Single crystalline Si thin films on insulating substrates (SOI) have a variety of potential applications to such as high mobility TFT and to high efficiency and low cost solar cells. Since the SOI is limited to a thin layer, it is needed to develop a low temperature epitaxial growth technology to form active layers thicker than several micorns at low temperatures. The purpose of this study is to develop a deposition technique of single crystalline Si thin films by a reactive CVD method [1] at temperatures less than 600○C utilizing gas-phase reaction (SiH4, F2).Deposition of Si films was performed on a single crystalline Si (100) wafer.Substrate-temperature was varied between 100 and 700○C, reaction-pressure 1 and 500mTorr, flow-rate between SiH4/F2 = 1/1 and 1/3, and the geometry of the substrate and the gas-outlet were optimized.First, it was found that deposition rate was sensitive to the distance between thegas-outlet and the substrate and to the total pressure. For four different combinations of pressures, 250 and 500 mTorr and distances, 50 and 150 mm. The deposition took place only for the combination of 500 mTorr and 50 mm, and otherwise the deposition rate was significantly lower or etching of Si wafer was observed. The deposition rate for gas flow ratio, SiH4/F2 of 1/1 was 1.7 nm/s at a substrate-temperature of 400○C, while for higher F2 flow rate ratio, SiH4/F2 = 1/2 and 1/3, the deposition rates were 8.3×10-3 nm/s and etching, respectively. Raman measurements show that crystallinity depends on the substrate-temperature; broad amorphous signal appears at 300, microcrystalline signal at 300 and 500○C and sharp crystalline at 400○C. RHEED observation shows a halo-pattern of amorphous-Si at 200○C, a mixed pattern of streak and spot without 2×1 superstructure at 300○C, a 2×1 streak-pattern at 400○C and a spot-pattern at 500○C. The reason of the narrow temperature window for epitaxial layer is a characteristic feature of low temperature epitaxy as reported before [2]. It is noteworthy the deposition rate of epitaxy obtained in this work is quite high, 1.7 nm/s even at 400○C. These observations are ascribed to the gas phase reaction between SiH4 and F2 and successive surface reactions. The SiH4 and F2 cause an exothermic reaction in the gaseous phases to generate radicals such as SiHx, H and F. The SiHx acts as a film precursor and others act as etchant. Under the conditions which radical density ratio SiHx/F increases, therefore, the deposition rate decreases or etching occurs. The material properties also will be discussed in relation to the growth mechanism.[1]J. Hanna et al., J. Non-Crst. Solids 114 (1989) 172-174[2]T. Kitagawa, M. Kondo et al, Appl. Surf. Sci. 159-160 (2000) 30-34

Selective area growth (SAG) of a-plane GaN grown on r-plane sapphire with a stripe orientation along <1-100> was investigated. The key technology of facet-control is optimizing the growth temperature and the reactor pressure. Our experiments reveal that the growth temperature determined facet form: in samples grown at 1000 °C, the structure consists of {11-22}and (000-1); with increasing growth temperature to 1050 °C, the area of {11-22} facet gradually decreases, and two new planes, (0001) and {11-20} facets form; eventually, in samples grown at 1000 oC, the {11-22} facet completely disappears, (0001) and {11-20} facet continue to increase to form a rectangle cross-section. The reactor pressure determines the ratio of the lateral growth rate and the vertical growth rate: with reactor pressure decreasing from 500 torr to 100 torr, the rectangle structure gradually decreases the height and increases the width, and the volume nearly keeps constant.

In this paper, we develop a new method based on ultra-low-energy ion implantation through a stencil mask to locally fabricate Si nanocrystals in an ultrathin silica layer. We perform a 1 keV Si implantation with doses of 5×1015 Si+/cm2, 7.5×1015 Si+/cm2 and 1×1016 Si+/cm2 in a 7 nm thick silicon oxide layer through stencil mask apertures ranging from 1μm up to 5 μm. After the mask removal the samples are furnace annealed at a temperature of 1050°C for 90 min under N2 atmosphere. The samples are then characterized by mapping the implanted and non-implanted areas by atomic force microscopy and photoluminescence spectroscopy. The intensity and the wavelength of the PL peak are found to depend on the implanted NCs cell size. A slight blue shift from 730 nm up to 720 nm is observed with decreasing cell size. Simultaneously, the PL intensity decreases and the signal vanishes for submicron features (which should contain 102 to 103 NCs). AFM microcopy performed on the implanted regions shows that the well-known oxide swelling usually observed after NCs synthesis decreases from 3.5 nm down to 0 as the cell size decreases. This result demonstrates that the effective implanted dose clearly decreases with the size of the cells. This effect is probably due to an electrostatic charging of the Si3N4 membrane despite the metallization treatments applied to the mask surface.

Epitaxial AlGaN/GaN layers grown by MBE on SiC substrates were irradiated with 150 MeV Ag ions at a fluence of 5×1012 ions/cm2. AlGaN/GaN MQWs were grown on Sapphire substrate by MOCVD and irradiated with 200 MeV Au8+ ions at a fluence of 5×1011 ions/cm2 . These samples were used to study the effects of SHI on optical properties of AlGaN/GaN based nano structures. RBS/Channelling strain measurements were carried out at off normal axis of irradiated and unirradiated samples. In as grown samples, AlGaN layer is partially relaxed with a small compressive strain. After irradiation this compressive strain increases by 0.22% in AlGaN layer. Incident ion energy dependence of dechannelling parameter shows E1/2 dependence, which corresponds to the dislocations. Defect densities were calculated from the E1/2 graph. As a result of irradiation defect density increased on both GaN and AlGaN layer. Optical properties of AlGaN/GaN MQWs before and after irradiation have been analyzed using PL. This study shows that SHI increase the confinement effects in the MQWs and intensity of the active layer of the MQWs luminescence is increased by one order. This may be due to the induced strain in GaN and AlGaN layers. Some unwanted yellow luminescence has also been introduced by the SHI possibly due the point defects or defect luminescence from the induced dislocations in GaN bulk epitaxial layers. In this study, we present some new results concerning high energy irradiation on AlGaN/GaN heterostructures and MQWs characterized by RBS/Channelling and PL.

In this study, we deposited zinc oxide (ZnO) thin film by atomic layer deposition (ALD) as an active channel layer in thin film transistor (TFT) using two different oxidizers, water (H2O-ALD) and oxygen radical (PA-ALD). The fabricated TFTs were annealed at various temperatures, in an oxygen ambient gas. The electrical properties of TFTs with PA-ALD ZnO film annealed at the temperature up to 400[oC] improved without any degradation of the subthreshold swing or any large shift of the threshold voltage. Through this study, we found that the high performance ZnO TFTs is possibly obtained using PA-ALD at low temperature, and the electrical properties are dependent on the annealing temperature.

Glancing-incidence X-ray diffraction (GIXRD) has been applied to the investigation of depth-dependent stress distributions within electroplated Cu films due to overlying capping layers. 0.65 μm thick Cu films plated on conventional barrier and seed layers received a CVD SiCxNyHz cap, an electrolessly-deposited CoWP layer, or a CoWP layer followed by a SiCxNyHz cap. GIXRD and conventional X-ray diffraction measurements revealed that strain gradients were created in Cu films possessing a SiCxNyHz cap, where a greater in-plane tensile stress was generated near the film / cap interface. The constraint imposed by the SiCxNyHz layer during cooling from the cap deposition temperature led to an increase in the in-plane stress of approximately 180 MPa from the value measured in the bulk Cu. However, Cu films possessing a CoWP cap without a SiCxNyHz layer did not exhibit depth-dependent stress distributions. Because the CoWP capping deposition temperature was much lower than that employed in SiCxNyHz deposition, the Cu experienced elastic deformation during the capping process. Cross-sectional transmission electron microscopy indicated that the top surface of the Cu films exhibited extrusions near grain boundaries for the samples undergoing the thermal excursion during SiCxNyHz deposition. The conformal nature of these caps confirmed that the morphological changes of the Cu film surface occurred prior to capping and are a consequence of the thermal excursions associated with cap deposition.

The introduction of large single crystal and high performance CdZnTe (CZT) grown by the traveling heater method (THM) in 2006 has defied conventional myths about the capability of this crystal growth method with respect to the production of spectroscopic grade CZT and its commercialization prospect in medical imaging application. Since then, a lot of progresses have been made, both in the crystal growth and the devices sides. This paper focuses on the development of THM CZT in recent years. Crystalline defects which challenge the thickness scalability of large volume CZT detectors along with efforts and achievements in overcoming these challenges are discussed. Advances in THM CZT crystal growth include 100mm diameter ingot and state-of-the-art device fabrication will also be presented.

A system to grow heteroepitaxial thin-films of solid oxide fuel cell (SOFC) cathodes on single crystal substrates was developed. The cathode composition investigated was 20% strontium-doped lanthanum manganite (LSM) grown by pulsed laser deposition (PLD) on single crystal (111) yttria-stabilized zirconia (YSZ) substrates. By combining electrochemical impedance spectroscopy (EIS) with x-ray photoemission spectroscopy (XPS) and x-ray absorption spectroscopy XAS measurements, we conclude that electrically driven cation migration away from the two-phase gas-cathode interface results in improved electrochemical performance. Our results provide support to the premise that the removal of surface passivating phases containing Sr2+ and Mn2+, which readily form at elevated temperatures even in O2 atmospheric pressures, is responsible for the improved cathodic performance upon application of a bias.

Silver (Ag) nanoparticles dispersed in an amorphous silica (SiO2) matrix or coated by a SiO2 layer were synthesized by flame spray pyrolysis (FSP). The coated nanoparticles were produced by using a modified enclosed FSP setup, in which the SiO2 precursor was injected through a ring above the FSP nozzle at various burner-ring-distances (BRDs), after the core Ag nanoparticles had been formed. The produced nanoparticles were characterized by XRD, BET, TEM and UV/vis analysis. The Ag particle size was possible to be controlled by tuning the FSP parameters. For the SiO2 coated nanoparticles, larger Ag core sizes were obtained for higher BRDs. All the produced nanoparticles exhibited the characteristic plasmon resonance frequency of Ag nanoparticles.

A method for simply and controllably modifying the surface of polyaniline nanofibres is described. The technique can be used to attach substituents bearing both acid and amine functional groups, making the materials suitable for further modification. Acid/amine functionalisation is achieved by a simple reflux reaction and therefore is a quick and easily scalable process. The modified nanofibres maintain their ability to switch between different states displaying distinctly different properties, thus making them suitable for adaptive sensing applications. As an example, we demonstrate how biomolecules can be attached to these functionalised nanofibres, to produce conducting polymer-based biosensors.

Two theoretical Monte Carlo models have been presented for description of the positive and negative magnetocaloric effects of Heusler Ni-Mn-X (X = Ga, In) alloys undergoing first order coupled magnetostructural phase transition. For both models all quantities such as temperature dependence of the magnetic contribution to the total specific heat, magnetization and isothermal magnetic entropy change are in qualitative agreement with the available experimental data.

The wide bandgap polar semiconductors GaN and ZnO and their related alloys exhibit fascinating properties in terms of bandgap engineering, carrier confinement, internal polarisation fields, and surface terminations. With a small lattice mismatch of ∼1.8 % between GaN and ZnO and the possibility to grow MgZnO lattice-matched to GaN, the system AlGaN/MgZnO offers the opportunity to design novel optoelectronic devices circumventing the problem of p-type doping of ZnO. In such AlGaN/MgZnO heterostructures with either hetero- or isovalent interfaces, tuning of band offsets is possible in various ways by polarisation fields, surface termination, strain, and composition. These aspects need to be fully understood to be able to make full use of this class of heterostructures. We report on the growth of ZnO films by chemical vapor deposition on p-type GaN and AlGaN grown by metal-organic vapor deposition on sapphire templates and on the fabrication of corresponding light-emitting diode (LED) structures. Electrical and optical properties of the n ZnO/p-GaN and n-ZnO/p-AlGaN LEDs will be compared and the observed differences will be discussed in terms of the band alignment at the heterointerface.

Transition metal multilayers are prime candidates for high reflectivity soft x-ray multilayer mirrors. In particular, Cr/Sc multilayers in the amorphous state have proven to give the highest reflectivity in the water window. We have investigated the influence of impurities N and O as residual gas elements on the growth, structure, and optical performance of Cr/Sc multilayers deposited in high vacuum conditions by a dual cathode direct current magnetron sputter deposition. Multilayer structures with the modulation periods in the range of 0.9–4.5 nm and Cr layer to bilayer thickness ratios in the range of 0.17–0.83 were deposited with an intentionally raised base pressure (pB), ranging from 2 × 10−7 to 2 × 10−5 Torr. Compositional depth profiles were obtained by elastic recoil detection analysis and Rutherford backscattering spectroscopy, while the structural investigations of the multilayers were carried out using hard x-ray reflectivity and transmission electron microscopy. By investigating stacked multilayers, i.e., several multilayers with different designs of the modulation periods, stacked on top of each other in the samples, we have been able to conclude that both N and O are incorporated preferentially in the interior of the Sc layers. At pB ≤ 2 × 10−6 Torr, typically <3 at.% of N and <1.5 at.% of O was found, which did not influence the amorphous nanostructure of the layers. Multilayers deposited with a high pB ∼2 × 10−5 Torr, a N content as high as ∼37 at.% was measured by elastic recoil detection analysis. These multilayers mainly consist of understoichiometric face-centered cubic CrNx/ScNy nanocrystalline layers, which could be grown as thin at 0.3 nm and is explained by a stabilizing effect on the ScNy layers during growth. It is also shown that by adding a background pressure of as little as 5 × 10−6 Torr of pure N2 the soft x-ray reflectivity (λ = 3.11 nm) can be enhanced by more than 100% by N incorporation into the multilayer structures, whereas pure O2 at the same background pressure had no effect.

Single-chamber micro solid oxide fuel cells (SC-μSOFCs) with coplanar electrodes were fabricated using a robotically controlled direct-write microfabrication approach. Viscoelastic, gel-based inks were employed to create homogeneous electrodes of controlled width and interelectrode distance as well as uniform cross-sectional thickness. Electrode powders, NiO-YSZ (yttria-stabilized zirconia) for the anode and (La0.8Sr0.2)0.98MnO3-YSZ for the cathode, were first dispersed with a cationic polyethyleneimine solution. Polyacrylic acid was added to induce a fluid-to-gel transition. The rheology of the fabricated inks was characterized. The inks were then extruded through cylindrical micronozzles and deposited onto YSZ electrolyte substrates using a robotic deposition apparatus. Thickness and width of the sintered electrodes were close to the diameter of the extrusion nozzle. The improved shape retention of the deposited electrodes also enabled the fabrication of continuous electrodes with square cross-section. The cathode adhered very well to the electrolyte during sintering. However, the mismatch between the thermal expansion coefficient of anode and electrolyte seems to cause detaching and breaking of the anode so that electrochemical characterization of the fabricated cells was not yet possible.

We used thickness gradients for high throughput optimization of adhesion in film stacks. The idea is based on the so-called superlayer test where a top layer under high compression exerts a load onto the lower interfaces and may cause delamination and buckling. Thus, on one hand, the thickness gradient of the superlayer results in the gradient of the load. On the other hand, the adhesion gradient can be realized by changing the thickness of an adhesion enhancer (or an adhesion reducer). When applied in two perpendicular directions (cross-gradient), the gradient of the superlayer in one direction and of the adhesion enhancer in the other, the plane of the sample represents a map where the line of delamination relates the interfacial toughness to the thickness of the enhancer.In our tests we used Mo superlayers under compressive stress of the order of ˜1.5 GPa on a Si wafer with a native oxide. The adhesion reduction was observed with this methodology when Ag layer up to 10 nm thick was deposited onto the substrate prior to Mo deposition. The delamination occurred at Ag thicknesses starting from ˜6 nm. This thickness of Ag corresponds to the islands coalescence and formation of a continuous film which immediately results in adhesion reduction. The other test was performed with a step gradient of Ti enhancer placed under a 10 nm thick Ag layer in otherwise the same arrangement. A single test showed that 2.8 Å of Ti was sufficient to improve the adhesion between Ag and SiOx by several times.

A simple and low-cost technique is demonstrated to fabricate sub-10 nm planar nanofluidic channels. Native oxide on silicon surface is etched with a multiple hydrofluoric (HF)-etch / SiO2-regrowth process. Shallow Si trenches with 3 nm to 24 nm depths are obtained at an etch rate of 1 nm per HF dip. The trenches are uniform with a surface r.m.s. roughness of 0.4 - 0.6 nm. A low-temperature and low-voltage anodic wafer bonding process is then used to form planar nanofluidic channels. Minimum aspect ratio (depth/width) of the fabricated sub-10 nm nanochannels is around 0.001-0.002.

Minoritary carrier’s transport properties are of fundamental importance in the HBT’s physics. The base transit time is a key parameter to improve microwave figure of merit. Some recent minoritary electron mobilities measurements versus acceptor doping level using magneto transport method exhibit a dramatic increase at very high majority carrier concentration. This effect has been attributed to the coupling of polar optical phonons with hole plasmons (LOPC) which controls the balance between enegy gain by electric field acceleration and energy loss by polar optical phonon emission. We present minoritary mobilities as a function of majority carrier doping calculated in the frame of electrons and holes Monte Carlo modelling including LOPC.

We have investigated different methods for preparing CdTe/CdS cross sections for electrical measurements, including the following: cleaving; using GaAs substrates; and sandwiching the structure between the substrate and a glass slide, and polishing with diamond discs and alumina suspension. The latter method proved to be the most reliable, with a success rate of over 90%.

We investigated cross sections of CdTe/CdS samples with scanning Kelvin probe microscopy (SKPM) using two different methods: applying the alternate bias with a frequency equal to 18.5 kHz, or equal to the frequency of the second cantilever resonance peak. The results showed that using the second resonance frequency produced a smoother signal, allowing the calculation of the electric field inside the device using just the raw SKPM data.

We were able to measure the distribution of the electrical potential inside working devices. Then, by taking the first derivative of the potential, we calculated the electric field and determined the location of the p-n junction.

Recent interest in phase change materials (PCMs) for non-volatile memory applications has been fueled by the promise of scalability beyond the limit of conventional DRAM and NAND flash memory [1]. However, for such solid state device applications, Ge2Sb2Te5 (GST), GeSb, and other chalcogenide PCMs require doping. Doping favorably modifies crystallization speed, crystallization temperature, and thermal stability but the chemical role of the dopant is not yet fully understood. In this work, X-ray Absorption Fine Spectroscopy (XAFS) is used to examine the chemical and structural role of nitrogen doping (N-) in as-deposited and crystalline GST thin films. The study focuses on the chemical and local bonding environment around each of the elements in the sample, in pre and post-anneal states, and at various doping concentrations. We conclude that the nitrogen dopant forms stable Ge-N bonds as deposited, which is distinct from GST bonds, and remain at the grain boundary of the crystallites such that the annealed film is comprised of crystallites with a dopant rich grain boundary.

Metal tetrahydroborates remain interesting materials as potential hydrogen storage media. In the present paper, we analyze thermodynamic stability of borohydrides of Al and Zr by means of extensive density functional calculations. We show that solid phases of these compounds are formed by dispersive Van der Waals forces. These compounds are thermodynamically unstable at room temperature with respect to decomposition to boron and hydrogen. Their stability is explained by formation of diborane as a necessary step in the decomposition path, pointing out to the kinetic factors that are important for the stability analysis of metal borohydrides.