RU2806151C1 - Photocathode - Google Patents

Abstract

FIELD: vacuum emission optoelectronics.

SUBSTANCE: invention can be used as a sensor-conversion medium of photocathodes for photomultiplier tubes (PMTs) or electron-optical converters (EOCs) in the design of night vision devices and equipment. The technical result is an increase in the threshold sensitivity of the night vision receiver due to the possibility of recording images of objects in the spectral range of 1.25-1.6 microns, the range of increased radiation intensity of the night sky.

EFFECT: germanium/polycrystalline diamond heterostructure is used as a sensor-transforming medium, which makes it possible to expand the “red limit” of the spectral sensitivity range to 1.6 mcm.

1 cl, 4 dwg

Description

This invention relates to vacuum emission optoelectronics. It can be used as a sensor-transforming medium for photocathodes in the design of night vision instruments and devices. Currently, when forming sensor-conversion layers of photocathodes of electron-optical converters (EOCs) and photomultiplier tubes (PMTs), alkali metals (so-called bi-alkali and multi-alkaline photocathodes) are usually used [1-2]. The advantages of using alkaline materials in this capacity are the broadband nature of their spectral photosensitivity (0.4...1.1 microns) and a fairly high threshold sensitivity (associated with the low electron affinity energy of a wide range of alkali metals). The disadvantages of photocathodes based on such media include their unsatisfactory resistance (“fear”) to powerful pulsed or stationary light exposure (the photosensitive medium temporarily “blinds” or degrades).

Among the solid-state materials of sensor-transforming media of photodetectors sensitive in the spectral range of 0.5...1.6 microns, the most striking representative is high-purity germanium [3]. Moreover, due to the peculiarities of its crystallography and p-n band structure, germanium transition photodetectors practically do not require cooling under operating conditions down to temperatures of ~ 40°C. In addition, the internal quantum efficiency of high-purity germanium in the spectral range of 1.25-1.55 μm reaches a value of ~ 0.8. However, numerous attempts to use germanium as a sensor-converting layer of photocathodes of emission image detectors ended in failure. The reason is that the electron affinity energy of germanium is quite high (~ 4 eV), and the traditional approach to reducing it by using nanometer-thick bilayer coatings with alkali metals (for example, antimony-cesium) has not led to success. Photoemissive night vision devices with photocathodes based on semiconductor sensor-converting layers based on gallium arsenide are known [4-7]. Despite the high electron affinity energy values of GaAs, it was possible to adapt it for use as sensor-converting layers of photocathodes sensitive in the spectral range of 0.5...0.9 μm. The use of a nanometer-thick bilayer coating of the surface of gallium arsenide with an alkali metal oxide, oxygen - cesium [8], made it possible to reduce the electron affinity energy of gallium arsenide from 4.07 eV to acceptable values of ~ 1.0-1.5 eV. Semiconductor GaAs photocathodes, when exposed to powerful optical radiation, although temporarily become inoperative (for time intervals of several tens of seconds), but after the cessation of powerful light exposure in real time, they relax to a stationary state. The red limit of the photosensitivity of the GaAs photocathode is ~0.9 µm.

The disadvantages of existing alkaline and semiconductor (for example, GaAs) photocathodes include the limitation of the “red” limit of the spectral range of their photosensitivity to 0.9...1.1 microns.

It is known, however, that the intensity of night sky radiation in the spectral range of 1.3...1.6 microns is two to three orders of magnitude higher than the night sky radiation in the range of 0.9...1.1 microns. And since in night vision devices, images of objects and terrain are formed in reflected fluxes of radiation emitted by the night sky, the development of the spectral range of 1.3...1.6 microns by photoemission night vision devices will lead to a significant (at least two orders of magnitude) increase in intensity images, and therefore the threshold sensitivity of the receiver and the image contrast near the photosensitivity threshold.

The objective of the present invention is to expand the spectral range of sensitivity of the sensor-converting medium of photocathodes of vacuum emission radiation and image detectors for infrared (IR) image intensifier tubes and IR photomultipliers, up to the “red” limit of 1.55 microns.

This goal is achieved by the fact that the photocathode is based on a semiconductor material with a surface activated for photoelectron emission processes [8], the sensor-transforming medium is made on the basis of a germanium/polycrystalline diamond heterostructure.

The expectation of positive success is associated with the presence of a negative electron affinity energy at a number of crystallite faces of the diamond film [(100) and (111)], as well as small values of the gap in E _c (the energy level of the bottom of the conduction band) at the germanium/diamond heterointerface. Indeed: the electron affinity energy of germanium is ~4 eV, the electron affinity energy of a number of diamond faces is ~3.8 eV, and the electron affinity energies of the (100) and (111) faces of diamond crystallites are negative.

In fig. Figure 1 shows a schematic representation of the architecture of an image detector with a photocathode based on a germanium/diamond heterostructure IR image intensifier tube as part of a vacuum-tight housing (1), an input window (2), a claimed Ge/C* heterostructure photocathode (3), and an electron flow multiplier (4). (MCP - microchannel plate), cathode-luminescent screen (5), fiber-optic plate, VOP (6), contacts (ϕ _i ).

In fig. Figure 2 shows the band diagram of a germanium/polycrystalline diamond heterojunction for the (111) and (100) faces of crystallites of a polycrystalline diamond film. The band gap of Ge is 0.8 eV (i.e., its “red boundary” is ~ 1.55 μm), the band gap of diamond is ~ 5.5 eV (i.e., the spectral region of its transparency is the IR, visible and UV ranges , up to 0.25-0.30 microns). The qualitative nature of the band diagram of the Ge/C* tetherjunction shown in the figure allows us to assert that when nonequilibrium electron-hole pairs in Ge are excited by optical radiation up to the “red” boundary of 1.55 μm, the effective implementation of photoelectron emission processes from the Ge/C* heterostructure is possible into a vacuum, both in the “transmission” and “reflection” operating modes.

In fig. 3a, b show photographs of films of high-purity germanium grown on quartz substrates (3a) with a thickness of 250 nm, and a Ge/C* heterostructure grown on the back side of the quartz input window (3b).

In fig. Figure 4 shows the elemental composition profiles of a germanium film with an adhesion layer of monocrystalline silicon.

To implement the proposed photocathode for a vacuum emission radiation detector or image receiver (IR photomultiplier, or IR image intensifier), it is necessary to form a sensor-transforming medium on a substrate that is transparent for operation of the device in the spectral region of 0.5...1.6 μm, or in region 1 ,25…1.60 µm (area of optimal operation for registering and recognizing images of objects in the reflected light of the night sky (night vision receivers). It should be noted that the use of the substrate must be compatible with the technological conditions of film formation (i.e. the substrate must withstand growth temperature of the constituent layers of the Ge/C* heterostructure, and the chemical effects of the growth reagents used).It is proposed to form a sensor-transforming medium based on the Ge/C* heterostructure through the sequential use of germanium molecular beam epitaxy (MBE) and plasma-enhanced vapor phase epitaxy (PECVD) diamond film doped with nitrogen, C*: N. It is proposed to choose quartz or MgF ₂ as a substrate for the sensor-transforming layers.

The ranges of possible technological regimes for the process of molecular beam epitaxy of germanium films are as follows: temperature range of film growth - 450-700°C, residual gas pressure - 10 ^-6 ... 10 ^-9 mm Hg. Art., growth rate - 50...250 nm/hour.

The ranges of possible technological modes for the PECVD process of growth of polycrystalline diamond film are as follows: temperature range - 650-850°C, residual gas pressure - 10 ^-2 ... 10 ^-6 mm Hg. Art., growth rate - 100...400 nm/hour, initial growth reagents - methane and hydrogen, doping reagent - nitrogen (donor) or boron (acceptor).

The performance of the method proposed above for implementing a sensor-transforming structure has been tested in the range of some variations in the technological modes for its production. Thus, the tested ranges of variations in the technological regimes of the process of molecular beam epitaxy of germanium films were: temperature range - 550-650°C, residual gas pressure - 10 ^-7 ... 10 ^-8 mm Hg. Art., growth rate - 100...150 nm/hour.

The ranges of variations in the technological modes of the PECVD process of polycrystalline diamond film growth were: temperature range - 750-850°C, residual gas pressure - 10 ^-2 ... 10 ^-5 mm Hg. Art., growth rate - 200...300 nm/hour, initial growth reagents - methane and hydrogen, alloying reagent - nitrogen (gas) and boron (thermal spray of trimethyl borate).

The optimal regimes for the sequential growth of films of high-purity germanium and polycrystalline diamond were selected:

- formation of an adhesion layer on the back side of the quartz input window of the image intensifier tube - from a silicon film with a thickness of ~ 150 nm,

- formation of a sensor-transforming layer based on high-purity germanium - thickness 250 nm,

- Ge film growth temperature - 750°C,

- pressure of residual gases in the growth chamber (with growth of Ge) - 10 ^-8 mm Hg. Art.,

- PECVD formation of a polycrystalline diamond film on the surface of a germanium film:

- pressure of residual atmospheric gases - no more than 10 ^-5 mm Hg. Art.,

- initial growth reagent - high-purity methane,

- reducing-synthesizing medium - hydrogen (in a ratio of 10:1 with the growth reagent),

- doping reagent - nitrogen (~ 1% of the growth reagent),

- growth temperature - 800...850°C,

- microwave power - 1.5 kW,

- microwave frequency - 2.4 GHz,

- duration of the growth process (C*) - 6 hours (including heating, holding and cooling).

The result of the sequence of the listed technological processes is the formation of a Ge/C* photocathode formed on the pedestal of the back side of the quartz input window of the image intensifier tube.

Measurements of the quantum efficiency coefficient (with respect to photoconductivity processes) of a high-purity germanium film grown in the MBE process in the above modes on the quartz glass of the input window of the image intensifier tube showed a value of ~ 60%, however, the value of the photoelectron emission coefficient of photoelectrons into vacuum from the surface of the germanium film did not exceed ~ 3…5%.

On the contrary, the results of measurements of the quantum efficiency coefficient of the Ge/C* sensor-converting heterostructure formed on the back side of the quartz glass of the input window of the image intensifier tube showed a value of ~ 45%, while the value of the coefficient of photoelectron emission of nonequilibrium electrons into vacuum was ~ 20...30% , which was 4-6 times higher than the photoemission values from the surface of the germanium film.

Below in Fig. 3a, b show photographs of films of high-purity germanium grown on quartz substrates (Figure 3a) with a thickness of 250 nm, and a Ge/C* heterostructure grown on the back side of the quartz input window, with a germanium film thickness of 250 nm, and a polycrystalline diamond film with a thickness of ~ 1.2…1.8 µm (Figure 3b).

In fig. Figure 4 shows profiles of the elemental composition of a germanium sensor film with an adhesive layer of single-crystalline silicon of nanoscale (150 nm) thickness.

The high values of the quantum efficiency of the germanium film in the IR spectral range up to 1.55 microns, the high quality of the surface morphology of the germanium film, and the high values of the photoelectron emission coefficient of the Ge/C* heterostructure allow us to count on the efficiency of use in the spectral range of 1.25- 1.55 microns (the region of maximum radiation intensity of the night sky) of the proposed sensor-converting layer and, based on it, a photocathode in vacuum emission image receivers of the image intensifier tube architecture.

Information sources:

1. Yu.K. Gurevich. Optical-electronic night vision devices. Moscow. Fizmatlit, 2014

2. Beguchev V.P., Burlakov I.D. //Night-vision devices. - M. MIREA, 2015

3. Masini G., Cencelli V., et. al.//Appl. Phys. Let's. 2002. Vol. 80. P. 3268

4. Koshavtsev N.F., Koshavtsev A.N., Fedotova S.F. //Applied Physics, 1999, No. 3, p. 66

5. Sheer J.J., van Laar J. GaAs-Cs: A new type of photoemitter. - Solid State Commun., 1965, v. 3, p. 189-193.

6. Tumbull A.A., Evans G.B. Photoemission from GaAs-Cs-O. - J. Phys. D: Appl. Phys., 1968, v. 1, no.2, p. 155-160.

7. Bakin V.V. Semiconductor surfaces with negative electron affinity / V.V. Bakin, A.A. Pakhnevich, A. G. Zhuravlev, A.N. Shornikov, I.O. Akhundov, O.E. Tereshchenko, V.L. Alperovich, H. E. Scheibler and A.S. Terekhov.//. e-J. Surf. Sci. Nanotech. - 2007. - Vol. 5. - p. 80-88.

8. Bell R.G. Emitters with negative electronic means. Per. from English M.: Energy, 1978, p. 192.

Claims (1)

A photocathode representing a sensor-transforming medium based on a semiconductor material with a surface activated for photoelectron emission processes, formed on a substrate transparent to radiation in the spectral range of 0.5-1.6 microns, characterized in that the said sensor-transforming medium is made on the basis of a heterostructure germanium/polycrystalline diamond.