EP0230818B1 - Photo cathode with internal amplification - Google Patents

Description

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

• une couche, dite couche fenêtre, constituée de semi-conducteur de type P⁺ dont la bande interdite est suffisamment large pour que cette couche soit transparente pour les longueurs d'onde de la lumière à détecter, et qui est collée sur une paroi de verre recevant la lumière à détecter ;
• une couche, dite couche d'absorption, constituée d'un semi-conducteur de type P⁺ dont la bande interdite a une largeur suffisamment faible pour convertir en paires électron-trou les photons de la lumière à détecter ;
• une couche, dite couche d'émission, constituée d'un matériau donnant à l'extrémité de la couche d'absorption une affinité électronique négative pour émettre dans le vide les électrons libérés dans la couche d'absorption.

It is known to produce a photocathode comprising mainly:

• a layer, called window layer, made of P⁺ type semiconductor whose band gap is wide enough for this layer to be transparent for the wavelengths of light to be detected, and which is bonded to a glass wall receiving the light to be detected;
• a layer, called absorption layer, consisting of a P⁺ type semiconductor whose forbidden band 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 for emitting 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 narrow to convert the photons of the photons into electron-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 two sublayers of N-type semiconductor material having respectively two different compositions at the interface of two so-called sublayers, and such that, when this multiplication layer is polarized, the electrons are accelerated in the direction in which they are to be removed 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 from the above description and the accompanying figures:
FIG. 1 given by way of illustration represents an embodiment of a photocathode, which is not part of the invention, and two diagrams of the energy levels E of charge carriers inside this photocathode, respectively without polarization and with polarization.

FIGS. 2 to 4 each represent an exemplary embodiment of a 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, on the other hand, with polarization.

• Une première couche 1, transparente pour toutes les longueurs d'onde de la lumière à détecter, constituée d'un matériau semi-conducteur de type P⁺ collé sur une paroi de verre non représentée, et recevant à travers cette paroi des photons 8;
• Une seconde couche 2, dite couche d'absorption, constituée d'un matériau semi-conducteur de type P⁺, pour convertir chaque photon 8 en une paire électron-trou;
• Une troisième couche 3, dite couche de multiplication des électrons, constituée d'un matériau semi-conducteur du type N⁻ dont la composition varie continument;
• Une quatrième couche 4, dite couche de transport, constituée d'un matériau semi-conducteur de type P⁺, et qui ne fait que transmettre les électrons libérés par les photons 8 dans la couche 3 ;
• Une électrode métallique 5, reliée à la borne positive d'un générateur de tension V dont la borne négative est reliée à la première couche 1, afin de polariser les quatre couches 1, 2, 3, 4 pour accélérer les électrons libérés par la lumière à détecter;
• Une dernière couche 6, donnant à la surface de la quatrième couche 4 la propriété d'affinité électronique négative permettant d'émettre dans le vide les électrons 7 transmis par la couche 4.

Figure 1a shows a section of a portion of a first embodiment of a photocathode given by way of illustration. 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 detect;
• A last layer 6, giving the surface of the fourth layer 4 the property of negative electronic affinity enabling 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 forbidden bandwidth than the previous one and 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.

• une première couche 30 semblable à la première couche 1 du premier exemple de réalisation, transparente à la lumière à détecter ;
• Une seconde couche 31, qui est une couche d'absorption semblable à la couche 2 du second exemple de réalisation ;
• Dix couches de multiplication des électrons, constituée de 20 sous-couches : 32, 33, 34, 35,..., 36, 37, 38, 39, 40 ;
• Une couche de transport 41, semblable à la couche 4 du premier exemple de réalisation mais qui est reliée à la borne positive du générateur de tension V car il n'y a pas d'électrode métallique dans cet exemple de réalisation ;
• Une dernière couche 42 procurant une affinité électronique négative à la couche 41.

FIG. 2a represents a section of a portion of an exemplary embodiment of a photocathode according to the invention. This 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 since 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 various points of this 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, he suffers, on the descent , a brutal acceleration which allows it to release an additional electron by impact ionization, and these two electrons then cross 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.

• Deux premières couches 50 et 51 semblables aux couches 30 et 31 du second exemple de réalisation ;
• Deux dernière couches 56 et 57 semblables aux deux dernières couches 41 et 42 du second exemple de réalisation, la couche 56 étant polarisées par un générateur de tension V par rapport à la première couche 50 ;
• Dix couches 52, 53,...,54, 55, chacune de ces couches étant constituées d'un matériau semi-conducteur de type N⁻ ayant une composition variant graduellement pour procurer une largeur de bande interdite croissante en direction de la couche 56, c'est-à-dire dans la direction où les électrons sont à évacuer; chaque couple de couches 52+53,...,54+55 constitue une couche de multiplication des électrons.

FIG. 3a represents another exemplary embodiment of the photocathode according to the invention. This example includes:

• First two layers 50 and 51 similar to layers 30 and 31 of the second embodiment;
• Two last 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 with respect to the first layer 50;
• Ten layers 52, 53, ..., 54, 55, each of these layers being made of an N⁻ type semiconductor material having a composition varying gradually to provide an increasing band gap towards the layer 56 , that is to say in the direction where the electrons are to be removed; each pair of layers 52 + 53, ..., 54 + 55 constitutes an electron multiplication layer.

FIG. 3b represents a diagram of the energy levels of the carriers in this 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 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 another exemplary embodiment of the photocathode according to the invention. This example differs only from the previous example by an additional layer 60 inserted inside the layer 56. The additional layer 60 consists of a P⁺ type semiconductor material having a band gap greater than that of the material layers 56 and 51 to create a potential barrier in the valence band, to stop most of the 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 (8)

1. An internal amplification photocathode, comprising: a layer (2 and 31) termed the absorption layer, constituted by a semiconductor material of the P type, whose forbidden band has a width sufficient to convert the photons of the light to be detected into electron-hole pairs; means (5 or 41) making possible the polarization of the photocathode; and a layer (6 and 42) endowing the surface of the photocathode with a negative affinity property in order to facilitate the emission into the vacuum of the electrons furnished by the photocathode; characterized in that it furthermore comprises at least one layer (32 + 33 and 52 + 53), termed the layer for the multiplication of electrons by ionization, constituted by two sub-layers (32, 33, 52 and 53) of an N type semiconductor material respectively having two compositions different to the interface of the two said sub-layers, and in that, when this multiplication layer (32, 33, 52 and 53) is polarized, the electrons are polarized in the direction whither they are to be evacuated and the holes are accelerated less than the electrons.
2. The photocathode as claimed in claim 1, characterized in that the said sub-layers (32 and 33) of semiconductor materials of the N type respectively have two different and homogeneous compositions.
3. The photocathode as claimed in claim 2 characterized in that the said multiplication layer (32 and 33) is constituted by a first 0.05 micron sub-layer (32) of Ga_0.9Al_0.1As and of a second 0.05 micron sub-layer (33) of Ga_0.7Al_0.3As.
4. The photocathode as claimed in claim 1, characterized in that the said sub-layers (52 and 53) of the said layer for the multiplication of electrons by ionization are each constituted by a semiconductor material of the N type having a composition varying continuously in such a manner that the width of its forbidden band increase in the direction whither the electrons are to be evacuated.
5. The photocathode as claimed in claim 4, characterized in that each multiplication sub-layer (52 and 53) is constituted by Ga_{1 - x}Al_x with x varying linearly between 0.3 and 0 in the direction whither the electrons are to be evacuated and having a thickness of 0.03 micron.
6. The photocathode as claimed in claim 4, characterized in that each multiplication sub-layer (52 and 53) is constituted by In_xGa_{1 - x}As_{1 - y}P_y with x and y varying in such a manner that the semiconductor material of the N type of the multiplication (52 and 53) is lattice adapted to the material of the absorption sub-layer (31), and has a thickness of 0.03 micron.
7. The photocathode as claimed in claim 1, characterized in that it furthermore comprises a layer (60) in order to diminish the hole current, constituted by a material of the p⁺ type having a forbidden band width greater than that of the absorption layer (51) and whose thickness is sufficiently small in order to permit the passage through it of the electrons (7) by the tunnel effect with an increased probability and sufficiently thick to halt a major fraction of the hole current.
8. The photocathode as claimed in claim 7, characterized in that the layer (60) in order to diminish the hole current is constituted by a layer of Ga_0.6.Al_0.4As with a thickness of less than 0.0045 micron.

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