Doping of photocathodes with materials that have large piezoelectric coefficients has been proposed as an alternative means of increasing the desired photoemission of electrons. Treating cathode materials to increase emission of electrons is called “activation” in the art. It has been common practice to activate photocathodes by depositing thin layers of suitable metals (usually, cesium). Because cesium is unstable in air, fabrication of cesiated photocathodes and devices that contain them must be performed in sealed tubes under vacuum. It is difficult and costly to perform fabrication processes in enclosed, evacuated spaces. The proposed piezoelectrically enhanced photocathodes would have electron-emission properties similar to those of cesiated photocathodes but would be stable in air, and therefore could be fabricated more easily and at lower cost.
Candidate photocathodes include nitrides of elements in column III of the periodic table — especially compounds of the general formula AlxGa1–xN (where 0≤x≤1). These compounds have high piezoelectric coefficients and are suitable for obtaining response to ultraviolet light. Fabrication of a photocathode according to the proposal would include inducement of strain in cathode layers during growth of the layers on a substrate. The strain would be induced by exploiting structural mismatches among the various constituent materials of the cathode. Because of the piezoelectric effect in this material, the strain would give rise to strong electric fields that, in turn, would give rise to a high concentration of charge near the surface.
Examples of devices in which piezoelectrically enhanced photocathodes could be used include microchannel plates, electron-bombarded charge-coupled devices, image tubes, and night-vision goggles. Piezoelectrically enhanced photocathode materials could also be used in making highly efficient monolithic photodetectors. Highly efficient and stable piezoelectrically enhanced, ultraviolet-sensitive photocathodes and photodetectors could be fabricated by use of novel techniques for growing piezoelectrically enhanced layers, in conjunction with thinning and dopant-selective etching techniques.
This work was done by Robert A. Beach, Shouleh Nikzad, Lloyd Douglas Bell, and Robert Strittmatter of Caltech for NASA’s Jet Propulsion Laboratory.
This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to the Patent Counsel, NASA Management Office–JPL. NPO-40407
This Brief includes a Technical Support Package (TSP).
Piezoelectrically Enhanced Photocathodes
(reference NPO-40407) is currently available for download from the TSP library.
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Overview
The document is a Technical Support Package from NASA's Jet Propulsion Laboratory (JPL) focused on Piezoelectrically Enhanced Photocathodes, specifically highlighting the development and advantages of stable, high-efficiency III-Nitride photocathodes. It is part of NASA's efforts to advance technologies with potential applications in various fields, including astrophysics, advanced lithography, and life detection.
The document outlines the motivation behind the research into photocathodes, emphasizing their importance in photoemissive devices. These devices are crucial for applications that require efficient light-to-electricity conversion, such as in space exploration and advanced imaging technologies.
A significant portion of the document is dedicated to experimental data, including a summary table that presents various samples of III-Nitride materials, detailing their characteristics such as doping levels, thickness, and performance metrics like threshold energy and peak output. For instance, the table includes samples with different doping concentrations and their corresponding threshold energies, which range from approximately 4.1 eV to 5.2 eV, and peak outputs that vary significantly, indicating the potential for optimizing these materials for specific applications.
The document also includes a schematic of the emission measurement setup, which illustrates the methodology used to assess the performance of the photocathodes. This setup is essential for understanding the efficiency and effectiveness of the materials being tested.
Additionally, the document emphasizes the collaborative nature of the research, acknowledging the support from the National Aeronautics and Space Administration and the California Institute of Technology. It highlights the importance of compliance with export regulations and the proprietary nature of the information contained within.
Overall, the Technical Support Package serves as a comprehensive resource for understanding the advancements in photocathode technology, particularly the promising characteristics of III-Nitride materials. It aims to facilitate further research and development in this area, with the potential for significant technological, scientific, and commercial applications. The document encourages collaboration and innovation, inviting interested parties to engage with JPL for further information and support.
