JPH07161287A - Photoelectron emitting surface and photomultipler using it - Google Patents

Abstract

PURPOSE:To provide a photoelectron emitting surface, where an electron can be emitted at low bias voltage, and a photomultiplier with high sensitivity further with low noise remarkably reducing a dark current. CONSTITUTION:In a condition that bias voltage is applied in a direction of accelerating a photoelectron through an electrode, when light hnu is incident from a side of an electron emitting layer 4, the incident light, permeating through the electron emitting layer 4 and an i-type intermediate layer 3, is absorbed by a light absorbing layer 2 to excite a photoelectron (e). This photoelectron (e) is accelerated by an electric field formed by applying the bias voltage, to reach an interface between the light absorbing layer 2 and the i-type intermediate layer 3. In an energy barrier generated in this interface, the width of the barrier is narrowed than before by the intensified internal electric field of the i-type intermediate layer 3, and since the photoelectron easily passes through this barrier by a tunnel effect, the photoelectron is emitted by applying the bias voltage far lower than before. When the bias voltage is decreased, the injecting of a hole is suppressed, and a dark current due to this hole is remarkably reduced.

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 which emits photoelectrons upon incidence of light and a photomultiplier tube using the photoelectron emitting surface.

[0002]

2. Description of the Related Art Conventionally, a bias voltage is applied to a hetero-stack structure of a p-type III-V group compound semiconductor to cause photoelectrons to transition to the L valley from the valley of the conduction band by an internal electric field and then to be emitted to the outside. The transition electron type photoemissive surface is US.
PAT. 3,958,143.

This is because, for example, a p ⁺ InP substrate 1, a p ⁻ InGaAsP light absorption layer 2, a p ⁻ InP electron emission layer 4 and a Ag having a thickness of 100 Å or less, as shown in the schematic sectional view of FIG.
A structure composed of the thin film Schottky electrode 5 is well known.

FIG. 9 is an energy band diagram when no bias voltage is applied to the transition electron type photoelectron emission surface.
Shown in. In this case, the photoelectrons e excited in the conduction band in the p ⁻ InGaAsP light absorption layer 2 cannot reach the surface of the p ⁻ InP electron emission layer 4 due to the energy barrier in the conduction band.

Next, FIG. 10 shows an energy band diagram in the case where an appropriate bias voltage is applied and an internal electric field is formed on the photoelectron emitting surface. In this case, as shown in the figure, the p ^-- InGaAsP light absorption layer 2 and the p ^-- In
The conduction band discontinuity ΔE between the P electron emission layers 4 becomes small. Therefore, the photoelectrons e excited in the conduction band in the p ^-- InGaAsP light absorption layer 2 can move to the p ^-- InP electron emission layer 4, and are further accelerated by the electric field formed inside the semiconductor to reach L from the valley of the conduction band. After transitioning to the valley, it is released to the outside.

[0006]

However, in the transition electron type photoelectron emission surface of such a structure, p ^- I is essentially present.
Since the conduction band energy barrier ΔE exists at the interface between the nGaAsP light absorption layer 2 and the p ⁻ InP electron emission layer 4, and the width d is such that photoelectrons cannot pass through the tunnel effect, p ⁻ InGaAsP The photoelectrons e excited in the light absorption layer 2 need to be accelerated by an electric field enough to overcome the discontinuity of the conduction band.

In the transition electron type photoelectron emission surface of this structure,
Photoelectron e overcomes this discontinuity in the conduction band and p ^- InP
A certain high bias voltage is required to move to the electron emission layer 4, and the increase in dark current due to the leakage current of holes from the Schottky electrode 5 due to this high bias voltage has been a serious problem.

As a photoelectron emission surface for reducing the dark current, an additional layer of a thin film capable of forming a potential barrier of the highest possible limit inside the valence band while maintaining electron permeability in the light absorption layer. What was formed is disclosed in Japanese Examined Patent Publication No. 62-133633. FIG. 11 shows the structure of this photoelectron emitting surface and the energy band diagram when the bias voltage is applied.

As is apparent from this figure, this photoelectron emission surface is a so-called transmissive photoelectron emission surface where light is incident from the window layer 21 side and photoelectrons are emitted from the electron emission layer 20 side. However, light which is incident from the electron emission layer side and photoelectrons are emitted to the electron emission layer side, that is, a so-called reflection type photoelectron emission surface in which the dark current is sufficiently reduced has not been realized yet.

The present invention has been made in order to realize a reflection type photoelectron emission surface in which the dark current is sufficiently reduced.
It is an object of the present invention to provide a photoelectron emitting surface capable of emitting electrons at a bias voltage lower than that of a conventional one, and to provide a highly sensitive and low noise photomultiplier tube in which dark current from the photoelectron emitting surface is significantly reduced. .

[0011]

In order to achieve the above object, the photoelectron emission surface of the present invention has a hetero-laminated structure of III-V group compound semiconductors and absorbs incident light to excite photoelectrons. The photoelectron emission surface is (a) a light absorption layer having a small energy gap formed on the substrate, and (b) is laminated on the upper surface of the light absorption layer, has a larger energy gap than the light absorption layer, and has a high resistance. An i-type intermediate layer, (c) an electron emission layer stacked on the upper surface of the i-type intermediate layer, (d) a first electrode in contact with the substrate, and (e) a second electrode in contact with the electron emission layer. It is characterized in that incident light enters from the electron emission layer side and photoelectrons are emitted to the electron emission layer side.

Here, the electron emission layer may be an n-type layer having a thickness of 100 Å or less.

The second electrode in contact with the electron emission layer is
At least a part of the surface of the electron emission layer may be formed to be exposed, incident light may enter from the exposed surface of the electron emission layer, and photoelectrons may be emitted from the exposed surface of the electron emission layer.

The second electrode in contact with the electron emission layer is
It may be made of a metal of Al, Au, Ag, Mo, Ni or W or a silicide or an alloy thereof.

In order to achieve the above object, the photomultiplier tube of the present invention is characterized by including the above-mentioned photoelectron emitting surface.

[0016]

When light is incident on the photoelectron emission surface according to the present invention from the electron emission layer side with a bias voltage applied in the direction of accelerating photoelectrons via the first and second electrodes, the incident light emits electrons. The light passes through the layer and the i-type intermediate layer, is absorbed by the light absorption layer, and excites photoelectrons.

On the other hand, when a bias voltage is applied, an internal electric field is formed on the photoelectron emission surface, and band bending occurs due to depletion near the surface, but energy is generated at the interface between the light absorption layer in the conduction band and the i-type intermediate layer. Barriers exist.

The excited photoelectrons are accelerated by the electric field formed by the application of the bias voltage, and the photoabsorption layer and i
Reach the interface of the mold interlayer. Since the energy barrier existing at this interface has a narrower width than that of the conventional one due to the strong internal electric field of the i-type intermediate layer, photoelectrons easily pass through this barrier by the tunnel effect.

The photoelectrons that have passed through the interface between the light absorbing layer and the i-type intermediate layer cross the interface between the i-type intermediate layer and the electron-emitting layer, transition from the bottom of the Γ valley of the conduction band to the bottom of the L valley, and then the electrons. It easily reaches the surface of the release layer and is released to the outside.

Since the photoelectrons e easily pass through the energy barrier of the conduction band due to the tunnel effect, the photoelectrons are emitted by applying a bias voltage much lower than the conventional one. Then, when the bias voltage is lowered, injection of holes due to application of the bias voltage is suppressed, and the dark current due to the holes is significantly reduced.

Among the photoelectron emitting surfaces according to the present invention,
In the case of using the ultrathin n-type semiconductor layer as the electron emission layer, since the internal electric field strength of the electron emission layer is low, injection of holes from the electrode in contact with the electron emission layer is further suppressed, and the dark current is further reduced.

Further, the photomultiplier tube having the above-mentioned photoelectron emitting surface operates while the dark current is greatly reduced as compared with the conventional one.

[0023]

Embodiments of the present invention will now be described in detail with reference to the accompanying drawings. In the description of the drawings, the same elements will be denoted by the same reference symbols, without redundant description.

A schematic cross-sectional view of an embodiment of the photoelectron emitting surface according to the present invention is shown in FIG. This photoelectron emission surface is p ⁺ type In
A p ⁻ InGaAsP light absorption layer 2 on the P semiconductor substrate 1,
A high resistance i-type InP intermediate layer 3 is formed thereon, and p ⁻ I is formed thereon.
III-V with nP electron emission layer 4 epitaxially grown
It has a hetero-stacked structure of a group compound semiconductor. Further, an Al Schottky electrode 5 formed in a pattern in which at least a part of the surface of the electron emitting layer 4 is exposed is formed on the electron emitting layer 4, and an A Schottky electrode 5 is formed on the lower surface of the p ⁺ type InP semiconductor substrate 1.
The uGe ohmic electrode 6 is formed in layers. The surface of the electron emission layer 4 is coated with Cs as an alkali metal 7 in a very thin manner in order to further reduce the work function.

Now, assuming that near infrared light hν enters the photoelectron emitting surface to which an appropriate bias voltage is applied from the exposed surface of the p ⁻ InP electron emitting layer 4, the near infrared light hν is p
^- through the InP electron emitting layer 4 and the i-type InP intermediate layer 3,
It is absorbed by the p ⁻ InGaAsP light absorption layer 2 and excites the photoelectrons e.

The photoelectrons e are accelerated by an electric field formed by applying a bias voltage, and the p ⁻ InGaAsP light absorption layer 2, the i-type InP intermediate layer 3, and the p ⁻ InP electron emission layer 4 are formed.
Of the conduction band, and after transitioning from the bottom of the Γ valley of the conduction band to the bottom of the L valley, the surface of the p ⁻ InP electron emission layer 4 is reached.

The surface of the p ^- InP electron emission layer 4 has a suitable C
The work function is lowered by the application of s, and the reached photoelectrons e are easily emitted to the outside from the exposed surface of the electron emission layer 4.

That is, the photoelectron emission surface of this embodiment is a so-called reflection type photoelectron emission surface in which incident light is incident from the electron emission layer 4 side and photoelectrons are emitted to the electron emission layer 4 side.

The characteristic point of this embodiment is that
Even if the bias voltage is low, the photoelectron e is p ^- InGaA
This is to pass through the interface between the sP light absorption layer 2 and the i-type InP intermediate layer 3 by the tunnel effect.

FIG. 2 shows an energy band diagram when the bias voltage on the photoelectron emission surface of this embodiment is not applied. An i-type InP intermediate layer 3 is introduced between the p ⁻ InGaAsP light absorption layer 2 and the p ⁻ InP electron emission layer 4. However, in the state where this bias voltage is not applied, the photoelectrons e excited in the p ⁻ InGaAsP light absorption layer 2 cannot reach the surface of the p ⁻ InP electron emission layer 4 due to the conduction band barrier.

FIG. 3 shows an energy band diagram when a bias voltage on the photoelectron emission surface of this embodiment is applied. An internal electric field is formed by applying a bias voltage, and band bending occurs due to depletion near the surface.

P ^- InGaAsP light absorption layer 2 and p ^- In
The conduction band discontinuity between the P electron emission layers 4 is as shown in FIG. 3 due to the presence of the i-type InP intermediate layer 3. Although there is an energy barrier in the conduction band as in the conventional case, the width d of the barrier becomes narrower than in the conventional case because the InP intermediate layer 3 is i-type and therefore has a strong internal electric field. As a result, the photoelectrons e can easily pass through the energy barrier in the conduction band by the tunnel effect even if the magnitude of the bias voltage is about the same as the conventional one.

Therefore, the photoelectrons e excited in the p ^-- InGaAsP light absorption layer 2 can easily reach the surface of the p ^-- InP electron emission layer 4. Here, since the width of the energy barrier of the conduction band is narrower than in the conventional case, the bias voltage required for the photoelectrons e to pass through the barrier can be set to be much lower than in the conventional case. This suppresses the injection of holes from the Schottky electrode 5 due to the application of the bias voltage, and significantly reduces the dark current caused by the holes. Therefore, it is possible to reduce the dark current, which is the biggest drawback of this type of transition electron photoemissive surface.

Of course, in order to further reduce the energy barrier ΔE at the interface of the conduction band, it is effective to use the i-type InP intermediate layer 3 as a graded composition layer in which the composition is gradually changed from InGaAsP to InP. It's easy to imagine. However, what is essential is that, since the intermediate layer 3 is composed of a high-resistance i-type semiconductor, photoelectrons can easily pass through the energy barrier of the conduction band by the tunnel effect, and thus the bias voltage can be set low, so that the photoelectron emission is reduced. That is, it is possible to suppress the injection of holes into the surface.

Next, FIG. 4 shows a schematic sectional view of an embodiment of the second photoelectron emitting surface according to the present invention. This photoelectron emission surface is formed on the p ⁺ type InP semiconductor substrate 1 by p ⁻ InGaAs.
P light absorption layer 2, high resistance i-type InP intermediate layer 3 thereon,
It has a III-V group compound semiconductor hetero layered structure in which an n-type InP electron emission layer 8 having a thickness of 100 Å or less is epitaxially grown thereon. Furthermore, on the electron emission layer 8,
The Al Schottky electrode 5 formed in a pattern in which at least a part of the electron emission layer 8 is exposed is formed, and p ⁺ type In
On the lower surface of the P semiconductor substrate 1, AuGe ohmic electrodes 6 are formed in layers. On the surface of the electron emission layer 8, Cs is used as an alkali metal 7 in order to further lower the work function.
Is applied very thinly.

Now, assuming that near infrared light hν enters the photoelectron emitting surface to which an appropriate bias voltage is applied from the exposed surface of the p ⁻ InP electron emitting layer 4, the near infrared light hν is n.
Through the i-type InP electron emission layer 8 and the i-type InP intermediate layer 3,
It is absorbed by the p ⁻ InGaAsP light absorption layer 2 and excites the photoelectrons e. The photoelectrons e are accelerated by the electric field formed by the application of the bias voltage, cross the interface between the p ⁻ InGaAsP light absorption layer 2 and the i-type InP intermediate layer 3, and cross the conduction band Γ.
After transitioning from the bottom of the valley to the bottom of the L valley, the surface of the n-type InP electron emission layer 8 is reached.

The surface of the n-type InP electron emission layer 8 has a suitable C
The work function is lowered by the application of s, and the reached photoelectrons e are easily emitted into the vacuum from the exposed surface of the n-type InP electron emission layer 8. Therefore, obviously, the photoelectron emission surface of FIG. 4 is also a so-called reflection type photoelectron emission surface.

FIG. 5 shows an energy band diagram when the bias voltage of the photoelectron emission surface according to the present invention is not applied, and FIG. 6 shows an energy band diagram when the bias voltage is applied. Since the electron emission layer is an n-type semiconductor, Al
In the vicinity of the Schottky electrode 5, the strength of the internal electric field is reduced as apparent from FIG. Therefore, the injection of holes from the Schottky electrode 5 is further suppressed. As a result, it is possible to suppress the electrons generated by the collisional ionization due to the acceleration of the injected holes h and further reduce the dark current due to the hole injection.

By the way, the photoelectron emitting surface of the present invention is of a reflection type, whereas the photoelectron emitting surface of the present invention is a transmissive type photoelectron emitting surface for reducing the dark current, while the valence electrons are maintained in the photoabsorption layer while maintaining the electron transparency. Japanese Patent Publication No. 62-133633 discloses that a thin film additional layer capable of forming a potential barrier of the highest possible limit is formed inside the band. FIG. 11 shows the structure of this photoelectron emitting surface and the energy band diagram when the bias voltage is applied.

As is clear from this figure, on this photoelectron emission surface, light enters from the window layer 21 side and photoelectrons are emitted to the electron emission layer 20 side. Therefore, the photoelectron emission surface of FIG. 11 is an invention of a so-called transmissive photoelectron emission surface, in which incident light is incident from the electron emission layer side and photoelectrons are emitted to the electron emission layer side as in the present invention. The structure is essentially different from that of the reflective photoelectron emission surface.

Further, the photoelectron emission surface of FIG.
It is intended to suppress the traveling of the holes h injected from the hole 9 and not to suppress the injection of the holes h itself. This is essentially different from the photoelectron emission surface of the present invention that suppresses the injection of hydrogen.

In this embodiment, InP / InGaAs is used.
Although the P heterostructure has been described as an example, the material of the photoelectron emission surface is not limited to this, and it is described in US Pat. PAT. 3,9
Of course, it can be applied to all the materials as disclosed in 58,143. Also, the Schottky electrode 5
Alternatively, the material used for the ohmic electrode 6 is not limited to that of the embodiment, but may be made of a metal of Al, Au, Ag, Mo, W, or Ni, a silicide, or an alloy thereof. Further, these electrodes 5 and 6 are not limited to the Schottky electrode and the ohmic electrode, respectively, and any electrodes having an action capable of effectively forming an electric field inside the hetero-laminated structure of the semiconductor may be used. Of course. Further, it goes without saying that the alkali metal 7 is not limited to Cs. The ohmic electrode 6 in the above embodiment is used.
Is Au to which Ge is added so as to have ohmic characteristics.
It consists of

FIG. 7 is a top sectional view of an embodiment in which the reflection type photoelectron emitting surface 30 of FIG. 1 or 4 is applied to a sand-on type photomultiplier tube. The structure of the photoelectron emitting surface is the same as that shown in FIG.

Light hν is incident on the photoelectron emission surface 30 according to the present invention through the side tube 9 made of glass, and photoelectrons e are emitted in the direction of the arrow. The emitted photoelectrons e enter the first dynode 10, and secondary electrons are emitted in the number of about 2 to 3 times the number of incident photoelectrons e. The secondary electrons further enter the second dynode 11 and similarly emit 2-3 times as many secondary electrons. By repeating this, finally, in the anode 12, 10 of the photoelectrons e emitted from the photoelectron emitting surface are finally produced. ^It is detected as a photocurrent of about ⁶ times, which makes it possible to detect light with extremely high sensitivity.

At this time, if the dark current on the photoelectron emission surface 30 is large, these are also multiplied and detected at the anode 12 in a mixed manner with a signal, resulting in S /
This leads to a decrease in N. Therefore, in such a photomultiplier tube, the large dark current on the photoelectron emission surface 30 is a fatal defect as a photodetector.

The photoelectron emission surface 30 of FIG. 1 or FIG. 4 used in this embodiment has a dark current significantly reduced as compared with the conventional one as described above. The double tube has the excellent characteristics of high sensitivity and low noise.

[0047]

As described above, the photoelectron emission surface according to the present invention has a high resistance i-type intermediate layer between the photoabsorption layer and the electron emission layer, and is therefore resistant to the discontinuity of the conduction band. Since an internal electric field is generated and the photoelectrons can pass through the energy barrier due to the tunnel effect, it becomes possible to emit the photoelectrons with a bias voltage lower than the conventional one. Therefore, it is possible to significantly reduce the dark current due to the application of the bias voltage.

Furthermore, the electron emission layer has a thickness of 100.
By providing the n-type layer of Å or less, the injection of holes from the electrode in contact with the electron emission layer can be further suppressed, and thus the dark current can be further reduced.

As a result, by using the photoelectron emitting surface according to the present invention, it is possible to realize a highly sensitive and low noise photomultiplier tube in which the dark current from the photoelectron emitting surface is significantly reduced as compared with the conventional one.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view of an embodiment of a photoelectron emitting surface according to the present invention.

FIG. 2 is an energy band diagram when a bias voltage on the photoelectron emission surface of FIG. 1 is not applied.

3 is an energy band diagram when a bias voltage is applied to the photoelectron emission surface of FIG.

FIG. 4 is a schematic cross-sectional view of an embodiment of a second photoelectron emission surface according to the present invention.

5 is an energy band diagram when a bias voltage on the photoelectron emission surface of FIG. 4 is not applied.

6 is an energy band diagram when a bias voltage is applied to the photoelectron emission surface of FIG.

FIG. 7 is a top sectional view of an example in which the photoelectron emission surface according to the present invention is applied to a side-on type photomultiplier tube.

FIG. 8 is a schematic cross-sectional view of a conventional transition electron type photoelectron emission surface.

9 is an energy band diagram when a bias voltage on the photoelectron emission surface of FIG. 8 is not applied.

10 is an energy band diagram when a bias voltage is applied to the photoelectron emission surface of FIG.

FIG. 11 is a schematic cross-sectional view of a photoelectron emission surface on which a thin film additional layer is formed and an energy band diagram when a bias voltage is applied to the photoelectron emission surface.

[Explanation of symbols]

1 ... p ⁺ type semiconductor substrate, 2 ... p ⁻ type semiconductor light absorption layer, 3
... i-type semiconductor intermediate layer, 4 ... p ^- type semiconductor electron emission layer, 5
... Schottky electrode, 6 ... ohmic electrode, 7 ... alkali metal, 8 ... n-type semiconductor electron emission layer, 9 ... glass side tube, 1
0 ... 1st dynode, 11 ... 2nd dynode, 12 ... Anode, 16, 18 ... P ⁺ type | mold light absorption layer, 17 ... Additional layer, 19
... electrode, 20 ... emission layer, 21 ... window layer, 30 ... photoelectron emission surface.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI technical display location // H01L 31/10

Claims (5)

[Claims] 1. A photoabsorption layer having a small energy gap formed on a substrate at a photoelectron emission surface that has a hetero-stack structure of a III-V group compound semiconductor and that absorbs incident light to excite photoelectrons. An i-type intermediate layer laminated on the upper surface of the light absorbing layer, having a larger energy gap than the light absorbing layer and having a high resistance; an electron emission layer laminated on the upper surface of the i type intermediate layer; One electrode and a second electrode in contact with the electron emission layer, wherein the incident light is incident from the electron emission layer side, and the photoelectrons are emitted to the electron emission layer side. Emission surface. 2. The photoelectron emitting surface according to claim 1, wherein the electron emitting layer is an n-type layer having a thickness of 100 Å or less. 3. The second electrode in contact with the electron emission layer is formed in a pattern in which at least a part of the surface of the electron emission layer is exposed, and the incident light enters from the exposed surface of the electron emission layer, The photoelectron emitting surface according to claim 1, wherein the photoelectrons are emitted from an exposed surface of the electron emitting layer. 4. The photoelectron according to claim 3, wherein the second electrode in contact with the electron emission layer is made of a metal of Al, Au, Ag, Mo, Ni or W, a silicide, or an alloy thereof. Emission surface. 5. A photomultiplier tube comprising the photoelectron emitting surface according to claim 1.