Search

Ion-beam amorphization of crystalline silicon is reviewed. All the peculiar features of the process (temperature effect, incubation fluence, superlinear behavior of the amorphous fraction as a function of the ion fluence, dose–rate effects, ion mass/energy dependence, doping influence, etc.) can be explained within the classical theory of nucleation and growth based on capillarity. Nucleation and growth rates depend on the free energy of the amorphous clusters and on the kinetics balance between damage creation at the prompt stage of each ion collision cascade and competitive re–crystallization induced by atomic jumps of long living defects at the cluster surface. The model explains the damage accumulation kinetics either in dilute or dense collision cascade. It is an extension of the theoretical approach describing the reverse process, i.e. the ion–beam assisted nucleation and growth of crystalline clusters in the amorphous material. The description is independent on the atomistic nature of the involved defects.

The disordering process in crystalline GeSb2Te4 films has been studied by means of ion irradiation with 150 keV Ar+ ions. The effect of the interfaces and the role of the crystal microstructure has been investigated. The disordering path observed in a randomly oriented polycrystalline material with trigonal structure involves the transition to the disordered rocksalt structure (at fluence 7x1013 cm-2) and then to the amorphous phase (at 1.5x1014 cm-2). In GeSb2Te4 epitaxially grown on Si(111) the formation of the disordered rocksalt phase occurs at much higher fluence (3x1014 cm-2) and it is preceded by the conversion of the stable phase into the ordered rocksalt structure (at 5x1013 cm-2), with the formation of ordered vacancy layers, associated to a local variation of the stoichiometry. Even by increasing the fluence up to 3.5x1014 cm-2, the films remains mainly crystalline. The observed behaviour has been attributed to the effect of the interfaces and suggests the possibility to promote switching between the crystalline phases by interface engineering.

The crystallization behavior of amorphous nano regions (20–100 nm in diameter) embedded in a textured epitaxial Ge2Sb2Te5 (GST) 25 nm thick film grown on a Si (1 1 1) substrate has been investigated in situ by transmission electron microscopy (TEM) analysis. The amorphous regions were obtained by irradiation with 30 keV Ge+ at a fluence of 1.5 × 1014 ions cm−2 of masked samples. The adopted configuration simulates the GST structure of a device in the RESET state, it is then of relevance for understanding their data retention characteristics. The in situ TEM analysis indicates that the amorphous to crystal transition for 20 nm dots is characterized by a growth velocity of 3.6 pm s −1 at 75 °C, probably related to the external partially damaged area. At 90 °C annealing crystallization is completed for 20 nm dot. In the case of 50 and 100 nm diameter amorphous dots a growth velocity of about 2.6 pm s −1 was observed at 90 °C and of 170 pm s −1 at 110 °C. The transition is governed only by rearrangement (nucleation is absent) of the atoms located at the boundary of the amorphous dot with the surrounding crystalline regions. In some cases a preferential growth along surfaces normal to the [1 1 1] and to the [1 2 1] directions has been found. The crystallographic characterization of the regrowth crystal indicates a good matching with the zone axis of the surrounding material although the seed, a Si (1 1 1) wafer, at the bottom interface is damaged by the implant.

3C-SiC layers (7 and 15 μm thick), epitaxially grown on silicon, were covered with gold nanoparticles by immersion in a solution containing HF and KAuCl4. The surface of the layers played a crucial role in the morphology of the deposited metal network and large gold agglomerates developed on the pronounced antiphase domain boundaries of the thinner layer. Preferential growth was not observed on the smooth surface of the 15 μm thick layer. Rutherford backscattering spectrometry and electron microscopy outlined a progressive nucleation that takes place for less than 60 s immersion times under a kinetic control process. For longer deposition time each cluster grows under a diffusion control process, resulting in flower-like gold particles. However, the nucleation and growth processes can be strongly modified by multiple immersions in solution after rinsing in water. The adopted procedure allows the tailoring of the particles size and avoids the aggregation, therefore improving by about 1 order of magnitude t...

The morphology of gold nanoparticles (AuNPs) deposited on a (100) silicon wafer by simple immersion in a solution containing a metal salt and hydrofluoric acid (HF) is altered by HF treatment both before and after deposition. The gold clusters are characterized by the presence of flat regions and quasispherical particles consistent with the layer-by-layer or island growth modes, respectively. The cleaning procedure, including HF immersion prior to deposition, affects the predominantly occurring gold structures. Flat regions, which are of a few tens of nanometers long, are present after immersion for 10 s. The three-dimensional (3D) clusters are formed after a cleaning procedure of 4 min, which results in a large amount of spherical particles with a diameter of ≈15 nm and in a small percentage of residual square layers of a few nanometers in length. The samples were also treated with HF after the deposition and we found out a general thickening of flat regions, as revealed by TEM and AFM analysis. This result is in contrast to the coalescence observed in similar experiments performed with Ag. It is suggested that the HF dissolves the silicon oxide layer formed on top of the thin flat clusters and promotes the partial atomic rearrangement of the layered gold atoms, driven by a reduction of the surface energy. The X-ray diffraction investigation indicated changes in the crystalline orientation of the flat regions, which partially lose their initially heteroepitaxial relationship with the substrate. A postdeposition HF treatment for almost 70 s has nearly the same effect of long duration, high temperature annealing. The process presented herein could be beneficial to change the spectral response of nanoparticle arrays and to improve the conversion efficiency of hybrid photovoltaic devices.

Platinum nanostructures are of large interest because of their electrocatalytic properties, enhanced by the high surface/volume ratio, and find applications in numerous fields from fuel cells and electrolyzers, to gas biosensors and nano-biomedicine. In this work we present a simple procedure, based on spontaneous galvanic displacement, to obtain Pt nanoparticles (PtNPs) directly on Si substrate. The nucleation and growth processes involved in the deposition have been studied by Transmission Electron Microscopy and Rutherford Backscattering Spectrometry. The nucleation is quite complex and includes, first, the generation of primary non-growing clusters, with size smaller than 2 nm. Then, for longer deposition times, larger particles are formed, resulting from dynamic coalescence of primary clusters with/or single ion attachment processes. The growth of such larger particles follows the prediction of the classical diffusion limited model. Moreover, the large nanoparticles are poly-crystalline and exhibit a nano-porous structure, in agreement with a mixed aggregative-diffusion limited growth. The nano-porous structure implies a high internal surface area, therefore making these particles particularly promising as efficient catalysts.

The variation of the electrical and optical properties under 150 keV Ar+ ion irradiation has been studied in Ge2Sb2Te5 polycrystalline films, either in the rocksalt or in the trigonal structure, by in situ reflectivity measurements and ex situ resistance measurements. As the irradiation dose increases, the disorder introduced in the crystalline films increases and the reflectivity decreases, down to a minimum value that corresponds to complete amorphization. Large differences are found by changing the irradiation temperature, for the two crystalline structures . Indeed, the measured amorphization threshold is the same for the two crystalline phases and equal to 1x1013 cm-2 under irradiation at 77K, whilst at room temperature the trigonal phase requires a dose almost double than the rocksalt phase to be amorphized. By structural analyses we found that, before amorphization, ion irradiation induces a transition from the trigonal to the rocksalt structure. The van der Waals gaps present in the trigonal phase might act as preferential sinks for the displaced and mobile atoms, thus promoting this transition. By further increasing the irradiation dose the formed disordered rocksalt phase converts into the amorphous phase. Ion irradiation also affects the electrical properties of the material: the disorder modifies the temperature dependence of resistance of the trigonal Ge2Sb2Te5 and induces a change of sign (from metallic to insulating behavior) at a dose of 2x1013 cm-2, well below the amorphization threshold.

Multidirectional SPEG (Solid Phase Epitaxial Growth) of silicon has been investigated in micro and nanoamorphous structures generated on a crystalline substrate by a nano-sized ion beam, Gaussian shaped and with a standard deviation of about 5 nm. The 20 keV Ga+ ions were implanted at a fluence of 5 × 1014 ions cm−2 in a bulk Si(1 0 0) single crystal. Two structures were used for the implants: circular regions of 100 nm and 1 μm diameters respectively and straight lines 10 nm in width and few microns in length along (1 0 0) or (1 1 0) directions. The lateral spread of ions has been taken into account in the damage estimation. Transmission Electron Microscopy indicates that the structures are made of an amorphous core surrounded by a defective and filamentary shell. The recovery of the damaged outer regions promptly occurs during the early stages of the thermal treatment at 500–600 °C for all the structures. By prolonging annealing time, re-crystallization of the amorphous cores is achieved too by the movement of the underneath crystal–amorphous interface. The re-growth is almost defects free when the contribution of the crystalline seed below the structures is present, defective and twin mediated if it misses as in the thinnest regions of the specimen.

Electrically trimmable resistors have been manufactured using the phase change alloy Ge 2 Sb 2 Te 5 . High resistivity values (about 1 mΩ cm) and almost zero temperature coefficient of resistance (TCR) have been obtained using resistors with hcp phase of metallic behavior and converting a small region of the film into the fcc phase, with opposite trend of resistance versus temperature. The conversion into the fcc phase has been obtained through two electric pulses, the first for melting and quenching in the amorphous phase and the second for crystallization in the fcc semiconductive phase. Theoretical considerations together with the experimental measurements allow to determine the proper ratio between the metallic and the semiconductive phase required to obtain a target resistance and temperature dependence. For example, a resistor almost independent on the temperature can be achieved by electrical trimming, increasing the resistance of about 10% of the initial value. The resistor layout has been optimized through electro-thermal simulations, to achieve the modification of the resistors structure only in a confined region. Experimental results confirm the theoretical expectations and demonstrate that phase change materials can be effectively used to obtain reversible electrically trimmable resistors with adjustable TCR.

The scattering mechanisms limiting electronic transport in substrate-supported graphene have been investigated by mapping the electron mean free path (l) in graphene on substrates with different dielectric permittivities, i.e. SiO2(κSiO2=3.9), 4H-SiC (0001) (κSiC=9.7) and SrTiO3(001) (κSrTiO3=330). From the analysis of the local l versus gate bias curves, histograms and nanoscale maps of the densities of charged impurities (NCI) have been extracted. l was found to increase from (107±6) nm to (218±11) nm moving from SiO2 to SiC substrate, whereas a decrease to (179±5)nm was found for gra- phene on SrTiO3. Furthermore, a clear correlation between the minima in the l maps and the maxima in the NCI maps was obtained for graphene on SiO2 and SiC, whereas the two maps are uncorrelated in graphene on SrTiO3. These results indicate that charged impurity scattering is the main source of the lateral inhomogeneity of l on lower κ substrates, like SiO2 and SiC, whereas a different scattering mechanism (probably resonant scattering) must be invoked as the origin of the nanoscale variations of l in graphene on high-κ substrates like SrTiO3. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

$\mathrm{G}{\mathrm{e}}_{2}\mathrm{S}{\mathrm{b}}_{2}\mathrm{T}{\mathrm{e}}_{5}$ polycrystalline films either in the trigonal stable phase or in the metastable rock-salt structure have been irradiated with 150 keV ${\mathrm{Ar}}^{+}$ ions. The effects of disorder are studied by electrical, optical, and structural measurements and density functional theory (DFT) simulations. In the metastable structure, the main effect of ion irradiation is a progressive amorphization, with an optical threshold at a fluence of $3\ifmmode\times\else\texttimes\fi{}{10}^{13}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}2}$. For the trigonal structure, a metal-insulator transition and a crystalline transition to rock-salt structure occur prior to amorphization, which requires a fluence of $8\ifmmode\times\else\texttimes\fi{}{10}^{13}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}2}$. The bonds of Te atoms close to the van der Waals gaps, present in the trigonal phase and identified by Raman spectroscopy, change as a function of the disorder induced by the irradiation. Comparison with DFT simulations shows that ion irradiation leads to the gradual filling of the van der Waals gaps with displaced Ge and Sb lattice atoms, giving rise first to a metal-insulator transition $(9%$ of displaced atoms) correlated to the modification of the Te bonds and then induces a structural transition to the metastable rock-salt phase $(15%$ of displaced atoms). The data presented here not only show the possibility to tune the degree of order, and therefore the electrical properties and the structure of phase change materials by ion irradiation, but also underline the importance of the van der Waals gaps in determining the transport mechanisms and the stability of the crystalline structure.

Scientific Reports 6: Article number: 23843; published online: 01 April 2016; updated: 16 August 2016 In this Article, Stefania Privitera and Emanuele Rimini are incorrectly listed as being affiliated with ‘Paul-Drude-Institut für Festkörperelektronik, Hausvogteiplatz 5-7, 10117 Berlin, Germany’. The correct affiliation is listed below

Scientific reports 6, 23843 (2016). doi:10.1038/srep23843, Phase Change Materials (PCMs) are unique compounds employed in non-volatile random access memory thanks to the rapid and reversible transformation between the amorphous and crystalline state that display large differences in electrical and optical properties. In addition to the amorphous-to-crystalline transition, experimental results on polycrystalline GeSbTe alloys (GST) films evidenced a Metal-Insulator Transition (MIT) attributed to disorder in the crystalline phase. Here we report on a fundamental advance in the fabrication of GST with out-of-plane stacking of ordered vacancy layers by means of three distinct methods: Molecular Beam Epitaxy, thermal annealing and application of femtosecond laser pulses. We assess the degree of vacancy ordering and explicitly correlate it with the MIT. We further tune the ordering in a controlled fashion attaining a large range of resistivity. Employing ordered GST might allow the realization of cells with larger programming windows., Published by Nature Publishing Group, London

Electrostatic properties, quantum capacitance (C-q) and local density of states (LDOS) are evaluated for graphene on 4H-SiC(0 0 0 1) by measuring the local capacitance with Scanning Capacitance Spectroscopy (SCS). Two distinct samples were used for comparative study, viz., graphene exfoliated and deposited on 4H-SiC(0 0 0 1)-DG, and graphene grown epitaxially on 4H-SiC(0 0 0 1)-EG. We observed a distinctly lower screening length (r(scr)) and C-q while wider variations in the LDOS for EG. Such differences are attributed to the peculiar interface between EG/4H-SiC(0 0 0 1), which is known to be more or less defective having the presence of positive charges.

A systematic study of silver-catalyzed etching in (100) and (111) n-type Silicon substrates is carried out by varying the amount and the morphology of electroless deposited silver clusters. Deposition rate is very fast during the first few seconds and then it levels at a lower value of one order magnitude for both oriented wafers. For small and well separated clusters the Si nanowires formed by the etching are aligned along the (100) axes, while for large and interconnected they are oriented along the normal to the surface. For the (100) Si substrate partially covered with small silver clusters the etching proceeds both in vertical and horizontal [100] directions. For larger coverage (70%) the Ag nanoparticles sink all together along the vertical direction. For the (111) Si substrate, when the Ag nanoparticles are isolated they sink in all the equivalent [100] directions with an irregular path; when the Ag nanoparticles begin to coalesce uniform domains of Si nanowires aligned along [100] directions are formed. When the silver clusters are well interconnected (70% of coverage), the etching is vertical. Silver coverage is then another experimental parameter to induce a preferred orientation of the etched Si nanowires.

We present a nanoscale morphological and structural characterization of few layers of graphene grown by thermal decomposition of off-axis 4H-SiC (0001). A comparison between transmission electron microscopy (TEM) in cross-section and in plan view allows to fully exploit the potentialities of TEM. Such a comparison was used to get information on the number of graphene layers as well as on the rotational order between the layers and with respect to the substrate. Some peculiar structures observed by TEM (wrinkles) could only be systematically measured by atomic force microscopy (AFM). In particular, the density and the height of the wrinkles in the few layers of graphene was investigated.

Graphene films were grown on thin polycrystalline Ni using a buried amorphous carbon (a-C) layer as C source. Rapid thermal processes (RTP) at temperatures from 600 to 800 degrees C were used to promote C diffusion into Ni and its subsequent segregation on Ni surface, during the sample cool down. RIP at 800 degrees C was the optimal condition for graphene film formation. Micro-Raman spectroscopy showed that the grown film is mostly composed by multilayers of graphene. Atomic force microscopy showed that the film presents peculiar corrugations (wrinkles), isotropically oriented and with heights ranging from from similar to 1 to similar to 15 nm. Selected area diffraction by transmission electron microscopy on the MLG membranes shows a rotational disorder between the stacked graphene layers.

We investigated a simple and low-cost route for the formation of metallic nanodots on Si substrates ordered in size and position and laterally isolated by SiO(2). The method was based on a two-step process: (i) the formation of a nanopattern of ordered cylindrical pores on oxidized Si substrates through self-assembly of diblock copolymers, and successive oxide dry etching down to the Si; (ii) the deposition of gold nanodots and thermal diffusion over the nanopatterned oxide substrates. After diffusion, the nanodot density outside the nanopores was found to decrease, and most of the nanodots were found to saturate the nanopores. The process was followed in situ by transmission electron microscopy (TEM) and ex situ by scanning electron microscopy (SEM) analysis for different thermal budgets. This patterned substrate can be used for catalyst mediated growth, for example, through vapor-liquid-solid (VLS), of nanowires for the formation of absorber materials in novel photovoltaic architectures.

The damaging process of GeTe up to amorphization has been studied by introducing controlled levels of disorder by irradiation with 150 keV Ar+ ions. In situ reflectivity measurements and ex-situ resistance and Raman spectroscopy analysis have been employed to study the impact of ion bombardment on the electrical conduction properties and on the bonding. The results obtained are indicative for three different stages of film damage. The first step appears to be dominated by point defects, affecting the temperature coefficient of resistance (TCR) and inducing a transition from positive (metallic conduction) to negative TCR values (conduction dominated by localized states), whilst the material still remains crystalline. The second step is characterized by the annealing of the defects induced, presumably, by the formation of complex defects that act as sinks for point defect recombination. This process is facilitated by the high atomic mobility. The third phase of damage starts at a fluence of 3.5 × 1014 cm−2 and finally converts the material to the amorphous state, characterized by higher resistance and decreased optical reflectivity. The modifications observed upon ion irradiation provide important insights into the possible states that can be achieved in crystalline GeTe through different local atomic arrangements towards amorphization.

The atomic diffusion and compositional variations upon melting have been studied by transmission electron microscopy and electron energy loss spectroscopy in Ge rich GeSbTe films, with a composition optimized for memory applications. Melting and quenching has been achieved by laser pulses, in order to study pure thermal diffusion without electric field induced electromigration. The effect of different laser energy densities has been investigated. The diffusion of Ge atoms in the molten phase is found to be a prominent mechanism and, by employing finite elements computational analysis, a diffusion coefficient of Ge on the order of 5 × 10−5 cm2 s−1 has been estimated.

Single layers of graphene obtained by mechanical exfoliation of highly oriented pyrolytic graphite and deposited on a SiO 2 /n + -Si substrate were irradiated with high-energy (500 keV) C ions. Controlled amounts of defects were introduced in graphene by a precise control of the ion fluence, ranging from 1 x 10 13 to 1 × 10 14 cm ―2 . Scanning capacitance spectroscopy (SCS) was used as non-destructive characterization technique to probe the effect of irradiation defects on the electrical properties of graphene. A wider variation between the capacitance curves measured at different positions on irradiated graphene is found in comparison with the pristine graphene. The quantum capacitance per unit area C' q was extracted from raw data. In particular, an increase of C' q is associated to the damaged regions.

We have carried out an investigation of graphene/4H-SiC (0001) interface by nanoscale current transport measurements. Graphene was deposited by mechanical exfoliation of HOPG. Novel Scanning Probe Current Mapping and Scanning Probe Current Spectroscopy are found to be useful for non-destructive characterization of graphene. Presence of graphene lowers Schottky barrier height on 4H-SiC (0001). Observed barrier heights (0.8 ± 0.1 eV) are comparable but higher than reported in literature for 6H-SiC (0001). Current mapping of graphene deposited on 4H-SIC(0001). Highly conductive regions in the current maps correspond to graphene in the morphological maps.

Irradiation with high energy (500 keV) C+ ions at fluences from 11013 to 11014 cm-2 was used to introduce controlled amounts of defects in single layers of graphene deposited on a SiO2(100 nm)/n+Si substrate. Scanning Capacitance Spectroscopy (SCS) was used as non-destructive characterization technique to probe the effect of irradiation on the electrical properties of graphene. In particular, a comparative study between the local capacitance of pristine graphene and irradiated graphene is presented, showing that lateral variations in irradiated graphene are distinctly higher. The local quantum capacitance per unit area C’q of graphene was extracted from raw data. While a narrow distribution of C’q values was obtained in pristine graphene, two distinct distributions were obtained in irradiated monolayers, associated to locally damaged and not damaged regions, respectively.

A nanoscale investigation on the capacitive behavior of graphene deposited on a SiO2/n(+) Si substrate (with SiO2 thickness of 300 or 100 nm) was carried out by scanning capacitance spectroscopy (SCS). A bias V(g) composed by an AC signal and a slow DC voltage ramp was applied to the macroscopic n(+) Si backgate of the graphene/SiO(2)/Si capacitor, while a nanoscale contact was obtained on graphene by the atomic force microscope tip. This study revealed that the capacitor effective area (A(eff)) responding to the AC bias is much smaller than the geometrical area of the graphene sheet. This area is related to the length scale on which the externally applied potential decays in graphene, that is, the screening length of the graphene 2DEG. The nonstationary charges (electrons/holes) induced by the AC potential spread within this area around the contact. A(eff) increases linearly with the bias and in a symmetric way for bias inversion. For each bias V(g), the value of A(eff) is related to the minimum area necessary to accommodate the not stationary charges, according to the graphene density of states (DOS) at V(g). Interestingly, by decreasing the SiO(2) thickness from 300 to 100 nm, the slope of the A(eff) versus bias curve strongly increases (by a factor of approximately 50). The local quantum capacitance C(q) in the contacted graphene region was calculated starting from the screening length, and the distribution of the values of C(q) for different tip positions was obtained. Finally the lateral variations of the DOS in graphene was determined.

In this paper, we present the results regarding the electrical and optical characterization of bidimensional arrays of silicon single photon avalanche photodiodes (SPAD). Low average dark count rates and crosstalk probability, good timing jitter and high quantum detection efficiency in the visible range have been measured over 5 × 5 SPAD arrays with 40 μm single pixel active area diameter. Moreover, a very good uniformity of these parameters has been found over the single pixels. These good electro-optical performances make our photodiodes attractive for the fabrication of arrays with a larger number of SPAD. In view of the design of a dense array to be used as a silicon photomultiplier (SiPM) for medical diagnostic tools, 5 × 5 SPAD arrays have been also tested in the photon-resolving mode. Charge spectra show that the arrays, in this configuration, allow to resolve several photoelectron peaks with a good single photoelectron resolution.

In this letter we present the results regarding the electrical and optical characterization of Geiger mode silicon avalanche photodiodes (GMAP) fabricated by silicon standard planar technology. Low dark count rates, negligible afterpulsing effects, good timing resolution and high quantum detection efficiency in all the visible range have been measured. The very good electro-optical performances of our photodiodes make them attractive for the fabrication of arrays with a large number of GMAP to be used both in the commercial and the scientific fields, as telecommunications and nuclear medical imaging.

The galvanic displacement deposition of silver on H-terminated Si (100) in the time scale of seconds is instantaneous and characterized by a cluster density of 10(11)-10(12) cm(-2). The amount of deposited Ag follows a t(1/2) dependence in agreement with a Cottrell diffusion limited mechanism. At the same time, during the deposition, the cluster density reduces by a factor 5. This behavior is in contrast with the assumption of immobile clusters. We show in the present work that coalescence and aggregation occur also in the samples immersed in the diluted hydrofluoric acid (HF) solution without the presence of Ag(+). Clusters agglomerate according to a process of dynamic coalescence, typical of colloids, followed by atomic redistribution at the contact regions with the generation of multiple internal twins and stacking-faults. The normalized size distributions in terms of r/rmean follow also the prediction of the Smoluchowski ripening mechanism. No variation of the cluster density occurs for samples immersed in pure H2O solution. The different behavior might be associated to the strong attraction of clusters to oxide-terminated Si surface in presence of water. The silver clusters are instead weakly bound to hydrophobic H-terminated Si in presence of HF. HF causes then the detachment of clusters and a random movement on the silicon surface with mobility of about 10(-13) cm(2)/s. Attractive interaction (probably van der Waals) among particles promotes coarsening.

The structural transformations occurring in Ge2Sb2Te5 films heated at temperature up to 400 °C, and under hydrostatic pressure up to 12 GPa, have been investigated through in-situ X ray diffraction measurements. The adopted experimental conditions are close to those experienced by the phase change material during the SET (crystallization)/RESET (amorphization) processes in a nonvolatile memory device. The compression enhances the thermal stability of the amorphous phase, which remains stable up to 180 °C at 8 GPa and to 230 °C at 12 GPa. The structure of the crystalline phases is also modified, with the formation of a CsCl-type structure instead of rock-salt and of a GeS-type structure at the temperature at which usually the trigonal stable phase is formed. Overall, the stability of the stable phase appears to be more affected by the compression. We argue that the presence of weak bonds associated to the van der Waals gaps is a determining factor for the observed reduced stability.

Silicon Carbide (SiC) is the most promising material for the fabrication of a new category of sensors and devices, to be used in very hostile environments (height temperature, corrosive ambient, presence of radiation ...). The fabrication of Silicon Carbide sensors requires new processes able to realize microstructures on bulk material or on the silicon carbide surface. A very promising method to create microstructures in SiC is the electrochemical etch in aqueous solution of hydrofluoric acid. In this paper the bulk micromachining of SiC substrate and the structural and electrical characterization of porous silicon carbide formation are reported. These results were used to create a novel method for the bidimensional pattering of SiC substrate. In particular a novel Si-based hard mask to transfer the design onto the SiC substrate was used. The hard mask is formed by a sandwich structures made by subsequent deposition of VAPOX, POLY Si, SiO2.

We outline the possible applications of newly developed single photon avalanche detectors (SPAD) operating at low voltage and fabricated in silicon planar technology, reporting on tests done with scintillators. We envisage the integration of large arrays of identical sensors, employing a common read-out mode in order to achieve a high resolution and high sensitivity solid state photomultiplier. The applications of such a device would be many, particularly if we consider that we are also working on the production of microoptical components, perfectly suited for the lossless coupling of these photosensors to several light sources (lasers, ordinary and scintillating fibers, scintillators, optical devices), by exploiting the promising deep lithography with ions (DLI) technology. Crossed time-of-flight and charge measurements between SPADs and usual photomultipliers, coupled to scintillators, have allowed to compare their relative performance. Specific issues regarding the SPAD and the scintillating fiber readout are presented, also in view of possible applications in the field of particle/radiation tracking systems.

Substitutional impurities (B, Ga) in Si experienced an off-lattice displacement during ion-irradiation using a H+ or He+ beam at room temperature in random incidence. Samples were prepared by solid phase epitaxy (SPE) of pre-amorphized Si subsequently implanted with B and Ga at a concentration of about 1 × 1020 at. cm−3 confined in a 300 nm thick surface region. The lattice location of impurities was performed by a channelling technique along different axes (, ) using the 11B(p,α)8Be reaction and standard RBS for B and Ga, respectively. The normalized channelling yield χ of the impurity signal increases with the ion fluence, indicating a progressive off-lattice displacement of the dopant during irradiation in random incidence, until it saturates at χF

The electrical properties and the structure of isothermally annealed thin films of Ge2+xSb2Te5 (x=0 and 0.5) have been studied by in situ electrical measurements, x-ray diffraction, and transmission electron microscopy analyses. Phase separation has been observed in samples with an excess of Ge; by annealing amorphous Ge2.5Sb2Te5 films at temperatures in the range 130–160 °C, the material cannot be completely converted into the metastable face-centered-cubic (fcc) structure. At temperatures higher than 160 °C, the residual amorphous material may be converted into a fcc structure with a lower lattice parameter.

Design and fabrication of single photon avalanche detector (SPAD) in planar technology is reported. Device design and critical issues in the technology are discussed. Experimental test procedures are described for dark-counting rate, afterpulsing probability, photon timing resolution, and quantum detection efficiency. Low-noise detectors are obtained, with dark counting rates down to 10 c/s for devices with 10 /spl mu/m diameter, down to 1 kc/s for 50 /spl mu/m diameter. The technology is suitable for monolithic integration of SPAD detectors and associated circuits.

Silicon quantum dots have been deposited on top of a 3-nm tunnel oxide by Low Pressure Chemical Vapour Deposition (LPCVD) and coated with a 7-nm Chemical Vapour Deposited (CVD) oxide. This stack was then incorporated in Metal-Oxide-Semiconductor structure and used as floating gate of a memory cell. The presence of 3 nm of tunnel oxides allows the injection of the charge by direct tunnel (DT) using low voltages for both program and erase operations. The charge stored in the quantum dots is able to produce a well-detectable flat band shift in the capacitors or, equivalently, a threshold voltage shift in the transistors. Furthermore, due to the presence of SiO 2 between the grains, the lateral charge loss is reduced and, thus, long retention time are possible. In this work we present good memory action characterised by low write voltages, write times of the order of milliseconds and long retention time in spite of the low tunnel oxide thickness. © 2002 Elsevier Science B.V. All rights reserved.

The oxidation of epitaxial thin Si 1-x Ge x layers (0.06 < x < 0.30) at 1000 °C in dry O 2 , for times between 20 and 240 s, has been investigated. The analysis of the thin oxides ( 4-20 nm) has been performed using X-ray photoelectron spectroscopy and Rutherford backscattering spectrometry. Although most of the Ge is found to pile-up at the oxide/Si 1-x Ge x interface, our data indicate the formation of both SiO 2 and GeO 2 for all investigated samples and oxidation times. Moreover, the oxidation rate, enhanced with increasing the Ge concentration in the alloy, is reported. To our knowledge, this is the first experimental evidence of GeO 2 formation and rate enhancement in the regime of high temperature oxidation in dry O 2 and Si 1-x Ge x alloys with x < 0.5. The differences could be peculiar to the initial stage of oxidation, as well as to the rapid thermal process used in our case, but a clear answer is currently unavailable.

The electroless deposition of Gold on H-terminated Si(100) substrates has been investigated with 2.0 MeV He+ ions Rutherford Backscattering Spectrometry (RBS) and Transmission Electron Microscopy (TEM). A large number of Gold nanoparticles (in excess of 1011 cm−2) was instantaneously generated on the silicon substrate by immersion in a solution containing 1mM KAuCl4 and 4.8M HF. The gold atomic surface concentration, as given by the RBS measurements, increases with the square root of the immersion time from 1 to 5s. The experimental data are fitted with a diffusion coefficient for the metallic ions in the solution of 1.45 × 10−5 cm2 s−1. At longer deposition times, instead, the Au ions flux is constant, with a static boundary layer of few hundred microns thick in agreements with one dimensional diffusion limited mechanism.

Crystallization of amorphised thin silicon layer on insulator (SOI), and subsequently doped by As implantation has been investigated after furnace annealing at 680 degrees C or laser irradiation in the milliseconds regime. In partly crystallized intrinsic amorphous SOI grains are 3 times larger than the ones observed in implanted layers produced by chemical vapor deposition. In the As implanted SOI, the density of grains is reduced whilst their size increases up to a factor 8 at 3 x 10(15) As/cm(2). The presence of As increases both the nucleation barrier and the grain growth velocity. Similar effects were observed after laser annealing. (C) 2014 Elsevier B.V. All rights reserved.

[object Object]