Oriented Silicon Substrates Research Articles

Photocatalytic Performance of Sol-Gel Prepared TiO2 Thin Films Annealed at Various Temperatures.

Titanium dioxide (TiO2) in the form of thin films has attracted enormous attention for photocatalysis. It combines the fundamental properties of TiO2 as a large bandgap semiconductor with the advantage of thin films, making it competitive with TiO2 powders for recycling and maintenance in photocatalytic applications. There are many aspects affecting the photocatalytic performance of thin film structures, such as the nanocrystalline size, surface morphology, and phase composition. However, the quantification of each influencing aspect needs to be better studied and correlated. Here, we prepared a series of TiO2 thin films using a sol-gel process and spin-coated on p-type, (100)-oriented silicon substrates with a native oxide layer. The as-deposited TiO2 thin films were then annealed at different temperatures from 400 °C to 800 °C for 3 h in an ambient atmosphere. This sample synthesis provided systemic parameter variation regarding the aspects mentioned above. To characterize thin films, several techniques were used. Spectroscopic ellipsometry (SE) was employed for the investigation of the film thickness and the optical properties. The results revealed that an increasing annealing temperature reduced the film thickness with an increase in the refractive index. Atomic force microscopy (AFM) was utilized to examine the surface morphology, revealing an increased surface roughness and grain sizes. X-ray diffractometry (XRD) and UV-Raman spectroscopy were used to study the phase composition and crystallite size. The annealing process initially led to the formation of pure anatase, followed by a transformation from anatase to rutile as the annealing temperature increased. An overall enhancement in crystallinity was also observed. The photocatalytic properties of the thin films were tested using the photocatalytic decomposition of acetone gas in a home-built solid (photocatalyst)-gas (reactant) reactor. The composition of the gas mixture in the reaction chamber was monitored using in situ Fourier transform infrared spectroscopy. Finally, all of the structural and spectroscopic characteristics of the TiO2 thin films were quantified and correlated with their photocatalytic properties using a correlation matrix. This provided a good overview of which film properties affect the photocatalytic efficiency the most.

Epitaxial growth of high-quality yttria-stabilized zirconia films with uniform thickness on silicon by the combination of PLD and RF sputtering

High-quality (100)-oriented yttria-stabilized zirconia (YSZ) films with uniform thickness were epitaxially grown on silicon substrates by a two-step process of pulsed laser deposition (PLD) and radio-frequency magnetron sputtering (RFS). First, we fabricated a YSZ-seed layer on (001)-oriented silicon substrates using a PLD apparatus, and then transferred the YSZ-seed substrate to an RFS equipment to complete the growth of a thicker YSZ film. The dependence of YSZ crystal quality on RFS-growth temperature was investigated in the range of 200 to 850 °C. All the resulting YSZ films exhibited pure (100) orientation, and the full width at half maximum of the YSZ (200) X-ray rocking curve was as low as 0.64° for the YSZ film grown at 850 °C, and increased to 1.02° with the RFS-growth temperature declining to 200 °C. The results evidenced that the YSZ film can be epitaxially grown at 200 °C in the presence of a YSZ-seed layer. However, the splitting of the YSZ (200) X-ray diffraction peak appeared at 500 °C and 200 °C, which may be attributed to a transformation of the intrinsic stress in the YSZ films from compressive to tensile stress with decreasing temperature. In addition, the YSZ film had a uniform thickness distribution with an inhomogeneity of less than 5 % and a very flat and dense surface with root-mean-square surface roughness below 1 nm.

Correlation between microstructural and optoelectronic properties of porous Si/ SnO2 composites coated Al2O3

In this work, Al2O3 coating effect on morphology, structure and optoelectronic properties of Si/SnO2/Al2O3 porous matrix composites (PMCs) were investigated. A three-staked thin layers deposited on a 〈100〉 oriented silicon substrate made these composites. First, porous silicon layers were achieved by electrochemical etching method. Then Al2O3 and SnO2 layers were successively deposited by physical and chemical vapor deposition, respectively. Morphological and micro-structural properties of the as prepared composites were evaluated by Scanning electron microscope, energy dispersive X-ray spectroscopy and X-ray diffraction. Results proved that Al2O3 concentration alters notably the porosity of the PMCs.Variable angle spectroscopic ellipsometry (SE) revealed a high correlation between the optical constants (n, k) and the PMC microstructure. Impedance spectroscopy revealed a semiconductor-metallic transition at high frequency in the temperature range between 340 and 410 °C.

Magnetic field robust high quality factor NbTiN superconducting microwave resonators

We systematically study the performance of compact lumped element planar microwave Nb70Ti30N (NbTiN) resonators operating at 5 GHz in external in-plane magnetic fields up to 440 mT, a broad temperature regime from 2.2 K up to 13 K, as well as mK temperatures. For comparison, the resonators have been fabricated on thermally oxidized and pristine, (001) oriented silicon substrates. When operating the resonators in the multi-photon regime at T = 2.2 K, we find internal quality factors Q int ≃ 2 × 105 for NbTiN resonators grown on pristine Si substrates. In addition, we investigate the Q-factors of the resonators on pristine Si substrates at millikelvin temperatures to assess their applicability for quantum applications. We find Q int ≃ 2 × 105 in the single photon regime and Q int ≃ 5 × 105 in the high power regime at T = 7 mK. From the excellent performance of our resonators over a broad temperature and magnetic field range, we conclude that NbTiN deposited on Si (100) substrates, where the surface oxide has been removed, constitutes a promising material platform for electron spin resonance and ferromagnetic resonance experiments using superconducting planar microwave resonators.

Measurement of Residual Stress and Young's Modulus on Micromachined Monocrystalline 3C-SiC Layers Grown on and Silicon.

3C-SiC is an emerging material for MEMS systems thanks to its outstanding mechanical properties (high Young’s modulus and low density) that allow the device to be operated for a given geometry at higher frequency. The mechanical properties of this material depend strongly on the material quality, the defect density, and the stress. For this reason, the use of SiC in Si-based microelectromechanical system (MEMS) fabrication techniques has been very limited. In this work, the complete characterization of Young’s modulus and residual stress of monocrystalline 3C-SiC layers with different doping types grown on <100> and <111> oriented silicon substrates is reported, using a combination of resonance frequency of double clamped beams and strain gauge. In this way, both the residual stress and the residual strain can be measured independently, and Young’s modulus can be obtained by Hooke’s law. From these measurements, it has been observed that Young’s modulus depends on the thickness of the layer, the orientation, the doping, and the stress. Very good values of Young’s modulus were obtained in this work, even for very thin layers (thinner than 1 μm), and this can give the opportunity to realize very sensitive strain sensors.

Synthesis of ZnO sol-gel thin-films CMOS-Compatible.

Zinc oxide (ZnO) is a II–VI group semiconductor with a wide direct bandgap and is an important material for various fields of industry and high-technological applications. The effects of thickness, annealing process in N2 and air, optical properties, and morphology of ZnO thin-films are studied. A low-cost sol–gel spin-coating technique is used in this study for the simple synthesis of eco-friendly ZnO multilayer films deposited on (100)-oriented silicon substrates ranging from 150 to 600 nm by adjusting the spin coating rate. The ZnO sol–gel thin-films using precursor solutions of molarity 0.75 M exhibit an average optical transparency above 98%, with an optical band gap energy of 3.42 eV. The c-axis (002) orientation of the ZnO thin-films annealed at 400 °C were mainly influenced by the thickness of the multilayer, which is of interest for piezoelectric applications. These results demonstrate that a low-temperature method can be used to produce an eco-friendly, cost-effective ZnO sol–gel that is compatible with a complementary metal-oxide-semiconductor (CMOS) and integrated-circuits (IC).

Silicon-assisted surface enhanced fluorescence toward improved assay performances

A novel scheme of silicon-assisted surface enhanced fluorescence (SEF) is presented for SEF-based assays, where the blank signal suppression and the fluorescence signal enhancement is combined. The P-doped, (100) oriented silicon substrate is used to quench the fluorescence of Rose Bengal (RB) molecules attached to it, resulting in an effectively suppressed background signal, which is useful for a lower limit of detection (LOD). When a proper quantity of silver nanoparticles (AgNPs) is deposited on the RB-attached silicon substrate, a significant fluorescence enhancement of up to around 290 fold is obtained, which helps to improve the sensitivity in fluorescence-based assays. Besides, conventional gold nanoparticles (AuNPs) have also been demonstrated to exhibit excellent SEF effect using the presented scheme, providing improved stability and biocompatibility. The mechanism of the observed SEF effect has been investigated, and both the decreased apparent quantum yield and the silicon-induced electric field redistribution are considered to play important roles. The experimental results suggest that the presented scheme holds great potential in the SEF-based assays aiming at higher sensitivity and lower LOD.

Effects of annealing processes on the crystallization and magnetic properties of nickel ferrite thin films

Magnetic nickel ferrite (NFO), NiFe2O4, thin films were fabricated using the chemical solution deposition method on (100)-oriented silicon substrates. The effects of an annealing process on the crystallization and magnetic properties of NFO thin films were investigated. Two annealing methods were used, namely layer-by-layer multiple rapid-annealing process and single slow-annealing process. The results showed that the appropriate pyrolysis temperatures range from 400 to 500°C. NFO thin films crystallized well when they were annealed at 750°C or above. The NFO thin films fabricated using the single slow-annealing process (NFO-S) show better crystallinity compared to the films fabricated using the layer-by-layer multiple rapid-annealing process (NFO-R) when the annealing temperatures were low. NFO-R thin films show preferred (400) orientation growth and the relative intensity of the (400) peak raises with the annealing heating rate. Fine magnetic hysteresis loops and good magnetic properties were observed in both NFO-S and NFO-R thin films. A saturation magnetization of 277 emu cm−3 was derived for the NFO-S thin films, compared to a 230 emu cm−3 for the NFO-R thin films, which is a very high value compared with former reported values for NFO thin films.

Efficient Electrochemical Sensing Using Hybrid Boron-Doped Diamond/Graphene Nanowall Electrodes

Electrochemical sensors have attracted increasing interest due to their low cost, high sensitivity of detected analytes, and the possibility of sensor miniaturization, which allows for portable and field applications. These devices provide information about the system composition in real-time by combining a chemically selective layer with an electrochemical transducer. Thereby, the chemical energy of the specific interaction between the chemical substances and the sensor is converted into an analytically useful signal.Herein we demonstrate the electrochemical sensor based on boron-doped diamond/graphene nanowall electrodes (B:DGNW). A B:DGNW electrode is a hybrid electrode, which combines the extraordinary features of boron-doped diamond and a graphene nanowall on the same surface. B:DGNW is a sp2 -rich phase material with multilayered graphene walls oriented vertically to the substrate. Moreover, the presence of the boron-doped diamond phase enhances the electrochemical performance and kinetics of the electrode surface when compared with typical carbon nanowalls. The electrodes were fabricated in a one-step growth process using chemical vapor deposition without any additional modifications (thin films were grown on (100)-oriented silicon substrates).Two different sensing platforms will be presented: i) impedimetric biosensor for detection of pathogenic bacteria (e.g., Haemophilus influenzae), and ii) voltammetric sensor for determination of important nitroaromatic explosive compounds (2,4,6-trinitrotoluene (TNT), 2,4,6-trinitroanisole (TNA), 4,4',5,5'-tetranitro-1H,1'H-2,2'-biimidazole (TNBI)). The proposed B:DGNW electrodes were revealed to have a high electroactive area towards the voltammetric detection of various nitroaromatic compounds.The biosensor platform was obtained by B:DGNW surface functionalization firstly by electrografting of diazonium salt and secondly by the attachment of antibodies. The electrochemical biosensor shows high sensitivity and selectivity towards Haemophilus influenzae. The measurements were conducted in the presence of three non-target pathogens S. pyogenes, S. pneumoniae, and B. parapertussis. The proposed B:DGNW nonmodified carbon electrodes were revealed to have a high electroactive area towards the voltammetric determination of various nitroaromatic compounds, thus appearing to be an attractive nanocarbon surface for further applications. The B:DGNW electrode exhibits high sensing performance for TNT detection with linearity between 0.05 to 2 ppm and 4 to 15 ppm, and a limit of detection (LOD) of 73 ppb. The LOD for 2,4,6-trinitroanisole, in turn, is equal to 0.27 ppm, whereas the limit of detection of TNBI is equal to 0.52 ppm. To the authors' knowledge, this is the first time B:DGNW has been used to detect the nitroaromatic compounds electrochemically [1].AcknowledgmentThis work was supported by the Science for Peace Programme of NATO [Grant no. G5147], Polish National Science Centre [2016/22/E/ST7/00102], and the National Centre for Science and Development [347324/12/NCBR/2017]. The DS funds of the Faculty of Electronics, Telecommunications, and Informatics of the Gdansk University of Technology are also acknowledged.Reference:[1] A. Dettlaff et al., J. Hazard. Mater [In press]. Doi: 10.1016/j.jhazmat.2019.121672.

Negative capacitance phenomena in Au/SrTiO3/p-Si heterojunction structure

In this study, temperature- and frequency-dependent capacitance and conductance measurements of the Au/SrTiO3/p-Si heterojunction structure were examined. Strontium titanate (SrTiO3) thin films were deposited on cleaned p-type (100)-oriented silicon substrates using radio frequency (RF) magnetron sputtering method at a substrate temperature of 500 °C. The temperature-dependent reverse and forward bias C–V and G/ω$$-V$$ characteristics of the Au/SrTiO3/p-Si heterojunction structure was realized in the temperature range of 110–350 K (by step 30 K) at 1 MHz. The frequency-dependent reverse and forward bias C–V and G/ω$$-V$$ characteristics of the Au/SrTiO3/p-Si heterojunction structure was realized in the frequency range of 100–900 kHz (by step 100 kHz) at room temperature. It has been demonstrated that Au/SrTiO3/p-Si heterojunction structure exhibits negative capacitance (NC) behavior as a result of both temperature- and frequency-dependent capacitance measurements due to the reduction of interface charges, ionization states and series resistance in the forward bias voltage. In addition, it was shown that it exhibits an intersection point in the forward bias voltage of both temperature and frequency-dependent capacitance measurements. In temperature and frequency-dependent admittance spectroscopy measurements of Au/SrTiO3/p-Si heterojunction structure was observed NC phenomenon.

Influence of Orientation of a Silicon Substrate with a Buffer Silicon Carbide Layer on Dielectric and Polar Properties of Aluminum Nitride Films

Dielectric and polar properties of aluminum nitride (AlN) thin films epitaxially grown on differently oriented silicon substrates with the p-type conduction and a buffer silicon carbide (SiC) layer and on vicinal planes are investigated. The results of studies of the polar properties by two independent methods—the dynamic pyroelectric effect and the piezoresponse force microscopy—show that the SiC buffer layer application considerably improves polar properties of the aluminum nitride thin layer.

Bismuth induced crystallization of hydrogenated amorphous silicon thin films: effect of annealing time

This paper investigates the effect of annealing time and Bismuth (Bi) interlayer on the crystallization of hydrogenated amorphous silicon (a-Si:H) thin films. Bismuth thin films, 50 nm in thickness, were deposited by physical vapor deposition on ⟨100⟩ oriented silicon substrates. Afterwards, a-Si:H films were elaborated by plasma enhanced chemical vapor deposition (PECVD) at fixed growth conditions to cover the Bi coated Si substrates. To investigate the role of the deposited Bi interlayer as a Metal Induced Crystallization (MIC) of amorphous Si thin films, annealing experiments were performed under N2 atmosphere at 400 °C for different annealing times ranging from 2 h30 to 5 h. The effect of annealing time on the amorphous a-Si:H thin films in the presence of Bi interlayer has been evaluated in terms of crystallinity, preferential orientations, average surface roughness and atomic distribution using X-ray diffraction (XRD), Raman spectroscopy, atomic force microscopy (AFM), scanning electron microcopies (SEM) and energy dispersive X-ray analysis (EDS). These results showed that a-Si:H thin films with high crystallinity could be obtained at annealing time (ta) ≥ 2 h30. The electrical properties study of a-Si:H thin films revealed its correlation with their crystalline distribution which was enhanced due to the presence of the Bi interlayer.

Epitaxial growth and strain relaxation studies of BaTiO3 and BaTiO3/SrTiO3 superlattices grown by MBE on SrTiO3-buffered Si(001) substrate

In this work, the epitaxy of SrTiO3 and BaTiO3 perovskites on the (001)-oriented silicon substrate by molecular beam epitaxy is investigated. The heterostructures are studied by means of various structural and electrical characterization techniques. In this study especially, the authors reveal experimentally by nanobeam electron diffraction analysis the critical thickness prior relaxation of BaTiO3 grown on an SrTiO3/Si pseudosubstrate. They also propose to use a strain mediated superlattice composed of stacked [BaTiO3/SrTiO3] bilayers to prevent misfit dislocation formation. Using this approach, they could demonstrate high quality and dislocation free BaTiO3 ferroelectric layers integrated on silicon as confirmed by piezo-force microscopy techniques.

High-Performance Silicon-Compatible Large-Area UV-to-Visible Broadband Photodetector Based on Integrated Lattice-Matched Type II Se/n-Si Heterojunctions.

A gold-induced NH4Cl-assisted vapor-based route is proposed and developed to achieve vertically aligned submicron Se crystals on lattice-matched (111)-oriented silicon substrates, based on which a high-performance large-area silicon-compatible photodetector is constructed. Thanks to the energy band structure and the strongly asymmetrical depletion region, the fabricated Se/Si device maintains a similar wavelength cutoff to that of selenium devices before the IR region, along with a high-performance broadband photoresponse in the UV-to-visible region. The large-area photodetector maintains avery low leakage current under a -2 V bias, and a high on/off ratio of 103-104 is obtained with a high photocurrent of 62 nA at 500 nm. A photoresponse is clearly observed when the bias voltage is removed. The pulse response precisely provides a high response speed (τrise + τfall ≈ 1.975 ms), exceeding the fastest Se-based photodetectors in current reports. The enhanced photoelectric properties and the self-power photoresponse mainly derive from the integrated high-quality Se/n-Si p-n heterojunctions with both lattice match and type II energy band match.

Synthesis of Cadmium Oxide/Si Heterostructure for Two-Band Sensor Application

Cadmium oxide on Si heterostructure device has been constructed using (111) oriented silicon substrate by reactive PLD technique. The deposition of CdO film was carried out by irradiating high-purity Cd target using Nd:YAG laser in the presence of reactive oxygen gas. The deposited tin films were characterized by the X-ray diffraction, scanning electron microscopy and AFM. The electrical properties of deposited films were investigated reaching to the optimum pressure of oxygen at which the device has been prepared. The films were polycrystalline in nature having high orientation along (111) (200) and (220) planes. The films are formed to give the lower value of the resistivity which was found to be 7.56 × 10−3 Ω cm at the ambient of oxygen pressure around 350 Torr and without using post-deposition heat treatment. The electrical properties such as current–voltage in dark and illumination and capacitance–voltage measurement were carried out and found to be directly related to the substrate temperature. Also, the detector parameters have been characterized by measuring responsivity and detectivity which are found to be 0.38 A/W and 3.9 × 1012 cm Hz1/2 W−1, respectively.

Epitaxial growth of highly mismatched III-V materials on (001) silicon for electronics and optoelectronics

Monolithic integration of III-V on silicon has been a scientifically appealing concept for decades. Notable progress has recently been made in this research area, fueled by significant interests of the electronics industry in high-mobility channel transistors and the booming development of silicon photonics technology. In this review article, we outline the fundamental roadblocks for the epitaxial growth of highly mismatched III-V materials, including arsenides, phosphides, and antimonides, on (001) oriented silicon substrates. Advances in hetero-epitaxy and selective-area hetero-epitaxy from micro to nano length scales are discussed. Opportunities in emerging electronics and integrated photonics are also presented.

Influence of lanthanide ions on multiferroic properties of Ba2LnFeNb4O15 (Ln = Eu3+, Sm3+ and Nd3+) thin films grown on silicon by pulsed laser deposition

Highly c-oriented Ba2LnFeNb4O15-based multiferroic composite films with the lanthanide ion Ln=Eu3+, Sm3+ and Nd3+, have been grown by puled laser deposition on platinum-coated (001)-oriented silicon substrates. Investigation of the films functional properties demonstrated that both ferroelectricity and magnetism, originating respectively from the Ba2LnFeNb4O15 matrix and the BaFe12O19 inclusions, coexist simultaneously at room temperature and depend on the nature of the lanthanide ion. The evaluation of the degradation of the ferroelectric properties with the number of switching cycles showed that the synthesized films exhibited ferroelectric fatigue, with the notable exception of Ba2NdFeNb4O15 films, attesting their high ferroelectric switching endurance.

Calcination induced phase transformation of TiO2 nanostructures and fabricated a Schottky diode as humidity sensor based on rutile phase

The effects of annealing treatment on growth of TiO2 nanostructures (Ns) on the structural and morphological properties were studied. The phase transformation also investigated in this study. TiO2 Ns were fabricated on p-type (111)-oriented silicon substrates. Chemical bath deposition (CBD) method was employed to grow TiO2 Ns on the Si substrate at different annealing temperatures (without annealing, 350, 550, 750, and 950°C). The sample prepared at an annealing temperature of 550°C had a high crystallinity. A metal–semiconductor–metal type humidity sensor device was fabricated by depositing Pt contacts on top of the optimal sample (the sample annealed at 550°C) and the device working at room temperature, environmentally friendly features, and uncomplicated low-cost fabrication.

Cathodoluminescence and depth profiling studies of unintentionally doped GaN films grown by MOVPE

GaN layers are grown by metalorganic vapor phase epitaxy at 1050 °C on porous silicon and (111) oriented silicon substrates. AlN buffer layers of about 100 nm thickness were previously deposited on Si substrates. The effect of substrates on optical properties is revealed by Cathodoluminescence measurements (CL), recorded at room temperature and liquid nitrogen temperature. Various excitonic transitions are depicted. Spectral features associated with F°X energy around 3.4 eV and bound excitons (D°X and A°X in the range 3.29–3.35 eV) related to wurtzite GaN excitons are observed. Yellow band is located around 2.15 eV. CL depth profiling is also investigated at various e-beam energies (3–25 keV). The low-energy electron beam irradiation reveals an inhomogeneous distribution of point defects in depth, and high non-radiative recombination beyond a threshold energy. Good agreement between our experimental data and literature is obtained. Moreover, CL investigations prove that growth of GaN on (111) oriented Si substrate improve the crystalline quality of the layer.

Fabrication and characterization of metal–semiconductor–metal ultraviolet photodetector based on rutile TiO2 nanorod

The fabrication and characterization of a metal–semiconductor–metal ultraviolet photodetector are studied. The photodetector is based on TiO2 nanorods (NRs) grown on p-type (111)-oriented silicon substrate seeded with a TiO2 layer is synthesized by radio frequency reactive magnetron sputtering. A chemical bath deposition is used to grow TiO2 NRs on Si substrate. The structural and optical properties of the obtained sample are analyzed by using X-ray diffraction and field emission-scanning electron microscopy. Results show a tetragonal rutile structure of the synthesized TiO2 NRs. Optical properties are further examined using photoluminescence spectroscopy. A sharp and high-intensity UV peak at 367nm is observed in comparison with visible defect peaks centered at 432 and 718nm. Upon exposure to 365nm light (2.3mW/cm) at 5V bias, the device displays 76.06×102 sensitivity, internal photodetector gain of 77.06, photocurrent of the device is 2.62×10−5A and photoresponse peak of 69.7mA/W. The response and recovery times are calculated as 18.5 and 19.1ms upon illumination to a pulse UV light (365nm, 2.3mW/cm2) at 5V applied bias. These results demonstrate that the fabricated high-quality photodiode is a promising candidate as a low-cost UV photodetector for commercially-integrated photoelectronic applications.