Film Growth Technique Research Articles

Energy-driven multi-step structural phase transition mechanism to achieve high-quality p-type nitrogen-doped β-Ga2O3 films

It is a big challenge to grow β-Ga2O3 films of good p-type conductivity and stability due to the large acceptor ionization energy, strong hole-trapping effect, low hole mobility, and self-compensation effects. In this study, we demonstrate for the first time the success in the growth of high-quality p-type N-doped β-Ga2O3 films with an acceptor ionization energy of 0.165 eV, Hall resistivity of 17.0 Ωcm, Hall hole mobility of 23.6 cm2V−1s−1, hole concentration of 1.56 × 1016 cm−3 and good stability, and in the fabrication of high-performance p-type β-Ga2O3 films-based field effect transistors. The success paves the way to fabricate full β-Ga2O3 films based p-n diodes and will advance the applications of next-generation high-performance β-Ga2O3-based optoelectronic and electronic devices. In addition, we discover the growth mechanism of multi-step structural phase transitions from hexagonal P63mc GaN to rhombohedral R3¯c α-GaNxO3(1-x)/2 and finally to monolithic C2/m N-doped β-Ga2O3, which improves the crystalline quality, facilitates the acceptor doping, increases the acceptor activation efficiency, and thus enhances the p-type conductivity and stability of the N-doped β-Ga2O3 films. The novel p-type oxide film growth technique and mechanism open a new horizon to the other wide bandgap oxide semiconductors.

High-mobility patternable MoS2 percolating nanofilms

Fabrication of large-area and uniform semiconducting thin films of two-dimensional (2D) materials is paramount for the full exploitation of their atomic thicknesses and smooth surfaces in integrated circuits. In addition to elaborate vapor-based synthesis techniques for the wafer-scale growth of 2D films, solution-based approaches for high-quality thin films from the liquid dispersions of 2D flakes, despite underdeveloped, are alternative cost-effective tactics. Here, we present layer-by-layer (LbL) assembly as an effective approach to obtaining scalable semiconducting films of molybdenum disulfide (MoS2) for field-effect transistors (FETs). LbL assembly is achieved by coordinating electrochemically exfoliated MoS2 with cationic poly (diallyldimethylammonium chloride) (PDDA) through electrostatic interactions. The PDDA/MoS2 percolating nanofilms show controlled and self-limited growth on a variety of substrates, and are easily patterned through lift-off processes. Ion gel gated FETs are fabricated on these MoS2 nanofilms, and they show mobilities of 9.8 cm2·V−1·s−1, on/off ratios of 2.1 × 105 with operating voltages less than 2 V. The annealing temperature in the fabrication process can be as low as 200 °C, thereby permitting the fabrication of flexible FETs on polyethylene terephthalate substrates. The LbL assembly technique holds great promise for the large-scale fabrication of flexible electronics based on solution-processed 2D semiconductors.

Towards Tm2+ in Sialon Thin Films Using the Gradient Deposition Method

There are different material candidates for efficient industry compatible luminescence solar concentrators to be used in electricity generating glass applications. Among them, divalent thulium (Tm2+) doping has been shown to have a high potential. It has been reported to have 65% absorption of the solar spectral range when in di-halide hosts, while showing no self-absorption and a relatively neutral color [1]. These characteristics are possible because Tm2+ has a strong absorption from the UV up to about 900 nm. This absorption is generated by intense 4f13 → 4f125d1 electronic transitions and a single emission line at 1140 nm, credited to the 2F5/2 → 2F7/2 transition of the 4f13 configuration. However, this specific oxidation state of thulium is rather anomalous: so far it was only stabilized on di-halide crystals. These crystals are very hygroscopic in nature, therefore not sufficiently attractive for scaling up in industry standard thin film growth techniques. Thus, it is necessary to research on the possible extension in the list of materials able to host such a promising optically active element. However, the intrinsic complex nature of the lanthanide’s electronic structure has prevented a detailed understanding of the valence stability of its elements. Therefore, designing industry compatible hosts ensuring that such exotic ion is stable and capable of efficiently emitting NIR light is a challenging task.In this work, we show the progress made so far towards unraveling the conditions for stabilization of the Tm2+ in industry compatible thin film materials – more specifically, thin film materials of the SiAlON family. I report recent results on Tm doped SiAlON thin films grown by combinatorial magnetron reactive sputtering. The gradient characterization method [2], developed at the TU Delft’s luminescence materials research group, was used in order to relate position dependent film composition – determined by energy dispersive x-ray spectroscopy – with local transmission, photoluminescence emission and excitation data. This singular approach makes it possible to retrieve a wide composition range in a smaller number of different films. These results are discussed upon the valence stability model of lanthanides of Dorenbos [3], and the model of impurity ionization adopted in solid state statistics.

Novel process for the production of 3Y-TZP ceramics: comparison between ageing in artificial saliva and accelerated ageing

The degradation of tetragonal zirconia polycrystalline (3Y-TZP) ceramics prepared by a novel film growth technique, and by sintering at 1400 °C, was evaluated for one year using two processes: (i) degradation under oral conditions in artificial saliva (37 °C and pH 6.8) and (ii) accelerated degradation by autoclaving at 134 °C at 0.2 MPa of pressure. X-ray diffraction analysis of 3Y-TZP ceramics sintered at 1400 °C showed a phase transformation (tetragonal to monoclinic) in 3Y-TZP ceramics after the fifth month in artificial saliva. After 12 months in artificial saliva, there was ∼11% of monoclinic phase present in 3Y-TZP ceramics, but only ∼2% of monoclinic phase in samples that were subjected to accelerated degradation for 5 h. No correlation was found between the level of ageing during accelerated degradation and degradation in artificial saliva. There was no degradation of 3Y-TZP ceramics prepared by a novel film growth technique, independently of the degradation method used, which suggests that this material could be used for dental prosthetics.

Metal-ion subplantation: A game changer for controlling nanostructure and phase formation during film growth by physical vapor deposition

Up until recently, thin film growth by magnetron sputtering relied on enhancing adatom mobility in the surface region by gas-ion irradiation to obtain dense layers at low deposition temperatures. However, an inherently low degree of ionization in the sputtered material flux during direct-current magnetron sputtering (DCMS), owing to relatively low plasma densities involved, prevented systematic exploration of the effects of metal-ion irradiation on the film nanostructure, phase content, and physical properties. Employing only gas-ion bombardment results in an inefficient energy and momentum transfer to the growing film surface. Also, for enhanced substrate biasing, the higher concentration of implanted noble gas atoms at interstitial lattice positions causes elevated compressive stress levels. High-power impulse magnetron sputtering (HiPIMS), however, provides controllable metal-ion ionization and, more importantly, enables the minimization of adverse gas-ion irradiation effects. The latter can be realized by the use of pulsed substrate bias applied synchronously with the metal-ion-rich portion of each HiPIMS pulse (metal-ion-synchronized HiPIMS), based on the results of time-resolved ion mass spectrometry analyses performed at the substrate position. In this way, both the metal-ion energy and the momentum can be precisely controlled for one to exploit the benefits of irradiation by metal-ions, which are also the film-forming species. Systematic studies performed in recent years using binary and ternary transition metal-based nitrides as model systems revealed new phenomena with accompanying unique and attractive film growth pathways. This Perspective paper focuses on the effects of low-mass metal-ion irradiation and their role for the nanostructure and phase control. We review basic findings and present original results from ion mass spectrometry studies and materials characterization for the effect of metal-ion subplantation. Key correlations are highlighted, which, if properly engaged, enable unprecedented control over film nanostructure and phase formation and, hence, the resulting properties. We show generalization from the findings to present a new concept for thin film growth in a hybrid HiPIMS/DCMS configuration with metal-ion-synchronized bias. Based on the results obtained for TM-based nitrides, there are no evident physical limitations preventing the extension of this deposition process concept for other materials systems or other metal–ion-based thin film growth techniques. Further exciting findings could, thus, be anticipated for the future.

(Invited) Towards Tm2+ in Sialon Thin Films Grown by Sputtering Using the Gradient Deposition

There are different material candidates for efficient industry compatible luminescence solar concentrators to be used in electricity generating glass applications. Among them, divalent thulium (Tm2+) doping has been shown to have a high potential. It has been reported to have 65% absorption of the solar spectral range when in di-halide hosts, while showing no self-absorption and a relatively neutral colour [1]. These characteristics are possible because Tm2+ has a strong absorption from the UV up to about 900 nm. This absorption is generated by intense 4f 13 → 4f 125d 1 electronic transitions and a single emission line at 1140 nm, credited to the 2F5/2 → 2F7/2 transition of the 4f 13 configuration. However, this specific oxidation state of thulium is rather anomalous: so far it was only stabilized on di-halide crystals. These crystals are very hygroscopic in nature, therefore not sufficiently attractive for scaling up in industry standard thin film growth techniques. Thus, it is necessary to research on the possible extension in the list of materials able to host such a promising optically active element. However, the intrinsic complex nature of the lanthanide’s electronic structure has prevented a detailed understanding of the valence stability of its elements. Therefore, designing industry compatible hosts ensuring that such exotic ion is stable and capable of efficiently emitting NIR light is a challenging task.In this work, I show the progress made so far towards unravelling the conditions for stabilization of the Tm2+ in industry compatible thin film materials – more specifically, thin film materials of the SiAlON family. I report recent results on Tm doped SiAlON thin films grown by combinatorial magnetron reactive sputtering. The gradient characterization method [2], developed at the TU Delft’s luminescence materials research group, was used in order to relate position dependent film composition – determined by energy dispersive x-ray spectroscopy – with local transmission, photoluminescence emission and excitation data. This singular approach makes it possible to retrieve a wide composition range in a smaller number of different films. These results are discussed upon the valence stability model of lanthanides of Dorenbos [3], and the model of impurity ionization adopted in solid state statistics.

Atomic layer deposition of a uniform thin film on two-dimensional transition metal dichalcogenides

Two-dimensional transition metal dichalcogenides (2D TMDs) is one of the promising materials for future electronics since they have, not only superior characteristics, but also a versatility that conventional materials do not have with a few nanometer thickness. One of the prerequisites for applying these materials to device fabrication is to deposit an ultrathin film below 10 nm with excellent uniformity. However, TMD has quite a different surface chemistry and is fragile to external conditions compared to conventional materials. Thus, thin film deposition on 2D TMD with excellent uniformity using conventional deposition techniques is quite challenging. Currently, the most adequate deposition technique for sub-10 nm-thick film growth is atomic layer deposition (ALD). A thin film is formed on the surface by the reaction between chemical and surface species based on the self-limiting growth manner. Owing to its unique and superior growth characteristics, such as excellent uniformity and conformality, ALD is an essential deposition technique for nanoscale device fabrication. However, since 2D TMD has a lack of reaction sites on the surface, various studies have reported that ALD on 2D TMDs surfaces without any treatment showed an island growth mode or formation of clusters rather than continuous films. For this reason, recent studies have been focused on the deposition of an ultrathin film on 2D TMDs with excellent uniformity. For a decade, there have been various approaches to obtain uniform films on 2D TMDs using ALD. Among them, the authors focus on the most frequently researched methods and adsorption control of chemical species by modifying the process parameters or functionalization of new chemical species that can assist adsorption on the chemically inert 2D TMD surface. In this review, the overall research progress of ALD on 2D TMD will be discussed which would, in turn, open up new horizons in future nanoelectronics fabrication using 2D TMDs.

Ultrahigh-pressure fabrication of single-phase α-PbO2-type TiO2 epitaxial thin films

Materials that are thermodynamically stable at ultrahigh pressures (>10 GPa) often exhibit unique physical properties. However, few studies have addressed the fabrication of epitaxial thin films of ultrahigh-pressure phases. Herein, we combine epitaxial thin film growth techniques with ultrahigh-pressure synthetic methods. We demonstrate the synthesis of single-phase epitaxial thin films of an ultrahigh-pressure polymorph of TiO2, α-PbO2-type TiO2. A rutile TiO2(100) epitaxial thin film is used as a precursor, and a structural phase transition is induced at 8 GPa and 800–1000 °C. This study demonstrates a new synthetic route to obtain ultrahigh-pressure-phase materials. The fabrication of epitaxial thin film ultrahigh-pressure phases paves the way for investigating the physical properties that arise at surfaces and interfaces of materials.

Intrinsic 2D Ferromagnetism in V5Se8 Epitaxial Thin Films.

The discoveries of intrinsic ferromagnetism in atomically thin van der Waals crystals have opened a new research field enabling fundamental studies on magnetism at two-dimensional (2D) limit as well as development of magnetic van der Waals heterostructures. Currently, a variety of 2D ferromagnetism has been explored mainly by mechanically exfoliating "originally ferromagnetic (FM)" van der Waals crystals, while a bottom-up approach by thin-film growth technique has demonstrated emergent 2D ferromagnetism in a variety of "originally non-FM" van der Waals materials. Here we demonstrate that V5Se8 epitaxial thin films grown by molecular-beam epitaxy exhibit emergent 2D ferromagnetism with intrinsic spin polarization of the V 3d electrons despite that the bulk counterpart is "originally antiferromagnetic". Moreover, thickness-dependence measurements reveal that this newly developed 2D ferromagnet could be classified as an itinerant 2D Heisenberg ferromagnet with weak magnetic anisotropy, broadening a lineup of 2D magnets to those potentially beneficial for future spintronics applications.

Designing near-infrared electro-optical devices from the SrTiO3/LaAlO3 materials system

While the SrTiO3/LaAlO3 materials system has garnered intense research interest over the past decade owing to the discovery of a two-dimensional electron gas at the interface of these two band insulators, recent reports have focused on its optical properties. The silicon-compatibility of the SrTiO3/LaAlO3 system, together with its large conduction band offset and the ability to confine charge carriers in SrTiO3 quantum wells, makes it a potential candidate for use in a wide range of integrated photonics applications. Here, we present numerical simulations of the electrical, optical, and electro-optical performance of silicon-integrated SrTiO3/LaAlO3 electro-optical devices utilizing the quantum-confined Stark effect to electro-optically tune intersubband absorptions occurring at near-infrared optical wavelengths. We discuss optimal design parameters for the fabrication of experimentally realizable devices and attempt to minimize the energy consumption of a SrTiO3/LaAlO3 electro-optic modulator while maximizing performance. We predict modulation energies on the order of tens of pJ/bit for the proposed device. Our results indicate the feasibility of producing SrTiO3/LaAlO3-based integrated electro-optical devices using existing thin film growth and semiconductor processing techniques.

Growth of vanadium dioxide thin films on hexagonal boron nitride flakes as transferrable substrates

Vanadium dioxide (VO2) is an archetypal metal-insulator transition (MIT) material, which has been known for decades to show an orders-of-magnitude change in resistivity across the critical temperature of approximately 340 K. In recent years, VO2 has attracted increasing interest for electronic and photonic applications, along with advancement in thin film growth techniques. Previously, thin films of VO2 were commonly grown on rigid substrates such as crystalline oxides and bulk semiconductors, but the use of transferrable materials as the growth substrates can provide versatility in applications, including transparent and flexible devices. Here, we employ single-crystalline hexagonal boron nitride (hBN), which is an insulating layered material, as a substrate for VO2 thin film growth. VO2 thin films in the polycrystalline form are grown onto hBN thin flakes exfoliated onto silicon (Si) with a thermal oxide, with grains reaching up-to a micrometer in size. The VO2 grains on hBN are orientated preferentially with the (110) surface of the rutile structure, which is the most energetically favorable. The VO2 film on hBN shows a MIT at approximately 340 K, across which the resistivity changes by nearly three orders of magnitude, comparable to VO2 films grown on common substrates such as sapphire and titanium dioxide. The VO2/hBN stack can be picked up from the supporting Si and transferred onto arbitrary substrates, onto which VO2 thin films cannot be grown directly. Our results pave the way for new possibilities for practical and versatile applications of VO2 thin films in electronics and photonics.

Investigation of GaN/InGaN thin film growth on ITO substrate by thermionic vacuum arc (TVA)

Group-III nitride semiconductors covering infrared, visible and ultraviolet spectral range have direct bandgaps changing from 0.7 eV (InN) to 3.4 eV (GaN). With this feature, optoelectronic devices such as light emitting diodes, laser diodes and ultraviolet (visible rays–UV) photodetectors are made. It is possible to grow high-quality InGaN epitaxial layers by modern crystal growth techniques such as molecular beam epitaxy, radio frequency sputtering method and metal organic chemical vapor deposition. Compared with these methods, the Thermionic Vacuum Arc, which is promising thin film growth technique, is relatively inexpensive and quite effective approach for preparing InGaN thin films. The purpose of this research is to investigate the physical properties of the film. The XRD patterns of the InGaN thin films deposited on the ITO substrate exhibited polycrystal structure. The larger crystallite size and smaller FWHM indicate better crystallization. Microstrain values also exhibit good crystallite films respect to the low dislocation density. This film has a potential for photovoltaic devices based on the absorbance graph. It was observed that the compound is homogeneously dispersed on the surface and that there is a nanoporous structure.

Gas-phase method makes membranes selective

Permeable membranes could potentially lower the cost of industrial gas separations traditionally done via the energy-intensive process of cryogenic distillation. A number of metal-organic framework compounds known as ZIFs—zeolite-type structures consisting of transition metals connected by imidazolate linkers—show promise for this application, but the solution-based methods used to make them are tough to scale up reliably. So Xiaoli Ma, Michael Tsapatsis, and coworkers at the University of Minnesota came up with a simple vapor-phase ZIF-preparation method based on atomic layer deposition (ALD), a well-known film-growth technique. The group showed that the ZIF membranes they produced efficiently separate the gases propane and propylene (Science 2018, DOI: 10.1126/science.aat4123). The researchers started with a common substrate made by sandwiching two types of alumina. That material is gas permeable but does not separate propane from propylene. Using ALD, they deposited zinc oxide on the alumina, plugging ...

Epitaxial integration and properties of SrRuO3 on silicon

We report the integration of SrRuO3, one of the most widely used oxide electrode materials in functional oxide heterostructures, with silicon using molecular-beam epitaxy and an SrTiO3 buffer layer. The resulting SrRuO3 film has a rocking curve full width at half maximum of 0.01°, a resistivity at room temperature of 250 μΩ cm, a residual resistivity ratio (ρ300 Kρ4 K) of 11, and a paramagnetic-to-ferromagnetic transition temperature of ∼160 K. These structural, electrical, and magnetic properties compare favorably to the best reported values for SrRuO3 films on silicon and rival those of epitaxial SrRuO3 films produced directly on SrTiO3 single crystals by thin film growth techniques other than molecular-beam epitaxy. These high quality SrRuO3 films with metallic conductivity on silicon are relevant to integrating multi-functional oxides with the workhorse of semiconductor technology, silicon.

Controlled growth of complex polar oxide films with atomically precise molecular beam epitaxy

At heterointerfaces between complex oxides with polar discontinuity, the instability-induced electric field may drive electron redistribution, causing a dramatic change in the interfacial charge density. This results in the emergence of a rich diversity of exotic physical phenomena in these quasi-two-dimensional systems, which can be further tuned by an external field. To develop novel multifunctional electronic devices, it is essential to control the growth of polar oxide films and heterointerfaces with atomic precision. In this article, we review recent progress in control techniques for oxide film growth by molecular beam epitaxy (MBE). We emphasize the importance of tuning the microscopic surface structures of polar films for developing precise growth control techniques. Taking the polar SrTiO3 (110) and (111) surfaces as examples, we show that, by keeping the surface reconstructed throughout MBE growth, high-quality layer-by-layer homoepitaxy can be realized. Because the stability of different reconstructions is determined by the surface cation concentration, the growth rate from the Sr/Ti evaporation source can be monitored in real time. A precise, automated control method is established by which insulating homoepitaxial SrTiO3 (110) and (111) films can be obtained on doped metallic substrates. The films show atomically well-defined surfaces and high dielectric performance, which allows the surface carrier concentration to be tuned in the range of ~1013/cm2. By applying the knowledge of microstructures from fundamental surface physics to film growth techniques, new opportunities are provided for material science and related research.

Experimental Studies on role of pH, potential and concentration of buffer solution for chemical bath deposition technique

The ionization potential is usually found to be less in acid and more in base. The experiment proves that the ionization potential increases on dilution of acid to base and reduces from base to acid. The potential can be tailored according to the desired properties based on our choice of acid or base. The experimental study establishes a direct relationship between pH and electric potential. This work provides theoretical insights on the need for a basic media of pH 10 in chemical thin film growth techniques called Chemical Bath Deposition Techniques.

A Review of SnSe: Growth and Thermoelectric Properties

SnSe is a 2D semiconductor with an indirect energy gap of 0.86 - 1 eV; it is widely used in solar cell, optoelectronics, and electronic device applications. Recently, SnSe has been considered as a robust candidate for energy conversion applications due to its high thermoelectric performance (ZT = 2.6 in p-type and 2.2 in n-type), which is assigned mainly to its anhamornic bonding leading to an ultralow thermal conductivity. In this review, we first discuss the crystalline and electronic structures of SnSe and the source of its p-type characteristic. Then, some typical single crystal and polycrystal growth techniques, as well as an epitaxial thin film growth technique, are outlined. The reported thermoelectric properties of SnSe grown by using each technique are also reviewed. Finally, we will describe some remaining issues concerning the use of SnSe for thermoelectric applications.

Rapid, Selective, Ambient Growth and Optimization of Copper Benzene-1,3,5-Tricarboxylate (Cu–BTC) Metal–Organic Framework Thin Films on a Conductive Metal Oxide

Metal–organic frameworks (MOFs) are useful for thin film-based device integration as they show high adsorption capacity for selected gases. However, optimized device integration of MOFs for various platforms requires high-quality, well controlled thin film growth techniques that are flexible, scalable, and manufacturable. For this reason, there is a critical need for the ability to grow MOFs in the form of dense and uniform thin films efficiently on a wide range of substrates. In this work, copper benzene-1,3,5-tricarboxylate (Cu–BTC) MOF thin films are rapidly produced at room temperature by using a conductive aluminum-doped zinc oxide (AZO) as a seed layer to template Cu–BTC growth, with growth only occurring on the AZO layer via a hydroxy double salt intermediate. The formation pathway of the Cu–BTC films was investigated in detail because of the significant importance of improving the Cu–BTC film growth from the perspective of optimized device integration. We demonstrate that the structure of the resu...

Progresses on structure and functionality of vapor-deposited polymers as thin films and in multilayers

Functional organic thin films (< 100 nm) are typical components of current devices in a variety of fields, including microelectronics, biotechnology and microfluidics. The need for miniaturization and structuration has boosted the development of advanced thin film growth techniques that can be easily implemented in the manufacturing steps of current device production. This review aims at presenting the latest progresses made in the field of chemical vapor deposition (CVD) methods of organic thin films. In CVD processes, the chemicals are delivered through the vapor-phase to the substrate, without the use of solvents, with the advantage of enlarging the applicability of polymer thin films to fields where the presence of solvent is detrimental. Among other methods, the initiated Chemical Vapor Deposition (iCVD) will be reviewed. High control over chemical composition, structure and film functionality has been largely demonstrated by iCVD. This technique allows coating virtually any substrate with conformal polymers at very low energy consumption. The specific chemical composition and the nanoengineered thickness control are desirable parameters for driving application-specific properties into the material. Further development of this field will certainly lead to progresses on the use of polymers in functional devices, as electrolytes, stimuli-responsive materials, encapsulants for drug-delivery and as membranes or barriers for permeation.

合成マイカ上の原子レベルで平坦なNi膜基板の作成方法

Synthetic mica was employed as a substrate for thin film growth and atomically flat Nickel thin film was heteroepitaxially formed. In addition to the small rms surface roughness achieved (< 1 nm), the surface morphology appears to be comparable to that of commercially available gold mica. Film growth was not complicated and consisted of nickel deposition and substrate annealing. The authors expect that as long as synthetic mica and standard film growth techniques are available, our results can be reproduced easily.