RU2454750C2 - Photocathode - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: photocathode includes a dielectric or semiconductor substrate on which there is a layer of photoelectron-emitting n-type semiconductor with band gap Eg₁. Quantum dots are grown on the surface of the semiconductor layer from an n-type semiconductor with band gap of the substrate Eg₂, decorated by atoms of an electropositive metal with total thickness of up to 3.0 monolayers. Eg₂<Eg₁.

EFFECT: high quantum yield of photoemission and possibility of selectively analysing and detecting incident radiation in different spectral ranges.

4 cl, 5 dwg

Description

The invention relates to the field of electronic vacuum electronic equipment, namely to photocathodes operating in the visible and ultraviolet spectral ranges, which can be used in photoelectronic devices for recording weak signals, as well as electron-optical signal conversion used in optics, nuclear physics, automation, medicine, television technology and other fields.

A known photocathode for an electron tube (see patent US 3387161, IPC H01J 039/00, published 04.06.1968), the emission surface of which is made of a p-type semiconductor material having an acceptor impurity concentration of at least 10 ¹⁸ cm ^-3 and selected from the group consisting of semiconductor materials A ³ B ⁵ or their compounds having a band gap of 1.1-1.6 eV. The emission surface of the photocathode is activated by an alkali metal. Material A ^{3 is} selected from the group: B, Al, Ga, In. Material B ^{5 is} selected from the group: N, P, As, Sb.

The known photocathode has well reproducible semiconductor properties while providing a high quantum yield in the visible region of the spectrum. A disadvantage of the known photocathode is a sharp decrease in the quantum yield in the spectral range of wavelengths of 620-800 nm and in the near ultraviolet region of 300-400 nm.

A known photocathode based on a heterostructure (see patent RU 2335031, IPC H01J 1/35, published September 27, 2008), including a heterostructure containing a layer of p-type diamond with nanoscale topological inhomogeneities on its surface. Regularly located auto-emitting diamond tips or diamond nanocrystallites are used as nanoscale topological inhomogeneities, and the surface of the layer, with the exception of these inhomogeneities, is covered with a conductive amorphous carbon or nanocarbide film.

A disadvantage of the known photocathode is the increase in the work function of the photocathode due to the deposition of diamond-like films. The work function of the photocathode with a monolayer of diamond-like films deposited on it (diamond films or any other films of allotropic states of carbon) is 4.7 ± 0.2 eV. This leads to a decrease in the spectral sensitivity range of the photocathode, as well as to a decrease in the quantum yield of photoemission.

A known photocathode (see patent RU 2216815, IPC H01J 1/34, published November 20, 2003) is based on a lightly doped semiconductor structure with a pn junction, on which metal nanoparticles with linear dimensions of not more than 100 nm are applied, in order to increase the quantum yield in the visible region spectrum up to 60%. A disadvantage of the known photocathode is a narrow (about 40 nm) maximum in the spectral dependence of the quantum yield in the blue region of the spectrum at a wavelength of 455 nm.

Known photocathode (see patent US 6628072, IPC H01J 1/34, published 11/14/2002), coinciding with the claimed technical solution for most of the essential features and adopted as a prototype. The known prototype photocathode includes a layer made of a photoelectron emitting semiconductor substrate containing protrusions and depressions on the surface up to nanoscale in order to increase the surface / volume ratio, that is, increase the total area of the emitting surface for a fixed size of the substrate.

The disadvantage of this prototype photocathode is the use of only a geometric factor to increase the quantum yield of emitted photoelectrons, that is, by increasing the total surface area of the photocathode. Moreover, when using a substrate and nano-inhomogeneities from the same material, the possibility of the appearance of new mechanisms or channels of photoemission cannot be realized.

The objective of the invention is the development of a photocathode having several new photoemission channels of various physical nature, allowing to provide a given spectral sensitivity and a given quantum yield. This allows you to achieve the following technical results: firstly, selectively analyze and record the incident radiation in various spectral ranges from visible to ultraviolet, and secondly, increase the quantum yield of photoemission.

The problem is solved in that, firstly, the photocathode includes a dielectric or semiconductor substrate, on which a layer of n-type conductivity emitting photoelectrons semiconductor (with band gap Eg ₁ ) is deposited, on the surface of which quantum dots from an n-type semiconductor with band gap Eg ₂ , with Eg ₁ > Eg ₂ ), which ensures the availability of new photoemission channels with different physical nature. To increase the quantum yield of photoemission and create a new photoemission channel, a semiconductor substrate-quantum dot system is coated (decorated) with atoms of an electropositive metal with a total thickness of up to 3.0 monolayers. An increase in the degree of coverage by atoms of an electropositive metal to 3 monolayers leads to an increase in the quantum yield of photoemission.

With an increase in the number of deposited monolayers of more than 3 monolayers, the quantum yield of photoemission will decrease and be determined by the photoemissive properties of alkali (alkaline earth) metal and not solve the problem.

As an electropositive metal, which is applied to the semiconductor-quantum dot system, an alkali metal from the series Li, Na, K, Rb, Cs or an alkaline earth metal from the series Mg, Ca, Sr, Ba can be used. Quantum dots can also be coated with two different electropositive metals (e.g., Ba layer and Cs layer).

The application of electropositive metal atoms to the quantum dots provides a sharp increase in the electron density at the quantum dot, which was initially small (without coating). An increase in the electron density at the quantum dot leads to the appearance of new photoemission channels with a large quantum yield. In this case, the traditional photoemission channel from the semiconductor layer also sharply increases in quantum yield due to an inhomogeneous electric field caused by the presence of quantum dots.

In addition, the coating of electropositive metal atoms leads to a decrease in the work function of the photocathode, which is an important factor in increasing the quantum yield of the photocathode. The choice of coating from an electropositive metal is determined by the required spectral range of sensitivity of the photocathode and the choice of the dominant photoemission channel. The range of spectral sensitivity of the photocathode can be controlled and changed by selecting an electropositive metal.

The invention is illustrated in the drawing, where

figure 1 is a schematic sectional view of one embodiment of the inventive photocathode;

figure 2 is a schematic sectional view of another embodiment of the inventive photocathode;

figure 3 shows the band diagram of the semiconductor layer-quantum dot system;

4 is a zone diagram of a semiconductor-quantum dot system after decoration with electropositive metal atoms;

figure 5 shows the spectral dependence of the photocurrent for the case of excitation of photoemission from the first and second channels of photoemission.

In one embodiment of the inventive photocathode (see FIG. 1), it contains a photoelectron-emitting layer grown on a dielectric or semiconductor substrate 1, layer 2 of an n 3-type semiconductor A ³ B ⁵ (e.g. n-GaAs) on which quantum dots 3 are grown from semiconductor A ³ B ⁵ n-type conductivity (for example, InAs). Quantum dots 3 are coated with a single layer 4 of electropositive metal with a thickness of up to 3.0 monolayers. As an electropositive metal, an alkali metal (Li, Na, K, Rb, Cs) or an alkaline earth metal (Mg, Ca, Sr, Ba) can be deposited.

In another embodiment of the inventive photocathode (see FIG. 2), it contains photoelectron-emitting material 1 grown on a dielectric or semiconductor substrate 1 from an n ³ type semiconductor A ³ B ⁵ , for example, GaAs, on which quantum dots 3 from semiconductor A are grown ³ V ⁵ n-type conductivity (e.g. InAs). Quantum dots 3 are coated with a layer 4 of electropositive metal with a thickness of up to 3.0 monolayers, for example, of an alkali metal (Li, Na, K, Rb, Cs). On top of layer 4, layer 5 is made of another electropositive metal, for example, alkaline earth metal (Mg, Ca, Sr, Ba). The total thickness of electropositive metals is not more than 3.0 monolayers.

Consider the operation of the inventive photocathode, including a semiconductor or dielectric substrate 1, layer 2 of an n-type semiconductor material with a bandgap of Eg ₁ , on which quantum dots 3 of n-type semiconductor material with a bandgap of Eg ₂ and a work function are grown electron φ on the working surface of the photocathode with quantum dots 3, decorated with atoms of an alkaline or alkaline-earth metal 4. In this case, the following relation is satisfied:

The photoelectron emission is excited by irradiation with light from the side of the emitting surface of layer 2 at an angle to it. The angle between the normal to the surface and the exciting light can be from 0 ° to the Brewster angle. One of the main distinguishing features of the claimed photocathode is the choice of a semiconductor material of an n-type semiconductor. It is generally believed that the use of n-type semiconductors as the photocathode material is inefficient due to the presence of a negative charge on the surface, which prevents the escape of electrons from the semiconductor to vacuum. As a result, such photoelectronic emitters have a low quantum yield η <0.01.

Figure 3 shows the band diagram of the semiconductor-quantum dot system, which reflects the large work function φ ₁ and a small amount of energy E _B , which corresponds to the position of the bottom of the potential well of the quantum dot relative to the Fermi level E _F.

Figure 4 shows the band diagram of the semiconductor-quantum dot system after decoration with metal atoms, which reflects a decrease in the work function to φ ₂ and a significant increase in energy E _B. An increase in the depth of the potential well of quantum dots and energy E _B leads to an increase in the number of occupied quantum levels in the potential well and, accordingly, to a substantial increase by more than an order of magnitude of the density of occupied electronic states participating in the photoemission process (channel II). When the atoms of electropositive metals are decorated with a system, the semiconductor-quantum dot layer also sharply increases, more than an order of magnitude, the photomission from channel I, due to a decrease in the work function and due to an increase in the pulling electric inhomogeneous field of quantum dots.

It is known that the quantum yield η of photoelectron emission is determined by:

where: B ₁ - the probability of excitation of the filled electronic states of the semiconductor with light to an energy level with an energy higher than or equal to the work function;

In ₂ - the probability of reaching the surface electrons;

In ₃ - the probability of overcoming the surface barrier by electrons.

The probability В _{1 is} proportional to the density ρ (hν), cm ^-3 eV ^-1 , of occupied electronic states participating in photoelectron emission, and to the matrix element M ₁ , cm ³ , of the excitation of occupied electronic states of a semiconductor participating in photoelectron emission:

where: C ₀ - constant, eV.

For photoemission in the presence of two photoemission excitation channels, the probability B ₁ is determined by the sum of two probabilities corresponding to two photoemission channels (B ₁₁ and B ₁₂ ).

For photoelectron emission, which occurs when electrons are excited from the valence band (I channel), where the edge of the band is E _v , eV, and where the density ρ ₁ (E) of occupied electronic states at the edge of the band is zero at E = E _v , and then slowly increases by law:

where: C ₂ - constant, cm ^-3 eV ^-3/2 ;

E is energy, eV.

For photoelectron emission from quantum dots (channel II), entrainment of the number of occupied levels of dimensional quantization during decorating by atoms of electropositive metals leads to an increase in the density ρ ₂ (E) of occupied electronic states, which directly leads to an increase in the quantum yield of photoemission.

The matrix elements for the excitation of photoelectron emission from the valence band and the size quantization levels at quantum dots are denoted as M ₁₁ and M ₁₂ , cm ³ , respectively. So, in the inventive device there is a sharp increase, compared with the prototype photocathode, the density of occupied electronic states involved in photoelectron emission.

The probability B _{2 is} proportional to the diffusion length of the charge carriers. It is well known that the diffusion length in an n-type semiconductor material is always higher than in a p-type material. Thus, both conditions lead to an increase in the quantum yield for the claimed device. Probability B _{3 is} not worse than that of the prototype photocathode, since in the claimed photocathode it depends only on the excitation energy hν, work φ of the material yield.

Example 1. Consider photoemission from a semiconductor layer (n-GaAs) -quantum dots (InAs) system, decorated with electropositive metal atoms (Cs) with a coating of 0.8 monolayer. The band gap of the substrate is Eg ₁ = 1.43 eV, and the band gap of quantum dots is Eg ₂ = 0.40 eV. Cs coating leads to a decrease in the work function of the photocathode to 1.6 eV. This leads to the appearance of photoemission from the size quantization levels in InAs quantum dots in the wavelength range (500-800 nm) - channel II of photoemission, and photoemission from the GaAs valence band in the wavelength range (340-500 nm) - channel I of photoemission.

Claims (4)

1. A photocathode comprising a dielectric or semiconductor substrate, on which a layer of n-type conduction emitting photoelectrons semiconductor with a band gap E _{g1 is deposited} , on the surface of which emitting photoelectrons quantum dots are grown from an n-type semiconductor with a bandgap E _g2 , decorated atoms of an electropositive metal with a total thickness of up to 3.0 monolayers, with E _g2 <E _g1 . 2. The photocathode according to claim 1, characterized in that an alkali metal is used as the electropositive metal. 3. The photocathode according to claim 1, characterized in that the alkaline earth metal is used as the electropositive metal. 4. The photocathode according to claim 1, characterized in that the quantum dots are coated with two layers of different electropositive metals.

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