Si Photodiode Research Articles

Evaluating Zn 2 SiO 4 :Mn phosphor for use in medical imaging radiation detectors

SiO4:Mn phosphor was evaluated for use in radiation detectors of medical imaging systems. Zn2SiO4:Mn was used in the form of laboratory-prepared fluorescent layers (screens) with coating weights from 18 to 150 mg/cm2. The phosphor was excited to luminescence by low-energy X-raysusing X-raytube voltages ranging from 15 to 50 kVp. The number of emitted optical photons per incident X-rayquantum was thus determined for various X-rayenergies and phosphor coating weights. The optical emission spectrum was also measured and it was used to evaluate the spectral compatibility of Zn2SiO4:Mn with radiographic films, photocathodes and the Si photodiode. Finally, phosphor optical properties were estimated by fitting a theoretical model to experimental data. Results showed that Zn2SiO4:Mn is more efficient for low-energy X-rays. Its intrinsic conversion efficiency was found equal to 0.08, which is comparable to that of actually used phosphors. Zn2SiO4:Mn was also adequately compatible with orthochromatic films and the ES-20 photocathode, thus being appropriate for low-voltage radiography and fluoroscopy.

Spatial uniformity of responsivity for silicon, gallium nitride, germanium, and indium gallium arsenide photodiodes

For almost a decade, the National Institute of Standards and Technology (NIST) has supplied to its customers calibrated photodiode standards and special tests of photodetectors for absolute spectral responsivity from 200 nm to 1800 nm. During this time spatial responsivity measurements have been made on several dozen Hamamatsu silicon S1337-1010BQ photodiodes. We have found that the spatial responsivity changes with wavelength, sometimes significantly as the wavelength approaches the bandgap. The most significant changes appear to be caused by defects in the photodiode material and are not apparent over most of the wavelength region where the photodiode operates. The change in spatial uniformity with wavelength can significantly contribute to measurement uncertainties. These measurements have been repeated and the spatial responsivities have remained constant over several years. Measurements have also been made on other types of Si, GaN, Ge, and InGaAs photodiodes. The measurement equipment, method, and results are presented.

Influence of vignetting errors on the relative spectral responsivity of trap detectors

Originally developed for application with laser beams, trap detectors are widely used as transfer standards in modern monochromator radiometry. However, a non-optimized beam focus applied to trap detectors can result in significant spectral errors. This paper shows that a defocused and improperly aligned beam entering a Si reflection trap detector can cause an apparent modification of the measured relative spectral responsivity. This is true especially in the ultraviolet (UV) region of the spectrum, where the measured relative differences are of the order of several percent due to the significant influence of the direct band-band transitions of Si on the reflectance of the Si photodiode. The spectral range from 248 nm to 600 nm was investigated in detail. Calculated curves, simulated by a computerized geometrical model, were fitted to the measured data. The geometrical model uses experimental spectral reflectance data.

Determining detector requirements for medical imaging applications

The Cramér–Rao lower bound on estimator variance is used as a tool for assessing radiation detector requirements for nuclear medicine imaging. The classical bound for unbiased estimators is reviewed and its relationship to the well-known propagation-of-error formula is demonstrated. The uniform Cramér–Rao bound is described for situations where unbiased estimation is impractical such as in tomographic imaging. The bounds are used to evaluate the necessary detector requirements for a CsI(Tl)/Si photodiode scintillation camera, and to compare the performance of a Compton-scatter camera to that of a parallel-hole collimator/Anger camera system.

Chopped Radiation Measurements With Large Area Si Photodiodes.

Frequency dependent response characteristics of photocurrent meters using large area, radiometric quality Si photodiodes have been analyzed. The current responsivity, the voltage noise and drift amplification, and the gain and bandwidth of the photocurrent-measuring analog control loop were calculated. The photodiodes were selected for high shunt resistance. The effect of the photodiode junction capacitance on the response characteristics was also analyzed. As a result of photocurrent gain dependent frequency compensations, the noise boosting effect was minimized at the output of the current meter. The loop gain and bandwidth were maximized. High-accuracy photocurrent measurements can be achieved using the described procedures for both dc and modulated optical radiation.

Europium-activated phosphors for use in X-ray detectors of medical imaging systems

Three europium-activated phosphors Y2O2S:Eu, Y2O3:Eu, and YVO4:Eu emitting red light were studied to investigate their suitability for radiographic cassettes or digital imaging systems. Screens were prepared from phosphor powders with various coating thicknesses by sedimentation. To assess phosphor light producing efficiency in relation to patient dose, each screen was X-rayed using 40-120 kVp and the number of light photons emitted per X-ray incident was experimentally and theoretically evaluated. Additionally, the capability of the emitted light to sensitize films or to generate electrons in silicon photodiodes used in digital imaging systems was examined. Y2O2S:Eu screens were most efficient in light emission, and when combined with either red sensitive films or Si photodiodes, they were found superior to Y2O3:Eu or YVO4:Eu screens in film grain or electron signal generation. In many cases they were also found superior to terbium-activated phosphors. Provided that several problems related to industrial production (special dyes, reflective backing, crossover effects) are dealt with, those europium-activated screens could be employed in low-tube-voltage radiographic applications.

Selective epitaxial growth Si resonant-cavity photodetector

An novel resonant-cavity Si photodiode was fabricated using a selective epitaxial growth process. The photodiode shows a bandwidth over 5 GHz, and a quantum efficiency over 65% at 700 nm. Compared to the previously reported Si resonant-cavity Si photodetectors, this photodiode process is more compatible with Si integrated circuit technology.

Determination of surface activity and radiation spectrum characteristics inside buildings by a gamma locator

A gamma locator designed to conduct contamination survey inside buildings connected with nuclear production is developed. The device consists of a detector head and a remote control unit connected with a cable (>150 m). The detector head (200 × 200 × 250 mm 3; the weight is nearly 40 kg) is a collimated scintillated gamma detector installed on a scanning unit. The gamma detector is placed in a lead shielding with the collimator having an entrance angle near 10°. The detector head contains a TV camera and laser distance device. The remote control unit provides operation and processing of the acquired information. The gamma detector is based on a system of CsI(Tl) scintillators and an Si photodiode. The detector energy resolution (nearly 9% for radiation of 137Cs) provides identification of gamma radiating isotopes such as 57Co, 137Cs, 40K, 60Co, 131I and 154Eu. The gamma locator threshold of determination is 250 Bq/cm 2 for a 10 cm 3 scintillator and 137Cs radiation. The system allows one to measure the effective surface activity density (for 137Cs radiation) of all building surfaces and to reconstruct an exposed dose rate distribution within the volume of the investigated space.

Electrical behaviour of laser-damaged silicon photodiodes

A measurement of the electrical parameters degradation of Si photodiodes irradiated by laser visible light has been performed. The laser is a Q-switched Nd:YAG, frequency doubled, operated in single pulse mode of 4 ns duration. The applied fluence levels range up to 90 J/cm 2. Two kinds of irradiation process were applied: either a part of the detector active area was irradiated in single pulse mode, or a scanning of the whole detector active area was performed with successive identical pulses. It has been shown that the fluence necessary to induce significant changes (local decrease of 35%) in responsivity is several times the surface melting threshold fluence (0.5 J/cm 2). Conversely, the dark current is the most sensitive parameter, increasing by about four times for high irradiation. The in-depth dopant distribution is altered by high fluence irradiation in a way that cannot be explained by simple thermal modelling.

High-speed polysilicon resonant-cavity photodiode with SiO/sub 2/-Si Bragg reflectors

Previously, it has been shown that the bandwidth of Si photodiodes can be increased by more than an order of magnitude, without sacrificing responsivity, by a resonant-cavity structure that utilized GeSi-Si asymmetric Bragg reflectors. We report an interdigitated p-i-n polysilicon resonant-cavity photodiode, which employs a Si-SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Bragg reflector, that is more compatible with standard Si processing technology. For an absorbing region thickness of only 0.5 μm, a peak quantum efficiency of 40% was achieved and the dark current was <60 nA at 10 V. For 2 μm×2-μm finger width and spacing the bandwidth was 10 GHz.

Color-sensitive photodetector based on porous silicon superlattices

Color-sensitivity of Si photodiodes was achieved by integrating porous silicon (PS) Bragg reflectors and Fabry–Perot filters. The PS was formed in the p +-type part of the p +n junction which required illumination of the samples during anodization. The optimal illumination power density turned out to be a compromise: high power densities are necessary to enable high anodization current densities, but this results in a degraded filter performance. The PS layers had no significant influence on the electrical characteristics of the photodiodes, but as expected they strongly modified the spectral response. The results are in good agreement with the reflectance spectra of the filters. © 1997 Elsevier Science S.A.

Monolithically Integrated X-Ray Detector Arrays for Computed Tomography

AbstractThe design and performance of a high sensitivity monolithic X-ray detector array fabricated by depositing CdTe photodiodes on CdWO4 scintillators for computed tomography (CT) are presented. CdTe photodiodes offer improved sensitivity due to the better match to the CdWO4 scintillator emission spectrum than the Si photodiodes used in current CT scanners. A CdTe photodiode fabricated on glass mechanically stacked to a CdWO4 scintillator showed 30% improvement in signal strength over Si photodiodes stacked to CdWO4-. Monolithic X-ray detector arrays were fabricated by depositing a CdTe photodiode structure on CdWO4 using close-spaced sublimation technique. Preliminary results on the detector's X-ray response, signalto- noise ratio and the X-ray temporal response show that monolithic CdTe X-ray detectors are suitable for practical CT scanner applications.

Organic films deposited on Si p-n junctions: Accurate measurements of fluorescence internal efficiency, and application to luminescent antireflection coatings

Ultraviolet (UV) optical pumping followed by fluorescence wavelength downconversion of thin film organic light emitting materials deposited directly on the surface of Si p-n junction diodes is found to be an accurate and rapid means to determine the film internal fluorescence efficiency. By measuring the photoresponse of the Si detectors in the UV, we find that the organic light emitting films of aluminum tris-(8-hydroxyquinoline) (Alq3), N,N′-diphenyl-N,N′-bis-(3-methylphenyl)- 1,1′-biphenyl-4,4′-diamine, and bis-(8-hydroxyquinaldine)-chlorogallium (Gaq2′Cl), have internal fluorescence efficiencies of 0.30±0.05, 0.35±0.03, and 0.36±0.03, respectively. It has also been found that the organic films can be grown to a thickness which optimizes UV light conversion and can, at the same time, serve as antireflection coatings in the visible spectral region, thereby resulting in enhanced Si photodiode sensitivity extending from the UV to the infrared.

Absolute brightness measurement of X-ray emission from laser-produced plasma using thermoluminescent dosemeters

Thermoluminescent dosemeters of LiF:Mg,Ti, LiF:Mg,Cu,P and CaF2:Dy were used as secondary standards in calibration measurements of X-ray emission from a laser-produced aluminium plasma. Relative sensitivities of the thermoluminescent dosemeters were found to be independent of dose rate over a wide dose rate region. The photon detection efficiency of thermoluminescent dosemeters is compared to that of semiconductor Si photodiodes. The thermoluminescent dosemeters are presented as a powerful tool for testing of capability of a semiconductor photodiode detector of X-ray pulse energy emitted from the laser-produced plasma.

Resolution limits in radiation temperature measurements using unfiltered photodiodes as detectors

This paper attempts to determine the practical and theoretical resolution limits of black-body (BB) radiation temperature measurements. The calculations were restricted to unfiltered Si, Ge and InSb photodiodes and to the 100 °C to 1500 °C temperature range. A measurement geometry determined by the fixed-point BB construction was used with a view angle of 3° and an observed area of 1 mm2. Current best technology for detector long-term noise performance was assumed. The calculations show that the best resolutions are 15 mK to 40 mK for InSb, 6 mK to 15 mK for Ge and 2 mK to 8 mK for Si fullband pyrometers in the temperature ranges 100 °C to 280 °C, 280 °C to 500 °C and 500 °C to 1100 °C, respectively. These results require very good temperature stabilization (better than 0,01 K) of the detectors.

Detector calibration in the wavelength region 10 nm to 100 nm based on a windowless rare gas ionization chamber

A detector calibration facility based on a rare gas ionization chamber has been built at the Electrotechnical Laboratory, Japan. We have realized a windowless system by applying strong differential pumping, which features no spectral limitation and easy gas handling. In the auto-ionization region near the xenon absorption edge, photon flux was found to be underestimated and this limited the maximum available wavelength to 92 nm. The uncertainty (1σ) for a Si photodiode calibration was estimated to be within ±9% in the wavelength range 10 nm to 92 nm.

A Fourier-transform spectrometer for spectral-irradiance comparisons

A Fourier-transform spectrometer (FTS) for the wavelength region between 1100 nm (cut-off wavelength of the Si photodiode detector) and 190 nm (absorption edge of air) has been developed for use in spectral-irradiance comparisons. The main advantages of the FTS over traditional spectroradiometers based on a monochromator are: (i) high light throughput; (ii) circular entrance aperture; (iii) high wavelength accuracy; and (iv) multiplex advantage if the detector is the dominant source of noise. The main problems encountered in achieving highly accurate irradiance comparisons are discussed. The uncertainties due to the size of the source are studied. It was found that sources with a diameter of up to 20 mm can be compared with an uncertainty of 2,5% between 1100 nm and 250 nm, and 3% below 250 nm. In the case of a homogeneous radiance distribution the uncertainty is below 1,5 %.

Polarization characteristics of semiconductor photodiodes

The polarization characteristics of a pn-junction Si photodiode and a Au-GaAsP Schottky photodiode which were revealed when the detectors were used obliquely to the incident radiation were investigated theoretically and experimentally in the spectral region from the vacuum ultraviolet to the visible. The experimental results in both cases were well explained by an optical model in the ultraviolet to the visible spectral range. It was found that the effect of beam divergence on the detector response could be negligible in the visible and resulted in a smaller response below about 90 nm for both the Si and the Au-GaAsP photodiode. The silicon photodiode showed complex spectral variation of polarization properties between 100 nm and 400 nm.

Normal-incidence strained-layer superlattice Ge0.5Si0.5/Si photodiodes near 1.3 μm

Ge0.5Si0.5 strained-layer pin photodiodes, in which multiple strained layers serve as the absorption region, have been fabricated. These devices exhibit an optical response at wavelengths beyond 1.3 μm at normal incidence. The measured external quantum efficiencies at an applied bias of 4 V are 17% at 0.85 μm and 1% at 1.3 μm, respectively. Excellent electrical characteristics evidenced by the avalanche breakdown at 20 V have also been demonstrated.

Relative calibration of photodiodes in the soft-x-ray spectral range

A method of obtaining a relative calibration of Si photodiodes for the spectral range of soft x rays (1–30 keV) is presented. A simple mathematical model of the p-n diode is adopted which allows the response to be described in terms of a small set of parameters. The diffusion length as well as the thickness of a dead layer below the front surface of the diodes are obtained from measurements of angular dependences of the photoinduced current. It is shown that a precise characterization of the diode response and an accurate relative calibration can be obtained using this method. However, it was found that the presence of a dead layer a few tenths of a micrometer thick can pose severe restrictions on the use of planar diode arrays in x-ray tomography systems where uniformity of response is crucial. The method has been applied to the diode arrays equipping the x-ray tomography system built for the TCV tokamak, a magnetic fusion research device.