US 3825807 A
A radiation responsive device is disclosed having a barrier generating material, such as a dye or metal, interposed between two semiconductive elements, at least one of which is radiation penetrable. Metal layers are associated with the outer faces of the semiconductive elements so that the device can be biased by an externally applied potential. Radiation absorbed adjacent the barrier generating material produces a current gain in the forwardly biased device. The device can be fabricated by successively vacuum depositing upon a semiconductive element surface a layer of a barrier generating material, a semiconductive overlayer and a metal layer. The metal layer can be chosen to form a rectifying junction with the semiconductive overlayer. The barrier generating material is chosen to exhibit a surface photovoltage when applied to the surface of one of the semiconductive elements.
Claims available in
Description (OCR text may contain errors)
United States Patent 1191 Wolf a HIGH GAIN BARRIER LAYER SOLID STATE DEVICES Edward L. Wolf, Brockport, N.Y.
Eastman Kodak Company, Rochester, NY.
22 Filed: Jan. 15, 1973 21 Appl. No.: 323,607
References Cited UNITED STATES PATENTS 5/1966 Rose....
5/1966 Rose 317/234 8/1971 Jones... 96/1.6
OTHER PUBLICATIONS Nelson, Journal of the Optical Society of America, Vol. 46, No. 1, January 1956.
[111 3,825,807 1 51 Jul '23', 1974 Primary Examiner-Martin I-I. Edlow Attorney, Agent, or Firm-'Carl 0. Thomas [5 7] ABSTRACT A radiation responsive device is disclosed having a barrier generating material, such as a dye or metal, interposed between two semiconductive elements, at least one of which is radiation penetrable. Metal layers are associated with the outer faces of the semiconductive elements so that the device can be biased by an externally applied potential. Radiation absorbed adjacent the barrier generating material produces a current gain in the forwardly biased device. The device can be fabricated by successively vacuum depositing upon a semiconductive element surface a layer of a barrier generating material, a semiconductive overlayer and a metal layer. The metal layer can be chosen to form a rectifying junction with the semiconductive overlayer. The barrier generating material is chosen toexhibit a surface photovoltage when applied to the surface of one of the semiconductive elements.
16 Claims, 11 Drawing Figures PATENTEnJuLzamm I SHEET 3, 0F 9 kmwq ROOM L /GH7' DARK 6/. v
- SHEET-7M9 I lLLU/l l/A/A 7'70/V 30 DARK ROOM L/GHT PATENTEUJULZSISH 3.825.807 SHEET 9 OF 9 9mm EU N This application is a continuation-in-part of my copending application Ser. No. 230,266, filed Feb. 29,
This invention is directed to a radiation responsive device capable of modulating current flow therethrough as a function of incident radiation. This invention is also directed to a process for producing such a radiation responsive device.
A variety of photoconductive and semiconductive photoresponsivedevices are known to the art. These devices are structurally dissimilar from the radiation responsive devices of the present invention, and exhibit fabrication, structural and performance disadvantages not encountered in the practice of the present invention. Considering first a very simple conventional photo-diode, this device is comprised of a semiconductive element incorporating a P-N or P-I-N junction. When a photon strikes the junction a majority and a minority. charge carrier becomes available for current conduction; however, the photo-diode exhibits no inv ternal gain capability. The avalanche photo-diode overcomes the gain limitations of the simple photo-diode by choosing a junction containing monocrystalline semiconductive element that can be reverse biased to near its break-over (or avalanche) voltage and photoavalanched without destruction. The avalanche photodiode, being monocrystalline, exhibits size limitations and additionally requires to withstand photo-avalanche without destruction, a semiconductive element that is comparatively difficult and expensive to prepare. The
photo-transistor, which is simply a transistor adapted for base modulation by incident light, is a photosemiconductor capable of achieving limited gain. Photo-transistors are also monocrystalline devices-- -hence size limited, and, like the avalanche photodiode, comparatively difficult and expensive to form. All of these photo-semiconductors utilize a single semiconductive element incorporating one or more junctions formed therein. When used in the manner intended for photo-detection, one junction is reverse biased which must be capable of substantially blocking current flow in the absence of incident light.
Semiconductor-metal barrier layer photoconductive devices, more simply designated barrier layer photorectifier devices, are provided with a rectifying junctionat a metalsemiconductor interface rather than within the semiconductive element. These devices can be forward biased, but are more commonly reverse biased.
It is an object of this invention to provide a device that utilizes a novel radiation responsive mechanism for the modulation of current therethrough.
It is another object to provide a radiation responsive device of high sensitivity and gain capabilities.
It is still another object to provide a radiation sensitive device operable with comparatively low applied voltages. More specifically, it is an object to provide such a device which requires no significant reverse blocking capability and which can be utilized in a forward biased mode.
It is a separate object of this invention to provide a process for preparing such a radiation responsive device.
Only one semiconductive element is required to permit photodetection.
Photoconductor devices are known which produce a current gain in response to incident light. These devices differ from the photo-semiconductor and barrier layer photorectifierdevices above described in that they typically do not rely for operability on the presence of a junction. Significant gain is achieved simply by providing a photoconductive layer whose resistivity varies as a function of incident light. As distinguished from the photoresponsive devices noted above which modulate current flow at a junction or interface, photoconductor devices rely for current modulation on a photoresponsive effect that is distributed throughout the photoconductive layer.
The objects of this invention are accomplished in one aspect by providing a radiation responsive device comprising radiation penetrable metal layer means and radiation penetrable first semiconductive means of a first conductivity type conductively associated with the metal layer means. Second semiconductive means of a like conductivity type are conductively associated with an electrode means. Means are interposed between the first and second semiconductive means and cooperate therewith to limit current between the metal layer means and the electrode means as a function of incident radiation penetrating the metal layer means and the first semiconductive means.
In another aspect, this invention is directed to a method of making a radiation'responsive device comprising providing a substrate of a semiconductive material of a first conductivity type capable of transporting majority charge carriers in the presence of an applied electrical field. On the substrate is deposited a barrier generating material capable of initially accepting the majority charge carriersand capable of releasing the majority charge carriers in response to absorbed incident radiation. The barrier is overcoated with a radiation penetrable semiconductive overlayer also of a first conductivity type and also capable of transporting the majority charge carriers in the presence of an applied electrical field, and the overlayer is coated with a radiation penetrable conductive layer.
My invention may be better understood by reference to the following detailed description considered in conjunction with the drawings, in which FIG. 1 is a schematic diagram of a radiation responsive device according to this invention,
FIG. 2 is a schematic circuit diagram,
FIG. 3 is a plot of current versus voltage for three levels of illumination,
FIGS. 4 and 5 are plots of conductance versus voltage for two levels of illumination,
FIG. 6 is a plot of current versus voltage for three levels of illumination,
FIG. 7 is a plot of conductance versus voltage,
FIGS. 8, 9 and 10 are plots of current versus voltage for three levels of illumination, and
FIG. 11 is a log scale plot of current density versus illumination. I
In FIG. 1 a radiation responsive device 1 according to the present invention is schematically illustrated. For ease of comprehension and viewing the dimensions of the elements of the device have been exaggerated in width and are not drawn to scale. Electrode 3 lies in ohmic conductive contact with one major face 5 of the semiconductive element or substrate 7. A barrier generating material 9 overlies the remaining major face 11 of the semiconductive substrate. A radiation penetratable semiconductive element or overlayer l3 overlies the barrier material. The semiconductive elements are chosen to be of like conductivity type and are preferably comprised of similar semiconductive materials. A radiation penetrable metal layer overlies the semiconductive overlayer in conductive contact therewith. The metal layer forms a conductive interface 17 with the semiconductive overlayer that is preferably a rectifying junction. An electrode 19 lies in conductive contact with the metal layer.
To place the radiation responsive device 1 in operation, it is placed in an electrical circuit as is schematically indicated by leads 21 and 23 attached to elec' trodes 3 and 19, respectively. The device is preferably placed in a circuit supplying a d-c bias; however, an a-c or variable d-c bias can also be provided. Where the interface 17 between the metal layer 15 and semiconductive overlayer 13 forms a rectifying junction it is necessary to forward bias the junction. Where an a-c bias is provided it is only necessary that the junction be forwardly biased over some portion of a cycle. Where the metal layer' and semiconductive overlayer do not form a rectifying junction the polarity of applied potential is not material to the practiceof the invention, since the device responds to both directions of bias as if forward biased.
It is preferred to forward bias the device with an applied potential which is less than the conduction band gap potentiali.e., the potential difference between the conduction and valence bands-of the semiconductive elements present in the device. It is generally preferred to utilize forward biasing potentials of from 0.5 to 2.0 volts, although somewhat greater latitudes for biasing can be available, depending upon the particular semiconductive materials employed.
So long as the quantity of radiation reaching the barrier generating material 9 of the forwardly biased radiation responsive device is limited, very low levels of current are permitted by the device. Without intending to be bound by any particular theory of operation, it is believed that the behavior of the radiation responsive device can be explained as follows: The semiconductive substrate 7 and the semiconductive overlayer 13, being of like conductivity type, are provided with sufficient impurities to allow current conduction through the transport of like majority charge carriers in the presence of the electrical field provided by the forward bias rier generating material creates a space charge at the interface of the semiconductive elements and extending into the depletion layer of each of the semiconductive elements. The space charge effectively provides a barrier potential extending into the depletion region of the semiconductive elements that repels majority charge carriers which under the influence of the externally applied potential would otherwise be transported through the semiconductive elements between the electrode 3 and the metal layer 15.
The barrier generating material by reason of being decoupled from the electrode 3 and the metal layer 15 by the majority charge carrier depleted portions of the semiconductive elements behaves as though it lies at a potential which need not be intermediate between that of the metal layer and the electrode 3. If either of the semiconductive elements (and therefore their depletion regions) are omitted, the potential of the barrier generating material will be clamped to the potential of the adjacent electrode, and the device will no longer function to limit current as a function of incident radiation.
When the device is exposed to radiation as schematically illustrated by arrows 25, the radiation penetrates the metal layer 15 and the semiconductive overlayer 13 to impinge upon the barrier material and to penetrate the semiconductive substrate to a depth corresponding to the reciprocal of the absorption coefficient of the semiconductive material forming the semiconductive substrate. A very small amount of radiation incident upon the barrier material produces a large increase in current through the device between the electrodes 3 and 19 and in the leads 21 and 23 of the external circuit.
Again, without intending to be bound by any particular theory of operation, it is believed that the incident radiation absorbed adjacent the barrier generating material depletes trapped majority charge carriers. The trapped majority charge carriers are believed to be neutralized by minority charge carriers generated within the semiconductive elements by the absorbed incident radiation. The minority charge carriers are, of course, of opposite polarity to the space charge and therefore attracted to the barrier generating material. The physical effect can be viewed as a diminution of the space charge and a resultant reduction of the barrier potential. Thus, with the barrier potential reduced, majority charge carriers flow between the electrode 3 and the metal layer 15.
The semiconductive elements incorporated within the radiation responsive device of this invention can be of either N or P conductivity type and can be either monocrystalline or polycrystalline. The semiconductive elements can be of any conventional type known to the art. It is preferred to utilize semiconductive compounds of elements chosen from Groups ll and VI of the Periodic Table of elements, such as cadmium sulfide, cadmium selenide, zinc oxide and the like. It also contemplated to use semiconductive compounds of elements from Groups III and V of the Periodic Table of elements, such as gallium arsenide and similar semiconductive compounds. It is preferred to employ the same semiconductive compound to form both of the semiconductive elements.
Since the semiconductive element 7 is not required to be either monocrystalline or radiation penetrable, its dimensions can be varied widely in the practice of this invention. The semiconductive element 7 is preferably of a thickness greater than the width of the depletion layer fonned in use of the radiation responsive device for best performance, although this is not required. The depletion layer is typically a few thousand Angstroms in thickness, and frequently ranges from to 10,000 Angstroms in thickness, depending on the impurity concentration of the semiconductive element and the barrier material chosen. The semiconductive element or overlayer 13 is preferably polycrystalline and therefore can be widely varied in its areal dimensions. The semiconductive overlayer is chosen of a thickness to allow radiation to be transmitted to the barrier layer. The thickness of the semiconductive overlayer is preferably lessthan the reciprocal of the absorption coefficient for the semiconductive material from which it is formed. For ll-VI semiconductive materials, such as cadmium sulfide, the semiconductive overlayer is preferably in the range of from 25 to 70 Angstroms in thickness, but can be up to 500 Angstroms in thickness without excessively attenuating the transmitted radiation.
The barrier generating materials useful in the practice of this invention are those capable of accepting majority charge carriers from the semiconductive elements and releasing majority charge carriers as a function of absorbed incident radiation. These barrier generating materials exhibit a measurable surface photovoltage on the semiconductive substrate, preferably of at'least 0.1 volt and, most preferably, of at least 0.5 volt.
As employed herein the term surface photovoltage refers to the change in the contact potential difference that occurs when a layer of a barrier generating material present on a semiconductive substrate surface is initially dark and then subjected to actinic radiation.
' The contact potential difference is the difference between the potential at the surface of the semiconductive substrate in contact with the barrier generating material and the bulk of the semiconductive substrate. Barrier generating materials useful with N conductivity type semiconductive substrates are relatively electron accepting and therefore trap electrons to render the contacted surface of the semiconductive substrate more negative than the bulk of the semiconductive substrate. On the other hand, barrier generatingmaterials useful with P conductivity type semiconductive substrates are relatively electron donating, thus effectively trapping holes (positive charge carriers) to render the potential at the contacted surface of the semiconductive substrate more positive than in the bulk thereof. On subsequent radiation exposure barrier generating materials for N conductivity type semiconductive substrates become less negative and produce less negative contact potentials while barrier generating materials for P conducivity typesemiconductive substrates become less positive and produce less positive contact potentials.
The procedure for measuring contact potential differences is well known to those skilled in the art. A suitable procedure for determining contact potential differences utilized in determining the surface photovoltages of the barrier materials of this invention is described in an article titled Contact Potential Measurements on Clean CdS Surfaces, by C. L. Balestra and H. C. Gates in Surface Science, Volume 28, pages 563 To illustrate a surface photovoltage' determination technique which is preferred, a semiconductive element corresponding to the semiconductive substrate to be utilized in forming a radiation responsive device is mounted in a vacuum chamber above a sublimitation source for the barrier generating material and in proximity to a vibrating reference electrode, preferably of gold or platinum. The barrier generating material is deposited on a surface of the semiconductor substrate in a substantially monomolecular layer, giving substantially complete coverage of a surface of the semiconductive element while keeping the reference electrode free of barrier generating material. The contact potential difference between the reference electrode and the barrier generating material is then determined according to the procedure of the above article both while the barrier generating material is darkand after exposure of the barrier generating material to actinic radiation. The change in the contact potential difference provides a direct measure of the surface photovoltage.
Barrier generating materials exhibiting surface photovoltages which are particularly suited to the practice of this invention can be chosen from among known sensitizing dyes, such as pyrylium and thiapyrylium dyes, cyanine and merocyanine dyes; mono-methine cyanine dyes;-nitro-substituted cyanine dyes; pyrylium cyanine dyes; pyrrole dyes; pyrimidinedione dyes; imidazopyridine or imidazothiazole dyes; pyrrole-(2,3- b) quinoxaline dyes; nitro-substituted carbazole derived dyes; dyes containing desensitizing nuclei; desensitizing dyes used for reversal 'silver halide systems; dyes which desensitize negative silver halide systems; triarylmethane dyes, such as Rhodamine B and Crystal Violet; diarylmethane dyes; azine dyes, such as phenosafranine; anthraquinone dyes; formazan dyes; azo dyes; acridine dyes; xanthene dyes; phthalein dyes; and the like as well as dye mixtures. It is particularly preferred to choose dyes exhibiting surface photovoltages from among known sensitizing compounds for photoconductive compounds. For example, photosensitizers for 1I-V1 compound photoconductors can be selected which produce excellent surface photovoltages on I1-Vl compound semiconductive substrates employed in the practice of this invention.
Exemplary of dyes within the categories above noted are those disclosed in the following issued patents: Fox
and Jones U.S. Pat. No. 3,597,196; Davis et al. U.S. Pat. No. 3,141,770; VanAllan et al' U.S. Pat. No. 3,250,615; Webster and Heseltine U.S. Pat. No. 3,565,616; Carpenter, Mee and Heseltine U.S. Pat. No. 3,542,548; Brooher and Webster U.S. Pat. No. 3,565,615; British Pat. No. 964,873; Brooher, l-leseltine and Daniel U.S. Pat. No. 3,579,346; U.S. Pat. Nos. 3,560,207 and 3,560,208; U.S. Pat. No.'3,549,362; U.S. Pat. No. 3,542,548; U.S. Pat. No. 3,579,331; Belgium Pat. Nos. 695,364 and 695,366;'Belgium Pat. No. 705,117; U.S. Pat. No. 2,610,120; U.S. Pat. No. 2,670,286; U.S. Pat. No. 2,670,287; US. Pat. No. 2,732,301; Stewart U.S. Pat. No. 3,110,591; Kendall and Stewart U.S. Pat. No. 3,128,179; and Jones and Stewart U.S. Pat. No, 3,121,008.
Other materials useful as barrier generating materials are those metals known to form majority charge carrier trapping sites when diffused into semiconductive elements. Typical metals of this type include gold, platinum, silver and copper. As one alternative the semiconductive elements can be doped with these metals adjacent the barrier generating material. For example, after forming a sandwich of semiconductive elements and barrier generating material, this composite can be heated to drive a portion of the metal into the semiconductive elements.
In its preferred form the barrier generating material forms a barrier layer within the radiation responsive device that is a single molecule in thickness. Layer thicknesses of up to about molecules are contemplated. The barrier generating material need not be present over the entire major face 11 of the semiconductive substrate in order to allow the device to exhibit radiation responsive current gain. Current gain can be obtained when as little as 1 percent of this major face is provided with barrier generating material. It is generally preferred for the barrier material to cover at least 25 percent of the major face of the semiconductive substrate, and in the preferred form of the invention the barrier generating material substantially uniformly covers the entire major face of the semiconductive substrate.
The metal layer can form either an ohmic or a rectifying conductive contact with the semiconductive overlayer 13. It is preferred that the metal layer be chosen to form a rectifying conductive contact with the semiconductive overlayer; since this permits a potential barrier of greater magnitude to be formed by the barrier layer. Consequently larger forward bias potentials can be applied to the device without driving the device into conduction in the absence of incident radiation. Any metal or combination of metals can be used to form the metal layer known to be useful in forming ohmic and/or rectifying contacts to semiconductive elements. Preferred metals for forming rectifying contacts with the semiconductive overlayer are comparatively noble metals, such as gold, silver, copper, platinum and palladium. The metal layer, if desired, can be a composite of several layers of like or dissimilar metals and, as employed herein, the term metal layer is intended to be inclusive of such composite layers. The metal layer is chosen to be of a thickness so that it remains penetrable to incident radiation. In its preferred form the metal layer is a substantially transparent layer of from about 100 to about 600 Angstroms in thickness, and, more preferably, from about 200 to about 300 Angstroms.
The electrodes 3 and 19 schematically shown in FIG. 1 can be formed according to conventional practices of a single metallic element or as a composite of metal layers and elements, as is well understood in the art. In most instances the electrode 3 includes one or more metallic contact layers at the major face 5 of the semiconductive element 7. As is conventional practice for radiation receiving devices, the electrode 19 can be areally extended to produce a pattern or array covering portions of the outer surface of the metal layer 15. In some modes of use it is desirable to form a conductive contact with the metal layer 15 using a fluid electrolyte. In this instance the electrode 19 is formed by the electrolyte, or, alternatively viewed, the electrode 19 can be viewed as spaced from the metal layer '15 by a fluid electrolyte. In this instance the electrolyte and electrode 19 together form an electrode lying in conductive contact with the metal layer.
In FIG. 1 no housing or packaging for the radiation responsive device is shown. It is appreciated that the radiation responsive device can be housed or encapsulated using techniques well known to those skilled in the art. For example, the radiation responsive device of the present invention can employ the housing of any conventional photodiode or phototransistor. In a simple form the device can be encapsulated in an insulative plastic. Where the plastic is transparent it can cover the metal layer 15, although this is not essential.
A glass layer or sheet can be provided overlying the metal layer 15 and electrode 19, if desired. In protected environments, such as an inert atmosphere, no housing for the radiation responsive device is needed.
The devices of the present invention can be used to detect any source of actinic radiation. By proper choice of semiconductive materials the device can be specifically adapted for response to the ultra-violet, visible and/or infra-red portions of the spectrum. Or, alternately viewed, for a given radiation responsive device formed according to this invention a radiation source is chosen which delivers radiation within that portion or those portions of the spectrum which are readily absorbed by the semiconductive material. For example, relatively large band gap semiconductive materials, such as zinc oxide, readily absorb ultraviolet radiation while relatively small band gap materials, such as germanium, are particularly responsive to the infra-red spectrum. Cadmium sulfide devices respond well to the lower wavelengths of the visible spectrum.
Without intending to be bound by any particular theory of operation, it is believed that it is the incident radiation absorbed by the semiconductive material within' the depletion region and within a distance from the barrier generating material corresponding to the recipro cal of the absorption coefficient for the semiconductive material that principally determines the response of the devices according to this invention. It is in theory possible to have most of the incident radiation absorbed within the depletion region of the semiconductive overlayer so that little radiation penetrates the barrier generating material to reach the semiconductive substrate. It is preferred to have the incident radiation absorbed principally by the semiconductive subtrate, since the semiconductive substrates of the devices formed according to the process herein disclosed are believed to permit somewhat higher minority charge carrier mobilities. In practice it is believed that both the semiconductive elements absorb significant portions of the incident radiation within their respective depletion regions and therefore contribute to reducing the potential barrier to current conduction.
While any method known to those skilled in the art can be used to prepare radiation responsive devices according to the present invention, it is preferred to form the radiation responsive devices of the present invention according to an inventive procedure hereinafter set forth. A semiconductive substrate 7, which can be either polycrystalline or monocrystalline and which can, optionally, have attached thereto the electrode 3, is placed in a vacuum chamber. A barrier material source, a semiconductive material source and, optionally, a metal layer source are also located within the vacuum chamber. By preferred practice the barrier material is chosen to sublime or otherwise vaporize in the vacuum chamber on heating to form a thin molecular barrier layer of from I to 5 molecules in thickness over some or all of one major surface of the semiconductive substrate. Immediately thereafter, while the barrier layer coated substrate is still in the vacuum chamber, a thin, radiation penetrable overlayer of the semiconductive material is deposited on the barrier. The semiconductive material is preferably deposited by being heated so that it can be vapor deposited. However, other deposition techniques, such as sputtering and the like can also be employed. The metal source is then activated to form the metal layer by vacuum deposition,
. 9 sputtering or the like. Subsequent attachment of the electrode 19 to the metal layer, attachment of the electrode 3 and/or' packaging can all be accomplished by techniques generally known to thoseskilled in the art.
While the radiation responsive device has been specifically described as capable of receiving radiation through only one of its semiconductive elements, it is appreciated that the radiation responsiveness of the device can be further increased by forming the device so EXAMPLE 1 A cadmium sulfide-Rhodamine B-cadmium sulfidegold radiation responsive device is fabricated according to the following procedure: An N conductivity type cadmium sulfide element corresponding to the semiconductive substrate 7 in FIG.] is chosen which is monocrystalline and has a resistivity of approximately 0.4 ohm-centimeters, an electron mobility of approximately 300 cm /volt-second and a carrier concentration' at -300K of X cm. The semiconductive substrate is provided with two parallel, square major faces and exhibits dimensions of 0.635 cm on an edge and is 0.159 cm in thickness, The major faces lie in the 0001 or A face of the cadmium sulfide crystal.
The semiconductive substrate is etched for thirty seconds in a solution consisting of equal parts by volume of concentrated hydrochloric acid, glacial acetic acid and distilled water, thenrinsed in distilled water and acetone. The semiconductive substrate is blotted on soft paper, dried and immediately mounted in a liquid nitrogen trapped diffusion pump evaporator with an etched major face approximately 5.8 cm-above'an' elec trically heated evaporation boat (R. D. Mathis type 819C) containing a few milligrams of purified Rhodamine B and approximately 381cm above an electrically heated baffled box" evaporation source containing large crystals of UHP grade cadmium sulfide. The heating elements of both evaporation sources are formed of tantalum and are outgassed by heating after being filled with a charge of the material to be evaporated. The evaporation system is closed and evacuated to between 4 and 5 X 10 torr. over a period of several hours. Sublimation of a barrier layer of approximately 4.5 Angstroms of Rhodamine B (between one and two molecules in thickness) onto the etched major face of the substrate is initiated by electrically heating the evaporation boat containing this material. The temperature of the Rhodamine B source is monitored with a chromel-alumel thermocouple. The first evidence of a deposit appears at an indicated temperature of 130 to 150C. The thickness of the barrier layer is determined by a frequency change of about 8 hertz-on .a 5 megahertz quartz crystal thickness monitor located about 7.6 cm from the dye source. No rise in pressure is measuredduring sublimation of the dye.
Next, an estimated Angstroms of cadmium sulfide is evaporated onto the barrier layer to form the semiconductive overlayer. This evaporation is carried out slowly at approximately 0.5 Angstromsper second and typically results in a pressure rise in the evaporation system of from about 4 X 10 to about 7 X 10. torr.
After allowing the evaporation sources to cool, dry nitrogen gas is introduced into the evaporation system and it is then opened. The sample is lightly clamped to a stainless steel screen with an array of 0.05'cm diameter holes so that the cadmium sulfide overlayer is in contact with the screen. The screen and sample are again placed in the evaporation system about 32 cm above an evaporation boat which has been previously 4 outgassed and which contains gold. The screen allows deposition of a pattern of spaced 0.05 cm gold dots to be formedon the semiconductive overlayer surface. A
heat shield is located about 1.9 cm above the gold boat to reduce the radiant energy falling on the semiconductor overlayer surface. The system is then evacuated to approximately 10 torr. and 300 Angstroms of gold .is
deposited to form spaced dots on the semiconductive overlayer corresponding to the holes in the stainless steel screen. I 0
Referring to FIG. 2, the device is mounted in the circuit schematically shown for the purpose of measuring and plotting its current and voltage (I-V) characteristies as a function of incident radiation. The circuit of gallium liquid eutectic layer at its interface with an element serving as electrode 3. Point 36 is in common with the lead Y1. Additional leads X2 and Y2 are provided between leads X1 and Y1; Leads X1 and X2 are connected to either side of the radiation responsive device to measure its I-V characteristics. Between leads Y1 and Y2 is disposed a resistor 40 to permit current measurement. Between points 34 and 36 is located a resistor-capacitor parallel network generally designated 42 and comprising resistor 44 and capacitor 46. In series with network 42 is another resistor 48. Between resistor 48 and network 42 is lead Y3. The Xl-X2 and Y1-Y2 terminals are connected to the X and Y axis inputs of a conventional X-Y recorder. AC voltage source 30 and variable DC potential source 32 apply voltage to points-34 and 36 within a range of about i 1.5 volts.
In dI/dV measurements, the AC voltage at YlY2, which is in synchronous phase with the AC voltage source 30, is detected with a lock-in detector. The output of the detector is plotted on the Y axis of. the X-Y recorder. In an exemplary reading, the frequency of source 30 is 2.3 kilohertz, although the AC voltage applied to the device 1 can be varied. The capacitive admittance of the device can be measured by setting'the lock-in detector to respond to the out-of-phase voltage across Yl-Y2. Measurements of small changes in conductance or capacitance induced by light are made in an AC bridge configuration by using the lock-in detector on the AC voltage between Y2 and Y3. The lock-in detector can be used as a null detector and by changing the phase control of the lock-in detector both the inphase and out-of-phase null conditions can be precisely determined, allowing an accurate bridge measurement of the conductance and capacitance of the device 1.
The spectral response of the device 1 is confirmed by focusing light from the exit slit of a monochromator such as the Eastmen Kodak Company Monochromator Model 2, Ser. No. l with all slits at 1 mm using a conventional tungsten light source. The device is noted to be radiation responsive over the entire spectral range of from about 4,800 to 6,500 Angstroms investigated.
FIG. 3 shows the I-V characteristics of the device 1 as fabricated above and forward biased so that the gold contact is positive with respect to the electrode 3. Curve A illustrates the current and voltage relationship in the dark; Curve B illustrates this relationship at room illumination and Curve C illustrates this relationship under intense illumination obtained from a B&L microscope illuminator. The incident light intensities are estimated as 3.2 X I photons/cm sec and 5 X I0 photons/cm sec in Curves B and C, respectively. The gain of the device in electrons per photon is estimated by comparing Curves A and B at 1.7 volts. The increase in current-upon illumination is 0.9 X 10' amps or 5.6 X 10 electrons/sec. Since the junction area is 2 X 10' cm*, the number of photons incident per second on the junction in room light is 6.4 X l0 photons/sec. The gain is thus the quotient of the increase in current divided by the incident photons per second or 8.7 X 10 From other measurements it is believed that the metal layer 15 of the device allows only about 10 percent of the incident photons to reach the semiconductive overlayer. With a transparent metal layer, gain approaches 10 electrons per photon.
EXAMPLE 2 Two devices are made concurrently similarly as the radiation responsive device in FIG. 1, except that the substrate of only one of these devices, D, is placed in a position to receive the Rhodamine B dye coating. The semiconductive substrate of the non-dye device, ND, is placed just, below the 819C dye boat and a few centimeters laterally displaced so that it receives no dye. It is approximately 32 centimeters above the cadmium sulfide baffled box source as compared with the substrate of the D device, which is approximately 38 cm above the cadmium sulfide source. The ND device has a cadmium sulfide overlayer which has a thickness of 36 Angstroms as compared to 25 Angstroms for the D device.
FIGS. 4 and 5 contrast the differential conductance dI/dV curves at forward bias (gold electrode positive) and the radiation responsiveness of the ND and D devices, respectively. In FIG. 4 the lower curve is in the dark and the upper curve in room illumination of roughly 3.2 X l0 photons/cm sec. These curves are noted to be quite close together, indicating a photocurrent that is small as compared to the dark current. Note that the differential conductance at 0.4 volts is near 1.4 X 10" mho.
In FIG. 5 a generally lower conductance level is evident-e.g. about 1.5 X 10 mho in the dark at 0.4 volts, and, at the same time, the radiation responsiveness of the device D is much larger than that of the device ND. The upper curve 57 is obtained with room illumination while the lower curve 59 is obtained in the dark. The sawtooth pattern of the upper surve arises from the fact that as the voltage is being linearly increased with time at a rate of 0.25 volts/minute room light is periodically removed by covering the device with a dark cloth. Considering the behavior at V 1.5 volts, note that the conductance in room light is increased by a factor of about 7, that the conductance increase is relatively rapid, being completed in about 5 sec., and the decay, while having a rapid start has a much slower final component persisting for several minutes.
The l-V plot for the device D is shown in FIG. 6 for dark, room light, and illumination, as curves 61, 63 and 65, respectively. It is noted that the dark current at 1.0 volt forward bias is less than 1 percent of the current under strong illumination.
EXAMPLE 3 A radiation responsive device is fabricated with no cadmium sulfide overlayer or dye coating. It is prepared by cleavage of a bar of cadmium sulfide containing approximately 2 X 10 donors/cm in a vacuum of I 1.0 X 10 torr. during evaporation of gold onto the sample through a stainless steel screen containing 0.05 cm diameter holes. Since the crystalline surface is formed in the vacuum chamber, no etching is required. The resulting cadmium sulfide substrate has an intimate gold cadmium sulfide junction with no intervening oxide or other layer. Otherwise the device of this example is prepared similarly to that of Example 1.
The dI/dV characteristic of the Example 3 device is graphically represented in FIG. 7. This device appears to have no photoconductiveresponse at forward bias, apart from a small increase in current evidently due to heating during illumination.
EXAMPLE 4 A device is fabricated according to the method described with reference to Example. I,except that the dye is 4-(4-dimethylaminobenzylidene)-2-phenyl-2- oxazolin-S-one. Sublimation occurs upon the heating of the dye to about C. The I-V characteristics are shown in FIG. 8 with various illuminations.
EXAMPLE 5 A device is prepared using the procedure described in Example 1, except that the dye is 3-ethyl-5-[(3- ethyl-2-benzothiazolinylidene) ethylidene] rhodanine. Sublimation occurs upon heating to about 200C. The l-V characteristics are shown in FIG. 9.
EXAMPLE 6 A device is made using the method of Example 1, except that the dye is Rhodamine 6G. Sublimation occurs near C. The I-V characteristics are shown in FIG.
EXAMPLE 7 A radiation responsive device is made similarly as the device of Example I, using Rhodamine B, except that the overlayer is approximately 60 Angstroms in thickness. The I-V characteristics of this device in the dark and under illumination generally resemble those shown in FIG. 6, but show a greater increase in current in going from dark to room illumination. The increase in current density at 1.0 volt forward bias with illumination intensity L of 4970 Angstrom photons, making correction forthe absorption of the gold electrode, is
fications can be effected within the spirit and scope of the invention.
1 claim: 1. A radiation responsive device comprisingmetal layer means of a thickness permitting radiation penetration,
first semiconductive means of a first conductivity type conductively associated with said metal layer means, said first semiconductive means being of a thickness permitting radiation penetration,-
second semiconductive means of like conductivity electrode means conductively associated with said second semiconductive means, and
a sensitizing dye interposed between said first and second semiconductive means and cooperating therewith to limit current betweensaid metal layer means and said electrode means as a function of incident radiation penetrating said metal layer means and said first semiconductive means, said sensitizing dye exhibiting a surface photovoltage at its interface with said semiconductive means of at least 0.1 volt. v i 2. A radiation responsive device according to claim 1 in which said metal layer means forms a rectifying junction with said firstsemiconductive means.
3. A radiation responsive device according to claim 1 in which said metal layer means is a noble metal.
4. A radiation responsive device according to.claim l in which said metal layer means is formed of a metal chosen from the group consisting of gold, copper, platinum,.palladium and silver.
5. A radiation responsive device according to claim 1 in which said metal layer means is from 100 to 600 Angstroms in thickness.
6. A radiation responsive device according to claim 1 in which said first and second semiconductive means are of N 9 conductivity type.
7. A radiation responsive device according to claim 1 in which said first and second semiconductive means are comprised of a lI-VI compound.
8. A radiation responsive device according to claim l in which said first and second semiconductive means are formed of cadmium sulfide.
9. A radiation responsive device according to claim l in which said first semiconductive means is up to 500 Angstroms in thickness.
10. A radiation responsive device according to claim 1 in which said current limiting means exhibits a surface photovoltage at its interface with at least one of said semiconductive means of at least 0.5 volt.
11. A radiation responsive device according to claim 1 in which said barrier means is a triarylmethane dye.
first semiconductive means of N conductivity type forming a rectifying junction with said metal layer means, said first semiconductive means being of a thickness permitting radiation penetration,
second semiconductive means of N conductivity yp I electrode means conductivelyv associated with said second semiconductive means, and
barrier means interposed between said first and second semiconductive means and cooperating therewith to limit current between said metal layer means and said electrode means as a function of incident radiation penetrating said metal layer means and said first semiconductive means, said barrier means exhibiting a surface photovoltage at its interface with said semiconductive means of at least 0.1 volt.
16. A photoresponsive device comprising metal layer means of a thickness permitting radiation penetration,
N conductivity type cadmium sulfide overlayer forming a rectifying junction with said metal layer means, said overlayer being of a thickness permitting radiation penetration,
an N conductivity type cadmium sulfide substrate, electrode means forming an 'ohmic conductive contact with said substrate, and" a sublimable dye interposed between said substrate and said overlayer having a thickness of less than 5 molecules and exhibiting a surface photovoltage of at least 0.1 volt.