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EP-4365924-B1 - SUBSTRATE STACK EPITAXIES FOR PHOTOCATHODES FOR EXTENDED WAVELENGTHS

EP4365924B1EP 4365924 B1EP4365924 B1EP 4365924B1EP-4365924-B1

Inventors

  • PANTHA, BED
  • BECKER, JACOB J.
  • BURNSED, JON D.

Dates

Publication Date
20260513
Application Date
20231030

Claims (7)

  1. A method of forming a photocathode, the method comprising: forming an improved substrate stack by forming: a GaAs substrate (404); and more additional layers on the GaAs substrate (404), the more additional layers configured to provide an improved substrate stack (402) surface with predetermined characteristics for forming a semiconductor device on the improved substrate stack (402) surface; and forming an InGaAs p-type photocathode (106) on the improved substrate stack (402) surface, the InGaAs p-type photocathode (106) having a predetermined percentage of In, characterised in that forming the more additional layers on the GaAs substrate (404) comprises forming a plurality of step graded InGaAs layers (408-1 - 408-n) which gradually increase an amount of Indium doping from layer to layer from a first layer (408-1) more proximate the GaAs substrate (404) to a final layer (408-n) less proximate the GaAs substrate (404), to approach or match the Indium percentage in the InGaAs p-type photocathode.
  2. The method of claim 1, wherein forming the more additional layers on the GaAs substrate (404) comprises forming the plurality of step graded InGaAs layers (408-1 - 408-n) and an overshoot layer (408-OS) which gradually increase an amount of Indium doping from layer to layer from the first layer (408-1) more proximate the GaAs substrate (404) to the overshoot layer (408-OS) between the final layer (408-n) and the InGaAs p-type photocathode (106), wherein the overshoot layer (408-OS) has a first amount of Indium doping that is greater than a second amount of doping in the InGaAs p-type photocathode (106) to reduce strain in layers from the GaAs substrate (404) to the InGaAs p-type photocathode (106).
  3. The method of claim 2, wherein the overshoot layer (408-OS) is at least 150% as thick as the final step graded InGaAs layer (408-n).
  4. The method of claim 1, wherein forming an InGaAs p-type photocathode (106) comprises forming an active layer (416) doped exponentially by p-type impurities with levels of doping increasing away from an interface between the InGaAs p-type photocathode (106) and the substrate stack.
  5. The method of claim 1, further comprising forming a fully strained GaAs layer (414) to be between an etch stop layer (412) and an active layer (416) of the InGaAs p-type photocathode (106), wherein the etch stop layer (412) is between and immediately adjacent to the overshoot layer (408-OS) and the fully strained GaAs layer (414).
  6. The method of claim 5, further comprising forming a CsO layer (424) on the fully strained layer for activation.
  7. The method of claim 5, further comprising forming a window layer (418) on the active layer of the InGaAs p-type photocathode (106).

Description

BACKGROUND Background and Relevant Art Nightvision systems allow a user to see in low-light environments without external human visible illumination. This allows for covert vision in a low-light environment to prevent flooding the environment with human visible light. Some nightvision systems function by receiving low levels of light reflected off of, or emitted from objects and providing that light to an image intensifier (sometimes referred to as I2). The image intensifier has a photocathode. When photons strike the photocathode, electrons are emitted into a vacuum tube, and directed towards a microchannel plate to amplify the electrons. The amplified electrons strike a phosphor screen. The phosphor screen is typically chosen such that it emits human visible light when the amplified electrons strike the phosphor screen. The phosphor screen light emission is coupled, typically through an inverting fiber-optic, to an eyepiece where the user can directly view the illuminated phosphor screen, thus allowing the user to see the objects. Spectral response from the state-of-the-art Gen III (GaAs) photocathodes cuts-off at around 900nm. This may be satisfactory for implementing devices configured to observe objects that would normally be visible to humans in lighted conditions. However, this spectrum cut-off may be unsuitable for other uses. For example, it may be useful to have a device that functions with wavelengths up to a 1550 nm. This wavelength is particularly useful as it represents a maximum wavelength suitable for eye-safe lasers for manufacturing long-range rangefinders and/or laser guidance and laser painting systems. Thus, if a user desires to have a traditional nightvision system that also allows for viewing certain laser-based systems, this may not be possible with current technology. To the extent that current systems are able to function up to 1550 nm, those systems are generally manufactured using inferior manufacturing techniques which may reduce sensitivity overall, or at least portions of, the usable spectrum. The subject matter claimed herein is not limited to embodiments that solve any disadvantages or that operate only in environments such as those described above. Rather, this background is only provided to illustrate one exemplary technology area where some embodiments described herein may be practiced. From US 5 268 570 A a method for forming a photocathode is known. The document discloses a buffer layer that can be epitaxially grown via the grading technique or the super lattice technique. The graded technique comprises starting with a GaAs substrate, and gradually increasing the percentage of indium in an InGaAs compound during growth of the buffer layer. The super lattice technique comprises growing extremely thin alternating layers of GaAs and InGaAs, in the same atomic concentration as will be used in an active layer compound. From US 6 110 758 A a method for making a transmission mode photocathode with active layer for night vision. The method comprises a compositional grading of active layers of the photocathode to reduces crystal stress between a window layer and an active layer. From CN 113 690 119 A a laminated composite GaAs-based photoelectric cathode with enhanced near-infrared response is known. The method comprises a linearly graded buffer layer that is grown on a GaAs buffer layer. BRIEF SUMMARY According to the present invention there is provided a method of forming a photocathode as defined in present claim 1. Preferred features are specified in the dependent claims. Additional preferred features and advantages will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of the teachings herein. BRIEF DESCRIPTION OF THE DRAWINGS In order to describe the manner in which the above-recited and other advantages and features can be obtained, a more particular description of the subject matter briefly described above will be rendered by reference to specific embodiments which are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments and are not therefore to be considered to be limiting in scope, embodiments will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: Figure 1 illustrates an example nightvision system;Figure 2 illustrates a block diagram of portions of a nightvision system;Figure 3 illustrates an improved photocathode;Figure 4 illustrates an improved epitaxial structure for forming an improved photocathode; andFigure 5 illustrates a method of forming an improved photocathode. DETAILED DESCRIPTION One embodiment illustrated herein includes a manufacturing process which, instead of GaAs structures used in photocathodes, uses high quality metamorphic InGaAs structures. Using these materials, embodiments can vary a band gap of the material from about 1.4 to 0.7 eV (i.e., 1.1 eV