WO1995015575A1

WO1995015575A1 - Sensor with improved photocathode having extended blue-green sensitivity, and method of making

Info

Publication number: WO1995015575A1
Application number: PCT/US1993/011733
Authority: WO
Inventors: Hyo-Sup Kim
Original assignee: Litton Systems, Inc.
Priority date: 1993-12-03
Filing date: 1993-12-03
Publication date: 1995-06-08
Also published as: AU6695194A

Abstract

A sensor (20) having both near infra-red and extended blue-green sensitivity, and a photocathode (26) for such a sensor, includes a GaAs active layer (56) configured to provide a greater spectral response in the visible portion of the spectrum while still providing an effective response in the near infra-red portion of the spectrum, and an AlGaAs window layer (58) configured to transmit a greater portion of the visible spectrum to the active layer. Consequently, the sensor (and photocathode) provide a spectral response in the near infra-red portion of the spectrum, like conventional Gen 3 sensors and photocathodes, and also provides a strong spectral response in the visible portion of the spectrum from the deep red wavelengths to about the blue-green transition wavelengths. As a result, a sensor using such a photocathode can respond to or image both near infra-red wavelengths, as well as responding to or imaging a major portion of the visible spectrum. The inventive sensor can thus replace two conventional sensors (visible light and near infra-red light sensors) with a single sensor, which single sensor is also free of transition difficulties encountered with two conventional sensors during periods of changing illumination.

Description

SENSOR WITH IMPROVED PHOTOCATHODE HAVING EXTENDED BLUE-GREEN SENSITIVITY. AND METHOD OF MAKING.

Cross Reference to Related Application

This invention is related to the subject matter of an earlier invention disclosed, described, and claimed in

United States patent application No. 07/811,781, entitled,

"Transmission Mode InGaAs Photocathode for Night Vision System," filed 20 December 1991, now issued on

, as United States Patent No. , and also being an invention made by the named inventor of this application.

Background of the Invention

Field of the Invention

The present invention is in the field of sensors. More particularly, the present invention relates to sensors which are responsive to infra-red radiation to provide, for example, an electrical signal indicative of the level of infra-red light flux falling on the sensor, or to provide an image in visible light which is replicative of a scene too dim to be viewed with the unaided eye. Still more particularly, the present invention relates to a photocathode for use in such sensors to provide electrons in response to the photons of infra-red and visible light which fall on the sensor. Yet more particularly, the present invention relates to such a photocathode which is uniquely responsive to both infra-red light, and to visible light as far into the visible spectrum as the blue-green transition. Accordingly, the present invention provides a sensor responsive to both near infra-red and to visible light from the deep red end of the spectrum to the blue-green transition, or farther. Further, the present invention relates to a method of making such a photocathode.

Related Technology Sensors which are responsive to infra-red and other frequencies of light have a variety of uses. For example, a sensor which is responsive to infra-red light may include an anode from which an electrical signal is derived which is indicative of the flux or intensity of infra-red radiation falling on the sensor. Such a sensor may find use as a flame or fire detector. Such a detector may be used in a fire protection and warning system, for example, aboard a ship or aircraft. Similarly, such a sensor may be used as part of a process monitoring system to indicate continued operation of an industrial furnace. Alternatively, such a sensor may be provided with an array of anodes, which provide electrical signals indicative of the varying light fluxes falling on various corresponding portions of the photocathode. This type of sensor may be used to produce a course video image, for example, or may be used as a threat monitor aboard aircraft to trigger a warning signal of an approaching threat missile because of its heat signature. Because the array of anodes provide an indication of the position as well as existence of a threat, the sensor can provide also an indication of the direction from which a threat approaches.

One use of such sensors involves the use of a double-YAG laser providing coherent laser light at a 532 nm wavelength. This laser light is used in air pollution monitoring equipment, which by absorption, transmission, or back-scatter techniques, for example, provides an output response indicative of air pollution levels and characteristics. The present sensor, and photocathode for such a sensor, is able to provide a response to the 532 nm laser light used in such equipment.

Similarly, sensors which are responsive to low-level visible or infra-red light are used in night vision systems. Night vision systems are commonly used by military and law enforcement personnel for conducting operations in low light conditions, or at night. Night vision systems are also used to assist pilots of helicopters or airplanes in flying at night.

A night vision system converts the available low-intensity ambient light at both the deep-red end of the visible spectrum, and at the near infra-red portion of the invisible spectrum to a visible image. These systems require some residual light, such as moon light or star light, in which to operate. The ambient light is intensified by the night vision scope to produce an output image which is visible to the human eye. The present generation of night vision scopes utilize image intensification technologies to intensify the low-level deep-red visible and near infra-red invisible light. This image intensification process involves conversation of the received ambient light into electron patterns, intensification of the electron patterns while retaining the relative intensity levels and contrast of the scene, and projection of the electron patterns onto a phosphor screen for conversion of the electron patterns into a visible-light image for the operator. This visible-light image is then viewed by the operator through a lens provided in an eyepiece of the system.

The typical night vision system has an optics portion and a control portion. The optics portion comprises lenses for focusing on the desired target, and an image intensifier tube. The image intensifier tube performs the image intensification process described above, and includes a photocathode liberating photo-electrons in response to light photons to convert the light energy received from the scene into electron patterns, a micro channel plate to multiply the electrons, a phosphor screen to convert the electron patterns into visible light, and a fiber optic transfer window to invert the image. The control portion includes the electronic circuitry necessary for controlling and powering the optical portion of the night vision system.

A factor limiting the performance of conventional image intensification tubes is the photocathode component. The most advanced photocathodes are those of the third-generation, or Gen 3 tubes, and have a cut-off of their spectral response at the long-wavelength or near infra-red end corresponding to light having a wavelength of 900 nanometers. Thus, infra-red light having wavelengths above that range cannot be seen using the Gen 3 tube. At the short wave length end, conventional Gen 3 tubes have a spectral drop off of response at the deep red visible wavelength of about 600 nanometers.

It follows from the above that when a conventional Gen 3 photocathode is used to make a sensor, such as a video camera tube, the video image produced is only from the near infra-red and deep-red end of the visible spectrum. Such a sensor or video camera tube is not usable to provide an image from most of the visible-light spectrum. In other words, the image from such a camera tube never appears to be a day-light image. Even when used under conditions which would allow visible-light imaging, the image provided by such a sensor has the limitations of its limited spectral response. Under conditions allowing visible light imaging, personnel equipped with such night vision equipment simply shut off the equipment and use their natural vision. However, for personnel using a video system to view a night scene, the transition to day-light imaging is not so simple or inexpensive. That is, video imaging systems, such as industrial security systems, for example, which desirably include both low-light imaging and visible light imaging for surveyance purposes, conventionally use an infra-red video camera, and a separate visible-light video camera. As the sun sets or rises, the security personnel must switch from one camera to the other, and experience a transition period during which neither camera provides the sensitivity and visual acuity which would be desired.

This same dichotomy is generally applicable between sensors responsive in the near infra-red (and only deep-red visible) wavelengths, and sensors responsive in the visible (but not infra-red) wavelengths, because each of the sensors have limited spectral response bands. In many cases, it is necessary to provide separate video cameras, or separate sensors of other types, when a response is required in both the visible wavelengths and the infra-red wavelengths of the spectrum. This duplication of sensors is expensive, adds weight to mobile systems, adds complexity in all cases, and frequently results in dithering back and forth between the sensors in an attempt to avoid the transition period described above.

A conventional photocathode for an infra-red type of sensor is known in accord with United States patent No.

3,959,045, issued 25 May 1976, to G. A. Antypas. The photocathode taught by the '045 patent is one version of the now-conventional Gen 3 photocathode described above. This photocathode has an active layer about 2 microns thick. Also, this photocathode has a spectral response band extending from the near infra-red wavelengths to the deep-red visible wavelengths, as was explained above with respect to conventional photocathodes. When used to provide a night vision scope, the conventional photocathode according to the '045 patent gives an image from the near infra-red to the deep red visible portion of the spectrum. However, if such a photocathode were used to make an infra-red video camera tube, the resulting image would be from the same portion of the spectrum. For visible light imaging through the video system using such a video camera, a separate visible-light video camera would be required. Summary of the Invention In view of the above, a need exists to provide a sensor which is responsive in a spectral band inclusive of the near infra-red wavelengths, and also inclusive of a substantial portion of the visible spectrum of wavelengths. A photocathode for such a sensor would provide the described spectral response band, and would liberate photo-electrons with a comparatively high quantum efficiency throughout this comparatively broad response band in order to provide a sensor with uniformly good sensitivity throughout the broad spectral response band.

Additionally; a need exists for a photocathode and method of making the photocathode for such a sensor.

Accordingly the present invention provides according to a particularly preferred exemplary embodiment of the invention, a sensor having a photocathode and providing an output response in response to incidence of light on the sensor in the near infra-red and visible spectrum about to the blue-green transition, the sensor photocathode including an active layer of gallium arsenide having a thickness in the range of from about 0.2 to about 0.5 nanometers.

Further, the present invention provides such a sensor photocathode in which a window layer is disposed adjacent to the active layer of the photocathode and includes aluminum gallium arsenide with a concentration of aluminum of at least 80 atomic percentage. The comparatively high aluminum content of the window layer of the photocathode according to the present invention makes this window layer more transparent to light in the green and blue portions of the visible light spectrum particularly, in order to improve the spectral response of the photocathode in this wavelength.

Also, the present invention provides a method of making such a photocathode, and a sensor including such a photocathode. In view of the above, it will be apparent that an advantage of the present invention resides in the provision of a photocathode and a sensor having such a photocathode, which is particularly responsive to incident light in both the near infra-red portion of the spectrum, but is also responsive to light in the visible portion of the spectrum from the deep red wavelengths to about the blue-green transition wavelengths. That is, the present invention provides a photocathode and a sensor having such a photocathode which is responsive to the near infra-red and a major portion of the visible spectrum. Thus, the single photocathode, and the sensor having such a photocathode, may replace two or more conventional photocathodes and sensors having such photocathodes or other means for responding to incident light, which have narrower spectral bands of response to light. It will be apparent that a sensor made with the use of a photocathode according to the present invention may be a night vision sensor, for example. Other types of sensors which may be made with the photocathode of the present invention include threat detectors, monitor devices, and video camera tubes responsive in both the near infra-red and visible portions of the spectrum. The sensors which may be made with the use of the present invention are not limited to night vision devices.

These and additional objects and advantages of the present invention will be apparent from a reading of the present detailed description of a single particularly preferred exemplary embodiment of the present invention, taken in conjunction with the appended drawing Figures, in which the same reference numeral refers to the same feature, or to features which are analogous in structure or function to one another.

Description of the Drawing Figures

Figure 1 provides a schematic representation of a sensor embodying the present invention; Figure 2 schematically represents a cross sectional view of a photocathode portion of the sensor seen in Figure 1, at a particular stage of manufacture;

Figure 3 presents another cross sectional vies of a photocathode portion of the sensor seen in Figure 1 at a stage of manufacture subsequent to that seen in Figure 2;

Figure 4 presents a cross sectional view similar to Figures 2 and 3, but showing a finished photocathode according to the present invention;

Figure 5 graphically depicts the steps of making a sensor according to the present invention; and

Figure 6 graphically presents information concerning the spectral response of the best conventional Gen 3 photocathode, and of a photocathode (and sensor) according to the present invention.

Detailed Description of the Preferred Exemplary Embodiments of the Invention Viewing Figure l, a sensor 10 embodying the present invention is depicted schematically. The sensor 10 is shown to be an image intensification tube for a night vision scope 12, but it will be understood that the invention is not so limited. In overview, the night vision scope 12 is seen to include an objective lens or lens system, which is schematically depicted by the single lens 14, and by which light from an object 16 or scene to be viewed is received into the night vision device 12. The light from the object which is received through lens 14 is focused through a glass face plate 18 of an image intensification tube 20. The tube 20 is powered by a conventional image tube power supply 22, connected to the tube 20 by plural power supply conductors 24. The power supply 22 maintains a electrostatic voltage gradient in the image intensification tube 20, and provides a current flow which is necessary to provide a shower of electrons in a pattern which replicates the image of the viewed object 16, as will be further explained. It should be. noted that the light received from the object 16 may be low intensity visible light, but more likely will be infrared light, which is rich in the night sky. The human eye can image light in the 400 to 700 nanometer range. That is, the visible spectrum extends from about 700 nanometers at the deep-red end to about 400 nanometers in wavelength at the violet end.

The light which is received through the face plate 18, which may be light of exceedingly low intensity or invisible infra-red light, is incident upon a photocathode portion 26 of the image intensification tube 20. The photocathode 26 is responsive to incident photons of particular frequencies and wavelengths to emit photoelectrons in response to the photons, as is indicated by the arrows 28. The photoelectrons 28 move rightwardly, viewing Figure 1, under the influence of the prevailing electrostatic field from power supply 22 and into a microchannel plate 30. This microchannel plate 30 is specially constructed to provide secondary emission electrons in response to the photoelectrons 28. As is indicated by the arrows 32, a shower of photoelectrons and secondary emission electrons is provided by the microchannel plate 30. The pattern of the shower 32 of electrons replicates the pattern of the photons falling on the photocathode 26. This shower of electrons 32 is directed to a phosphorescent screen 34 where it produces a visible image replicative of the image falling on the photocathode 26, but several orders of magnitude as intense.

It will be noted that the image intensification tube 20 includes a housing, indicated by the dashed line 36 which is closed at one end by the face plate 18, and which may be closed at the other end by the phosphorescent screen 34. Light from the phosphorescent screen 34 may be conducted through a fiber optic image invertor 38, and through an eyepiece optical system, schematically indicated by the lens 40, to the eye of an observer 42. The observer then sees an enlarged and intensified image 16' of the object 16.

It will be under stood that the sensor 10 could be configured as a device which is to provide an electrical response to incident light of selected wavelengths, rather than a visible response to the incident light. In such a case, the sensor 10 could include, for example, an anode, or array of anodes, which are schematically indicate at the position of the phosphorescent screen 34. Such an anode, or array of anodes, would produce an electric current indicative of the photon flux incident upon the photocathode 26. This electric current can be extracted from the sensor 10 via a conductor 46, or via a bundle of such conductors connecting to corresponding ones of the plural anodes in the anode array 44. A few of the exemplary uses of such a sensor were discussed above.

Viewing now more particularly Figure 2, and the first six steps enumerated in Figure 5, the structure resulting from these manufacturing steps is depicted. The final end product resulting from these manufacturing steps is the sensor 10, which may take the form of the image intensification tube 20 discussed above, or may take the form of a sensor providing an electrical or other output in response to a light input of particular wavelengths, also as pointed out above. Figure 2 shows a manufacturing intermediate structure 48, including elements which will become part of the photocathode 26, and which includes a gallium arsenide (GaAs) substrate 50 upon which a buffer layer 52 of high quality single crystalline GaAs has been formed by MOCVD technique. The substrate 50 is preferably a low defect density single crystal wafer in the crystal orientation of (001) . The buffer layer 52 effectively reduces the crystal quality degradations which could result from crystalline defects in the GaAs substrate material 50. The buffer layer 52 also minimizes contamination from the substrate 50 of the subsequent layers of material to be grown atop this substrate. Preferably, the buffer layer 52 is about 1.0 microns thick.

Atop the buffer layer 52 is placed a stop layer 54, which is about 0.5 microns thick, and which is preferably in the range of from about 50 to about 60 atomic percent aluminum in a stop layer of aluminum gallium arsenide (AlGaAs) . As will be better understood in view of following explanation, the etch rate of this stop layer can be controlled by varying the proportion of aluminum in this layer. On the stop layer 54 is placed an active layer 56 of GaAs, which is about a micron or more in thickness. This active layer 56 is doped with a p-type of impurity, such as zinc, for example, to produce a negative electron affinity for the active layer 56. Preferably, the active layer 56 is doped at a concentration of about 1 x 10¹⁹ dopant atoms per cubic centimeter of GaAs material in the active layer 56. This active layer will be controlled in thickness, as is explained below, in order to be sufficiently thin as to maximize the yield of photoelectrons arriving at the lower surface of the active layer 56. That is, a portion of photoelectrons generated at the interface of the active layer and a window layer (which is described below) from blue-green light photons are absorbed in the adjacent thickness of the active layer. Dependent upon the spectral response desired for a particular photocathode, the thickness of the finished active layer should be between 0.2 and 0.7 micron. For a high sensitivity to blue-green light, the active layer should be between 0.4 and 0.5 micron thick. Most preferably, the active layer 56 is 0.45 micron thick. In every case, the finished thickness of the active layer 56 will be substantially thinner than the thickness of conventional photocathodes, which at their thinnest are about 1.2 microns thick.

On the active layer 56 is formed a window layer 58 of AlGaAs, which is also of a thickness of less than or equal to about one micron. Preferably, this window layer has a thickness of about 0.5 to about 0.7 micron. This window layer 58 is doped also with a p-type of impurity, preferably to a concentration of impurity atoms of about 1 x ιo¹⁸ dopant atoms per cubic centimeter of AlGaAs in the layer 58, or lower. In order to make the window layer 58 more transparent to light in the shorter wavelengths, such as light as short in wavelength as the blue-green transition, and blue light, the layer 58 is formed with a concentration of aluminum in the AlGaAs of at least eighty (80) percent. Preferably, the layer 58 of AlGaAs has a concentration of Al in the range of 83 to 90 atomic percent. Because of considerations having to do with preparation of a high quality interface with the active layer and minimization of difficulties in the photocathode fabrication process, concentrations of aluminum in the window layer of greater than 90 percent have not been used by the applicant. However, future improvements in materials and fabrication techniques may allow such higher concentrations of aluminum in the window layer. Accordingly, the present photocathode is preferably make with a concentration of aluminum in the window layer of 80 percent or higher according to the available fabrication technology. Atop the window layer 58 is formed a temporary top layer 60 of GaAs. Consideration of Figure 5 will show that the steps and structure so far described are depicted diagrammatically as steps 1 through 6. The top layer 58 is subsequently etched away using a suitable concentration of a conventional etchant, such as NH₄OH and H₂0₂, Subsequently, as is seen in the remainder of Figure 5, and in Figure 3, a thin anti-reflective layer 62 of Si₃N₄ is deposited on the window layer 58. A thin passivating layer 64 of Si0₂ is placed over the anti-reflective layer 62. Next, the resulting assembly is thermal compression bonded to a glass face plate 18, as is indicated in Figure 3 by the pressure arrows 66 and the heat arrows 68. Preferably, the glass face plate 18 is fabricated of 7056 borosilicate glass. Such a glass is available from Corning Glass.

The assembly seen in Figure 3 then has the substrate 50 etched away using a suitable concentration of a conventional etchant, such as NH₄0H and H₂0₂. The stop layer 54 is removed using Hcl solution. Subsequently, the thickness of the active layer 56 is adjusted in two steps using suitable etchants, as set out above, and as is further explained below. The thickness of the active layer 56 is preferably reduced to about 0.45 microns.

As stated above, preferably the active layer thickness is controlled to be in the range of from about 0.2 micron to about 0.7 micron. However, more preferably, the thickness of the active layer 56 is controlled to be in the range of from about 0.4 micron to 0.5 micron. Most preferably, the active GaAs layer 56 has a thickness of 0.45 micron. This thickness for the active layer 56 is achieved in two steps. In a first step the thickness of layer 56 is reduced to about 0.65 micron (i.e., 0.2 microns, plus the intended final thickness of the active layer 56) with an etchant solution of NH₄OH and H₂θ₂. In a second step, an etchant solution of H₂S0₄ and H₂0₂ is used to further adjust the active layer thickness, preferably to 0.45 microns. This second etch needs to be conducted just before the photocathode assembly is loaded into the vacuum exhaust system in order to minimize contamination of the active layer surface.

The resulting thin active layer (much thinner than conventional photocathodes) reduces the probability of the energetic photoelectrons resulting from photons in the green, blue-green, and blue regions of the visible spectrum being recombined into the crystal lattice of the active layer near the interface with the window layer 58. Next, the active layer 56 is thermally surface cleaned in a very high vacuum exhaust station to remove surface oxides and absorbed gas species. The active layer 56 is next activated with Cs and 0₂ to enhance the photosensitivity of the photocathode 26.

The resulting photocathode assembly is seen in Figure 4. This photocathode assembly is next configured at its periphery to mate with the sensor body 36, which is not detailed in the drawing Figures, and a peripheral electrode is applied for connection of electrostatic charge from the power supply 22 to the photocathode. This resulting finished photocathode assembly is then bonded to the sensor body 36 to complete the sensor 10 for use in a night vision system, or for other uses, as described above.

Viewing now Figure 6, a graphical representation of the spectral response of a conventional Gen 3 photocathode is presented at line 70. This conventional photocathode, and any sensor made with this photocathode, has a long wavelength response cut off at about 900 nanometers, and a short wavelength cut off at about 600 nanometers. Thus, this conventional photocathode has a spectral response band width of about 300 nanometers. In contrast, the photocathode according to the present invention has a long wavelength cut off at or about 900 nanometers, and a short wavelength cut off of about 450 nanometers. Consequently, the inventive photocathode has a spectral response band width of about 450 nanometers. Compared to the conventional photocathode, the inventive photocathode has a spectral response band width which is about 50 percent wider. This spectral response band of the present inventive photocathode extends into the visible spectrum to about the blue-green transition, or possibly into the blue portion of the visible spectrum. Consequently, a sensor made with the present inventive photocathode can provide a response from both the near infra-red portion of the spectrum, and from the visible portion of the spectrum from the deep-red wavelengths to and including the blue- green transition wavelengths.

While the present invention has been depicted, described, and is defined by reference to a particularly preferred embodiment of the invention, such reference does not imply a limitation on the invention, and no such limitation is to be inferred. The invention is capable of considerable modification, alteration, and equivalents in form and function, as will occur to those ordinarily skilled in the pertinent arts. The depicted and described preferred embodiment of the invention is exemplary only, and is not exhaustive of the scope of the invention. Consequently, the invention is intended to be limited only by the spirit and scope of the appended claims, giving full cognizance to equivalents in all respects.

Claims

I Claim:

1. A photocathode comprising: a layer of gallium arsenide (GaAs) , and means for making said layer of GaAs provide a response in a band including both the near infra-red and visible wavelengths to about the blue-green transition.

2. The photocathode of Claim 1 wherein said response band extends from substantially 450 nm to substantially 900 nm, inclusive.

3. The photocathode of Claim 2 wherein said means for making said layer provide said response includes having said layer of GaAs define a thickness less than 1.2 microns.

4. The photocathode of Claim 3 wherein said layer of GaAs defines a thickness of less than 0.7 micron.

5. The photocathode of Claim 4 wherein said layer of GaAs defines a thickness in the range of from about 0.2 micron to about 0.7 micron.

6. The photocathode of Claim 5 wherein said layer of GaAs has a thickness of substantially 0.45 micron.

7. The photocathode of Claim 1 wherein said layer of GaAs is doped with a p-type of impurity.

8. The photocathode of Claim 7 wherein said p-type of impurity in said layer of GaAs is present in a concentration of substantially 1 x 10¹⁹ dopant atoms per cubic centimeter of GaAs material.

9. The photocathode of Claim 8 wherein said p-type impurity is zinc.

10. The photocathode of Claim 1 wherein said means for making said layer provide said spectral response includes a window layer of aluminum gallium arsenide (AlGaAs) adjacent to and forming an interface with said layer of GaAs.

11. The photocathode of Claim 10 wherein said window layer includes a concentration of Al of at least about 80 atomic percent.

12. The photocathode of Claim 11 wherein said window layer includes a concentration of Al of at least 83 atomic percent.

13. The photocathode of Claim 12 wherein said window layer includes a concentration of Al in the range of from about 83 atomic percent to about 90 atomic percent.

14. The photocathode of Claim 10 wherein said window layer is doped with a p-type of impurity.

15. The photocathode of Claim 14 wherein said p-type of impurity in said window layer is present in a concentration of substantially 1 x 10¹⁸ dopant atoms per cubic centimeter of AlGaAs material in the window layer.

16. The photocathode of Claim 10 wherein said window layer defines a thickness of less than about one micron.

17. The photocathode of Claim 16 wherein said window layer defines a thickness in the range of from about 0.5 micron to about 0.7 micron.

18. A method of making a sensor including a photocathode having a response in both the near infra-red and visible portion of the spectrum, comprising: providing a gallium arsenide (GaAs) temporary substrate; forming a buffer layer of high-quality single crystalline GaAs on said temporary substrate; forming a stop layer of aluminum gallium arsenide (AlGaAs) on said buffer layer; forming a layer of GaAs about a micron or more in thickness on said stop layer; and forming a window layer of AlGaAs with a concentration of aluminum in the AlGaAs of at least eighty (80) atomic percent on said GaAs layer to form a photocathode workpiece;

19. The method of Claim 18 further including the steps of: forming a anti-reflective layer of Si₃N₄ on said window layer; and forming a thin passivating layer of Si0₂ over said anti-reflective layer.

20. The method of Claim 19 additionally including the step of: thermal compression bonding said photocathode workpiece to a transparent face plate.

21. The method of Claim 20 further including the steps of; removing said temporary substrate and said buffer layer; and removing said stop layer.

22. The method of Claim 18 subsequently including the step of decreasing the thickness of the GaAs layer to a thickness of less than about 1.2 microns.

23. The method of Claim 22 additionally including the step of reducing the thickness of said GaAs layer to a thickness in the range of from about 0.2 micron to about 0.7 micron.

24. The method of Claim 23 further including the step of reducing the thickness of said GaAs layer to a thickness of substantially 0.45 micron.

25. A method of making a sensor which includes a photocathode and provides a spectral response band extending from the near infra-red to substantially the blue-green portion of the electromagnetic spectrum, and including the steps of making said photocathode for said sensor, said method comprising the steps of: providing a gallium arsenide (GaAs) temporary substrate; forming an layer of GaAs having a thickness of substantially less than about 1.2 microns on said temporary substrate; and forming a window layer of AlGaAs with a concentration of aluminum in the AlGaAs of at least eighty (80) atomic percent on said GaAs layer.

26. The method of Claim 25 additionally including the step of reducing the thickness of said GaAs layer to a thickness in the range of from about 0.2 micron to about 0.7 micron.

27. The method of Claim 26 further including the step of reducing the thickness of said GaAs layer to a thickness of substantially 0.45 micron.

28. A sensor for providing a photo-electron spectral response to incident light both in the near infra-red and visible portions of the electromagnetic spectrum from the deep-red wavelengths to about the blue-green transition wavelengths (substantially 900nm to substantially 450 nm) , said sensor including a photocathode which comprises: a layer of gallium arsenide (GaAs) , and means for making said layer of GaAs provide said spectral responsive to near infra-red and visible wavelengths.

29. The sensor of Claim 28 wherein said means for making said GaAs layer provide said spectral response includes said layer of GaAs defining a thickness which is substantially less than 1.2 microns.

30. The sensor of Claim 29 wherein said layer of GaAs defines a thickness of less than 0.7 micron.

31. The sensor of Claim 30 wherein said layer of GaAs defines a thickness in the range of from about 0.2 micron to about 0.7 micron.

32. The sensor of Claim 31 wherein said layer of GaAs has a thickness of substantially 0.45 micron.

33. The sensor of Claim 28 wherein said means for making said GaAs layer provide said spectral response includes a window layer of aluminum gallium arsenide (AlGaAs) adjacent to and forming an interface with said GaAs layer.

34. The sensor of Claim 33 wherein said window layer includes a concentration of Al of at least about 80 percent.

35. The sensor of Claim 34 wherein said window layer includes a concentration of Al of at least 83 percent.

36. The sensor of Claim 35 wherein said window layer includes a concentration of Al in the range of from about 83 percent to about 90 percent.

37. A photocathode sensitive to a spectral region extending from substantially 900nm to at least 450 nm comprising: a layer of GaAs having a sufficiently small thickness to permit a substantial portion of photoelectrons generated by radiation in said spectral region to escape said GaAs layer; and a window layer of AlGaAs adjacent to said GaAs layer which is substantially transparent within said spectral region.

38. The photocathode of Claim 37 wherein said GaAs layer thickness is between 0.2 micron and 1.2 microns.

39. The photocathode of Claim 38 wherein said GaAs layer thickness is less than 0.7 micron.

40 The photocathode of Claim 39 wherein said GaAs layer thickness is less than 0.5 micron.

41. The photocathode of Claim 40 wherein said GaAs layer thickness is substantially 0.45 micron.

42. The photocathode of Claim 37 wherein said window layer is composed of AlχGaAs^χ, wherein X is greater than

0.80.

43. The photocathode of Claim 42 wherein X is in the range of from about 0.83 to about 0.90.