ICMCTF 2023 Session CP-ThP: Functional Thin Films and Surfaces (Symposium C) Poster Session

Transparent conducting oxides (TCOs) are needed for a wide range of applications in thin film devices such as thin film transistors, solar cells, and smart windows.

Tin doped indium oxide (ITO) is the most commonly used TCO material since it possesses high electrical conductivity and optical transmittance. Nevertheless, indium is scarce and expensive, hence, it would be desirable to find a cheaper and more abundant TCO material that could replace ITO. Aluminum doped zinc oxide (AZO) stands as one of the most suitable candidates due to its earth abundance, high stability, and non-toxicity. Despite its advantages, AZO suffers from low spatial uniformity when it is grown as a thin film using sputter deposition. This issue has been attributed to the bombardment of the film by high-energy negative oxygen ions generated at the sputter target and accelerated in the plasma region between the target and the substrate [1].

In this study, the spatial uniformity of the key properties of sputtered AZO films was studied as a function of the discharge type (DC, pulsed-DC, and RF), and sputtering parameters. A ceramic target containing 2 wt % of Al was used for all experiments. Usually, these kind of depositions are performed using rotation of the substrate to seek uniformity, in this study however, the rotation of the substrate was stopped for a better mechanism growth understanding, obtaining a thickness gradient.

Both pulsed-DC and RF discharges produced films possessing high electrical conductivity (4x10^-4 Ω.cm) and an average optical transmittance above 85% when the substrate temperature was above 100°C. All films deposited at room temperature presented lower electrical conductivity and lower transmittance. A DC discharge led to inferior film properties for TCO applications.

Films deposited at room temperature and using Pulsed-DC, and DC discharges showed an edge effect with an increase of the sheet resistance in the wafer region that is closer to the target, therefore, low distance from target to substrate (DTS). This undesirable characteristic was improved once the temperature of the substrate increased. Films deposited using RF discharge showed the best performance regarding optical and electrical properties.

[1] – K. Norrman, P. Norby and E. Stamate, J. Mater. Chem. C 10 (2022) 14353.

Frost damage can be classified into two types: white frost, which occurs when water vapor sublimates on the surface of an object to form ice, and water frost, which occurs when water droplets condense and solidify. Currently, there are no excellent sensors that can detect frost, and frost can only be observed visually, which makes field observations difficult and makes the actual state of frost damage unresolved. The presenters have developed a sensor that can detect water and adsorbed water molecules by measuring the electric current that flows spontaneously due to galvanic action when an aggregate of water or adsorbed water molecules comes into contact with adjacent arrays of alternating thin wires made of different metals at regular intervals (minimum value 100 nm) on an insulating substrate. Previous results have shown that the current peaks appear before and after the water undergoes a supercooled state and solidifies, indicating that frost formation detection may be possible, and furthermore, the current peaks and microscopic solidification process have been clarified. As a response to water solidification, the current response is decreasing despite the growth of water droplets. The authors believe that the temperature dependence of the conductivity of water is responsible for the decrease in the current response. However, although the temperature dependence of conductivity in water has been clarified, the temperature dependence of conductivity in the supercooled state of water has not been clarified.

In this study, we aim to clarify the temperature dependence of conductivity in supercooled water by cooling the sensor surface temperature step by step from 0°C and by clarifying the sensor response behavior.

Harvesting the Gibbs free energy contained in the salinity gradient of waste organic solutions will not only relieve the environmental burden but also render a new clean energy resource to accomplish the never-ending energy demand. Taking inspiration from the electrocytes in the bioelectricity systems (e.g. electrical eel) which possess numerous subnanoscale rectified ion channels acting as an ion-selective filter allowing unidirectional ion transport, we sought a feasible strategy to design an ionic-diode membrane, ZIF-8/PSS@ANM, consisted of a continuous layer of the subnanochannel zeolitic imidazolate framework-8 (ZIF-8)/polystyrene sulfonate (PSS) and a highly ordered aluminum nanochannel membrane (ANM), as osmotic energy conversion generator. The SEM results indicate that a large-scale, continuous, defect-free ZIF-8/PSS membrane was successfully prepared. Furthermore, the BET result verifies that the as-synthesized MOF membrane possesses subnanoscale (~4.1 Å) channel windows with a high surface area (~1290 m²/g). The as-developed ZIF-8/PSS₂@ANM demonstrates a vivid diode-like ion current rectification effect even in methanol solutions with a ratio as high as ~5.57 in 1 mM LiCl (Fig. 1a), enabling the ion transport magnification at the subnanoscale confinement, due to the multiple broken symmetries (Fig. 1b). We, thereby, probe the application of this subnanoscale ionic-diode membrane in osmotic energy conversion, and exceptionally, an unprecedented osmotic power density of ~5.28 W/m² at 50-fold LiCl gradient in methanol was achieved (Fig. 2a), exceeding the bandgap of commercial benchmark value. The unbelievable osmotic power achieved can be plausibly elucidated to be associated with the space charges carried by PSS enhancing the ionic selectivity and accelerating ion migration, abundant ordered subnanoscale window-cavity channels of the ZIF-8 for screening dehydrated cations (Fig. 2b), and ionic-diode effect for amplifying the generated ionic current. The heterogeneous subnanochannel MOF membrane we designed will likely ignite valuable insight not only to help alleviate the environmental burden but also to open up a new avenue towards a new energy platform for meeting the need of the ever-growing energy demand.

In recent years, metal silicides have been widely used in ultra-large scale integrated-circuit (ULSI). Shallow junction generation has always been a major challenge with ever-shrinking device dimensions, and size effects must be overcome for silicide generation, thermal stability, and electrical properties. Due to the size effects, evolution of metal silicides from TiSi₂, CoSi₂, to NiSi has been used as the source material for the formation of a low-resistivity form of metal silicide on top of the gate and source/drain for connecting the tungsten contact plug. However, characteristics of TiSi₂ have led to it having the potential to be a plug/interconnect material in nanoscale devices.

Because the TiSi₂ used in logic device is normally fabricated using a self-alignment silicide process, the process includes Ti/TiN deposition, first-step rapid thermal annealing (RTP-1), strip of unreacted Ti/TiN, second-step rapid thermal annealing (RTP-2), and final-step annealing for smoothing the top-cap dielectric. The process needs to be optimized using a design of experiment method. The influence of arsenic doping dosage, the thickness of titanium, the temperature of rapid thermal annealing on the sheet resistance of a polysilicon gate was experimentally analyzed. Experimental results revealed that thickness of titanium, the temperature of RTP-2, and the interaction between the thickness of titanium and the temperature of RTP-2dominated the sheet resistance of TiSi₂. An optimum RTP-2 temperature was also required for reducing sheet resistance of TiSi₂. A low sheet resistance was yielded for titanium thickness = 32-35 nm, RTP-1 = 720-750°C for 75 sec, and RTP-2 = 860°C for 20 sec. Heavily doping of the polysilicon gate with arsenic suppressed the formation of C54-TiSi₂. The lowest sheet resistance of 3.91 Ω/sq. was obtained with an arsenic dosage of 1´10¹⁴ /cm². The characteristics of the TiSi₂ supports its capability as a contact material for next generation devices.

Keywords: Titanium self-aligned silicide, TiSi₂, Sheet resistance, Designing experimental

The article describes the study of electrical properties investigations of solar cells based on organic p-n junction. Organic layer systems will be produced using spin-coating method with starting material in form of mixture of polymeric materials: Poly[2,6-(4,4-bis-(2-ethylhexyl)-4Hcyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7(2, 1,3-benzothiadiazole)] - PCPDDTBT) and poly[2,5-bis(2-octyldodecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione-3,6-diyl)-alt-(2.2';5',2'';5'',2'''-quaterthiophene-5.5'''-diyl)] - DPP4T. The p-n heterojunction materials were selected on the basis of gap matching component acting as an electron donor to a wavelength of 700 nm, corresponding to the maximum photon density in the spectrum of sunlight (about 1.8 eV) to the acceptor material. Acceptor material was selected on the basis of the band gap width and the electron affinity energyionization energies suitably greater than the electron affinity of the excited state of the donor, and ionization potential, ensuring dissociation of the exciton and transfer of the electron from the LUMO orbital of the donor into the LUMO orbital of the acceptor. The investigations presented in article included the production of organic solar cells (OSC) with bulk heterojunction (BHj) and determination ofthe structure and morphology of the deposition layers, chemical composition analizses, optical and electrical properties of the BHj thin films and I–V characteristics of created OSC using PV Test Solar Cell I–V Tracer System and Keithley 2410 source meter under Standard Test Conditions (AM 1.5, 100 W/m2 ). The conducted basic research brings knowledge of controlling the structure and properties of the thin films of the semiconducting organic material (containing bulk heterojunctions). Analysis of the results of electrical properties testing will allow for a thorough examination mechanisms of electronic transitions, electron-electron and electron-phonon interactions in p-n heterojunctions combining organic materials.

Various two-dimensional (2D) van der Waals systems including graphene, hBN, MoS₂, WS₂, and topological insulators form heterostructures with the high quality in atomic-layer scale. Understanding the growth kinetics of the layered heterostructure films is essential to control the atomic layer growth. We studied the growth kinetics of Bi₂Se₃ film on graphene by using AFM images and DFT calculations as well as by in-situ x-ray scattering.

During growth of 2D materials, abrupt growth of multilayers is practically unavoidable even under well-control. Delicate control of growth reaches its limits for complicated crystal structure. In epitaxial growth of Bi₂Se₃ thin film, we observe that the multilayer growth pattern deduced from in-situ x-ray diffraction requires nontrivial interlayer diffusion process. We expect that an intriguing diffusion process occurs at step edges where a slowly downward-diffusing Se adatom having a high step-edge barrier interacts with a Bi adatom pre-existing at step edges. The Se−Bi interaction lowers the high step-edge barrier of Se adatoms. This drastic reduction of the overall step-edge barrier and hence increased interlayer diffusion modifies the overall growth significantly. Thus, a step-edge-barrier reduction mechanism assisted by hetero adatom-adatom interaction could be widely utilized for multilayer growth of 2D heteroatomic materials.

KEYWORDS: heteroatom epitaxial growth, kinetic multilayer growth model, step-edge barrier

Single crystal Ti₂O₃ with a trigonal corundum structure exhibits a metal to insulator transition (MIT) between 400K and 500K without a structural phase transition. Upon cooling Ti₂O₃ undergoes a transition from metallic state to a nonmagnetic insulating state showing a ultranarrow bandgap (~0.1eV). Compared to other MIT oxides such as V₂O₃ and VO₂ that undergoes structural phase transition during MIT, the Ti₂O₃ shows pure electronic MIT process without having a structural transition. This purely electronic MIT is unique and would be useful for many electronic and photonic devices. We have deposited epitaxial corundum structured Ti₂O₃ films on c-plane sapphire substrates using pulsed laser deposition and investigated their structural, electrical, and optical properties as a function of the film growth parameters. We have found that a MIT temperature is varied with a growth temperature and the MIT is suppressed when the films are grown at 480 °C, showing conducting behavior at all temperatures. This metallic ground states were further investigated by X-ray diffraction and spectroscopic ellipsometry measurements to provide crystal structure and broadband optical properties of Ti₂O₃ films. Results show that the electrical properties are governed by the lattice parameter ratio(c/a) of crystal structure and the imposed strain causes an increase in the c-axis length as the temperature is decreased, and thereby suppresses the MIT. We will present details of the deposition conditions on the structural, electronic, and optical properties of Ti₂O₃ films.

This work was supported by the Office of Naval Research (ONR) through the Naval Research Laboratory basic research program.

Tungsten trioxide (WO₃) films are of great interest due to their electronic properties, which can be tuned by surface nanostructuring leading to enhanced efficiency for gas sensing, energy storage or electrochromic applications. Resistive heating of tungsten (W) filaments at pressures of few Pa in an oxygen O₂ atmosphere has already demonstrated its capability to form porous, micro/nano-structured, cheap and fast films making it suitable for industrial applications.

In this work, stoichiometric WO₃ films were produced by applying a current into a W filament in an O₂ atmosphere. The pressures were varied from 2 to 20 Pa. The increase of the pressure above 7.5 Pa led to amorphous WO₃ films with a filamentous morphology. As a function of the pressure, the film morphology and the work functions (W_F) were analyzed using Scanning Electron Microscopy (SEM) and in-situ Ultraviolet Photoelectron Spectroscopy (UPS). In addition to the high surface-to-volume ratio of the films, the W_F exhibited an increase from 5.8 eV, for a conventional WO₃ film, to a maximum of 8.7 eV at 20 Pa. This change corresponds to an increase of the W_F of about 50 %, making our films suitable for a large variety of applications.

Noble metals have long been known to be an excellent basis for electrocatalysts. While the effectivity of catalysts depends on the reaction they are used for, studies have shown that for the oxygen reduction reaction (ORR), Au (100) is the most active face of Au in alkaline media. This work investigates magnetron sputtered epitaxial Au-films on MgO (100) for electrocatalysis. We show that the deposition parameters and their effect on the surface morphology are a key factor to optimize catalytic activity. We further explore various surface treatment methods to improve the adhesion of Au as well as its surface morphology without the use of a transition metal seed layer. The samples are characterized by atomic force microscopy, X-ray diffraction and cyclic voltammetry to establish a correlation between the surface topography and electrocatalytic activity.

This research is divided into two parts. Both use the hydrophobic small molecule as a double-layer hole transport material provided by Professor Yung-Chung Chen from Kaohsiung University of Science and Technology. In the first part, we investigate the effects of the three p-type small molecules (CL-1~CL3) with tetraphenylethylene as the core and different aromatic rings attached to the side chain. The side chains are benzene, naphthalene, and pyrene. The tetraphenylethylene core has highly distorted nature, even without alkyl solubilizing groups, it can still have good solubility, and then through the modification of side chain groups, the energy level and hole mobility can be fine-tuned. Under the condition of AM1.5, NiOX/ CL-3 double-layer hole transport layer has the best power conversion efficiency of 20.15% in the trans-structured perovskite solar cell.

ZnO has semi-conductivity and piezoelectricity at the same time that makes it a promising material for piezotronics. Zinc Oxide (ZnO) nanorod array was grown on silicon substrates by a hydrothermal method. From SEM top-view image, well aligned ZnO nanorods were deposited on the silicon substrate. The XRD patterns showed that the nanorods behaved highly (002) oriented. Resonant Raman spectroscopy revealed that the degree of (002) orientation was decreasing with raising the vanadium concentration of growth solution. The Photoluminescence spectrum showed typical ZnO UV emission and the 6% sample has an obvious red shift.