Ion Beam Sputtering System Research Articles

Impact of B4C buffer layer on interface diffusion in Cr/Sc multilayers: combined study by x-ray reflectivity, scattering and fluorescence

In thin film multilayer based optical componentsof x-ray imaging system, diffusion of one material into the other degrades the reflectivity of the mirrors severely. Along with this thermodynamically driven diffusion, there are also growth generated interface roughness of different special frequencies and microstructures which can increase the diffused scattering from the multilayer and reduce the resolution of an image. Generally grazing incidence x-ray reflectivity in specular geometry (specular GIXR) and diffused x-ray scattering measurement in rocking scan geometry yield information regarding microstructure and overall diffusion at the interfaces of a multilayer. In this paper it is shown that grazing incidence x-ray fluorescence (GIXRF) measurement in standing wave condition alongwith the above measurements can give precise information regarding element-specific diffusion at the interfaces of a multilayer structure. Periodic multilayers made of 75 Cr/Sc bilayers with bilayer thickness ∼4 nm with and without B4C barrier layer of 0.2 nm thickness at the interfaces have been prepared using ion beam sputtering system and characterized by GIXR, diffused x-ray scattering and GIXRF measurements using synchrotron x-ray radiation just above the Cr K-edge. From the above measurements, drastic reduction in interface diffusion of Cr and improvement of interface morphology after addition of B4C barrier layer at the interfaces of Cr/Sc multilayers have been observed which is also corroborated by cross-sectional transmission electron microscopy of the multilayers. Finally, in the water window soft x-ray region of 2.3–4.4 nm performance of these multilayers have been tested and the Cr/B4C/Sc multilayer with improved interface quality has been found to yield ∼30.8% reflectivity at 3.11 nm wavelength which is comparable with the best reported reflectivities in the literature at this wavelength.

High-Throughput Study of Amorphous Stability and Optical Properties of Superlattice-Like Ge-Sb-Te Thin Films.

A high-throughput ion beam sputtering system is used to synthesize compositional gradient superlattice-like (SLL) thin film libraries of Ge-Sb-Te alloys over the entire phase diagram. The optical properties and structural evolution of the Ge-Sb-Te combinatorial SLL thin film are investigated. A systematic screening over the annealing temperature, annealing time, and modulation period has elucidated the critical factors that affect the stability of the metastable phase and optical properties. It is found that amorphous stability and optical constant are highly dependent on the modulation period and chemical composition of the thin film. This data-driven approach offers new perspectives for accelerating the development of new materials with excellent optical and amorphous stability and for exploring their mechanisms, by greatly expanding the dataset of Ge-Sb-Te alloys with SLL structures through high-throughput experiments.

Investigation of Exchange Bias in Irmn/Nife Bilayer Thin Films Using Magnetoresistance Technique

A bottom pinned Si/SiO2/Ta(20)/IrMn(15)/NiFe(10)/Ta(5) film has been fabricated using an ion beam sputtering system at room temperature. Post-deposition magnetic annealing was performed on sputtered multilayer thin film in a vacuum of 2´10-6 mbar at 250˚C for one hour in an in-plane magnetic field of 1.8 kG. The X-ray diffraction and X-ray reflectivity measurements have been performed for the structural and thickness analysis, respectively. The roughness of individual layers was determined using XRR measurements. The longitudinal MOKE measurements have been performed to measure the exchange bias, coercivity, and magnetic anisotropy at room temperature. The magnetoresistance measurement technique is employed to demonstrate the thermal evaluation of the exchange bias effect, training effect, and magnetization reversal asymmetry at room temperature and low temperature. The magnetoresistance measurement revealed a large exchange bias, pronounced magnetization reversal asymmetry, and enhancement in the training effects at 20 K. The experimental result exhibits the occurrence of temperature-dependent change from uniaxial to biaxial exchange anisotropy.

High-throughput investigation of structural evolution upon solid-state in Cu–Cr–Co combinatorial multilayer thin-film

Cu–Cr–Co combinatorial multilayer thin-films were prepared by a high-throughput ion beam sputtering system. Based on the thickness ratio among the individual nanoscale monolayers (Cu, Cr, Co), the resulting stoichiometry covered the entire phase diagram. The chemical composition and structure of Cu–Cr–Co combinatorial chip upon solid-state reaction were studied by lab-based micro-X-ray fluorescence (μ-XRF) and high-throughput synchrotron X-ray diffraction (XRD), respectively. A composition-structure map for Cu–Cr–Co combinatorial chip was developed through automated data analysis employing hierarchical clustering techniques. The structural evolution of Cu–Cr–Co combinatorial chip as a function of heat-treatment temperature, time, and modulation period were studied systematically. Furthermore, the effect of the elemental distribution in-depth direction was investigated to gain more insights regarding phase transformation. This work provides an efficient method and new perspectives for the design and optimization of the composition and structure of high-performance thin-films.

Phase Evolution and Amorphous Stability upon Solid-State Reaction in Superlattice-Like Ge–Sb–Te Combinatorial Thin Films

In this paper, the superlattice-like (SLL) Ge–Sb–Te combinatorial thin films were prepared by using a high-throughput ion beam sputtering system. The phase evolution and amorphous stability of such...

Investigation of DIBS-Deposited CdZnO/ZnO-Based Multiple Quantum Well for Large-Area Photovoltaic Application

Multiple quantum wells (MQWs) of CdZnO/ZnO are realized, for the first time, by dual ion beam sputtering (DIBS) system at different deposition conditions in terms of ion beam power, substrate temperature, and time cessation between deposition of successive layers. The effects of DIBS deposition conditions are analyzed by secondary ion mass spectroscopy (SIMS) and high-resolution transmission electron microscopy (HRTEM) and discussed systematically. The SIMS analysis has been used for depth profiling of CdZnO/ZnO-based MQWs structure. The deposition of CdZnO/ZnO-based MQW structure performed at 100 °C with time cessation of 30 min between successive layer growth and ion beam power of 14 W has displayed the best results in terms of distinct well and barrier layers formation. This work also includes an analytical study of CdZnO/ZnO-based MQW solar cell (MQWSC), in which a study is performed for solar irradiance dependence of performance parameters to explore the potential use of CdZnO/ZnO-based MQWSC for concentrator solar cell (SC). The short-circuit current density increases from 0.12 to 57.98 mA/cm2, the open-circuit voltage rises from 2.60 to 2.77 V, and the photon conversion efficiency is from 2.85% to 3.04%, as solar irradiance increases from 0.1 to 50 suns. The results show that the performance of SCs can be improved by using concentrators and also explore the possibility of efficiently absorbing short-wavelength photons.

Interface evolution of Cr/Ti multilayer films during continuous to discontinuous transition of Cr layer

Cr/Ti multilayers with moderate to low bi-layer thickness have been deposited by Ion beam sputtering system and have been investigated thoroughly by specular grazing incidence X-ray reflectivity (GIXR) and diffused X-ray scattering (both in coplanar and non-coplanar configurations) measurements. The investigation is important for development of Cr/Ti soft X-ray multilayer mirrors for “water window” applications in the wavelength regime of 2.2–4.4 nm. Initially few multilayers have been deposited and the highest reflectivity at first Bragg peak positions of the GIXR plots have been obtained for the Cr layer to bi-layer thickness ratio of 0.4. Subsequently, keeping the ratio fixed a set of five samples with bi-layer thickness varying from 3.8 nm to 2.1 nm have been prepared. For the all five ML samples reported in this communication, Ti layer thickness is greater than 1.3 nm, however, the Cr layer thickness varies from 1.5 to 0.8 nm in the above range of bi-layer thickness and it undergoes continuous-to-discontinuous transition. Cr films undergo significant change in morphology as the thickness changes and thus the interfaces are also evolved differently. Analysis of the diffused X-ray scattering data shows that interface imperfection of Ti-on-Cr interfaces is dominated by interface diffusion at low Cr thickness (discontinuous) regime and by interface roughness at high Cr thickness (continuous) regime, while in case of Cr-on-Ti interfaces, interface roughness dominates throughout the whole thickness regime. For the sample with highest bi-layer thickness of 3.8 nm high Bragg peak reflectivity value close to the ideal case is obtained manifesting high thickness uniformity, very low interface imperfection and high vertical correlation length across the interfaces for this sample which is also confirmed by cross-sectional Transmission Electron Microscopy. A 50 bi-layer sample prepared with the above bi-layer thickness shows high reflectivity of 5.6% at 2.84 nm water window soft X-ray wavelength.

Dwelling time calculation for film uniformity correction by ion beam sputtering process on larger optical materials

It is difficult to obtain film with uniform thickness and constant performance for optical components with larger diameter. It is a critical technology to improve the film uniformity during the deposition process. The ion beam sputtering system was used to prepare the film on the surface of large-aperture optical elements. The performance of film including thickness and roughness were measured by step profiler and optical profilometer instrument. The film thickness correction model was proposed in this paper. The optimized dwell time ratio was obtained by the film thickness data obtained from former experiments, and the dwell time distribution was calculated, after several iteration with the model, uniform film can be achieved on optical elements over 300mm in diameter. Taking the Si film on the surface of fused silica as an example. The research results show that, different film thickness between 100nm and 400nm can be obtained, when the film thickness is more than 200 nm, the film uniformity from more than 13% to less than 0.5%. The surface roughness variation of optical material before and after film deposition were studied, which provide stable process for obtaining high performance film on larger optical components.

Thin film deposition from dual ion beam sputtering system

In this report, we study different device applications which utilizes oxide layers grown by dual ion beam sputtering (DIBS) system. DIBS system is noteworthy since it produces high-quality thin films with reasonably better compositional stoichiometry, small surface roughness and good adhesion to the substrate even for films grown at room temperature. DIBS system parameters affect the oxide and non-oxide layer in a very significant manner and control of these parameters allows for fine-tuning the as-grown thin films specialize for specific applications.

Two-dimensional electron gases in MgZnO/ZnO and ZnO/MgZnO/ZnO heterostructures grown by dual ion beam sputtering

This work reports on the formation of high-density (~1013–1014 cm−2) two-dimensional electron gas (2DEG) in ZnO-based heterostructures, grown by a dual ion beam sputtering system. We probe 2DEG in bilayer MgZnO/ZnO and capped ZnO/MgZnO/ZnO heterostructures utilizing MgZnO barrier layers with varying thickness and Mg content. The effect of the ZnO cap layer thickness on the ZnO/MgZnO/ZnO heterostructure is also studied. Hall measurements demonstrate that the addition of a 5 nm ZnO cap layer results in an enhancement of the 2DEG density by about 1.5 times compared to 1.11 1014 cm−2 for the uncapped bilayer heterostructure with the same 30 nm barrier thickness and 30 at.% Mg composition in the barrier layer. From the low-temperature Hall measurement, the sheet carrier concentration and mobility are both found to be independent of the temperature. The capacitance–voltage measurement suggests a carrier density of ~1020 cm−3, confined in 2DEG at the MgZnO/ZnO heterointerface. The results presented are significant for the optimization of 2DEG for the eventual realization of cost-effective and large-area MgZnO/ZnO-based high-electron-mobility transistors.

Realization of synaptic learning and memory functions in Y2O3 based memristive device fabricated by dual ion beam sputtering

Single synaptic device with inherent learning and memory functions is demonstrated based on a forming-free amorphous Y2O3 (yttria) memristor fabricated by dual ion beam sputtering system. Synaptic functions such as nonlinear transmission characteristics, long-term plasticity, short-term plasticity and ‘learning behavior (LB)’ are achieved using a single synaptic device based on cost-effective metal-insulator-semiconductor (MIS) structure. An ‘LB’ function is demonstrated, for the first time in the literature, for a yttria based memristor, which bears a resemblance to certain memory functions of biological systems. The realization of key synaptic functions in a cost-effective MIS structure would promote much cheaper synapse for artificial neural network.

Fiber-Optic Thermal Sensor for TiN Film Crack Monitoring.

The study focuses on the thermal and temperature sensitivity behavior of an optical fiber sensor device. In this article, a titanium nitride (TiN)-coated fiber Bragg grating (FBG) sensor fabricated using an ion beam sputtering system was investigated. The reflection spectra of the FBG sensor were tested using R-soft optical software to simulate the refractive index sensitivity. In these experiments, the temperature sensitivity of the TiN FBG was measured at temperatures ranging from 100 to 500 °C using an optical spectrum analyzer (OSA). The results showed that the temperature sensitivity of the proposed TiN FBG sensor reached 12.8 pm/°C for the temperature range of 100 to 300 °C and 20.8 pm/°C for the temperature range of 300 to 500 °C. Additionally, we found that the produced oxidation at temperatures of 400–500 °C caused a crack, with the crack becoming more and more obvious at higher and higher temperatures.

Integrated Linear-processing Ion-beam Sputtering System Using a Meter-scale Ribbon-beam

We present a system capable of performing linear PVD processing by DC ion beam sputtering, suitable for reactive or high-vacuum deposition of dielectrics, metals, or magnetic materials onto conductive, insulating, or magnetic substrates. This equipment can be installed on a single vacuum flange for mounting on a variety of vacuum systems, but linear flow systems are preferred. Our recently developed ion source can generate ribbon ion beams of high current and low divergence over a wide range of energies, but is optimized for about 1 to 5 keV, and high ion currents of up to 1A per meter breadth; breadths of several meters are possible. The current density is an order of magnitude higher than existing systems. Uniformity is intrinsically good and can be controlled in real-time. The ion source and the sputter target are mounted close to each other and to the target; the cross section of the equipment does not change as the equipment is scaled up to large dimensions. The ion beam impinges on the sputter target at the optimum angle and energy, in a field-free region, and in high vacuum. This combination of conditions is unique, and dramatically raises the sputter yield. Free choice of beam energy permits higher thermal efficiency. High vacuum means that the deposited atoms comprising the film retain the full energy of several eV that was produced in the sputtering process. The geometry further permits extremely simple and efficient jetting of reactive gases such as oxygen for the production of oxide films. The single broad-beam source has far higher current density and a far more compact layout than existing ion-beam sputtering systems, and can be installed in linear processing systems in a similar manner to linear magnetrons.

Investigation of barrier inhomogeneities and interface state density in Au/MgZnO: Ga Schottky contact

The electrical characteristics of Au/MgZnO:Ga (GMZO) Schottky contact, which is fabricated using a dual ion beam sputtering system, have been investigated using current–voltage (I–V) and capacitance–voltage (C–V) measurements over a wide temperature range of 80 to 300 K. The apparent Schottky barrier height (SBH) and ideality factor obtained from the I–V measurements are observed to increase and reduce, respectively, with increasing measurement temperature. This anomalous observation in the behaviour of the SBH is in good agreement with the predictions of a double Gaussian distribution (DGD) of the inhomogeneous SBH at a metal–semiconductor (MS) interface. The values of the SBH as determined from C–V measurements are expectedly higher than those extracted from I–V measurements. The DGD model is observed to fit with experimentally obtained data for the temperature-dependent SBH with mean values of the SBH of 0.95 and 0.54 eV and standard deviations of 0.131 and 0.072 eV in the temperature range of 160–300 K and 80–160 K, respectively. The larger value of the SBH standard deviation confirms more SBH inhomogeneity at the MS interface, and these inhomogeneities are attributed to the presence of deep level or surface level interface states. The calculated interface states density is seen to vary from 6.46 × 1014 eV−1 cm−2 at EC-0.27 eV to 1.58 × 1014 eV−1 cm−2 at EC-0.74 eV, where EC is the bottom of a conduction band at 300 K.

Growth and characterization of dual ion beam sputtered Cu2ZnSn(S, Se)4 thin films for cost-effective photovoltaic application

A systematic growth optimization of Cu2ZnSn(S,Se)4 (CZTSSe) thin films by dual ion beam sputtering system from a single CZTSSe target is presented. It is observed that the ratio of Cu/(Zn+Sn) varies from 0.86 to 1.5 and that of (S+Se)/metal varies between 0.62 and 0.97 when substrate temperature (Tsub) is increased from 100 to 500°C. The crystal structure of all CZTSSe films are identified to be preferentially (112)-oriented, polycrystalline in nature, and without the existence of secondary phases such as Cu2(S,Se) or Zn(S,Se). The full-width at half-maximum of (112) diffraction peak is the minimum with a value of 0.12° and the maximum crystallite size 75.11nm for CZTSSe grown at 300°C. Morphological investigation reveals the achievement of the largest grain size at Tsub=300°C. The band gap of CZTSSe thin films at room temperature, as determined by spectroscopic ellipsometry, varies from 1.23 to 1.70eV, depending on Tsub. The optical absorption coefficient of all CZTSSe thin films is >104cm−1.

Plasmon generation in sputtered Ga-doped MgZnO thin films for solar cell applications

The crystalline, electrical, morphological, optical properties and plasmonic behaviour of Ga doped MgZnO (GMZO) thin films grown at different substrate temperatures (200–600 °C) by a dual ion beam sputtering (DIBS) system are investigated. Transmittance value of more than ∼94% in 400–1000 nm region is observed for all GMZO films. The particle plasmon features can be detected in the absorption coefficient spectra of GMZO grown at 500 and 600 °C in the form of a peak at ∼4.37 eV, which corresponds to a plasmon resonance peak of nanoclusters formed in GMZO. The presence of such plasmonic features is confirmed by ultraviolet photoelectron spectroscopy measurements. The values of particle plasmon resonance energy of various nanoclusters are in the range of solar spectrum, and these can easily be tuned and excited at the desirable wavelengths while optimizing the efficiency of solar cells (SCs) by simple alteration of DIBS growth temperature. These nanoclusters are extremely promising to enhance the optical scattering and trapping of the incident light, which increases the optical path length in the absorber layer of cost-effective SCs and eventually increases its efficiency.

Study of interface correlation in W/C multilayer structure by specular and non-specular grazing incidence X-ray reflectivity measurements

W/C/W tri-layer thin film samples have been deposited on c-Si substrates in a home-built Ion Beam Sputtering system at 1.5 × 10−3 Torr Ar working pressure and 10 mA grid current. The tri-layer samples have been deposited at different Ar+ ion energies between 0.6 and 1.2 keV for W layer deposition and the samples have been characterized by specular and non-specular grazing incidence X-ray reflectivity (GIXR) measurements. By analyzing the GIXR spectra, various interface parameters have been obtained for both W-on-C and C-on-W interfaces and optimum Ar+ ion energy for obtaining interfaces with low imperfections has been found. Subsequently, multilayer W/C samples with 5-layer, 7-layer, 9-layer, and 13-layer have been deposited at this optimum Ar+ ion energy. By fitting the specular and diffused GIXR data of the multilayer samples with the parameters of each interface as fitting variables, different interface parameters, viz., interface width, in-plane correlation length, interface roughness, and interface diffusion have been estimated for each interface and their variation across the depth of the multilayers have been obtained. The information would be useful in realizing W/C multilayers for soft X-ray mirror application in the <100 Å wavelength regime. The applicability of the “restart of the growth at the interface” model in the case of these ion beam sputter deposited W/C multilayers has also been investigated in the course of this study.

The ion beam sputtering facility at KURRI: Coatings for advanced neutron optical devices

We describe a film coating facility for the development of multilayer mirrors for use in neutron optical devices that handle slow neutron beams. Recently, we succeeded in fabricating a large neutron supermirror with high reflectivity using an ion beam sputtering system (KUR-IBS), as well as all neutron supermirrors in two neutron guide tubes at BL06 at J-PARC/MLF. We also realized a large flexible self-standing m=5 NiC/Ti supermirror and very small d-spacing (d=1.65nm) multilayer sheets. In this paper, we present an overview of the performance and utility of non-magnetic neutron multilayer mirrors fabricated with the KUR-IBS

Growth and characterizations of dual ion beam sputtered CIGS thin films for photovoltaic applications

The growth of CIGS thin films on soda-lime glass substrates at different substrate temperatures by dual ion beam sputtering system in a single-step route from a single quaternary sputtering target with the composition of Cu (In0.70 Ga0.30) Se2 was reported. The effects of the substrate temperature on structural, optical, morphological and electrical properties of CIGS films were investigated. Stoichiometry of one such film was investigated by X-ray photoelectron spectroscopy. All CIGS films had demonstrated a strong (112) orientation located at 2θ ~26.70o, which indicated the chalcopyrite structure of films. The value of full-width at half-maximum of (112) peak was reduced from 0.58° to 0.19° and crystallite size was enlarged from 14.98 to 43.05 nm as growth temperature was increased from 100 to 400 °C. However, atomic force microscope results showed a smooth and uniform surface at lower growth temperature and the surface roughness was observed to increase with increasing growth temperature. Hall measurements exhibited the minimum film resistivity of 0.09 Ω cm with a hole concentration of 2.42 × 1018 cm−3 and mobility of 28.60 cm2 V−1 s−1 for CIGS film grown at 100 °C. Film absorption coefficient was found to enhance nominally from 1 × 105 to 2.3 × 105 cm−1 with increasing growth temperature from 100 to 400 °C.

Investigation of the optical and the electrical characteristics of thin titanium dioxide films

This study dealt with the deposition of nanotitanium films with different thicknesses by using the ultrahigh vacuum ion beam sputtering system. It is found that the thickness of the titanium films was in the nanoscale thus the electric resistivity of the titanium films decreased. Moreover, the deposited titanium was transformed into titanium oxide in an oxygen atmosphere and by using a rapid annealing process. The photoelectronic properties of the nanotitanium thin film was enhanced, when the nanotitanium film was transformed to titanium dioxide.