Photoconductive Target Research Articles

2-7 サチコンのセットアップ焼付の改善

2-7 サチコンのセットアップ焼付の改善

Hot Carrier Energy and Energy Relaxation Length in a-Se Photoconductive Target

The a-Se HARP (High-gain Avalanche Rushing amorphous Photoconductor) target was prepared, and the avalanche characteristics were investigated. In this paper, to study avalanche multiplication at a high electric field, the lucky-drift model was used. In addition, the energy-and field-dependent energy relaxation length was considered. The avalanche multiplication factor was obtained from the current-voltage characteristic of the a-Se HARP target. The threshold field of the 0.4-μm-thick a-Se HARP target was 0.88×108V/m. The hot carrier energy at the threshold field for the avalanche multiplication in the 0.4-μm-thick a-Se HARP target was 0.21 eV. The hot carrier energy in the a-Se layer increases linearly as the electric field increases. The hot carrier energy also saturates as the avalanche multiplication factor increases. In addition, the energy relaxation length between the inelastic scattering events in the a-Se layer saturates as the hot carrier energy and the avalanche multiplication factor increase.

Paradoxes of photoconductive target and optical control of secondary ion yield

This study of the photoconductivity of semiconductors, in particular, cadmium chalcogenides as materials for targets of vacuum image converters followed the path of overcoming paradoxes. The concepts developed by the classics of photoelectricity also help to understand the paradoxes of the new secondary-ion photoelectric effect, especially, its spectral characteristic. The optical channel of secondary ion yield control via a photoconductive target opens the way to a new branch of nanotechnology, i.e., optoionics.

Scintillator avalanche photoconductor with high resolution emitter readout for low dose x-ray imaging: Lag

A new concept of indirect conversion flat-panel imager with avalanche gain and field emitter array (FEA) readout is being investigated. It is referred to as scintillator avalanche photoconductor with high resolution emitter readout (SAPHIRE). The present work investigates the temporal performance, i.e., lag, of SAPHIRE. Since the temporal performance of the x-ray detection materials, i.e., the structured scintillator and avalanche amorphous selenium (a-Se) photoconductor, has been studied previously, the investigation is focused on lag due to the FEA readout method. The principle of FEA readout is similar to that of scanning electron beam readout used in camera tubes, where the dominant source of lag is the energy spread of electrons. Since the principles of emission and beam focusing methods for FEA are different from thermionic emission used in camera tubes, its electron beam energy spread and hence lag is expected to be different. In the present work, the energy spread of the electrons emitted from a FEA was investigated theoretically by analyzing different contributing factors due to the FEA design and operations: The inherent energy spread of field emission, the FEA driving pulse delay, and the angular distribution of emitted electrons. The electron energy spread determined the beam acceptance characteristic curve of the photoconductive target, i.e., the accepted beam current (I(a)) as a function of target potential (V(t)), from which lag could be calculated numerically. Lag calculation was performed using FEA parameters of two prototype HARP-FEA image sensors, and the results were compared with experimental measurements. Strategies for reducing lag in SAPHIRE were proposed and analyzed. The theoretical analysis shows that the dominant factor for lag is the angular distribution of electrons emitted from the FEA. The first frame lags for two prototype sensors with 4 and 25 microm HARP layer thicknesses were 62.1% and 9.1%, respectively. A lag clearance procedure can be implemented by turning on all the FEA pixels simultaneously between subsequent frames without negative impact of readout speed. For large-area SAPHIRE, the bias electrode for the HARP needs to be divided into strips to allow parallel readout. With typical cardiac detector parameters, SAPHIRE with 128 parallel strips can provide real-time readout (30 frames/s) with first frame lag of -4%. The investigation of lag in SAPHIRE shows that the angular distribution of emitted electrons from FEA can result in substantial lag if the readout was performed pixel by pixel. Effective strategies for reducing lag include dividing the bias electrode into multiple strips to allow parallel readout and the incorporation of rapid charge clearance procedure between subsequent frames or rows.

High-Gain Avalanche Rushing amorphous Photoconductor (HARP) detector

We have been studying a very sensitive image sensor since the early 1980s. In 1985, the author found for the first time that an experimental pickup tube with an amorphous selenium photoconductive target exhibits high sensitivity with excellent picture quality because of a continuous and stable avalanche multiplication phenomenon. We named the pickup tube with an amorphous photoconductive layer operating in the avalanche-mode “HARP”: High-gain Avalanche Rushing amorphous Photoconductor. A color camera equipped with the HARP pickup tubes has a maximum sensitivity of 11 lx at F8. This means that the HARP camera is about 100 times as sensitive as that of CCD camera for broadcasting. This ultrahigh-sensitivity HARP pickup tube is a powerful tool for reporting breaking news at night and other low-light conditions, the production of scientific programs, and numerous other applications, including medical diagnoses, biotech research, and nighttime surveillance. In addition, since the HARP target can convert X-rays into electrons directly, it should be possible to exploit this capability to produce X-ray imaging devices with unparalleled levels of resolution and sensitivity.

Dependence of Thickness on Avalanche Characteristics of Te-Doped Amorphous Selenium Photoconductive Target

The thickness dependence of the avalanche characteristics of a tellurium (Te)-doped amorphous selenium (a-Se) high-gain avalanche rushing amorphous photoconductor (HARP) target is investigated. To improve the quantum efficiency of the a-Se HARP photoconductive target, a Te-doped a-Se photoconductive layer is sandwiched within a-Se HARP target. The avalanche multiplication factor and hole ionization coefficient of the a-Se HARP target are obtained using the result of photocurrent measurement. The multiplication factor in the avalanche mode exponentially increases with increasing electric field by avalanche multiplication phenomena over the threshold field. The quantum efficiency of the 8-µm-thick a-Se HARP target in the avalanche mode is higher than that of the thin HARP target below 2 µm thickness. Also the spectral response, decay lag, and light-transfer characteristics are studied.

Sensitivity to red/green/blue illumination of amorphous selenium based photodetector driven by nitrogen (N)-Doped CVD diamond

An imaging device that could detect weak light of approximately 0.001 lx with high SN ratio is capable of opening up the possibility of creating artificial visions as a substitute for a human eye. In terms of sensitivity and SN ratio, amorphous selenium (a-Se) is known to be one of the best candidates among other photoconductive target. Combined with a-Se, a diamond cold cathode would be one of the best options in order to steadily emit electrons under low vacuum. The a-Se film is modified with Arsenic (As) and Tellurium (Te) to prevent crystallization and narrow the optical gap for red light detection. An RGB filtered light from a halogen light source was illuminated onto this modified a-Se film and the photo-response was measured for each color. The device could detect extremely weak illumination of under 0.39 lx, which is approximately the darkness under moonlight. Results of the photo-response compared between films with and without Te clarified that Te dopants enhances red light detection significantly. The illuminated light was then pulsed at 10 mHz and the current to time stability was observed. It showed clear correspondence between the intensity of the luminance and current, which the Te modified samples showed good response to other colored light as well.

50 × 50 μ m pixel magnetic focus field emitter array image sensor with high-gain avalanche rushing amorphous photoconductor target

A 50×50μm pixel field emitter array image sensor with a highly sensitive avalanche-mode photoconductive target and a scanning electron focusing system consisting of permanent magnets were fabricated and tested as a step toward the development of ultrahigh-sensitivity compact image sensors for high-definition television cameras. The experimental results revealed that the magnetic focusing conditions of the prototype sensor were in accordance with those estimated from the simulation, and the prototype sensor could obtain both enough resolution for its pixel size and high sensitivity by focusing the electrons emitted from the field emitter array onto the highly sensitive photoconductive target.

高感度ＣｄＴｅ光導電膜を用いたビジコン型Ｘ線イメージセンサ

A one-inch X-ray vidicon (imaging camera tube) with a polycrystalline cadmium telluride (CdTe) photoconductive target has been developed. This vidicon has a 10.5μm thick CdTe layer deposited by RF sputtering and can attain an X-ray signal current of about 200 nA/cm2 for 2.58 mC/kg/min (10R/ min). This value is higher than the 45 nA/cm2 of conventional PbO X-ray vidicons. The dark current is about 5 nA/cm2 at a target voltage of 35 V at room temperature. The resolution of the device exceeds 251p/mm (20μm). In addition, by using a sputtering technique which can obtain large and uniform film, simultaneous deposition of many conventional and large vidion targets can easily be performed.

X-ray imaging camera tube using sputter-deposited CdTe/CdS heterojunction

CdTe heterojunction devices have been fabricated for the first time by an RF sputter deposition method for application to X-ray imaging sensors. The electrical resistivities of sputter-deposited polycrystalline CdS and CdTe films are greater than 10/sup 6/ Omega -cm and 10/sup 9/ Omega -cm, respectively. The fabricated CdS/CdTe heterojunction sensor shows a good diode characteristic and a high sensitivity to X-ray radiation. An X-ray imaging camera tube consisting of CdS/CdTe heterojunction photoconductive target shows three times larger responsivity to X-rays than the conventional PbO X-ray tube. The dark current density of the device is observed to be lower than 10 nA/cm/sup 2/ at 20 V target voltage at room temperature. >

Ultra-High-Sensitive Image Pickup Tubes Using Avalanche Multiplication in a-Se.

ABSTRACTExtremely high-sensitive image pickup tubes with sensitivities 1000 times higher than those of conventional tubes are fabricated using the avalanche phenomenon in a-Se as photoconductive targets. The excess avalanche noise of a video signal is found to be much less than that expected, based on the carrier ionization rates. The frequency spectra of the noise currents of both the pickup tubes and sandwich-type photocells are examined. The results are compared with those of a simulation, and it is found that the excess noise can be reduced by the charge-storage operation of imaging devices.

If Progressive Scanning is So Good, How Bad is Interlace?

While much HDTV equipment can indeed be made to function very well in progressive mode (assuming cost and extended bandwidth do not represent any practical impediment), the fact remains that the camera and VTR (the very basis of HDTV shoot and capture) are both subject to fundamental physical and technical limitations. This article discusses the effects of the nonideal beam spot, the low-velocity-beam interface with the photoconductive target, the self-sharpening effect of electron beam scanning, the astigmatism of the beam spot profile, and the way all of these affect the three primary dimensions of the real-world television picture: horizontal, vertical, and temporal resolution. When the totality of the camera's resolution (in all dimensions, simultaneously) is examined, it can be seen that the gap between progressive and interlace narrows considerably. Camera sensitivity, however, emerges as the singular pivotal issue. A substantial loss is incurred in progressive scanning because of degradation in signal-to-noise performance. This sensitivity penalty actually worsens in the future HDTV charge-coupled device (CCD), if progressively scanned. Consideration of such progressive scanning would push the advent of the solid-state imager into the far future.

Thin films for rewritable optical recording by use of photoconductive effect.

A TV camera tube equipped with a photoconductive thin-film target is, in a sense, a two-dimensional rewritable optical memory. To apply the target of a camera tube to memory devices, however, its storage time, that is, time constant, must be sufficiently long to store optical information. This is, in general, possible by optimizing the composition and structure of photoconductive targets.In this article, various kinds of photoconductive materials are reviewed in detail. To realize optical recording media, it is advantageous to combine a photoconductive film with another material. Various combinations of photoconductor and insulator, elastomer, thermoplastics, electro-optical single crystal, and ferroelectric ceramics are also discussed.

An amorphous silicon imaging tube using high-velocity beam scanning

An imaging tube using hydrogenated amorphous silicon as the photoconductive target with a blocking-type structure and a high secondary electron emission yield surface is successfully operated using a high-velocity electron-beam scanning mode. The internal electric field in the photoconductor is the opposite of those used in conventional tubes in that the secondary electron current is obtained as a video signal. This imaging tube overcomes a lot of the disadvantages of conventional imagers, especially very small decay lag at both high- and low-illumination levels, a high uniform resolution of more than 700 TV lines, and no burning or blooming.

Surface problem in imaging devces.

Imaging devices in the invisible light regions of ultraviolet, infrared and low light level are described and discussed, considering, in particular, the surface problems of photocondutive material. In the MIS blocking junction of a photoconductive target for an ultraviolet vidicon, a thin insulating film of MgF2 on a a-Se photoconductor and SiXNY on a-Si : H are shown to produce interfacial fields increasing sensitivity in short wave length region. In an infrared vidicon with a PbO-S target, it is shown that PbS, which forms a thin sucface layer on the textured Pb0 crystals, plays an important role and brings infrared sensitivity and resistivity sufficiently close to vidicon operation. For low light level use. SiO2-Si interface problems of a silicon intensifier target are described, and new type target with a hydrogenated amorphous silicon to crystalline silicon heterojunction is presented and discussed.

Photoconductive properties of thin-film heterojunction ZnSe-(Zn1-xCdxTe)1-y(In2Te3)y

We have developed a high quantum efficiency and a low dark current photosensor with thin-film heterojunction ZnSe-(Zn <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</inf> Cd <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</inf> Te) <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-y</inf> (In <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> Te <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> ) <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</inf> . This heterojunction has a quantum efficiency of above 0.9 in the wavelength range of 400-750 nm and the dark current is 1.5-5.0 nA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at reverse bias of about 15 V. These excellent characteristics can be obtained by adopting the blocking type heterostructure and optimizing the composition <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</tex> . The roles of each layer of this heterojunction are analytically discussed. The ZnSe film works as a blocking layer for the holes injected from the transparent electrode, reducing the dark current. The (Zn <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</inf> Cd <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</inf> Te) <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-y</inf> (In <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> Te <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> ) <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</inf> film is a photosensitive layer and blocks the electrons injected from this side of electrode because of its p-type conductivity. The several photoconductive properties of the heterojunction are shown to be dominated by the deep trap states lying near the middle of forbidden energy gap. This photosensor can be applied to several devices such as photocells, facsimile sensors, photoconductors for electroxerography, and photoconductive targets of camera tubes.

The heterojunction ZnSe-(Zn 1− xCd xTe) 1− y(In 2Te 3) y having high sensitivity in the visible light range and its applications

The effect of a ZnSe film on the heterostructural ZnSe-(Zn 1− x Cd x Te) 1− y (In 2Te 3) y film was studied from the standpoint of crystal growth. It is made clear that the ZnSe film improves the crystallinity of the Zn 1− x Cd x Te) 1− y (In 2Te 3) y film evaporated on the ZnSe film and improves the spectral sensitivity of the heterostructure in the visible to the near-infrared light range. This heterostructure which consists of many elements can be fabricated by simple evaporation and anneal methods. One of the applications of this heterostructure is the photoconductive target in a camera tube. A camera tube with such a heterostructure is called “Newvicon” and has excellent features such as high sensitivity, low dark current, no burn-in and small lag. This heterostructure can be also applied to illuminometers photometers and solid state imagers.

Single-Tube Color Imager Using Hydrogenated Amorphous Silicon

Reactively sputtered hydrogenated amorphous silicon film is used as the photoconductive target of a vidicon type image pickup tube. It is indicated that hydrogen partial pressure in the discharge gas has to be 40% or more in order to attain suitable image pickup tube characteristics. Photo-electric properties and lag-characteristics are enormously improved by lightly doping boron into amorphous silicon film, and a blocking type target structure is effective for suppressing dark current of the pickup tube. Using doped amorphous silicon film with a color-filter-integrated face plate, a single-tube color imager is fabricated.

1-Inch Saticon for high-definition color television cameras

A high-resolution 1-in TV camera tube with the Saticon photoconductive layer has been developed for live color image pickup in the high-definition television system based on 2:1 interlaced 1125 scanning lines and 60 fields per second. The inherent high-resolution capability of the Saticon photoconductive target is fully demonstrated by the use of a diode-operation electron gun having no beam crossover point. The electrons emitted from a flat barium-impregnated tungsten cathode are formed into a narrow and laminar electron beam of high current density and low beam temperature using a fine beam-defining aperture about 12 µm in diameter in the positively biased G <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</inf> disc electrode. The tube has an outstanding resolving power and a low lag characteristic as well as sufficient beam current margin to handle the standard peak signal current of 0.5 µA required for a reasonably high <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S/N</tex> ratio in the wide-band video signal system. The applicability of the design concept to the tubes for standard TV systems is also discussed.

Development of a Saticon Single Color Pickup Tube for a Color TV Camera

To satisfy the need for a smaller and lighter color TV camera with reduced power consumption, a pickup tube for an 18-mm electrostatic-focus magnetic-deflection pickup tube has been developed. A new photoconductive target, based on the Saticon <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">*</sup> target, that is appropriate for a pickup tube of a consumer-use TV camera has also been developed. The combination of a high-performance stripe filter integrated faceplate and a new type of photoconductive target was effective in solving a number of problems.