Si Photodiode Research Articles

Bandstruwre dependence of impact ionisation in semiconductors from high pressure studies

Abstract Hydrostatic pressure has been used to study the bandstructure dependence of impact ionisation in Si, GaAs and Ge photodiodes. The results were modeled using lucky drift and Monte Carlo calculations, with ionisation threshold energies calculated from model pseudopotential bandstructures. The results for Si, GaAs and Ge were significantly different, and were interpreted in terms of the different band structures. For Si, multiplication is dominated by electron ionisation in the A valleys due to the large intervalley separations. However for GaAs and Ge ionisation gives rise to final electron states which are not only in the lowest conduction-band valley as previously assumed, but throughout the Brillouin zone. This has important implications for our understanding of impact ionisation and for modelling of avalanche multiplication in semiconductor alloys and multilayers.

Multiple- and single-shot autocorrelator based on two-photon conductivity in semiconductors

Two-photon-induced conductivity has been observed in Si and GaAsP photodiodes and in a CdS photo-conductive cell at room temperature and applied to the ultrashort optical pulse measurement with a simplified Michelson-type arrangement. The efficiency of the two-photon conductivity is found to be 3 x 10(-14)I A/W, where I is the intensity of the incident pulse in watts per squared centimeter. A single-shot pulse-width measurement has also been performed by using a two-dimensional silicon CCD array. Use of these inexpensive photodetectors with nonlinear characteristics has proved to be convenient for intensity correlators because of their instantaneous response to temporal coincidence of optical pulses and ease of optical arrangement.

Lateral coupling between a silicon-oxinitride waveguide and an amorphous Si photodiode

We have measured the absorption coefficient at λ=850 nm for an amorphous Si (a-Si:H) p-i-n photodiode laterally coupled to a silicon-oxinitride single mode planar waveguide both for TE and TM modes. The experimental results are compared to calculations considering a five-layer system with a complex index of refraction for the absorbing layer. Theoretical and experimental results agree well considering the inaccuracies in the determination of the layer thicknesses.

Hybrid low-noise charge preamplifiers for heavy-ion detectors

The characteristics of hybrid low-noise general-purpose preamplifiers are described. The basic performances are reported. They have been used in connection with ionization chambers, large-area Si detectors and photodiodes.

X-ray and charged particle detection with CsI(Tl) layer coupled to a Si:H photodiode layers

A compact real-time X-ray and charged-particle imager with digitized position output can be built either by coupling a fast scintillator to a photodiode array or by forming one on a photodiode array directly. CsI(Tl) layers 100-1000- mu m thick were evaporated on glass substrates from a crystal CsI(Tl). When coupled to a crystalline Si or amorphous silicon (a-Si:H) photodiode and exposed to calibrated X-ray pulses, their light yields and speed were found to be comparable to those of a crystal CsI(Tl). Single beta particle detection was demonstrated with this combination. The light spread inside evaporated CsI(Tl) was suppressed by its columnar structure. Scintillation detection gives much larger signals than direct X-ray detection due to the increased energy deposition in the detector material. Fabrication of monolithic-type X-ray sensors consisting of CsI+a-Si:H photodiodes is discussed. >

Linearity of a Silicon Photodiode at 30 MHz and Its Effect on Heterodyne Measurements.

The effect of optical irradiance on the linearity of a Si photodiode was studied. These results are compared for light modulated at 30 MHz and at dc as the optical irradiance was varied over a 9 decade range. We discuss how these results affect the use of this detector as a heterodyne receiver. As the optical irradiance varied from 10−2 to 10+3 mW/cm2, while maintaining constant total power, the photocurrent was constant to about 1%, but as the power density increased further, the photocurrent increased about 13%. At the highest densities that we could achieve, about 6×107 mW/cm2 there was only slight evidence of the onset of saturation. These results are of importance in our work to use optical heterodyne detection to measure filter transmittances over a wide dynamic range. The results provide guidelines for achieving maximum accuracy when using this particular diode as an optical heterodyne receiver.

Radiation Detection with Evaporated Csl(TI) Coupled to a-Si:H Photodiode Layers

ABSTRACTCsl(TI) layers 100–1000 μm thick were evaporated on glass substrates from a crystal Csl(TI). When they were exposed to calibrated X-ray pulses, their scintillation properties were found to be comparable to those of a crystal Csl(TI). Single p particles from radioisotopes were successfully detected by these layers coupled to a crystalline Si photodiode. The light spread inside evaporated Csl(TI) was measured by an amorphous Si (a-Si:H) photodiode array coupled to evaporated Csl(TI) layers. Monolithic X-ray detectors were fabricated by evaporating Csl(TI) on a-Si:H photodiodes directly. The signal yield and noise of this prototype were 1.5×10+4 electrons/MeV and 3×10+4 electrons FWHM, respectively. Larger signal size and lower noise are expected by optimizing the photodiode design.

Effects of Humidity and Cleaning on the Sensitivity of Si Photodiodes

The photodiode self-calibration technique requires the application of a water drop to the surface of the photodiode to be calibrated. By adsorption of water in the silicon oxide the reflectivity and hence the sensitivity of the photodiode changes. We show that the reflectivity of the detector can be restored to its former value by cleaning with pure ethanol. Influences of the room humidity on photodiode responsivity are discussed.

Cryogenic Microbolometers as Detectors in the VUV

For detector calibration in the VUV spectral range, electrical substitution radiometers with low uncertainty for radiation powers between 10 nW and 1 μW are being developed. The microbolometers utilize the low thermal capacitance of sapphire plates at the temperature of liquid helium. On the back side of the plate with an absorbing front surface a semiconductor resistor is used as thermometer and the calibration is achieved with an electrical heater. First measurements in the spectral range around 1 keV show satisfactory agreement with the results from a self-calibration technique of a Si photodiode.

Prospects of CsI(Tl)-photodiode detectors for low-level spectroscopy

A detailed study of the potential of CsI(Tl)—photodiode detectors for applications in low-level spectroscopy was carried out. The energy resolution of small crystals (few cm 3) for γ-rays in the MeV range can be parametrized as ΔE(FWHM)/ E = 5% × E[MeV] −0.69. In kg size detectors, where resolution is limited by electronic noise, the use of new large-area low-capacitance pin Si photodiodes led to a significantly improved energy precision of ΔE (FWHM) = 150 keV, nearly independent of energy. Unfavourable light collection geometries, like in large-acceptance well detectors, do not substantially degrade the energy measurement. Scintillation light yields vary slowly with temperature and time so excellent long-term stability can be expected. For the first time a systematic investigation of inherent crystal radioactivity was possible due to the improved energy resolution obtained with large crystals. Six α-active radionuclides of the Th-decay-chain were identified. The isotopes are in equilibrium with each other but not in secular equilibrium with 232Th. The total α-activity varies from crystal to crystal between 2.4 and 5.5 decays/h cm 3) corresponding to 228Th concentrations of 1.1–2.5×10 −18 g/g. Activity from 238U-chain isotopes is not sent at a level two orders of magnitude below the Th-activity. Prospects are discussed for further improved energy resolution and for applications in low-level spectroscopy, specifically for a search for 100Mo double beta decay.

Selective Epitaxial Growth in Deep Trenches for Integrated Photodetector Applications

Selective epitaxial growth of Si in deep trenches is characterized using a combination of deep level transient spectroscopy, scanning electron microscopy, defect etch, and current‐voltage measurements. The experimental results indicate that the majority of the defects observed are located near the sidewalls of the deep trenches. In the areas far away from the sidewalls, the deep level defect concentration is measured to be less than 1010 cm−3. The success of this effort could lead to an integrated structure of the Si photodiode and the Si VLSI.

Reflectometer for measurements of scattering from photodiodes and other low scattering surfaces

We have designed and tested a simple instrument to measure the diffuse reflectance of good quality optical surfaces such as the surfaces of semiconductor detectors with uncertainties of ~3%. Measurements have been performed on Si photodiodes and on a sample of known reflectance at two different wavelengths.

Characteristics of semiconductor photodiodes in the VUV region

The responsivities as well as the losses of photons through reflection and absorption of a Si photodiode with a SiO2 film, and of a GaAs photodiode with a Au film are estimated using an optical model in the wavelength region from 9 to 800 nm. The model is compared with experiments performed in the visible-far ultraviolet region, and good agreements between the calculation and the experiments are obtained except for scale differences due to the surface recombination of carriers. It is shown that the rate of the surface recombination in a Si photodiode can be determined through a measurement of the internal quantum efficiency in the visible-far ultraviolet region. Thus, the spectral responsivities of semiconductor photodiodes in the VUV region can be roughly predicted.

Detectivity of gallium arsenide on silicon substrate photoconductive detectors

Planar photoconductors made with GaAs on silicon substrate have been studied with respect to static and dynamic responsivities as well as noise levels, in the 1Hz–100kHz frequency range. The results obtained have led to the determination of the specific detectivity which is in turn compared to those of GaAs planar photoconductors and Si photodiodes.

Ge photodiodes as transfer standards for radiant power measurements in the field of fibre optics

The demand for precise radiometric measurements in testing fibre-optical communication networks requires transfer standards of radiant power to fit the variety of available power meters to the radiometric units realised by the national standard laboratories. This paper is focused on Ge photodiodes as transfer standards covering the complete spectral range of interest, including the optical windows of fibre-optical communication at 850, 1300 and 1550 nm. Aspects of correct determination of the absolute spectral responsivity, temperature dependence, ageing, interreflections, inhomogeneity, non-linearity, and optimum detector size are discussed on the basis of measurements. Deviations from the ideal detector behaviour are explained by simple models. Some points of view on the use of Si and InGaAs photodiodes are also given.

Photovoltaic potential induced by 10 mu m laser radiation in Si photodiodes

Photon drag and photovoltaic response observed in p/sup +//n and n/sup +//p silicon junctions excited by moderated-power-density 10- mu m radiation are discussed. A multistep multiphotonic process through intermediate states located within the energy gap is proposed as the mechanism responsible for the photovoltaic potential. >

Performance of an automated rotating-detector ellipsometer

A computer-controlled rotating-detector ellipsometer (RODE) has been constructed using a windowless planar-diffused Si photodiode, a stepping motor, an operational amplifier, and a personal computer (PC) equipped with an A/D-D/A converter. The detector is mounted to the output shaft of the stepping motor at an optimum angle of incidence of ∼59° and is rotated through 360° about the incident beam as an axis. The detector output signal voltage is measured and stored in the PC at every 2° increment of the RODE angle θ. Fourier analysis of the recorded data provides a handedness-blind determination of the state of polarization of the incident light. A beam from a HeNe laser (λ=632.8 nm) is transmitted through the polarizing optics of an ellipsometer to provide the polarization states needed for calibration and testing. The calibration parameters mL and θr are determined by rotating the detector about a linearly polarized beam of light of zero reference azimuth. The RODE is subsequently tested and found to correctly measure the first two normalized Stokes parameters of a number of states with a residual rms error of ∼0.002. This limit on precision is dictated mainly by the 12-bit A/D converter. A small angular misalignment of the rotation axis of the detector with respect to the light-beam axis introduces odd harmonics in the detector signal; hence, its effect is readily isolated by appropriate data reduction. Thin-film coatings on the detector surface that significantly improve the performance of the RODE are proposed.

Spectral quantum efficiencies of semiconductor photodiodes in the far ultraviolet region

The spectral quantum efficiencies and reflectances of p-i-n-type Si photodiodes and a Schottky-type GaAsP photodiode were measured in the wavelength region between 105 to 300 nm. These results are compared with an optical model using complex refractive indices. The spectral relative external quantum efficiencies are well interpreted within the model by assuming that the production energy of an electron-hole pair, ε, is wavelength independent. It is suggested that surface recombination of minority carriers occurs in some devices. The internal quantum efficiency for the GaAsP photodiode is shown to be constant in the photon energy range below about 5.4 eV. The recombination loss and the ε value are separately estimated.

Uncertainty in the calibration of a photodiode-type power meter due to the spectral width of a light source

The error due to the spectral width of a light source in the calibration of a photodiode-type power meter has been studied. Three types of photodiodes, namely, Si, Ge, and InGaAs, have been examined in the wavelength region 800-1650 nm. Theoretical calculations have shown that an error of more than 5% may arise with a Si or Ge photodiode in a specific wavelength region. The measured difference between two photodiode-type power meters agreed with the theoretically calculated value to 0.5%.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A programmable optical demultiplexer using a 32–element photodiode array

AbstractThis paper describes the design and performance of a programmable optical demultiplexer which can compensate the influence of: (1) scattering in the wavelength of an optical source; and (2) wavelength fluctuation of the optical source caused by temperature change, by controlling the branching characteristics in accordance with the wavelength of the optical source. In this multiplexer, a 32‐element Si photodiode array was used to carry out programmable control of the branching characteristics. This optical demultiplexer has the following merits: (1) a wide channel passband is not needed; and (2) since the passband‐center wavelength of this multiplexer is adjusted to the wavelength of the optical sources, fine tuning of the branching characteristic in fabrication is not needed. The fabricated programmable optical demultiplexer has a channel wavelength spacing of 4.0 nm, an insertion loss of 8.0 dB which includes a loss of 3.9 dB due to the detection loss caused by the gap between two photodiodes composing the PD array, and a passband width of 2.8 nm. It was confirmed also by a digital transmission experiment (32 mB/s, RZ signal) that this demultiplexer can maintain its optimum branching characteristics for any wavelength deviation in the optical sources.