Negative differential resistance in a reverse-biased diode with inhomogeneous base

The possibility of a negative differential resistance (NDR) region appearing in the reverse branch of the current-voltage characteristic of a semiconductor diode with a wide-bandgap layer in the base is theoretically predicted. The NDR formation is related to the fact that increasing reverse bias leads to a decrease in the thermal production of charge carriers in the narrow-bandgap part of the base (with a thickness on the order of the Debye screening length). It is shown that the NDR region appears, provided that the carrier lifetime in the diode base is determined by the Auger processes.     

A biological porin engineered into a molecular, nanofluidic diode

We changed the nonrectifying biological porin OmpF into a nanofluidic diode. To that end, we engineered a pore that possesses two spatially separated selectivity filters of opposite charge where either cations or anions accumulate. The observed current inhibition under applied reverse bias voltage reflects, we believe, the creation of a zone depleted of charge carriers, in a sense very similar to what happens at the np junction of a semiconductor device.     

Study of temperature dependence of positive charge generation in thin dielectric film of MOS structure under high-fields

In this work, the temperature effect on positive charge generation under high-field electron injection in MOS structures is studied using a new multilevel current stress technique. In our experiments, we used commercially made test MOS capacitors based on thermal silicon dioxide film with thickness in the range from 7 to 100 nm. Using the proposed multilevel current stress technique one can investigate the positive charge generation in MOS structure gate dioxide under high electric fields. The technique is different from the standard constant current stress technique, as it includes two levels of current: the stress current level, which provides the generation of positive charge, and the additional measuring current level, which allows to monitor the change of dielectric charge. Due to the additional measuring current level, it is possible to decrease significantly the error of positive charge density measurement in the dielectric. The latter error is caused by the positive charge trapped in the initial stage of electron injection while the steady-state of the stress mode is being reached. It was found that the rise of sample temperature leads to a decrease of the density of the positive charge generated in SiO₂ film under high-field injection of electrons from the silicon substrate. A model of the positive charge generation in MOS structures under high-field electron injection has been proposed. The new model of the positive charge generation in MOS structures takes into account the temperature effect and the subsequent phenomena of dielectric charge state change: the band-to-band impact ionization in SiO₂ with the creation of electron-hole pairs and the subsequent trapping of holes in traps in the oxide; the trapping of injected electrons by trapped holes as well as the thermal release of trapped holes from the traps.     

Measurements of the photocurrent enhancement of reverse-biased silicon photodiodes in the 0.05-1.5-keV photon-energy range

The measured photocurrents from two different p-n-junction silicon photodiodes at 170-? (73-eV) and at 8.34-? (1480-eV) light are presented. One is a standard extreme-UV photodiode fabricated on low resistivity silicon (70-100 Ω cm), and the other is fabricated on high-resistivity silicon (> 2 × 10(4) Ω cm). The photocurrents from the diode on high-resistivity silicon are at least an order of magnitude greater than the photocurrents from the diode on low-resistivity silicon when a reverse bias of 40 V is applied to each. This photocurrent enhancement is 15.4 ± 4 at 8.34 ? and 12.5 ± 2 at 170 ?.     

Experimental factors controlling analyte ion generation in laser desorption/ionization mass spectrometry on porous silicon

Desorption/ionization on porous silicon (DIOS) is a relatively new laser desorption/ionization technique for the direct mass spectrometric analysis of a wide variety of samples without the requirement of a matrix. Porous silicon substrates were fabricated using the recently developed nonelectrochemical H2O2-metal-HF etching as a versatile platform for investigating the effects of morphology and physical properties of porous silicon on DIOS-MS performance. In addition, laser wavelength, mode of ion detection, pH, and solvent contributions to the desorption/ionization process were studied. Other porous substrates such as GaAs and GaN, with similar surface characteristics but differing in thermal and optical properties from porous silicon, allowed the roles of surface area, optical absorption, and thermal conductivities in the desorption/ionization process to be investigated. Among the porous semiconductors studied, only porous silicon has the combination of large surface area, optical absorption, and thermal conductivity required for efficient analyte ion generation under the conditions studied. In addition to these substrate-related factors, surface wetting, determined by the interaction of deposition solvent with the surface, and charge state of the peptide were found to be important in determining ion generation efficiency.     

Demonstration of feasibility of operating a silicon ZIP detector with 20 eV threshold

100-g silicon detectors (known as “ZIPs,” Z-resolving Ionization and Phonon detectors) developed by the Cryogenic Dark Matter Search Experiment have been tested at charge bias voltages of up to 200 V/cm, significantly above their usual operating range (3–6 V/cm). Thermal gain factors in excess of 50 were observed due to the primary ionization drifting in the large applied field, with only minimal increase in phonon noise. The observed thermal gain corresponds to an intrinsic threshold of 20 eV, resulting in detectors that have direct application for use in a neutrino magnetic moment measurement based on a 40-MCi tritium source.     

Carrier transport mechanisms and photovoltaic characteristics of Au/toluidine blue/n-Si/Al heterojunction solar cell

Toluidine blue (TB)/n-silicon heterojunction solar cell was fabricated by depositing TB film on n-silicon wafer using thermal deposition technique. X-ray diffraction patterns of the TB film show presence of crystals with size 30 nm dispersed in amorphous matrix. The current–voltage–temperature performance of Au/TB/n-Si/Al device was studied in dark and under illumination conditions. The device showed diode behavior. The diode parameters such as ideality factor, barrier height, series and shunt resistance were determined using a conventional I–V–T characteristics. The analysis of the diode characteristics in forward bias direction confirmed that the transport mechanisms of the Au/TB/n-Si/Al solar cell at applied potential?<?0.1 V is thermionic emission and at high electric field?>?0.1 V is Ohmic conduction. The operating conduction mechanisms in reverse bias direction are Pool–Frenkel effect followed by Schootky field lowering mechanism. The small value of activation energy in reverse bias direction indicates that the conduction process is expected to be by tunneling of electrons between nearest-neighbor sites and it is temperature independent. The photo conduction characteristics of the diode suggests its application as a solar cell.     

Simulation of forward dark current voltage characteristics of tandem solar cells

The transport mechanisms tailoring the shape of dark current-voltage characteristics of amorphous and microcrystalline silicon based tandem solar cell structures are explored with numerical simulations. Our input parameters were calibrated by fitting experimental current voltage curves of single and double junction structures measured under dark and illuminated conditions. At low and intermediate forward voltages the dark current-voltage characteristics show one or two regions with a current-voltage exponential dependence. The diode factor is unique in tandem cells with the same material in both intrinsic layers and two dissimilar diode factors are observed in tandem cells with different materials on the top and bottom intrinsic layers. In the exponential regions the current is controlled by recombination through gap states and by free carrier diffusion. At high forward voltages the current grows more slowly with the applied voltage. The current is influenced by the onset of electron space charge limited current (SCLC) in tandem cells where both intrinsic layers are of amorphous silicon and by series resistance of the bottom cell in tandem cells where both intrinsic layers are of microcrystalline silicon. In the micromorph cell the onset of SCLC becomes visible on the amorphous top sub-cell. The dark current also depends on the thermal generation of electron-hole (e-h) pairs present at the tunneling recombination junction. The highest dependence is observed in the tandem structure where both intrinsic layers are of microcrystalline silicon. The prediction of meaningless dark currents at low forward and reverse voltages by our code is discussed and one solution is given.     

Effect of the dielectric-substrate interface on charge accumulation from vacuum ultraviolet irradiation of low-k porous organosilicate dielectrics

We compare the effect of various dielectric-substrate interfaces on charge accumulation during vacuum ultraviolet irradiation of capped low-k porous organosilicates to find that more charges are trapped in a dielectric stack deposited on silicon compared with the same stack deposited on copper. Insertion of a 5-nm interfacial thermal oxide layer further increases the amount of trapped charges in the dielectric. The difference between the photoemission and injection currents determines the number of charges trapped in the dielectric as a result of irradiation. Fewer charges are trapped when the injection current increases.     

Dopant concentration dependence of the response of SiC Schottky diodes to light ions

The responses of Silicon Carbide (SiC) Schottky diodes of different dopant concentration to ¹²C ions at 14.2, 28.1 and 37.6 MeV incident energies are compared. The relation between the applied reverse bias and the thickness of the depleted epitaxial region is studied for different dopant concentrations. The experimental data show that SiC diodes with lower dopant concentration need lower reverse bias to be depleted. Moreover it has been observed that the energy resolution, measured as a function of the applied reverse bias and of the ions incident energies, does not depend on the dopant concentration. The radiation damage, produced by irradiating SiC diodes of different dopant concentration with ¹⁶O ions at 35.2 MeV, was evaluated by measuring the degradation of both the signal pulse-height and the energy resolution as a function of the ¹⁶O fluence. Diodes having a factor 20 lower dopant concentration exhibit a radiation hardness reduced by 60%. No inversion in the signal at the breakdown fluence was observed for ¹⁶O ions stopped inside the diode epitaxial region.     

Study of space charge in gallium nitride by the thermal step method

In this paper, we report the electric investigation of thin nitride gallium films by the capacitance voltage technique and the thermal step method (TSM). The C–V analysis at 1 MHz of Au/GaN diode reveals MOS behaviour and shows strong capacitance hysteresis.

This may be due to the presence of trapped charge in this structure. The space charge dynamics is studied by thermal step method at different applied voltages. The TS currents are reverted from negative ones to positive ones above inversion threshold of +0.2 V. This change corresponds to charge modulation from accumulation to the inversion one, in good agreement with the C–V characteristics. The stored charge in this sample is related to the nature of gallium nitride and to the manufacturing processes. The results confirm the possibility to apply the TSM for the measurement of the space charge in the semiconductor materials.     

Dynamic electrowetting on nanofilament silicon for matrix-free laser desorption/ionization mass spectrometry

Dynamic electrowetting on nanostructured silicon surfaces is demonstrated as an effective method for improving detection sensitivity in matrix-free laser desorption/ionization mass spectrometry. Without electrowetting, silicon surfaces comprising dense fields of oriented nanofilaments are shown to provide efficient ion generation and high spectral peak intensities for deposited peptides bound to the nanofilaments through hydrophobic interactions. By applying an electrical bias to the silicon substrate, the surface energy of the oxidized nanofilaments can be dynamically controlled by electrowetting, thereby allowing aqueous buffer to penetrate deep into the nanofilament matrix. The use of electrowetting is shown to result in enhanced interactions between deposited peptides and the nanofilament silicon surface, with improved signal-to-noise ratio for detected spectral peaks. An essential feature contributing to the observed performance enhancement is the open-cell nature of the nanofilament surfaces, which prevents air from becoming trapped within the pores and limiting solvent penetration during electrowetting. The combination of nanofilament silicon and dynamic electrowetting is shown to provide routine detection limits on the order of several attomoles for a panel of model peptides.     

Bias spectroscopy and simultaneous single-electron transistor charge state detection of Si:P double dots

We report a detailed study of low-temperature (mK) transport properties of a silicon double-dot system fabricated by phosphorous ion implantation. The device under study consists of two phosphorous nanoscale islands doped to above the metal-insulator transition, separated from each other and the source and drain reservoirs by nominally undoped (intrinsic) silicon tunnel barriers. Metallic control gates, together with an Al-AlO(x) single-electron transistor (SET), were positioned on the substrate surface, capacitively coupled to the buried dots. The individual double-dot charge states were probed using source-drain bias spectroscopy combined with non-invasive SET charge sensing. The system was measured in linear (source-drain DC bias V(SD) = 0) and non-linear (V(SD) ≠ 0) regimes, allowing calculations of the relevant capacitances. Simultaneous detection using both SET sensing and source-drain current measurements was demonstrated, providing a valuable combination for the analysis of the system. Evolution of the triple points with applied bias was observed using both charge and current sensing. Coulomb diamonds, showing the interplay between the Coulomb charging effects of the two dots, were measured using simultaneous detection and compared with numerical simulations.     

Reducing the effects of mismatch between zinc oxide and silicon by silane plasma modification

We investigated the mismatch between zinc oxide (ZnO) and silicon (Si) upon reduction by silane plasma modification in a plasma-enhanced chemical vapor deposition system. This plasma treatment was only carried out for 10?s and the Si–H bonds that were provided by the silane plasma modification as dangling bonds on the Si wafer in addition to functioning as a conjunction layer to reduce the defects. The X-ray diffraction analysis of the ZnO/p-type silicon structure produced by silane plasma modification has a slightly lower full width at the half maximum, which improved the ZnO film’s crystalline properties. After the silane plasma modification ZnO/Si diode is produced, the measured current–voltage characteristics gave favorable rectifying properties and reverse bias had a low leakage current. The ZnO/Si diode under illumination increased the short-circuit current (I_sc) from 7.32 to 19.75?mA/cm², which is an improvement compared with a conventional bare ZnO/Si diode because of the reduced ZnO/Si interface states. Therefore, the silane plasma modification diminishes the effects of the interface and improves the ZnO/Si diode performance.     

Addressing a model of multistreamer switching in high-voltage silicon <Emphasis Type="Italic">p-n</Emphasis> junctions above the Zener breakdown threshold

It is shown that a model of multistreamer switching from the blocking to conducting state in high-voltage diode structures cannot consistently explain the phenomenon of ultrafast switching of such silicon structures into the conducting state, which was experimentally observed in the rapid growth of the reverse bias voltage in strong (～1 MV/cm) electric fields.     

A SPICE-Compatible New Silicon Nanowire Field-Effect Transistors (SNWFETs) Model

Extraction of carrier mobilities of silicon nanowire FETs (SNWFETs) with Schottky source and drain contacts is performed using a newly developed compact model, which is suitable for efficient circuit simulation. The SNWFET model is based on an equivalent circuit including a Schottky diode model for two metal-semiconductor contacts and a SPICE LEVEL 3 MOSFET model for an intrinsic NW. The Schottky diode model is based on our recently developed Schottky diode model that includes thermionic field emission for reverse bias and thermionic emission mechanism for forward bias. It also includes a new analytical Schottky barrier height model dependent on the gate voltages as well as the drain-source voltages. The results simulated from the SNWFET model reproduce various, previously reported experimental results within 10% errors. The mobilities extracted from our model are compared with the mobility calculated without considering the Schottky contacts.     

Differentiation of domains in composite surface structures by charge-contrast X-ray photoelectron spectroscopy

An external bias is applied to two samples containing composite surface structures, while recording an XPS spectrum. Altering the polarity of the bias affects the extent of differential charging in domains that are chemically or electronically different to create a charge contrast. By utilizing this charge contrast, we show that two distinct silicon nitride and silicon oxynitride domains are present in one of the composite samples. Similarly, we use this technique to show that titanium oxide and silicon oxide domains exist as separate chemical entities in another composite sample.     

Optically Induced Measurement of Electron and Hole Drift Velocities in a Germanium 〈100〉 CDMS Detector at 50 mK

We report on precise drift velocity measurements of electrons and holes in 50 mK, ultrapure (≈10¹⁰ net shallow impurities per?cm³) germanium 〈100〉 CDMS dark matter detectors as a function of electric field up to 4?V/cm. A laser diode connected to an optical fiber extending from room-temperature to the detector creates electron-hole pairs on one surface of the crystal. High-speed electronics measure the drift current as the generated carriers travel to the opposite face of the crystal. CDMS detectors measure the ionization and phonon response of particle interactions within the crystal. Stable charge collection is necessary for successful background discrimination when looking for a possible dark matter signal. While biased, however, ionization performance degrades over time due to the build-up of space charge. Free electrons and holes created by particle interactions are subject to drift-diffusion dynamics occurring simultaneously with the trapping of carriers to localized surface and bulk states. The combination of these processes determine the evolution of space charge within the crystal, making it important that we understand carrier transport under our unique operating condition of low-temperature and low-field. We find good agreement between our measured drift velocities and our theoretical predictions, indicating carrier scattering is dominated by spontaneous phonon emission. In addition, we present preliminary measurements of effective longitudinal carrier trapping lengths for both n-type and p-type crystals at 50?mK.     

Interface‐state density and noise behavior of gamma‐irradiated MOSFET'S

Abstract

MOSFET's and MOS memory devices suffer the increase of interface state density and oxide trapped charge density after ionization radiation such as gamma rays. The drain‐current dependence and the gate‐voltage dependence of the flicker noise of the irradiated MOSFET's are illustrated in this paper. The annealing effect, the bias effect, and the total dose effect on the noise behavior are shown. It is also shown that the change of the interface state density is essential to the noise change and the change of oxide trapped charge density may cause the “apparent” change of MOSFET's flicker noise after gamma irradiation.     

Edge sensitivity of “edgeless” silicon pad detectors measured in a high-energy beam

We report measurements in a high-energy beam of the sensitivity of the edge region in “edgeless” planar silicon pad diode detectors. The edgeless side of these rectangular diodes is formed by a cut and break through the contact implants. A large surface current on such an edge prevents the normal reverse biasing of this device above the full depletion voltage, but we have shown that the current can be sufficiently reduced by the use of a suitable cutting method, followed by edge treatment, and by operating the detector at a low temperature. A pair of these edgeless silicon diode pad sensors was exposed to the X5 high-energy pion beam at CERN, to determine the edge sensitivity. The signal of the detector pair triggered a reference telescope made of silicon microstrip detector modules. The gap width between the edgeless sensors, determined using the tracks measured by the reference telescope, was then compared with the results of precision metrology. It was concluded that the depth of the dead layer at the diced edge is compatible with zero within the statistical precision of ±8 μm and systematic error of ±6 μm.