ICMCTF 2025 Session MB-ThP: Functional Thin Films and Surfaces Poster Session

Light-driven energy conversion devices call for the atomic-level manipulation of defects associated with electronic states in solids. However, previous approaches to producing oxygen vacancy (V_O) as a source of sub-bandgap energy levels have hampered the precise control of distribution and concentration in V_O.

Here, a new strategy to spatially confine V_O at the homo-interfaces is presented by exploiting the sequential growth of anatase TiO₂ under dissimilar thermodynamic conditions. Remarkably, metallic behavior with high carrier density and electron mobility is observed after sequential growth of the TiO₂ films under low pressure and temperature (L-TiO₂) on top of high-quality anatase TiO₂ epitaxial films (H-TiO₂), despite the insulating properties of L-TiO₂ and H-TiO₂ single layers. Multiple characterizations elucidate that the V_O layer is geometrically confined within 4 unit cells at the interface, along with low-temperature crystallization of upper L-TiO₂ films; this two-dimensional V_Olayer is responsible for the formation of in-gap state, promoting photocarrier lifetime (~ 300 %) and light absorption. These results suggest a synthetic strategy to locally confine functional defects and emphasize how sub-bandgap energy levels in the confined imperfections influence the kinetics of light-driven catalytic reactions.

This work is performed by the collaboration with Mr. Minwook Yoon,Dr. Yunkyu Park, Ms. Hyeji Sim, Ms. Hee Ryeung Kwon, Dr. Yujeong Lee, Prof. Ho Won Jang, Prof. Si-Young Choi.

The increasing focus on sustainable energy has driven advancements in renewable technologies, with quantum dot solar cells gaining particular interest in photovoltaics for their ability to efficiently convert sunlight into electricity. Early cells used II-VI semiconductors with high crystallinity and luminescence but were limited by toxicity and complex synthesis.In contrast, all-inorganic perovskite quantum dots such as CsPbX₃ (X=Cl, Br, I) have gained prominence due to their excellent photoelectric properties, low cost, and easy to be manufactured. Moreover, compared to organic-inorganic perovskites, all-inorganic perovskites are more stable under high temperature and with extremely high quantum yield.Consequently, they are gradually becoming mainstream in research and development.

Recent advances in silicon-based hybrid solar cells, distinguished by high photovoltaic efficiency, low production costs, and strong environmental resilience, position them as promising candidates for solar energy conversion.[1] Solution-processed few-layer MoS₂ sheets enhance solar capture, but improved charge separation is essential, and their moisture sensitivity limits stability by attracting electrons. [2] This study introduces MoS₂/Mo₂C/carbon colloid dots (CCDs) heterostructures within a PEDOT:PSS matrix, utilizing Mo₂C electron-transport channels to facilitate the transfer of photoexcited electrons from MoS₂. This configuration fosters positive trion formation via interactions with defect-bound excitons on CCD surfaces, reducing recombination rates and enhancing photovoltaic performance. [3, 4] To elucidate carrier transfer mechanisms in these heterostructures, MoS₂@CCD heterojunctions with Mo₂C bridging interfaces facilitate efficient electron transfer. Photoluminescence (PL) enhancement factors (β) were used to characterize trion emissions across HT interfaces compared to intrinsic trion emissions in CCDs, by analyzing three distinct MoS₂@CCD blends within a PEDOT:PSS matrix, grounded in fundamental transport phenomena. [5] This design achieves a 16.1% efficiency, 1.6 times higher than conventional hybrid solar cells, with outstanding long-term stability, advancing photophysical bound-carrier research in photovoltaics.

[1]T. C. Wei, S. H. Chen, C. Y. Chen, Mater. Chem. Front. 2020, 4, 3302.

[2]G. H. Lee, X. Cui, Y. D. Kim, G. Arefe, X. Zhang, C. H. Lee, F. Ye, K. Watanabe, T. Taniguchi, P. Kim, ACS Nano. 2015, 9, 7019.

[3]T. Kim, S. Fan, S. Lee, M.-K. Joo, Y. H. Lee, Sci. Rep. 2020, 10, 13101.

[4]A. Behranginia, P. Yasaei, A. K. Majee, V. K. Sangwan, F. Long, C. J. Foss, T. Foroozan, S. Fuladi, M. R. Hantehzadeh, R.

Shahbazian‐Yassar, Small. 2017, 13, 1604301.

[5]L. Zhang, A. Sharma, Y. Zhu, Y. Zhang, B. Wang, M. Dong, H. T. Nguyen, Z. Wang, B. Wen, Y. Cao, Adv. Mater. 2018, 30, 1803986.

Colloidal quantum dots (QDs) are semiconductor nanoparticles composed of a core, shell, and organic ligands. They have unique optical and electrical properties due to quantum confinement effects, which enable the bandgap to vary with particle size. This characteristic allows easy modification of emission wavelengths, producing various colors of light. QDs are compatible with solution process and notable for their narrow full-width at half-maximum for the high color purity. Due to these advantages, quantum dot light emitting diodes (QLEDs) that use QDs as light emitting layers are being recognized as a promising next-generation display technology. In the field of AR/VR devices, Organic Light Emitting Diode on Silicon (OLEDoS) has received significant attention recently. This technology uses silicon as a substrate and emits light from the top with micropatterned structure, thus research on top-emitting devices is essential. However, there is still limited research on QLEDs in this area.

We investigated the influence of chlorine incorporation on solution-processed amorphous indium-gallium-zinc oxide (a-IGZO) thin-film transistors (TFTs). During TFT fabrication, materials inevitably interact with unintended elements. Notably, chlorine is an essential component in various stages of TFT fabrication, including as a precursor for metal oxide deposition and as a dry etching gas, making exposure to chlorine nearly unavoidable. Therefore, understanding chlorine’s role in affecting the electrical and material properties of TFT devices is essential.

In this study, we immersed a-IGZO films in NaCl solution to incorporate chlorine. X-ray photoelectron spectroscopy (XPS) analysis revealed that chlorine formed bonds with metals, increasing both metal-oxygen (M-O) bonds and oxygen vacancies (Vo). Additionally, we observed a degradation in IGZO’s electrical performance, attributed to structural bonding changes due to chlorine incorporation. Our initial findings indicated that the electrical properties deteriorated as the NaCl soaking time increased. To verify whether these effects were solely due to water exposure, we also examined the electrical properties of a-IGZO films soaked in deionized water. Compared to the pristine device, the saturation mobility and subthreshold slope of a-IGZO TFTs soaked in water for 1 hour decreased from 0.17 to 0.07 cm²/Vs and increased from 0.79 to 1.11 V/decade, respectively. However, when soaked in NaCl solution, these values further degraded to 0.02 cm²/Vs and 2.08 V/decade, respectively, confirming that chlorine penetration, rather than water exposure alone, caused the observed degradation. This degradation was associated with an increase in carrier concentration, corresponding with a widening bandgap from 3.46 eV to 4.29 eV. However, XRD analysis showed that soaking in NaCl solution did not alter the film's crystallinity. Furthermore, we examined the impact of chlorine diffusion on IGZO films deposited via the sputtering process. These findings suggest that chlorine exposure during fabrication must be carefully controlled to achieve the desired electrical performance targets for a-IGZO TFTs.

All-solid-state supercapacitors represent a promising advancement in energy storage technology, providing superior energy density, enhanced safety, and a compact design compared to conventional supercapacitors and batteries. Their potential as flexible, bendable, and wearable energy storage solutions have garnered significant interest. The capacitance and energy density of supercapacitors can be improved through the introduction of novel electrode materials or by utilizing electrolytes with high potential windows. In this study, we successfully fabricated a high-quality porous thin film of tungsten diselenide (WSe₂) on a flexible graphite substrate using an environmentally friendly DC magnetron sputtering technique, without the need for additives or binders, under optimized conditions. The resulting thin film exhibited a nanoflake morphology with an increased surface area, which provided a greater number of active sites for ion adsorption and desorption, thereby enhancing both capacitance and energy density. The WSe₂@graphite composite demonstrated a remarkable specific capacitance of 310 F/g at a scan rate of 10 mV/s, with 95% capacitance retention after 5000 charge-discharge cycles in a three-electrode configuration. An all-solid-state flexible symmetric supercapacitor (FSS) device was subsequently constructed, utilizing WSe₂ as both the cathode and anode, separated by a highly flexible 6M KOH/PVA solid-state gel electrolyte. This device achieved a high cell voltage of 1.4 V and an excellent specific capacitance of 38.932 F/g at a scan rate of 50 mV/s. Comprehensive electrochemical performance analyses, including charge-discharge measurements at varying current densities, revealed a specific capacitance of 17 F/g, an energy density of 4.62 mWh/g, and a power density of 3457 mW/g, along with outstanding electrochemical stability of 92% after 5000 cycles at a current density of 5 mA/g. The exceptional electrochemical performance, combined with the flexible characteristics of the WSe₂@graphite thin-film-based symmetric supercapacitor, positions this device as a promising candidate for the development of next-generation flexible, bendable, and wearable energy storage systems.

Colloidal quantum dots (QDs) are semiconductor nanoparticles with unique optical and electrical properties. By controlling particle size, QDs can exhibit various colors and provide excellent color reproducibility. Due to these advantages, quantum dot light-emitting diodes (QLEDs) using QDs as the emissive layer are studied actively. In QLEDs, the electron transport layer (ETL) is essential for electron transport and charge balance, and optimizing ETL can enhance device stability and efficiency. In general, ZnO nanoparticles (NPs) are commonly used as ETL for their high electron mobility and transmittance. However, ZnO NPs aggregate easily at room temperature, leading to reduce stability. Therefore, SnO₂, which offers high electron mobility, transmittance, and excellent stability, is gaining attention as an ETL material. Typically, the ETL is deposited via solution processes like spin coating, but this method has challenges such as difficulty in thickness control, poor crystallinity and uniformity of the thin films. In this study, we deposited SnO₂ as the ETL using RF sputtering process for high reproducibility and excellent crystallinity. It is well known that crystallinity of inorganic materials are directly related to their electrical properties. To adjust the physical and chemical properties of SnO2thin film, we controlled the substrate temperature and Ar/O2ratio during RF sputtering while fabricating inverted devices with the structure of ITO/SnO₂/QDs/CBP/MoO₃/Al. As the substrate temperature increased, the crystallinity of sputtered SnO₂ thin film improved, which leaded the enhancement of electron mobility and improvement of electrical properties of devices. QLEDs employing the optimized SnO₂ ETL exhibited more than 120,000 cd/m² and a current efficiency of 15 cd/A which showed comparative performance with QLEDs using soluble SnO2NPs as an ETL. Additionally QLEDs with sputtered ETL showed better stability due to the uniform SnO₂ layer, which is advantage for practical display mass production.

Plasma-resistant Y₂O₃ coating is essential for extending the durability and replacement cycles of semiconductor components that face intense etching conditions. Plasma etching typically involves both physical ion bombardment and chemical reactions with surface. To counter these effects, recent advancements in Y₂O₃ coating focus on enhancing etch resistance and film density through physical vapor deposition (PVD) methods. While several studies have aimed to further improve the plasma resistance of PVD Y₂O₃ coatings by increasing hardness, our observations suggest that beyond a certain hardness threshold (>900 HV), the relationship between hardness and plasma resistance became weak. Consequently, this study focuses on the characteristics of residual surface stress as a primary factor influencing plasma resistance. The residual stress in the coating was measured using X-ray diffraction (XRD) equipment and calculated based on the peak shift observed with varying psi angles. Comparing residual stress and plasma resistance in PVD Y₂O₃ coatings manufactured under identical conditions, we found that coatings with tensile surface stress exhibited approximately 25% better plasma etch resistance than those with compressive stress. Although both coatings displayed similar grain size and hardness, the superior plasma-resistant coating demonstrated a tensile surface stress of around 600 MPa, whereas the less resistant sample had a compressive stress of approximately 300 MPa. This enhanced resistance in tensile-stressed coatings can be attributed to channeling effects, where the increased atomic spacing prevents accelerated plasma ions from interacting directly with atoms, allowing them to pass through specific crystallographic directions without obstruction. This study aims to establish a better understanding of the correlation between surface residual stress and plasma etch resistance in PVD Y₂O₃ coatings and to propose new criteria for evaluating such coatings, ultimately contributing to enhanced performance in etching equipment.

Aluminium oxide (Al₂O₃) is a well-known insulating material employed in a wide range of applications, both as structural component as well as in thin film form. Al₂O₃ can be stabilized in several polymorphs, in addition to an amorphous modification. Especially the amorphous state of Al₂O₃ exhibits interesting features, considering the absence of crystalline defects for diffusion of charge carriers paired with the difficulties in stabilizing crystalline Al₂O₃ during physical vapor deposition (PVD). Furthermore, amorphous materials are free of pinholes, which is favourable for a number of applications. Consequently, it is crucial to investigate economically and sustainably viable deposition techniques to grow insulating Al₂O₃ thin films.

Therefore, this study focuses on the effect of alloying elements such as silicon and yttrium-zirconium (YZr) on the thermal stability of amorphous Al₂O₃ based thin film materials up to 1200°C. The amorphous Al₂O₃ thin films have been synthesised via a reactive Modulate Pulse Power (MPP) sputtering processes. In all depositions, an in-house developed sputter system, equipped with a 3” Al target, was used in a mixed Ar/O₂ atmosphere. To this end, two types of targets were employed: an Al-Si target and Al-YZr target. The impact of the deposition parameters on the structure, morphology, and electrical resistivity at high temperatures was investigated using high-resolution characterization methods such as XRD, SEM, HR-TEM or in-situ set-ups for annealing treatments. The insulating behaviour of the coatings was analysed using in-situ impedance spectroscopy across a temperature range. Ti/Pt electrode pads were deposited on the thin films using a lithography process for the purpose of electrical characterization. In addition, the bonding type was investigated via XPS, which was also employed to determine the chemical composition across the thickness of the coating.

The increasing expectations and requirements for engineering materials are steadily compelling researchers to evolve and innovate further. Adding transition metals to coating architectures is becoming increasingly attractive as it improves structural and mechanical properties. In this work, CrYN thin films incorporating transition metals Nb, Ta, and V were deposited on a 316L stainless steel substrate using Closed Field Unbalanced Magnetron Sputtering (CFUBMS) with a DC and pulsed-DC power supply. The microstructural properties of the thin films were analyzed using scanning electron microscopy (SEM), while X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) provided a comprehensive understanding of the coating structure by providing information on crystallographic and surface chemical properties. Mechanical properties were evaluated using nanoindentation testing, which provided accurate measurements of hardness and elasticity, while scratch testing assessed critical load values. In addition, four-point bending tests were performed at room temperature to characterize the CrYN:Nb/Ta/V transition metal nitrides (TMNs), providing a more comprehensive analysis of the mechanical behavior (flexural strength and elastic modulus) and adhesion properties of the coating. The mechanisms of coating damage (crack formation and density, spalling, flaking, and separated coating particles) were analyzed as a result of four-point bending tests. The Taguchi approach was employed to investigate how deposition parameters—such as target current, duty cycle, and pulse frequency—affect elastic modulus and bending strength. Superior structural (homogeneous and dense film) and mechanical properties (CrYN:Nb/Ta/V high hardness values of 21.4, 18.2, 16.1 GPa, and bending strengths of 707, 711, and 697 MPa, respectively) were obtained. The positive correlation between hardness and bending strength points to an enhancement in the overall durability of the thin film.

Quantum dots (QDs) are nanometer-sized semiconductor particles, and Quantum Dot Light Emitting Diodes (QLEDs) are electroluminescent devices that use QDs as an emitting layer. As QD size decreases, the quantum confinement effect enhances the discreteness of energy levels, leading to an increased bandgap. Consequently, by manipulating the size of QDs, it is possible to produce various colors of light and enhance color purity by narrow full width at half maximum. ZnMgO NPs, which are currently used as the electron transport layer (ETL) in QLEDs, are actively researched due to their high electron mobility and chemical stability. However, there are inevitable oxygen vacancies in thin films using ZnMgO NPs, which reduce the performance of QLEDs by exciton quenching. In this study, we used ZnMgO NPs as the ETL to fabricate InP QD-based QLEDs, which consisted of multilayers: ITO/ZnMgO/red InP QDs/CBP/MoO₃/Al. First, we formed ZnMgO NPs film on ITO glass and passivate halides on ZnMgO NPs to reduce oxygen vacancies. New Zn-halide and Mg-halide peaks were observed in the x-ray photoelectron spectroscopy. Additionally, photoluminescence (PL) measurements showed that halide-treated ZnMgO NPs exhibited a higher PL intensity compared to untreated ZnMgO NPs. These results indicate that the halide treatment effectively reduces oxygen vacancies in ZnMgO NPs, and its effect was verified with the inverted structured QLEDs. The maximum luminance of QLEDs with halide-treated ZnMgO NPs (h-QLEDs) showed 1,134 cd/m², compared to 696 cd/m² for the QLEDs with pristine ZnMgO NPs (p-QLEDs). After aging for 48 hours in a nitrogen atmosphere, h-QLEDs showed 1,290 cd/m², but the performance of p-QLEDs decreased dramatically to 64.67 cd/m². The experimental results indicated that the halide-treated ZnMgO NPs enhance the optical properties and stability of QLEDs, which can contribute QDs display commercialization.

Magnetoelectric Sensors (ME) have the potential to contribute to sustainable development because of their peculiar features such as lower power requirement, enhance energy efficiency, reduced environmental impact, applications in renewable energy, enable precision agriculture, infrastructure monitoring, health monitoring, optimize waste management, and contribution to overall resource conservation. The present study reports the fabrication of highly flexible, cost-effective, nano-structured magnetic field sensor comprising AlN/Ni-Mn-In ME heterostructure fabricated over magneto strictive Ni foil. The ultra-low magnetic field up to or less than ~1 µT has been easily detected from the fabricated sensor. Further the surface acoustic wave (SAW) delay line-based piezo resonator has been fabricated over highly flexible AlN/Ni-Mn-In/Kapton for flexible MEMS application. The fabricated device resonates at ~1.40 GHz. The effect of the external magnetic field on the resonance frequency (fR) of the device has been investigated and tunability (∆fR/fR) ~9% was observed. The device displays high sensitivity of ~0.94 Hz/nT at room temperature. The alteration in the fR can be attributed to the ∆E-effect in the AlN/Ni-Mn-In heterostructure. The flexibility of the fabricated magnetic field sensors has been investigated in terms of the bending cycles and bending angle. The sensor characteristics remain unchanged up to ~ 2500 bending cycles. The integration of these novel ferromagnetic shape memory alloy (FSMA, i.e., Ni-Mn-In) based flexible piezo-resonator into various systems can enhance efficiency, reduces environmental impact, and contributes significantly to the overarching goal of sustainable development.

Keywords: Ferromagnetic shape memory alloys, flexible magnetic sensor, lead-free piezoelectric, magnetostrictive effect, surface acoustic waves (SAW).

Flexible optoelectronic devices based on transparent substrates are attracting interest for potential applications in wearable sensors, flexible displays and transparent electronic devices. Among various fabrication techniques, magnetron r.f. sputtering stands out for preparation of high-quality films. The principal advantages of this technique are high adhesion, low density of defects, high compatibility with flexible substrates, and control over the deposited material composition.

Flexible photodetectors based on NiO_x deposited by magnetron r.f. sputtering at room temperature and 50°C, using powers in the range of 40-80W over ITO-PET and ITO-PEN substrates were fabricated. The obtained films were evaluated using ellipsometry to determine their thicknesses and optical constants. Thermal evaporation was used to deposit Au as top electrodes.

To assess the performance as photodetectors in the UV-visible spectrum, the fabricated devices were electrical characterized by current-voltage measurements in dark and under the incidence of different light emitting diodes. In addition, the fabricated flexible photodetectors were tested for mechanical and electrical stability after cyclic bending stress.

Diego Lundquist,C. Abinash Bhuyan, Mary Becker, Jennifer Brumley, Sanjay K. Behura*

Department of Physics, San Diego State University, San Diego, CA 92182, United States

Hexagonal boron nitride (h-BN) is a two-dimensional material that has recently been the focus of research for hosting single photon emitters at room temperature. Its spin-optical properties make an ideal candidate for quantum technologies. Existing research relies on exfoliated hBN for its application which restricts its scalability for various practical applications. Also, the current synthesis of chemical vapor deposition grown hBN is on catalytic substrates such as Cu, Al, Ni, and Co. As a result, hBN film is transferred onto desired substrates for photonics applications. This study focuses on synthesizing large-scale hBN film on dielectric substrates, which allows for direct characterization of its high optical properties. Through the optimization of a two-zone, low-pressure chemical vapor deposition (LPCVD), hBN grown directly on SiO₂/Si substrates using the sublimation of ammonia borane complex as precursor and ultra-high pure H₂ as carrier gas. CVD-grown hBN films were characterized to confirm the large-area growth. Typical optical and scanning electron microscopic images reveal the uniform growth of hBN on SiO₂/Si substrate. Raman spectroscopic measurement reveals the signature peaks at 1368 cm⁻¹ which corresponding to E^2g in-plane B-N vibrational modes in hBN.

Topological insulator, such as bismuth telluride (Bi₂Te₃), has narrow bulk band gap and has a Dirac-type surface stat. Thus, it can absorb middle or long-wavelength infrared light and has low resistivity. Graphene, which is a 2D material, exhibits broadband light adsorption and a rapid response due to its Dirac cone structure. Therefore, graphene is an expected material for broadband photodetectors. Its drawbacks are the properties of a high electron-hole recombination rate and a low photoresponse. However, reduced graphene oxide (rGO) has lower electron-hole recombination rate comparing with graphene due to its functional group and edge defects. Silicon has become very popular for many applications because of the unique advantages of CMOS compatibility and high integrated density. Silicon nanowire (SiNW) array has low reflectivity that can raise light harvesting efficiency. Thus, in this study an ultra-broadband photodetector was fabricated by combining with the Bi₂Te₃, rGO and SiNW array.

The silicon substrate is etched using metal-assisted chemical etching to an obtain SiNW array. Then, Bi₂Te₃ was deposited on SiNW array by chemical vapor deposition (CVD). Using photocatalytic reduction to reduce graphene oxide on a SiNW array to form a thin film. Finally, silver electrodes are deposited on both sides of the specimen to create a device. Optical sensing is performed using 940 nm near-infrared light and 5500 nm mid-infrared light.

The results show that, under suitable process conditions using by CVD, Bi₂Te₃ and tellurium (Te) precipitates with a size of approximately 500 nm can be formed on the SiNW array. It can reduce the reflectance of the device in the near- to mid-infrared range (1200–2500 nm). For sensing 940 nm near-infrared light, the light is primarily absorbed by the SiNW array, generating electron-hole pairs that increase carrier concentration and produce photocurrent. The rGO coating can reduce the contact resistance between the electrode and the silicon nanowires, enhancing the responsivity and sensitivity of the photodetector. When irradiated with mid-wavelength infrared light at 5500 nm, the Bi₂Te₃/SiNW array device also exhibits a rapid response characteristic. The reason is that, upon illumination, electron-hole pairs are generated in the Bi₂Te₃ particles. Electrons are conducted through the SiNW to the electrodes, producing a photocurrent in the external circuit. Notably, the rGO/Bi₂Te₃/SiNW device with a mesh-like rGO film, compared to a fully covered rGO film, demonstrates superior responsivity and faster response times in sensing both 940 nm near-infrared light and 5500 nm mid-infrared light.

Flexible and tunable bulk acoustic wave (BAW) resonators are opening new possibilities in flexible MEMS, wireless devices, and wearable magnetic sensors. This work presents a highly adaptable thin-film BAW resonator constructed with a PMN-PT piezoelectric layer positioned between magnetostrictive Ni−Mn−In electrodes on a flexible Ni substrate. This device, operating at a resonance frequency (f_R) of 5.31 GHz, demonstrates significant tunability via both magnetic and electric fields. A magnetic field of 1200 Oe produces an f_R shift of approximately 450 MHz, achieving a sensitivity of around 3.75 Hz/nT and an impressive 9.6% tunability. Similarly, a 10 V DC bias yields an f_R downshift of roughly 360 MHz, with electric field sensitivity and tunability measured at about 36 Hz/µV and 6.8%, respectively. The device’s resonance performance aligns with the modified Butterworth–Van Dyke model, and its quality and reliability remain intact through 3,000 bending cycles, underscoring its potential in advanced flexible electronics, tunable MEMS, and magnetic sensors.