JPH07245057A - Semiconductor photo-electric cathode - Google Patents

Abstract

PURPOSE:To reduce noise and dark current. CONSTITUTION:This cathode is provided with a light absorbing layer 110 generating a photoelectron by incident light, and electron transfer layers 130a and 130b making heterojunction with the light absorbing layer 110, and by an outside bias applied from electrodes 122 and 142, formed on the emission surface side on the electron transfer layers 130a, 130b, photoelectrons are transferred in the layers 130a, 130b, and are emitted from the emission surface 150. These electron transfer layers 130a, 130b of a semiconductor photo-electric cathode are composed by including a first graded layer 132 composed so as to become larger when the band gap thereof recedes from the light absorbing layer 110 and a second graded layer 134 formed on the upper side of the first graded layer 132 and composed so as to become smaller when the band gap nears the emission surface 150.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a structure of a photocathode which belongs to a semiconductor photocathode and forms an electric field inside the photocathode.

[0002]

2. Description of the Related Art A semiconductor photocathode is used to form an electric field inside a semiconductor by an external bias voltage to transfer photoelectrons to a discharge surface and discharge them into a vacuum. An example of this photocathode is US Pat. There are 3958143 (reference 1) and "JP-A-62-133633". This photocathode is called a TE photocathode, and its mechanism is described in several documents (for example, Document 2 “Semicond.Sci.Technol. 4”).
(1989) 498-499 ").

[0003]

One of the biggest drawbacks of this type of photocathode is the large dark current of the photocathode. For example, the experimental value of the dark current of the photocathode using InGaAs (light absorption layer) / InP system (electron transfer layer) is Id to 10 when the external bias voltage is 3.0 V at room temperature.
It is about ^-12 A / cm ² , and when the bias voltage is 5.0 V, this sharply increases to 10 ^-7 A / cm ² . When this photocathode is introduced into a PMT (photomultiplier tube) or II (image intensifier tube), etc., a remarkably large dark current is exhibited due to its multiplication function. Therefore, it becomes a big problem in practical use.

The dark current of the photocathode may have two components as its current source. One of them is a diffusion component caused by emission of thermoelectrons from the light absorption layer. The light absorption layer generally tends to select a material having a small band gap for the photocathode in order to make it more sensitive to long wavelengths. However, the relationship between the diffusion component Id and the temperature T is "I
As shown by "d-exp (-Eg / kT)", the band gap Eg and the operating temperature T change logarithmically. Therefore, if a material with a small band gap is used for the photocathode, the number of thermoelectrons will increase. Therefore, it is unavoidable to operate the photocathode at a low temperature to suppress an increase in dark current.

Another component of the dark current source is a component caused by avalanche multiplication by injecting holes from the surface Schottky electrode into the electron transport layer. That is, the holes collide with the lattice and are ionized to generate holes-electrons. This electric field component has only a weak dependence on the temperature, but has a strong dependence on the external bias voltage. The relationship between the component I ′d and the external bias voltage is “I′d˜Aex” using the maximum electric field strength E _max in the electron transport layer.
p [− (b / E _max ) ^m ] ”(reference“ Physic
s and Properties of Semiconductors-A Resume. 6 Pho
ton Spectra and Properties of Semiconductors ").
That is, the electric field component I ′d has a dependency of “E _max ˜ (V _B ) ^½ ” on the external bias voltage V _B. This equation can explain the experimental results well. Therefore, lowering the surface barrier of the photocathode by the external bias voltage can improve the escape probability of electrons and increase the sensitivity, but it increases the electric field component of the dark current.
Thus, there is a trade-off relationship between increasing the sensitivity and the electric field component of the dark current.

Therefore, an object of the present invention is to reduce the electric field component of dark current without lowering the photocathode sensitivity.

[0007]

In order to solve the above problems, a semiconductor photocathode of the present invention comprises a photoabsorption layer which generates photoelectrons by incident light, and an electron transport layer which forms a heterojunction with the photoabsorption layer. A semiconductor photocathode that has photoelectrons transferred in the electron transfer layer and emitted from the emission surface by an external bias applied from an electrode formed on the emission surface side on the electron transfer layer, the electron transfer layer comprising: , A first graded layer whose composition is such that its bandgap increases with distance from the light absorption layer, and it is formed above this first graded layer, and decreases with the bandgap approaching the emission surface. And a second graded layer having the above composition.

Between the first and second graded layers,
A layer having a maximum band gap in the electron transport layer may be formed.

The light absorption layer is formed by stacking Ge on a GaAs substrate, and the first graded layer is composed of GaA composed so that AlAs becomes larger as the distance from the light absorption layer increases.
The second graded layer is formed of a mixed crystal layer of s and AlAs, and the second graded layer is formed of a mixed crystal layer of GaAs and AlAs that is made smaller as AlAs approaches the emission surface. Also good. An AlAs layer may be formed between the first and second graded layers. It is also characterized in that a GaAs layer is further formed between the light absorbing layer and the first graded layer, the light absorbing layer side being doped with Ge, and the first graded layer side being intrinsic. good.

[0010]

In the present invention, since the graded layer is formed in the electron transport layer, the photoelectrons traveling in the electron transport layer have a low potential level with respect to the bottom of the conduction band even in a hot state. Has become. It also prevents injection of holes from the surface electrode into the electron transport layer. Therefore, avalanche multiplication due to holes and electrons is unlikely to occur.

[0011]

Embodiments of the present invention will be described with reference to the drawings.
In this example, the substrate is GaAs, the light absorption layer is p ⁺ Ge,
This is an example in which AlAs / GaAs and a mixed crystal thereof are used for the electron transport layer, and FIG. 1A shows the cross-sectional structure thereof. This semiconductor photocathode has a light absorption layer 1 on a substrate.
10 and an electron transport layer (reference numerals 131 to 135) are sequentially formed, and a Schottky electrode 141 is formed on the electron transport layer.
And the CsO film 150 is formed on the entire surface of the electrode pad 1
Bias voltage can be applied from the outside through the electrode 22 and the electrode pad 142. The substrate is omitted in the figure (on the left side of the figure). This photocathode can be manufactured as follows.

First, the Ge light absorption layer 110 is formed on the p-type GaAs semiconductor substrate by the epitaxial growth method. Zn may be used as a dopant and the doping concentration may be about 10 ¹⁸ cm ⁻³ . This film thickness is appropriately determined by the electron diffusion length in the light absorption layer, and is, for example, about 1 to 3 μm.

Next, an electron transport layer is formed. First, the GaAs layer 131 is grown on the electron transport layer (the light absorption layer 110).
The interface between and the GaAs layer 131 may have a stepwise potential). In order to reduce the discontinuity on the conduction band at the interface, for example, Ge is doped at about 10 ¹⁷ cm ⁻³ to form an n ⁻ GaAs layer, and then the intrinsic (“i”)
Abbreviated) GaAs layer is grown to 1 μm. Thus, the potential level of the conduction band does not form a barrier against electrons at the interface between the light absorption layer 110 and the electron transfer layer 130a.

Next, the graded layer 132 is formed so that the composition of AlAs is increased while gradually changing the composition of AlAs and GaAs. Graded layer 132
As the epitaxial growth method, MBE, vapor phase growth method, MOMBE method, or the like is used for the formation of the above, and a good graded layer can be formed by continuously controlling the supply of the group III raw material. The layer 132 may have a thickness of, for example, about 0.5 μm.

Then, the AlAs layer 133 which gives the maximum band gap in the electron transport layer is grown to about 1 μm. A
After the growth of the 1As layer 133, a graded layer 134 in which the composition of AlAs and GaAs changes so that GaAs gradually increases is formed. The thickness of the layer 134 is, for example, 0.
5 μm is sufficient. Next, the GaAs layer 135 is formed on the outermost surface of the semiconductor crystal. The thickness of the GaAs layer 135 is 0.1 μ
It may be m or less. In addition, layers 132 to 13 of the electron transport layer
5 may have a doping level (about 10 ¹⁷ cm ⁻³ or less) so that each of them has an i-p ⁻ type.

A surface Schottky electrode 141 is formed on the surface of the electron transport layer 132 by a vacuum deposition method or the like. Electrode 14
When 1 is thinly formed on the entire surface, Ag or the like is easily used as an electrode material, and when it is formed in a mesh lattice shape, a metal such as Al is easily used as an electrode material. A surface Schottky electrode pad 142 for attaching a lead wire for applying a bias voltage V _B is formed on a part of the surface of the surface Schottky electrode 141. Further, the bias applying electrode 122 is also formed on the light absorption layer 110. A for this material
uGe or the like may be used. The electrodes 122 are equivalent when formed on the substrate. After the wafer is processed in this manner, the surface cleaning treatment is performed in a vacuum, Cs is vapor-deposited, and O ₂ is adsorbed repeatedly to perform the work function lowering treatment to form the active layer 150 for electron emission. .

Each semiconductor layer is made of a general and easy-to-handle material, and a Schottky electrode 141 for forming a highly reliable Schottky barrier by the same manufacturing method as the conventional one is formed on the outermost GaAs layer 135. Can be formed. Further, as is clear from the composition of the semiconductor, it is possible to easily manufacture an excellent device in which the lattice constants of the respective layers are well matched.

The electron transport layer is formed in a heterojunction structure, and the electron transport layer has at least two graded layers 132 and 134. That is, the electron transport layer has three layers 131, 132 and 133 (electron transport layer 130) so that the composition gradually increases from GaAs to AlAs.
a), three layers of the layers 133, 134, and 135 (electron transfer layer 130) so that GaAs is gradually increased from AlAs.
b) and has an energy band structure having a curve such that the energy band gap has the maximum value between the graded layers 132 and 134.

FIG. 1 (b) shows an outline of a band diagram of this photocathode (VL indicates a vacuum level). By introducing a heterojunction in such a structure, that is, in the electron transport layer, avalanche multiplication caused by electrons, in which electrons eject ions of a lattice and ionize to generate electron-holes, and holes are prevented. Can be suppressed. At the same time, injection of holes from the surface electrode into the electron transport layer is blocked, and avalanche multiplication due to holes due to the high electric field in the electron transport layer is suppressed to increase dark current and noise. You can prevent it from happening.

FIG. 2 is a band diagram when the bias voltage V _B is applied. The operation of the photocathode will be described with reference to this figure.

Since the electron transport layers 130a and 130b both have low doping levels, in this case, the bias voltage V _B is applied almost uniformly, and the electron transport layers 131 to 133 are applied.
Has a voltage V ₁ and the electron transfer layers 133-135 have a voltage V ₂
Is applied. The applied bias voltage V _B is the potential difference V
₁ is the band gap of the AlAs layer 133 having the largest band gap among the electron transfer layers 131 to 135 (2.1
eV) or less and the potential difference V ₂ is the outermost surface layer GeAs13.
The voltage is set to be equal to or less than the bandgap of 5 (1.4 eV) (the magnitude of this bias voltage V _B will be described later).

The valence band (V.
B. ) 251 has a Schottky barrier significantly different from that in the case where a normal Schottky electrode on GaAs is formed even if a bias is applied. In the case of a layer having a band structure in which the valence band is flat when biased as usual, the valence band becomes parallel to the Fermi level E _F as indicated by the dotted line 252. On the other hand, the photocathode of FIG. 1 has two graded layers, and its energy band structure is shown in FIG.
As shown in (b), the level of the valence band is low. The characteristics of the Schottky barrier generated in the electron transfer layer by the electrode 141 differ depending on the thickness of the electron transfer layers 133 to 135. 3 and 4 show the difference, and FIG. 3 shows the case where the graded layer is thinner than that in FIG. In the case of FIG. 3, the Schottky contact of the electrode 141 brings the valence band 251 into a state close to a flat band.

When the light hν is incident from the substrate side, the incident light hν is absorbed by the light absorption layer 110 and photoelectrons e are generated in the light absorption layer 110. The photoelectrons e move to the electron transfer layer 130a, are transferred to the emission surface on the surface of the electron emission active layer 150 by the electric field formed inside the semiconductor by the external bias voltage V _B , and are emitted into the vacuum. The photoelectrons e are emitted into a vacuum, then multiplied by a multiplication function such as PMT or II, and taken out as a signal.

Here, from the light absorption layer 110 to the electron transfer layer 1
The photoelectrons that have moved to 30a are accelerated and transit to a higher band to be in a hot state. If the energy difference generated when the photoelectrons in this state transition to the low energy state is sufficiently large, avalanche multiplication due to the electrons occurs. The carriers generated by this become noise generated in the photocathode.

However, in the photocathode of FIG. 1, the band gap is the entrance of the electron transfer layer 130a (on the side of the light absorption layer 110).
Since it is formed so that it gradually increases, the bottom slope of the conduction band (CB) is small, and the energy difference caused by the transition of photoelectrons is small. Therefore, avalanche multiplication due to electrons is unlikely to occur at any position in the electron transport layer 130a, and holes are less likely to be generated. At the same time, it is in a cool state near the bottom of the band for electrons traveling in the conduction band. After all, generation of holes and electron multiplication are very unlikely to occur. Electron transport layer 1
The photoelectrons e moved to 33 to 135 are further accelerated by the electric field in this region, but the average energy is 0.4 to 0.6 eV from the measurement result of the emission energy distribution of the electrons e. It can be seen that avalanche multiplication due to electrons cannot occur.

Further, the above structure is considered to work as follows for holes.

The injection of holes into the electron transport layer 130b is defined by the height and thickness of the Schottky barrier generated in the electron transport layer by the electrode 141. In the example, the inclination of the band can be flattened on the valence band, and it can be seen that a potential barrier having a sufficient thickness is first formed for holes. Further, the reliability of the Schottky electrode is improved because the sufficient thickness of the potential barrier also compensates for the good formation of the Schottky electrode. As compared with the conventional case, since the injection of holes can be blocked by the potential barrier, the injection of holes into the electron transport layer 130b cannot occur as a matter of course, and the avalanche multiplication due to holes is suppressed. Therefore, the electric field component of the dark current can be suppressed low.

In addition to this, since the Schottky electrode 141 forming a highly reliable Schottky barrier can be formed, the injection of holes is reduced, the leak current is reduced, and the avalanche caused by holes is reduced. The multiplication is suppressed. Further, since there is no portion having a small band gap in the electron transfer layer as in the conventional example, the dark current can be reduced.

As described above, in the present invention, a material having a large band gap of p to i type is introduced into the electron transport layer,
By adjusting the material composition, the band gap is gradually increased from the light absorption layer side and is increased from the surface Schottky electrode side. This makes it possible to suppress the electric field component of the dark current, reduce the noise component accompanying the photoelectrons e emitted in vacuum, and obtain a signal with a large S / N from the photocathode. And PMT or I
When it is used for I or the like, it is multiplied by the multiplication function utilizing the secondary electron emission phenomenon and a signal with a large S / N can be obtained.

Even when the flat band is not formed in the valence band of the electron transfer layer 130b as shown in FIG. 4, as compared with the case where the flat band is formed only by the normal GaAs layer (reference numeral 252 in FIG. 2). However, since the Schottky barrier is thick, hole injection can be suppressed to a large extent. this is,
This is the effect of introducing a wide-gap material between the electron transfer layers 130a and 130b.

In the photoelectron emission phenomenon, a surface barrier exists for photoelectrons, so that it cannot be said that any photoelectrons can escape into the vacuum (right side of FIG. 1 or FIG. 2B). The height of the barrier makes the wavelength limit (called the threshold), and the thickness of the barrier determines the probability that an electron can escape into the vacuum (this probability is called the escape probability). This kind of photocathode exists as one of means for improving both of them. How much a bias voltage applied from the outside is required depends on the wavelength of incident light when GaAs is used as an example. About 8
In the wavelength region of 00 nm or less, when the electric field strength inside the semiconductor is 10 ⁴ V / cm, a sensitivity increase of 2.0 to 2.2 times is produced. The sensitivity saturates even if applied more than this (Reference 2). From this viewpoint, it is desirable that the bias voltage be as low as possible.

InGaAsP / InP system (1.4 μm
There is an experimental value showing the highest sensitivity at a bias voltage of 4.5 V (for use), but the dark current increases remarkably at this bias voltage. Considering the escape probability, the increase in sensitivity, and the increase in dark current, in the above wavelength range, a bias voltage of about 3.5 V is an operating point that can be determined from the S / N.
Further, as is clear from the band diagram of FIG. 2, the bias voltage V _B is “V _B = V ₁ + V” with respect to the maximum value Eg ₁ and the minimum value Eg ₂ of the band gap of the electron transport layer.
₂ <Eg ₁ + Eg ₂ , V ₁ <Eg ₁ , V ₂ <Eg ₂ (provided that Eg ₁ > Eg ₂ ) ”, and it is sufficient that the electron transfer layer has an electric field strength of about 1 × 10 ⁴ v / cm. Considering the manufacturing restrictions of materials, the thickness of the electron transport layer is
It can be calculated as 3.5 μm. This means that conventional manufacturing techniques can be used, without any problems,
Can actually be manufactured.

Further, as long as the production technique is improved and the material and the processing technique can be made finer, the present invention is not limited to the above embodiment. Rather, technological improvement tends to lower the bias voltage, and in that respect the conditions aimed at by the present invention can be easily satisfied, which is desirable.

As described in the above-mentioned conventional example, "Japanese Patent Laid-Open No. 62-13".
3633 ", it would be possible to block the flow of holes by the introduction of an additional layer forming a barrier-like potential. However, since the injection of holes from the electrode exists, accumulation of holes occurs due to the potential barrier in the conduction band. The potential is forced to change, and it lacks operational stability. Further, the introduction of the additional layer into the conduction band causes discontinuity in the conduction band (there are convex and concave types as potential barriers). Therefore, the electron transparency is about several percent in the case of 30 angstroms as in the example, and the quantum efficiency is largely reduced. When a plurality of additional layers are introduced, the function deteriorates beyond practical use.

However, in the present invention, since the structure in which the potential level is gently changed so as to have the maximum energy band gap is adopted for the electron transfer layer 130, the instability of operation and the decrease in quantum efficiency are described above. It is possible to suppress the electric field component of the dark current while suppressing the disadvantage of (1), and it becomes a photocathode with less noise and dark current.

The present invention is not limited to the above-described embodiment, but various modifications can be made.

The combinations of materials are not limited to those described above, but the combinations as shown in Table 1 (No. 1 to No. 6)
However, the graded layers 132 and 134 may be layer 1
Needless to say, it is a mixed crystal of 31,135 materials). These combinations have good lattice constant matching as in the above embodiment.

[0038]

[Table 1]

It should be noted that the sensitivity is extended to a long wavelength, and
Since it is desirable to make it easier for photoelectrons generated in the light absorption layer 110 to move to the electron transfer layer, the level of the conduction band is higher than that of electrons at the interface with the electron transfer layer 130a as in the above embodiment. It is desirable to have a composition that does not form a barrier.

Further, for example, the transmission type photocathode in which light is incident from the substrate side is shown as an example, but a reflection type photocathode in which light is incident from the Schottky electrode 141 side may be used. Further, each of the electron transport layers 130a and 130b may be composed of only a graded layer.

[0041]

As described above, according to the present invention, since the graded layer is formed in the electron transport layer, generation of holes and electron multiplication due to avalanche multiplication is less likely to occur. It is possible to obtain a stable operation photocathode with little dark current and noise.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an embodiment.

FIG. 2 is a diagram showing a state when a bias is applied.

FIG. 3 is a diagram showing a state of a valence band near the Schottky electrode 141 when the graded layer 134 is thin.

FIG. 4 is a diagram showing a state of a valence band near the Schottky electrode 141 when the graded layer 134 is thick.

[Explanation of symbols]

110 ... Light absorption layer, 130a, b ... Electron transfer layer, 13
1,135 ... GaAs layer, 133 ... AlAs layer, 13
2, 134 ... Graded layer, 141 ... Schottky electrode, 150 ... Electron emission active layer.

Claims (5)

[Claims] 1. A light absorption layer that generates photoelectrons by incident light, and an electron transfer layer that forms a heterojunction with the light absorption layer, and is applied from an electrode formed on the emission surface side of the electron transfer layer. A semiconductor photocathode that transfers the photoelectrons through the electron transfer layer by an external bias and emits the electrons from the emission surface, wherein the electron transfer layer has a band gap that increases with distance from the light absorption layer. A first graded layer having a composition and a second graded layer formed above the first graded layer and having a band gap that becomes smaller as the band gap approaches the emission surface. A semiconductor photocathode characterized by comprising: 2. The semiconductor photovoltaic device according to claim 1, wherein a layer having a maximum bandgap in the electron transport layer is formed between the first and second graded layers. cathode. 3. The light absorption layer is made of Ge on a GaAs substrate.
The first graded layer is formed by laminating AlAs and GaAs and
The second graded layer is formed of a mixed crystal layer of GaAs and AlAs, and the second graded layer is formed of a mixed crystal layer of GaAs and AlAs that is made smaller as AlAs approaches the emission surface. The semiconductor photocathode according to claim 1. 4. The semiconductor photocathode according to claim 3, wherein an AlAs layer is formed between the first and second graded layers. 5. A GaAs layer is further provided between the light absorption layer and the first graded layer, wherein the light absorption layer side is doped with Ge, and the first graded layer side is intrinsic. The semiconductor photocathode according to claim 3, wherein the semiconductor photocathode is formed.