Photodiodes array as potential diagnostic for measuring short bursts of K-alpha radiation from Ti targets irradiated with 45 fs laser pulses

We report on a study comparing absolute K-alpha yield from Ti foils measured with a calibrated system of an X-ray CCD coupled to a curved LiF Von-Hamos crystal spectrometer to the difference in the signals measured simultaneously with two similar photodiodes fitted with two different filters. Our data indicate that a combination of photodiodes with different filters could be developed into an alternative and inexpensive diagnostic for monitoring single shot pulsed emission in a narrow band of X-ray region.

Quantum efficiency in GaAs Schottky photodetectors with enhancement due to surface plasmon excitations

The overall quantum efficiency in surface plasmon (SP) enhanced Schottky barrier photodetectors is examined by considering both the external and internal yield. The external yield is considered through calculations of absorption and transmission of light in a configuration that allows reflectance minimization due to SP excitation. Following a Monte Carlo method, a procedure is presented to estimate the internal yield while taking into account the effect of elastic and inelastic scattering processes on excited carriers subsequent to photon absorption. The relative importance of internal photoemission and band-to-band contributions to the internal yield is highlighted along with the variation of the yield as a function of wavelength, metal thickness and other salient parameters of the detector. (C) 2002 Elsevier Science Ltd. All rights reserved.

Scheme for enhancing efficiency in resonant-cavity Schottky photodetectors

It is shown that structuring the top layers of a resonant cavity Schottky photodetector in a way that allows coupling between the wavevector of incident radiation and that of electron-collective oscillations (plasmons) at the surface of the metallic electrode leads to practically zero reflectance in the case of front illuminated devices. This is expected to result in a consequential enhancement in the quantum efficiency in these photodetectors. (C) 2001 Elsevier Science Ltd. All rights reserved.

Characterization of Nickel Germanide Schottky Contacts for the Fabrication of Germanium p-channel MOSFETs

Nickel germanide Schottky contacts, formed by rapid thermal annealing of thin nickel films, have been characterized on n-type germanium wafers for a range of RTA temperatures. The highest Schottky barrier heights for electrons (= 0.6-0.7 eV) were obtained for RTA temperatures of approximately 300°C. For this RTA schedule, the corresponding barrier height for holes is close to zero, ideal for Schottky contacted p-channel germanium MOSFETs. When the RTA temperature was increased to 400oC, a dramatic reduction in electron barrier height (< 0.1 eV) was observed. This RTA schedule, therefore, appears ideal for ohmic source/drain contacts to n channel germanium MOSFETs. From sheet resistance measurements and XRD characterization, nickel germanide formation was found to occur at 300oC and above. The NiGe phase was dominant for RTA temperatures up to at least 435oC.

The electrical characterization and response to hydrogen of Schottky diodes with a resistive metal electrode—rectifying an oversight in Schottky diode investigation

Schottky-barrier structures with a resistive metal electrode are examined using the 4-point probe method where the probes are connected to the metal electrode only. The observation of a significant decrease in resistance with increasing temperature (over a range of similar to 100 K) in the diode resistance-temperature (R(D)-T) characteristic is considered due to charge carrier confinement to the metal electrode at low temperature (high resistance), with the semiconductor progressively opening up as a parallel current carrying channel (low resistance) with increasing temperature due to increasing thermionic emission across the barrier. A simple model is constructed, based on thermionic emission at quasi-zero bias, that generates good fits to the experimental data. The negative differential resistance (NDR) region in the R(D)-T characteristic is a general effect and is demonstrated across a broad temperature range for a variety of Schottky structures grown on Si-, GaAs- and InP-substrates. In addition the NDR effect is harnessed in micro-scaled Pd/n-InP devices for the detection of low levels of hydrogen in an ambient atmosphere of nitrogen.

High sensitivity (1 ppm) hydrogen detection using an unconventional Pd/n-InP Schottky device

Hydrogen is detected using a Pd/n-InP Schottky diode in which the elongated, very thin Pd electrode is of greater resistance than the underlying semiconductor substrate. Four-probe measurements of the device resistance, as a function of hydrogen concentration, are made by contacting only the Pd electrode, with a sensitivity of 1 ppm being achieved. On hydrogen exposure the device resistance drops from an initial high value, characteristic of the Pd electrode alone, to a lower value due to a hydrogen-induced lowering of the Schottky barrier that opens up the InP substrate as a parallel current carrying channel.

Monte Carlo calculations of quantum yield in inhomogeneous PtSi/p-Si Schottky barriers

Monte Carlo calculations of quantum yield in PtSi/p-Si infrared detectors are carried out taking into account the presence of a spatially distributed barrier potential. In the 1-4 mu m wavelength range it is found that the spatial inhomogeneity of the barrier has no significant effect on the overall device photoresponse. However, above lambda = 4.0 mu m and particularly as the cut-off wavelength (lambda approximate to 5.5 mu m) is approached, these calculations reveal a difference between the homogeneous and inhomogeneous barrier photoresponse which becomes increasingly significant and exceeds 50% at lambda = 5.3 mu m. It is, in fact, the inhomogeneous barrier which displays an increased photoyield, a feature that is confirmed by approximate analytical calculations assuming a symmetric Gaussian spatial distribution of the barrier. Furthermore, the importance of the silicide layer thickness in optimizing device efficiency is underlined as a trade-off between maximizing light absorption in the silicide layer and optimizing the internal yield. The results presented here address important features which determine the photoyield of PtSi/Si Schottky diodes at energies below the Si absorption edge and just above the Schottky barrier height in particular.

Barrier characteristics of PtSi/p-Si schottky diodes as determined from I-V-T measurements

The current-voltage-temperature characteristics of PtSi/p-Si Schottky barrier diodes were measured in the temperature range 60-115 K. Deviation of the ideality factor from unity below 80 K may be modelled using the so-called T-0 parameter with T-0 = 18 K. It is also shown that the curvature in the Richardson plots may be remedied by using the flatband rather than the zero-bias saturation current density. Physically, the departure from ideality is interpreted in terms of an inhomogeneous Schottky contact. Here we determine a mean barrier height at T = 0 K, phi(b)(-0) = 223 mV, with an (assumed) Gaussian distribution of standard deviation sigma(phi) = 12.5 mV. These data are correlated with the zero-bias barrier height, phi(j)(0) = 192 mV (at T = 90 K), the photoresponse barrier height, phi(ph) = 205 mV, and the flatband barrier height, phi(fb) = 214 mV. Finally, the temperature coefficient of the flatband barrier was found to be -0.121 mV K-1, which is approximately equal to 1/2(dE(g)(i)/dT), thus suggesting that the Fermi level at the interface is pinned to the middle of the band gap.

ANALYSIS OF SURFACE-PLASMON POLARITON ENHANCEMENT IN PHOTODETECTION BY AL-GAAS SCHOTTKY DIODES

The surface plasmon polariton mediated photoresponse from Al-GaAs diodes is examined in a prism-air gap-diode configuration as a function of both the wavelength of the incident light and thickness of the Al electrode. The experimental data shows a pronounced dip in reflectance as a function of internal angle of incidence in the prism, due to the excitation of the surface plasmon polariton at the Al-air interface, and a corresponding peak in device photosignal. Careful modelling of reflectance and quantum efficiency data shows that the bulk of the signal is generated by light which is re-radiated from this surface mode into the semiconductor substrate where it is absorbed by the creation of electron-hole pairs in the depletion region. This holds for all the wavelengths used here (all are shorter than the GaAs absorption edge) and across the thickness range of the Al electrodes (20-50 nm). Quantum efficiencies in the range 0.5-22% and enhancement factors of typically 7.5 were recorded in this investigation.

SURFACE-MODE ENHANCED PHOTOCURRENT FROM PTSI/N-SI SCHOTTKY-BARRIER DETECTORS

Using the Otto geometry of attenuated total reflection (prism-air gap-sample), front illuminated PtSi/Si Schottky barrier detectors are shown to exhibit enhanced photocurrent at surface plasmon resonance in the near infrared region. Correlation of the measured photocurrent with the calculated transmittance of light into the Si substate is demonstrated. The transmittance, which is due to surface plasmon re-radiation, is the optical parameter of principal importance in photosignal generation since the photon energies used here are greater than the silicon intrinsic bandgap. The results presented here indicate clearly the important features in optimizing surface plasmon enhancement in photodetection both above and below the silicon absorption edge.

An alternative methodology in Schottky diode physics

The fabrication and electrical characterization of Schottky junction diodes have been extensively researched for three-quarters of a century since the original work of Schottky in 1938. This study breaks from the highly standardized regime of such research and provides an alternative methodology that prompts novel, more efficient applications of the adroit Schottky junction in areas such as chemical and thermal sensing. The core departure from standard Schottky diode configuration is that the metal electrode is of comparable or higher resistance than the underlying semiconductor. Further, complete electrical characterization is accomplished through recording four-probe resistance-temperature (R-D-T) characteristics of the device, where electrical sourcing and sensing is done only via the metal electrode and not directly through the semiconductor. Importantly, this results in probing a nominally unbiased junction while eliminating the need for an Ohmic contact to the semiconductor. The characteristic R-D-T plot shows two distinct regions of high (metal) and low (semiconductor) resistances at low and high temperatures, respectively, connected by a crossover region of width, DT, within which there is a large negative temperature coefficient of resistance. The R-D-T characteristic is highly sensitive to the Schottky barrier height; consequently, at a fixed temperature, R-D responds appreciably to small changes in barrier height such as that induced by absorption of a chemical species (e.g., H-2) at the interface. A theoretical model is developed to simulate the R-D-T data and applied to Pd/p-Si and Pt/p-Si Schottky diodes with a range of metal electrode resistance. The analysis gives near-perfect fits to the experimental R-D-T characteristics, yielding the junction properties as fit parameters. The modelling not only helps elucidate the underlying physics but also helps to comprehend the parameter space essential for the discussed applications. Although the primary regime of application is limited to a relatively narrow range (DT) for a given type of diode, the alternative methodology is of universal applicability to all metal-semiconductor combinations forming Schottky contacts. (C) 2015 AIP Publishing LLC.

Investigation of an electron string ion source with field emission cathode

The string mode of operation for an electron beam ion source uses axially oscillating electrons in order to reduce power consumption, also simplifying the construction by omitting the collector with cooling requirements and has been called electron string ion source (ESIS). We have started a project (supported by INTAS and GSI) to use Schottky field emitting cathode tips for generating the electron string. The emission from these specially conditioned tips is higher by orders of magnitude than the focused Brillouin current density at magnetic fields of some Tesla and electron energies of some keV. This may avoid the observed instabilities in the transition from axially oscillating electrons to the string state of the electron plasma, opening a much wider field of possible operating parameters for an ESIS. Besides the presentation of the basic features, we emphasize in this paper a method to avoid damaging of the field, emission tip by backstreaming ions. (C) 2008 American Institute of Physics.