Iii-v Cathodes Having A Built-in Gradient Of Potential Energy For Increasing The Emission Efficiency

Abstract

A gradient of potential energy was established in the active layer of a III-V photocathode for enhancing free electron diffusion toward the emissive surface of the cathode. The energy gradient was provided by decreasing the bandgap energy across the active layer which caused the conduction level to slope downwards from the substrate to the emissive surface through progressive changes in the concentration of the III-V elements forming the active layer. Alternatively, a nonuniform concentration of active layer dopant--heavy on the substrate side and light on the emissive side of the active layer--established a built-in electric field across the active layer. The graded bandgap and/or dopant levels promote free electron drift toward the outer surface of the active layer. Layers of cesium, cesium oxide, or both, were provided over the active layer to lower the work function of the photocathode emissive surface.

Description

The invention herein described was made in the course of Government Contract No. F33615-68-C-1396 with the United States Air Force.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to photocathodes and more particularly to photocathodes having a gradient of potential energy thereacross for establishing electron drift towards the emissive surface.

2. Description of the Prior Art

Heretofore, the application of external electric fields across cathodes to increase electron emission has been limited. Generally these fields are generated by applying a voltage across a pair of space SnO layers positioned on either side of the cathode. An inherent limitation in this structure is that electrons of low energy cannot penetrate the thin conductive layer placed over the emissive surface.

Internal electric fields established by nonuniform doping have been employed in the drift transistor art as described in John N. Shive's book entitled "The Properties, Physics, and Design of Semiconductor Devices" page 231. The purpose of the built-in electric field in this application was to accelerate minority carriers across the transistor junction. Further, the concept of a graded bandgap layer is known in the graded bandgap base (GBGB) transistor art. However, these sources of internal fields or potential energy gradients have not heretofore been employed to increase the emission efficiency of III-V photocathodes.

SUMMARY OF THE INVENTION

It is therefore an object of this invention to provide a more efficient III-V semiconductor cathode.

It is a further object of this invention to provide a III-V semiconductor cathode having a faster response time.

It is another object of this invention to provide a III-V semiconductor cathode having a unidirectional electron flow for avoiding the loss of free electrons in the interior portion of the active layer.

Briefly these and other objects are achieved by providing a substrate means, preferably a binary or compound III-V crystalline solid, upon which is grown a III-V semiconductor crystalline solid active layer formed by: a first constituent which is at least one element listed under column III of the periodic table; a second constituent which is at least one element listed under column V of the periodic table; and a third or additional material which is at least one nonuniformly present element which progressively changes in concentration across the active layer for establishing the gradient of potential energy. The gradient of potential energy across the graded active layer biases the movement of free electrons therein towards the emissive surface. In one embodiment the gradient is established by progressively decreasing the bandgap energy of the active layer, from the substrate side to the emissive surface side by progressively altering the concentrations of elements forming the compound semiconductor crystal. In another embodiment, the gradient may be established by nonuniformly doping the active layer with heavy concentrations of a suitable P-type dopant near the substrate region of the active layer and decreasing concentrations of the dopant toward the emissive surface. The dopant captures free electrons from the balance band, causing a nonuniformly distributed negative charged across the active layer. The simultaneously created holes in the the valance band of the semiconductor are mobile and by diffusion tend to distribute themselves evenly across the active layer. The negative charge concentration toward the substrate side of the active layer tends to prevent this diffusion of the positive holes. However, a sufficient diffusion of holes does take place to establish an electric field across the active layer which is positive on the emissive surface and negative adjacent to the substrate. The gradient of electron energy potential created by the graded band energy and dopant concentrations establish a unidirectional flow of electrons toward the emissive surface. This bias on the electron diffusion minimizes the loss of electrons into the substrate and the interior region of the active layer. The enhanced electron diffusion increases the emissive efficiency of the photocathode and establishes a faster response time between photon absorption and electron emission.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects and advantages of the present invention and the cause and operation of the gradient of energy potential will become apparent from the following description taken in conjunction with the drawings in which:

FIG. 1 is a schematic diagram, in section, of an infrared image converter showing the inventive photocathode, employed in a reflective optical system,

FIG. 2 is an enlarged sectional view of the inventive photocathode taken along lines 2--2 of FIG. 1,

FIG. 3 is an energy level diagram for the graded bandgap embodiment of the photocathode, and

FIG. 4 is an energy level diagram of the graded dopant embodiment of the photocathode.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1 there is shown an infrared image intensifier tube 10, representing one of the uses of the present invention. An object 12 to be viewed, gives off or reflects optical radiation in the near infrared spectrum. Rays emanating from object 12 are focused on the photosensitive surface of photocathode 14 through a lens 16 and a 45.degree. mirror 18 disposed within an evacuated envelope 20. Vacuum envelope 20 is preferably evacuated to a suitably low pressure such as 10.sup..sup.-9 Torr. The image of object 12 is reflected from mirror 18 onto photocathode 14 which converts the near infrared image into a corresponding electron image emitted from photocathode 14. The emitted electrons are accelerated by a relatively high voltage such as 30 kv. via accelerating electrodes 22, and focused onto a fluorescent screen 24. The optical image appearing on fluorescent screen 24 is a greatly intensified image of object 12 and is viewed through fluorescent screen 24 by the eye or a suitable optical pickup device. A reflection optical system is depicted in FIG. 1; however, a transmission optical system may be employed with a suitably thin photocathode active layer grown on a suitably transparent substrate.

Referring now to FIG. 2, photocathode 14 is shown in greater detail. Photocathode 14 includes a heavily doped P-type semiconductor substrate 32 with a graded active layer 34 thereover, both supported on a conductive electrode 36. Preferably, a layer 38 of alkali metal up to 5 monolayers thick is formed on the clean surface of graded active layer 34 for filling the surface energy states of active layer 34. A layer 40 of cesium oxide [Cs.sub.2 O] is then preferably formed on alkali metal layer 38 for lowering the work function of the photocathode as described by Ronald L. Bell in application Ser. No. 698, 941, filed Jan. 18, 1968, entitled "Photoemitter Having A P-Type Semiconductor Substrate Overlaid with Cesium and N-Type Cesium Oxide Layers," and assigned to the present assignee.

In operation, optical radiation in the near infrared range and forming the image to be converted, falls onto the emissive surface of photocathode 14 and passes through cesium oxide layer 40 and alkali metal layer 38 and is absorbed in P-semiconductor active layer 34. Upon absorption, the infrared radiation produces an electron hole pair pattern corresponding to the optical image. The electrons drift toward the cesium oxide emissive surface under the influence of the gradient of potential energy developed by the graded bandgap levels or graded dopant concentration in graded layer 34. The electrons so accelerated pass through alkali metal layer 38 and cesium oxide layer 40 and are emitted into the vacuum to form an electron image which is then accelerated to fluorescent screen 24.

Referring now to the energy level diagram shown in FIG. 3, graded active layer 34 is selected to have a semiconductive energy bandgap preferably equal to or slightly greater than the work function of cesium oxide layer 40. The junction of active layer 34 with cesium layer 38 and cesium oxide layer 40 causes severe band bending of the conduction band level 50 and the valance band level 52 in active layer 34. Substrate 32 is heavily doped with P-dopants such as Zn or Be to bring the fermi level within the semiconductive material near to the top of valance band 52. The preferred cesium oxide layer 40 is N-type to bring its fermi level near the bottom of its conduction band 54. Conduction band 50 across active layer 34 is shown sloping downward to the right toward the emissive surface as dictated by the progressively changing composition of active layer 34. The effect of the gradient will become particularly noticeable with a downwards slope of several times KT [thermal energy for the operating temperature] in a distance which is the inverse of the absorption depth for photon energies of greatest interest. A gradient energy potential of 1,000 volts per centimeter may easily be established in this embodiment. Of course, greater electric fields may be established if required, subject to the breakdown limitations of the graded layer material. For emission purposes it is desirable to maintain conduction level 50 at about 1 ev. adjacent to emissive layer 40. This boundary condition limits the steepness of the slope in graded active layer 34. Another consideration in determining the maximum slope of the gradient is how much shift in response frequency caused by the decreasing bandgap is permissible.

A number of techniques exist for making the graded bandgap embodiment. For instance bandgap could be gradually lowered by incorporating an increased fraction of the bandgap lowering element into the growth of the compound active layer 34 on substrate layer 32. All of the techniques available must allow for controlled variation of the bandgap by variation of the growth parameters. Some variations in growth parameters occur naturally, such as the depletion of one component of the melt in the case of liquid epitaxy, or different incorporation rates from a given environment with varying temperature in the case of liquid or vapor epitaxy. Another possibility involving the dynamics of growth initiation is that a composition and bandgap gradient may be induced in the base layer simply by growing from an environment of constant composition onto a substrate of higher bandgap than the equilibrium solid deposited. Further, films of these materials formed by evaporation of compounds or their elements in high vacuum may be deposited in a controlled way to produce graded properties of the types discussed above.

More specifically, GaAs.sub. 0.85 Sb.sub. 0.15 was grown on GaAs substrates by liquid phase epitaxy resulting in the epitaxial layer having a bandgap gradient which varies from about 700 volts per centimeter near the substrate [1-2.mu.from the substrate] to about 20 volts per centimeter at a distance greater than 10.mu. from the substrate. As another example, a 2.mu. GaAs.sub. 0.94 Sb.sub. 0.06 photocathode was grown on a GaAs substrate resulting in a bandgap gradient of approximately 200 volts per centimeter. This example had a surprisingly good performance comparable to highly cleaned GaAs. In both of these examples, the first and second constituents were Ga and As respectively, while the third or additional material was Sb. The above examples had a measured sensitivity of approximately 700 .mu.amps per meter, with the second example having a lower threshold. Generally, the column III elements Ga, In, and Al; and the column V elements As, Sb, and P, are the preferred constituents of the III-V crystal because they may be combined in many combinations and proportions to produce binary or compound substrates and active layers. In some applications the remaining III-V elements B, Tl, N, and B.sub.1 may also be used.

FIG. 4 shows the energy level diagram for the graded dopant concentration embodiment. Except for the dopant, the composition of the III-V semiconductor remains constant throughout the active layer which in this embodiment may be a binary III-V crystalline solid. The concentration of the P-dopant, heaviest near substrate 32 and lightest near the emissive surface, causes conduction band 50 to slope downward toward the emissive surface. Thermal electrons from valance band 52 are captured by the dopant centers producing a nonuniform negative charge distribution across active layer 34. The positive holes generated by the escaping thermal electrons tend to diffuse themselves uniformly across the thickness of the active layer. An electric field develops between the stationary negatively charged dopants and the diffusing positively charged holes. The presence of this electric field limits the diffusion process resulting in an equilibrium condition. Thus substantial electric fields are generated causing conduction band 50 and valance band 52 to slope as shown. More specifically, a field of 750 volts/cm. was developed across a 2-micron active layer due to a Zn dopant gradient of from 10.sup.20 atoms/cc. on the substrate side to 10.sup.18 atoms/cc. on the vacuum side of the active layer. As the net bandgap between conduction band 50 and valance band 52 remains constant throughout the sloping region, no shift in response peaks is notice in the graded dopant embodiment. Many possibilities are available for accomplishing this gradiation of concentration. In the case of vapor phase epitaxy, a gradient of dopant could be effected by reducing the dopant content of the growth atmosphere, for example, by reducing the rate of injection of an organozinc vapor into the growth area, or by lowering the temperature of a zinc metallic or alloy doping source, during the growth process.

Clearly, various changes may be made in the structure and embodiments shown herein without departing from the concept of the present invention. For example, other III-V semiconductor materials and dopants may be employed. In some cases, the cesium layer may be employed along without the cesium oxide or both layers eliminated altogether. Thin active layers are preferably grown on a III-V substrate; however a thick active layer could be mounted on glass or other suitable material. Further, the features and advantages of each modification may be employed with the other modifications. For instance, the graded bandgap and graded dopants embodiments may be used separately or combined in a single photocathode.

It will be apparent to those skilled in the art that the objects of this invention have been achieved by providing a gradient of potential energy across the active layer for accelerating the free electrons thereacross and improving the response time of the cathode. The force generated by the gradient of energy potential biases the free electron motion toward the emissive surface and avoids electron loss in the substrate and interior regions of the active layer.

Claims

What is claimed is:

1. In a III-V semiconductor cathode for providing free electrons and having a built-in gradient of potential energy for enhancing the flow of free electrons therethrough towards an emissive surface, the combination comprising:

support means; and

an active layer supported by the support means and formed by a III-V semiconductor crystalline solid comprising a first constituent which is at least one element listed under column III of the periodic table and a second constituent which is at least one element listed under column V of the periodic table and a third constituent which is at least one additional element for determining the conduction band profile of the active layer in accordance with the concentration of the third constituent, the concentration of the third constituent being higher proximate one major surface of the active layer and lower proximate the other major surface of the active layer causing a decrease in the conduction band across the active layer thereby establishing the gradient of energy potential which promotes free electron drift toward the emissive surface.

2. The III-V cathode as specified in claim 1, wherein a thin cesium layer is formed over the emissive surface of the active layer to decrease the work function of the cathode.

3. The III-V cathode as specified in claim 2 wherein a cesium oxide layer is provided over the cesium layer to further reduce the work function.

4. The III-V cathode as specified in claim 1, wherein the support means is a III-V crystalline solid substrate upon which the active layer is grown by epitaxy.

5. The III-V cathode as described in claim 1 wherein the first constituent is at least one element selected from the group consisting of Al, Ga, and In, and the second constituent is at least one element selected from the group consisting of P, As, and Sb.

6. The III-V cathode as specified in claim 5, wherein the additional material is at least one element selected from either of the two groups of claim 5.

7. The III-V cathode as described in claim 1 wherein the cathode is a photocathode, and the changing composition of the active layer causes the conduction band to have a slope at least KT in a distance which is the inverse of the absorption depth for the incident photons.

8. The photocathode as described in claim 1 wherein the third constituent is a P-dopant material which decreases in concentration across the active layer from about 10.sup.20 atoms/cc. near the support means to about 10.sup.18 atoms/cc. near the emissive surface.