Si Photodiode Research Articles

Evaluation of CsI(Tl)-PIN diode array tiling schemes

Methods of tiling scintillator-photodiode detector arrays were investigated. Si photodiodes coupled to CsI(Tl) scintillation crystals are being considered for high-resolution (/spl sim/1 mm) gamma-ray imaging. Large area (/spl sim/200 cm/sup 2/) detectors can be assembled by tiling together modular arrays. The goal of the tiling schemes was to maintain uniform pixel pitch and detector performance across the seam where the arrays join. The perimeter of the array has an insensitive area that can be an appreciable fraction of a pixel size when using 1 mm pixels. One tiling scheme used smaller pixels at the edge of the arrays to compensate for this insensitive area (partial PIN method). A second approach used a scintillator light-sharing technique to cover the insensitive area (segmented method). The partial pixel method resulted in worse energy resolution and sensitivity in the partial pixels compared to the full pixels (20% and 30% FWHM at 140 keV for edge and corner pixels, respectively, versus 12% in full pixels, with relative corner pixel sensitivity of /spl sim/50%), but successfully maintained spatial resolution and pixel pitch. The segmented crystal light-sharing method demonstrated 17%-20% FWHM energy resolution (at 140 keV) in the edge pixels, /spl sim/85% relative sensitivity, and provided good pixel identification (i.e., good spatial resolution). However, the segmented crystal method is considerably more complex to implement, compared with the partial pixel method.

Radiation damage induced in Si photodiodes by high-temperature neutron irradiation

Results are presented of a detailed study of the effects of high-temperature 4-MeV neutron irradiation on the performance degradation of Si pin photodiodes together with the radiation-induced defects, observed by deep level transient spectroscopy (DLTS). It was found that the dark current increases after irradiation, while the photocurrent decreases. After irradiation, two majority electron capture levels with (E c -0.22 eV) and (E c -0.40 eV) were induced in the n-Si substrate, while one minority hole capture level with (E v + 0.37 eV) was found. Additionally, the degradation of the device performance and the introduction rate of the lattice defects decreases with increasing irradiation temperature. For a 250 °C irradiation, the reduction of the reverse current is only 20% of the starting value. This result suggests that the creation and recovery of the radiation damage proceeds simultaneously at high temperatures.

Radiation damage of Si photodiodes by high-temperature irradiation

Results are presented of a detailed study of the effects of high-temperature 4-MeV neutron irradiation on the performance degradation of Si pin photodiodes. The macroscopic device performance is correlated with the radiation-induced defects observed by DLTS. It was found that the dark current increases after irradiation, while the photocurrent decreases. After irradiation, two majority electron capture levels (Ec 0.40 eV) were induced in the n-Si substrate. Additionally, degradation of device performance and the rate of introduction of lattice defects decrease with increasing irradiation temperature. For 250 °C irradiation, the reduction of the photocurrent is only 10% of the starting value. This result suggests that the creation and recovery of radiation damage proceed simultaneously at high temperatures.

Radiation damage in Si photodiodes by high-temperature irradiation

Results are presented of a detailed study of the effects of high-temperature 4-MeV neutron irradiation on the performance degradation of Si pin photodiodes together with the radiation-induced defects, observed by deep level transient spectroscopy. The degradation of the device performance and the introduction rate of the lattice defects decrease with increasing sample temperature during irradiation. For a 250°C irradiation, the reduction of the reverse current is only 20% of the starting value. This result suggests that the creation and recovery of the radiation damage proceeds simultaneously at high temperatures. Carbon-related complex as hole capture levels is also mainly responsible for the device degradation for high-temperature neutron irradiation.

Absolutely calibrated, time-resolved measurements of soft x rays using transmission grating spectrometers at the Nike Laser Facility

Accurate simulation of pellet implosions for direct drive inertial confinement fusion requires benchmarking the codes with experimental data. The Naval Research Laboratory (NRL) has begun to measure the absolute intensity of radiation from laser irradiated targets to provide critical information for the radiatively preheated pellet designs developed by the Nike laser group. Two main diagnostics for this effort are two spectrometers incorporating three detection systems. While both spectrometers use 2500 lines/mm transmission gratings, one instrument is coupled to a soft x-ray streak camera and the other is coupled to both an absolutely calibrated Si photodiode array and a charge coupled device (CCD) camera. Absolute calibration of spectrometer components has been undertaken at the National Synchrotron Light Source at Brookhaven National Laboratories. Currently, the system has been used to measure the spatially integrated soft x-ray flux as a function of target material, laser power, and laser spot size. A comparison between measured and calculated flux for Au and CH targets shows reasonable agreement to one-dimensional modeling for two laser power densities.

Impact of lattice defects on the performance degradation of Si photodiodes by high-temperature gamma and electron irradiation

Results of a detailed study of the effects of high-temperature γ-ray and electron irradiation on the performance degradation of Si pin photodiodes are presented. The macroscopic device performance will be correlated with the radiation-induced defects, observed by DLTS. After irradiation, two majority electron capture levels with ( E c−0.22 eV) and ( E c−0.40 eV) were induced in the n-Si substrate, while one minority hole capture level with ( E v+0.37 eV) was found. It was found that the dark current increases after irradiation, while the photo current decreases. Additionally, the degradation of the device performance and the introduction rate of the lattice defects decrease with increasing irradiation temperature. At 200°C irradiation, the reduction of the photocurrent is only 10% of the starting value. This result suggests that the creation and recovery of the radiation damage proceeds simultaneously at high temperatures.

The commissioning results of the first undulator beamline at the Pohang Light Source

The initial commissioning results obtained from the first undulator beamline at the Pohang Light Source are reported. The beamline was designed to cover a photon energy range of 50–1500 eV with high spectral resolution and high photon flux (H.J. Shin, J. Korean Phys. Soc. 34 (1999) 350). It is equipped with a variable-included angle plane-grating monochromator. The measured spectra of the inner shell excitations in inert Ar and Ne gases and molecular N 2 show that the spectral resolving power is about 6500 below 600 eV and larger than 4000 at higher photon energy ranges. The photon flux measured with an Si photodiode is of the order of 10 11–10 12 (photons/s/0.1% BW/200 mA).

Calculating the optimum shape of a lightguide for collecting light from a fine-grained, large-area scintillator based on a Si photodiode

Calculating the optimum shape of a lightguide for collecting light from a fine-grained, large-area scintillator based on a Si photodiode

Selective UV Detector as a Combination of Waveguide Structures of Eu3+-Doped Zinc Borate Glasses with Si Photodiode

Selective, low cost ultraviolet detectors with a sensitivity of up to 3 mA/W in the UVC energy range were fabricated by means of Eu3+-doped zinc borate glass (ZBG) fibers and Si-photodiodes. Europium-doped ZBG shows highly efficient emission (613 nm) from Eu3+ by irradiation with high-energy UV light. The Eu3+-doped ZBG fiber converts the incoming UV light into visible light of 613 nm and guides it to a silicon photodiode. Eu3+-doped ZBG fibers ZnxB1—xO1.5—0.5x of varying composition (x = 0.33–0.52) were produced by the melt spinning method from glass melt in air atmosphere. The fibers were then connected to the silicon photodiode surface. The wavelength of maximum sensitivity of our detector is in the far ultraviolet and can be varied between 240 and 270 nm by changing the Zn content in the glass matrix.

Growth kinetics in levitated and quenched Nd-Fe-B alloys

We investigated the growth kinetics and the effect of quenching conditions on rapid solidification of undercooled Nd-Fe-B melts with compositions near the Nd-/sub 2/-Fe/sub 14/-B (2-14-1) phase. We prepared melt drops of various undercooling levels (up to 300 K below the liquidus temperature) were prepared by the electromagnetic levitation method and subsequently quenched them onto chill substrates. We measured the solidification kinetics of the undercooled melts in situ using a high-resolution Si photodiode. In accordance with the nucleation theory, the properitectic /spl gamma/-Fe phase nucleates at first during the undercooling process. There were two different solidification routes, with the observed route depending on the undercooling level of the levitated melt prior to quenching. The peritectic reaction is favored in melts with high undercooling levels prior to quenching. Low previous undercooling levels lead to primary solidification of the 2-14-1 phase on quenching. The thickness of the homogeneous 2-14-1 phase zone, grown directly at the substrate side, depends strongly on the undercooling level prior to solidification. We estimated the growth velocity of the 2-14-1 phase from temperature-time-characteristics to be of the order of 1 mm/s. These investigations give rise to improved understanding about the high sensitivity of the microstructure of Nd-Fe-B alloys on different rapid solidification procedures.

A Monte Carlo optical code for the study of gamma cameras based on a scintillator read out by a silicon photodetector array

A Monte Carlo optical code for the optimization of scintillation gamma cameras based on a single scintillator coupled to arrays of Si photodiodes (PDs) or Si drift detectors (SDDs) have been developed. The simulator allows to calculate the expected gain (e-/keV), position resolution and energy resolution of the gamma detector for different geometries and boundary conditions at the crystal interfaces. A validation of the algorithm has been carried out by comparing the results of the simulations with the corresponding experimental results obtained on existing prototypes of detectors recently developed. An example of application of the proposed code in the study of a small scintillation camera is also reported.

Difference in silicon photodiode response between collimated and divergent beams

A theoretical model, whose validity was confirmed experimentally, has been used to investigate the polarization properties of silicon photodiodes as functions of the angle of incidence with wavelengths ranging from 10 nm to 800 nm. From this, spectral differences in the responses of silicon photodiodes between a collimated beam and a beam with angle divergence are derived. The calculation shows that divergent beams (both isotropic and Gaussian beams) usually give lower responses than those for a collimated beam except in a limited spectral region (approximately 120 nm to 220 nm for a Si photodiode coated with a SiO2 layer of thickness 27 nm). The decrease in response of the Si photodiode with 27 nm SiO2 for the isotropic divergent beam with half apex angle of the beam cone of 60° reaches a maximum of 14 % at a wavelength of 44 nm and the increase reaches a maximum of 7.5 % at 163 nm. Similar results for a Si photodiode coated with an 8 nm oxide layer and a divergent beam with Gaussian angular distribution were also obtained.

Ultraviolet calibration of detectors with respect to a cryogenic radiometer

A facility has been designed to perform a variety of detector characterizations in the air-ultraviolet in the spectral range 230 nm to 400 nm. The facility allows the measurement of spatial uniformity, linearity and the spectral responsivity of different detectors, irradiated by a He-Cd laser or by a monochromator and Xe discharge lamp. With this technique, a set of Si photodiodes, Si detectors in trap configuration, and ultraviolet (UV) radiometers built with SiC detectors in an integrating sphere, have been calibrated and their absolute responsivity at the He-Cd laser UV line has been determined with respect to an absolute cryogenic radiometer.

Cryogenic radiometer facility of the CENAM and first international comparison

A new modern cryogenic radiometer facility has recently been installed at the Centro National de Metrologia (CENAM), Querétaro, Mexico. The radiometer head is a CryoRad II (CRI Inc., Boston, USA). In a specially designed laboratory with clean-room atmosphere, the facility is now ready for calibrations of detectors with power-stabilized laser radiation. This cryogenic radiometer facility will be the basis for most of the radiometric and photometric calibrations of the CENAM. A first radiometric comparison between the CENAM and the Physikalisch-Technische Bundesanstalt (PTB), Germany, was carried out with trap detectors at the He-Ne laser wavelength of 632.8 nm. Two reflection trap detectors, each with three photodiodes, were used: one with 10 mm × 10 mm and the other with 18 mm × 18 mm Si photodiodes. The results were very encouraging. At power levels of about 100 µW the calibrations showed a relative difference of 7 × 10-5 or even less. The PTB calibrations are based on a modified LaseRad II (CRI) radiometer that has been compared with cryogenic radiometers of other types and from other national metrology institutes.

X-ray detection with a scintillating YAP-window hybrid photomultiplier tube

A YAP(YAlO/sub 3/:Ce)-scintillating window, coated on its inner surface with an S20-photocathode, seals a cross-focusing hybrid photomultiplier tube (HPMT) equipped with a small p-i-n anode of 2-mm diameter. This new radiation detector separates X-ray lines down to about 2-keV peak energy from the HPMT noise. Its detection efficiency for high gamma energies depends on the YAP-window thickness and amounts to about 18% attenuation at 400-keV energy in the present version. Competitive radiation detectors like Si photodiodes and Si drift chambers are discussed and compared to our prototype, with particular attention given to their energy resolution and noise performance, which limits their active area considerably.

Resonant-cavity-enhanced high-speed Si photodiode grown by epitaxial lateral overgrowth

We report a resonant cavity Si photodiode grown by merged epitaxial lateral overgrowth. At a reverse bias of 5 V, the dark current was 2.7 pA and the bandwidth exceeded 34 GHz. The peak quantum efficiencies ranged from 42% at 704 nm to 31% at 836 nm. This is the highest speed reported for a Si p-i-n photodiode and the highest bandwidth-efficiency product for any Si-based photodetector.

Observation of the photovoltaic effect in n-ZnSe/p-GaAs heterostructures

We report the photovoltaic properties of an n-ZnSe/p-GaAs heterostructure. The n-type ZnSe films were grown by molecular beam epitaxy using Zn and Se effusion cells and ZnCl 2 as an n-type dopant source. Dark and illuminated I– V characteristics, as well as the spectral response are presented. Two transport mechanisms (recombination and tunneling) were observed in the dark I– V curves as a function of temperature. The photovoltaic parameters of the heterojunction were an open-circuit voltage of 500 mV, a short-circuit current density of 15 mA/cm 2, and a fill factor FF=0.53, resulting in an efficiency close to 4%. The spectral response of the structure extends from the band gap of GaAs to energies higher than that of the band gap of ZnSe, in this high photon energy region being larger than that of a commercial p–i–n Si photodiode.

Design and implementation of high-speed planar Si photodiodes fabricated on SOI substrates

We report high-speed interdigitated (nonmetalized) p-i-n Si photodiodes fabricated on SOI substrates that operate at low bias voltages and that offer easy integration with transistor fabrication processes. Devices fabricated with a finger spacing of 2 /spl mu/m and a 3.75-/spl mu/m-thick active layer achieved a 1.1-GHz bandwidth at a bias of -3 V with a peak efficiency of 29% (0.2 A/W) at 850 nm. Photodiodes with the same geometry that were fabricated on a 2.71-/spl mu/m-thick active layer exhibited a 3.4-GHz bandwidth and a quantum efficiency of 24% (0.16 A/W) at 840 nm when biased at -3 V. The dark current of the photodiodes was less than 25 pA at -3 V, and the capacitance of the photodiodes was less than 265 fF at an applied bias of -3 V.

Application of a plastic scintillating fiber array for low-energy x-ray imaging

Interest in digital imaging has led to the development of new detectors in the form of large-area displays. Most of the recent improvements are based on charge-coupled devices, a:Si photodiodes arrays, and so on. Some of these photodetectors must be coupled to scintillating screens to convert the ionizing radiation into light. Fiber-optic screens offer an advantage for achieving this interface because the length (thickness) of the interaction medium does not contribute too much to the degradation of the spatial resolution. We discuss the possibility of using a plastic scintillating fiber (PSF) array for x-ray detection and imaging in the 10-keV range. Modulation-transfer-function (MTF) measurements of the PSF array are compared with the optics MTF of the imaging system (without the sample); cross talk in the fiber array is negligible, even though the fiber array thickness is 20 mm. The optimal thickness of the array is estimated experimentally.

Experimental determination of detector gain, zero frequency detective quantum efficiency, and spectral compatibility of phosphor screens: comparison of CsI : Na and Gd 2O 2S : Tb for medical imaging applications

Gd 2O 2S : Tb and CsI : Na phosphors mostly used in X-ray medical imaging detectors were compared at various X-ray tube voltages between 50 and 150 kVp. Laboratory prepared test screens of 80, 110, and 150 mg/cm 2 coating weight were evaluated on the basis of the number of emitted photons per incident X-ray (NEP) and DQE, both expressing the signal-and-noise transfer properties of image receptors. NEP and DQE were determined by a method employing luminescence and emission spectrum measurements. The spectral compatibility of the two phosphors with films, photocathodes and the Si photodiode was also examined. CsI : Na exhibited high performance in the 60–90 kVp X-ray tube voltage range while Gd 2O 2S : Tb was found better at higher X-ray energies. The CsI : Na – film combination could provide an excellent X-ray radiographic detector, if effectively protected from humidity. Gd 2O 2S : Tb is more suitable for high tube voltage digital imaging but also adequate for conventional imaging.