Increase In Dark Current Research Articles

Electronic Properties of Furan-Quinone Pigments. I. Charge-Transport Studies

The electrical properties of a furan-quinone pigment, 8, 13-dioxodinaphtho (2, 1−b:2′, 3′−d)-furan-6-(2-pyridyl)-carboxamide (DFC), have been studied by means of photoinduced condenser discharge and transient photocurrent techniques. It is shown that the electron range in DFC, ∼10−8 cm2 V−1, is 3 orders of magnitude longer than that of holes, ∼10−11 cm2 V−1. The quantum efficiency of the photogeneration of charge carriers in DFC increases linearly with the square of the electric field, while the dark current increases linearly with the square root of the field. The dark current is determined by the number of free carriers in the bulk and may be described in terms of thermally assisted tunnelling.

Radiation Sensitivity of Silicon Imaging Sensors on Missions to the Outer Planets

An investigation of radiation damage mechanisms and the magnitude of their effects on the operating characteristics of imaging sensors utilizing a mosaic array of silicon diodes in a television pickup tube is reported. The effects produced by bombardment of bare-silicon-diode arrays or vidicon tubes with 85kV x-rays, Cobalt 60 gamma rays, 1 MeV and 11 MeV electrons, 3 MeV and 142 MeV protons, and reactor neutrons are presented. Results show that the dark (reverse bias) current of a diode array increases less than a factor of 2 if the fluence level is less than 104 rads of gamma radiation or 4 ×1011 1 MeV electrons/cm2. For these same dose levels, the quantum efficiency increases rather than decreases. Continued bombardment in excess of these fluences decreases the quantum efficiency and increases the dark current. Electrons of 11 MeV were more damaging than 1 MeV electrons by a factor of 10 to 20 while protons of 3 MeV more effectively produced dark current increases and quantum efficiency degradation in silicon diode arrays than 1 MeV electrons by a factor of 105 in fluence. Protons of 142 MeV were less effective than 3 MeV protons in producing the same effects by a factor of 102 in fluence. Measurements of these degradation effects at 217 K indicated that the dark current is reduced by a factor of 200 to 500 relative to room-temperature values.

Soft X-ray effects upon silicon-diode arrays aged in camera tubes

The rate of dark current increase of silicon-diode-array camera- tube targets has been found to be as high as 10 nA/h when operated with electrode potentials as high as 1000 volts. Irradiation of the target by soft X-rays is shown to cause the fast-state density at the silicon-silicon dioxide interface to increase. The increased fast-state density is largely responsible for the increased dark current. The X-rays are generated by the impact of the returning electron beam upon high-potential tube electrodes. Several means for reducing the X-ray flux at the target are discussed. The most effective means is a reduction in tube electrode potentials to low values where dark current aging rates as small as 0.001 nA/h have been achieved.

Ruggedized ceramic vidicon

A one-inch vidicon capable of withstanding sterilization treatments and severe environmental testing has been developed for space applications. The tube uses electrostatic focusing and magnetic deflection provided by photoetched deflection coils. The brazed ceramic-and-metal structure has an indium-sealed faceplate and is potted in a magnetic shield. The sterilization tests, consisting of ethylene-oxide exposure and a prolonged 135°C. dry-nitrogen bake, cause some increase in dark current and a small gain in sensitivity of the slow-scan photoconductor. The tube operates satisfactorily after undergoing high-amplitude vibration tests (e.g., 35 g at 2000 Hz and 25 g rms wide-band noise) and after 60 halfsine shock pulses having a 3000-g amplitude) 30 with 100-µ sec rise time, the other 30 with 225- µsec rise time).

A Miniaturized Semiconductor Radiation Detector

Small cylindrical p-i-n semiconductor radiation detectors have been fabricated for use with internally administered radioisotope tracers and for internal dose measurements. The p-i-n semiconductor (1–4) radiation detector, because of its wide depletion region, has adequate sensitivity for either beta or gamma radiations. For this application, such detectors have been fashioned into cylinders having both diameter and height of 2 mm. Increased sensitivity may be achieved by increasing the height of the cylinder. The cylinders have been incorporated into polyethylene 4-foot catheters with an outside diameter of 3 mm.; these are sealed at the detector end and have standard microdot connectors at the other end. The connectors can be joined directly to BNC outlets through the use of adapters. Two types of catheters have been built. In one, only the p-i-n detector is incorporated in the catheter, while in the second, a microminiaturized emitter-follower and preamplifier are also incorporated. The present configuration of the electronics has a diameter of 4.1 mm., which can easily be reduced by a factor of two. The length of the emitter-follower is 5.3 mm. and that of the two-transistor preamplifier is 2.2 cm. The advantage gained through the use of the microminiaturized electronics is that a greater signal pulse height is achieved, thereby increasing the absolute sensitivity of the probe. The response of the detectors to radiation has been recorded both by counting individual pulses and by recording the change in dark current when operated with a back bias. The pulses recorded when exposed to a Co60 or Cs137 beam have a height of about ten times those due to noise in the case of the detector only. The increase in dark current produced when a back bias of 10 v is applied is 0.1 microampere for an approximate dose rate of 100 r/minute air. Through the use of a potentiometer circuit the dark current may be balanced out and the system may then be used for very low dose rates. Of course, pulse counting gives the ultimate in sensitivity, but, due to the situation of detector or electronics, one is limited in the range of high dose rates to about 1 r/minute. The magnitude of the back current is linear with dose rate in the range of 1 to 100 r/minute when a 100 kvp x-ray beam (h.v.1. = 1.5 mm. A1) is used.

The Effect of Oxygen Adsorption on Photo- and Semiconduction of β-Carotene

The presence of adsorbed oxygen gas increases the magnitude of the surface semiconduction currents in crystalline powder β-carotene by a factor of about 103. The activation energy for the surface semiconduction process is 1.52 ev in argon gas and is decreased to 1.29 ev in oxygen. The room temperature photo-current is increased by a factor of about 25 in oxygen over the value in argon. The activation energy of the photoconduction process is not affected by oxygen. The increase in dark current in an oxygen ambient requires a time of about 2 hr to come to the equilibrium value at room temperature. Radiation is not required to cause the increase. The effect of the oxygen can be reversed completely by heating the β-carotene to 120°C in an inert atmosphere (dry nitrogen or argon). The increase in dark current is linearly proportional to the partial pressure of oxygen in the chamber. The rate of desorption of the oxygen increases with the temperature. The resulting data yield an activation energy of desorption of 0.17 ev. It is suggested that this is the binding energy for the formation of an oxygen-carotene complex, and that the decrease in semiconduction activation energy is correlated to this binding energy. An analysis is given which leads to the conclusion that the mechanism of the current increase with formation of the oxygen complex is twofold; a decrease in the activation energy for semiconduction, and an increase in the mobility of the charge carriers. There is no evidence for a change in the charge creation process. The experimental results are considered to be a partial verification of Platt's theory on the donor-carotene-acceptor complex in photosynthesis since the triplet level of chlorophyll b has a value of 1.43 ev, midway between the value of 1.52 ev for the free carotene semiconduction activation energy and 1.29 ev for the corresponding value of the oxygen-carotene complex. It is also suggested that a possible mechanism for the carotene protection of cells from destructive photo-oxidation may be the formation of the oxygen-carotene complex.

Some Electrical Properties of Amorphous Selenium Films

The dark current and photocurrent characteristics of amorphous selenium and arsenic-selenium alloy films have been investigated. One millisecond after the application of an electric field, the dark current (neglecting capacitance charging) is several orders of magnitude higher than the steady-state dark current. A hysteresis effect is observed in the dark volt-ampere curves with the descending voltage branch exhibiting an exponential volt-ampere relationship. The voltage of a charged Se plate in the dark discharges logarithmically with time as anticipated from the exponential volt-ampere relationship. Upon admitting air to a freshly evaporated Se plate, a negative surface potential of 150 mv is observed. The phenomenon of ``fatigue'' (i.e., the temporary increase in dark current following an exposure to illumination) obtained with highly absorbed radiation indicates surface barrier effects exert a large measure of control over dark currents. Photogenerated holes and electrons in Se films are trapped with trapping parameters such that the hole range/unit field is on the order of 10−8 cm2/v, and the electron range is less than 10−9 cm3/v. The addition of As to the Se greatly increases the electron effective lifetime. Trapped holes are eliminated in the dark exponentially with time through a process having an activation energy of 0.77 ev. A 40-mv negative surface photovoltage is observed in Se films upon illumination. This voltage may be accounted for by hole diffusion away from the surface. The addition of As to Se extends the spectral response into the red end of the spectrum, results in the initiation of secondary photocurrent effects, intensifies fatigue effects, and increases the softening temperature of the film.

A Mechanism for Water Induced Excess Reverse Dark Current on Grown Germanium NV-P Junctions

An ionic conduction process in the multilayers of water adsorbed on a germanium n-p junction bar is proposed to explain the increase in reverse dark current observed when the relative humidity to which the unit is exposed is increased. Experimental evidence supporting this postulate is given and discussed.

Loss and Restoration of Photoconductivity in Red Mercuric Iodide

Single crystals of red mercuric iodide, which normally display photosensitivity when precipitated, lose their photosensitivity on aging. This loss is accompanied by a change from the mono- to the polycrystalline fibrous state with considerable randomness of orientation of the individual crystallites. The nonsensitive crystals can be resensitized by subjecting them to an electric field for a time of the order of a few minutes. The increase in photosensitivity is accompanied by a decrease in the dark current. These correlated phenomena may not occur till after the lapse of an appreciable time after the field is first applied. The rate of decrease of the dark current with time is greater the lower the temperature. The data also suggest, but do not conclusively demonstrate, that the dark current increases with decreasing temperature. The rate of resensitization and the magnitude of the attained photosensitivity are found to increase with (a) increase in applied field strength and (b) decrease in temperature. When the light is turned on the photoelectric current is found, under certain circumstances, to rise briefly to a maximum and then descend to a permanent value at which the total current is equal to the value of the dark current prevailing when the voltage was first applied.