Surface‐enhanced Raman scattering from gold‐coated germanium oxide nanowires

We utilized bulk‐synthesized nanowires (NWs) of germanium dioxide as nanoscale structures that can be coated with noble metals to allow the excitation of surface plasmons over a broad frequency range. The NWs were synthesized on substrates of silicon using gold‐catalyst‐assisted vapor–liquid–solid (VLS) growth mechanism in a simple quartz tube furnace setup. The resulting NWs have diameters of ∼100–200 nm, with lengths averaging ∼10–40 µm and randomly distributed on the substrate. The NWs are subsequently coated with thin films of gold, which provide a surface‐plasmon‐active surface. Surface‐enhanced Raman scattering (SERS) studies with near‐infrared (NIR) excitation at 785 nm show significant enhancement (average enhancement > 10⁶) with good uniformity to detect submonolayer concentrations of 4‐methylbenzenethiol (4‐MBT), trans‐1,2‐bis(4‐pyridyl)ethylene (BPE), and 1,2‐benzendithiol (1,2‐BDT) probe molecules. We also observed an intense, broad continuum in the Raman spectrum of NWs after metal coating, which tended to diminish with the analyte monolayer formation. Copyright © 2008 John Wiley & Sons, Ltd.     

Structural dependence of electrical properties of Ge films prepared by RF magnetron sputtering

This paper investigates the electrical properties of non-hydrogenated and hydrogenated germanium thin films deposited on silicon nitride coated glass in order to develop a material for the bottom cells of low cost monolithic tandem solar cells. Films were deposited by RF magnetron sputtering over a series of substrate temperatures up to 500°C. A structure-dependent conduction property of germanium films was found. As the substrate temperature increased from 255 to 400°C, both series of films first showed n-type conductivity with progressively increasing room-temperature dark resistivity that peaks around the type switch. Upon attaining p-type character the resistivity decreased rapidly with further increase in T _s. Accompanying these trends, the film grain orientation evolved from predominantly (220) to (111).     

Additive advantage in characteristics of MIMCAPs on flexible silicon (100) fabric with release‐first process

We report the inherent increase in capacitance per unit planar area of state‐of‐the art high‐κ integrated metal/insulator/metal capacitors (MIMCAPs) fabricated on flexible silicon fabric with release‐first process. We methodically study and show that our approach to transform bulk silicon (100) into a flexible fabric adds an inherent advantage of enabling higher integration density dynamic random access memory (DRAM) on the same chip area. Our approach is to release an ultra‐thin silicon (100) fabric (25 µm thick) from the bulk silicon wafer, then build MIMCAPs using sputtered aluminium electrodes and successive atomic layer depositions (ALD) without break‐ing the vacuum of a high‐κ aluminium oxide sandwiched between two tantalum nitride layers. This result shows that we can obtain flexible electronics on silicon without sacrificing the high density integration aspects and also utilize the non‐planar geometry associated with fabrication process to obtain a higher integration density compared to bulk silicon integration due to an increased normalized capacitance per unit planar area. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Highly transparent and conductive indium tin oxide thin films for solar cells grown by reactive thermal evaporation at low temperature

Transparent conductive tin-doped indium oxide (In₂O₃:Sn, ITO) thin films with various Sn-doping concentrations have been prepared using the low cost reactive thermal evaporation (RTE) technique at a low growth temperature of ~160 °C. The structural characteristics, optical and electrical properties of the ITO thin films were investigated. These polycrystalline ITO films exhibited preferential orientation along (222) plane and possessed low resistivities ranging from 3.51 to 5.71 × 10^?4 Ω cm. The decreased mobility was attributed to the scattering by ionized and neutral impurities at high doping concentrations. The optimized ITO thin film deposited with 6.0 wt% Sn-doping concentration exhibited a high average transparency of 87 % in the wavelength range of 380–900 nm and a low resistivity of 3.74 × 10^?4 Ω cm with a high Hall mobility of 47 cm² V^?1s^?1. A hydrogenated amorphous silicon and silicon–germanium (a-Si:H/a-SiGe:H) double-junction solar cell fabricated with the RTE-grown ITO electrodes presented a conversion efficiency of 10.51 %.     

Low‐temperature atomic layer deposition of MoOx for silicon heterojunction solar cells

The preparation of high‐quality molybdenum oxide (MoO_x) is demonstrated by plasma‐enhanced atomic layer deposition (ALD) at substrate temperatures down to 50 °C. The films are amorphous, slightly substoichiometric with respect to MoO₃, and free of other elements apart from hydrogen (&11 at%). The films have a high transparency in the visible region and their compatibility with a‐Si:H passivation schemes is demonstrated. It is discussed that these aspects, in conjunction with the low processing temperature and the ability to deposit very thin conformal films, make this ALD process promising for the future application of MoO_x in hole‐selective contacts for silicon heterojunction solar cells. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Aluminum-assisted crystallization and <Emphasis Type="Italic">p</Emphasis>-type doping of polycrystalline Si

Aluminum-doped p-type polycrystalline silicon thin films have been synthesized on glass substrates using an aluminum target in a reactive SiH₄+Ar+H₂ gas mixture at a low substrate temperature of 300 °C through inductively coupled plasma-assisted RF magnetron sputtering. In this process, it is possible to simultaneously co-deposit Si–Al in one layer for crystallization of amorphous silicon, in contrast to the conventional techniques where alternating metal and amorphous Si layers are deposited. The effect of aluminum target power on the structural and electrical properties of polycrystalline Si films is analyzed by X-ray diffraction, Raman spectroscopy, scanning electron microscopy and Hall-effect analysis. It is shown that at an aluminum target power of 100 W, the polycrystalline Si film features a high crystalline fraction of 91%, a vertically aligned columnar structure, a sheet resistance of 20.2 kΩ/□ and a hole concentration of 6.3×10¹⁸ cm⁻³. The underlying mechanism for achieving the semiconductor-quality polycrystalline silicon thin films at a low substrate temperature of 300 °C is proposed.     

Effect of annealing temperature on K-shell X-ray fluorescence parameters of zinc oxide thin films prepared by the sol-gel method

Zinc oxide thin films were grown on a glass substrate by a sol-gel process using a spin-coating technique. The obtained thin films were annealed between 350?°C and 550?°C in 50?°C steps and were then characterized using X-ray diffraction, scanning electron microscopy, and X-ray fluorescence techniques. The samples were stimulated by 59.5?keV gamma rays emitted from an Americium-241 annular radioisotope source. K X-rays emitted by samples were counted using an ultra-low energy germanium detector with a resolution of 150?eV at 5.96?keV. It was found that there was generally a decrease in both the Kβ/Kα X-ray intensity ratios and the K X-ray fluorescence cross sections for zinc oxide between 350?°C and 500?°C, but not at 550?°C. In addition, the X-ray diffraction patterns of the films showed that the transition phase from an amorphous to a polycrystalline hexagonal wurtzite structure was complete at an annealing temperature of 500?°C. The results show that variations in these parameters can be explained by the reorganization of atoms and the charge transfer process due to the effect of the annealing temperature on the elements forming the compounds.     

Probing the ferroelectric phase transition in sol–gel‐derived polycrystalline bismuth ferrite thin films

Polycrystalline BiFeO₃ (BFO) thin films were successfully grown on Pt/Ti/SiO₂/Si(100) and SrTiO₃ (STO) (100) substrates using the chemical solution deposition (CSD) technique. X‐ray diffraction (XRD) patterns indicate the polycrystalline nature of the films with rhombohedrally distorted perovskite crystal structure. Differential thermal analysis (DTA) was performed on the sol–gel‐derived powder to countercheck the crystal structure, ferroelectric (FE) to paraelectric (PE) phase transition, and melting point of bismuth ferrite. We observed a significant exothermic peak at 840 °C in DTA graphs, which corresponds to an FE–PE phase transition. Raman spectroscopy studies were carried out on BFO thin films prepared on both the substrates over a wide range of temperature. The room‐temperature unpolarized Raman spectra of BFO thin films indicate the presence of 13 Raman active modes, of which five strong modes were in the low‐wavenumber region and eight weak Raman active modes above 250 cm⁻¹. We observed slight shifts in the lower wavenumbers towards lower values with increase in temperature. The temperature‐dependent Raman spectra indicate a complete disappearance of all Raman active modes at 840 °C corresponding to the FE–PE phase transitions. There is no evidence of soft mode phonons. Copyright © 2008 John Wiley & Sons, Ltd.     

Raman characterization before and after rapid thermal annealing of CeO2 thin films grown by rf sputtering on (111) Si

We investigated by Raman spectroscopy (RS) the crystalline quality of CeO₂ thin films radio frequency magnetron sputtered on n‐type (111) Si substrates from CeO₂ target. The deposition temperature was in the range of 200–800 °C. We also realized structural investigations on CeO₂ layers after Rapid Thermal Annealing (RTA) performed in the range of 750–1000 °C for 30 s under nitrogen atmosphere. So this study displays that a high‐growth temperature and a high post‐growth‐RTA temperature improves the crystalline structure of the film. In fact, the best crystalline quality, which is close to the CeO₂ target taken as a reference, is obtained for a CeO₂ layer deposited at 800 °C and post‐annealed at 1000 °C for 30 s. Copyright © 2008 John Wiley & Sons, Ltd.     

Effective surface passivation of crystalline silicon using ultrathin Al2O3 films and Al2O3/SiNx stacks

We measure surface recombination velocities (SRVs) below 10 cm/s on p‐type crystalline silicon wafers passivated by atomic–layer–deposited (ALD) aluminium oxide (Al₂O₃) films of thickness ≥10 nm. For films thinner than 10 nm the SRV increases with decreasing Al₂O₃ thickness. For ultrathin Al₂O₃ layers of 3.6 nm we still attain a SRV < 22 cm/s on 1.5 Ω cm p‐Si and an exceptionally low SRV of 1.8 cm/s on high‐resistivity (200 Ω cm) p‐Si. Ultrathin Al₂O₃ films are particularly relevant for the implementation into solar cells, as the deposition rate of the ALD process is extremely low compared to the frequently used plasma‐enhanced chemical vapour deposition of silicon nitride (SiN_x). Our experiments on silicon wafers passivated with stacks composed of ultrathin Al₂O₃ and SiN_x show that a substantially improved thermal stability during high‐temperature firing at 830 °C is obtained for the Al₂O₃/SiN_x stacks compared to the single‐layer Al₂O₃ passivation. Al₂O₃/SiN_x stacks are hence ideally suited for the implementation into industrial‐type silicon solar cells where the metal contacts are made by screen‐printing and high‐temperature firing of metal pastes. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Quantitative analysis of hydrogen in amorphous silicon using Raman scattering spectroscopy

Hydrogenated amorphous silicon (a‐Si:H) films were studied using infrared and Raman spectroscopy. We have experimentally found that ratios of Raman scattering cross‐sections for Si–H to Si–Si bonds and for Si–H₂ to Si–Si bonds are equal to 0.65 ± 0.07 and 0.25 ± 0.03, respectively. It allows to measure the concentration of hydrogen in a‐Si:H films. The developed approach can be applied for in situ control of hydrogen in a‐Si:H films and also suitable for thin a‐Si:H films on substrates that are opaque in infrared spectral region. Copyright © 2013 John Wiley & Sons, Ltd.     

Electrocaloric effect in antiferroelectric PbZrO3 thin films

Antiferroelectric PbZrO₃ thin films have been deposited on Pt(111)/Ti/SiO₂/Si substrate by polymer modified sol–gel route. Temperature dependent P –E hysteresis loops have been measured at 51 MV/m within a temperature range of 40 °C to 330 °C. The maximum electrocaloric effect ～0.224 × 10^–6 K mV^–1 has been observed near the dielectric phase transition temperature (235 °C) of the thin films. The electrocaloric effect and its strong temperature dependence have been attributed to nearly first‐order phase transition. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

High mobility In2O3:H transparent conductive oxides prepared by atomic layer deposition and solid phase crystallization

The preparation of high‐quality In₂O₃:H, as transparent conductive oxide (TCO), is demonstrated at low temperatures. Amorphous In₂O₃:H films were deposited by atomic layer deposition at 100 °C, after which they underwent solid phase crystallization by a short anneal at 200 °C. TEM analysis has shown that this approach can yield films with a lateral grain size of a few hundred nm, resulting in electron mobility values as high as 138 cm²/V s at a device‐relevant carrier density of 1.8 × 10²⁰ cm^–3. Due to the extremely high electron mobility, the crystallized films simultaneously exhibit a very low resistivity (0.27 mΩ cm) and a negligible free carrier absorption. In conjunction with the low temperature processing, this renders these films ideal candidates for front TCO layers in for example silicon heterojunction solar cells and other sensitive optoelectronic applications. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Field emission measured from nanostructured germanium and silicon thin films

We have prepared nanostructured thin films of germanium and silicon. The films were grown by an ion beam sputtering technique followed by a rapid annealing step using an electron beam annealer. The annealing temperature is a comparatively low 500 °C, resulting in well defined nano-islands on the film surface. Electron field emission has been measured from the surfaces under high vacuum. The threshold electric field value for significant current flow was measured as 2.5 V μm⁻¹ for a silicon thin film which is comparable to other silicon technologies. A value of 0.5 V μm⁻¹ for a germanium thin film represents an order of magnitude improvement for related germanium nanostructured systems.     

Excellent organic/inorganic transparent thin film moisture barrier entirely made by hot wire CVD at 100 °C

We present a silicon nitride/polymer hybrid multilayer moisture barrier for flexible electronics made entirely by hot wire chemical vapor deposition (HWCVD) at substrate temperatures below 100 °C. Using the initiated CVD (iCVD) variant of HWCVD for the polymer layers, these can be extremely thin, while efficiently decoupling the defects in consecutive inorganic layers. Although a single layer of low temperature SiN_x is more prone to have pinholes than its state‐of‐the‐art high temperature equivalent, we have achieved a simple three‐layer structure consisting of two low‐temperature SiN_x layers with a polymer layer in between, which is pinhole free and shows a water vapor transmission rate (WVTR) as low as 5 × 10^–6 g/m²/day at a temperature of 60 °C and a relative humidity of 90%. This WVTR is low enough for organic devices. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Low temperature uniaxial growth of conducting LaNiO3 thin films on glass substrates with RbLaNb2O7 seed layer

Conducting LaNiO₃ thin films have been fabricated on the borosilicate glass substrates with and without the uniaxial oriented RbLaNb₂O₇ seed layer by an excimer laser assisted metal organic deposition process with a KrF laser irradiation at 400°C in air. The LaNiO₃ thin film prepared on the seed layer had a very high Lotgering factor at F(100)=0.971, indicating highly (100)-oriented growth. The obtained LaNiO₃ thin films with and without the seed layer showed low resistivity values, 4.42 and 1.02 mΩ cm at room temperature, respectively. The ρ value of the (100)-oriented LaNiO₃ film on the seed layer was comparably lowered to that of the films prepared at high process temperatures reported in the previous reports.     

Ultra‐thin nanocrystalline diamond films (<100 nm) with high electrical resistivity

Thick diamond films are known to exhibit remarkably high electrical resistivity and thermal conductivity. However, on thin films, difficulties are often observed to achieve such performances. In this study, the synthesis of ultra‐thin diamond films was optimized towards the possibility to maintain high dielectric performances on layers compatible with today requirements for Silicon‐On‐Diamond technology, and namely aiming at films with thicknesses equal or below 150 nm. The nucleation of diamond nanocrystals is crucial to obtain films with thickness lower than 100 nm. A Bias Enhanced Nucleation step (BEN) was improved to achieve nucleation densities above 10¹¹ cm^–2 although the process was also tuned to limit the size of the nanocrystals during this step. The control of the carbonization of the silicon substrate is also essential to reach such a density with a high reproducibility. The BEN is followed by a growth step with optimized conditions. The films were characterized by SEM and Spectroscopic Ellipsometry. Electrical conductivity measurements were conducted on thin diamond films and values obtained on layers below 100 nm were as high as 5 × 10¹³ Ω cm; a value significantly higher than the state of the art for such thin films. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Monitoring structural defects and crystallinity of carbon nanotubes in thin films

We report the influence of catalyst formulation and reaction temperature on the formation of carbon nanotube (CNT) thin films by the chemical vapour deposition (CVD) method. Thin films of CNTs were grown on Fe-Mo/Al₂O₃-coated silicon wafer by thermal decomposition of methane at different temperatures ranging from 800 to 1000°C. The electron microscopic investigations, SEM as well as HRTEM, of the as-grown CNT thin films revealed the growth of uniform multi-walled CNTs in abundance. The intensity ratio of D-band to G-band and FWHM of G-band through Raman measurements clearly indicated the dependency of structural defects and crystallinity of CNTs in thin films on the catalyst formulation and CVD growth temperature. The results suggest that thin films of multi-walled CNTs with negligible amount of defects in the nanotube structure and very high crystallinity can be obtained by thermal CVD process at 925°C.     

Synthesis of tin oxides SnO2–x in the entire composition range (x = 0 to 1) by ion‐beam sputter‐deposition

Ion‐beam sputter‐deposition (IBSD) was used to reactively deposit tin oxide crystalline films at oxygen fluxes of 3–15 sccm and at substrate temperatures of 100–600 °C. Analysing the samples by X‐ray diffraction and Raman spectro‐ scopy yields a map of the crystalline structures in dependence on the growth parameters. In addition to SnO₂, pure SnO films of high quality and an intermediate phase such as Sn₂O₃ or Sn₃O₄ can be reproducibly obtained. Thus, IBSD is, to our knowledge, the only thin‐film deposition technique verified yet to reliably produce samples in the entire composition range of tin oxides. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Structural study of TiO2 thin films by micro-Raman spectroscopy

The Raman spectroscopy method was used for structural characterization of TiO₂ thin films prepared by atomic layer deposition (ALD) and pulsed laser deposition (PLD) on fused silica and single-crystal silicon and sapphire substrates. Using ALD, anatase thin films were grown on silica and silicon substrates at temperatures 125–425 °C. At higher deposition temperatures, mixed anatase and rutile phases grew on these substrates. Post-growth annealing resulted in anatase-to-rutile phase transitions at 750 °C in the case of pure anatase films. The films that contained chlorine residues and were amorphous in their as-grown stage transformed into anatase phase at 400 °C and retained this phase even after annealing at 900 °C. On single crystal sapphire substrates, phase-pure rutile films were obtained by ALD at 425 °C and higher temperatures without additional annealing. Thin films that predominantly contained brookite phase were grown by PLD on silica substrates using rutile as a starting material.