reactive radio frequency (RF) magnetron sputtering. Prior to the TFT fabrication, ZnO layers were sputtered on to glass and silicon substrates, and the deposition parameters optimised for electrical resistivities suitable for TFT applications. The sputtering process was carried out at room temperature with no intentional heating. The aim of this work is to prepare ZnO thin films with stable semiconducting electrical properties to be used as the active channel in TFTs; and to understand the role of intrinsic point defects in device performance and stability. The effect of oxygen (O2) adsorption on TFT device characteristics is also investigated. The structural quality of the material (defect type and concentration), electrical and optical properties (transmission/absorption) of semiconductor materials are usually closely correlated. Using the Vienna ab-initio simulation package (VASP), it is predicted that O2 adsorption may influence film transport properties only within a few atomic layers beneath the adsorption site. These findings were exploited to deposit thin films that are relatively stable in atmospheric ambient with improved TFT applications. TFTs incorporating the optimised layer were fabricated and demonstrated very impressive performance metrics, with effective channel mobilities as high as 30 cm2/V-1s-1, on-off current ratios of 107 and sub-threshold slopes of 0.9 – 3.2 V/dec. These were found to be dependent on film thickness (~15 – 60 nm) and the underlying dielectric (silicon dioxide (SiO2), gadolinium oxide (Gd2O3), yttrium oxide (Y2O3) and hafnium oxide (HfO2)). In this work, prior to sputtering the ZnO layer (using a ZnO target of 99.999 % purity), the sputtering chamber was evacuated to a base pressure ~4 x 10-6 Torr. Oxygen (O2) and argon (Ar) gas (with O2/Ar ratio of varying proportions) were then pumped into the chamber and the deposition process optimised by varying the RF power between 25 and 500 W and the O2/Ar ratio between 0.010 to 0.375. A two-level factorial design technique was implemented to test specific parameter combinations (i.e. RF power and O2/Ar ratio) and then statistical analysis was utilised to map out the responses. The ZnO films were sputtered on glass and silicon substrates for transparency and resistivity measurements, and TFT fabrication respectively. For TFT device fabrication, ZnO films were deposited onto thermally-grown silicon dioxide (SiO2) or a high-k dielectric layer (HfO2, Gd2O3 and Y2O3) deposited by a metal-organic chemical deposition (MOCVD) process. Also, by using ab initio simulation as implemented in the “Vienna ab initio simulation package (VASP)”, the role of oxygen adsorption on the electrical stability of ZnO thin film is also investigated. The results indicate that O2 adsorption on ZnO layers could modify both the electronic density of states in the vicinity of the Fermi level and the band gap of the film. This study is complemented by studying the effects of low temperature annealing in air on the properties of ZnO films. It is speculated that O2 adsorption/desorption at low temperatures (150 – 350 0C) induces variations in the electrical resistance, band gap and Urbach energy of the film, consistent with the trends predicted from DFT results