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We present X-ray diffraction and Raman spectroscopy investigations of a (100)-oriented (Al{sub x}Ga{sub 1–x}){sub 2}O{sub 3} thin film on MgO (100) and bulk-like ceramics in dependence on their composition. The thin film grown by pulsed laser deposition has a continuous lateral composition spread allowing to determine precisely the dependence of the phonon mode properties and lattice parameters on the chemical composition. For x < 0.4, we observe the single-phase β-modification. Its lattice parameters and phonon energies depend linearly on the composition. We determined the slopes of these dependencies for the individual lattice parameters and for nine Raman lines, respectively. While the lattice parameters of the ceramics follow Vegard's rule, deviations are observed for the thin film. This deviation has only a small effect on the phonon energies, which show a reasonably good agreement between thin film and ceramics.
P-SnO/n-Si heterojunctions were constructed by using e-beam evaporation in combination with ultra-violet lithography technique. The current-voltage and capacitance-voltage characteristics of the pn heterojunctions were systematically investigated, through which the diode parameters, such as the turn-on voltage, forward-to-reverse current ratio, series resistance, ideality factor, and build-in voltage, were also determined. In particular, the pn heterojunctions presented a relatively good electrical rectifying behavior, with a forward-to-reverse current ratio up to 58 ± 5 at ±2.0 V. The relative permittivity and work function of the SnO films were measured to be 18.8 ± 1.7 and 4.3 eV, respectively. The energy band diagram of the heterojunctions was depicted in detail, which can interpret the rectifying behavior very well.
Recently, a large enhancement in the ferroelectric transition temperature of several oxides is reported by growing the respective thin films on appropriate substrates. This phenomenon is correlated with high residual strain in thin films often leading to large increase in the tetragonality of their crystal structure. However, such an enhancement of transition temperature is usually limited to very thin films of ∼10 nm thickness. Here, we report growth of fully strained epitaxial thin films of BaTiO{sub 3} of 400 nm thickness, which are coherently grown on MgO substrates by pulsed laser deposition technique. Conventional high resolution x-ray diffraction and also the reciprocal space map measurements confirm that the film is fully strained with in-plane tensile strain of 5.5% that dramatically increases the tetragonality to 1.05. Raman measurements reveal that the tetragonal to cubic structural phase transition is observed at 583 K, which results in an enhancement of ∼200 K. Furthermore, temperature dependent Raman studies on these films corroborate absence of all the low temperature phase transitions. Numerical calculations based on thermodynamical model predict a value of the transition temperature that is greater than 1500 °C. Our experimental results are therefore in clear deviation from the existing strain dependent phase diagrams.
We report on a method for “direct-write” conductive patterning via reduction of graphene oxide (GO) sheets using focused electron beam induced deposition (FEBID) of carbon. FEBID treatment of the intrinsically dielectric graphene oxide between two metal terminals opens up the conduction channel, thus enabling a unique capability for nanoscale conductive domain patterning in GO. An increase in FEBID electron dose results in a significant increase of the domain electrical conductivity with improving linearity of drain-source current vs. voltage dependence, indicative of a change of graphene oxide electronic properties from insulating to semiconducting. Density functional theory calculations suggest a possible mechanism underlying this experimentally observed phenomenon, as localized reduction of graphene oxide layers via interactions with highly reactive intermediates of electron-beam-assisted dissociation of surface-adsorbed hydrocarbon molecules. These findings establish an unusual route for using FEBID as nanoscale lithography and patterning technique for engineering carbon-based nanomaterials and devices with locally tailored electronic properties.
Thin film sample of Zn{sub 0.85}Mg{sub 0.15}O was prepared on Si (100) substrate by pulsed laser deposition and its structural, electronic, magnetic and ferroelectric properties were investigated. The Zn{sub 0.85}Mg{sub 0.15}O thin film sample is found to show room temperature multiferroic properties; suitable for device fabrication.
AlN films with various thicknesses have been grown on Si(111) substrates by pulsed laser deposition (PLD). The surface morphology and structural property of the as-grown AlN films have been investigated carefully to comprehensively explore the epitaxial behavior. The ∼2 nm-thick AlN film initially grown on Si substrate exhibits an atomically flat surface with a root-mean-square surface roughness of 0.23 nm. As the thickness increases, AlN grains gradually grow larger, causing a relatively rough surface. The surface morphology of ∼120 nm-thick AlN film indicates that AlN islands coalesce together and eventually form AlN layers. The decreasing growth rate from 240 to 180 nm/h is a direct evidence that the growth mode of AlN films grown on Si substrates by PLD changes from the islands growth to the layer growth. The evolution of AlN films throughout the growth is studied deeply, and its corresponding growth mechanism is hence proposed. These results are instructional for the growth of high-quality nitride films on Si substrates by PLD, and of great interest for the fabrication of AlN-based devices.
Previous attempts to harvest energy from aeroelastic vibrations have been based on attaching a beam to a moving wing or structure. Here, we exploit self-excited oscillations of a fluttering composite beam to harvest energy using piezoelectric transduction. Details of the beam properties and experimental setup are presented. The effects of preset angle of attack, wind speed, and load resistance on the levels of harvested power are determined. The results point to a complex relation between the aerodynamic loading and its impact on the static deflection and amplitudes of the limit cycle oscillations on one hand and the load resistance and level of power harvested on the other hand.
High-quality (001)-oriented (pseudo-cubic notation) ferromagnetic YTiO{sub 3} thin films were epitaxially synthesized in a layer-by-layer way by pulsed laser deposition. Structural, magnetic, and electronic properties were characterized by reflection-high-energy-electron-diffraction, X-ray diffraction, vibrating sample magnetometry, and element-resolved resonant soft X-ray absorption spectroscopy. To reveal ferromagnetism of the constituent titanium ions, X-ray magnetic circular dichroism spectroscopy was carried out using four detection modes probing complementary spatial scale, which overcomes a challenge of probing ferromagnetic titanium with pure Ti{sup 3+}(3d{sup 1}). Our work provides a pathway to distinguish between the roles of titanium and A-site magnetic rare-earth cations in determining the magnetism in rare-earth titanates thin films and heterostructures.
Lightly doped n-type GaSb substrates with p-type GaSb buffer layers are the preferred templates for growth of InAs/InGaSb superlattices used in infrared detector applications because of relatively high infrared transmission and a close lattice match to the superlattices. We report here temperature dependent resistivity and Hall effect measurements of bare substrates and substrate-p-type buffer layer structures grown by molecular beam epitaxy. Multicarrier analysis of the resistivity and Hall coefficient data demonstrate that high temperature transport in the substrates is due to conduction in both the high mobility zone center Γ band and the low mobility off-center L band. High overall mobility values indicate the absence of close compensation and that improved infrared and transport properties were achieved by a reduction in intrinsic acceptor concentration. Standard transport measurements of the undoped buffer layers show p-type conduction up to 300 K indicating electrical isolation of the buffer layer from the lightly n-type GaSb substrate. However, the highest temperature data indicate the early stages of the expected p to n type conversion which leads to apparent anomalously high carrier concentrations and lower than expected mobilities. Data at 77 K indicate very high quality buffer layers.
In relativistic backward wave oscillators (RBWOs), although the slow wave structure (SWS) and electron beam determine the main characteristics of beam-wave interaction, many other factors can also significantly affect the microwave generation process. This paper investigates the influence of voltage rise time on beam-wave interaction in RBWOs. Preliminary analysis and PIC simulations demonstrate if the voltage rise time is moderately long, the microwave frequency will gradually increase during the startup process until the voltage reaches its amplitude, which can be explained by the dispersion relation. However, if the voltage rise time is long enough, the longitudinal resonance of the finitely-long SWS will force the RBWO to work with unwanted longitudinal modes for a while and then gradually hop to the wanted longitudinal mode, and this will lead to an impure microwave frequency spectrum. Besides, a longer voltage rise time will delay the startup process and thus lead to a longer microwave saturation time. And if unwanted longitudinal modes are excited due to long voltage rise time, the microwave saturation time will be further lengthened. Therefore, the voltage rise time of accelerators adopted in high power microwave technology should not be too long in case unwanted longitudinal modes are excited.
We have fabricated Fe{sub 3}O{sub 4}/p-Si heterojunction using pulsed laser deposition technique and explored its electro-magnetic transport properties. The heterojunction exhibits backward rectifying property at all temperatures, and appraisal of giant junction magnetoresistance (JMR) is observed at room temperature (RT). Conspicuously, the variation and sign change of JMR as a function of electric field is observed at RT. The backward rectifying behavior of the device is ascribed to the highly doped p-type (p{sup ++}) semiconducting nature of Fe{sub 3}O{sub 4}, and the origin of electric field (voltage) dependence of magnetoresistance is explained proposing electronic band diagram of Fe{sub 3}O{sub 4}/SiO{sub 2}/p-Si heterojunction. This interesting result may have importance to integrate Si-based magnetoresistance sources in multifunctional spintronic devices.
The Laser Megajoule (LMJ) facility located at CEA/CESTA started to operate in the early 2014 with two quadruplets (20 kJ at 351 nm) focused on target for the first experimental campaign. We present here the first set of gated x-ray imaging (GXI) diagnostics implemented on LMJ since mid-2014. This set consists of two imaging diagnostics with spatial, temporal, and broadband spectral resolution. These diagnostics will give basic measurements, during the entire life of the facility, such as position, structure, and balance of beams, but they will also be used to characterize gas filled target implosion symmetry and timing, to study x-ray radiography and hydrodynamic instabilities. The design requires a vulnerability approach, because components will operate in a harsh environment induced by neutron fluxes, gamma rays, debris, and shrapnel. Grazing incidence x-ray microscopes are fielded as far as possible away from the target to minimize potential damage and signal noise due to these sources. These imaging diagnostics incorporate microscopes with large source-to-optic distance and large size gated microchannel plate detectors. Microscopes include optics with grazing incidence mirrors, pinholes, and refractive lenses. Spatial, temporal, and spectral performances have been measured on x-ray tubes and UV lasers at CEA-DIF and at Physikalisch-Technische Bundesanstalt BESSY II synchrotron prior to be set on LMJ. GXI-1 and GXI-2 designs, metrology, and first experiments on LMJ are presented here.
CaBi{sub 2}Nb{sub 2}O{sub 9} (CBNO) thin film, a lead-free ferroelectric material, was prepared on a Pt/Ta/glass substrate via pulsed laser deposition. The Ta film was deposited on the glass substrate for a buffer layer. A (115) preferred orientation of the polycrystalline CBNO thin film was verified via X-ray diffraction measurements. The CBNO thin film on a glass substrate exhibited good ferroelectric properties with a remnant polarization of 4.8 μC/cm{sup 2} (2P{sub r} ∼9.6 μC/cm{sup 2}), although it had lower polarization than the epitaxially c-oriented CBNO thin film reported previously. A mosaic-like ferroelectric domain structure was observed via piezoresponse force microscopy. Significantly, the polycrystalline CBNO thin film showed much faster switching behavior within about 100 ns than that of the epitaxially c-oriented CBNO thin film.
Orientation control of {100}-oriented epitaxial orthorhombic 0.07YO{sub 1.5}-0.93HfO{sub 2} films grown by pulsed laser deposition was investigated. To achieve in-plane lattice matching, indium tin oxide (ITO) and yttria-stabilized zirconia (YSZ) were selected as underlying layers. We obtained (100)- and (001)/(010)-oriented films on ITO and YSZ, respectively. Ferroelastic domain formation was confirmed for both films by X-ray diffraction using the superlattice diffraction that appeared only for the orthorhombic symmetry. The formation of ferroelastic domains is believed to be induced by the tetragonal–orthorhombic phase transition upon cooling the films after deposition. The present results demonstrate that the orientation of HfO{sub 2}-based ferroelectric films can be controlled in the same manner as that of ferroelectric films composed of conventional perovskite-type material such as Pb(Zr, Ti)O{sub 3} and BiFeO{sub 3}.
Scanning spreading resistance microscopy (SSRM) was performed on non-intentionally doped (nid) ZnO nanowires (NWs) grown by metal-organic chemical vapor deposition in order to measure their residual carrier concentration. For this purpose, an SSRM calibration profile has been developed on homoepitaxial ZnO:Ga multilayer staircase structures grown by molecular beam epitaxy. The Ga density measured by SIMS varies in the 1.7 × 10{sup 17 }cm{sup −3} to 3 × 10{sup 20 }cm{sup −3} range. From measurements on such Ga doped multi-layers, a monotonic decrease in SSRM resistance with increasing Ga density was established, indicating SSRM being a well-adapted technique for two dimensional dopant/carrier profiling on ZnO at nanoscale. Finally, relevant SSRM signal contrasts were detected on nid ZnO NWs, and the residual carrier concentration is estimated in the 1–3 × 10{sup 18 }cm{sup −3} range, in agreement with the result from four-probe measurements.
Polycrystalline β-PbO films were grown by pulsed laser deposition in atmospheres ranging from oxygen-poor (the oxygen pressure of 0.01 Pa) to oxygen-rich (13 Pa) conditions, and the oxygen chemical potential was further enhanced by ozone annealing to examine hole doping. It was found that each of the as-grown β-PbO films showed poor electrical conductivity, σ < 1.4 × 10{sup −7} S cm{sup −1}, regardless of the oxygen pressure. The density functional calculations revealed that native defects including Pb and O vacancies have deep transition levels and extremely high formation enthalpies, which indicates difficulty of carrier generation in β-PbO and explains the experimentally observed poor electrical conductivity. The analysis of the electronic structures showed that the interaction between Pb 6s and O 2p orbitals is weak due to the deep energy level of Pb 6s and does not raise the valence band maximum (VBM) level unlike that observed in SnO, which is also supported by ultraviolet photoemission spectroscopy measurements. The deep acceptor transition levels of the native defects are attributed to the deep VBM of β-PbO. On the other hand, annealing β-PbO films in reactive oxygen-containing atmospheres (i.e., O{sub 3}) led to a significantly enhanced electrical conductivity (i.e., σ > 7.1 × 10{sup 2} S cm{sup −1}) but it is the result of the formation of an n-type PbO{sub 2} phase because oxygen chemical potential exceeded the phase boundary limit. The striking difference in carrier generation between PbO and SnO is discussed based on the electronic structures calculated by density functional theory.
The valence band offset value of 2.3 ± 0.2 eV at epitaxial NiO/Al{sub 2}O{sub 3} heterojunction is determined from photoelectron spectroscopy experiments. Pulsed laser deposited thin film of NiO on Al{sub 2}O{sub 3} substrate is epitaxially grown along [111] direction with two domain structures, which are in-plane rotated by 60° with respect to each other. Observation of Pendellosung oscillations around Bragg peak confirms high interfacial and crystalline quality of NiO layer deposited on Al{sub 2}O{sub 3} substrate. Surface related feature in Ni 2p{sub 3/2} core level spectra along with oxygen K-edge soft X-ray absorption spectroscopy results indicates that the initial growth of NiO on Al{sub 2}O{sub 3} substrate is in the form of islands, which merge to form NiO layer for the larger coverage. The value of conduction band offset is also evaluated from the measured values of band gaps of NiO and Al{sub 2}O{sub 3} layers. A type-I band alignment at NiO and Al{sub 2}O{sub 3} heterojunction is also obtained. The determined values of band offsets can be useful in heterojunction based light emitting devices.