Oxygen Background Pressure Research Articles

Heteroepitaxial growth of Ga2O3 thin films on Al2O3(0001) by ion beam sputter deposition

Deposition of epitaxial oxide semiconductor films using physical vapor deposition methods requires a detailed understanding of the role of energetic particles to control and optimize the film properties. In the present study, Ga2O3 thin films are heteroepitaxially grown on Al2O3(0001) substrates using oxygen ion beam sputter deposition. The influence of the following relevant process parameters on the properties of the thin films is investigated: substrate temperature, oxygen background pressure, energy of primary ions, ion beam current, and sputtering geometry. The kinetic energy distributions of ions in the film-forming flux are measured using an energy-selective mass spectrometer, and the resulting films are characterized regarding crystalline structure, microstructure, surface roughness, mass density, and growth rate. The energetic impact of film-forming particles on the thin film structure is analyzed, and a noticeable decrease in crystalline quality is observed above the average energy of film-forming Ga+ ions around 40 eV for the films grown at a substrate temperature of 725 °C.

Synergistic effect of morphology and Co+2/Co+3 ratio on supercapacitor performance of Co3O4/vertical graphene nanosheets binder-free hybrid electrodes

Cobalt oxide (Co3O4) nanostructures decorated vertical graphene nanosheets (VGN) based hybrid electrodes are synthesized to fabricate binder-free high-performance supercapacitors. Tuning of microstructure, morphology, and stoichiometry of Co3O4 nanostructures is endeavoured by growing them in an oxygen medium using the pulsed laser deposition (PLD) technique. The variation in background oxygen pressure transformed the growth morphology of Co3O4 nanostructures from smooth film to nano-particulate caterpillar, flower, and cloud-like. Additionally, the variation in background oxygen pressure significantly influences the Co+2/Co+3 ratio. Moreover, the morphology variation led to a significant change in electrochemical active surface area (ECSA), and the electrochemical impedance characteristics varied from highly capacitive to nearly diffusive nature. These Co3O4/VGN hybrid electrodes exhibited an areal capacitance value of 32 mF/cm2 (1900 F/g) @ 20 mV/s with cyclic stability of 87 % after 3000 charge-discharge (CD) cycles in 1 M KOH electrolyte. Herein, a correlation between morphology, ECSA, and Co+2/Co+3 ratio with areal capacitance values is established. Finally, an asymmetric coin-cell device is fabricated using the Co3O4/VGN as a positive electrode and oxygenated VGN as a negative electrode, respectively. The coin-cell device exhibited a 4 mF/cm2 capacitance with energy and power densities of about 1.8 μWh/cm2 and 218.8 μW/cm2, respectively. Additionally, the coin-cell device displayed excellent cyclic stability of 92% after 5000 CD cycles. The potential utilization of this device is demonstrated by lighting an LED.

PLD of α-Ga2O3 on m-plane Al2O3: Growth regime, growth process, and structural properties

Phase-pure α-Ga2O3 thin films with high surface quality and crystallinity have been grown on m-plane sapphire using pulsed laser deposition (PLD). Therefore, the influence of growth temperature, oxygen background pressure, and film thickness on the structural properties is investigated to determine the growth window for phase-pure corundum structured α-Ga2O3. Samples were analyzed using x-ray diffraction (XRD), atomic force microscopy (AFM), and spectroscopic ellipsometry measurements. A distinct growth window in favor of phase-pure (10.0)-oriented α-Ga2O3 for growth temperatures above 480 °C and low oxygen partial pressures p(O2) of 3 × 10−4 mbar is identified. The growth rate increases significantly with increasing oxygen pressure. Furthermore, it shows an Arrhenius-like decrease for lower temperatures, caused by the increasing desorption of volatile Ga2O suboxides. It was found that for thicker layers, the growth of monoclinic β-Ga2O3 is promoted likely facilitated by the c-facets of the α-Ga2O3 grains. This leads to a (010)-oriented island growth corroborated by stripe-like features in AFM scans and a corresponding in-plane orientation confirmed by XRD ϕ-scans. For oxygen partial pressures above 3 × 10−4 mbar and Tg < 540 °C, the formation of mixed (10.0) α-Ga2O3 and spinel-defective (110)-γ-Ga2O3 manifesting as defective inclusions was observed independent of the layer thickness d. A corresponding p(O2) − d − T phase diagram for the growth of Ga2O3 on m-plane sapphire by PLD is provided.

Осаждение прозрачных покрытий Al-=SUB=-2-=/SUB=-O-=SUB=-3-=/SUB=- с экстремальными свойствами смачивания методом наносекундной лазерной абляции алюминия в фоновом кислороде

Transparent aluminum oxide nanostructures with extreme wetting properties were synthesized by nanosecond laser deposition in background oxygen. The transparency and morphology of the samples have been analyzed. Non-monotonic behavior of the transmittance coefficient was found when varying the background oxygen pressure in the range of 20-140 Pa due to differences in the kinetics of ablation product dispersion. The evolution of the contact wetting angle from ~ 5 to ~ 120◦ when the coatings are stored in air under normal conditions was investigated.

New Insight into the Gas Phase Reaction Dynamics in Pulsed Laser Deposition of Multi-Elemental Oxides.

The gas-phase reaction dynamics and kinetics in a laser induced plasma are very much dependent on the interactions of the evaporated target material and the background gas. For metal (M) and metal–oxygen (MO) species ablated in an Ar and O2 background, the expansion dynamics in O2 are similar to the expansion dynamics in Ar for M+ ions with an MO+ dissociation energy smaller than O2. This is different for metal ions with an MO+ dissociation energy larger than for O2. This study shows that the plume expansion in O2 differentiates itself from the expansion in Ar due to the formation of MO+ species. It also shows that at a high oxygen background pressure, the preferred kinetic energy range to form MO species as a result of chemical reactions in an expanding plasma, is up to 5 eV.

Influence of substrate temperature and oxygen pressure on the structural and optical properties of polycrystalline BaTiO3 thin films grown by PLD

The effect of substrate temperature and oxygen pressure on linear and nonlinear optical properties of pulsed laser deposited barium titanate BaTiO3 (BTO) thin films is reported in the present manuscript. BTO thin films have been deposited on n-type Si (100) and quartz substrates in the range of substrate temperatures from 400 to 700 °C and oxygen pressures from 0.005 to 1 mbar. The root means square (RMS) surface roughness is observed to decrease with the rise in substrate temperature and increased with an increase in oxygen pressure. The optimum background pressure and the substrate temperature are observed to be around ∼0.1 mbar and 700 °C, respectively. All the BTO thin films are found to be in the preferential (110) plane, from X-ray Diffraction (XRD), indicating the pseudocubic phase. Raman analysis also confirms the pseudocubic phase of BTO for all the thin films. The crystallite size, measured from XRD analysis is found to be increased with the increase in substrate temperature and is also affected by the background oxygen pressure. The thickness, linear absorption coefficient, and refractive index of the thin films are determined from UV–Visible–NIR spectroscopy. Transmission is observed to be 75–85% for all the thin films in the spectral range of 400–2000 nm. Nonlinear optical properties (NLO) are studied by an in-house assembled modified Z-scan set-up using continuous-wave (cw) Ar-Ion laser (λ = 488 nm) and found to be (57.54 ± 0.05) cm/W at the optimum deposition parameters.

Sinusoidal changes in transcutaneous oxygen pressure, suggesting Cheyne-Stokes respiration, are frequent and of poor prognosis among patients with suspected critical limb ischemia

Background and aimsTranscutaneous oxygen pressure (TcpO2) is used in patients with suspected critical limb ischemia (CLI). Sinusoidal changes (SC~) in TcpO2 are found in patients with Cheyne-Stokes respiration (CSR). We aimed to determine the characteristics of TcpO2 changes at rest in patients with suspected CLI, define the objective criteria for SC ~ TcpO2 patterns (SC+), and estimate the prevalence of SC+ in our population and its impact on the outcome. MethodsWe retrospectively analyzed 300 chest TcpO2 recordings performed in a 16-month period. We determined the presence/absence of SC ~ TcpO2 by visual analysis. We determined the acceptable error in the regularity of peaks of the cross-correlation with ROC curve analysis, among patients with typical SC ~ TcpO2 and non-sinusoidal patterns. Then, we defined SC + as a minimum of five peaks, a standard deviation of TcpO2 >1.25 mmHg, an error in regularity of peaks of the cross-correlation < 10%, and a cycle length between 30 and 100 s. In patients included until October 2019, we compared the outcome as a function of SC + or SC- with Cox models. ResultsMathematical detection of SC + found that 43 patients (14.3%) fulfilled all four defined criteria at the chest level, but only 23 did so at the limb level. In the follow-up of 207 patients, the presence of Sc ~ TcpO2 at the chest significantly increased the risk of mortality: hazard ratio: 2.69 [95%CI: 1.37–5.30]; p < 0.005. ConclusionsSC ~ TcpO2 is frequent, and is associated with a poor outcome in patients with suspected CLI.

Numerical modeling of the plasma plume propagation and oxidation during pulsed laser deposition of complex oxide thin films

Pulsed laser deposition is widely used to synthesize complex oxide thin films with advanced functional properties because of the versatility of the process, although the actual processes and mechanisms that take place are highly dynamic and nontrivial. Detailed understanding of the plume dynamics is required to achieve enhanced control of the growth of complex oxide thin films. However, analytical models of the plasma plume proposed in previous studies, the so-called shockwave model, drag model, and adiabatic thermalization model, only apply to a specific pressure regime and only focus on the propagation of the plasma front. Numerical modeling of the plasma dynamics has previously been pursued for the one-dimensional propagation of a Si plasma plume in a noble background gas. Here we have explored a more generalized numerical model for the dynamics of a ${\mathrm{TiO}}_{2}$ plasma plume by extending to three dimensions, multiple elements in the plasma plume, and including chemical reactions in an oxygen environment without the use of adaptive parameters, other than used for the description of the initial plume shape. Comparison between the simulations and the self-emission measurements of the ${\mathrm{TiO}}_{2}$ plasma plume shows good agreement. Our model enables detailed simulation of the oxidation state of the arriving particles (Ti, TiO, and ${\mathrm{TiO}}_{2}$) on the substrate surface depending on oxygen background pressure. Interestingly, low pressures of about 0.02 mbar result in fast oxygen particles arriving prior to the slower titanium particles, which could significantly influence the oxidation state of the initial substrate surface. The enhanced number of collisions for higher pressures of about 0.1 mbar leads to a large amount of the low-mass oxygen particles coming to a standstill 1--2 cm away from the ablated target. Furthermore, the three-dimensional nature of our model has enabled simulation of the lateral variations in composition of the deposited particles on the substrate surface. Although negligible variations in the Ti:TiO and Ti:${\mathrm{TiO}}_{2}$ ratios are present for high pressures over deposition areas with diameters up to 5 cm, significant variations can be observed for low pressures. These insights could play an important role in upscaling pulsed laser deposition from a scientific laboratory-based scale to an industrial large-area scale.

Amorphous ZnO/PbS Quantum Dots Heterojunction for Efficient Responsivity Broadband Photodetectors.

The integration of lead sulfide quantum dots (QDs) with a high-conductivity material that is compatible with a scalable fabrication is an important route for the applications of QD-based photodetectors. Herein, we first developed a broadband photodetector by combining amorphous ZnO and PbS QDs, forming a heterojunction structure. The photodetector showed detectivity up to 7.9 × 1012 and 4.1 × 1011 jones under 640 and 1310 nm illumination, respectively. The role of the oxygen background pressure in the electronic structure of ZnO films grown by pulsed laser deposition was systematically studied, and it was found to play an important role in the conductivity associated with the variation of the oxygen vacancy concentration. By increasing the oxygen vacancy concentration, the electron mobility of amorphous ZnO layers dramatically increased and the work function decreased, which were beneficial for the photocurrent enhancement of ZnO/PbS QD photodetectors. Our results provide a simple and highly scalable approach to develop broadband photodetectors with high performance.

Oxygen in Complex Oxide Thin Films Grown by Pulsed Laser Deposition: a Perspective

For thin film synthesis of complex oxides, one of the most important issues has always been how to oxidise the material. For a technique like pulsed laser deposition, a key benefit is the relatively high oxygen background pressure one can operate at, and therefor oxidation should be relatively straightforward. However, understanding the microscopic oxidation mechanisms turns out to be rather difficult. In this perspective, we give a brief overview of the sources of oxidation for complex oxide thin films grown by pulsed laser deposition. While it is clear what these sources are, their role in the kinetics of the formation of the crystal structure and oxygen stoichiometry is not fully understood.

Mechanism of Si doping in plasma assisted MBE growth of β-Ga2O3

We report on the origin of high Si flux observed during the use of Si as a doping source in plasma assisted MBE growth of β-Ga2O3. We show on the basis of secondary ion mass spectroscopy analysis that Si flux is not limited by the vapor pressure of Si but by the formation of volatile SiO. The low sublimation energy of SiO leads to a weak dependence of the SiO flux of Si cell temperature and a strong dependence on the background oxygen pressure. Extended exposure to activated oxygen results in reduction of SiO flux due to the formation of SiO2 on the Si surface. The work reported provides key understanding for incorporating Si into future oxide-based semiconductor heterostructure and device MBE growth.

Influence of background oxygen pressure and post - deposition annealing on the structural, morphological, optical and luminescence properties of laser ablated SrWO4 thin films

Poly-crystalline strontium tungstate films with tetragonal scheelite phase were prepared on quartz substrate using pulsed laser ablation technique at different background oxygen pressures. These films were then post-annealed at a temperature of 800 °C for 2 h in air. The films prepared under oxygen ambience show a textured nature with preferred direction of crystal growth along <004>.The low value of FWHM of the XRD peaks exhibited by the annealed films compared to that of the as-deposited films suggests an enhancement in crystalline quality of the films with annealing. AFM and FESEM micrographs show that the morphology of the as-deposited films exhibit a tendency to form bigger clusters whereas the annealed films depict a dense distribution of grains. The films prepared under oxygen ambience show higher values of transmittance compared to those prepared in oxygen free ambience. All the films show luminescence emission ~388 nm and 468 nm region originating from lattice defects and surface defects respectively. The chromaticity diagrams show that the CIE coordinate of the films are very close to blue light region, suggesting the suitability of these films for the fabrication of blue light emitting device applications.

Incongruent pulsed laser deposition strategy for thin film growth of Ca3Co4O9 thermoelectric compound

Abstract Pulsed laser deposition (PLD) is commonly used to grow thin film materials with complex stoichiometry and crystal structures, since it achieves a congruent transfer of the target cation elements to the substrate. Although the incongruent PLDs were previously observed via regulating the plasma/background interaction regimes, the incongruent PLDs were seldom investigated and applied practically for the thin film depositions. Herein, we demonstrate the incongruent pulsed laser deposition of Ca3Co4O9 thermoelectric thin films, via regulating the target compositions and the oxygen background pressures to preferential scattering the plasma cation compositions. To compare the deposition properties via incongruent and congruent thin film growth, we systematically investigate the crystal structures, surface morphologies, elemental distributions and thermoelectric transportation behaviors for Ca3Co4O9 thin films grown at various target compositions and background pressure ranges. A stoichiometric Ca3Co4O9 thin film composition is achievable via designing an incongruent PLD strategy, in which case the Ca/Co ratios in the targets are enlarged actively while the PLDs are performed at a pressure range of 100 Pa to further preferential scatter of the lighter Ca compared to Co. Nevertheless, the incongruently grown thin films exhibit lower thermoelectric performances, compared to the one at a similar composition grown via the congruent PLD strategy. This result may associate to the localized composition inhomogeneity in the Ca3Co4O9 grown incongruently, and their electronical resistivities are largely elevated by the resultant impurities that are hardly detectable by X-ray diffractions. The present work reveals the complexity in deposition mechanism and kinetics when performing incongruent PLDs, in particular, for growing multiple elemental thin film materials.

Effect of oxygen pressure on the structural and optical properties of BaSnO3 films prepared by pulsed laser deposition method

BaSnO3 thin films were deposited on quartz substrate by pulsed laser deposition technique under different background oxygen pressures and the effects of oxygen pressure on the structural, morphological and optical properties of BaSnO3 thin films are systematically investigated using different characterization techniques. The BaSnO3 films deposited without and with oxygen pressures are polycrystalline in nature with cubic crystalline phase. Moderate oxygen ambience favors enhanced crystallinity of the BaSnO3 films and 0.02 mbar is found to be optimum oxygen pressure for highest crystallinity. The surface morphology of the deposited films was strongly affected by the oxygen pressure in the deposition chamber. A systematic increase of film thickness and decrease of RMS surface roughness is observed with increase in oxygen pressure. XPS analysis reveals that barium is in the + 2 oxidation state and Sn is in the + 4 oxidation state in the film deposited at optimum oxygen pressure of 0.02 mbar. The deposited BaSnO3 films have photoluminescence emissions in the visible region and have high transmittance in the visible and infrared regions. The BaSnO3 films deposited at oxygen ambience shows a blue shift in the optical band gap. The optimized film shows high crystallinity, high value of transmittance and wide band gap energy which indicates its suitability for optoelectronic devices.

Designing zinc oxide nanostructures (nanoworms, nanoflowers, nanowalls, and nanorods) by pulsed laser ablation technique for gas‐sensing application

AbstractIn this study, pulsed laser ablation technique, also known as pulsed laser deposition (PLD), is used to design and grow zinc oxide (ZnO) nanostructures (nanoworms, nanowalls, and nanorods) by template/seeding approach for gas‐sensing applications. Conventionally, ZnO nanostructures used for gas‐sensing have been usually prepared via chemical route, where the 3D/2D nanostructures are chemically synthesized and subsequently plated on an appropriate substrate. However, using pulsed laser ablation technique, the ZnO nanostructures are structurally designed and grown directly on a substrate using a two‐step temperature‐pressure seeding approach. This approach has been optimized to design various ZnO nanostructures by understanding the effect of substrate temperature in the 300‐750°C range under O2 gas pressure from 10‐mTorr to 10 Torr. Using a thin ZnO seed layer as template that is deposited first at substrate temperature of ~300°C at background oxygen pressure of 10 mTorr on Si(100), ZnO nanostructures, such as nanoworms, nanowalls, and nanorods (with secondary flower‐like growth) were grown at substrate temperatures and oxygen background pressures of (550°C and 2 Torr), (550°C and 0.5 Torr), and (650°C and 2 Torr), respectively. The morphology and the optical properties of ZnO nanostructures were examined by Scanning Electron Microscope (SEM‐EDX), X‐ray Diffraction (XRD), and photoluminescence (PL). The PLD‐grown ZnO nanostructures are single‐crystals and are highly oriented in the c‐axis. The vapor‐solid (VS) model is proposed to be responsible for the growth of ZnO nanostructures by PLD process. Furthermore, the ZnO nanowall structure is a very promising nanostructure due to its very high surface‐to‐volume ratio. Although ZnO nanowalls have been grown by other methods for sensor application, to this date, only a very few ZnO nanowalls have been grown by PLD for this purpose. In this regard, ZnO nanowall structures are deposited by PLD on an Al2O3 test sensor and assessed for their responses to CO and ethanol gases at 50 ppm, where good responses were observed at 350 and 400°C, respectively. The PLD‐grown ZnO nanostructures are very excellent materials for potential applications such as in dye‐sensitized solar cells, perovskite solar cells and biological and gas sensors.

Impact of Background Oxygen Pressure on the Pulsed-Laser Deposition of ZnO Nanolayers and on Their Corresponding Performance as Electron Acceptors in PbS Quantum-Dot Solar Cells

ZnO films are commonly employed as n-type semiconductors in heterojunctions with PbS colloidal quantum-dot (CQDs) films because of their outstanding optical transparency and electron acceptor properties, yet studies of the impact of ZnO film microstructure, composition, and defect qualities on the solar-cell performance are quite limited. Here we report on the fabrication of ZnO films via pulsed-laser deposition and use these films to investigate how different morphologies affect the PbS CQD solar-cell performance. By modification of the background gas O2 pressures during the ZnO deposition process, the device performance approaching a 7.8% energy conversion efficiency is achieved with an O2 pressure of 100 mTorr. Higher or lower O2 pressures led to significantly lower device efficiency. We employ various materials and device characterizations to highlight the differences in the physical properties introduced by the fabrication oxygen pressure. In particular, we have found that the differences in the type...

Impact of Background Oxygen Pressure on the Pulsed-Laser Deposition of ZnO Nanolayers and on Their Corresponding Performance as Electron Acceptors in PbS Quantum-Dot Solar Cells

Impact of Background Oxygen Pressure on the Pulsed-Laser Deposition of ZnO Nanolayers and on Their Corresponding Performance as Electron Acceptors in PbS Quantum-Dot Solar Cells

Stability of Ultrathin Ceria Films on Pt(111) Exposed to Air and Treated in Redox Cycles

The stability of cerium oxide (ceria) is a major topic in the field of heterogeneous catalysis. When exposed to a reactive environment or treated in a redox step, ceria is prone to changes of its surface morphology, atomic structure, and composition, with a strong impact onto its catalytic properties. Here, we investigate the stability of Pt(111)-supported ultrathin ceria films upon air exposure and during redox cycles under ultrahigh vacuum (UHV) conditions. Scanning probe microscopy, X-ray photoemission spectroscopy, and low-energy electron diffraction show that upon air exposure, a clean ceria film surface gets contaminated by hydroxyls and carbon-containing species whereas a following annealing in molecular oxygen at around 650 °C removes such contaminants and allows to restore the original surface morphology and structure. When clean films are oxidized in a background oxygen pressure (reduced in UHV) at ∼650 °C, the film coverage increases (decreases). The decrease of the film coverage upon reduction...

Tuning LaAlO3 lattice structure by growth rate at the picometer scale in LaAlO3/SrTiO3 heterostructures

Here, we report an abnormal lattice reconstruction of LaAlO3 in the LaAlO3/SrTiO3 two-dimensional electron liquid heterostructure using the layer-by-layer pulsed laser deposition. We found that the lattice structure change of the top LaAlO3 layer is at the picometer level and is mainly controlled by the oxygen vacancies with an abnormal dependence on the oxygen background pressure. The lattice of the LaAlO3 film expands with more oxygen vacancies grown with high oxygen pressure and collapses with less oxygen vacancies grown with low oxygen pressure. The in situ monitoring of reflective high energy electron diffraction shows the lattice change coincident to the layer-by-layer deposition period change, revealing a non-equilibrium correlation between the lattice and the deposition environment, instead of the commonly assumed equilibrium correlation.

Epitaxial stabilization of pulsed laser deposited Srn+1IrnO3n+1 thin films: Entangled effect of growth dynamics and strain

The subtle balance of electronic correlations, crystal field splitting, and spin–orbit coupling in layered Ir4+ oxides can give rise to novel electronic and magnetic phases. Experimental progress in this field relies on the synthesis of epitaxial films of these oxides. However, the growth of layered iridates with excellent structural quality is a great experimental challenge. Here we selectively grow high quality single-phase films of Sr2IrO4, Sr3Ir2O7, and SrIrO3 on various substrates from a single Sr3Ir2O7 target by tuning background oxygen pressure and epitaxial strain. We demonstrate a complex interplay between growth dynamics and strain during thin film deposition. Such interplay leads to the stabilization of different phases in films grown on different substrates under identical growth conditions, which cannot be explained by a simple kinetic model. We further investigate the thermoelectric properties of the three phases and propose that weak localization is responsible for the low temperature activated resistivity observed in SrIrO3 under compressive strain.