Quantitative analysis of tungsten, oxygen and carbon concentrations in the microcrystalline silicon films deposited by hot-wire CVD

In order for hot-wire chemical vapor deposition to compete with the conventional plasma-enhanced chemical vapor deposition technique for the deposition of microcrystalline silicon, a number of key scientific problems should be cleared up. Among these points, the concentration of tungsten (nature of the filament), as well as the concentration of oxygen and carbon (elements issued when vacuum is broken between two runs), should not exceed threshold values, beyond which electronic properties of the films could be degraded, as in the case of monocrystalline silicon. Quantitative chemical analysis of these elements has been carried out using the secondary ion mass spectrometry technique through depth profiles. It has been shown that for a high effective filament surface area (S_f=27 cm²), the W content increases steadily from 5×10¹⁴ to 2×10¹⁸ atoms cm⁻³ when the filament temperature T_f increases from 1500 to 1800 °C. For a fixed T_f, the W content increases with the effective surface area S_f. Thus, considering our reactor geometry, the W content does not exceed the detection limit (5×10¹⁴ atoms cm⁻³) when T_f and S_f are limited to 1600 °C and 4 cm², respectively. For O and C elements, under deposition conditions of high dilution of silane in hydrogen (96%), O and C concentrations approaching 10²⁰ atoms cm⁻³ have been obtained. The introduction of an inner vessel inside the reactor, the addition of a load-lock chamber and a decrease in substrate temperature to 300 °C have led to a drastic decrease in these contents down to 3×10¹⁸ atoms cm⁻³, compatible with the realization of 6% efficiency HWCVD μc-Si:H solar cells.     

Dependence of the a-Si:H defect density of states on the magnetic field profile of an electron cyclotron resonance microwave plasma

The magnetic field profile of an electron cyclotron resonance microwave plasma was systematically altered to determine subsequent effects on a-Si:H film quality. The mobility gap deep density N_D deposition rate and light-to-dark conductivity were determined for the a-Si:H films. By variation of the magnetic field profile N_D could be altered by more than an order of magnitude, from 1 × 10¹⁶ to 1 × 10¹⁷ cm⁻³ at 0.7 mTorr and 1 × 10¹⁶ to 5 × 10¹⁷ cm⁻³ at 5 mTorr as determined by junction capacitance techniques. Two deposition regimes were found to occur for the conditions of this study. Highly divergent magnetic fields resulted in poor quality a-Si:H, while for magnetic field profiles defining a more highly confined plasma, the a-Si:H was of device quality and relatively independent of the magnetic field configuration. The data is interpreted as a consequence of silane depletion for highly divergent magnetic field profiles.     

Structural properties of swift heavy ion beam irradiated Fe/Si bilayers

The Fe/Si multilayers were prepared by electron beam evaporation in a cryo-pumped vacuum deposition system. Ag⁺ and Au⁺ ions of 100 MeV at two different fluencies such as 1 × 10¹² ions/cm² and 1 × 10¹³ ions/cm² at a pressure of 10^− 7 torr were used to irradiate the Fe/Si samples. The irradiated samples were analyzed by High-Resolution XRD and it reveals that the irradiated films are having polycrystalline nature and it confirms the formation of the β-FeSi₂. The structural parameters such as crystallite size (D), strain (ε) and dislocation density (δ) have been evaluated from the XRD spectrum. The role of the substrates and the influence of swift heavy ions on the formation of β-FeSi₂ have been discussed in this paper.     

Influence of conventional furnace and rapid thermal annealing on the quality of polycrystalline β-FeSi2 thin films grown from vapor-deposited Fe/Si multilayers

Thin films of polycrystalline β-FeSi₂ were grown on (100) Si substrates of high resistivity by electron beam evaporation of Si/Fe ultrathin multilayers and subsequent annealing by conventional vacuum furnace (CVF) and rapid thermal annealing (RTA) for 1 h and 30 s, respectively, in the temperature range from 600 to 900°C. X-ray diffraction, Raman spectroscopy, spectroscopic ellipsometry, resistivity and Hall measurements were employed for characterization of the silicide layers quality in terms of the annealing conditions. For the silicide layers prepared by CVF annealing, although the grain size increase with increasing the annealing temperature, the optimum temperature to obtain the higher material quality (carrier mobility of the order of 100 cm² Vs⁻¹ and carrier concentration of about 1 × 10¹⁷ cm⁻³) is about 700°C. At higher annealing temperatures, the quality of the material is degraded due to the presence of the oxide Fe₂O₃. In the case of the silicides prepared by RTA, the quality of the material is improved progressively with increasing the annealing temperature up to 900°C.     

Characteristics of the radiation damage seen in the silicon microstrip detector of Fermilab experiment E771

The central region of the silicon microstrip detector used in Fermilab experiment E771 was subjected to a peak fluence of 9.5 × 10¹³ p/cm² induced by 800 GeV protons over a two-month period. Fourteen 300 μm thick planes manufactured by Micron Semiconductor were operated at bias voltages ranging from 84 to 109 V. Analysis of data from low intensity beam triggers taken near the end of the run shows that the mean pulse height from our amplifiers began to decline at a fluence of approximately 2 × 10¹³ p/cm² and fell to near zero by 6 × 10¹³ p/cm². We show that the use of fast amplifiers contributed to this early loss of signal.     

Growth and spectroscopy of Dy doped in ZnWO4 crystal

Single-crystal ZnWO₄:Dy³⁺ was grown by Czochralski technique. The XRD, absorption spectra as well as fluorescence spectrum are investigated and the Judd–Ofelt intensity parameters Ω₂, Ω₄, Ω₆ are obtained to be 7.76 × 10⁻²⁰ cm², 0.57 × 10⁻²⁰ cm², 0.31 × 10⁻²⁰ cm², respectively. Calculated radiative transition rate, branching ratios and radiative lifetime for different transition levels of ZnWO₄:Dy³⁺ crystals are presented. Fluorescence lifetime of ⁴F_9/2 level is 158 μs and quantum efficiency is 66%.The most intense fluorescence line at 575 nm correlative with transition ⁴F_9/2 → ⁶H_13/2 is potentially for application of yellow lasers.     

Formation of low-resistivity poly-Si and SiNx films by Cat-CVD for ULSI application

In this paper, bulk-Si metal–oxide–semiconductor field effect transistors (MOSFETs) are fabricated using the catalytic chemical vapor deposition (Cat-CVD) method as an alternative technology to the conventional high-temperature thermal chemical vapor deposition. Particularly, formation of low-resistivity phosphorus (P)-doped poly-Si films is attempted by using Cat-CVD-deposited amorphous silicon (a-Si) films and successive rapid thermal annealing (RTA) of them. Even after RTA processes, neither peeling nor bubbling are observed, since hydrogen contents in Cat-CVD a-Si films can be as low as 1.1%. Both the crystallization and low resistivity of 0.004 Ω·cm are realized by RTA at 1000 °C for only 5 s. It is also revealed that Cat-CVD SiN_x films prepared at 250 °C show excellent oxidation resistance, when the thickness of films is larger than approximately 10 nm for wet O₂ oxidation at 1100 °C. It is found that the thickness required to stop oxygen penetration is equivalent to that for thermal CVD SiN_x prepared at 750 °C. Finally, complementary MOSFETs (CMOSs) of single-crystalline Si were fabricated by using Cat-CVD poly-Si for gate electrodes and SiN_x films for masks of local oxidation of silicon (LOCOS). At 3.3 V operation, less than 1.0 pA μm⁻¹ of OFF leakage current and ON/OFF ratio of 10⁷–10⁸ are realized, i.e. the devices can operate similarly to conventional thermal CVD process.     

Concepts of UCN sources for the FRM-II

Three concepts for sources of ultra-cold neutrons (UCN) for the reactor FRM-II at Garching near Munich are studied: one, Mini-D₂, is a source with 170 cm³ of solid deuterium in the beam tube SR4 and the second one a large solid-deuterium source (volume about 30 dm³), mounted in the beam tube SR5 as an advanced cold source with a number of neutron guides. The third one, Mark 3000, uses superfluid ⁴He at a cold-neutron guide. A UCN density of up to 7×10⁴ cm⁻³ may possibly be achieved in the storage volumes of Mini-D₂ yielding more than 10⁹ UCN for extraction to an attached experimental setup. The usable UCN flux at the periphery of the large deuterium source is predicted to be 2×10⁷ cm⁻² s⁻¹. Mark 3000, finally, is expected to yield a UCN density of about 10⁵ cm⁻³.     

Surface physical and chemical changes of pure iron after molybdenum ion implantation and their effects on the tribological behaviour I: Physical and chemical analysis

Molybdenum ions generated by a metal vapour vacuum arc (MEVVA) ion source were implanted into pure iron at doses of 1 × 10¹⁷ and 3 × 10¹⁷ ions cm⁻² with an extraction voltage of 45 kV. Auger electron spectroscopy (AES) sputtering depth profiles, X-ray photoelectron spectroscopy (XPS) analysis, X-ray diffraction (XRD) analysis, microhardness and the residual stress of the implanted specimen were studied. The results show that molybdenum atoms exist in the implanted layer at a maximum concentration 20 at.%. A new phase (Fe₃C) is formed in the specimens implanted higher doses due to carbon incorporation during sputtering of the natural oxide film from the implanted surface. The Fe₂Mo phase is formed in both dose regimes. Residual compressive stresses of 310 and 560 MPa were measured on the surfaces of the specimens after molybdenum ion implantation at 1 × 10¹⁷ and 3 × 10¹⁷ ions/cm² respectively due to a local expansion of the lattice in the near-surface region. Due to the existence of residual compressive stress and the formation of the new phases, the microhardness of pure iron specimens was increased from 264 to 325 and 333 kgf mm⁻² by molybdenum ion implantation at 1 × 10¹⁷ and 3 × 10¹⁷ ions cm⁻² respectively.     

Anomalous elastic response of amorphous alloys suggesting collective motions of many atoms

The dynamic Young’s modulus, E, of amorphous (a-) Zr₆₀Cu₃₀Al₁₀ (numbers indicate at.%) alloy was measured as a function of frequency, f, with a strain amplitude, _t, of 10⁻⁶, E(10⁻⁶,f), and also as a function of _t for f near 10² Hz, E(_t,10² Hz), by means of the vibrating reed methods. The elasticity study under the passing of electric current (PEC) was carried out too. E(10⁻⁶,f) is lower than E₀ for f between 10 and 10⁴ Hz showing local minima near 5×10, 5×10² and 5×10³ Hz, which are indicative of the resonant collective motion of many atoms, where E₀ is the static Young’s modulus. E(_t,10² Hz) increases showing saturation with increasing _t. Qualitatively, the outlines of E(10⁻⁶,f) and E(_t,10² Hz) observed for a-Zr₆₀Cu₃₀Al₁₀ are similar to those reported for various a-alloys. Quantitatively, a change in E(_t,10² Hz) for a-Zr₆₀Cu₃₀Al₁₀ is smallest among that reported for various a-alloys, presumably reflecting that the crystallization volume, (ΔV/V)_x, is smallest for a-Zr₆₀Cu₃₀Al₁₀. The effective charge number, Z^*, estimated from the change in E(10⁻⁶,10² Hz) due to PEC is 3.0×10⁵, which is comparable with Z^* reported for various a-alloys. We surmise that the number of atoms in the collective motions excited near 10² Hz is similar among various a-alloys. The E(10⁻⁶,f) data suggest that the spatial sizes of the density fluctuations may show a distribution.     

Thin film TiO2 on nickel(110): an STM study

Titanium oxide films grown on the surface of a Ni(110) single crystal have been investigated using STM, LEED and AES for Ti coverages ranging from 1 to 10 ml [1 ml of Ti is defined here as equivalent to the number of top layer Ni atoms of Ni(110)]. The oxide overlayers were prepared by vapour phase deposition of Ti followed by oxidation in 1×10⁻⁷ mbar O₂ at 800 K. Oxidation of Ti coverages between 1 and 10 ML results in STM images indicating the presence of two terminations coexisting on the surface. One termination consists of islands of epitaxial rutile TiO₂(110), the second having cell parameters of 2.98±0.1×3.15±0.2 Å. The latter unit cell is consistent with TiO(001) (2.99×2.99 Ų). On oxidation of higher Ti coverages (10 ml), only epitaxial rutile TiO₂(110) islands are observed.     

Investigation of stress behaviors and mechanism of void formation in sputtered TiSix films

We have investigated the stress behaviors and a mechanism of void formation in TiSi_x films during annealing. TiSi_x thin films were prepared by DC magnetron sputtering using a TiSi_2.1 target in the substrate temperature range of 200–500 °C. The as-deposited TiSi_x films at low substrate temperature (<300 °C) have an amorphous structure with low stress of 1×10⁸ dynes/cm². When the substrate temperature increases to 500 °C, the as-deposited TiSi_x film has a mixture of C49 and C54 TiSi₂ phase with stress of 8×10⁹ dynes/cm². No void was observed in the as-deposited TiSi_x film. Amorphous TiSi_x film transforms to C54 TiSi₂ phase with a random orientation of (311) and (040) after annealing at 750 °C. The C49 and C54 TiSi₂ mixture phase transforms to (040) preferred C54 TiSi₂ phase after annealing over 650 °C. By increasing substrate temperature, the transformation temperature for C54 TiSi₂ can be reduced, resulting in relieved stress of TiSi₂ film. The easy nucleation of the C54 phase was attributed to an avoidance of amorphous TiSi_x phase. We found that amorphous TiSi_x→C54 TiSi₂ transformation caused higher tensile stress of 2×10¹⁰ dynes/cm², resulting in more voids in the films, than C49→C54 transformation. It was observed that void formation was increased with thermal treatment. The high tensile stress caused by volume decreases in the silicide must be relieved to retard voids and cracks during C54 TiSi₂ formation.     

MOS structures with “chlorine” grown oxide and tungsten-gold metallization

Oxide layers produced by the thermal oxidation of silicon in an oxidising atmosphere containing trichloroethylene show better properties when used in MOS transistors. The dielectric strength is improved, and the surface state density is reduced from about 2.8 × 10¹¹ cm⁻² to 1.0 × 10¹¹ cm⁻² compared with normally oxidised silicon.     

The effect of rapid thermal annealing on structural and electrical properties of TiB2 thin films

This work reports on the effect of post-deposition rapid thermal annealing on the structural and electrical properties of deposited TiB₂ thin films. The TiB₂ thin films, thicknesses from 9 to 450 nm, were deposited by e-beam evaporation on high resistivity and thermally oxidized silicon wafers. The resistivity of as-deposited films varied from 1820 μΩ cm for the thinnest film to 267 μΩ cm for thicknesses greater than 100 nm. In the thickness range from 100 to 450 nm, the resistivity of TiB₂ films has a constant value of 267 μΩ cm.

A rapid thermal annealing (RTA) technique has been used to reduce the resistivity of deposited films. During vacuum annealing at 7 × 10⁻³ Pa, the film resistivity decreases from 267 μΩ cm at 200 °C to 16 μΩ cm at 1200 °C. Heating cycles during RTA were a sequence of 10 s. According to scanning tunneling microscopy analysis, the decrease in resistivity may be attributed to a grain growth through polycrystalline recrystallization, as well as to an increase in film density.

The grain size and mean surface roughness of annealed films increase with annealing temperature. At the same time, the conductivity of the annealed samples increases linearly with grain size. The obtained results show that RTA technique has a great potential for low resistivity TiB₂ formation.     

Ion-activated interface adsorption of oxygen during silver deposition on silicon

The process of ion-activated oxygen adsorption on silicon has been investigated using an experimental concept with simultaneous deposition of silver films. Auger electron spectroscopy in combination with sputter depth profiling is subsequently performed to determine the amount of oxygen adsorbed at the Ag---Si interface. Noble gas ions (⁴He⁺, ²⁰Ne⁺ and ⁴⁰Ar⁺)with energies between 50 and 175 keV were used, and it was found that for substrate temperatures of 300–700 K the oxygen adsorption depends strongly on ion mass, ion energy and ion flux density. For flux densities of 5 × 10¹¹ cm⁻² s⁻¹ or less, adsorption dominates and, in particular, for light-ion bombardment the majority of adsorbed oxygen atoms form chemical bonds with the silicon surface atoms (Si---O). However, for heavy ions, physical sputtering starts to compete and limits the effective rate of adsorption. At sufficiently high ion fluxes the adsorption starts to decrease, and for all ions and energies used in this work it is found that, if the electronic energy deposition density exceeds a critical value of about 1.2 × 10²¹ eV cm⁻² s⁻¹, dissociation of the Si---O bonds prevails with a corresponding loss in the adsorbed oxygen quantity.     

Implantation effect of diamond-like carbon films by 110 keV nitrogen ions

Diamond-like carbon films, grown on microscope slides by a dual-ion beam sputtering system, were implanted by 110 keV N⁺ under the doses of 1 × 10¹⁵, 1 × 10¹⁶ and 1 × 10¹⁷ions cm⁻² respectively. The implantation induced changes in electrical resistivity of the films and in infrared (IR) transmittance of the specimens were investigated as a function of implantation dose. The structural changes of the films were also studied using IR spectroscopy and Raman spectroscopy. It was observed that, with the increase of implantation dose, the diamond-like carbon films display two different stages in electrical and optical behaviours. The first is the increase of both the film resistivity and the IR transmittance of specimen at the dose of 1 × 10¹⁵ ions cm⁻² which, we consider, is attributed to the implantation-induced increase sp³ C---H bonds. However, when the doses are higher than 1 × 10¹⁵ ions cm⁻², the film resistivity and the IR transmittance of specimen decrea significantly and the decrease rates at dose range of 1×10¹⁶ to 1×10¹⁷ ions cm⁻² are smaller than those between 1×10¹⁵ and 1 × 10¹⁶ ions cm⁻². We conclude that the significant reductions of the two parameters at high doses are caused by the decreases of bond-angle disorder and of sp³ C---H bonds, the increases of sp² C---C bonds dominated the crystallite size and/or number and also the sp² C---H bonds. The smaller decrease rates at a dose range of 1 × 10¹⁶ to 1 × 10¹⁷ ions cm⁻² may be caused by further recombination of some retained hydrogen atoms to carbon atoms.     

First results from the Cuoricino experiment

At the end of 2001 the Milano Double Beta Decay (MI-DBD) experiment on double beta decay of ¹³⁰Te has been completed. The project Cryogenic Underground Observatory for Rare Events (CUORE), proposed as a natural extension of MI-DBD, will be a tightly packed array of 1000 TeO₂ bolometers, each being a cube 5 cm on a side with a mass of 790 g. The array will consists of 25 vertical towers, arranged in a square of 5 towers by 5 towers, each containing 10 layers of 4 crystals. A single CUORE tower has been constructed as a smaller scale experiment called CUORICINO. The technical feasibility of CUORE is now being tested in CUORICINO, running since few weeks. The CUORICINO experiment consists of 44 TeO₂ detectors 5×5×5 cm³ and 18 TeO₂ detectors 3×3×6 cm³ for a total mass of approximately 41 kg. An analysis of the detector performances is presented together with the new limit obtained on neutrinoless double beta decay of ¹³⁰Te.     

Auger electron spectroscopy of high-speed-deformed Ni/Cu layers

Ni/Cu double- and multilayers subjected to high-speed deformation were investigated by Auger electron spectroscopy (AES) using depth profiling. Ni and Cu thin films were alternately deposited on a 0.3 mm thick Ni substrate using RF magnetron sputtering. The thickness of the double-layer was 90 nm, while that of the multilayer was 160 nm. High-speed compression was performed using bullet masses from 30.0 to 57.4 g at varying bullet speeds between 16.8 and 48.5 m s⁻¹. The strain rate ranged from 6.7×10⁴ to 8.4×10⁵ s⁻¹. Upon high-speed deformation, the thickness of the Ni/Cu double-layer was reduced to about 80% of its original value. The Cu thin film was compressed to a greater extent relative to the Ni thin film (by about 15%), which may be due to the difference in malleability between the two metals. At a strain rate of 8.4×10⁵ s⁻¹, the Ni/Cu double-layer virtually disappeared. Ni/Cu interdiffusion was enhanced by high-speed deformation. The degree of interdiffusion appeared to be greater at lower strain rates. Cu_0.5Ni_0.5 and Cu_0.75Ni_0.25 thin film alloys formed in the high-speed-deformed multilayer sample, indicating that high-speed compression could potentially be used to prepare thin film metal alloys.     

Heavy ion radiation damage in double-sided silicon strip detectors

A ²⁵²Cf fission fragment source was used to produce heavy-ion radiation damage in a double-sided silicon strip detector. It was found that a good quality fission fragment spectrum (as determined by the peak to valley ration N_L/N_V) could not be achieved for radiation incident on the p⁺ face of the detector. However, for radiation incident on the n⁺ face, the ratio N_L/N_V remained adequate up to an accumulated dose of 4×10⁶ fragments mm⁻². For the measurement of alphas, typical resolution deteriorated from an initial 30 keV FWHM to 50 keV FWHM at a dose of 8×10⁶ fragments mm⁻² for incident on the n⁺ face, and 6×10⁶ for radiation incident on the p⁺ face. The interstrip resistance in one region of the n⁺ face broke down completely after a relatively small radiation doses incident on that face. Further investigation of this is still required.     

Up-conversion dynamics in GdAlO3:Er single crystal fibre

Green fluorescence has been obtained under continuous laser excitation in the 780–860 nm range in GdAlO₃:Er³⁺. With the help of the Judd-Ofelt treatment we built a model based on population rate equations to describe its time evolution. We found the intensity parameters to be Ω₂ = 2.045 × 10⁻²⁰ cm², Ω₄ = 1.356 × 10⁻²⁰ cm² Ω₆ = 1. 125 × 10⁻²⁰ cm². Even if a two-photon absorption and a looping mechanism are necessary to well describe the dynamics, the main process responsible for up-conversion is energy transfer between erbium ions.