EP0230818A1 - Photo cathode with internal amplification - Google Patents

Abstract

The invention relates to a photocathode for television picture tubes or image intensifier tubes. In an exemplary embodiment, this photocathode comprises: - A transparent layer (1); - An absorption layer (2) whose prohibited bandwidth is such that it converts the electron photons (8) of the light to be detected into electron-hole pairs; - A layer (3) called electron multiplication, having a non-uniform composition providing a variable forbidden bandwidth which allows impact ionization, without noise; - A layer (4) called the transport layer; - A polarization electrode (5), allowing the electrons released by the photons to be accelerated, to multiply them by ionization in the multiplication layer (4); - A layer (6) providing a negative electronic affinity for emitting electrons (7) in a vacuum. Application to television cameras and image intensifier tubes.

Description

The invention relates to a photocathode for a picture tube and an image intensifier tube.

It is known to produce a photocathode comprising mainly:
a layer, called a window layer, consisting of P⁺ type semiconductor, the band gap of which is sufficiently wide for this layer to be transparent for the wavelengths of the light to be detected, and which is bonded to a wall of glass receiving the light to be detected;
- A layer, called absorption layer, consisting of a P⁺ type semiconductor whose band gap has a sufficiently small width to convert into photon-hole pairs the photons of the light to be detected;
- A layer, called the emission layer, made of a material giving the end of the absorption layer a negative electronic affinity to emit in a vacuum the electrons released in the absorption layer.

The maximum detectable wavelength is limited by the width of the forbidden band of the material constituting the absorption layer. By applying a positive polarization at the end of this layer opposite to the window layer, it is possible to use materials having a small prohibited bandwidth while retaining a good emission efficiency and therefore it is possible to detect longer wavelength light.

A polarization of the absorption layer can be applied by means of a connection with this layer, or by a very thin metal electrode interposed between this layer and the layer resignation. Such a photocathode is described in the article by: JJ ESCHER et al, IEEE-EDL2, 123-125 (1981).

To make a shooting tube with a very low level of illumination, in particular to make an image intensifier tube, it is known to place, downstream of the photocathode, a wafer of microchannels supplied by a high voltage generator and allowing a multiplication of the electrons emitted in the vacuum by the photocathode. Such a microchannel wafer is very effective in multiplying these electrons but imposes numerous technological constraints, in particular imposes the use of a high voltage generator. The object of the invention is to produce a photocathode with internal amplification making it possible to use a microchannel wafer having a lower gain, and therefore imposing less technological constraints, or even to eliminate the microchannel wafer. The object of the invention is a photocathode comprising an absorption layer of particular structure providing a multiplication of the electrons without notably multiplying the current of holes, the latter causing a dark current which constitutes a noise.

According to the invention an internally amplified photocathode, comprising a so-called absorption layer made of a P⁺ type semiconductor material, the forbidden band of which is sufficiently small to convert the photons of the photon into hole-pairs. light to be detected, is characterized in that it further comprises at least one so-called electron multiplication layer by ionization, consisting of a N⁻ type semiconductor material where the composition is not uniform and such that, when this multiplication layer is polarized, the electrons are accelerated in the direction in which they are to be evacuated and the holes are less accelerated than the electrons; and comprises means making it possible to polarize the multiplication layer.

The invention will be better understood and other details will appear with the aid of the above description and the accompanying figures:

FIGS. 1 to 4 each represent an exemplary embodiment of the photocathode according to the invention and two diagrams of the energy levels E of the charge carriers inside these exemplary embodiments, on the one hand without polarization and, d on the other hand, with polarization.

Figure 1a shows a section of a portion of a first embodiment of the photocathode according to the invention. This first example includes:
- A first layer 1, transparent for all the wavelengths of the light to be detected, consisting of a P⁺ type semiconductor material bonded to a wall of glass not shown, and receiving through this wall photons 8 ;
- A second layer 2, called absorption layer, made of a P⁺ type semiconductor material, for converting each photon 8 into an electron-hole pair;
- A third layer 3, called the electron multiplication layer, consisting of a N⁻ type semiconductor material whose composition varies continuously;
- A fourth layer 4, called the transport layer, made of a P⁺ type semiconductor material, which only transmits the electrons released by the photons 8 in the layer 3;
- A metal electrode 5, connected to the positive terminal of a voltage generator V, the negative terminal of which is connected to the first layer 1, in order to polarize the four layers 1, 2, 3, 4 to accelerate the electrons released by the light to be detected;
- A last layer 6, giving the surface of the fourth layer 4 the property of negative electronic affinity allowing the electrons 7 transmitted by layer 4 to be emitted in a vacuum.

FIG. 1b represents a diagram of the energy levels E of the charge carriers inside this exemplary embodiment when it is not polarized. In this figure, the curve E _c represents the minimum level of energy of the conduction band, E _v represents the maximum level of energy of the valence band, E _F1 represents the Fermi level of layer 1, E _F5 represents the Fermi level of the metal electrode 5, E _c6 represents the minimum level of energy of the conduction band of the last layer 6, and E _vi represents the potential of the vacuum. The energy levels of the valence bands of the metal electrode 5 and of the last layer 6 and are not shown because they are very low.

In this figure, it appears that layer 1 has a large forbidden bandwidth, corresponding to the transparency of this layer for the light to be detected. Layer 2 has a smaller band gap than the previous one and which makes it possible to detect all the wavelengths of the light to be detected. Layer 3 has a conduction band and a valence band whose energy levels are respectively lower than those of the conduction band and the valence band of the two preceding layers and whose forbidden bandwidth varies linearly with decreasing from layer 2 to layer 4, that is to say in the direction where the electrons are to be removed. In this example, layer 3, on the side of layer 2, has a width of the forbidden band equal to that of layer 1 whereas on the side of layer 4 this width is equal to that of layer 4. In the layer 3, the slope of the curve E _c is almost zero, while the slope of the curve E _v is positive in the direction of layer 4.

Layer 4 has the same energy levels as layer 2, for its conduction band and its valence band, because in this example layers 2 and 4 are made with the same material and the same doping. In the absence of polarization, the Fermi E _{F1 level} of layer 1 and the Fermi E _{F5 level} of layer 5 are aligned and there are two potential steps in the conduction band and in the valence band, in the diagram area corresponding to layer 3.

FIG. 1c represents a diagram of the energy levels of the carriers in the same exemplary embodiment but when a polarization is applied. If V is the value of the voltage applied between the metal electrode 5 and the first layer 1, the Fermi E _{F5 level} of the electrode 5 is lowered by a value qV with respect to at the Fermi E _{F1 level} of the first layer, q being the value of the charge of an electron. The energy level curves of the conduction band and the valence band of layers 3, 4, and 5 are lowered. The curve of the minimum energy level of the conduction band of layer 3 presents a strong negative slope in the direction of layer 4, corresponding to an acceleration of the electrons in the direction of layer 4. When this acceleration is sufficiently significant the electrons are multiplied by impact ionization. On the other hand, the curve of the maximum energy level of the valence band of layer 3 has a much smaller negative slope, because the gradual variation of the composition of the material gives it a strong positive slope in the absence of polarization . This much lower slope gives the holes an acceleration in the direction of layers 2 and 1, much lower than that provided to the electrons. The holes are therefore multiplied in a much lower ratio than the electrons, which avoids increasing the noise of the photocathode.

The curves of the extrema of the energy levels of the conduction band and the valence band of layer 4 are connected to the curves of the extrema of the energy levels of the conduction band and of the valence band of the third layer 3 with a threshold which is practically zero, which allows an easy passage of electrons and holes between layers 3 and 4. The electrons then pass through layer 5 and layer 6 and are ejected in vacuum, their acceleration being sufficient to tunnel through the potential well located at layer 5 and the potential step located at layer 6, layers 5 and 6 being extremely thin.

In this exemplary embodiment, layer 1 consists of Ga _0.6 Al _0.4 As with doping of 5.10 5.1 zinc atoms per cm³ and has a thickness of the order of 1 micron. Layer 2 consists of GaAs with a doping of 10¹⁹ zinc atoms per cm³, and has a thickness of 2 microns. Layer 3 consists of Ga _1-x Al _x As in which x varies from 0.6 to 0 from layer 3 to layer 4 and whose doping consists of 10¹⁵ zinc atoms per cm³. The thickness of layer 3 is 1 micron. It is chosen so as to be a little less than the diffusion length of the carriers. Layer 4 is made of the same material as layer 2 and has a thickness of 0.1 microns. Its surface is covered with a very thin layer of silver or a silver mesh to constitute the metal electrode 5, then is covered with a layer of Cs + O to give it a negative electronic affinity. An alternative embodiment may consist in eliminating the metal electrode 5 and in applying the polarization by connecting the positive terminal of the generator V to the layer 4.

The value of the bias voltage V is chosen such that the slope of the curve E _v of the minimum level of energy of the conduction band of layer 3 is negative in the direction of layer 4 in order to accelerate the electrons. In an exemplary embodiment, this bias voltage is of the order of 15 volts.

FIG. 2a represents a section of a portion of a second embodiment of the photocathode according to the invention. This second example includes:
- A first layer 30 similar to the first layer 1 of the first embodiment, transparent to the light to be detected;
- A second layer 31, which is an absorption layer similar to layer 2 of the second embodiment;
- Ten electron multiplication layers, consisting of 20 sublayers: 32, 33, 34, 35, ..., 36, 37, 38, 39, 40;
- A transport layer 41, similar to layer 4 of the first exemplary embodiment but which is connected to the positive terminal of the voltage generator V because there is no metal electrode in this exemplary embodiment;
- A last layer 42 providing a negative electronic affinity to layer 41.

The 10 electron multiplication layers are identical and each has two sublayers. For example, the layer multiplication 32-33 comprises a first sublayer 32 and a second sublayer 33 which are made of two semiconductor materials of type N type having respectively two different compositions corresponding to two different widths for the band gap, and these two widths being greater than that of the material of the absorption layer 31.

FIG. 2b represents a diagram of the energy levels of the carriers at the different points of this second exemplary embodiment, in the absence of polarization. It appears that the curves E _c and E _v of the extrema of the energy levels of the conduction band and of the valence band in the layers 32 to 40 comprise steps of potential, corresponding to the sublayers 33, 35,. .., 37, 39 which have a prohibited bandwidth greater than that of the sublayers 32, 34, ..., 36, 38, 40.

FIG. 2c represents a diagram of the energy levels of the charge carriers at different points of this exemplary embodiment, when a polarization of value V is applied to the layer 41 relative to the layer 30. In the zone corresponding to the layers 32 at 40 the curves E _c and E _v of the extrema of the energy levels of the conduction band and of the valence band have a negative slope corresponding to an acceleration of the electrons in the direction of layer 41, that is to say say in the direction where the electrons are to be evacuated, and an acceleration of the holes in the direction of the layer 31. This acceleration is sufficient for the electrons to tunnel through the steps of potential located at the limit between the sublayers 32 and 33, 34 and 35, ..., 38 and 39. Each time an electron crosses one of the descending potential steps located at the limit of the sublayers 33 and 34, 35 and 36, ..., 39 and 40, it undergoes, upon descent, a brutal acceleration which allows it to release an additional electron by impact ionization, and these two electrons then take the next step by creating two other additional electrons.

The holes undergo a much less effective multiplication because the tunnel effect is weaker because of their effective mass more higher than that of electrons. On the other hand, the materials constituting the sub-layers 32, 33, ..., 39, 40 are chosen such that the potential steps in the valence band are lower than in the conduction band to communicate to the holes a weaker acceleration than with electrons.

Theoretically, the number of electrons can be multiplied up to twice, each time a potential step is crossed, if the polarization is sufficient. The multiplication factor can theoretically reach 10³ for 10 multiplication layers, each comprising two sublayers. In an exemplary embodiment, the polarization V is of the order of 20 volts, each sublayer 32, 34, ..., 36, 38, 40 consists of Ga _0.9 Al _0.1 As having a width band gap of 1.56 ev and a thickness of 0.05 micron, and each sublayer 33, 35, ..., 37, 39 consists of Ga _0.7 Al _0.3 As having a band width prohibited of 1.8 ev and a thickness of 0.05 micron.

The thickness of a set of two successive sublayers has a value which is chosen to be of the same order of magnitude as the mean free path of ionization by impact of electrons.

The ideal composition of the materials constituting these two types of sub-layers would be such that the difference in level of their conduction bands is greater than the ionization energy of the material having the smallest prohibited bandwidth among these two materials.

In the absence of an ideal composition, a composition is chosen such that the potential discontinuity in the conduction band is greater than in the valence band, so that the impact ionization of the electrons is more effective than that of the holes. The energy missing from the hot electrons produced by the tunnel effect, to effect impact ionization, is supplied to the electrons by the polarization field. The choice of the composition of materials and the choice of polarization are within the reach of ordinary skill in the art.

FIG. 3a represents a third embodiment of the photocathode according to the invention. This third example includes:
- First two layers 50 and 51 similar to layers 30 and 31 of the second embodiment;
- Last two layers 56 and 57 similar to the last two layers 41 and 42 of the second embodiment, the layer 56 being polarized by a voltage generator V relative to the first layer 50;
- Ten electron multiplication layers: 52, 53, ..., 54, 55; each of these layers being made of an N⁻ type semiconductor material having a composition varying gradually to obtain a band gap increasing in the direction of the layer 56, that is to say in the direction where the electrons are to be evacuated.

FIG. 3b represents a diagram of the energy levels of the carriers in this third exemplary embodiment, in the absence of polarization. In the zone corresponding to the electron multiplication layers, 52 to 55, the curves E _c and E _v of the energy level extrema have a sawtooth shape consisting of a slope and a steep flank. Each sawtooth has a positive slope for the conduction band and a negative band for the valence band, in the direction where the electrons are evacuated.

In this embodiment the thickness of each electron multiplication layer, 52 to 55, is 0.03 micron and its composition is Ga _1-x Al _x As with x varying linearly from 0 to 0.3 to 0 in the direction from layer 51 to layer 56, that is to say in the direction where the electrons are to be removed.

FIG. 3c represents a diagram of the energy levels of the carriers in this third exemplary embodiment, when the polarization V is applied. The reduction qV of the Fermi E _F56 energy of the layer 56 compared to the Fermi E _{F50 level} of the layer 50 modifies the slope of the saw teeth, this slope becoming negative for the conduction band. In the conduction band the electrons descend along the slopes of the saw teeth without colliding with the vertical flanks whereas in the valence band, the holes meet the flanks of the saw teeth which constitute potential markets. Each time an electron jumps from one sawtooth to the next, it undergoes a sudden acceleration which allows it to release another electron by ionization.

The ideal composition of the materials would be such that the height of the potential steps would be greater than the ionization energy of the material with the smallest band gap, that is to say Ga _0.7 Al _0.3 As in this example. In the absence of an ideal composition, a composition is chosen such that the potential discontinuity in the conduction band is as large as possible. The energy missing from an electron that has just crossed a potential discontinuity, to effect impact ionization, is supplied by the polarization field. The choice of materials and the choice of polarization fulfilling these conditions are within the reach of ordinary skill in the art. For 20 multiplication layers thus produced, the multiplication factor is theoretically of the order of 10⁶.

This variant embodiment may include other materials. For example, In Al As for layer 50, In P or In Ga As for layer 51, In _x Ga _1-x As _1-y P _y for layers 52 to 55, preferably with x and y varying according to the known art in such a way that the material of the layers 52 to 55 is adapted in mesh with the material of the absorption layer 51, and of In P for the layer 56. In this example the difference in bandwidth prohibited is 0.8 eV and the bias voltage is approximately 20 V for 20 multiplication layers 52, 53, ... having a thickness of the order of 0.03 microns.

The bias voltage V to be applied between the layers 56 and 50 of this variant embodiment is of the order of: V = n. E _g , where n is the number the multiplication layers 52, 53, ..., 55 and where E _g is the forbidden bandwidth necessary to release an electron by impact in one of these multiplication layers.

This alternative embodiment may include other materials: Ga Al As for layer 50, Ga As for layer 51, Ga _1-x Al _x As with x varying from 0 to 1 for the layers 52, ..., 55, and Ga As for layer 56.

FIG. 4a shows a section of a portion of a fourth embodiment of the photocathode according to the invention. This fourth example differs only from the third example by an additional layer 60 inserted inside the layer 56. The additional layer 60 is made of a P⁺ type semiconductor material having a band gap greater than that of the material of layers 56 and 51 to create a potential barrier in the valence band, to stop most holes. This barrier makes it possible to reduce the current of holes which is the source of unnecessary electrical consumption and of a dark current, since it creates electron-hole pairs by ionization.

The thickness of this layer 60 must be large enough to stop the holes and, on the other hand, it must be sufficiently small so that this layer 60 is practically transparent to the electrons, the latter passing through it by tunnel effect. This difference in transparency is obtained thanks to the large difference in effective mass between the electrons and the holes. This additional layer may consist, for example, of Ga _0.6 Al _0.4 As having a thickness of 0.003 microns and doped with 10¹⁹ zinc atoms per cm³.

Such a layer 60 can also be provided in layers 4 and 41 of the first and second embodiments of the photocathode according to the invention.

The invention is not limited to the exemplary embodiments described above. Many variants are within the reach of those skilled in the art, in particular as regards the number of electron multiplication layers and the materials constituting them.

The invention can be applied in particular to shooting tubes for television cameras and to image intensifier tubes, for shooting at low light levels.

Claims (10)

1. Internally amplified photocathode, comprising a so-called absorption layer (2; 31) made of a P⁺ type semiconductor material, the band gap of which is sufficiently small to convert the photons into electron-hole pairs of the light to be detected, characterized in that it also comprises at least one layer (3; 32-33) called electron multiplication by ionization, consisting of a N⁻ type semiconductor material where the composition n is not uniform and such that, when this multiplication layer (3; 32-33) is polarized, the electrons are accelerated in the direction in which they are to be evacuated and the holes are less accelerated than the electrons; and includes means (5 or 41) for polarizing the multiplication layer (3; 32-33). 2. Photocathode according to claim 1, characterized in that it comprises a single multiplication layer (3) consisting of a semiconductor material whose composition varies continuously so that the width of its forbidden band decreases in the direction where the electrons are to be removed, and whose thickness is less than the diffusion length of the carriers. 3. Photocathode according to claim 2, characterized in that the multiplication layer (3) consists of Ga _1-x Al _x As with x varying linearly from 60% to 0% in the direction where the electrons are to be removed and from which the thickness is 1 micron. 4. Photocathode according to claim 1, characterized in that it comprises a plurality of multiplication layers (32-33, ...), each consisting of two sublayers (32, 33) of semiconductor material of type N⁻ and having respectively two different compositions. 5. Photocathode according to claim 4, characterized in that each multiplication layer (32-33) consists of a first sublayer (32) of 0.05 micron of Ga _0.9 Al _0.1 As and d '' a second sub-layer (33) of 0.05 micron of Ga _0.7 Al _0.3 As. 6. Photocathode according to claim 1, characterized in that it comprises several layers (52 to 55) of electron multiplication by ionization, each consisting of a N⁻ type semiconductor material having a composition varying continuously in such a way that the width of its forbidden band increases in the direction where the electrons are to be evacuated. 7. Photocathode according to claim 6, characterized in that each multiplication layer (52 to 55) consists of Ga _1-x Al _x As with x varying linearly from 0.3 to 0 in the direction where the electrons are to be removed , and having a thickness of 0.03 micron. 8. Photocathode according to claim 6, characterized in that each multiplication layer (52 to 55) consists of In _x Ga _1-x As _1-y P _y with x and y varying according to the known art so that the material of the multiplication layers (52 to 55) is matched in mesh with the material of the absorption layer (50), and having a thickness of 0.03 microns. 9. Photocathode according to one of claims 1 to 4, characterized in that it further comprises a layer (60) for reducing the current of holes, made of a P⁺ type material having a higher band gap width to that of the absorption layer (51), and whose thickness is sufficiently small to allow the passage of electrons (7) by tunnel effect with a high probability, and strong enough to stop a major part of the current of holes. 10. Photocathode according to claim 9, characterized in that the layer (60) for reducing the current of holes consists of a layer of Ga _0.6 Al _0.4 As of thickness less than 0.0045 micron.

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