JP2591533B2 - Radiation detecting element and method of manufacturing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is used for high-energy physics experiments using accelerators, and is used for α-ray, β-ray, γ-ray, X-ray
The present invention relates to a radiation detecting element that detects a beam, a neutron beam, and the like.

[0002]

2. Description of the Related Art As a radiation detector, a gas drift detector is used.
Semiconductor detectors using semiconductors such as Si have been increasingly used instead of chambers and other devices requiring a large volume. In this method, a reverse voltage is applied to a Si pn junction, and when radiation is incident on the junction, an electron-hole pair is generated and a current flows. Good linearity of radiation energy and detection current.

However, since silicon or germanium is used in the detection unit, the carrier has a low saturation mobility (the saturation electron mobility of silicon is 1 × 10 ⁷ cm / sec, and the response speed as a radiation detecting element is slow. Even a high-purity intrinsic substance has a resistivity of about 10 ⁵ Ω · cm, and germanium is even lower.If an electrode is attached to the Si or Ge semiconductor in the intrinsic region and a current is applied as it is, a dark current is too large. As in the case of the element to be detected, a voltage is applied so that a pn junction is created and a reverse bias is applied. However, this method can detect radiation only in the depletion layer of several tens of μm. The sensitivity is often not sufficient.

It is already known that when single-crystal diamond is used for the semiconductor in the detecting portion of the radiation detecting element, a device having a high response speed and high sensitivity can be obtained (Japanese Patent Laid-Open No. Sho 62-1).
98780, EP52397). This is because the carrier has a very high saturation mobility in single crystal diamond (2.5 × 10 ⁷ cm / sec). When diamond is used as the detection element, a reverse bias voltage is not applied to the pn junction, but an intrinsic region is used for the detection unit, and only a voltage is applied thereto. One of the reasons is that the resistivity of diamond in the intrinsic region is sufficiently high to reduce dark current. Another reason is that it is difficult to form a pn junction in the case of diamond. However, in the case of detecting radiation, it is better to detect radiation over the entire thickness of the element. If the detection can be performed only in the vicinity of the pn junction, the sensitivity will inevitably decrease. When radiation passes perpendicular to the element surface, in the case of Si, electron-hole pairs are generated only in the depletion layer of the pn junction. However, in the case of not using a pn junction like diamond, electron-hole pairs are generated in the entire thickness through which radiation passes. Therefore, such a detector has higher sensitivity. However, it must be a single crystal diamond with few defects. Such are not readily available and difficult to manufacture.

In recent years, a method for inexpensively synthesizing film-like polycrystalline diamond from the gas phase has been developed. An attempt was made to manufacture a radiation detecting element using polycrystalline diamond using this method, but no good results were found. This is because polycrystalline diamond has extremely low sensitivity to radiation.

[0006]

The reason why polycrystalline diamond has low sensitivity to radiation is that grain boundaries existing between crystal grains hinder the movement of electrical carriers generated by radiation. Since the speed is greatly reduced every time the light passes through the crystal grain boundary, the effective carrier mobility is reduced, and the performance higher than that of the Si detector cannot be obtained. It is an object of the present invention to provide a radiation detecting element using polycrystalline diamond which has a high response speed, a high sensitivity, and is inexpensive.

[0007]

Means for Solving the Problems The inventors of the present invention provided electrodes on both sides of a polycrystalline diamond film grown by a vapor phase synthesis method so that the grain boundaries became columnar, and the diamond grain boundaries were formed. Voltage in a direction that does not cross
When the radiation detection sensitivity was measured, a radiation detection element having practical sensitivity was obtained.

In the polycrystalline diamond used in the present invention, the diamond grains in the crystal have a large grain size, and the diamond grains grow in columns in the growth direction. In some cases, there is no grain boundary in the longitudinal direction of the columns. It is. Alternatively, a conductive layer doped with impurities is formed in the vicinity of the electrode so that the carrier is not decelerated in the vicinity of the electrode even if there is a grain boundary. Since there is almost no electric field in the conductive layer, even if carriers are generated by radiation, they are recombined and do not become a signal, but carriers reaching the conductive layer from the high-resistance layer can pass through the conductive layer as current. It has the effect of extracting a signal without deactivating or decelerating the carrier.

Electrodes are attached to the two opposing surfaces in the growth direction of such polycrystalline diamond grown in a columnar shape and having no grain boundaries in the column direction. This allows current to flow from electrode to electrode without crossing the grain boundaries. Therefore, the mobility of the carrier increases. This makes it possible to manufacture a high-sensitivity radiation detection element that has not been obtained with polycrystalline diamond.

The thickness of the semiconductor diamond layer used for the detecting element is preferably at least 10 μm, more preferably at least 100 μm, in consideration of the cross-sectional area of collision with radiation. The best is 200-300 μm. It is desirable to be 1 mm or less from the viewpoint of economy. Here, the direction in which the current flows, that is, the growth direction, is referred to as the vertical direction, and the direction perpendicular thereto is referred to as the horizontal direction.

If the grain size of the diamond particles in the polycrystalline diamond in the horizontal direction is too small, the carrier may obliquely collide with the grain boundaries even if there are no grain boundaries in the direction (vertical direction) in which the current flows. Therefore, it is better that the particle size is large and the number of grain boundaries is small in the horizontal direction. Preferably, the minimum lateral grain size in the polycrystalline diamond of the present invention is at least 1 μm. A preferred transverse particle size range is 5 to 50 μm. The optimum value is 20 μm.

The initial layer of diamond grown by the vapor phase synthesis method often has a small crystal grain size. In addition, since many defects and irregularities remain on the interface between the diamond and the substrate during the growth of the diamond and on the growth surface, the mobility of the carrier may decrease, and as a result, the sensitivity may decrease. To overcome this, the inventor has devised two solutions.

One method for preventing this is to remove the small grain boundaries by polishing the vicinity of one side or both sides of the diamond film obtained by the vapor phase synthesis, which has many small grain boundaries, so as to be smooth. As a result, the remaining portion has no grain boundary in the vertical direction, and is effective in obtaining high sensitivity. It is preferable that the interface with the substrate at the time of growth be cut off by 5 μm or more. The smoothness of the polished surface is desirably 0.2 μm or less.

Another method for preventing carrier deactivation at the interface between the diamond layer and the substrate or at the growth surface is to add a diamond boundary near the substrate interface or the growth surface to a grain boundary or a portion having many defects during the vapor phase synthesis. The purpose is to dope a sufficient amount of impurities to form a diamond conductive layer having a high conductivity, and to separate the generation and movement of carriers in the diamond semiconductor layer related to sensitivity and response speed. The diamond layer in this portion may have a grain boundary. In this area, carriers flow across the grain boundaries. However, since the carrier density is high, even if carriers are generated by radiation, they are immediately recombined, and do not contribute to the detection current. However, loss of signals at the grain boundaries can be prevented, which indirectly contributes to improvement in sensitivity.

In order to impart conductivity to diamond,
Boron, aluminum, lithium, phosphorus, sulfur, selenium, etc. are conceivable as impurity elements to be doped, but boron can effectively reduce the electric resistivity with the smallest addition and degrade the crystallinity of diamond This is preferable because it does not occur. The thickness of the diamond conductive layer should be 5 in order to suppress a decrease in mobility at the edge of the diamond layer.
It is preferably at least μm. However, if the conductive layer is thick, the part that receives radiation becomes thin, so that the conductive layer should not be too thick. As described above, the problem that a large number of crystal grain boundaries exist on both surfaces of the growth layer can be solved by two methods. However, these methods can be used alone or in combination. That is, when the conductive layer is provided at the end of the growth layer, the electrode may be attached without polishing or the electrode may be attached after polishing.

The semiconductor layer used for the radiation detecting element is
In order to detect a small signal, the dark current must be small, and the higher the electrical resistivity, the better. So use the intrinsic region of the diamond. However, it may not be high enough to charge the semiconductor layer. This can be distinguished by considering the contents of the electrical resistivity. Electrical resistivity is the reciprocal of the product of carrier density and mobility. This is a condition that requires high mobility and low carrier density. It is preferable that the electrical resistivity of the diamond semiconductor layer of the detecting portion is 10 ⁷ Ω · cm or more. A more preferable range of the electric resistivity is 10 ⁹ to 10 ¹² Ω · cm. Of course, when the conductive layer diamond is grown near the end face of the diamond layer, the electric resistivity of the conductive layer is low, and the preferable resistivity is 10 ⁻² to ² Ω · cm.

In the device of the present invention, the resistivity of the diamond layer can be controlled by doping a small amount of impurities into the diamond of the detecting portion during vapor phase synthesis. By adding a small amount of B, the resistance value decreases,
The resistance value is increased by adding N. This controls the number of carriers and does not change the mobility.

[0018]

The diamond gas phase synthesis method used in the present invention is a plasma CVD method or a thermal CV heating a thermionic emission material.
Any of the diamond gas phase synthesis methods such as the D method, the combustion flame method, the ion beam method, and the laser CVD method can be used. The substrate may be any material that can withstand the temperature required for the synthesis of diamond, but is most preferably a heat-resistant material such as Si, Mo, or SiC. The carbon raw material may be any gaseous substance containing carbon such as hydrocarbons, alcohols and ketones, and is used after being diluted with hydrogen or an inert gas.
An oxygen-containing gas can be added to improve the crystallinity of diamond.

A method for manufacturing the diamond detector of the present invention by polishing the growth end face will be described. FIG. 1 shows a substrate 1
Is shown. This is, as mentioned earlier, Si, Mo, Si
C and the like. On this, a polycrystalline diamond is grown by the method described above. Initially, polycrystals having a small grain size tend to have fine grain boundaries and grow, but columnar crystals without grain boundaries grow gradually in the growth direction. Therefore, the diamond film 2 is grown so as to be thicker than necessary. This is shown in FIG. After this, the substrate 1
Is removed. FIG. 3 illustrates this. Since the first portion contains many non-columnar polycrystals and the last portion has many defects and irregularities, both surfaces of the diamond layer are polished. The growth side is polished first, and then the surface adhering to the substrate is polished. This is shown in FIG. As described above, this must be polished so that no grain boundaries remain in the vertical direction. Therefore, the surface in contact with the substrate must be ground off by 5 μm or more. The smoothness after polishing is
The thickness should be 0.2 μm or less. Thereafter, electrodes 3 and 4 are formed on both surfaces of the diamond layer 2. This is, for example, T
i, Ni, etc. This is shown in FIG. The electrodes are orthogonal to the longitudinal direction of the columnar polycrystal. Thus, current can flow from electrode to electrode without crossing the polycrystalline grain boundaries. High response speed due to high carrier mobility.

As shown in FIGS. 1 to 5, a columnar polycrystal can be more actively grown in addition to growing a columnar polycrystal from a substrate naturally. As shown in FIG. 6, at the stage when small polycrystalline grains are grown on the substrate, the growth is temporarily stopped, and the crystal grains not oriented in (100) are selectively removed. Then, only the crystal grains oriented in (100) are left. At this time, it can be confirmed that particles oriented to (100) remain by a method such as X-ray diffraction. This is the state shown in FIG. Thereafter, when the crystal growth of diamond is started again, only crystal grains whose (100) plane is parallel to the substrate surface grow. Since the crystal orientation is the same, the crystal grains are likely to be large and have no grain boundaries in the growth direction. It may be grown in this way. Subsequent steps are the same. As shown in FIG. 3, both sides are polished and electrodes are attached.

The method of manufacturing a detector according to the present invention in which conductive layers are provided on both surfaces of a diamond growth layer will be described with reference to FIGS. FIG. 9 shows the substrate 1. FIG. 10 shows a thin polycrystalline diamond conductive layer 7 grown on a substrate. FIG.
Reference numeral 1 denotes a polycrystalline diamond semiconductor layer 8 having a high resistivity grown on a polycrystalline diamond conductive layer. FIG.
Reference numeral 2 denotes a polycrystalline diamond conductive layer 9 further grown on the high resistivity polycrystalline diamond semiconductor layer.
The intermediate diamond semiconductor layer 8 is polycrystalline, but has no grain boundaries in the vertical direction. The conductive layers on both sides may have fine grain boundaries, but that is not a problem. Next, the substrate 1 is removed by treatment with an acid or the like. This is the state shown in FIG.
Both sides may be polished, but need not be polished. In this example, it is not polished. Next, electrodes are deposited on both sides of the electrode. As shown in FIG.

The radiation detector is used as usual. Appropriate voltage is applied between the electrodes. When radiation is incident on the diamond layer, electron-hole pairs are excited by the radiation, and a current proportional to the energy of the radiation flows between the electrodes. Thereby, the presence and energy of the radiation can be detected. Since diamond has extremely high withstand voltage, it is preferable to use a high voltage applied to improve the response speed. It is preferable to apply an electrolysis of 2000 V / cm or more to the thickness of the diamond. Radiation that can be detected by the detector of the present invention is:
It contains any one or more of α rays, γ rays, X rays, β rays, and neutron rays.

[0023]

【Example】

Example 1 Polycrystalline diamond was synthesized by a known microwave plasma CVD method using a substrate obtained by processing polycrystalline Si to a size of 11 × 11 × 1 mm and polishing the growth surface with diamond abrasive grains. . As pigments, isopropyl alcohol 5 SCCM above, hydrogen 500 SCC
M, 100 SCCM of argon supplied, microwave (2.45 GHz) output 700 W, gas pressure 70 Torr
After growing at 200 r for 200 hours, a diamond film having an average of 580 μm was obtained.

When the cross section of the diamond film was polished and the grain boundaries were observed with an optical microscope, it was found that the grain boundary was
It was found that the film was grown in a columnar shape over 0 μm or more. The substrate was removed with an acid and the interface between the growth surface and the substrate was polished to finally obtain a diamond layer having a size of 9 × 9 × 0.45 mm. The smoothness of both sides of the diamond layer was 0.1 μm or less, and observation with an optical microscope confirmed that the diamond layer was composed of only columnar crystal grain boundaries without any grain boundaries that interrupted the current passing through the upper and lower surfaces. The average thickness after polishing was 450 μm, the interface with the substrate was shaved off by an average of 12 μm, and the surface roughness (Rmax) was 0.1 μm or less on both the substrate interface side and the growth surface side. Ni and Ti are laminated and vapor deposited on both sides of the diamond film, and 8 × 8 mm
Were formed as radiation detecting elements. When a voltage of 300 V was applied between both electrodes of the detection element and a gamma ray having an energy of about 4 MeV was irradiated, a detection current of 30 μA on average was obtained. The dark current when not irradiating γ-rays is 0.
01 μA or less.

Example 2 A radiation detector was prepared in the same manner as in Example 1, except that the polishing method and the type of the conductive layer were changed. In growing the conductive layer, in addition to the above conditions, 5 SCCM of diborane gas (B ₂ H ₆ ) diluted to 500 ppm with hydrogen was supplied. The electrode formation and sensitivity test method are the same as in 1. In other words, a voltage of 300 V was applied between the electrodes, and 4 MeV gamma rays were irradiated. The current value of the detection sensitivity is a value obtained by subtracting the dark current. Table 1 shows sample preparation conditions and detection sensitivity.

[0026]

[Table 1]

NO. 1 indicates that the thickness of the diamond growth layer is 3
40 μm, the end face was not polished and no conductive layer was provided near the end, and the sensitivity was as small as 3 μA. This is because the mobility is lowered due to defects or grain boundaries near both surfaces of the polycrystalline diamond. A voltage is applied near both surfaces, and the generation of carrier pairs by radiation is small. NO. Reference numeral 2 shows a diamond-grown layer alone having a thickness of 300 μm, which is obtained by polishing the end face on the substrate side. The end face on the growth side is not polished and no conductive layer is provided. The sensitivity is considerably higher at 18 μA. NO. 3
The diamond-grown layer has a thickness of 320 μm and is polished on both sides. There is no conductive layer. The sensitivity was 35 μA and the NO. It is about twice as large as 2. These indicate that it is effective to polish the end face of the diamond growth layer. NO. No. 4 has a diamond growth layer thickness of 170 μm
And polished on both sides. The sensitivity is best at 39 μA. NO. 5 is a diamond growth layer having a thickness of 37 μm.
Thus, a conductive layer having a thickness of 20 μm was formed on the substrate side without polishing the end face. The sensitivity is quite good at 22 μA. N
O. Numeral 6 is a diamond growth layer having a thickness of 370 μm, which is not polished and provided with a 20 μm thick conductive layer on the substrate side and a 30 μm conductive layer on the growth surface side. 36μ sensitivity
This is also excellent at A. From this result, it can be seen that the diamond growth layer should be polished on both sides and the conductive layer should be formed on both sides.

[0028]

According to the present invention, a low-cost, large-area, high-sensitivity, high-speed response radiation detecting element can be obtained. Polycrystalline diamond is used for radiation detection, but since the vicinity of both end faces is polished away or a conductive layer is formed near both end faces, the carrier does not cross the grain boundary, or even if it crosses the grain boundary, there are many carriers. So that
Substantially high mobility. Therefore, the response speed is faster than that of the silicon detection element. Also, the detection part is p
Since not only the depletion layer near the n-junction but also most of the diamond layer receives radiation and generates electron-hole pairs, it is also highly sensitive in this sense. Since it is diamond, it has excellent environmental resistance. If a suitable material is selected for the electrodes and other parts, it can be used even in a high-temperature or corrosive atmosphere. It is much cheaper than one using single crystal diamond. In the case of single crystal diamond, there is no large one, but in the case of the present invention, polycrystalline diamond grown on a substrate is used, so that a large-area detection element can be obtained. It is convenient that the radiation detection element has a certain area.

[Brief description of the drawings]

FIG. 1 is a sectional view of a substrate on which diamond is to be grown.

FIG. 2 is a cross-sectional view of a state where polycrystalline diamond is grown on a substrate.

FIG. 3 is a cross-sectional view showing a state where a substrate is removed.

FIG. 4 is a sectional view showing a state where both surfaces of a diamond layer are polished.

FIG. 5 is a sectional view showing a state where electrodes are attached to both surfaces of a diamond layer.

FIG. 6 is a cross-sectional view showing an initial state in which minute diamond grains having various crystal orientations begin to grow on a substrate. Those shaded are those oriented to (100).

FIG. 7 shows (1) of diamond grains grown on a substrate.
FIG. 2 is a cross-sectional view of a state excluding one oriented in a (00) direction.

FIG. 8 is a cross-sectional view showing a polycrystalline diamond grown from a (100) -oriented crystal.

FIG. 9 is a sectional view of a substrate on which diamond is to be grown.

FIG. 10 is a cross-sectional view showing a state where a diamond conductive layer is grown thinly on a substrate.

FIG. 11 is a cross-sectional view showing a state where a high-resistance diamond layer is grown on a diamond conductive layer.

FIG. 12 is a cross-sectional view of a thin conductive diamond grown on a high-resistance diamond layer.

FIG. 13 is a cross-sectional view showing a state where a substrate is removed from diamond.

FIG. 14 is a cross-sectional view showing a state in which electrodes are provided on both surfaces of diamond.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Substrate 2 Diamond layer 3 Electrode 4 Electrode 7 Diamond conductive layer 8 Diamond high resistance layer 9 Diamond conductive layer

 ────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-62-198780 (JP, A) JP-A-57-115876 (JP, A) JP-B-50-29355 (JP, B1) JP-B-50 25829 (JP, B1)

Claims (6)

(57) [Claims] 1. A radiation detecting element comprising at least two metal electrode layers and a semiconductor layer sandwiched between them, wherein a voltage is applied between the metal electrodes so that a current flows between the electrodes when radiation is incident on the semiconductor layer. Therefore, high-resistance polycrystalline diamond is used as a semiconductor layer, and the polycrystalline diamond does not have a grain boundary in a direction in which a voltage is applied, so that current can flow from electrode to electrode without penetrating the grain boundary. Radiation detection element characterized by the following. 2. A radiation detecting element comprising at least two metal electrode layers and a semiconductor layer sandwiched between them, wherein a voltage is applied between the metal electrodes so that a current flows between the electrodes when radiation enters the semiconductor layer. Then, using a polycrystalline diamond as a semiconductor layer, a conductive diamond layer is provided on one or both surfaces of a high-resistance polycrystalline diamond used as a radiation detecting semiconductor layer, and the high-resistance semiconductor layer is a conductive diamond layer. A radiation detection element, which is in contact with a metal electrode layer through the radiation detection element. 3. A column-shaped high-resistance polycrystalline diamond layer perpendicular to the surface is grown on a substrate by a vapor phase synthesis method using at least a carbon-containing gas as a raw material, and after removing the substrate, metal electrode layers are formed on both surfaces of the diamond layer. A method for manufacturing a radiation detecting element, comprising: 4. A column-shaped high-resistance polycrystalline diamond layer perpendicular to the surface is grown on a substrate by a vapor phase synthesis method using at least a carbon-containing gas as a raw material, and the substrate is removed.
A method for manufacturing a radiation detecting element, wherein at least one of a growth surface side of a diamond layer and an interface side with a substrate is shaved off by 5 μm or more, and metal electrodes are provided on both surfaces of the diamond layer. 5. A thin conductive diamond layer is formed on a substrate using a gas containing at least carbon and boron as a raw material,
Thereafter, using a carbon-containing gas as a raw material, a columnar high-resistance polycrystalline diamond layer perpendicular to the surface is grown on the conductive diamond layer by a vapor phase synthesis method, and after removing the substrate, metal electrodes are formed on both surfaces of the diamond layer. A method for producing a radiation detecting element, comprising providing a layer. 6. A thin conductive diamond layer is formed on a substrate using a gas containing at least carbon and boron as a raw material,
Thereafter, using at least a carbon-containing gas as a raw material, a columnar high-resistance polycrystalline diamond layer perpendicular to the plane is grown on the conductive diamond layer by a vapor phase synthesis method, and at least carbon and boron are further formed on the high-resistance diamond layer. A method for producing a radiation detection element, comprising: forming a conductive diamond layer using a substrate containing the same as a raw material; removing the substrate; and providing metal electrode layers on both surfaces of the diamond layer.