Zinc Tin Oxide Thin Film Research Articles

Tail state mediated conduction in zinc tin oxide thinfilm phototransistors under below bandgap optical excitation

We report on the appearance of a strong persistent photoconductivity (PPC) and conductor-like behaviour in zinc tin oxide (ZTO) thinfilm phototransistors. The active ZTO channel layer was prepared by remote plasma reactive sputtering and possesses an amorphous structure. Under sub-bandgap excitation of ZTO with UV light, the photocurrent reaches as high as ~ 10−4 A (a photo-to-dark current ratio of ~ 107) and remains close to this high value after switching off the light. During this time, the ZTO TFT exhibits strong PPC with long-lasting recovery time, which leads the appearance of the conductor-like behaviour in ZTO semiconductor. In the present case, the conductivity changes over six orders of magnitude, from ~ 10−7 to 0.92/Ω/cm. After UV exposure, the ZTO compound can potentially remain in the conducting state for up to a month. The underlying physics of the observed PPC effect is investigated by studying defects (deep states and tail states) by employing a discharge current analysis (DCA) technique. Findings from the DCA study reveal direct evidence for the involvement of sub-bandgap tail states of the ZTO in the strong PPC, while deep states contribute to mild PPC.

SLOT-DIE-COATED ZINC TIN OXIDE FILM FOR CARBON-BASED METHYLAMMONIUM-FREE PEROVSKITE SOLAR CELLS

At the laboratory scale, the spin coating technique is commonly used for depositing an electron transporting layer (ETL) in perovskite solar cells (PSCs). However, this technique is unsuitable for scaling up production. To enable large-scale deposition, slot-die coating, — a promising scalable technique and low operational cost processing, — has been proposed for depositing thin and uniform films across large areas. In this work, scalable slot-die coating processes of amorphous zinc tin oxide (ZTO) as ETLs are illustrated for carbon-based methylammonium-free PSCs. The amorphous ZTO was selected because of its excellent optical and electronic properties such as high electrical conductivity, high electron mobility, and high transparency. Slot-die-coated ZTO thin films were prepared from a proprietary homemade slot-die setup on a 3-axis computer numerical control (CNC) platform. Various thicknesses of ZTO thin films were utilized by changing the speed of the slot-die head. The device using slot-die-coated ZTO films with a film thickness of 48[Formula: see text]nm was found to exhibit the best device maximum power conversion efficiency of 9.92%, which is comparable to that of the device using spin-coated ZTO film. This work demonstrates the potential of the slot-die coating technique to replace the conventional spin-coating method in the fabrication of high efficiency and scalable PSCs.

Improving the thermoelectric properties of zinc tin oxide thin films by varying post growth annealing duration

In this research work, we have successfully improved the thermoelectric properties of Zinc tin oxide thin films by controlling the post annealing duration. Initially, zinc tin oxide thin films were grown by direct evaporation of zinc and tin metal powder using the thermal evaporation method. After deposition, post growth annealing of the grown thin films was performed at 500 °C for different annealing durations (0.5, 1, 1.5 & 2 h). XRD data show the structural transformation from amorphous to mixed phases of Zn2SnO4, ZnSnO3, ZnO, and SnO2 with the increase in post-growth annealing duration. While the sample annealed for 2 h represent only two phases of ZnO and ZnSnO3. UV–vis data shows the decrease in band gap value from 3.8 to 3.61 eV with the increase in annealing duration. Seebeck data suggested the improvement in the Seebeck coefficient value from 24 to 104 μV/°C with the increase in annealing time duration while the electrical conductivity value was also improved from 34 to 63 S/cm. This increase in Seebeck coefficient and electrical conductivity was associated with the phase transformation into ZnSnO3 by suppression of un-wanted secondary phases leads to a high power factor up to 3.1 × 10−4 Wm−1K−2. In addition, we have also performed Raman and SEM as supplementary measurements to support our purposed argument.

Nonlinear growth of zinc tin oxide thin films prepared by atomic layer deposition

Zinc tin oxide (ZTO) thin films can be deposited by atomic layer deposition (ALD) with adjustable electrical, optical and structural properties. However, the ternary ALD processes usually suffer from low growth rate and difficulty in controlling film thickness and elemental composition, due to the interaction of ZnO and SnO2 processes. In this work, ZTO thin films with different Sn levels are prepared by ALD super cycles using diethylzinc, tetrakis(dimethylamido)tin, and water. It is observed that both the film growth rate and atom composition show nonlinear variation versus [Sn]/([Sn]+[Zn]) cycle ratio. The experimental thickness measured by spectroscopic ellipsometry and X-ray reflectivity are much lower than the expected thickness linearly interpolated from pure ZnO and SnOx films. The [Sn]/([Sn]+[Zn]) atom ratios estimated by X-ray photoelectron spectroscopy have higher values than that expected from the cycle ratios. Hence, to characterize the film growth behavior versus cycle ratio, a numerical method is proposed by simulating the effect of reduced density and reactivity of surface hydroxyls and surface etching reactions. The structure, electrical and optical properties of ZTO with different Sn levels are also examined by X-ray diffraction, atomic force microscope, Hall measurements and ultraviolet–visible–infrared transmittance spectroscopy. The ZTO turns out to be transparent nanocrystalline or amorphous films with smooth surface. With more Sn contents, the film resistivity gets higher (>1 Ω cm) and the optical bandgap rises from 3.47 to 3.83 eV.

Ultrafast Carrier Dynamics of Amorphous Zinc Tin Oxide Graded Thin Films

We report the fabrication of amorphous zinc tin oxide (ZTO) thin films using rf magnetron sputtering and rapid postannealing. A vertically graded composition alloy (Zn1–xSnxO) is constructed in a s...

Study of Self-Patterned Zinc Tin Oxide Thin-Film Transistors Prepared from Photocurable Precursor Solution with Photoacid Generator

Solution-processed zinc tin oxide (ZTO) thin-film transistors (TFTs) have great potential uses in next-generation wearable and flexible electronic products. Zinc and tin precursor materials are naturally abundant and have low fabrication costs. To integrate a single ZTO TFT into logic circuits including inverters, NAND, and NOR gates will require the development of a facile patterning process to replace conventional and complicated photolithography techniques which are usually time-consuming and toxic. In this study, self-patterned ZTO thin films were prepared using a photo-patternable precursor solution including a photoacid generator, (4-methylthiophenyl)methyl phenyl sulfonium triflate. Solution-processed ZTO precursor films fabricated with the photoacid generator were successfully micropatterned by UV exposure, and transitioned to a semiconducting ZTO thin film by heat treatment. The UV-irradiated precursor films became insoluble in developing solvent as the generated proton from the photoacid generator affected the metal-containing ligand and changed the solubility of the metal oxide precursors. The resulting ZTO thin films were utilized as the active layers of n-type TFTs, which exhibited a typical n-type transfer, and output characteristics with appropriate threshold voltage, on/off current ratio, and field-effect mobility. We believe that our work provides a convenient solution-based route to the fabrication of metal-oxide semiconductor patterns.

Influence of oxygen flow during sputtering process on the electrical properties of Ga-doped InZnSnO thin film transistor

This work presents the electrical properties of amorphous gallium-doped indium zinc tin oxide (Ga-IZTO) thin film transistors (TFTs), as a function of O2 flow rate (Ar/O2), during the sputtering process of the channel layer. As the O2 flow rate increased from 30:0 to 30:7, the saturation mobility ( SAT) monotonically degraded from 31.5 cm2V−1s−1 to 12.0 cm2V−1s−1, the threshold voltage (V TH) shifted from 0.8 V to 6.8 V, and the V TH shift (ΔV TH) induced by the negative bias stress improved from −7.0 V to −2.8 V. X-ray diffraction analysis indicated that themicrostructure of all the films is amorphous, which is independent of O2 flow rate. Transmittance spectra reflected that the average transmittance of the thin film is sensitive to the O2 flow rate and decreases as the O2 flow rate increases. XPS analysis revealed that the density of oxygen vacancies is reduced, and the oxygen lattices are enhanced as the O2 flow rate increases. The Hall effect measurements indicated that the carrier concentration (n) was reduced with increasing O2 flow rate. These results show that the electrical properties of Ga-IZTO TFTs can be easily tuned by the O2 flow rate during the sputtering process. The related mechanism for the variations of electrical properties of Ga-IZTO TFT has been discussed in detail.

Spectroscopic Ellipsometry Characterization of As-Deposited and Annealed Non-Stoichiometric Indium Zinc Tin Oxide Thin Film.

A spectroscopic ellipsometry study on as-deposited and annealed non-stoichiometric indium zinc tin oxide thin films of four different compositions prepared by RF magnetron sputtering was conducted. Multi-sample analysis with two sets of samples sputtered onto glass slides and silicon wafers, together with the analysis of the samples onto each substrate separately, was utilized for as-deposited samples. Annealed samples onto the glass slides were also analyzed. Spectroscopic ellipsometry in a wide spectral range (0.2–6 eV) was used to determine optical constants (refractive index n and extinction coefficient k) of these films. Parameterized semiconductor oscillator function, together with Drude oscillator, was used as a model dielectric function. Geometrical parameters (layer thickness and surface roughness) and physical parameters (direct optical bandgap, free carrier concentration, mobility, and specific electrical resistivity) were determined from spectroscopic ellipsometry data modeling. Specific electrical resistivity determined from the Drude oscillator corresponds well with the results from electrical measurements. Change in the optical bandgap, visible especially for annealed samples, corresponds with the change of free carrier concentration (Moss–Burstein effect). Scanning electron microscope did not reveal any noticeable annealing-induced change in surface morphology.

Improved photovoltaic performance of CdTe-based solar cells: Roles of using a hole-blocking layer and nanoscale imaging of barrier height at interfaces

The performance of photovoltaic devices is hindered by the presence of barrier height at the interfaces as well as the presence of structural defects. CdTe solar cells, based on a CdS/CdTe heterojunction and CdCl2 vapour treatment, exhibit high efficiency. In this work, we show that the use of a hole-blocking layer has a potential to further increase the efficiency of CdTe-based cells. As a case study, we have fabricated multi-juncton CdTe-based solar cells on both pristine- and textured-silicon substrates. Here we use an n-type zinc tin oxide (ZTO) thin film as the transparent conducting oxide (TCO) layer and an n-type hole-blocking layer of MoO3 on a p-type CdTe:Cu absorber layer. In addition, we map the nanoscale barrier height at each interface, i.e. ZTO/MoO3 and MoO3/CdTe:Cu by Kelvin probe force microscopy (KPFM). We also investigate the real time photo-generated charge carrier dynamics across the heterojunctions using photo-KPFM which plays a major role in the cell efficiency. Quantitative analysis shows that barrier height between the interfaces decreases after light illumination. Based on these findings, we have fabricated solar cells on pristine- and textured-Si substrates and the maximum efficiency is found to be 8.2% for the textured-Si substrate. The present study demonstrates the fabrication of efficient hole-blocking CdTe-based solar cells and provides insights on how local barrier height affects their macroscopic performance.

Lowering the Trap-State Density of Transparent Amorphous Oxide Semiconductor-Based Thin Film Transistors Through Microwave Irradiation.

In this study, a low-thermal-budget microwave irradiation (MWI) technique was applied as a post-deposition annealing (PDA) process to lower the trap densities that exist in transparent amorphous oxide semiconductor thin film transistors (TAOS TFTs). As channel layers of TAOS TFTs, two types of indium gallium zinc oxide (IGZO) with different compositions as well as aluminum zinc tin oxide (AZTO) and zinc oxide (ZnO) thin films were deposited with various thicknesses through radio frequency (RF) magnetron sputtering at 25°C. Cost-effective and energy-efficient MWI was conducted to enhance the electrical performance of transistors by removing traps and defects. The electrical characteristics of IGZO (1:1:1 and 4:2:3)-, ZnO-, and AZTO-based TFTs treated by MWI were evaluated by measuring the transfer curves. In particular, the relation between the interface trap density (Dit) and bulk trap density (Nt) of microwave-irradiated TFTs was quantitatively evaluated by the subthreshold swing (SS) variation based on channel thickness. The results indicated that of the four types of channel layers, the performance of IGZO (4:2:3) TFTs was the best and that of AZTO TFTs was the worst, in terms of electrical properties such as on/off current ratio, mobility SS, and trap density. In particular, it was demonstrated that the trap density of MWI-treated TAOS TFTs was much lower than that of conventional furnace annealing (CFA)-treated devices. Despite the short annealing duration of a few minutes, the MWI more effectively reduced the trap sites than did the furnace treatment, and significantly enhanced the electrical properties of the TAOS TFTs. It is expected that high-performance TAOS TFTs can be fabricated by applying MWI, which is a highly efficient and low-thermal-budget annealing method, to the PDA process and can thus reduce trap density.

Work Function Tuning of Zinc–Tin Oxide Thin Films Using High-Density O2 Plasma Treatment

Work function tuning has a significant influence on the performance of semiconductor devices, owing to the formation of potential barriers at the interface between metal-semiconductor junctions. In this work, we introduce a technique for tuning the work function of ZnSnO thin films using high-density O2 plasma treatment. The work function and chemical composition of the ZnSnO thin film surfaces were investigated with regards to plasma treatment time through UPS/XPS systems. The optical band gap was estimated using Tauc’s relationship from transmittance data. The work function of Zn0.6Sn0.4O thin film increased from 4.16 eV to 4.64 eV, and the optical band gap increased from 3.17 to 3.23 eV. The surface of Zn0.6Sn0.4O thin films showed a smooth morphology with an average of 0.65 nm after O2 plasma treatment. The O2 plasma treatment technique exhibits significant potential for application in high-performance displays in optical devices, such as thin-film transistors (TFTs), light-emitting diodes (LEDs), and solar cells.

Effects of Zn Ratio Tuning on the Structural and Transport Properties of Amorphous Indium Zinc Tin Oxide Thin Films

In this study, various indium zinc tin oxide (IZTO) films were fabricated using co-sputtering to explore amorphous transparent conducting oxides as alternatives to crystalline indium tin oxide (ITO). Various Zn ratios were tuned to study their effects on the structural evolution of IZTO. Amorphous IZTO (aIZTO) films were obtained when the Zn concentration was higher than approximately 17 at.%. X-ray diffraction and transmission electron microscopy analyses indicated that amorphous structures were formed due to the high crystallization temperatures of IZTO and nanocrystalline In2Zn5O8, which also caused aIZTO to exhibit small band gaps. On average, 87% transmittance was achieved in the visible light range. The aIZTO films had substantially higher transmittance in the infrared region than the ITO films did. Samples with Zn ratios between approximately 17 at.% and 27 at.% exhibited resistivity (approximately 3 × 10−4 Ω-cm), mobility (7 cm2 V−1 s−1), and carrier concentrations (4 × 1021 cm−3) comparable to those of crystalline ITO. Thus, the aIZTO films are promising alternative TCO materials. The mechanisms of competition between the crystallinity, oxygen deficiency, high-mobility In2Zn5O8, and electron and hole generation rates were applied to shed light on the excellent electrical and transport properties of aIZTO films.

Low voltage, high gain inverters based on amorphous zinc tin oxide on flexible substrates

Deposition of semiconductors on bendable substrates is a crucial step toward flexible circuitry and deposition by a roll-to-roll process. Since most bendable substrates have limited temperature stability (normally degradation starts between 150 °C and 300 °C), processing temperatures are typically below that of rigid substrates. Amorphous oxide semiconductors (AOSs) such as indium gallium zinc oxide (IGZO) can be fabricated at room temperature (RT) and exhibit electron mobilities >10 cm2 V−1 s−1 being a pre-requisite for application in backplanes of displays. While IGZO is already commercially exploited, the search for alternative materials is highly relevant because indium and gallium are rare and expensive. Zinc tin oxide (ZTO) is a promising AOS since zinc and tin are highly abundant and cheap. In this letter, we discuss RT-fabricated n-type ZTO thin films used as the channel material in flexible inverter circuits based on junction field-effect transistors. RT-fabricated nickel oxide is used as a semitransparent p-type gate material. The devices are fabricated on flexible polyimide and exhibit an excellent peak gain magnitude of 464 and uncertainty level as low as 130 mV at a supply voltage of only 3 V. They are characterized before and after bending at various radii. Even after bending at 2 mm radius, the inverters behave still very well.

Morphology and optical analysis of defect levels in ultrasonically-sprayed zinc tin oxide thin films

We prepared zinc-doped tin oxide (ZTO) films of various concentrations of zinc using the ultrasonic spray pyrolysis technique. The surface morphology and roughness were studied by Field-Emission Scanning Electron Microscope and atomic force microscope. Systematic x-ray diffraction (XRD) measurements revealed that all films have tetragonal rutile structure and mixed preferred orientation along the (200) and (110) planes. The proper substitution of zinc ions into tin sites and the absence of any secondary phase in the films were proved by the analysis of the x-ray photoelectron spectroscopy (XPS) measurements. The detailed analysis of the optical absorption spectra revealed the presence of four direct optical transitions within the energy range 3.8–5.1 eV. The defect levels of the neutral and double-ionized oxygen vacancy states were determined within the energy bandgap by the comprehensive analysis of the photoluminescence spectra. The existence of high concentration of oxygen vacancies is supported by the XPS results. The observed transitions were used to sketch the band diagram of ZTO films.

Development and characterization of (Zn,Sn)O thin films for photovoltaic application as buffer layers

In order to study the Zinc Tin Oxide (ZTO) as an alternative buffer layer to replace CdS in thin films solar cells, we synthesized the (Zn,Sn)O material using the vacuum thermal evaporation of the metal compounds, Tin and Zinc, followed by annealing treatments. The obtained samples were divided into two groups according to the amount of Tin used during the thermal evaporation while the amount of Zinc remains invariant. The first group of ZTO thin films was made using a Tin amount of 0.312 g and annealed in air at different temperatures (samples A). The second group was made using a less amount of Tin (0.156 g) (samples B). Part of these samples were annealed in air atmosphere (samples B1) whereas the other sample was annealed under vacuum then annealed in air (sample B2). The annealing process was mainly dedicated to the oxidation treatment of Sn/Zn metallic bilayers but it also plays an important role to improve the crystallinity of the as-deposited films. The structural, optical and electrical properties of the prepared films were characterized using X-ray diffraction, UV–VIS–NIR spectrophotometry and impedance spectroscopy, respectively. We have succeeded to obtain a stable cubic spinel structure of Zn2SnO4 phase (sample B2). This thin film showed interesting properties such as a direct band gap energy of value of 3.20 eV, a high optical transmission of 80% and a n-type electrical conductivity.

Complementary Inverter Circuits Based on p-Cu2O and n-ZTO Thin Film Transistors

This paper describes the fabrication of copper oxide and zinc tin oxide complementary inverters where both the p-type and n-type channels were deposited by RF magnetron sputtering. We have designed low-voltage and high gain complementary inverters by combining a set of p channel copper oxide and n channel zinc tin oxide thin film transistors (TFTs) with different aspect ratios, thus reducing the difference in mobility and threshold voltage between both types of TFTs. The complementary inverters with four different geometric aspect ratios were fabricated using top-contact configuration. The voltage gain increase with aspect ratio and a maximum value of 4.2 was reached for an aspect ratio of 2 (Wp:Wn = 7200 μm/3600 μm). The voltage gain was found to be dependent on the SS value of p-type TFTs. It is also noticed that an inverter having a geometric aspect ratio of 2 gave better voltage gain and noise margin values. For a VDD = 20 V the NMH and NML values are 7 V and 6.5 V, respectively.

Yb-doped zinc tin oxide thin film and its application to Cu(InGa)Se2 solar cells

The use of rare earth elements with semiconductor materials has attracted immense interest due to their unique properties. In this study, we investigated the characteristics of an ytterbium (Yb)-doped zinc tin oxide (Yb:ZTO) thin film and its application as a potential down-converter of Cu(InGa)Se2 (CIGS) thin-film solar cells. Yb:ZTO thin films were deposited by reactive sputtering of Zn and Sn metal with oxygen flow. A few pieces of Yb were embedded in a Zn metal target; thus Yb elements were supplied during Zn sputtering. The relative composition of Zn and Sn was controlled by changing the sputtering power (10–70 W) of Sn, in relation to the fixed sputtering power for Zn (70 W). In addition, the substrate temperature was varied from room temperature to 400 °C. It was confirmed that a smaller amount of Sn with lower sputtering power led to more incorporation of Yb into ZTO. X-ray photoelectron spectroscopy analysis confirmed the incorporation of Yb into ZTO, and photoluminescence measurement demonstrated Yb emission. Grazing incidence X-ray diffraction indicated the shift of ZTO emission peaks induced by the difference in the composition of Zn and Sn. Finally, CIGS solar cells with an Yb:ZTO layer were fabricated. The results suggested that cells with the highest Yb photoluminescence emission showed the highest short-circuit current density and cell efficiency.

P‐1.13: The Conductivity Modulation of Amorphous Zinc Tin Oxide Thin Film by aluminum (Al) reaction method

We propose a method to form low‐resistance amorphous zinc tin oxide thin film (a‐ZTO) by aluminum (Al) reaction method. After the optimized reaction process, a‐ZTO film exhibits a low sheet resistance of 0.71(KΩ/□). The metallic tin and the large amount of oxygen vacancies formed on the surface of a‐ZTO film contribute to the high conductivity. An optimized aluminum reaction method for fabrication of low‐resistance with high thermal stability a‐ZTO film is well demonstrated.

Influence of grain size on local work function and optoelectronic properties of n-ZTO/p-Si heterostructures

In this work, we investigate on tunability of the local work function, morphological, structural, electrical, and optical properties of ZTO (zinc tin oxide) thin films. Using Kelvin probe force microscopy, we demonstrate that local work function of ZTO films can be tuned from 5.04 to 4.94 eV by varying the film thickness from 60 to 240 nm. X-ray diffraction studies reveal the amorphous nature of ZTO films, whereas AFM measurements confirm their granular nature. It is observed that the average grain size increases (from 24 to 58 nm) with increasing film thickness. In addition, a systematic red shift in the band gap as well as a decrease in the resistivity takes place with increasing thickness. Moreover, current-voltage characteristics show a systematic reduction in the turn-on potential and an enhancement in the leakage current with increasing ZTO film thickness. Above observations are explained in the framework of ZTO thickness-dependent variation in the grain size. This study will be of importance to make a transparent conductive oxide-based electrode in photovoltaic applications and ZTO-based optoelectronic devices.

Optical transitions and photoluminescence of fluorine-doped zinc tin oxide thin films prepared by ultrasonic spray pyrolysis

Abstract Thin films of fluorine-doped zinc tin oxide (FZTO) were deposited on borosilicate glass by ultrasonic spray pyrolysis. Structural and elemental investigations revealed that all films have a single-phase polycrystalline tetragonal rutile structure and proved the successful incorporation of zinc ions into tin sites and the fluorine ions into the sites of oxygen vacancies. A comprehensive analysis of the optical transitions in FZTO films was presented. All films show high optical transparency (≥85%) in the visible range. Four direct optical transitions were observed in the range 3.8 - 5.1 eV by spectrophotometric measurements, in addition to three transitions identified by photoluminescence. From these transitions observed for the first time in FZTO, three states of point defects were determined in the bandgap and attributed to the neutral, singly- and doubly-ionized oxygen vacancies. The ascription of these states to oxygen vacancies was supported by the x-ray photoelectron spectroscopy measurements. The band diagram of FZTO was sketched, demonstrating the observed transitions. The obtained results are expected to have their imprint on understanding the optical processes in FZTO and evaluating the future utility of it in various optoelectronic applications.