JPH05234501A - Photoelectron emitting surface and electron tube using the same - Google Patents

Abstract

PURPOSE:To provide a photoelectron emitting surface having remarkably higher sensitivity in comparison with a conventional photoelectron emitting surface using a semiconductor and able to extend a critical wave length to a far longer wavelength, and an electron tube using this surface. CONSTITUTION:Since a GaAs layer 22a of a heterojunction semiconductor multi- layer film to be a light absorbing layer 22 has a film thickness of 300Angstrom or less which is shorter than the de Brogie wave length of an electron, it forms a potential well being interposed by adjacent Al0.65Ga0.35As layers 22b each having a large energy gap, and a sub band in accordance with a quantum level is formed in the GaAs layer 22a. Since the Al0.65Ga0.35As layer 22b has a film thickness equal to or thicker than 45Angstrom which an electron can not pass through due to a tunnel effect, this sub band is all the time filled with bound electrons.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photoelectron emitting surface for emitting photoelectrons upon incidence of photons and an electron tube constructed using the photoelectron emitting surface.

[0002]

2. Description of the Related Art Conventionally, for the purpose of obtaining a highly efficient photoelectron emitting surface, a technique using a semiconductor multilayer film as a light absorbing layer has been disclosed in Japanese Patent Laid-Open Nos. 62-133634 and 62-62.
It is disclosed in Japanese Patent No. 133633. This first conventional photoelectron emission surface has the energy band structure shown in FIG. That is, the layer 6 having a small energy gap
Has a thickness (300 angstroms or less) sufficient to absorb incident photons and excite photoelectrons.
The layer 7 having a large energy gap has a sufficient thickness (45 angstroms or less) to allow photoelectrons to pass therethrough by the tunnel effect. Then, these layers 6 and 7 are alternately laminated to form a photoelectron emitting surface. This photoelectron emission surface utilizes the fact that the light absorption coefficient for incident photons is increased as compared with the conventional light absorption layer made of a homogeneous semiconductor material by alternately stacking two types of layers. In order for the photoelectrons excited by the incident photons to efficiently reach the emission surface, the thickness of the layer 7 having a large energy gap that serves as a barrier must be sufficiently thin. In practice, this thickness is 45 angstroms. Must be very thin with: Only photoelectrons that could pass through the layer 7 having a large energy gap due to the tunnel effect reach the emission surface, and Cs _x O formed on the emission surface.
It is released into the vacuum through the _y- layer 8. On this photoelectron emission surface, the absorption of incident photons and the generation of electron-hole pairs are caused between the valence band and the conduction band of the layer 6 having a small energy gap to improve the efficiency of photoelectron emission. ing. There is also a conventional second photoelectron emission surface having the energy band structure shown in FIG. This photoelectron emission surface has a band structure when the film thickness of the layer 9 having a small energy gap is thinner than that of the conventional photoelectron emission surface layer 6. Also on this photoelectron emission surface, absorption of incident photons and generation of electron-hole pairs are caused between the valence band and conduction band of the layer 9 having a small energy gap, and C
By emitting photoelectrons from the s _x O _y layer 11, the efficiency of photoelectron emission is improved.

In addition, a long-wavelength light receiving element using a quantum well structure has been disclosed by BF Levine et al. (Appl. Phys. Lett. 58).
(14) 1991). The band structure of the valence band of this light receiving element (conventional third photoelectron emission surface) is shown in FIG. The absorption of incident photons is performed between subbands of the valence band formed in the layer 12 having a small energy gap and serving as a potential well, and the absorption of the incident photons excites photoelectrons. The excited photoelectrons move in this case to the X-valley of the valence band of the layer 13 having a large energy gap that serves as a potential barrier. In such a light receiving element, photoelectrons are detected as a signal when they reach the electrode formed on the surface, so the electric field formed inside the semiconductor is generally small, and noise is kept as low as possible. It is common to operate with.

[0004]

However, in the above-mentioned first conventional photoelectron emission surface, when the film thickness of the layer 6 having a small energy gap is reduced, both the valence band and the conduction band are sub-due to the quantum effect. There is an essential defect that a band is formed, the absorption threshold of incident photons becomes high, and incident photons having a long wavelength cannot be absorbed.

Further, in the above-mentioned second conventional photoelectron emission surface, since the layer 9 having a small energy gap is thin, the energy difference E _sg between the valence band subband and the conduction band subband is _equal to the bandgap. It becomes larger than E _g , and the absorption threshold wavelength of the incident photon becomes shorter.

Further, since a high dark current is generated in the above-mentioned third conventional photoelectron emitting surface (light receiving element), it is necessary to cool it to a considerably low temperature with liquid nitrogen or the like before use.
This point has been a drawback when used by incorporating it into a general device.

[0007]

The present invention has been made to solve such a problem, and the light absorption layer is formed of a heterojunction semiconductor multilayer film in which different kinds of semiconductors having different energy gaps are multilayered. The semiconductor having a small energy gap has a film thickness of 300 angstroms or less, and the semiconductor having a large energy gap has a film thickness of 45 angstroms or more.

Further, the light absorption layer is formed by a homojunction semiconductor multilayer film in which homojunctions of semiconductors having different conductivity types are multi-layered, and the film thickness of one conductivity type semiconductor which becomes a potential well is 300 angstroms or less. The film thickness of the other semiconductor serving as the potential barrier is 45 angstroms or more.

Further, the photoelectrons excited by the incident photons are characterized by being accelerated by the internal electric field and transiting to a higher energy band.

[0010]

[Function] The incident photons are absorbed and the photoelectrons are excited between the sub-bands of the conduction band or between the sub-bands and the bottom of the conduction band formed in the semiconductor multilayer film. It is sensitive to incident light of long wavelength. Further, the excited photoelectrons are accelerated by the internal electric field, so that the photoelectrons are easily emitted into the vacuum.

[0011]

1 is a sectional view showing the structure of a photoelectron emitting surface according to an embodiment of the present invention.

A light absorption layer 2 is formed on the P ⁺ -GaAs substrate 21.
2 is formed. The light absorption layer 22 includes an undoped GaAs layer 22a having a thickness of 30 Å and Al _0.65 Ga _0.35 As having a thickness of 500 Å.
The layer 22b is formed of a heterojunction semiconductor multilayer film of 40 layers each. Note that some dissimilar joints are omitted in the drawing. P ⁻ -GaAs is formed on the light absorption layer 22.
The contact layer 23 is formed to a thickness of 3000 angstrom. Furthermore, the ^P - -GaAs on the surface of the contact layer 23 is patterned in a mesh shape A
The l-Schottky electrode 24 is formed. The surface of these materials is activated with Cs and O ₂ and is very thin.
_{The x} O _y film 25 is formed, and the work function is reduced. On the other hand, an ohmic electrode 26 is formed on the back surface of the P ⁺ -GaAs substrate 21, and a bias is applied between the Schottky electrode 24 and the ohmic electrode 26 by the power supply 27.

FIG. 2 shows a band structure when a bias voltage is not applied to the photoelectron emitting surface according to this embodiment. Energy bands corresponding to the respective layers in FIG. 1 are designated by the same reference numerals as those of the respective layers.

In this band structure, the energy difference between the conduction bands of the heterojunction semiconductor multi-layer film which becomes the light absorption layer 22 is smaller than the minimum energy gap of the semiconductor used. Since the GaAs layer 22a of the heterojunction semiconductor multilayer film has a film thickness of 300 angstroms or less, which is shorter than the de Broglie wavelength of electrons, it is sandwiched between the adjacent Al _0.65 Ga _0.35 As layers 22b having a large energy gap to form a potential well. Subbands corresponding to the quantum levels are formed in the layer 22a. At this time, since the Al _0.65 Ga _0.35 As layer 22b has a film thickness of 45 angstroms or more, which prevents electrons from passing through due to the tunnel effect, this subband is always filled with bound electrons.

The photoelectron emission surface according to the present embodiment is intended to excite the bound electrons into subbands of other quantum levels by absorption of incident photons. JP 62-
It is obvious that the photoelectron emission surface disclosed in Japanese Patent No. 133634 or the like is essentially different from the photoelectron emission surface that absorbs incident photons and excites photoelectrons between the valence band and the conduction band of a semiconductor. is there. Therefore, in the conventional photoelectron emission surface, the wavelength λ corresponding to the energy gap of the semiconductor used is
Although it has no sensitivity to incident photons having a wavelength longer than = 1.24 / E _{g, the} photoelectron emission surface according to the present example has a sensitivity to incident photons having a wavelength longer than the energy gap of the semiconductor used. Even with sensitivity. Also, the wavelength can be arbitrarily changed by appropriately designing the heterojunction semiconductor multilayer film of the light absorption layer 22.

FIG. 3 shows an energy band structure when a bias voltage is applied to the photoelectron emission surface according to this embodiment. Also in this figure, energy bands corresponding to the respective layers in FIG. 1 are denoted by the same reference numerals as those of the respective layers.

The photoelectrons excited by the incident photon hν between the subbands of the GaAs layer 22a in the light absorption layer 22 are A
l _0.65 Move to the X valley of the conduction band of the Ga _0.35 As layer 22b. However, since a high electric field (≧ 10 ³ V / cm or more) is formed inside the semiconductor by applying the bias voltage, the photoelectrons are accelerated and immediately transit to the Γ valley of the higher energy band. In this embodiment, Al _0.65 Ga
_{Since 0.35} As layer 22b is an indirect transition semiconductor,
However, when a direct transition semiconductor is used, Γ−X
Alternatively, it is natural that the transition is Γ-L. The photoelectrons that are accelerated by the internal electric field and move in the higher energy band fall to the potential well of the GaAs layer 22a because the potential energy of the photoelectrons is kept sufficiently high when they cross the heterojunction semiconductor multilayer film. It is possible to reach the emission surface. A small part of the photoelectrons that reach the emission surface is absorbed by the Schottky electrode 24, but most of them pass between the electrode patterns,
It is released into the vacuum through the Cs _x O _y film 25. At this time as well, the potential energy of the photoelectrons is kept sufficiently high, so that the photoelectrons are emitted into the vacuum very efficiently.

As described above, in the photoelectron emission surface according to the present embodiment, the photoelectrons excited by the incident photons between the subbands of the conduction band are accelerated by the internal electric field, and the photoelectrons are transitioned to a higher energy band and then moved into a vacuum. It emits light, and has, for example, a mechanism that is essentially different from the photodetector using the conventional quantum well structure described above reported by BF Levine et al. In other words, the photoelectron emission surface excites photoelectrons by absorbing incident photons between the subbands of the conduction band, rather than photoexcitation between so-called bands between the valence band and the conduction band as in the conventional case. It has photosensitivity to long wavelengths without using a semiconductor with a small gap. Further, the excited photoelectrons are accelerated by the internal electric field, and are emitted into the vacuum after transiting to the higher energy band. Therefore, as described above, the potential energy of the photoelectrons is higher than that of the conventional one, and it is very efficient in the vacuum. Can be released to. Therefore, it has a remarkably high sensitivity as compared with a conventional photoelectron emitting surface using a semiconductor, and its limit wavelength can be set to a much longer wavelength. Furthermore, the photoelectron emission surface according to the present invention can be formed as a photoelectron emission surface having a peak at an arbitrary wavelength by appropriately designing the kind and structure of the heterojunction semiconductor multilayer film which becomes the light absorption layer 22. Also, since the type, layer thickness, and number of layers of the heterojunction semiconductor multilayer film have a high degree of freedom, the sensitivity wavelength range can be arbitrarily adjusted from narrow to wide.

Further, by multiplying the photoelectrons emitted into the vacuum into secondary electrons, it is possible to provide a long-wavelength light receiving element with high sensitivity and low noise, which can operate even at a relatively high temperature. That is, the photoelectron emission surface according to the above embodiment is useful when applied to an electron tube. In the case of application to a photomultiplier tube, the photoelectron emission surface according to the above embodiment is applied to the photoelectric conversion surface, the photoelectrons emitted from this photoelectron emission surface are subjected to secondary electron multiplication by the dynode, and the electron group is formed at the anode. Is detected. Further, when applied to an image pickup tube, the photoelectron emission surface according to the above-described embodiment is applied to the image input unit, the photoelectrons emitted from this photoelectron emission surface are focused by the electron lens, and the image picked up on the fluorescent surface is obtained. Form an image. Image of multiplying photoelectrons focused by electron lens by microchannel plate (MCP)
The same applies to the intensifier tube (I · I tube). When applied to a phototube,
The photoelectron emission surface according to the above embodiment is applied to the light receiving surface, and the photoelectrons emitted from this photoelectron emission surface are detected by the anode. Each electron tube having the photoelectron emitting surface according to the above-mentioned embodiment has a remarkably high sensitivity in the long wavelength region, as compared with the conventional electron tube. Therefore, when such an electron tube is used for photometry or imaging under low illuminance, its effect is great.

In the above embodiment, the GaAs layer 2 is used.
The light absorption layer 22 was formed by using two kinds of heterojunctions composed of 2a and Al _0.65 Ga _0.35 As layer 22b as a semiconductor multilayer film, but the light absorption layer 22 is not limited to this, and other III-V group compound semiconductors or Si may be used. , Ge and their mixed crystals may be used, and it is not essential to limit the types to two. Therefore, even if the light absorption layer is formed by using these materials, the same effect as that of the above-mentioned embodiment can be obtained.
Further, the light absorption layer may be formed of the same kind of semiconductor multilayer film having p-type, n-type and i-type junctions, and in this case, the same effect as that of the above-described embodiment can be obtained.

The Schottky electrode 24 has been described by taking Al as an example in the above embodiment, but the kind is not limited as long as it is a metal that forms a good Schottky junction with the semiconductor used and its pattern. The shape can be arbitrary. Even if such a Schottky electrode is formed, the same effect as that of the above embodiment can be obtained.

[0022]

As described above, according to the present invention, incident photons are absorbed and photoelectrons are excited between the subbands of the conduction band formed in the semiconductor multilayer film or between the subbands and the bottom of the conduction band. That is, it has sensitivity to incident light having a wavelength longer than the wavelength corresponding to the energy gap of the semiconductor used. Further, the excited photoelectrons are accelerated by the internal electric field, so that the photoelectrons are easily emitted into the vacuum.
Therefore, it has a remarkably high sensitivity as compared with the conventional photoelectron emitting surface using a semiconductor, and it becomes possible to set the limit wavelength to a much longer wavelength.

Further, the electron tube having such a photoelectron emitting surface operates even at a relatively high temperature, and exhibits a remarkably high sensitivity in the long wavelength region in comparison with the conventional one. Further, in the photoelectron emitting surface according to the present invention, the wavelength range of sensitivity can be arbitrarily adjusted from narrow to wide by appropriately designing the type and structure of the semiconductor multi-layer film serving as the light absorption layer.

[Brief description of drawings]

FIG. 1 is a sectional view showing a structure of a photoelectron emission surface according to an embodiment of the present invention.

FIG. 2 is a band structure diagram of the photoelectron emission surface according to the present embodiment when there is no bias.

FIG. 3 is a band structure diagram when the photoelectron emission surface according to the present embodiment is biased.

FIG. 4 is a band structure diagram of a conventional first photoelectron emitting surface.

FIG. 5 is a band structure diagram of a conventional second photoelectron emitting surface.

FIG. 6 is a band structure diagram of a third conventional photoelectron emitting surface.

[Explanation of symbols]

21 ... P ⁺ -GaAs substrate, 22 ... Light absorption layer, 22a ...
Undoped GaAs, 22b ... Al _0.65 Ga _0.35 As,
23 ... Contact layer, 24 ... Schottky electrode, 25 ... C
s _x O _y film, 26 ... Ohmic electrode, 27 ... Power supply.

Claims (8)

[Claims] 1. A photoelectron emission surface that emits photoelectrons when photons are incident on the photoabsorption layer, wherein the photoabsorption layer is formed of a heterojunction semiconductor multilayer film in which heterojunctions of semiconductors having different energy gaps are multilayered. The thickness of the semiconductor with a small gap is 3
The photoelectron emission surface is characterized in that the thickness of the semiconductor having a large energy gap is 00 angstroms or less and the film thickness is 45 angstroms or more. 2. A photoelectron emission surface that emits photoelectrons when photons are incident on the photoabsorption layer, wherein the photoabsorption layer is formed by a homojunction semiconductor multilayer film in which homojunctions of semiconductors having different conductivity types are multilayered. The photoelectron emission surface is characterized in that the film thickness of the semiconductor of the conductivity type is 300 angstroms or less, and the film thickness of the other semiconductor is 45 angstroms or more. 3. The photoelectron emitting surface according to claim 1, wherein the photoelectrons excited by the incident photons are accelerated by an internal electric field and transit to a higher energy band. 4. The energy difference between the respective conduction bands of the semiconductor multilayer film forming the light absorption layer is smaller than the minimum energy gap of the semiconductor. Photo-emission surface of. 5. The photoelectron is excited by absorbing an incident photon between subbands of the conduction band or between the subband and the bottom of the conduction band formed in the semiconductor multilayer film. The photoelectron emitting surface according to claim 2 or 3. 6. The photoelectron emitting surface according to claim 1, wherein the semiconductor multilayer film is formed of a III-V group compound semiconductor or a mixed crystal thereof. 7. The photoelectron emitting surface according to claim 1, 2 or 3, wherein the semiconductor multilayer film is formed of Si, Ge or a mixed crystal thereof. 8. Claim 1 or claim 2 or claim 3.
An electron tube comprising the described photoelectron emitting surface.