Pulsed Laser Deposition Chamber Research Articles

Topological signatures of Mo2TiC2O2

Mo2TiC2 is the Ordered Double Transitional Metal Layered Carbides (ODTMLC) derived from its parent MAX phases Mo2TiAlC2 by a wet chemical etching. Its oxidation was done by a new ablated plasma thrust method in which the MXenes were at 750 °C under an oxygen background in the pulsed laser deposition chamber. The reflective high electron energy diffraction technique assures the oxidation at the ambient gas pressure p = 0.1 mbar, which was described in the previous paper. The obtained Mo2TiC2O2 was transferred for their topological test under angle-resolved photoemission spectroscopy, circular dichroism test, and Chemical Potential (CP) analysis. An indirect energy bandgap of 125 meV was obtained. The sine function of α along with period π and β with period 2π shows that there is a possibility of helical spin textures in both α (electron-like pocket around Γ̄) and β (elliptical electron-like pocket around M̄). The CP analysis shows the possibility of at least 100 meV bandgap creation on a single surface so that the surface charges will flow without any effect of bulk. The Mo2TiC2O2 can be used as topological insulating material.

Controlling crystallites orientation and facet exposure for enhanced electrochemical properties of polycrystalline MoO3 films

This study focuses on the development and optimization of MoO3 films on commercially available FTO substrates using the pulsed laser deposition (PLD) technique. By carefully selecting deposition conditions and implementing post-treatment procedures, precise control over crystallite orientation relative to the substrate is achieved. Deposition at 450 °C in O2 atmosphere results in random crystallite arrangement, while introducing argon instead of oxygen to the PLD chamber during the initial stage of sputtering exposes the (102) and (011) facets. On the other hand, room temperature deposition leads to the formation of amorphous film, but after appropriate post-annealing treatment, the (00k) facets were exposed. The deposited films are studied using SEM and XRD techniques. Moreover, electrochemical properties of FTO/MoO3 electrodes immersed in 1 M AlCl3 aqueous solution are evaluated using cyclic voltammetry and electrochemical impedance spectroscopy. The results demonstrate that different electrochemical processes are promoted based on the orientation of crystallites. When the (102) and (011) facets are exposed, the Al3+ ions intercalation induced by polarization is facilitated, while the (00k) planes exposure leads to the diminished hydrogen evolution reaction overpotential.

Tin(IV) Oxide Electron Transport Layer via Industrial-Scale Pulsed Laser Deposition for Planar Perovskite Solar Cells.

Electron transport layers (ETL) based on tin(IV) oxide (SnO2) are recurrently employed in perovskite solar cells (PSCs) by many deposition techniques. Pulsed laser deposition (PLD) offers a few advantages for the fabrication of such layers, such as being compatible with large scale, patternable, and allowing deposition at fast rates. However, a precise understanding of how the deposition parameters can affect the SnO2 film, and as a consequence the solar cell performance, is needed. Herein, we use a PLD tool equipped with a droplet trap to minimize the number of excess particles (originated from debris) reaching the substrate, and we show how to control the PLD chamber pressure to obtain surfaces with very low roughness and how the concentration of oxygen in the background gas can affect the number of oxygen vacancies in the film. Using optimized deposition conditions, we obtained solar cells in the n-i-p configuration employing methylammonium lead iodide perovskite as the absorber layer with power conversion efficiencies exceeding 18% and identical performance to devices having the more typical atomic layer deposited SnO2 ETL.

Thin Films Fabricated by Pulsed Laser Deposition for Electrocatalysis

Thin Films Fabricated by Pulsed Laser Deposition for Electrocatalysis

ITO Top‐Electrodes via Industrial‐Scale PLD for Efficient Buffer‐Layer‐Free Semitransparent Perovskite Solar Cells

AbstractThe deposition of transparent conductive oxides (TCO) usually employs harsh conditions that are frequently harmful to soft/organic underlayers. Herein, successful use of an industrial pulsed laser deposition (PLD) tool to directly deposit indium tin oxide (ITO) films on semitransparent vacuum‐deposited perovskite solar cells without damage to the device stack is demonstrated. The morphological, electronic, and optical properties of the PLD deposited ITO films are optimized. A direct relation between the PLD chamber pressure and the solar cell performance is obtained. The semitransparent perovskite solar cells prepared exclusively by vacuum‐assisted techniques had fill factors of 78% and exceeded 18% in power conversion efficiencies. This demonstrates that the direct deposition of TCO‐based top electrodes without protective buffer layers is possible and leads to efficient devices.

RHEED analysis of the oxidized M′2M″xXyene sheets by ablated plasma thrust method in pulsed laser deposition chamber

M′2M″xXyene (M′ and M″ are the early transitional metals and X is carbide with x = 1 for y = 2 and x = 2 for y = 3) are the ordered double transitional metal layered carbides derived from their parent MAX phases M′2M″xAlXyene by a wet chemical etching method. Their oxides are predicted to have topological properties for which they should be annealed at around 800 °C in an oxygen background. This paper includes the new ablated plasma thrust method for the ionization and adsorption of oxygen on the M′2M″xXyene substrate in the pulsed laser deposition chamber. We have found that the background pressure has a negative effect and the substrate temperature has a positive effect on plume expansion. The density profile of the background gas is highly affected by deposition temperature. Similarly, it is found that the density of plasma generated by longer wavelengths is not affected significantly due to the inverse bremsstrahlung process. A shorter wavelength produces the bremsstrahlung process as photoionization takes place. At a certain time (200 ns), the pressure of background gas and plasma pressure are equal (snow-plogh effect) so that all the wavelengths then produce electrons (highest for shorter wavelengths), thereby increasing its density. The energy necessary for the oxidation of the substrate is provided by the energy of the ablated species. The adsorption is assured by the reflective high electron energy diffraction technique, and it is found that the ambient gas pressures p = 0.1 mbar and 0.2 mbar are appropriate for the adsorption process. The obtained M′2M″xXyene oxides can be used for their topological test.

A Case Study to Address: “Is Your Pulsed Laser Deposition Chamber Clean?”

AbstractPulsed laser deposition (PLD) is one of the important techniques for the growth of oxide thin films, interfaces, and superlattices. It can also be utilized to implement diverse combinatorial approaches. Thin film growth using PLD hinges on various parameters that decide the composition, structure, quality, and finally the physical properties of the films, interfaces, and superlattices. In this paper it is demonstrated how the growth conditions inside the chamber during the growth can be judged from outside by combining in situ and ex situ techniques. An example of the growth of LaVO3‐SrTiO3 interface is given to elucidate the effect of repetitive growth on the chamber condition and hence on the reproducibility of the physical properties of the samples. The experiments suggest noticeable change in transport properties with successive deposition processes.

Movable holder for a quartz crystal microbalance for exact growth rates in pulsed laser deposition.

Controlling the amount of material deposited by pulsed laser deposition (PLD) down to fractions of one atomic layer is crucial for nanoscale technologies based on thin-film heterostructures. Albeit unsurpassed for measuring growth rates with high accuracy, the quartz crystal microbalance (QCM) suffers from some limitations when applied to PLD. The strong directionality of the PLD plasma plume and its pronounced dependence on deposition parameters (e.g., background pressure and fluence) require that the QCM is placed at the same position as the substrate during growth. However, QCM sensors are commonly fixed off to one side of the substrate. This also entails fast degradation of the crystal, as it is constantly exposed to the ablated material. The design for a movable QCM holder discussed in this work overcomes these issues. The holder is compatible with standard transfer arms, enabling easy insertion and transfer between a PLD chamber and other adjoining vacuum chambers. The QCM can be placed at the same position as the substrate during PLD growth. Its resonance frequency is measured in vacuum at any location where it can be in contact with an electrical feedthrough, before and after deposition. We tested the design for the deposition of hematite (Fe2O3), comparing the rates derived from the QCM and from reflection high-energy electron diffraction oscillations during homoepitaxial growth.

2×2R45∘ surface reconstruction and electronic structure of BaSnO3 film

We studied surface and electronic structures of barium stannate (BaSnO$_3$) thin-film by low energy electron diffraction (LEED), and angle-resolved photoemission spectroscopy (ARPES) techniques. BaSnO$_3$/Ba$_{0.96}$La$_{0.04}$SnO$_3$/SrTiO$_3$ (10 nm/100 nm/0.5 mm) samples were grown using pulsed-laser deposition (PLD) method and were \emph{ex-situ} transferred from PLD chamber to ultra-high vacuum (UHV) chambers for annealing, LEED and ARPES studies. UHV annealing starting from 300$^{\circ}$C up to 550$^{\circ}$C, followed by LEED and ARPES measurements show 1$\times$1 surfaces with non-dispersive energy-momentum bands. The 1$\times$1 surface reconstructs into a $\sqrt{2}$$\times$$\sqrt{2}R45^\circ$ one at the annealing temperature of 700$^{\circ}$C where the ARPES data shows clear dispersive bands with valence band maximum located around 3.3 eV below Fermi level. While the $\sqrt{2}$$\times$$\sqrt{2}R45^\circ$ surface reconstruction is stable under further UHV annealing, it is reversed to 1$\times$1 surface by annealing the sample in 400 mTorr oxygen at 600$^{\circ}$C. Another UHV annealing at 600$^{\circ}$C followed by LEED and ARPES measurements, suggests that LEED $\sqrt{2}$$\times$$\sqrt{2}R45^\circ$ surface reconstruction and ARPES dispersive bands are reproduced. Our results provide a better picture of electronic structure of BaSnO$_3$ surface and are suggestive of role of oxygen vacancies in the reversible $\sqrt{2}$$\times$$\sqrt{2}R45^\circ$ surface reconstruction.

High Oxygen Exchange Activity of Pristine La0.6Sr0.4FeO3–δ Films and Its Degradation

The polarization resistance of La0.6Sr0.4FeO3–δ (LSF) thin film electrodes was investigated by electrochemical impedance spectroscopy. Different states were considered, ranging from pristine films right after deposition to films degraded under various conditions. Impedance measurements performed inside the pulsed laser deposition chamber (in-situ) revealed the existence of a very facile, low resistive state of as-deposited thin films with up to two orders of magnitude lower resistances compared to conventional ex-situ measurements and literature data. Ex-situ and in-situ measurements show different dependencies of polarization resistances, suggesting a different oxygen exchange reaction mechanism on the as-deposited films. The time dependence of the polarization resistance indicates that two degradation mechanisms with different timescales are responsible for the much higher resistances found in ex-situ measurements. Degradation mechanisms are discussed and conclusions with respect to the rate determining step of oxygen exchange on different surfaces are drawn.

Experimental setup combining in situ hard X-ray photoelectron spectroscopy and real-time surface X-ray diffraction for characterizing atomic and electronic structure evolution during complex oxide heterostructure growth.

We describe the next-generation system for in situ characterization of a complex oxide thin film and heterostructure growth by pulsed laser deposition (PLD) using synchrotron hard X-rays. The system consists of a PLD chamber mounted on a diffractometer allowing both real-time surface X-ray diffraction (SXRD) and in situ hard X-ray photoelectron spectroscopy (HAXPES). HAXPES is performed in the incident X-ray energy range from 4 to 12 keV using a Scienta EW4000 electron energy analyzer mounted on the PLD chamber fixed parallel with the surface normal. In addition to the standard application mode of HAXPES for disentangling surface from bulk properties, the increased penetration depth of high energy photoelectrons is used for investigation of the electronic structure changes through thin films grown deliberately as variable thickness capping layers. Such heterostructures represent model systems for investigating a variety of critical thickness and dead layer phenomena observed at complex oxide interfaces. In this new mode of operation, in situ HAXPES is used to determine the electronic structure associated with unique structural features identified by real-time SXRD during thin film growth. The system is configured for using both laboratory excitation sources off-line and on-line operation at beamline 33-ID-D at the Advanced Photon Source. We illustrate the performance of the system by preliminary scattering and spectroscopic data on oxygen vacancy ordering induced perovskite-to-brownmillerite reversible phase transformation in La2/3Sr1/3MnO3 films capped with oxygen deficient SrTiO3-δ (100) layers of varying thickness.

Low resistivity of pulsed laser deposited CdxZn1-xO thin films

In this work, group II-VI metal oxide thin films of cadmium zinc oxide (CdxZn1-xO), have been deposited using pulsed laser deposition (PLD) growth technique in an oxygen-deficient environment. The effect of cadmium content on structural, optical and electrical properties of the prepared CdxZn1-xO film samples has been delineated by way of varying CdO composition in ceramic target samples. All film samples grown at substrate temperatures of 200–500 °C are revealed to have single phase wurtzite structure with a preferential orientation along (002) plane. By slightly incrementing the substrate temperature from 250 °C to 300 °C in the PLD chamber, the growth process has been further mapped out to investigate changes in said properties. XRD observations are further confirmed by Raman studies, which revealed E2 (low), E2 (high) and A1 (LO) modes for all samples. Enhancement in Cd-content is found to promote the degree of crystallinity in film samples, as suggested by atomic force microscopy images. The optical band gap is estimated at 3.26, 3.16 and 3.12 eV, respectively for increasing atomic cadmium concentration in CdxZn1-xO film samples deposited at 250 °C. Importantly, low resistivity films with resistivities in the order of 10−2-10−3 Ω-cm were obtained, while electron mobility showed an increasing trend with respect to the presence of cadmium content in films. PL measurements were conducted in order to unravel the origin of n-type conductivity in CdxZn1-xO films. This study brings to focus growth aspects of the CdxZn1-xO material system required for the development of thin film heterostructures for optoelectronic applications.

(Invited) Towards an Improved Understanding of Electrochemical Oxygen Exchange Reactions on Mixed Conducting Oxides

In solid oxide fuel cells or electrolysis cells with ionic and electronic conducting electrodes, the electrochemical reactions take place at the gas/solid interface of the mixed conducting electrode. This is in contrast to the situation met in aqueous electrolytes, where the reactions occur at the electrode/electrolyte interface. Thus, modified concepts are required to describe the reaction rates of such reactions on mixed conducting oxides. Here, it is discussed how overpotential and gas partial pressure affect reaction rates. A rate equation is suggested that also includes defects and the meaning of the empirical reaction orders is illustrated. Three experiments are presented that emphasize different aspects required for a mechanistic understanding of such reactions: A simultaneous variation of overpotential and partial pressure reveals the role of defects in oxygen evolution on (La,Sr)FeO3-δ electrodes. Bias dependent impedance measurements with an analysis of the chemical capacitance gives information on the concentration of minority defects in Sr(Ti,Fe)O3- δ. Modification of (La,Sr)CoO3- δ thin film surfaces by Sr or Co and impedance measurements in the pulsed laser deposition chamber help understanding catalytically active sites.

(Invited) Towards an Improved Understanding of Electrochemical Oxygen Exchange Reactions on Mixed Conducting Oxides

It is generally agreed that the oxygen reduction reaction on oxide surfaces is strongly affected by defects, and the same is true for the counter reaction of oxygen evolution from an oxide. Relevant defects include the “standard” point defects also present in the bulk, i.e. oxygen vacancies and electrons or holes, but also defects within the ideal termination of an oxide, e.g. cation segregates, and higher dimensional defects such as dislocations or grain boundaries. However, it is far from trivial to clearly identify the role of specific defects on the oxygen reduction or release reaction. Often, one neither knows exact defect concentrations nor can one vary those in a defined manner without affecting the entity of all defects. This also hampers development of theoretical models describing the current voltage characteristics of oxygen reduction or evolution, i.e. oxygen exchange, at gas/solid interfaces. In this contribution, two types of experiments are employed to shed further light on the role of defects in oxygen exchange of mixed conducting oxides such as Sr doped LaCoO3 (LSC) and LaFeO3 (LSF). i) The first approach makes use of the fact that both oxygen partial pressure p(O2) and overpotential affect the oxygen chemical potential and thus defect concentrations in oxides. However, the oxygen reduction or evolution kinetics may still be affected very differently by overpotential and p(O2), since only the latter also changes the concentration of reacting molecules. By properly combining oxygen partial pressure changes and overpotential, we could tune the chemical potentials of LSF and gas phase independently and thus we extracted “true” reaction orders of O2 and defects in the oxygen reduction and evolution reaction. Additional bias dependent three point impedance measurements revealed the chemical capacitance of LSF and this was used for calculating the absolute bulk defect concentration in dependence of the oxygen chemical potential. This analysis also revealed that defect interaction strongly affects the thermodynamic data of LSF and a new defect chemical model including non-dilute defects is suggested. ii) The second type of experiments deals with the effect of certain cations in the topmost layer of LSC. It is already known that Sr segregation to the surface of mixed conducting oxides may reduce their activity towards oxygen reduction. A kind of SrO termination layer seems to be unavoidable on LSC at higher temperatures but still additional degradation may take place and reasons are not fully understood. A novel type of experiment was thus developed, with impedance spectroscopic analysis of samples inside a PLD (pulsed laser deposition) chamber (in-situ impedance PLD). In this manner, the surface of an LSC electrode in an YSZ based electrochemical cell could be decorated by very tiny amounts of other oxides (SrO, Co3O4, La2O3) and the effect on oxygen exchange kinetics could be detected immediately by impedance measurements. Already small fractions of monolayers of Sr strongly modified the polarization resistance, thus indicating that only a small number of very active sites was present on LSC and those became deactivated by Sr. Co, on the other hand, can significantly activate the LSC electrode surface.

Real-time impedance monitoring of oxygen reduction during surface modification of thinfilmcathodes.

Improvement of solid oxide fuel cells strongly relies on the development of cathode materials with high catalytic activity for the oxygen reduction reaction. Excellent activity was found for perovskite-type oxides such as La1-xSrxCoO3-δ (LSC), but performance degradation, probably caused by surface composition changes, hinders exploitation of the full potential of LSC. This study reveals that the potentially very high activity of the LSC surface can be traced back to few very active sites. Already tiny amounts of SrO, for example, 4% of a monolayer, deposited on an LSC surface, lead to severe deactivation. Co, on the other hand, causes (re-)activation, suggesting that active sites are strongly related to Co being present at the surface. These insights could be gained by a novel method to measure changes of the electrochemical performance of thin film electrodes in situ, while modifying their surface: impedance spectroscopy measurements during deposition of well-defined fractions of monolayers of Sr-, Co- and La-oxides by single laser pulses in a pulsed laser deposition chamber.

Growth of epitaxial tetragonal (Bi,K)TiO3 films and their ferroelectric and piezoelectric properties

(Bi,K)TiO3 films were grown on (001)cSrRuO3 // (100)SrTiO3 substrates by pulsed laser deposition (PLD) at various temperatures. The K/Ti and Bi/Ti ratios of the films were almost the same at 350 and 500 °C, but decreased markedly at 650 °C. The ferroelectric property was ascertained for epitaxial tetragonal (Bi,K)TiO3 films grown at 500 °C from polarization–electric field relationships, and their saturation polarization (Psat) and coercive field (Ec) at a maximum electric field of 800 kV/cm were 18 µC/cm2 and 300 kV/cm, respectively. To improve the ferroelectric property, the films were heat-treated at 650 °C while keeping the films in the PLD chamber after film deposition. These films also maintained a (001)-oriented epitaxy and had a tetragonal symmetry. The Psat and Ec obtained at an electric field of 800 kV/cm were changed to 22 µC/cm2 and 95 kV/cm, respectively. Moreover, these Psat and Ec values increased to 31 µC/cm2 and 165 kV/cm, respectively, at a maximum electric field of 1500 kV/cm. These heat-treated (Bi,K)TiO3 films showed piezoelectricity with an apparent piezoelectric coefficient (d33(AFM)) of 22 pm/V.

Influence of the Cation Ratio on Optical and Electrical Properties of Amorphous Zinc-Tin-Oxide Thin Films Grown by Pulsed Laser Deposition.

Continuous composition spread (CCS) methods allow fast and economic exploration of composition dependent properties of multielement compounds. Here, a CCS method was applied for room temperature pulsed laser deposition (PLD) of amorphous zinc-tin-oxide to gain detailed insight into the influence of the zinc-to-tin cation ratio on optical and electrical properties of this ternary compound. Our CCS approach for a large-area offset PLD process utilizes a segmented target and thus makes target exchange or movable masks in the PLD chamber obsolete. Cation concentrations of 0.08-0.82 Zn/(Zn + Sn) were achieved across single 50 × 50 mm(2) glass substrates. The electrical conductivity increases for increasing tin content, and the absorption edge shifts to lower energies. The free carrier concentration can be tuned from 10(20) to 10(16) cm(-3) by variation of the cation ratio from 0.1 to 0.5 Zn/(Zn + Sn).

Effect of working pressure and annealing temperature on microstructure and surface chemical composition of barium strontium titanate films grown by pulsed laser deposition

Barium strontium titanate (BST) (Ba1-x Sr x TiO3) thin films have been extensively used in many dielectric devices such as dynamic random access memories (DRAMs). To optimize its characteristics, a microstructural control is essential. In this paper, Ba0.6Sr0.4TiO3 thin film has been deposited on the SiO2/Si substrate by the pulsed laser deposition (PLD) technique at three different oxygen working pressures of 100, 220 and 350 mTorr. Then the deposited thin films at 100 mTorr oxygen pressure were annealed for 50 min in oxygen ambient at three different temperatures: 650, 720 and 800°C. The effect of oxygen working pressure during laser ablation and thermal treatment on the films was investigated by using X-ray diffraction (XRD), atomic force microscopy (AFM) and scanning electron microscopy (SEM) analysis methods. X-ray photoelectron spectroscopy analysis was used to determine the surface chemical composition of the samples. The results indicate that the deposited BST film at low working pressure (100 mTorr) in PLD chamber shows a lower surface roughness than other working pressures (220 and 350 mTorr). The as-deposited films show an amorphous structure and would turn into polycrystalline structure at annealing temperature above 650°C. Increase of temperature would cause the formation of cubic and perovskite phases, improvement in crystalline peaks and also result in the decomposition of BST at high temperature (above 800°C). In addition, rising of temperature leads to the increase in size of grains and clusters. Therefore more roughness was found at higher temperatures as a result of a more heterogeneous growth and less tensions.

Superconducting properties of SiC-buffered-MgB2tapes

Production of <TEX>$MgB_2$</TEX> film on metallic Hastelloy with SiC as the buffer layer was achieved by means of hybrid physical-chemical vapor deposition technique, whereas SiC buffer layers with varied thickness of 170 and 250 nm were fabricated inside a pulsed laser deposition chamber. Superconducting transition temperature and critical current density were verified by transport and magnetic measurement, respectively. With SiC buffer layer, the reduced delaminated area at the interface of <TEX>$MgB_2$</TEX>-Hastelloy and the slightly increased <TEX>$T_c$</TEX> of <TEX>$MgB_2$</TEX> tapes were clearly noticed. It was found that the upper critical field, the irreversibility field and the critical current density were reduced when <TEX>$MgB_2$</TEX> tapes were buffered with SiC buffer layer. Clarifying the mechanism of SiC buffer layer in <TEX>$MgB_2$</TEX> tape in affecting the superconducting properties is considerably important for practical applications.

Parametric study of Y-doped BaZrO3 thin film deposited via pulsed laser deposition

This study investigates the microstructure morphologies of Y-doped barium zirconate (BZY) thin films via a pulsed laser deposition (PLD) process. BZY thin film crystallinity was investigated at different substrate temperatures in a PLD chamber. BZY thin films deposited under 0.013 mbar of oxygen pressure are highly dense regardless of the substrate temperature (25 °C ≤ T ≤ 600 °C), while those deposited at low temperature (T ≤ 200 °C) have porous structures at an oxygen partial pressure of 0.13 mbar. In addition, BZY thin films deposited at 0.26 mbar in low/intermediate temperature ranges (25 °C ≤ T ≤ 400 °C) have porous and columnar structures. Regardless of the chamber pressures, BZY thin films with dense structures can be obtained when the substrate temperature exceeds 500 °C.