JPH0760635B2 - Photoelectron emitter - Google Patents

Description

The present invention relates to a photoelectron radiator having a structure in which a conductive material and a semiconductor are joined.

[Conventional technology]

Materials with a small energy gap and a low work function have not been discovered. Therefore, as a conventional photoelectron radiator having sensitivity to long-wavelength light, for example, those shown in FIGS. 2 to 4 are known.

Figure 2 shows the energy band diagram of the NEA type photoemissive surface with CsO activity on GaAs. In the figure, reference numeral 41 indicates a P-type GaAs semiconductor substrate, and reference numeral 42 indicates a CsO compound layer bonded to its surface by adsorption. Here, E _C is the top energy of the conduction band, E _F is the Fermi energy, E _V is the bottom energy of the valence band, and E ₀ is the vacuum level. According to this, the work function can be lowered due to the junction between the surface level and the Cs-O compound layer.

FIG. 3 is an energy band diagram of a photoelectron emission surface of a type in which a PN junction is formed with Ge and CsO activation is performed. As illustrated, the P-type Ge semiconductor 51 and the N-type Ge semiconductor 52 are PN-junctioned, and an electrode (not shown) is formed on the opposite side of the P-type Ge semiconductor 51 from the junction. Further, a partial electrode (not shown) is formed on the side opposite to the junction of the N-type Ge semiconductor 52 (the surface side as a photoelectron emission surface) so as not to interfere with electron emission or light incidence. Further, the surface barrier of the N-type Ge semiconductor 52 is further reduced by the adsorption of the CsO layer. The depletion layer 53 is formed by the above junction and bias. According to this, the work function can be substantially reduced by the action of the PN junction (or Schottky junction) and the reverse bias.

Figure 4 is aimed at detection of light of a wavelength longer than that of the type shown in Figure 3. In the figure, the surface barrier is lowered by the Schottky junction between the InGaAs and InP junctions and the CsO activated metal. FIG. 3 is an energy band diagram of a photoelectron emission surface aiming at FIG.

In the figure, the P-type InGaAs semiconductor 61 and the InP semiconductor 62 are joined, and an electrode (not shown) is formed on the side opposite to the junction of the semiconductor 61. Further, a partial electrode (not shown) is formed on the opposite side of the junction of the semiconductor 62 (on the surface side as a photoelectron emission surface) so as not to interfere with electron emission or light incidence. Furthermore, the surface of the semiconductor 62 is
The adsorption of the CsO layer further reduces the surface barrier. A depletion layer 63 is formed by the above junction and bias. According to this, a substance with a small energy gap and a substance with a large energy gap
Bond it so that the barrier is not formed in the conduction band as much as possible,
Further, the barrier on the surface can be lowered by bias or the like.

Photoelectron emitters of the type that have been put to practical use in these studies are to make photoelectrons by band-to-band transition of semiconductors, transfer the photoelectrons into a substance with low electron affinity by various means, and emit external electrons. Is characterized by.

Some photodetectors realize the generation of photoelectrons not by the band-to-band transition of a semiconductor but by a barrier formed by a Schottky junction between a semiconductor and a metal. Here, a photoelectron or a hole is created by radiating internal photoelectrons to the Schottky barrier, and this is a detector having sensitivity to long-wavelength light. There is no conventional example that emits radiation.

[Problems to be Solved by the Invention]

As described above, in the conventional photoelectron emitting surface, the limit wavelength of photoelectron emission is not longer than the wavelength determined by the energy gap of the semiconductor. Moreover, when the surface barrier is present, the limit wavelength is shortened by the surface barrier. Therefore,
In order to construct a photoelectron emitting surface sensitive to long-wavelength light, it is necessary to use a semiconductor having a small energy gap to lower the substantial surface barrier by the method described above.

However, in the case where the Cs-O layer as shown in FIG. 2 is used to reduce the substantial surface barrier, the semiconductor must have an extremely clean surface, and the Cs-O layer and the Cs-O layer must be very clean. Therefore, it is necessary to join without providing an energy barrier in the conduction band. This technology is extremely advanced, and available semiconductors are extremely limited.

In order to form the photoelectron emitting surface of the type shown in FIG. 3, the PN junction thereof needs to have an extremely high breakdown voltage characteristic.
This is because in order for the photoelectrons to be emitted from the semiconductor surface while retaining the energy obtained at the PN junction, the thickness of the N layer on the surface and the depleted layer must be less than the mean free path of the photoelectrons. Then, a reverse bias voltage that exceeds the surface barrier must be applied to the thin depleted layer, and the electric field strength becomes extremely high. Therefore, the Zener breakdown usually occurs and the reverse bias voltage cannot be applied. In general, a semiconductor having a smaller energy gap is more likely to undergo Zener breakdown, which hinders the realization of a long-wavelength photoemission surface by this method. In addition, even if the Zener breakdown does not occur, the increase in the reverse saturation current directly increases the dark current, so this is a problem when the PN junction is formed with a small energy gap. Thus, it is difficult and impractical to form this type of photoemissive surface.

In the photoelectron emitting surface of the type shown in FIG. 4, the point is to make a junction without forming a barrier in the conduction band.
If there is a barrier, the photoelectron needs to have more energy than that, so the limiting wavelength is shortened accordingly. In general,
This barrier becomes quite large, and there are almost no combinations that extend the limiting wavelength to the infrared. In addition, recombination centers are generally likely to be formed at the interface of the heterogeneous semiconductor junction, and photoelectrons cannot be efficiently transmitted. Therefore, what is realized as a photoelectron emitting surface of this type is semiconductors having very similar properties. As in the example shown here, there are cases where the junctions have relatively different properties, but the sensitivity is low, and most of the others are cases where a ternary and four-element semiconductor of the same series is joined to a III-V semiconductor. . In this case, the mixed crystal ratio is limited, and there are many problems such as requiring extremely high technology.

These problems result from the fact that the generation of photoelectrons depends on the transition between semiconductor hands, and therefore it is necessary to use a semiconductor with a small energy gap and to lower the surface barrier at the same time. ing.

[Means for Solving the Problems]

The photoelectron radiator according to the present invention has a structure having a semiconductor junction between one conductive material and the other semiconductor, and internal photoelectrons are emitted from the one conductive material into the other semiconductor by irradiation light. The photoelectrons are configured to be emitted as external electrons from the surface of the other semiconductor as hot electrons by accelerating and traveling in the thickness direction of the other semiconductor, and the other semiconductor is a semi-insulating semiconductor. By using a substrate having a sufficient thickness that does not cause a tunnel effect penetrating the whole, it is more sensitive to infrared light than semiconductor absorption edge energy.

[Action]

In the photoelectron emitter of the present invention, photoelectrons are generated by internal electron emission at the semiconductor junction arranged inside the photoelectron emitter, so that the semiconductor constituting the mechanism for causing external photoelectron emission is independent of the energy gap. Therefore, it facilitates the formation of a photoelectron emitting surface sensitive to long-wavelength light.

〔Example〕

First, the basic contents of the present invention will be described prior to the description of specific embodiments.

A feature of the photoelectron radiator of the present invention is that photoelectrons generated by internal photoelectron emission at a semiconductor junction are made to emit external electrons. In other words, conventionally, there is a Schottky photo diode in which a metal and a semiconductor are joined as a photodetector having sensitivity to long-wavelength light, but this is a detector using only internal photoelectron emission. On the other hand, the photoelectron radiator of the present invention is characterized in that the photoelectrons emitted from the internal photoelectrons are further emitted to the external electrons. Therefore, the essence is that it has a structure in which electrons are carried in one direction by electric field acceleration and are continuously connected to internal electron emission → electric field acceleration → external electron emission.

The photoelectron radiator of the present invention has a structure having a semiconductor junction, and an energy barrier is formed at the interface.
When one of the conductor materials is irradiated with light having a higher energy than the barrier, the electrons of the one conductor material cross the barrier and are internally photoelectron-emitted to the other semiconductor. The other semiconductor that has received photoelectrons has an electric field that accelerates the photoelectrons in the thickness direction of the semiconductor and carries them to the surface in one direction, and the surface has a structure with a substantially negative surface barrier. Therefore, the photoelectrons are emitted into the vacuum as external photoelectrons. Therefore, in this case, the limit wavelength of the photoelectron emission surface is determined by the height of the barrier of the semiconductor junction, not by the conventional energy gap of the semiconductor. In other words, conventionally, in order to design a photoelectron radiator having sensitivity to long-wavelength light, it was necessary to always use a semiconductor with a small energy gap, but this photoelectron radiator does not need to do so, and the height of the barrier is That is, it is possible to independently consider the limiting wavelength and the reduction of the surface barrier. Also, the height of the barrier can be changed by changing the combination of semiconductor junctions.

Various conventionally known techniques can be used to lower the surface barrier. In the case of this photoelectron emitter, the technical difficulties in terms of material in the conventional method are freed. The semiconductor junction for generating photoelectrons may be semiconductors, but may be a so-called Schottky junction between semiconductor and metal. Furthermore, the direction in which the light is incident when the light is incident on the junction is not essential, and a type that emits external photoelectrons on the same surface as the light incident surface (a so-called reflection type) and a type that emits external photoelectrons on the opposite surface ( Any of these so-called transmissive types is possible, and these structural modifications are included in the scope of the present invention. Further, the photoelectrons emitted from the internal electrons may be guided to the surface as an induced current and may be emitted to the outside by utilizing the low electron affinity layer on the surface. The gist of the present invention is to externally emit photoelectrons emitted as internal electrons.

Next, the present invention will be described in more detail with reference to specific examples in the case where a semiconductor and a metal are Schottky bonded.

The embodiment of the present invention shown in FIG. 1 is an example in which a semi-insulating semiconductor substrate (for example, a semi-insulating GaAs substrate) is used as a semiconductor and a mesh electrode is formed on the surface. That is, FIG. 1 shows that when a metal and a biased semi-insulating semiconductor are joined together, electrons emitted from the metal to the semiconductor by internal photoelectrons are given energy above the surface barrier by a strong electric field in the semi-insulating semiconductor, FIG. 3 is an energy band diagram of a photoelectron emission surface configured to efficiently emit external photoelectrons. In the figure, the semi-insulating semiconductor substrate 21 to which a strong electric field is applied is, for example, GaAs, and is joined to the metal 22 to form a barrier to be exceeded by internal photoelectron emission. Further, an electrode 23 is provided on the surface, and a bias is applied to the semi-insulating substrate 21. The electrode 23 has a thin film shape or a mesh shape and does not interfere with external photoelectron emission.

When using a semi-insulating substrate, a semiconductor to a sufficiently high bias voltage internal photoemission electrons from the back surface of the metal 22 in order to be able to apply a (e ^-) is now being field acceleration hot electron Then, the photoelectron is emitted into the vacuum by getting enough energy to overcome the barrier on the surface. In this case, if the distance required to pass the semiconductor substrate 21 is longer than the average travel distance of hot electrons,
The photoelectrons are wasted because they disappear. Ideally, it may be thin, but on the other hand it must also have a thickness for acceleration, and it should not be so thin as to cause a tunnel effect. Even if the thickness is as thick as 400 μm, the sensitivity is enough to withstand practical use. (Sensitivity: 0.1mA / W, Wavelength: 1200n
m). Depending on the semi-insulating substrate 21 used, it is also possible to increase the amount of electrons emitted by photoelectron by avalanche amplification during acceleration by a high electric field. The electric field strength required for this purpose is about 100 KV / cm to 500 KV / cm, and it is possible to generate this electric field with a single structure as shown in Fig. 1, but in order to create a local electric field at the required location. Other joints may be added.
The material of the metal 22 is not particularly limited. This is because normally, if a semiconductor and a metal are bonded together without any special treatment, the height of the barrier formed by the interface state is almost one third of the semiconductor energy gap, regardless of the metal. This is because. Therefore, in this case, the barrier height Ga
Energy gap of As = 1/3 of 1.4eV, that is,
It becomes about 0.47 eV, and the limiting wavelength becomes 2.64 μm. The height of the barrier caused by bonding with a metal is defined as
There is also a method of material treatment that is determined by the difference in electron affinity between a metal and a semiconductor. In this case, a metal is selected so that the height of the barrier becomes a desired height. Here, an example of the reflection type in which the incident light and the direction in which the external photoelectrons are emitted is the same has been shown, but as described above, the transmission type photoelectrons can be formed by using a thin layer of a semitransparent metal. It may be used as a radiator. In this example, the electrode on the surface is a mesh electrode, but this is to apply a bias voltage to the GaAs substrate 21 without photoelectrons losing energy in the electrode,
If the electrons are thin enough to pass through without losing energy, there is no need to make a mesh. Further, the electrodes are not limited to metals such as Al and Ag.

The embodiments described above are examples of methods of providing a substantially negative surface barrier to a semiconductor joined to a metal, and therefore the present invention is not limited to these methods, and any method may be used. Furthermore, although the III-V group semiconductor such as GaAs is mentioned in the above-mentioned specific examples, any material may be used in view of the essence of the present application. For example, a mixed crystal of a III-V group tertiary quaternary compound may be used. However, I
Another advantage arises when using the II-V group. Γ
The −L transition has a merit that, in addition to emission of photoelectrons from a higher level of energy, the life of carriers can be extended and more efficient photoelectron emission can be performed. This was made possible by the discovery of the inventor of the present application, and applying it to a normal photodiode or the like was filed as a separate invention,
This property is a general property peculiar to the III-V group, and it should be used as an effective carrier propagating means in further emitting external photoelectrons emitted from internal photoelectrons as in the present application. Is naturally conceivable. The essence of the present invention resides in a structure in which the electrons, which have been internally photoelectron-emitted, are internally accelerated and then externally emitted.

〔The invention's effect〕

It is difficult to create a photoelectron emitting surface having a sensitivity in the infrared region by the conventional method. At the laboratory level, it has been reported that it has a sensitivity up to a considerably long wavelength, but a practical one has a sensitivity to a wavelength longer than 1 μm.
It only consists of Ag, O, Cs called S-1 photoemission surface. Moreover, this sensitivity is extremely low as compared with the sensitivity of other photoelectron emitting surfaces in the visible range. A photomultiplier tube using a photoelectron emission surface is indispensable for weak light detection because it has less noise, but since there is no good photoelectron emission surface in the infrared region, various studies using weak light have been performed. And the commercialization is delayed. Currently used in the infrared region
The internal photoelectric type detector such as InSb and PbS is a detector having a quantum efficiency close to 1 and high sensitivity, but is not suitable for photodetection at a weak light level. This is because many of these internal photoelectric type detectors are used in photoconductive type operation, and dark current is extremely large. Therefore, it is extremely difficult to detect the weak photocurrent contained in the output signal.
Further, even in a photodetector using the photovoltaic effect, it is difficult to externally amplify a weak signal that is taken out to a signal level that is easy to handle with low noise. This is because much noise is generated in the amplifier.

On the other hand, if the photoelectron emitter of the present invention is applied to a photomultiplier tube, the detection efficiency tends to be lower than that of the internal photoelectric detector, but the secondary electron multiplication with extremely low noise is used. Therefore, weak light can be detected. Therefore, according to the present invention, various researches and devices in the weak light region, which are currently limited to the visible region, can be extended to the infrared region as they are. For example, in the study of physical properties, it has been possible to study the impurity level using luminescence in the infrared region, although the sensitivity of the luminescence detector was conventionally low and difficult. Alternatively, a weak light camera in the infrared region can be obtained by combining with an image pickup system. It is possible to observe a hot object at a weak light level and to perform night vision by irradiating infrared rays. Furthermore, if it is applied to a streak camera that uses light emitted by electron emission to deflect the electrons, the fastest photodetector in the infrared region can be realized.

[Brief description of drawings]

FIG. 1 is an energy band diagram of an embodiment of the present invention, and FIGS. 2 to 4 are energy band diagrams of a conventional example.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Tsuneo Uchihara 1 1126 Ichinomachi, Hamamatsu City, Shizuoka Prefecture 1126 Hamamatsu Photonics Co., Ltd. Co., Ltd. (72) Kenichi Sugimoto 1 1126, Nomachi, Hamamatsu, Shizuoka Prefecture 1126 Hamamatsu Photonics Co., Ltd. (72) Inventor Koichiro Ohba 1 126, Nomachi, Hamamatsu, Shizuoka Prefecture Hamamatsu Photonics Co., Ltd. ( 72) Inventor Toshihiro Suzuki 1 126-1, Nomachi, Hamamatsu-shi, Shizuoka Prefecture Hamamatsu Photonics Co., Ltd. (56) References Japanese Patent Publication Sho 44-7677 (JP, B1)

Claims (3)

[Claims] 1. A structure having a semiconductor junction between one conductive material and the other semiconductor, wherein photoelectrons internally emitted from the one conductive material into the other semiconductor by irradiation light are It is configured such that external photoelectrons are emitted from the surface of the other semiconductor as hot electrons by accelerating the electric field in the thickness direction of the other semiconductor to allow the semiconductor to run, and the other semiconductor is a semi-insulating semiconductor, A photoelectron emitter which is made more sensitive to infrared light than semiconductor absorption edge energy by using a substrate having a sufficient thickness that does not cause a tunnel effect penetrating the whole. 2. The photoelectron radiator according to claim 1, wherein an avalanche multiplication is caused by setting the internal electric field strength to 100 KV / cm or more. 3. A photoelectron emitter according to claim 1, wherein a reverse bias junction is provided and is used as a part of electric field acceleration.