3C-SiC nanowires and layers grown on Si: attractive material for biosensor applications

Nonotechnology is becoming an interesting field of research for biomolecular and medical diagnostics. The repeated screening is of crucial importance in the diagnosis of cancer and malignant tumours, since the pathologies at the early stages can be treated with the highest success probability. Many innovative approaches are emerging for the overcoming of this challenge, such as nanostructured surfaces for the enhancement of proteomic analysis, nanowires (NW) as biologically gated transistor, transductor for molecular binding events into real-time electrical signals and cantilevers for mechanical-based detection of biomolecules The interface of a biological system with tailored made detectors at the molecular scale opens the possibility to develop a entirely new class of devices and personal monitoring systems. The materials selected for these nanostructures must be biocompatible to ensure they are nontoxic and non-invasive for the organism, and must be capable to work in a very harsh environment. Silicon carbide (SiC) is mechanically robust, chemically inert, non toxic and biocompatible, so is a good material for biomedical purpose and for biosensor and bioelectronic applications. Several medical tools already uses SiC as bio-compatible coating, such as biomedical needles used in open heart surgery monitoring or temperature sensors based on bulk SiC. Nanosensors for ultrasensitive detection of proteins down to individual virus particles are also realised. Covalent bonding between specific molecules and stable interfaces are required for the realisation of biosensors based on molecular recognition, and since it was also demonstrated that SiC surfaces can be functionalized in order to react to specific biomolecules, this material is an optimal candidate for these kind of medical applications. Here we report a study on properties b-SiC-NWs and SiC layers deposited on silicon. Nanowires has been prepared with carbon oxide and nickel as catalytic element in nitrogen or argon atmosphere at the temperature between 1050 to 1100?C. Nanowires has been characterised by X-ray diffraction (XRD), by Scanning Electron Microscopy (SEM), Cathodoluminescence (CL) and by means Transmission Electron Microscopy (TEM). XRD patterns confirmed the characteristic peaks at 2q =35.6? (111), 41.4? (200), 59.9? (220), 75.5? (222) indexed as b-SiC. The room temperature CL spectrum revealed a broad peak centred at about 2.34 eV, related to the indirect band gap transition in b-SiC, and an intense blue band at about 2.68 eV. TEM images showed the crystalline core, having a <111> growth axis, sheathed by amorphous oxide. Typical SiC planar defects were present mainly on (111) planes, either stacking faults or rotational twins. Selected area electron diffraction from single NWs indicated the main phase as b-SiC. The SiC thin films were deposited on 2?? 001 Si wafers by means of VPE technique in a home made reactor with induction heating. A growth procedure at atmospheric pressure involving several steps (thermal etching, carburisation, epitaxial growth) was implemented. The precursor of choice were dilute SiH4 and C3H8 while H2 was used as carrier gas. The layers have been characterised by XRD, SEM, AFM. X-Ray diffraction evidences SiC(001) film is well oriented whit respect to the substrate having a narrow peak. SEM and AFM observations indicate a smooth film with good morphology

Epitaxial preparation of germanium cells for photovoltaic and thermophotovoltaic applications

Germanium is widely used in the advanced III-V photovoltaic technology based on arsenides and phosphides to realise triple junction (TJ) solar cells for space application and in thermophotovoltaic (TPV) devices. Nowadays, Ge cells are realised by diffusion and the cell conversion efficiency is partially limited by the broad doping profile thus obtained. Better performances could be achieved by means of homo-epitaxy of Ge on the Ge substrate, as improved thickness and doping profile control can be obtained with the epitaxial process. TJ cells, made of a InGaP/InGaAs/Ge monolithic array, have reached an efficiency value over 40% under concentration. The AM1.5 current density of the TJ cells are in the range of 15 mA/cm2 and the limiting subcell is the GaAs one: theoretical models suggest that the Ge subcell could produce a current density up to 40 mA/cm2, so that a large amount of the Ge potential is not fulfilled in the TJ cell. Epitaxial deposition of Ge would permit novel cell design (e.g. stacking 2 Ge cells) in order to obtain higher open circuit voltage. Ge cells are also employed in TPV devices to produce electricity from a heating source, thus fulfilling all the energetic content of a particular fuel and obtaining both heat and power from a single source. In this application, the advantages of Ge compared to the more common GaSb are a larger wafer size and a cheaper price. Ge epitaxial layers were deposited on both Ge and GaAs substrates by means of a home made Metal-Organic Vapor Phase (MOVPE) reactor using isobutylgermane (iBuGe). The samples were characterised by X-Ray Diffraction (XRD), Atomic Force Microscopy (AFM) and Transmission Electron Microscopy (TEM). Ohmic contacts were deposited on the samples in order to perform electrical characterization and to realise a simple p-n junction which showed photovoltaic effect. This work analyses the epitaxial growth of Germanium and the effect of the growth parameters (iBuGe partial pressure, temperature) on growth rate, surface morphology and material quality. In the temperature interval between 500 and 600?C a mass transport controlled regime was observed. Surface morphology showed a dependence on both the growth rate and on the substrate orientation: by using a low iBuGe partial pressure, a large density of holes were observed both by TEM and AFM. The holes almost disappeared by increasing the growth rate up to a limit of about 1mm/h, after which the surface roughness increases, degrading sample quality. XRD showed a nearly perfect crystallographic structure for the samples deposited on exaclty oriented (001) Ge substrates, while a larger diffraction peak was obtained for samples grown on (001) Ge substrated 6?off toward (110) direction. On the latter, a rougher and wave-like surface was observed by AFM while on exaclty oriented the surface was mirrorlike. n/p junctions were characterised by means of I-V and C-V techniques, and an optimal rectifying behaviour was obtained. The illuminated Ge n/p junctions reported a VOC of about 170 mV

TEM and SEM-CL studies of SiC Nanowires

3C-SiC and 3C-SiC/SiO2 core-shell nanowires (NWs) grown on silicon substrates by three different processes, based on the use of i) carbon monoxide, ii) silane with propane and iii) carbon tetrachloride precursors, are analysed by structural and optical techniques. Spectroscopic cathodoluminescence studies show a luminescence enhancement in core-shell structures, ascribed to an effective role of the shell as both carrier injecting barrier and passivation layer. In NWs grown using CCl4 precursor, a peculiar luminescence with dominant red component at about 2 eV has been detected and ascribed to point defects related to an unintentional oxygen incorporation.vedi abstract ingles

Growth and characterization of 3C-SiC grown using CBr4 as a precursor

The growth of silicon carbide on silicon is being studied for many diverse applications and so the search for precursors that could be used to grow with improved or novel physical, structural and morphological properties is a relevant issue in this field. Here we present a study of the use of CBr4 as a precursor in the deposition of 3C-SiC in a cold walled MOVPE reactor. The growth has been studied in a range of temperatures between 1100 and 1250 ?C, on differently oriented substrates. Additionally, the effect of the C:Si ratio in the gas phase was examined by the addition of propane to the reaction mixture. At lower temperatures faceted crystals grew as islands on the substrate; faceting and 2D planar growth was obtained if higher growth temperatures were applied and at higher C:Si ratios. Atomic force and scanning microscopies revealed interesting growth habits of the island type crystals. Transmission electron microscopy in cross-section confirms that these islands are 3C-SiC and have a high crystal perfection. The crystal habit has been characterised and will be presented. Carbon tetrabromide has revealed itself to be a useful precursor for the growth of SiC and, with a judicious control of the growth conditions could be applied to the growth of thin films and nanocrystals

Surface Functionalization of 3C-SiC Nanowires

One dimensional nanostrucures have potential applications in nanoscale electronic, optoelectronic or sensing devices. Core-shell nanowire (NW) structures of SiO2/&#946;-SiC and SiC-NWs are interesting for fundamental studies and technological applications: 3C-SiC is particularly appealing because of its good physical, chemical properties and biocompatibility, offering opportunities for nano-scale devices operating in biological environment. Moreover, functionalized 3C-SiC nanowires have the potential to act as highly sensitive detector elements in bio-chemical field. Here, we report on the preliminary results of the functionalization of 3C-SiC nanowires with an optically active, thiophene-based, &#960;-conjugated oligomer (PyT4). Oligothiophenes are semiconducting and fluorescent materials, widely used in organic electronics and biodiagnostic. SiC/SiO2 core/shell NWs grown by a Chemical Vapour Deposition (CVD) process on n-type Si (001) substrates, using carbon monoxide (CO) as the carbon source and nickel nitrate as the catalyst. The synthesis, performed at temperatures between 1050-1100?C. 3C-SiC NWs were grown in a home-made Vapor Phase Epitaxy (VPE) reactor using propane and silane as precursors (both diluted 3% in hydrogen) and a few nm of Ni as catalyst, deposited on Si(100) substrate using e-beam system. The nickel-deposited substrate is preheated at 1100?C for 5 minutes before introducing reagents for the grow time of 10 minutes. SiC/SiO2 core/shell NWs were then reacted with the triethoxysilane terminated with PyT4 to yield the hybrid NWs. The covalent grafting of the fluorophores was confirmed by fluorescence microscopy. The nanowires were further characterised by X-ray diffraction, Scanning Electron Microscopy, Cathodoluminescence and Transmission Electron Microscopy

Curvature and stress analysis in 3C-SiC layers grown on (001) and (111) Si substrates

Silicon carbide is an attractive material for the realization of devices and Micro Electro Mechanical Systems working in harsh environment and for biocompatible applications. More than 100 polytypes of SiC exist but the SiC cubic phase (3C-SiC) has drawn particular attention because it can be deposited on Si, enabling the low cost and large area growth and the use of conventional microfabrication processes. Unfortunately, high lattice and thermal mismatch (20% and 8%, respectively) hinder SiC growth on Si, leading to high residual stress and creating a highly defective layer at the interface, which must be effectively controlled for many applications. The residual stress can potentially lead to macroscopic wafer bending, while variation of stress through the film thickness could generate undesired deformation for SiC microstructures. In the standard 3C-SiC growth, a thin carbonization layer can help to relieve the lattice mismatch and thus high quality 3C-SiC/Si layers are commonly realized. However, large bending and warping of the structures are still reported, with complex shape depending on the growth recipes. This hinders the following-on wafer level processes.. To assess these key stress and deformation issues, we tested several kinds of pre-growth (carbonization) procedures, adding various amount of SiH4 to C3H8. 3C-SiC layers were grown on (001) and (111) Si substrates by Vapor Phase Epitaxy (VPE) using SiH4 and C3H8, diluted in H2. The mechanical deformation of the samples was measured by an optical technique called Makyoh, through which 3D deformation maps of the entire wafers were obtained. Curvature radius and stress profiles, parallel and perpendicular to the gas flow direction, were extracted from these maps and compared with those for the different growth conditions. These were correlated to the results of X-Ray Diffraction (XRD) and Raman spectroscopy (RS). XRD was used to check the crystal quality of the layers and, in transmission geometry, to assess whether the observed deformation was plastic or elastic. The 2\u27\u27 wafers were mapped in several points with RS and the peak position was related to the residual internal stress. It was observed that, for the same pre-growth procedures, the substrate curvature (convex, concave) is dependant on the (001) or (111) Si substrate used. In particular, the addition of SiH4 during carbonization ramp induced increased deformation for SiC/Si(001), while decreased deformation for SiC/Si (111). These results agree with the stress analysis from RS and XRD

Relazione sulle attivit? completate nel 4? semestre, periodo 20/09/2009-20/02/2010 del progetto 1

Secondo il piano temporale dettagliato del progetto gli obiettivi individuati nel semestre 20/08/2009-20/02/2010 sono: A.1.1: progetto preliminare di componenti termo-fotovoltaici A.4.1: progetto preliminare di caldaia termofotovoltaicaSecondo il piano temporale dettagliato del progetto gli obiettivi individuati nel semestre 20/08/2009-20/02/2010 sono: A.1.1: progetto preliminare di componenti termo-fotovoltaici A.4.1: progetto preliminare di caldaia termofotovoltaic

SiC epitaxial growth on Si(100) substrates using carbon tetrabromide

3C-SiC films were grown on Si by VPE using CBr4 as the carbon source, at temperatures ranging from 1100 to 1250?C. XRD, TEM, AFM, and SEM results indicate that the epitaxy proceeds as a 3D growth of uncoalesced islands at low temperature, whereas a continuous layer with hillocks on top is obtained above 1200?C. The shape and faceting of the islands are analyzed by AFM, showing (311) preferred facets.vedi abstract ingles

Effect of temperature on the mutual diffusion of Ge/GaAs and GaAs/Ge

GaAs/Ge heterostructures are commonly used in high efficiency solar cell: structures such as InGaP/GaAs/GaAs or GaAs/Ge and triple junction InGaP/GaAs/Ge, are continuously improving their efficiency. Sharp heterointerfaces and abrupt dopant profiles are essential in order to obtain a good control of the heteroepitaxy. One common problem in the Ge/GaAs, GaAs/Ge, Ge/GaAs/Ge, Ge/GaAs/GaAs, GaAs/Ge/GaAs and GaAs/Ge/Ge heterostructures is the interdiffusion of the elements in the different layers, i.e. Ge in GaAs and Ga and As in Ge. Since Ge acts as dopant in GaAs, and vice-versa, the film/substrate interdiffusion of Ge and GaAs is able to change the carrier concentrations in the layers. In order to assess this problem we grew Ge/GaAs and GaAs/Ge heterojunctions by metal-organic vapour phase epitaxy (MOVPE) using iso-butylgermane, arsine and trimethylgallium in hydrogen atmosphere at low pressure, varying the deposition temperature. The use of low temperature GaAs and Ge buffer layers was investigated in order to limit the interdiffusion. Different experimental techniques, including Secondary Neutral Mass Spectrometry (SNMS), High Resolution X-ray Diffraction (HR-XRD), Transmission Electron Microscopy (TEM), Rutherford Backscattering Spectrometry combined with Channeling technique (RBS/C) and Atomic Force Microscopy (AFM) have been used to investigate the samples. HR-XRD profiles show the good crystalline quality of the epitaxial layers, with a lattice mismatch between the layer and the substrate as calculated form the peak separation corresponding to perfectly adapted layers RBS/channeling spectra show no significant difference between the samples grown at different temperatures, indicating that the presence of extra defects, strain, or misorientation at the interface is below the detection limit. The depth profile analysis of samples measured by SNMS show remarkable interdiffusion of all components at the heterointerfaces between the layers and substrates, indicates a variation with temperature. The results confirmed that a low temperature GaAs buffer layer could efficiently reduce GaAs/Ge mutual diffusion. The same is not true for a low temperature Ge buffer layer in Ge/GaAs epitaxy. TEM was used to assess the crystal quality of the grown layers and composition distribution by X-ray microanalysis, that confirmed elements interdiffusion as measured by SNMS