High Spectral Response Research Articles

Gamma-ray detection with an UV-enhanced photodiode and scintillation crystals emitting at short wavelengths

A low-noise ion implanted photodiode with high spectral response in the deep blue/UV region has been tested as read-out device for scintillation crystals with matching emission spectra (YAP(Ce), GSO(Ce), BGO and CsI(Tl)). This gamma-ray detector concept is attractive in many industrial applications where compactness, reliability and ambient temperature operation are important. The results show that the amount of detected scintillation light energy falls rapidly off as the wavelength of the scintillation light decreases. It is concluded that the dynamic spectral response of the photodiode, due to increasing carrier collection times, is considerably less than the DC response at short wavelengths. The diode is not useful in pulse mode operation with scintillation crystals emitting at wavelengths below about 400 nm. For read-out of CsI(Tl) with 661.6 keV gamma-radiation, however, the photodiode concept shows better energy resolution (7.1%) than other detectors.

Growth of polycrystalline silicon thin films on glass

Polycrystalline silicon (poly-Si) thin films were grown by plasma-enhance CVD from fluorinated precursors on glass substrate by two-step growth (TSG), i.e. (1) growth of seeds on glass and (2) epitaxial-like growth on the seeds. The seed crystal with high crystallinity was grown by layer-by-layer technique. An optimum condition to promote epitaxial-like growth on the seeds having a certain texture was also found by varying the mixing ration of source gases, SiF 4 and H 2, under the real time observation with a spectroscopic ellipsometer (SE). Smooth interface on atomic scale between the seeds and the layer grown epitaxial-like made by TSG was established by measurements with SE and SIMS. High spectral response obtained in a fully poly-Si p-i-n diode proves that high quality poly-Si is able to be grown on glass by TSG.

Low Temperature Processed Metal-Semiconductor-Metal Phtodetectors on ZnSe/SI-GaAs (100)

AbstractLow temperature (LT) processed ZnSe MSM photodetectors can be used for detecting Gamma rays or X-rays using scintillation crystals in many space and medical applications. Metalsemiconductor-metal (MSM) photodetectors were fabricated on undoped ZnSe grown by molecular beam epitaxy (MBE) on semi-insulating (100) GaAs substrates. The MSM photodetectors consist of interdigitated metal fingers with 2 μm, 3 μm, and 4 μm spacing on one chip. Probimide and SiO2 thin films were deposited to aid the LT lift-off process before the pattern generation. An interdigitated structure was achieved by photolithography and reactive ion etching. Pd Schottky metal was deposited at a substrate temperature near 77 K using a lift-off technique. The LT metallization provides an improved interface between metal and semiconductor interface. Continuous wave signal to noise ratio (SNR) of 1.57×104 was obtained for 2 μm interdigitated photodetectors, operated under 180 nW optical power at a wavelength of 400 nm. The detectors showed good DC saturation characteristics indicating a low surface recombination. Saturation current without illumination remained at around less than 1 pA for a ± 10 V biasing. Detectors exhibited linearity with light intensity and DC bias voltage suggesting no gain mechanism involved, and showed a high spectral responsivity (0.6 (A/W)) at a wavelength of 450 nm at 5V applied bias.

Improvement of efficiency of ITO/Se thin film solar cells using tellurium stimulator

AbstractIndium Tin Oxide ITO/Se thin film solar cells have been fabricated by vacuum deposition. Prior to deposition of selenium film, ultrathin tellurium film has been deposited on ITO thin film. Tellurium film acts as a stimulator for bringing improvement in crystallite size, adhesion to ITO and electrical behaviour of selenium thin films. The maximum efficiency of the ITO/Se thin film solar cells without tellurium stimulator and with tellurium stimulator under illumination AM1 (100 mW cm−2) are found to be 1.68% and 2.27% while under fluorescent tubelight (0.15 mW cm−2) are found to be 5.85% and 7.05% respectively. The ITO/Se thin film solar cells exhibits high spectral response at short wavelength. The barrier height of the ITO/Se thin film solar cells was determined.

Ground Based Measurements of Atmospheric Gases by Spectroscopic Methods

AbstractSpectroscopic methods afford a number of advantages for making atmospheric measurements. It is a passive technique which is capable of making real time, in situ measurements. Since each molecules has a characteristic spectrum it is possible to make simultaneous measurement of a number of species. In addition, unequivocal identification of the target species is possible if the instrument has sufficient spectral resolution. Detection limits as low as 1 part in 1013 have been reported as well as dynamic ranges as high as 108. Temporal resolution of fractions of a second can be achieved for “eddy correlation” flux measurements. – Fourier Transform Infrared Spectroscopy (FTIR) is, perhaps, the most universal method and is particularly useful in identifying new species in heavily polluted air. However, it lacks the sensitivity required for measurements of trace gases in clean air. Differential Optical Absorption Spectroscopy (DOAS) operates in the visible and the near UV which limits the number of measurable species. But it is capable of measuring highly reactive gases such as OH, NO3, and HONO. Tunable Diode Laser Absorption Spectroscopy (TDLAS) has very high spectral resolution, sensitivity and time response. Its main disadvantage is the need for a number of laser diodes to cover a wide spectral range. Laser Induced Fluorescence (LIF) is also capable of both high sensitivity and specificity. However, rather complex excitation sources have to be devised for each gas being investigated. Photoacoustic Absorption Spectroscopy (PAS) is, in principle very simple and capable of simultaneous measurements. In practice, however, it suffers from lack of resolution and serious interferences from CO2 and H2O.