High-quality Crystalline Films Research Articles

Achieving vertical orientation films with superior environmental stability in quasi-2D lead iodide perovskites

The incorporation of a spacer cation to improve the stability of lead halide perovskites entails a trade-off, notably compromising film quality and device performance, particularly in quais-2D variants employing a higher proportion of hydrophobic, larger spacer cations. Addressing this challenge in quasi-2D perovskite-based devices requires enhancement in film morphology, crystallinity, and vertical orientation to optimize charge transport. Herein, we present a simple one step spin coating synthesis approach for producing PEA2MAn-1PbnI3n+1 (n = 5–2) thin films from a single precursor solution. Notably, our method operates at room temperature without the need of usually toxic anti-solvent or specialized additives, resulting in high quality crystalline films that are vertically oriented and smooth. We systematically investigate the impact of annealing processes on crystal structure, including gradient annealing (GA) as well as thermal annealing at 100℃ for durations of 10 and 15 minutes. Our findings indicate that the films with n = 4 exhibits superior crystallinity and vertical crystallographic orientation, alongside reduced roughness, and minimal contamination of the lower n mixtures. Gradient annealing notably enhances the crystallinity of n = 3 films. While the surface of all the films appears smooth with flake-like structure, the n = 2 sample notably exhibits pitting.

Beyond Structural Stabilization of Highly‐Textured AlN Thin Films: The Role of Chemical Effects

AbstractThe crystalline quality and degree of c‐axis orientation of hexagonal AlN thin films correlate directly with their functional properties. Therefore, achieving AlN thin films of high crystalline quality and texture is of extraordinary importance for many applications, but particularly in electronic devices. This systematic study reveals, that the growth of c‐axis‐orientated AlN thin films can be governed by a chemical stabilization effect in addition to the conventionally known structural, strain‐induced, stabilization mechanism. The promotion of in‐plane growth of AlN grains with c‐axis out‐of‐plane orientation is demonstrated on Y, W, or Al seed layers with different thicknesses and crystallinity preliminary exposed to N2 at room temperature. It is established that the stabilization mechanism is chemical in nature: the formation of an N‐rich surface layer on the metal seed layers upon exposure to N2 pre‐determines the polarity of AlN islands at the initial stages of thin film growth while the low energy barrier for the subsequent coalescence of islands of the same polarity contributes to grain growth. These results suggest that the growth of c‐axis oriented AlN thin films can be optimized and controlled chemically, thus opening more pathways for energy‐efficient and controllable AlN thin film growth processes.

Treatment and aging studies of GaAs(111)B substrates for van der Waals chalcogenide film growth

GaAs(111)B are commercially available substrates widely used for the growth of van der Waals chalcogenide films. Wafer-scale, high-quality crystalline films can be deposited on GaAs(111)B substrates using molecular beam epitaxy. However, two obstacles persist in the use of GaAs(111)B: first, the surface dangling bonds make it challenging for the growth of van der Waals materials; second, the As-terminated surface is prone to aging in air. This study investigated a thermal treatment method for deoxidizing GaAs(111)B substrates while simultaneously passivating the surface dangling bonds with Se. By optimizing the treatment parameters, we obtained a flat and completely deoxidized platform for subsequent film growth, with highly reproducible operations. Furthermore, through first-principle calculations, we find that the most energetically favorable surface of GaAs(111)B after Se passivation consists of 25% As atoms and 75% Se atoms. Finally, we discovered that the common storage method using food-grade vacuum packaging cannot completely prevent substrate aging, and even after thermal treatment, aging still affects subsequent growth. Therefore, we recommend using N2-purged containers for better preservation.

Study on the effects of Si-doping in molecular beam heteroepitaxial β-Ga2O3 films

β-Ga2O3, an emerging wide bandgap semiconductor material, holds significant potential for various applications. However, challenges persist in improving the crystal quality and achieving controllable doping of β-Ga2O3. In particular, the relationship between these factors and the mechanisms behind them are not fully understood. Molecular beam epitaxy (MBE) is viewed as one of the most sophisticated techniques for growing high-quality crystalline films. It also provides a platform for studying the effects of doping and defects in heteroepitaxial β-Ga2O3. In our study, we tackled the issue of Si source passivation during the MBE growth of Si-doped β-Ga2O3. We did this by using an electron beam vaporize module, a departure from the traditional Si effusion cell. Our research extensively explores the correlation between Si doping concentration and film properties. These properties include microstructure, morphology, defects, carrier conductivity, and mobility. The results from these investigations are mutually supportive and indicate that a high density of defects in heteroepitaxial β-Ga2O3 is the primary reason for the challenges in controllable doping and conductivity. These insights are valuable for the ongoing development and enhancement of β-Ga2O3-based device techniques.

Direct synthesis of WSe2/PtSe2 heterostructures

The results of a successful synthesis of a WSe2/PtSe2 heterostructures are presented. High quality crystalline films were achieved through a one–step selenization of a pre-deposited tungsten film with pre-deposited platinum as an underlayer. The role of the PtSe2 layer, formed during selenization, was to assists the growth of crystalline WSe2. The existence of WSe2 was confirmed using Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The crystallinity of the samples was investigated using X-ray diffraction (XRD). Surface measurements were performed using atomic force microscopy (AFM).

ELECTROCHEMICAL SYNTESIS OF A CATHODE MATERIAL BASED ON Co-Se ALLOY

The present work is devoted to the electrochemical deposition of thin CoSe films and study of the influence of various factors on the composition of the resulting film. It was established that during the deposition of both individual components and in their joint presence, the electroreduction potentials (Se, Co and CoSe) are respectively equal to 0.35, -0.2, 0.23 V. Anodic dissolution of CoSe is observed at a potential of 0.75 V. The electrochemical reduction of selenium and cobalt occurs in 2 stages. The results demonstrated that both the temperature and the electrode material have a significant effect on the kinetics of CoSe electrodeposition. At temperatures above 600C, the quality of the resulting films deteriorates. High-quality crystalline films with high adhesion surface are obtained in the temperature range of 25–600 C. Pt, Ni were taken as a cathode material and Pt served as the anode. The optimal electrolyte composition and electrolysis mode for obtaining thin CoSe films close to the stoichiometric composition were also determined. 0.01 M H2SeO3, 0.1 М CoCl2, 100 ml H2O, i=0.2-0.4 mА/sm2, Т=25–600С, Cathode - Pt, Ni. anode Pt

Robust Antiferromagnetic FeRh Films on Mica.

FeRh shows an antiferromagnetic to ferromagnetic phase transition above room temperature, which permits its use as an antiferromagnetic memory element. However, its antiferromagnetic order is sensitive to small variations in crystallinity and composition, challenging its integration into flexible devices. Here, we show that flexible FeRh films of high crystalline quality can be synthesized by using mica as a substrate, followed by a mechanical exfoliation of the mica. The magnetic and transport data indicate that the FeRh films display a sharp antiferromagnetic to ferromagnetic phase transition. Magnetotransport data allow for the observation of two distinguishable resistance states, which are written after a field-cooling procedure. It is shown that the memory states are robust under the application of magnetic fields of up to 10 kOe.

Quantum correction to the anomalous Hall effect in PtMnGe thin films

The role of quantum correction such as weak localization in anomalous Hall effects (AHEs) is an important issue. In this work, we have investigated the robust quantum correction to the AHE in magnetic PtMnGe (PMG) thin films of high crystalline quality. The PMG films have not shown long-range magnetic order. The observed temperature and magnetic field dependence of the sheet conductivities in the ultrathin PMG films are explained based on the perturbation theory, where the interplay of spin-dependent scattering and the spin Zeeman splitting effect are taken into consideration. It is found that, as long as the magnetic scattering time is comparable to the spin-orbit scattering time, the conductivity correction due to the Cooperons between antiparallel spins that involves spin-flip scattering can be negligible in the two dimensions. Therefore, even when the spin-dependent scattering exists, the quantum correction to the transport behavior is robust in the two-dimensional PMG system. Our work makes a significant step toward clarifying the role of quantum correction in the AHE of the magnetic materials.

Early-stage growth of GeTe on Si(111)-Sb

The advent of germanium telluride as a promising ferroelectric Rashba semiconductor for spintronic applications requires the growth of nanometer-thick films of high crystalline quality. In this study, we have elucidated the initial growth stages of GeTe on Si(111)-Sb by scanning tunneling microscopy and low energy electron diffraction. We demonstrate the presence of an initial 0.35-nm-thick GeTe buffer layer followed by the 2D growth of GeTe via Frank-Read sources of atomic steps. As shown by core level spectroscopy, Sb is acting as a surfactant during growth up to a 5-nm-thick film. X-ray diffraction, transmission electron microscopy, and low energy electron microscopy evidence that numerous mirror domains and in-plane misorientations appear early in the growth process and are gradually buried at the film/substrate interface. The use of a miscut Si substrate close to Si(111) allows suppressing these defects from the early beginning of growth.

A-phase tantalum film deposition using bipolar high-power impulse magnetron sputtering technique

In this study, a method for forming a highly adhesive and ductile α-phase tantalum film on 304 stainless steel using the bipolar high-power impulse magnetron sputtering (bipolar HiPIMS) technique was investigated. The growth of brittle β-phase tantalum, which is commonly formed when using conventional direct current magnetron sputtering methods (DCMS) at room temperature, was prevented by the high-energy ions directed toward the growing films. The results showed the formation of an extremely thin (<70 nm) α-phase tantalum film without any additional processes, such as substrate biasing or heating. In addition, a high adhesion strength exceeding 45 MPa, broadened intermixed layer, and nanocrystalline microstructure were achieved by the effect of positive voltage. These results indicate that the bipolar HiPIMS technique is a facile deposition method for modifying the substrate surface and forming high-quality crystalline films. Our tantalum films are optimized to protect substrates against corrosion in severely harsh mechanical, chemical, and thermal environments. To confirm the superiority of the bipolar HiPIMS technique over HiPIMS and DCMS, the properties of the films formed using the three methods were analyzed via X-ray diffraction, Auger electron spectroscopy, atomic force microscopy, scanning electron microscopy, and pull-off adhesion testing.

Asymmetric liquid crystalline dibenzo[a,h]anthracenes for organic semiconductors with high mobility and thermal stability.

A series of asymmetric organic semiconductors based on N-shaped dibenzo[a,h]anthracene (DBA), Ph-DBA-Cn (n = 8, 10, 12), were developed. All Ph-DBA-Cn compounds had good chemical stability and smectic LC qualities, and thermally stable crystal phase can be maintained under 190 °C due to the suppressed molecular motions by the bent DBA core. High-quality crystalline films can be fabricated using a blade-coating technique. It was revealed that the average mobility of all Ph-DBA-Cn organic thin-film transistors (OTFTs) was estimated to be over 2.8 cm2 V-1 s-1, and a Ph-DBA-C8 device in particular afforded exceptional mobility of up to 11.8 cm2 V-1 s-1. The highly-ordered and uniaxially-oriented crystalline films composed of bilayer units were revealed to be responsible for their excellent electrical device performances. Furthermore, all Ph-DBA-Cn OTFTs can retain operational characteristics up to 160 °C over 1 cm2 V-1 s-1. These findings will be crucial for the development of high-mobility and thermally durable OSCs for practical electronics.

A practical guide to pulsed laser deposition.

Nanoscale thin films are widely implemented across a plethora of technological and scientific areas, and form the basis for many advancements that have driven human progress, owing to the high degree of functional tunability based on the chemical composition. Pulsed laser deposition is one of the multiple physical vapour deposition routes to fabricate thin films, employing laser energy to eject material from a target in the form of a plasma. A substrate, commonly a single-crystal oxide, is placed in the path of the plume and acts as a template for the arriving species from the target to coalesce and self-assemble into a thin film. This technique is tremendously useful to produce crystalline films, due to the wide range of atmospheric conditions and the extent of possible chemical complexity of the target. However, this flexibility results in a high degree of complexity, oftentimes requiring rigorous optimisation of the growth parameters to achieve high quality crystalline films with desired composition. In this tutorial review, we aim to reduce the complexity and the barrier to entry for the controlled growth of complex oxides by pulsed laser deposition. We present an overview of the fundamental and practical aspects of pulsed laser deposition, discuss the consequences of tailoring the growth parameters on the thin film properties, and describe in situ monitoring techniques that are useful in gaining a deeper understanding of the properties of the resultant films. Particular emphasis is placed on the general relationships between the growth parameters and the consequent structural, chemical and functional properties of the thin films. In the final section, we discuss the open questions within the field and possible directions to further expand the utility of pulsed laser deposition.

Comprehensive Analysis of Metal Modulated Epitaxial GaN.

While metal modulated epitaxy (MME) has been shown useful for hyperdoping, where hole concentrations 40 times higher than other techniques have been demonstrated, and the ability to control phase separation in immiscible III-nitrides, the complexity of the dynamically changing surface conditions during the cyclic growth is poorly understood. While MME is capable of superb crystal quality, performing MME in an improper growth regime can result in defective material. These complications have made the transfer of MME knowledge challenging. This work provides a comprehensive study of the conditions necessary for achieving the benefits of MME while avoiding undesirable defects. The effects of growth temperature, Ga/N ratio, and excess Ga dose per MME growth cycle on the morphological, structural, electronic, and optical properties of unintentionally doped (UID) MME grown gallium nitride (GaN) have been investigated. Optimal structural and electrical quality were achieved for GaN films grown at ∼650 °C, at pre-bilayer Ga coverage and at the moderate droplet regime. However, high defect concentrations were observed at the lowest growth temperatures, and counter to traditional MBE, as the excess Ga dose transitioned from bilayer coverage to the low droplet regime. Optoelectronic properties were optimal for films grown at intermediate growth temperatures, an excess Ga dose condition just before the droplet formation, and, at a III/V ratio of 1.3. Optimization of growth temperatures, Ga/N ratios, and excess Ga dose results in a range of growth conditions achieving smooth surfaces, step-flow surface morphology, and high crystalline quality films with low threading dislocation densities, allowing researchers to utilize the extensive advantages of MME.

Utilization of nanoporous biosilica of diatoms as a potential source material for fabrication of nanoelectronic device and their characterization

Biosilica obtained from frustules of diatoms have very delicate nanostructures similar to many micro- and nanofluidic devices. In the present investigation, a cheaper and ecofriendly metal-oxide-semiconductor (MOS) tool has been fabricated exploiting frustules of the diatom Halamphora subturgida. The structural, optical, and chemical characterizations of the cleaned frustules of cultured diatom were investigated through various techniques. Electron microscopic images revealed intricate morphology of the nanostructured silica with very minute pores making the frustules very hard but light material. UV-Vis spectroscopy showed maximum absorbance at 223 nm. The absorption maximum for photoluminescence was centered at 462 nm and that of cathodoluminescence was at 439.9 nm and 466.6 nm. Bulk of extracted silicon dioxide (SiO2) nano-powder from diatom frustules was used as a source material for preparing a high-quality crystalline film on P-type silicon (p-Si) by vapor-liquid-solid (VLS) method. The crystalline quality of the film was tested by X-ray diffraction (XRD) and the crystalline size obtained was 62 ± 2.4 nm. From scanning electron microscopic (SEM) investigation, the growth of a continuous film from diatom biosilica on p-Si substrate was revealed. The thickness of the as-grown SiO2 film was 22.2 ± 1.6 nm, obtained from spectroscopic ellipsometry (SE) study. The performances of fabricated Al/SiO2/Si metal-oxide-semiconductor capacitor were tested by measuring leakage current (~ 43 ± 8 × 10−11 A μm−2 at +2 V), capacitance-voltage, constant current (0.1 μA), and voltage stress (at − 2 V) for reliable gate dielectrics applications in MOS devices. Overall, the process was cost-effective and provides an alternative technique to design high-quality diatom-derived “Bio-SiO2 films” on p-Si substrate.

Formamidine-assisted fast crystallization to fabricate formamidinium-based perovskite films for high-efficiency and stable solar cells

A FA-assisted fast crystallization (FAAFC) method involving adding FA into an antisolvent to achieve homogeneous nucleation and a high-quality crystalline film.

Ultrathin regime growth of atomically flat multiferroic gallium ferrite films with perpendicular magnetic anisotropy

Room temperature magnetoelectric multiferroic thin films offer great promises for the spintronics industry. The actual development of devices, however, requires the production of ultrathin atomically smooth films of high crystalline quality in order to increase spin transfer efficiency. Using both high-resolution transmission electron microscopy and atomically resolved electron energy loss spectroscopy, we unveil the complex growth mechanism of a promising candidate, gallium ferrite. This material, with its net room-temperature magnetization of approximately $100\phantom{\rule{0.16em}{0ex}}\mathrm{emu}/\mathrm{c}{\mathrm{m}}^{3}$, is an interesting challenger to the antiferromagnetic bismuth ferrite. We obtained atomically flat gallium ferrite ultrathin films with a thickness control down to one fourth of a unit cell. Films with thicknesses as low as 7 nm are polar and show a perpendicular magnetic anisotropy of $3\ifmmode\times\else\texttimes\fi{}{10}^{3}\phantom{\rule{0.16em}{0ex}}\mathrm{J}/{\mathrm{m}}^{3}$ at 300 K, which makes them particularly attractive for spin current transmission in spintronic devices, such as spin Hall effect based heavy-metal / ferrimagnetic oxide heterostructures.

Role of plasma properties in controlling crystallinity and phase in oxide films grown by plasma-enhanced atomic layer epitaxy

Plasma-enhanced atomic layer epitaxy (PEALE) is a cyclic atomic layer deposition process that incorporates plasma-generated species into one of the cycle substeps to achieve layer-by-layer crystalline growth. The addition of plasma generally provides unique gas phase chemistries and a substantially reduced growth temperature compared to thermal approaches. Indeed, when properly configured, PEALE systems can deliver high-quality crystalline films with structural characteristics that rival those grown by conventional thermal equilibrium growth processes such as molecular beam epitaxy or metalorganic chemical vapor deposition. However, the inclusion of plasma also adds a complex array of reaction pathways that can be challenging to understand and control. This work focuses on the use of plasma diagnostics to inform the choice of process conditions for PEALE. Optical emission and vacuum ultraviolet emission spectroscopy, as well as spatially resolved Langmuir probe measurements, are employed to characterize an inductively coupled plasma source used for the growth of epitaxial TiO2 and Ga2O3 films on sapphire. Under plasma conditions with large concentrations of atomic oxygen and significant ion energy (30–50 eV), highly crystalline TiO2 and Ga2O3 films were grown, indicating that both reactive neutral chemistry and ion energy are important in these processes.

Influence of nitrogen ion species on mass-selected low energy ion-assisted growth of epitaxial GaN thin films

The application of an energy and mass selected ion beam assisted deposition setup to investigate the influence of hyperthermal atomic and molecular nitrogen ion species during the initial stages of epitaxial GaN thin film growth is reported. By using a compact quadrupole mass filter system a hyperthermal ion beam is mass filtered. The GaN thin film depositions with varying ion-to-atom arrival ratio and kinetic energies of 40 eV and 80 eV are performed to highlight the distinct impact of the ion species on the early stages of the growth. It is evaluated that precise selection of ion beam parameters enables the preferred growth of either the metastable zinc blende GaN phase or the stable wurtzite GaN phase on 6H-SiC(0001) substrates. Further, using molecular nitrogen ion species thin films of high crystalline quality could be deposited even at ion kinetic energies of 80 eV, while films with atomic nitrogen ion assistance feature reduced crystalline quality.

Ferromagnetic resonance studies of strain tuned Bi:YIG films

Bismuth-doped Yttrium iron garnet (Bi:YIG) thin films known for large magneto-optical activity with low losses still need to get probed for its magnetization dynamics. We demonstrate a controlled tuning of magnetocrystalline anisotropy in Bi-doped Y3Fe5O12 (Bi:YIG) films of high crystalline quality using growth induced epitaxial strain on [1 1 1]-oriented Gd3Ga5O12 (GGG) substrate. We optimize a growth protocol to get thick highly-strained epitaxial films showing large magneto-crystalline anisotropy, compare to thin films prepared using a different protocol. Ferromagnetic resonance measurements establish a linear dependence of the out-of-plane uniaxial anisotropy on the strain induced rhombohedral distortion of Bi:YIG lattice. Interestingly, the enhancement in the magnetoelastic constant due to an optimum substitution of Bi3+ ions with strong spin orbit coupling does not strongly affect the precessional damping (∼). Large magneto-optical activity, reasonably low damping, large magnetocrystalline anisotropy and large magnetoelastic coupling in Bi:YIG are the properties that may help Bi:YIG emerge as a possible material for photo-magnonics and other spintronics applications.

Scalable Epitaxial Growth of WSe2 Thin Films on SiO2/Si via a Self-Assembled PtSe2 Buffer Layer

The growth of large-area epitaxial transition metal dichalgogenides (TMDCs) are of central importance for scalable integrated device applications. Different methods have been developed to achieve large-sized high quality films. However, reliable approaches for centimeter-sized or even wafer-level epitaxial growth of TMDCs are still lacking. Here we demonstrate a new method to grow inch-sized epitaxial WSe2 films on SiO2/Si substrates at a much lower temperature with high repeatability and scalability. High quality crystalline films are achieved through direct selenization of a tungsten film with platinum as the underlayer. The self-assembled PtSe2 buffer layer, formed during selenization, assists epitaxial growth of WSe2. Using fabricated WSe2 films, excellent performance memory devices are demonstrated. As a member of the TMDC family, our findings based on WSe2 may also be applied to other TMDC materials for large-scale production of high quality TMDC films for various applications.