Low-temperature Thin Films Research Articles

Effects of Grain Size on Various Electrical Trap Extraction Methods in Low-Temperature Polysilicon Thin Films

Effects of Grain Size on Various Electrical Trap Extraction Methods in Low-Temperature Polysilicon Thin Films

Low-loss and low-temperature Al2O3 thin films for integrated photonics and optical coatings

Amorphous aluminum oxide (Al2O3) is a key material in optical coatings due to its notable properties, including a broad transparency window (ultraviolet to mid-infrared) and excellent durability. Moreover, its higher refractive index contrast relative to silica cladding layers and high solubility of rare-earth ions make it well suited for optical waveguides and the development of various functionalities in integrated photonics. In many coatings and integrated photonics applications, the substrates are temperature and stress sensitive, while relatively thick (∼1 μm) alumina layers are required; thus, it is crucial to fabricate low optical loss alumina thin films at low deposition temperatures, while maintaining high deposition rates. In this study, plasma-assisted reactive magnetron sputtering, operated in an alternating current mode, is investigated as a reliable, straightforward, and wafer-scale compatible technique for the deposition of high optical quality and uniform Al2O3 thin films at low temperature. One-micrometer-thick amorphous Al2O3 planar waveguides, deposited at 150 °C and a rate of 23.3 nm/min, exhibit optical losses below 1 dB/cm at 638 nm and as low as 0.1 dB/cm in the conventional optical communication band.

Three-type precursors for low-temperature solution-processed void-and-crack-free copper(I) iodide films: comparison of electrical conductivities and optical transparency

Abstract Copper(I) iodide is a wide-bandgap (colorless) p-type semiconductor with a high Seebeck coefficient. Although copper(I) iodide is promising for fabricating transparent thermoelectric devices and hole-transfer layers of solar cells, the insolubility in common solvents due to 3-dimensional coordination networks has been a drawback to constructing low-temperature solution-processed thin films. Moreover, it is challenging to fabricate void-and-crack-free copper(I) iodide thin films through a convenient spin-coating process. In limited solvents of acetonitrile and diethyl sulfide, copper(I) iodide is dissolved by forming soluble copper(I) iodide complexes; however, void-and-crack-free copper(I) iodide thin films have never been prepared. In this study, we report that copper(I) iodide–alkanolamine complexes are soluble in alcohols and the spin-coated complexes undergo thermal decomposition to a copper(I) iodide thin film at moderately low temperatures until 150 °C. We discover that the copper(I) iodide–alkanolamines show different properties such as solubility and melting/decomposition temperatures depending on their structures. Specifically, by using 1-amino-2-propanol, we obtain void-and-crack-free and transparent copper(I) iodide thin films with controlled thicknesses of >50 nm. The conductivity, carrier density, mobility, and Seebeck coefficient of the copper(I) iodide thin film are 9.35 S·cm−1, 6.38 × 1019 cm−3, 0.96 cm2·V−1·S−1, and 192 µV·K−1, respectively.

Mechanistic insights into low-temperature epitaxial growth of aluminum nitride films on layered transition metal dichalcogenides

It has been demonstrated that the WS2 monolayer is an excellent template for AlN epitaxy at 400 °C low temperature. Low-temperature AlN thin films exhibit much superior crystalline quality than those grown directly on sapphire substrates. In addition to the small lattice mismatch between AlN and WS2 monolayer, we proposed a growth mechanism to explain the excellent van der Waal epitaxy by looking at the initial growth. This growth model reveals that transition metal dichalcogenides (TMDCs) are promising buffer layers for the deposition of III-nitrides but also suggests the novel combination of AlN and TMDCs in the research of future 2D field-effect transistors due to the extremely low leakage current of high-quality AlN films.

Low-Temperature CVD-Grown Graphene Thin Films as Transparent Electrode for Organic Photovoltaics

Good conductivity, suitable transparency and uniform layers of graphene thin film can be produced by chemical vapour deposition (CVD) at low temperature and utilised as a transparent electrode in organic photovoltaics. Using chlorobenzene trapped in poly(methyl methacrylate) (PMMA) polymer as the carbon source, growth temperature (Tgrowth) of 600 °C at hydrogen (H2) flow of 75 standard cubic centimetres per minute (sccm) was used to prepare graphene by CVD catalytically on copper (Cu) foil substrates. Through the Tgrowth of 600 °C, we observed and identified the quality of the graphene films, as characterised by Raman spectroscopy. Finally, P3HT (poly (3-hexylthiophene-2, 5-diyl)): PCBM (phenyl-C61-butyric acid methyl ester) bulk heterojunction solar cells were fabricated on graphene-based window electrodes and compared with indium tin oxide (ITO)-based devices. It is interesting to observe that the OPV performance is improved more than 5 fold with increasing illuminated areas, hinting that high resistance between graphene domains can be alleviated by photo generated charges.

Electrical properties of low-temperature SiO2 thin films prepared by plasma-enhanced atomic layer deposition with different plasma times

Silicon dioxide (SiO2) thin films were prepared by plasma-enhanced atomic layer deposition (PEALD) at a low temperature of 150 °C using di-isopropylaminosilane and oxygen with different plasma times. While SiO2 films deposited with a short plasma time of 0.5 s exhibited high leakage current, SiO2 films deposited with a plasma time of 7 s at 150 °C showed excellent dielectric properties, including a low current density of 4.8 × 10−9 A/cm2 at 1 MV/cm and a high breakdown field of 10.5 MV/cm, comparable to those of PEALD-SiO2 films deposited at 350 °C. As the plasma time increased from 0.5 to 7 s, the dielectric constant of SiO2 films decreased from 7.5 to 4.0, which was close to the value of stoichiometric SiO2. Appropriate conduction mechanisms of these SiO2 films with differing electrical characteristics by plasma time were examined. Analyses by x-ray photoelectron spectroscopy and secondary ion mass spectrometry revealed that the quality of SiO2 films largely depended on the amount of defects such as hydroxyl and hydrogen-related species generated by low-temperature deposition.

Low-temperature and high-performance ZnSnO thin film transistor activated by lightwave irradiation

Ultra-high-resolution and flexible display applications require high-performance oxide thin film transistors (TFTs) fabricated at low temperatures. This study fabricated low-temperature and solution-processed high-mobility ZnSnO (ZTO) thin films and TFTs by a lightwave-activation annealing process. The effect of Sn content on the microstructures and electrical properties of lightwave-activated ZTO (LW-ZTO) films and TFTs was investigated. The lightwave-activation process triggered the efficient decomposition of chloride-based precursors and enhanced the performance of ZTO TFTs at a low temperature (230 °C). The optimized ZTO TFTs exhibited excellent electrical properties, including a high saturation mobility of 45.1 cm2/Vs and an on-current/off-current ratio of 105-106 at a low operating voltage of 3 V. Furthermore, the LW-ZTO devices demonstrated high operational stability with a small Vth shift of 0.231 V under bias stress for 5400 s. The performance of the ZTO TFTs was attributed to the low-temperature lightwave-activation process, stemming from the synergic effects of light and heat. These promising results highlight the potential applications of low-temperature lightwave-activated oxide transistors in next-generation cost-effective and low-power flexible electronics.

Enhanced Interfacial Integrity of Amorphous Oxide Thin-Film Transistors by Elemental Diffusion of Ternary Oxide Semiconductors.

Low-temperature solution-processed oxide semiconductor and dielectric films typically possess a substantial number of defects and impurities due to incomplete metal-oxygen bond formation, causing poor electrical performance and stability. Here, we exploit a facile route to improve the film quality and the interfacial property of low-temperature solution-processed oxide thin films via elemental diffusion between metallic ion-doped InOx (M:InOx) ternary oxide semiconductor and AlOx gate dielectric layers. Particularly, it was revealed that metallic dopants such as magnesium (Mg) and hafnium (Hf) having a small ionic radius, a high Gibbs energy of oxidation, and bonding dissociation energy could successfully diffuse into the low-quality AlOx gate dielectric layer and effectively reduce the structural defects and residual impurities present in the bulk and at the semiconductor/dielectric interface. Through an extensive investigation on the compositional, structural, and electrical properties of M:InOx/AlOx thin-film transistors (TFTs), we provide direct evidences of elemental diffusion occurred between M:InOx and AlOx layers as well as its contribution to the electrical performance and operational stability. Using the elemental diffusion process, we demonstrate solution-processed Hf:InOx TFTs using a low-temperature (180 °C) AlOx gate dielectric having a field-effect mobility of 2.83 cm2 V-1·s-1 and improved bias stability. Based on these results, it is concluded that the elemental diffusion between oxide semiconductor and gate dielectric layers can play a crucial role in realizing oxide TFTs with enhanced structural and interfacial integrity.

Alternative Organic Spacers for More Efficient Perovskite Solar Cells Containing Ruddlesden-Popper Phases.

The halide perovskite Ruddlesden-Popper (RP) phases are a homologous layered subclass of solution-processable semiconductors that have aroused great attention, especially for developing long-term solar photovoltaics. They are defined as (A')2(A)n-1PbnX3n+1 (A' = spacer cation, A = cage cation, and X = halide anion). The orientation control of low-temperature self-assembled thin films is a fundamental issue associated with the ability to control the charge carrier transport perpendicular to the substrate. Here we report new chemical derivatives designed from a molecular perspective using a novel spacer cation 3-phenyl-2-propenammonium (PPA) with conjugated backbone as a low-temperature strategy to assemble more efficient solar cells. First, we solved and refined the crystal structures of single crystals with the general formula (PPA)2(FA0.5MA0.5)n-1PbnI3n+1 (n = 2 and 3, space group C2) using X-ray diffraction and then used the mixed halide (PPA)2(Cs0.05(FA0.88MA0.12)0.95)n-1Pbn(I0.88Br0.12)3n+1 analogues to achieve more efficient devices. While forming the RP phases, multiple hydrogen bonds between PPA and inorganic octahedra reinforce the layered structure. For films we observe that as the targeted layer thickness index increases from n = 2 to n = 4, a less horizontal preferred orientation of the inorganic layers is progressively realized along with an increased presence of high-n or 3D phases, with an improved flow of free charge carriers and vertical to substrate conductivity. Accordingly, we achieve an efficiency of 14.76% for planar p-i-n solar cells using PPA-RP perovskites, which retain 93.8 ± 0.25% efficiency with encapsulation after 600 h at 85 °C and 85% humidity (ISOS-D-3).

Low temperature homoepitaxy of (010) β -Ga2O3 by metalorganic vapor phase epitaxy: Expanding the growth window

In this work, we report on the growth of high-mobility β-Ga2O3 homoepitaxial thin films grown at a temperature much lower than the conventional growth temperature window for metalorganic vapor phase epitaxy. Low-temperature β-Ga2O3 thin films grown at 600 °C on Fe-doped (010) bulk substrates exhibit remarkable crystalline quality, which is evident from the measured room temperature Hall mobility of 186 cm2/V s for the unintentionally doped films. N-type doping is achieved by using Si as a dopant, and a controllable doping in the range of 2 × 1016–2 × 1019 cm−3 is studied. Si incorporation and activation is studied by comparing the silicon concentration from secondary ion mass spectroscopy and the electron concentration from temperature-dependent Hall measurements. The films exhibit high purity (low C and H concentrations) with a very low concentration of compensating acceptors (2 × 1015 cm−3) even at this growth temperature. Additionally, an abrupt doping profile with a forward decay of ∼ 5 nm/dec (10 times improvement compared to what is observed for thin films grown at 810 °C) is demonstrated by growing at a lower temperature.

Microstructure and Electrical Properties of Low-temperature Solution-processed Sol-gel KNN Thin Films

We report on an environmentally friendly and versatile chemical solution deposition route to K0.5Na0.5NbO3 (KNN) thin films. The excess amounts of K and Na in KNN precursor solutions was found to be strong influence on perovskite KNN single-phase thin films. It was revealed from Raman spectroscopic analysis data that a change in scattering mode was observed for the KNN thin films fabricated under various processing conditions. This change was due to the chemical composition fluctuation of K and Na in the KNN thin films during heat treatment. The leakage current and ferroelectric properties of the thin films were strongly affected by the excess amounts of K and Na as well. KNN thin films with 20 mol% excess K and Na exhibited a leaky ferroelectric polarization–electric field (P–E) hysteresis. Leakage current density of the film was 3.85´10-8 A/cm2 at applied field of -60 kV/cm.

Low-temperature ZrO2 thin films obtained by polymeric route for electronic applications

In this work, zirconium oxide (ZrO2) films obtained by the polymer-assisted chemical solution method were evaluated. Thin films are obtained using dip-coating with dipping rates from 1 to 100 mm/min, and annealing temperatures from 150 to 450 °C. The films present amorphous structure even with annealing at 450 °C, bandgap of 5.4 eV and a non-porous surface. The zirconia films were electrically characterized applied to a metal–insulator–metal capacitor (MIM-c) configuration. The dielectric layer presents high capacitance and impedance, highly dependent on the dipping rates and annealing temperature. The outcomes from the MIM-c investigated demonstrate that this low-temperature zirconia film may be an alternative for application in flexible electronic devices as insulating layer. More interesting results like higher capacitance and higher operation frequencies are obtained for zirconia layers obtained at 350 °C due to better dipole formation in the film enhanced by the thinner films and a better elimination of polymeric ligands in the film.

Low-Temperature Solution Approaches for the Potential Integration of Ferroelectric Oxide Films in Flexible Electronics.

This technical review presents the state of the art in low-temperature chemical solution deposition (CSD) processing of ferroelectric oxide thin films. To achieve the integration of multifunctional crystalline oxides with flexible and semiconductor devices is, today, crucial to meet the demands of coming electronic devices. Hence, amorphous metal-oxide-semiconductors have been recently introduced in thin-film electronics. However, their benefits are limited compared with those of ferroelectric oxides, in which intrinsic multifunctionality would make possible multiple operations in the device. However, ferroelectricity is linked to a noncentrosymmetric crystal structure that is achieved, in general, at high temperatures, over 500 °C. These temperatures exceed the thermal stability of flexible polymer substrates and are not compatible with those permitted in the current fabrication routines of Si-based devices. In addition, the manufacturing of flexible electronic devices not only calls for low-temperature fabrication processes but also for deposition techniques that scale easily to the large areas required in flexible devices. In this regard, CSD processes are the best positioned today to integrate metal oxide thin films with flexible substrates as a large-area, low-cost, high-throughput fabrication technique. Here, we review the progress made in the last years in fabricating at low-temperature crystalline ferroelectric oxide thin films via CSD methods, highlighting the recent work of our group in the preparation of ferroelectric oxide thin films on flexible polyimide substrates.

Low-temperature crystalline lead-free piezoelectric thin films grown on 2D perovskite nanosheet for flexible electronic device applications

A monolayer of Sr2Nb3O10 (SNO) is deposited on the Pt/Ti/SiO2/Si (Pt-Si) or Pt/Ti/polyimide (Pt-PI) substrate by using the Langmuir-Blodgett method and employed as a seed-layer for the growth of a crystalline (Na1−xKx)NbO3 (NKN) film at 350 °C. The crystalline NKN film is grown along the [001] direction on the SNO/Pt-Si (or SNO/Pt-PI) substrate. Due to the presence of oxygen vacancies in the SNO seed-layer, the NKN film exhibits low ferroelectric properties and large leakage current. To ameliorate these properties, the SNO/Pt-Si substrate is annealed in a 50 Torr oxygen atmosphere at 300 °C, which removes the oxygen vacancies. Consequently, the NKN film deposited on this substrate exhibits promising electrical properties, namely a dielectric constant of 278, dissipation factor of 1.7%, a piezoelectric constant of 175 pmV−1, and a leakage current density of 6.47 × 10−7 Acm−2 at −0.2 MV·cm−1. Similar electrical properties are obtained from the NKN film grown on the flexible SNO/Pt-PI substrate at 350 °C. Hence, the NKN films grown on the SNO seed-layer at 350 °C can be applied to electronic devices with flexible polymer substrates.

High-performance energy storage and breakdown strength of low-temperature laser-deposited relaxor PLZT thin films on flexible Ti-foils

The microstructure, ferroelectric, electric-field breakdown strength, and energy-storage properties of relaxor Pb0.9La0.1(Zr0.52Ti0.48)O3 (PLZT) thin films grown on flexible Ti foils using pulsed laser deposition were systematically investigated. Low temperature deposited PLZT thin films showed very slim polarization hysteresis loops with a high difference between maximum and remanent polarizations and low remanent polarization through modulating the film structure with a small columnar-grain size. An ultrahigh recoverable energy density (Ureco) of 40.9 J/cm3, excellent energy efficiency (η) of 80.2% and large breakdown strength (EBD) of 3000 kV/cm were achieved in a PLZT film deposited at the low temperature of 480 °C. More importantly, this film shows excellent charge-discharge cycling endurance with a small variation of both Ureco and η values (less than 3%) after 1010 cycles and good thermal stability under a wide operating temperature range from room temperature to 200 °C. These results indicate that the relaxor PLZT films deposited on thin Ti foils, even at low temperature, are a promising strategy to enhance energy-storage performance for pulse-power energy-storage systems with broad temperature range applications, especially in applications where the device weight is critical (lightweight) due to the thin and low density of Ti foils.

Wet-Chemical Synthesis of ZnO Nanowires on Low-Temperature Photo-Activated ZnO-rGO Composite Thin Film with Enhanced Photoconduction

A homogeneous dispersion of graphene oxide (GO) and zinc acetate was dip-coated and subsequently photo-annealed by deep UV irradiation to form ZnO-rGO (DUV) nanocomposite thin film. ZnO nanowires were then grown by wet-chemical synthesis on the ZnO-rGO (DUV) thin film. For comparison, ZnO nanowires were also grown on bare ZnO and ZnO-rGO (HT) thin films thermally annealed at 500°C. X-ray diffraction and Raman spectroscopy of ZnO-rGO (DUV) thin film revealed crystallization of ZnO and reduction of GO. The photoluminescence spectra of ZnO nanowires grown on ZnO-rGO (DUV) thin film showed high crystallinity. The current–voltage characteristics of ZnO nanowires grown on ZnO-rGO (DUV) thin film in dark and under UV showed a 15-fold increase in photocurrent as compared to ZnO nanowires grown on ZnO and ZnO-rGO (HT) thin films. The low-temperature photo-activated ZnO-rGO thin films can be used as a template to grow one-dimensional ZnO nanowires with improved structural and optical properties and can pave the way for high-performance flexible device fabrication.

Cesium-Doped Vanadium Oxide as the Hole Extraction Layer for Efficient Perovskite Solar Cells.

In this study, we report the utilization of low-temperature solution-processed Cs-doped VOX thin films as the hole extraction layers (HELs) in perovskite solar cells (PSCs). It is found that the VOX:yCs (where y is the mole ratio of Cs versus V and y = 0.1, 0.3, and 0.5) thin films possess better electrical conductivities than that of the pristine VOX thin film. As a result, the PSCs incorporated with the VOX:yCs HEL exhibit large fill factors and high short-circuit currents, with consequently high power conversion efficiencies, which is more than 30% enhancement as compared with pristine VOX HEL. Our studies provide a facial way to enhance the electrical conductivity of the hole extraction layer for boosting device performance of perovskite solar cells.

Low-temperature solution-processed sol-gel K-rich KNN thin films for flexible electronics

ABSTRACTSputtered lead-free piezoelectric materials like potassium sodium niobate (K1-xNaxNbO3 or KNN) have received significant technological interest in recent years in light of several reports of piezoelectric constants comparable to lead zirconium titanate (PZT). Potential applications include self-powered sensors, actuators, and low acoustic impedance transducers. For large area printed applications, it is vital to develop low-temperature solution processed deposition methods. In this work, sol-gel synthesis of K-rich (70:30) KNN was carried out under an argon atmosphere, using acetate precursors, followed by precipitation of white KNN powder upon careful drying. Powder X-ray diffraction (XRD) scans of the product with a Cu Kα source after calcination revealed a dominant (110) peak, accompanied by smaller (100) and (010) peaks, in agreement with published standard KNN data. The composition of K-rich phase was confirmed using energy dispersive X-ray spectroscopy (EDX). To produce thin films, the sol was spin coated on a surface-treated Au-coated Si substrate, followed by slow annealing to obtain low surface roughness films (RMS roughness ﹤∼10 nm) of thickness ∼200 nm after solvent removal. Atomic force microscopy (AFM) scans revealed an unremarkable amorphous film. However, deposition of the sol on the Au-coated backside of Si wafer under similar processing conditions revealed limited polycrystalline film formation observed using optical profilometry. Thin film XRD measurements of the deposited film reveal orthorhombic phase growth of KNN, though the unannealed film was more amorphous than the calcined KNN film. Preliminary piezoresponse force microscopy (PFM) scans were used to estimate a piezoelectric constant (d33) ∼ 2.7 pC/N, consistent with the general expectation of lower piezoelectric constants for thin sol-gel films. The highest processing temperature used at any step during the deposition process was 90°C, consistent with the applications involving flexible polyimide substrates. This low-temperature thin-film growth suggests a potential route towards integration of large area piezoelectric generators for environmentally-friendly autonomous flexible sensor applications, with better control of phase and composition during the solution-phase deposition of KNN.

Analysis of defects in low-temperature polycrystalline silicon thin films related to surface-enhanced Raman scattering

The analysis of Raman scattering (RS) spectroscopy is presented for low-temperature polycrystalline silicon (poly-Si) thin films on glass substrates fabricated by excimer laser crystallization. In this material, RS is enhanced by specific protrusions at the grain boundary (GB). As a result, the Si lattice mode predominantly reflects the characteristics of GB and its neighborhood. A combination of low-damage hydrogenation and RS analysis enables the detection of lattice defects as Si–hydrogen (H) local vibration modes (LVMs). The characteristics of LVMs peculiar to this material are examined by chemical etching and postannealing. One of the dominant LVMs centered at ∼2000 cm−1 is assigned to H-terminated dangling bonds in the amorphous structures at GB, which is also enhanced by protrusions. The other dominant band centered at ∼2100 cm−1 is attributed to the strained Si–Si lattice near the Si/underlayer interface in grains that is broken and stabilized by extrinsic H atoms.

Low temperature thin films for next-generation microelectronics (invited)

In this article the current methodologies for low-temperature thin film deposition in microelectronics are reviewed. The paper discusses the high temperature processes in microchip manufacturing and describes the thermal budget fitting issue. The quest for low temperature deposition techniques is motivated in the perspective of contemporary trends in microchip technology such as 3D integration and the ending miniaturization. Reduced temperature depositions tend to deliver lower quality films. This is illustrated with the relation between deposition temperature and thin dielectric film quality (dielectric strength). Existing and emerging technologies for low-temperature thin film deposition are reviewed with an emphasis on their applicability in microelectronic fabrication.