Color imaging charge-coupled array and method of imaging

Abstract: A multi-color imaging charge-coupled array (10) comprises a plurality of photosensitive layers (16,20,24), each sensitive to a specific range of wavelengths (B,G,R) as would be found in a full-color image. The various photosensitive layers (16,20,24) are separated by boundary layers (18,22,26) of high band-gap energy, so that charge packets formed within individual layers are insulated from charge packets formed in other layers within the same pixel. The combination of photosensitive layers (16,20,24) and high band-gap boundary layers (18,22,26) cause charge packets to be formed in potential wells within each pixel area of the charge-coupled array (10).

Description: The present invention relates to a charge-coupled array (CCD) having a multi-layer structure for imaging full-color images. Charge-coupled arrays (CCD's) are generally defined as layered structures for the selective retention of charge in specific areas thereof, wherein the structure itself may serve as a shift register for selective serial readout of data in the form of the charged areas. Discrete areas within the CCD are caused to store charge because the electrical potential is lower in the particular area holding the charge than in the surrounding material. When it is desired to read out the charges as a serial signal, the charge in each area can be transferred from one area to its neighbor by suitably changing the potential on it and its neighbor so that charge flows from one area to the next. By applying charges to a linear series of electrodes along the array, typically with some time period of overlap of charging adjacent electrodes, individual charge packets can be caused to move along the CCD. The system for transferring discrete charges from one area to the next until the charges are read out as a serial signal is colloquially known as the "bucket-brigade" method of readout. An early patent describing the shift register operation of a CCD is US-A-3,758,794. One special type of CCD is the photosensitive, or "imaging," CCD, wherein discrete areas of charge, which may be read out as serial signals, are initially created in the CCD by the imagewise focusing of light onto the structure. With certain materials, such as aluminum, gallium, and arsenic compounds and combinations in an ordered crystal structure, photons focused on the material will create electron-hole pairs in areas corresponding to the distribution of light in the image. Thus, the CCD forms a linear array of photosensors upon which narrow sections of an image can be recorded, while the CCD can output data related to the image. Such CCD's, particularly monochrome CCD's, are in common use in facsimile machines, digital copiers, and other scanners. Typical examples of such an imaging CCD are shown in US-A-3,971,003; US-A-4,658,278; and US-A-4,667,213. US-A-4,613,895 discloses an image sensing element in a solid state imaging device and provided with a plurality of superposed channels disposed at respective distances from a light receiving surface of the device, each of such channels having a different characteristic spectral response due to the differential absorption of light by a semiconductor. By so disposing the channels, the device becomes a color imaging sensor having optimized resolution. The top channel, i.e. the channel nearest the surface of the device, may be either a "surface" channel or a "buried" channel, the lower channel(s) being buried channels. Depending upon the design of the element, either electrons or holes may be accumulated as photocharges in respective superposed channels. The color photocharges generated in respective channels of such an image sensing element are simultaneously moved in a plurality of superposed channels by a multiple superposed channel signal handling device such as a multiple channel charge coupled device (CCD), thus the solid state imaging device does not require special timing networks to correct for phase differences between color signals which result from a common point within an image. US-A-5,138,416 discloses a multilayer color photosensitive element made from group III-V alloy semiconductors. Charge is collected from the photosensitive element according to an amount of time it takes for red, green and blue light to travel through the layers. No filters or extra memory or patterning of the layered semiconductor during growth is required to create full color separations on a pixel by pixel basis. In accordance with the present invention, there is provided a photosensitive charge-coupled array, defining a plurality of exposable image areas on a main surface thereof, comprising: a first photosensitive layer parallel to the main surface and comprising a material having a band-gap energy associated therewith to generate electron-hole pairs in an area thereof exposed to light within a predetermined first range of wavelength; a second photosensitive layer parallel to the main surface and comprising a material having a band-gap energy associated therewith to generate electron-hole pairs in an area thereof exposed to light within a predetermined second range of wavelength different from the first range of wavelength; a first boundary layer disposed between the first photosensitive layer and the second photosensitive layer and comprising a material having a higher band gap energy associated therewith than the material of the first photosensitive layer and the second photosensitive layer; and a series of electrodes disposed on the main surface, each electrode corresponding to an exposable image area, the electrodes being adapted for enabling the shifting of image data in the first photosensitive layer and the second photosensitive layer along the discrete exposable image areas within the array. Also according to the present invention, there is provided a method of deriving color component data of an image, according to claim 10 of the appended claims. Preferably, the step of applying a second compensatory voltage to the electrode at the end of the linear array, for causing the electron-hole pairs within the first layer to be accessed. Preferably, the method further comprises the steps of forming in the linear charge-coupled array a third layer adapted to form electron-hole pairs when exposed to light of a third wavelength, the third wavelength being unequal to the first and second wavelength, the third layer being separated from the first and second layer by a second boundary layer, the exposure step creating electron-hole pairs in the third layer in proportion to the components of light of the third wavelength received thereon. Preferably, the method further comprises the step of applying a third compensatory voltage to the electrode at the end of the linear array, for causing the electron-hole pairs within the third layer associated with an individual pixel to be accessed. In the drawings: Figure 1 is a cross-sectional, elevational view showing the layers of a linear imaging charge-coupled array according to the present invention; Figure 2 is a graph showing the energy levels of various layers within the charge-coupled array as a function of depth; and Figures 3A-C are a series of comparative graphs showing the effects of a compensatory voltage on the energies of various layers in the charge-coupled array in the present invention. Figure 1 is a simplified cross-sectional view of a portion of the multi-layer structure of a CCD according to the present invention. Section 10 shows the area around a number of discrete locations, representing individual pixels, in a linear array CCD. Each discrete area associated with a pixel has associated therewith an electrode disposed effectively on the surface thereof, such as shown as 12a, 12b, and 12c, with a last electrode in the row indicated as 12z. These electrodes may be transparent or otherwise arranged so as not to interfere with light being exposed onto the CCD 10 from the top of the diagram, as would be apparent to one skilled in the art. The layers of the section of the CCD, from the top surface downward, are as follows: a top insulative layer 13, a high bandgap layer 14, a blue detecting layer 16, a first large band gap boundary layer 18, a green detecting layer 20, a second large band gap boundary layer 22, a red detecting layer 24, a third large band gap layer boundary 26, and the bulk semiconductor 28 upon which all of the layers are ultimately disposed. Even though the electrodes 12a ... 12z are disposed on an insulating layer 13, the electric fields associated with each electrode pass through the insulating layer 13 in a manner known in the art. In an initial exposure step, such as in a facsimile machine or digital copier, the top of the CCD structure is exposed to multicolor light, as shown in Figure 1. One pixel of image data is assigned to each electrode 12a, 12b, etc. in the array. However, it should be noted that with each pixel will be data associated with each of the three primary colors; the image data will be information regarding a relative proportion of blue signal, green signal, and red signal from the particular portion of the image creating the pixel. The exposed light will penetrate into the multi-layer structure in such a way that, for each pixel, the blue component of the light will affect blue layer 16 of the structure, the green component will affect layer 20, and the red component will effect layer 24. In photosensitive imaging CCD's, the energy associated with light will act to create specific quantities of electron-hole pairs, or "charge packets," in the photosensitive layer of the structure. Thus, the corresponding layers sensitive to individual primary colors in the multi-layer structure of CCD 10 will, upon exposure to light, be energized (that is, charge packets of a certain charge magnitude will be created) to the extent that individual primary colors are present in the light in the particular pixel. It will be noticed, in Figure 1, that the light-sensitive layers in CCD 10 are arranged so that the layers requiring longer wavelengths of light, green and red, are respectively disposed deeper within the multilayer structure. For this reason, short-wavelength, high energy light such as blue will be absorbed in upper layers, while longer wavelength, lower energy light will pass through these upper layers and penetrate into the multi-layer structure to a layer wherein the particular wavelength of light may cause electron-hole pairs to be created. In this way, a color separation is possible for each pixel. Figure 2 is a graph showing the relationship of necessary electron voltage to create electron hole pairs in various layers of the multi-layer structure of CCD 10 as a function of the depth D of the particular layer. As can be seen, predetermined electron voltages are required for the blue, green, and red levels as the depth of the structure increases. Also significant is the fact that the intervening layers of high band-gap energy within the structure, that is, boundary layers 18, 22, and 26, will not create electron-hole pairs at even the highest energy of any of the photosensitive layers 16, 20, or 24. These high-band-gap boundary layers create barriers between the various photosensitive layers, which will create electron-hole pairs at lower energies. Thus, as can be seen in Figure 2, the photosensitive layers are disposed in, in an energy sense, "wells" so that the charge packets within each individual photosensitive layer will be electrically isolated by high band-gap layers, which border each layer on both sides thereof. The purpose of the "wells" is to prevent the creation of electron-hole pairs in any given photosensitive layer from flowing into adjacent photosensitive layers sensitive to other colors. In this way, each individual photosensitive layer in the structure may in effect operate as an independent CCD, as will be explained below. Once the CCD has been exposed, as in part of a cycle wherein one of a series of line images forming a complete image are exposed onto the linear array of the CCD in a line-scan imager, there begins a readout step in which the charge values in each of the photosensitive layers of the CCD are read out in the familiar "bucket-brigade" fashion to form a series of signals at the end of the CCD. In the illustrated embodiment, the series of electrodes 12a, 12b,... 12z may be connected to an external circuit (not shown) to apply appropriate voltages to discrete areas of the array; thus the electrodes 12a, 12b,... 12z enable the shifting of charge packets across the array. In the case of the multicolor CCD of the present invention, each photosensitive layer will act as an independent CCD and the readout step will result in three parallel data streams, each data stream representing one color of a given pixel. This bucket-brigade technique generally consists of applying a series of potentials to the electrodes such as 12a, 12b, and 12c in a sequence so that, by decreasing the potential to one side of a given pixel, the charge packet in one particular pixel area of the CCD will move over by the length of one pixel. This process is repeated, as is known in the art, until all of the image data, in the form of discrete charge packets associated with each pixel, is read out at the end of the CCD. In the case of the multicolor CCD of the present invention, this technique of applying voltages in sequence to various of the electrodes along the line of the CCD will, as mentioned above, result in three streams of charge packets moving in parallel across the respective layers of the CCD. That is, the stream of image data through photosensitive layer 16 will be read out as a series of the blue component of the pixels across the line, layer 20 will be the green components of each pixel, and so on. These three parallel data streams through the CCD can, in turn, be read out of the entire CCD in an order in which the color components of each pixel are read out as triplets of three charge packets each, each triplet representing the three primary color components for each pixel. This sequential readout of the pixels from the photosensitive layers in sequence may be carried out readily by applying to the CCD at the end of the linear array a series of compensatory voltages which will cause the potential high band gap barriers between the wells associated with each color in each pixel to be broken down to such an extent that the pocket of electron-hole pairs within each well can be "liberated" from the layer and to be measured and converted into a (typically digital) signal. In other words, the application of the compensatory voltage eliminates the effect of the high band gap barriers in order to access the separate charge packets within each photosensitive layer as the data is read out. At the end of the CCD array 10 beyond the last electrode 12z is an output terminal 32, in the form of a metallic insert which is disposed, as shown, in an opening formed in the insulative layer 13. The electronics which is necessary to provide the voltage sequence displayed in Figure 3 and associated detection electronics may be provided elsewhere on the substrate or off the CCD structure as part of the image-processing circuitry. Alternately, there may also be provided, as is known in the art of CCD's, an oppositely-doped layer (now shown) between the layer 14 and the output terminal 32, which functions as a diode to temporarily retain a charge packet as needed as a series of charge packets are read out by external electronics from the device. Output terminal 32 acts as the interface between the CCD and other electronic components which may be used to process the charge packets being read out. Typically, such components include a counter or digitizer for measuring the magnitude of individual charge packets and converting such magnitudes into digital signals. When charge packets from any layer are liberated at the end of the array 10, these charge packets will migrate to output terminal 32. The last electrode in the row, shown as electrode 12z, is used to feed in a compensatory voltage into the last pixel area of the CCD, in order to "liberate" one charge packet or another from a selected layer 16, 20, or 24, depending on the magnitude of the compensatory voltage. Figures 3A, 3B, and 3C show the effect of applying compensatory voltages, in sequence, in order to liberate the charge packets in various layers for read out. Each Figure 3A, 3B, and 3C is the ionization potential of each layer as a function of the depth thereof in the structure, as was Figure 2. The external application of the compensatory voltage through electrode 12z effectively lowers the barriers between the wells. In Figure 3A, the ionization potential of the high band gap layers is shown by the dotted line marked E, which is also shown as the top of the barriers between the wells in Figure 2. The effect of the compensatory voltages is to progressively lower these barriers by energizing high band gap layers so that the charge in each layer may "escape" in sequence to output terminal 32. In Figure 3A is shown, in solid lines, the effect of applying a compensatory voltage V 1 of such magnitude that the potential barrier is, at a layer adjacent blue photosensitive layer 16, less than the potential of the electron-hole pairs in the blue photosensitive layer 16. Comparing Figure 3A to Figure 1, it is apparent that, with the application of compensatory voltage V 1 , the potential in band gap layer 14 is lower than that in the blue photosensitive layer 16. For this reason, any electron-hole pairs created in photosensitive layer 16 for a given pixel will be liberated to be read out through output terminal 32, since this liberation occurs at the end of the array 10. Thus, information for the blue pixel will form, in time sequence, the first element of the triplet of pixel data for that given pixel. Figure 3B shows the potential of the function of depth upon the application of a second compensatory voltage V 2 . The line V 2 may also be seen as the dotted line in Figure 3A. Voltage V 2 will be of such a magnitude that the potential in band gap layer 18 will be less than that in photosensitive layer 20 for green components of the pixel. Because of this lower potential, the electron-hole pairs in layer 20 will be caused to escape to output terminal 32. Thus, after the application of compensatory voltage V 2 , the electron-hole pairs relating to the green component of the pixel may be read out. Figure 3C shows the effect of a third compensatory voltage V 3 which has the effect of making the potential in band gap layer 22 lower than the potential in red photosensitive layer 24. And once again, the charge packet created in photosensitive layer 24 for a given pixel, representing the red component for that given pixel, may be read out through output terminal 32. In summary, the sequential application of stepwise higher compensatory voltages permit the electron-hole pairs for each photosensitive layer for a given pixel to be read out in an orderly fashion for measuring, and then for conversion to a digital signal for purposes of subsequent image processing. Once the first triplet of color data for a pixel is read out, the applied compensatory voltage is set to zero, the next pixel is moved to the end of the CCD, and the readout cycle begins again. In order to create the desired structure of layers sensitive to particular desired primary colors, there are many possible direct band-gap materials available, such as(Al x Ga 1 - x ) y In 1- y As. It is also possible to use indirect gap materials for the photosensitive layers 16, 20, and 24, such as ordered crystals of Al x Ga 1- x As. The proportion of aluminum to gallium in this ordered crystal structure (given as x ) may be varied for sensitivity to a particular energy associated with a desired wavelength for a given layer. The use of indirect gap materials is possible because there is a large increase in absorption for energies at and above the material's direct gap. The small amount of absorption below the direct gap can be corrected for once the signals are read out and converted to digital form. In brief, the best known proportions of aluminum to gallium are shown in the table below for a desired wavelength sensitivity: Al X Ga 1-X As X = Direct gap eV λ (µm) Color (approx.) 0.45 2 .62 red 0.6 2.2 .56 yellow 0.75 2.4 .52 green 0.85 2.6 .48 blue 0.9 2.8 .44 violet For the boundary layers 18, 22, and 26, it is desired to provide a crystalline structure which has a considerably higher energy level than any of the photosensitive layers, as can be seen in Figure 2. These layers of high energy are required to form the potential barriers between the respective wells which represent the photosensitive layers. The preferred material for these large band gap boundary layers is an ordered crystal of gallium phosphide (GaP). It is also significant that the variations in composition among the layers is used to facilitate discrimination by color among the layers, which in turn enables the photosensitive layers to be of substantially equal thickness. Prior art structures have relied on making various layers sensitive to different colors by varying the thickness of particular layers.

Claims: A photosensitive charge-coupled array (10), defining a plurality of exposable image areas on a main surface thereof, comprising: a first photosensitive layer (16) parallel to the main surface and comprising a material having a band-gap energy associated therewith to generate electron-hole pairs in an area thereof exposed to light within a predetermined first range of wavelength (B); a second photosensitive layer (20) parallel to the main surface and comprising a material having a band-gap energy associated therewith to generate electron-hole pairs in an area thereof exposed to light within a predetermined second range of wavelength (G) different from the first range of wavelength; a first boundary layer (18) disposed between the first photosensitive layer and the second photosensitive layer and comprising a material having a higher band gap energy associated therewith than the material of the first photosensitive layer and the second photosensitive layer; and a series of electrodes (12a, 12b, 12c, 12d) disposed on the main surface, each electrode corresponding to an exposable image area, the electrodes being adapted for enabling the shifting of image data in the first photosensitive layer and the second photosensitive layer along the discrete exposable image areas within the array. An array as in claim 1, wherein the first photosensitive layer (16) is disposed closer to the surface of the array than the second surface and the first range of wavelength (B) includes shorter wavelengths than the second range of wavelength (G). An array as in claim 1 or 2, further comprising a uniform conductor disposed adjacent the first photosensitive layer, the second photosensitive layer, and the boundary layer at one end thereof. An array as in claim 1, 2 or 3, wherein the first photosensitive layer and the second photosensitive layer are of substantially the same thickness. An array as in any of claims 1 to 4, wherein the first photosensitive layer and the second photosensitive layer comprise different relative proportions of aluminum and gallium and/or aluminum, gallium, and arsenide. An array as in any of claims 1 to 5, wherein the boundary layer comprises gallium phosphide. An array as in any of the preceding claims, further comprising an electrode for applying a compensatory voltage to a portion of the array, whereby the band-gap energy of the boundary layer may be selectably decreased. An array as in any of the preceding claims, further comprising: a third photosensitive layer (24), adapted to generate electron-hole pairs in an area thereof exposed to light within a predetermined third range of wavelength (R) different from the first range of wavelength and the second range of wavelength; and a second boundary layer (22) disposed between the first photosensitive layer and the second photosensitive layer, adapted to not generate electron-hole pairs when exposed to light of the first range of wavelength, the second range of wavelength, and the third range of wavelength. An array as in claim 8, wherein the first, second, and third photosensitive layers are respectively disposed farther from the exposable surface of the array and are respectively sensitive to longer wavelengths of light. A method of deriving color component data of an image, comprising the steps of: forming a linear charge-coupled array including a main surface, a set of exposable image areas in the main surface, a series of electrodes (12a, 12b, 12c, 12d) disposed on the main surface, each electrode corresponding to an exposable image area, a first layer (16) parallel to the main surface, the layer comprising a material having a band-gap energy associated therewith to form electron-hole pairs when exposed to light of a first wavelength (B), a second layer (20) parallel to the main surface, the layer comprising a material having a band-gap energy associated therewith to form electron-hole pairs when exposed to light of a second wavelength (G) not equal to the first wavelength, and a boundary layer (18) disposed between the first layer and the second layer, the boundary layer comprising a material having a higher band-gap energy than that of the first layer or the second layer; exposing the linear charge-coupled array, so that electron-hole pairs are created in exposable areas in the first and second layers in proportion to the components of light of the first and second wavelengths received thereon; applying, to an electrode at an end of the linear array, a compensatory voltage for causing the electron-hole pairs within the second layer associated with an individual pixel to be accessed; and applying, to the series of electrodes on the linear charge-coupled array, a series of shift voltages for causing the electron-hole pairs to migrate to an output terminal (32).

Inventors: MANTELL DAVID A, MANTELL, DAVID A.
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1993
Main classification: H01L27
IPC codes: H01L31, H01L27, H01L29, H01L21