J. P. Conde

Simple procedure for generating sequences of daily radiation values using a library of Markov transition matrices

This article describes how sequences of daily global radiation can be generated for any location, using as input only the average monthly radiation for that location (or the average monthly number of sunshine hours—insolation). The generated sequences are statistically indistinguishable from real ones and the method derived here is, therefore, a way of obtaining radiation sequences for locations were such sequences have not been measured, and for which many types of long-term performance calculations could not be made until now. The method is based on the observation that (1) there is a significant correlation only between radiation values for consecutive days and (2) that the probability of occurrence of radiation values is the same for months with the same Kt (clearness index). The method employs a library of Markov transition matrices, each corresponding to a specific interval in Kt. This article explains the derivation of the matrices, how they are to be used to generate radiation sequences, and compares synthetized and measured sequences.

AMORPHOUS AND MICROCRYSTALLINE SILICON FILMS GROWN AT LOW TEMPERATURES BY RADIO-FREQUENCY AND HOT-WIRE CHEMICAL VAPOR DEPOSITION

The effect of hydrogen dilution on the optical, transport, and structural properties of amorphous and microcrystalline silicon thin films deposited by hot-wire (HW) chemical vapor deposition and radio-frequency (rf) plasma-enhanced chemical vapor deposition using substrate temperatures (Tsub) of 100 and 25 °C is reported. Microcrystalline silicon (μc-Si:H) is obtained using HW with a large crystalline fraction and a crystallite size of ∼30 nm for hydrogen dilutions above 85% independently of Tsub. The deposition of μc-Si:H by rf, with a crystallite size of ∼8 nm, requires increasing the hydrogen dilution and shows decreasing crystalline fraction as Tsub is decreased. The photoconductivity, defect density, and structure factor of the amorphous silicon films (a-Si:H) are strongly improved by the use of hydrogen dilution in the Tsub range studied. a-Si:H films with a photoconductivity-to-dark conductivity ratio above 105, a deep defect density below 1017 cm−3, an Urbach energy below 60 meV and a structure fa...

Detection of DNA and proteins using amorphous silicon ion-sensitive thin-film field effect transistors

Amorphous silicon-based ion-sensitive field-effect transistors (a-Si:H ISFETs) are used for the label-free detection of biological molecules. The covalent immobilization of DNA, followed by DNA hybridization, and of the surface adsorption of oligonucleotides and proteins were detected electronically by the a-Si:H ISFET. The ISFET measurements are performed with an external Ag/AgCl microreference electrode immersed in 100mM phosphate buffer electrolyte with pH 7.0. Threshold voltage shifts in the transfer curve of the ISFETs are observed resulting from successive steps of surface chemical functionalization, covalent DNA attachment to the functionalized surface, surface blocking, and hybridization with a complementary target. The surface sensitivity achieved for DNA oligonucleotides is of the order of 1pmol/cm(2). Point-of-zero charge estimations were made for the functionalized surfaces and for the device surface after DNA immobilization and hybridization. The results show a correlation between the changes in the point-of-zero charge and the shift observed in the threshold voltage of the devices. Electronic detection of adsorbed proteins and DNA is also achieved by monitoring the shifts of the threshold voltage of the ISFETs, with a sensitivity of approximately 50nM.

Optoelectronic and structural properties of amorphous silicon–carbon alloys deposited by low-power electron-cyclotron resonance plasma-enhanced chemical-vapor deposition

The optoelectronic and structural properties of hydrogenated amorphous silicon-carbon alloys ~a-SiC:H! are studied over the entire compositional range of carbon content. The films are prepared using low-power electron-cyclotron resonance ~ECR! plasma-enhanced chemical vapor deposition. The carbon content was varied by using different methane ~or ethylene-!-to-silane gas phase ratios and by introducing the methane ~or ethylene! either remotely into the plasma stream or directly through the ECR source, together with the excitation gas ~hydrogen!. Regardless of the deposition conditions and source gases used, the optical, structural and transport properties of the a-SiC:H alloys followed simple universal dependencies related to changes in the density of states associated with their structural disorder. The deep defect density from photothermal deflection spectroscopy, the ESR spin density, the steady state and the transient photoluminescence, the dark and photoconductivity, the temperature of the hydrogen evolution peaks and the bonding from infrared spectroscopy are correlated to the Urbach tail energy, the B factor of the Tauc plot and E04 ~defined as the energy at which the absorption coefficient is equal to 10 4 cm 21 !. Silicon-rich and carbon-rich regions with very different properties, corresponding approximately to carbon fractions below and above 0.5, respectively, can be distinguished. The properties of the ECR a-SiC:H alloys are compared with those of alloys deposited by rf glow discharge.

TRANSPORT AND PHOTOLUMINESCENCE OF HYDROGENATED AMORPHOUS SILICON-CARBON ALLOYS

The optoelectronic properties of hydrogenated amorphous silicon–carbon alloys (a‐SiC:H) are studied over the entire compositional range of carbon content. The films are prepared using radio‐frequency glow discharge and optimization was made with respect to deposition power and pressure, hydrogen dilution, and methane (or ethylene) ‐to‐silane gas phase ratio. Regardless of the deposition conditions and source gases used, the optical, structural, and transport properties of the a‐SiC:H alloys followed simple universal dependencies related to changes in the density of states associated with their structural disorder. The Urbach tail energy Eu and the B factor of the Tauc plot correlate with E04 (defined as the energy at which the absorption coefficient is equal to 104 cm−1) taken from photothermal deflection spectroscopy measurements. Up to E04pds≊2.6 eV, Eu increases monotonically from 50 up to ≊200 meV, while the B factor decreases from ≊800 down to ≊200 cm−1/2 eV−1/2. Above E04pds≊2.6 eV, both Eu and B re...

Amorphous silicon air-gap resonators on large-area substrates

Air-gap resonators composed of bridges of hydrogenated amorphous silicon and aluminum bilayers on glass and polyethylene terephthalate substrates are realized using surface micromachining techniques. The resonance frequency of structures on plastic substrates was found to vary inversely with the square of the length of the bridge span. On glass substrates, this dependence is seen for lengths >100 μm, while for lengths <100 μm, a decrease in the frequency is attributed to compressive stress. Resonance frequencies of ∼500 kHz to ∼2 MHz were measured in bridges with lengths from 160 μm down to ∼80 μm.

Spin dependent tunnel junctions for memory and read-head applications

Spin dependent tunnel junctions with TMR exceeding 30-40% can now be prepared with AlO/sub x/ and AlN barriers, with junction resistance tuned from 30-40 /spl Omega//spl times//spl mu/m/sup 2/ to 10/sup 8/ /spl Omega//spl times//spl mu/m/sup 2/. Thermal stability is better than 3000% for thicker barriers (>11 /spl Aring/) but is degraded to 220/spl deg/C for 6 /spl Aring/ barriers. A 9 bit MRAM cell is demonstrated using tunnel junctions and vertically integrated a:Si diodes. Junction switching is achieved with on chip 20 ns field pulses. Requirements for tunnel junctions for 100 Gb/in/sup 2/ read head applications are discussed. First prototypes of tunnel junction read heads with 600 /spl Aring/ read gaps were fabricated, where the TJ is at the air bearing surface. TMR loss was observed during the final head lapping steps.

Amorphous and microcrystalline silicon films deposited by hot‐wire chemical vapor deposition at filament temperatures between 1500 and 1900 °C

The optical, electronic and structural properties of thin films deposited by Hot‐wire chemical vapor deposition with filament temperatures, Tfil, between 1500 and 1900 °C from silane and hydrogen are studied. The substrate temperature, Tsub, was kept constant at 220 °C. Amorphous silicon films (a‐Si:H) are obtained at high filament temperatures, low deposition pressures and low hydrogen‐to‐silane flow rate ratio (Tfil∼1900 °C, p<30 mTorr and FH2/FSiH4≤1). At these deposition conditions, high growth rates are observed (rd≥10 As−1) both with and without hydrogen dilution, and no silicon deposition was observed on the filaments. However, if a lower filament temperature is used (Tfil∼1500 °C) a transition from a‐Si:H to microcrystalline silicon (μc‐Si:H) occurs as the pressure is decreased from above 0.3 Torr to below 0.1 Torr. The highest dark conductivity and lowest activation energy, of ∼1 Scm−1 and <0.1 eV, respectively, were observed for μc‐Si:H deposited at p∼50 mTorr. In this Tfil regime, μc‐Si:H growt...

Amorphous silicon electrostatic microresonators with high quality factors

Phosphorus-doped amorphous-silicon thin-film micromachined mechanical bridge resonators are processed at low temperatures (⩽110 °C) on glass substrates. The microelectromechanical structures are electrostatically actuated, and the resulting deflection is monitored optically. Resonance frequencies in the megahertz range are observed with quality factors up to 5000 when measured in vacuum. The energy dissipation processes in amorphous-silicon thin-film microbridges are discussed. The dominant intrinsic dissipation mechanism is surface loss.

Electrostatic actuation of thin-film microelectromechanical structures

Microbridge and cantilever electrostatic actuators are fabricated using thin film technology and surface micromachining at low temperatures (⩽100 °C) on glass substrates. Electrostatic actuation is accomplished by applying a voltage, combining a dc component to a low frequency ac component, between the microstructure and an underlying gate counterelectrode. The movement is optically detected by focusing a laser beam on the top of the structure and monitoring the deviation of the reflected light, which is proportional to the electrostatically induced deflection. The absolute value of the deflection is obtained using a calibrated piezoelectric actuator sample holder. The response of the structure is measured with a precision better than 5 A. The deflection of the microstructures is studied as a function of the magnitude of the electrostatic load, and of the type (bridge or cantilever) and geometrical dimensions of the structure. The mechanical movement is analyzed using an electromechanical model and mechan...