Growth Conditions Of Thin Films Research Articles

FEATURES OF THE ELECTRICAL PROPERTIES OF HETEROJUNCTIONS N-GAAS-P-(GAAS)1 - X - У(GE2)X(ZNSE)Y

The optimal technological conditions for growing multicomponent epitaxial films of solid solutions (GaAs)1 - x - y(Ge2)x(ZnSe)y with specified physical properties by liquid-phase epitaxy have been determined. It has been established that the above conditions for growing thin films from a tin solution-melt at temperature intervals of 730-630, 650-550 °С with a substrate cooling rate of 1 deg/min are the most optimal. In this case, the films had a thickness of 10 μm and p-type conductivity. For ohmic contacts to such semiconductor solid solutions, Sn, Au, In, (In-Ga) and (Ge-Ag) alloys were used. It was determined that the mobility of current carriers depends on the composition, the structural perfection of the epitaxial layers and the ionization energy of the atoms of the constituent components have values of 0.19, 0.07, 0.029 eV. It has been established that in heterostructures of the n-GaAs-p-(GaAs)1 - x - y(Ge2)x(ZnSe)y type, obtained at Т = 7500 °С, the current transmission is determined by tunneling-recombination, and in the samples obtained at Т = 7300 °С, by currents limited volume charges. It was also determined that a region with a higher resistivity is formed at the heterointerface, the thickness of which, depending on the growth conditions of thin films, is from 0.2 to 0.5 μm.

Influence of ex-situ thermal treatment on the chemical states, microstructure and ferroelectrics properties of polycrystalline BiMnO3-δ thin films

Nanoelectronic devices based on oxide films require materials that may exhibit combined properties such as ferroelectricity, ferromagnetism, and/or ferroelasticity at the same phase. In this sense, multiferroic perovskite BiMnO3-δ thin films exhibit a relevant combination of ferroelectricity and ferromagnetism suitable for device applications. The purpose of this work is to investigate the growth conditions of BiMnO3 thin films using radio-frequency magnetron sputtering and the role of ex-situ thermal treatments under different conditions, which resulted in a P21/c to C2 phase transformation and BiMnO2.93 composition. The crystal structure of the BiMnO3-δ thin films was investigated using X-ray diffraction, high-resolution transmission electron microscopy, ultraviolet-visible spectrophotometer, X-ray photoelectron spectroscopy, and resistivity measurement, to correlate structural and transport properties. The ex-situ thermal treatment was performed in a bismuth and oxygen atmosphere in order to compensate for Bi and O deficiencies. Thus, the result was not an ideal stoichiometric, BiMnO3-δ proved to enhance the ferroelectric properties and other physical properties. The optimized thin films exhibit ferromagnetism up to 37 K, while ferroelectricity remains up to room temperature.

(Invited) Ferroelectric Hafnium Oxide for Non-Volatile Memory Applications: From Single Capacitors to Memory Arrays

This year marks the 10th anniversary of the public announcement of the discovery of ferroelectricity in HfO2-based thin films. Tremendous strides have been made in both the physical understanding and the technological use of ferroelectric HfO2. These unique fluorite-structured ferroelectrics have outstanding potential for commercial applications due to their scalability, CMOS-compatibility, and facile manufacturability. Such advantages make ferroelectric HfO2 particularly attractive for nonvolatile memories technologies such as ferroelectric random access memory (FRAM) and ferroelectric field effect transistors. Due to the wide variety of conditions in which ferroelectricity can emerge in HfO2 thin films, there has been a vast effort by the international scientific community to enhance and optimize the ferroelectric properties using a wide variety of approaches. Advancements in thin film ferroelectric capacitor and memory array technologies are therefore progressing rapidly.The structural properties of HfO2 are a major focal point for technology development because the competition between several crystalline phases in HfO2 thin films can have a major impact on device performance. For thin film memory capacitors, stabilizing the polar orthorhombic phase and eliminating secondary paraelectric phases is paramount in achieving robust polycrystalline HfO2 ferroelectrics. Doping and the presence of a capping electrode during crystallization have been extensively demonstrated to influence the emergence and stability of the ferroelectric orthorhombic phase. Greater attention has been placed recently on the thin film growth conditions in atomic layer deposition and physical vapor deposition, especially with regard to the oxidation conditions that have been shown to have a substantial impact on the device performance of ferroelectric HfO2 and Hf0.5Zr0.5O2 capacitors. Composition, film growth, and the crystallization processes are critical aspects for thin film ferroelectric capacitor development and the latest developments will be comprehensively reviewed in the context of how the material properties relate to nonvolatile memory performance.Developments in thin film HfO2 ferroelectric capacitors are now coming to fruition with the emergence of ferroelectric random access memory arrays and demonstrators. Thin film ferroelectric HfO2 based capacitors are successfully being demonstrated in back-end-of-line (BEOL) processes at the 130nm CMOS technology. The performance of these early demonstrator memory arrays show very promising long term data retention, large memory windows with good statistical distributions, and projections for area scaling to nanoscale dimensions. The transition from large-scale micron-sized laboratory devices to smaller scale integrated capacitors has proven to be a major step forward in endurance cycling with >1011 cycles being reported in memory arrays. The most recent integration of thin film ferroelectric HfO2 capacitors into FRAM memory arrays and what future developments are expected for HfO2 ferroelectric random access memories will be presented. Figure 1

Machine learning predictions of surface migration barriers in nucleation and non-equilibrium growth

Machine learning is playing an increasing role in the discovery of new materials and may also facilitate the search for optimum growth conditions for crystals and thin films. Here, we perform kinetic Monte-Carlo simulations of sub-monolayer growth. We consider a generic homoepitaxial growth scenario that covers a wide range of conditions with different diffusion barriers (0.4–0.55 eV) and lateral binding energies (0.1–0.4 eV). These simulations are used as a training data set for a convolutional neural network that can predict diffusion barriers and binding energies. Specifically, a single Monte-Carlo image of the morphology is sufficient to determine the energy barriers with an accuracy of approximately 10 meV and the neural network is tolerant to images with noise and lower than atomic-scale resolution. We believe this new machine learning method will be useful for fundamental studies of growth kinetics and growth optimization through better knowledge of microscopic parameters.

Control of Competing Thermodynamics and Kinetics in Vapor Phase Thin-Film Growth of Nitrides and Borides.

Thin-film growth is a platform technique that allows the preparation of various undeveloped materials and enables the development of novel energy generation devices. Preferred phase formation, control of crystalline orientation and quality, defect concentration, and stoichiometry in thin films are important for obtaining thin films exhibiting desired physical and chemical properties. In particular, the control of crystalline phase formation by utilizing thin-film technology favors the preparation of undeveloped materials. In this study, thin-film growth of transition metal nitride and rare-earth metal boride was performed using remote plasma–assisted molecular beam epitaxy and hybrid physical–chemical vapor deposition techniques, and was successfully achieved by tuning the competition between thermodynamics and kinetics during vapor-phase thin-film growth. Growth conditions of high crystalline quality titanium nitride thin films and high phase purity ytterbium boride thin films were not thermodynamically favorable. Appropriate control of the contribution degree of thermodynamics and kinetics during vapor-phase thin-film growth is crucial for fabricating high phase purity and high crystalline quality thin films.

Structural-crystalline, optical, topographical properties of ZnO thin film produced in presence of various oxygen

Changes in growth conditions of ZnO thin films produced in the presence of different oxygen, changes in important properties such as crystal, surface properties, and absorption properties of the films were examined and reported. It is inferred from the XRD experimental results that the oxygen we applied to the films plays a role in the crystal structure changes of the films (grain size, strain value, dislocation density etc.) The highest RMS roughness value is 8.58 nm, the lowest RMS roughness value is 1.08 nm corresponding to non-flow and 1 sccm flow film respectively. AFM proved that films with nano-structured, tightly packed, grain properties were obtained in the produced films. Inference from UV analysis made is that the oxygen applied to the film caused small changes in the optical band gap values (in the range of about 3.30-3.32 eV). Except for 3 sccm oxygen state, all the films obtained were tightly packed, granulated and almost homogeneous and the nano property was clearly seen. All the results obtained show that the oxygen applied in the ZnO film process causes some changes in the physical properties of the film and this has an effect on the film quality and it is seen that these results can contribute to the production of the devices using ZnO.

Characterization of microscopic ferromagnetic defects in thin films using magnetic microscope based on Nitrogen-Vacancy centres

In this work we present results acquired by applying magnetic field imaging technique based on Nitrogen-Vacancy centres in diamond crystal for characterization of magnetic thin films defects. We used the constructed wide-field magnetic microscope for measurements of two kinds of magnetic defects in thin films. One family of defects under study was a result of non-optimal thin film growth conditions. The magnetic field maps of several regions of the thin films created under very similar conditions to previously published research revealed microscopic impurity islands of ferromagnetic defects, that potentially could disturb the magnetic properties of the surface. The second part of the measurements was dedicated to defects created post deposition - mechanical defects introduced in ferromagnetic thin films. In both cases, the measurements identify the magnetic field amplitude and distribution of the magnetic defects. In addition, the magnetic field maps were correlated with the corresponding optical images. As this method has great potential for quality control of different stages of magnetic thin film manufacturing process and it can rival other widely used measurement techniques, we also propose solutions for the optimization of the device in the perspective of high throughput.

Screening and optimization of processing temperature for Sb2Se3 thin film growth protocol: Interrelation between grain structure, interface intermixing and solar cell performance

Deliverability of controllable deposition processes represents the current state-of-the-art strategy for the development of emerging 1D crystal-structure photovoltaic (PV) materials. For Sb2Se3, a compound with tremendous potential for next-generation cost-efficient thin-film PV, the current reported findings proposed that columnar grain growth protocol is required to promote efficient carrier transport and to achieve highly efficient solar cells. It is still a matter of debate, however, whether the growth of the thin-film absorber should follow the weak contacted nanorod grain structure or the columnar-sintered large grain protocol, in relation with the processing temperature, the under-layer junction partner, and interface intermixing. In this work, close-spaced sublimation processing temperature in conjunction with CdS and TiO2 junction partner layers is systematically investigated towards optimization of Sb2Se3 thin film growth conditions. The desideratum of columnar-sintered large grains protocol is validated and insights into the mechanism of absorber-buffer interface intermixing are provided. Following a systematical technological approach, a solar cell efficiency of 5.3% is demonstrated. Combining classical and advanced measurement techniques, insights into physicochemical processes and device functionality in connection with grain structure, defect density, carrier concentration, and recombination mechanism in Sb2Se3 absorber are provided.

Magnetic anisotropy and electrical property of CoZrTaB thin films deposited by oblique sputtering

Soft magnetic materials have been studied extensively in recent years due to their applications in micro-transformers, micro-inductors, spin dependent memories etc. Achieving high uniaxial anisotropy is one of the most important properties for these materials. Oblique angle sputtering without the presence of the magnetic field enables the ease thus lower cost of the adoption of magnetic materials in applications. With a highly soft magnetic material, one can operate the devices made out of these films at giga hertz regions without significant loss of efficiency thus desirable for applications such as power conversion and radio frequency (RF) devices. This work focuses on analyzing different growth conditions of thin films of CoZrTaB and the resulting magnetic and electrical properties of the films. Thin films are grown by oblique-angle sputtering, where the sputtering gun forms an angle with respect to the sample substrate normal, ranging from 0 to 75 degrees. External magnetic field normally applied in order to form magnetic anisotropy is not used during film sputtering process. Using vibrating sample magnetometer, it was observed that films resulting from small oblique angles have no clear magnetic anisotropy developed; whereas in samples deposited at large angles close to 60°, there is clear magnetic anisotropy observed. Scanning electron microscopy imaging of the cross-section of film suggests the formation of tilted columns, which is likely to be the reason for magnetic anisotropy. Resistivity of the films was measured systematically and found to increase as the magnitude of oblique angle during sputtering increased.

Study of CuSbSe2 thin films grown by pulsed laser deposition from bulk source material

In this paper, we discuss the details of growth of CuSbSe2 chalcostibite thin films via pulsed laser deposition using bulk ball milled compound. Thin film growth conditions, in particular pulsed laser energy was optimized to obtain the near stoichiometric thin films. Films were grown at room temperature as well as at 400 °C and as grown films were post annealed at 400 °C to see annealed induced effects. Material characteristics of the thin films were evaluated in detail using X-ray diffraction, X-ray photoelectron spectroscopy, UV–Vis–NIR spectroscopy and Raman spectroscopy studies. Scanning electron microscopy and dynamic force microscope measurements revealed the morphology as well as smoothness of the thin films. Hall coefficient values measured positive and carrier concentration value of 1.2 × 1018 cm−3 for the films grown at 400 °C. Absorption coefficient values of the order of ~105 cm−1 in the entire visible region and band gap of ~1.2 eV observed infer favorable photovoltaic characteristics.

Defect-Engineered Electroforming-Free Analog HfOx Memristor and Its Application to the Neural Network.

The thin-film growth conditions in a plasma-enhanced atomic layer deposition for the (3.0-4.5) nm thick HfO2 film were optimized to use the film as the resistive switching element in a neuromorphic circuit. The film was intended to be used as a feasible synapse with analog-type conductance-tuning capability. The 4.5 nm thick HfO2 films on both conventional TiN and a new RuO2 bottom electrode required the electroforming process for them to operate as a feasible resistive switching memory, which was the primary source of the undesirable characteristics as the synapse. Therefore, electroforming-free performance was necessary, which could be accomplished by thinning the HfO2 film down to 3.0 nm. However, the device with only the RuO2 bottom electrode offered the desired functionality without involving too high leakage or shorting problems, which are due to the recovery of the stoichiometric composition of the HfO2 near the RuO2 layer. In conjunction with the Ta top electrode, which provided the necessary oxygen vacancies to the HfO2 layer, and the high functionality of the RuO2 as the scavenger of excessive incorporated oxygen vacancies, which appeared to be inevitable during the repeated switching operation, the Ta/3.0 nm HfO2/RuO2 provided a highly useful synaptic device component in the neuromorphic hardware system.

Machine-learning-assisted thin-film growth: Bayesian optimization in molecular beam epitaxy of SrRuO3 thin films

Materials informatics exploiting machine learning techniques, e.g., Bayesian optimization (BO), have the potential to reduce the number of thin-film growth runs for optimization of thin-film growth conditions through incremental updates of machine learning models in accordance with newly measured data. Here, we demonstrated BO-based molecular beam epitaxy (MBE) of SrRuO3, one of the most intensively studied materials in the research field of oxide electronics, mainly owing to its unique nature as a ferromagnetic metal. To simplify the intricate search space of entangled growth conditions, we ran the BO for a single condition while keeping the other conditions fixed. As a result, high-crystalline-quality SrRuO3 film exhibiting a high residual resistivity ratio of over 50 as well as strong perpendicular magnetic anisotropy was developed in only 24 MBE growth runs in which the Ru flux rate, growth temperature, and O3-nozzle-to-substrate distance were optimized. Our BO-based search method provides an efficient experimental design that is not as dependent on the experience and skills of individual researchers, and it reduces experimental time and cost, which will accelerate materials research.

Kinetic modeling of phase separation and surface segregation in growing a-C:Ni thin films

The processes of phase separation and surface segregation which take place during two-component nanocomposite Metal:Carbon thin film growth are analyzed. The mathematical model is proposed which includes the processes of adsorption, phase separation, surface segregation, surface and bulk diffusion. The process of phase separation is described using Cahn-Hilliard equation and the process of surface segregation is described by Gibbsian segregation model. Diffusion in growing film is allowed to occur in a layer of an arbitrary thickness near the surface. The formations of metal clusters, columns or surface layer in matrixes of carbon are shown by the numerical simulations depending on thin film growth conditions. The influence of film composition and deposition temperature on film morphology is analyzed by the presented simulations. The simulation results are compared with experimental data taken from literature where C:Ni thin films were grown at different contents of Ni and substrate temperatures. The simulation results are in a good agreement with the corresponding experimental data.

Study of CBD-CdS/CZTGSe solar cells using different Cd sources: behavior of devices as a MIS structure

CdS buffer layer plays an important role in thin film solar cells; as a result, a deep understanding of its physical properties and impact on solar cell performance is highly required. In this work, we report results on growth conditions of CdS thin films deposited by chemical bath method to be used as insulators in MIS structure instead of the traditionally assumed p–n junction. Three different Cd sources—CdCl2, Cd(NO3)2, and CdSO4—are considered for CdS processing. The impact of Cd source on CdS physical properties as well as on the electro-optical device properties of CdS/CZTGSe solar cells is evaluated. Furthermore, interface state properties are calculated as a function of temperature. These states are a result of the CdS interaction with both the TCO and Kesterites, depending on the Cd source used in solution of the chemical bath deposition. Besides, some important requirements to be considered in order to improve device efficiency are discussed in the present work.

Greater Porosity with Redox Reaction Speeds Up MOF Color Change

In this issue of Chem , Dincă and colleagues show that immobilization of redox-active naphthalene diimide moiety into a mesoporous metal-organic framework structure (MOF-74) allows the rapid and reversible transparent-to-dark electrochromism and has a potential application in smart windows.

Growth and characterization of α and β-phase tungsten films on various substrates

The growth conditions of tungsten thin films were investigated using various substrates including Si, Si/SiO2, GaAs, MgO, and Al2O3, and recipes were discovered for the optimal growth conditions of thick metastable β-phase tungsten films on Si, GaAs, and Al2O3 substrates, which is an important material in spin orbit torque studies. For the Si/SiO2 substrate, the crystal phase of the tungsten films was different depending upon the tungsten film thickness, and the transport properties were found to dramatically change with the thickness owing to a change in phase from the α + β phase to the α-phase. It is shown that the crystal phase changes are associated with residual stress in the tungsten films and that the resistivity is closely related to the grain sizes.

Investigation of Photoelectrochemical Deposition of Zn(S,O) Via Nitrate Ions Reduction in the Presence of Thiourea

Cadmium-free buffer layers in high efficiency Cu(In,Ga)Se2 (CIGSe) solar cells have already been obtained via Chemical Bath Deposition of Zn(S,O)1. Electrodeposition is a low cost technique, which allows a relatively easy industrial implementation and the preparation of conformal films whose properties and thickness can be easily controlled. These advantages make the method attractive for the synthesis of buffer layers in CIGSe solar cells. Generally, the sulfur precursor which is used to electrodeposit Zn(S,O) is sodium thiosulfate (Na2S2O3)2. Nevertheless, in our case, this precursor did not lead to good results. Furthermore, to date, no attempts have been carried out to electrodeposit Zn(S,O) buffer layers using thiourea as sulphur (S) precursor. The effect of thiourea concentration (0-1 M) on the electrochemical behaviour and the growth conditions of thin films have been studied on glass/Mo/CIGSe absorbers. CIGSe being a p-type semiconductor, photoelectrochemical studies were performed on this substrate using a Xenon lamp or a LED panel to electrodeposit Zn(S,O). When CIGSe substrate is enlighten, a cathodic photocurrent appears in the external circuit and the surface electrons react with the oxidant leading to its reduction. In this work, first, the photoelectrochemical behaviour of CIGSe has been investigated by mean of chopped voltammetry in acidic medium for pH values of 1 and 5 (Fig. 1). The comparison of voltammograms obtained in dark and light conditions allowed the determination of the potential range in which the CIGSe is photoelectrochemically active. The electrodeposition of Zn(S,O) in acidic nitrate medium has then been investigated using Zn(NO3)2 as both zinc and oxygen precursor. As previously shown3, nitrate reduction is catalysed by zinc ions following the global reaction (1): Zn2+ + NO3 - + 2 e- → ZnO + NO2 - (1) As the reduction of nitrates locally increases the pH at the interface, thiourea hydrolysis is expected and, consequently, the generation of S2- ions which can react with Zn2+ ions to precipitate ZnS. Simultaneously, Zn2+ ions react with interfacial OH- leading to ZnO precipitation. In order to have a better understanding of the role of thiourea, a thermodynamical study of the system has been carried out at 80 °C showing a strong complexation of Zn2+ions with thiourea. Voltamperometric measurements show that the electrochemical behaviour of the bath is dependent on thiourea concentration. Films were deposited in a potential range of – 0.9 V/MSE to -1.2 V/MSE for different thiourea concentrations on CIGSe substrates and have been characterized through SEM, EDX and XRD analysis. The SEM images show conformal films with a thickness between 1 μm and 7 μm. Fig. 2 shows the surface and cross section morphology for a film obtained at – 0.9 V/MSE for 5 minutes and a thiourea concentration of 0.5 M. The film growth is very rapid leading to conformal films containing Zn and O as main components and an increasing amount of S proportional to thiourea concentration. Nevertheless, the S incorporation remains quite low. The XRD analysis revealed that the films are composed of well crystallized ZnO. Moreover, these studies highlight the predominant complexing role of thiourea which influences the growth and morphology of the deposited films leading to nanoporous ZnO. In order to have a better understanding of the role of thiourea, XPS analyses have been performed and will be discussed in this paper.

Stress engineering using low oxygen background pressures during Volmer–Weber growth of polycrystalline nickel films

A robust strategy for controlling the level of residual stress in polycrystalline films remains elusive, owing to the complex coevolution of the surface, microstructure, and intrinsic stress during Volmer–Weber film growth. Recent improvements in the understanding of stress evolution mechanisms have led to the possibility of engineering the intrinsic stress through the control of thin film growth conditions. Here, the authors demonstrate stress engineering during deposition of polycrystalline Ni films through control of the oxygen partial pressure. The physical mechanisms of stress management during codeposition of nickel and oxygen are investigated using in situ stress measurements and ex situ structural and chemical characterizations. The intrinsic stress in Ni films is affected by grain growth during deposition (which causes a tensile stress) and by Ni adatom trapping at grain boundaries and oxygen incorporation in the Ni lattice (which cause a compressive stress). The authors show direct evidence that a small amount of oxygen suppresses grain growth during deposition. They suggest that the presence of chemisorbed oxygen limits surface diffusion of Ni adatoms, thereby limiting adatom trapping at grain boundaries. The presence of oxygen therefore affects the mechanisms for development of both tensile and compressive stresses, providing a direct method for engineering the residual stress in as-deposited Ni films. Finally, the authors demonstrate a process for evaporative deposition of “zero” stress Ni films by introducing a very low level of background impurities, with the resultant films containing only 1.2 at. % oxygen.

Synthesis of semimetal A3Bi (A = Na, K) thin films by molecular beam epitaxy

Three-dimensional (3D) Dirac cones are predicted to reside in semimetals A3Bi (A=Na, K). By using molecular beam epitaxy (MBE) and scanning tunneling microscopy (STM), we have successfully established the growth conditions for Na3Bi thin films on Si(111)-7×7, and determined that the lattice of Na3Bi is rotated by 30 degree with respect to that of Si(111)-7×7. The Na3Bi/Si(111)-7×7 thin film was further used as the substrate for the growth of K3Bi. The 3D Dirac-cone-like electronic band structures of Na3Bi and K3Bi have been clearly revealed by angle resolved photoelectron spectroscopy (ARPES).

Semimetal Na3Bi Thin Film Grown on Double-Layer Graphene by Molecular Beam Epitaxy

Atomically flat thin films of topological semimetal Na3 Bi are grown on double-layer graphene formed on 6H–SiC(0001) substrates by molecular beam epitaxy. By combined techniques of molecular beam epitaxy, scanning tunneling microscopy and angle resolved photoelectron spectroscopy, the growth conditions for Na3 Bi thin films on double-layer graphene are successfully established. The band structure of Na3 Bi grown on graphene is mapped along Γ̄–M̄ and Γ̄–K̄ directions. Furthermore, the energy band of Na3 Bi at higher energy is uncovered by doping Cs atoms on the surface.