JPH0734480B2 - CdTe radiation detection element - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a CdTe radiation detecting element, and more particularly to a CdTe radiation detecting element having an improved energy resolution of radiation by forming electrodes in a specific crystal plane orientation. . INDUSTRIAL APPLICABILITY The radiation detection element of the present invention can be suitably applied to various radiation detection devices such as a measurement field requiring high resolution, such as a detector for X-ray CT, a γ-ray spectrum meter, and a detector for X-ray measuring instrument.

Background of the Invention As a radiation detector, three types of an ionization chamber utilizing an ionization action in a gas, a proportional counter and a GM tube were used in the past.
Further, a scintillator using a substance that emits light upon incidence of radiation and a thermoluminescence dosimeter using a thermoluminescence phenomenon were used, but they were not always satisfactory in terms of price and sensitivity. Therefore, since the 1960s Si and
Semiconductor radiation detectors such as Ge have attracted attention, and today Si and
Ge radiation detectors are used in a wide range of fields.
However, while these Si and Ge radiation detectors have better resolution than previous ones, both Si and Ge have small bandgap, so noise due to thermal excitation is large at room temperature and cannot be used unless cooled to a low temperature. It has a serious drawback.

Under such circumstances, attention has been paid to CdTe, which is a II-VI group compound semiconductor, and it has entered the stage of practical application. One of the characteristics of CdTe
One is that the bandgear is as large as 1.53 eV, which allows it to be used at room temperature. Furthermore, the average atomic number of CdTe is as large as 50, which results in a large absorption coefficient of radiation and high sensitivity in a thin layer. in this way
Radiation detection elements using CdTe single crystals are attracting great attention because they have the advantages of high radiation detection efficiency and the possibility of downsizing the detector.

Conventional technology and problems Conventional CdTe radiation detection elements are used to arbitrarily cut out a single crystal ingot prepared by the Britzmann method or the like, mainly for convenience of cutting, such as a single crystal growth direction or a removable surface of a large area. It is manufactured by cutting, grinding or etching the surface, and then forming electrodes.

There are still many issues that need to be improved in terms of performance for such CdTe radiation detection elements, but one of the important issues is the improvement of energy resolution during radiation detection.

SUMMARY OF THE INVENTION In such a situation, the present inventors have addressed the problem of improving the energy resolution of a CdTe radiation detection element.
The energy resolution in radiation detection largely depends on the mean free path λ of the carrier. λ is expressed as follows: λ = μτE where μ = mobility of the carrier in the crystal τ = average life of the carrier E = electric field strength applied to the crystal μ and τ are constants determined by the type of crystal, so the λ value is To increase it, the electric field strength E may be increased. However, if the electric field strength E is increased, the leak current at the time of voltage load increases, so that the electric field strength E cannot be increased conventionally.

Therefore, if the leak current at the time of voltage load can be reduced, the voltage load can be increased, and as a result, the energy resolution at the time of radiation detection can be improved.

Under such consideration, the inventors of the present invention have conducted extensive research on a method of reducing the leak current when a voltage is applied. Through repeated research, there is a correlation between the plane orientation of the CdTe crystal and the leakage current, and a new finding that the leakage current can be reduced by using the {111} plane as the electrode formation surface Got Until now, there have been almost no studies in which electrodes are formed paying particular attention to the plane orientation in the crystal and the correlation with the radiation characteristics is examined, and this finding is extremely significant in this field.

Based on the above findings, the present invention provides a CdTe radiation detection element in which electrodes are formed on opposite surfaces of a CdTe single crystal body.
Provided is a CdTe radiation detection element characterized by forming an electrode on the {111} plane of a Te single crystal.

DETAILED DESCRIPTION OF THE INVENTION FIG. 1 is a diagram showing the operating principle of a CdTe radiation detecting element according to the present invention. When a semiconductor material is irradiated with radiation, bound electrons in the valence band pass through the energy gap and are released into the conduction band by the action of photoelectric effect or the like to form an electron-hole pair.
In the case of CdTe, the electron-hole pair production energy is 4.65 eV, which is an order of magnitude smaller than that in the case of gas (up to 30 eV). Some of the generated carriers neutralize and disappear by the recombination effect,
In the presence of a voltage load, many are accelerated and reach the electrodes for ionizing current.

The {111} plane of CdTe has an A plane and a B plane, and the A plane has a C plane.
Only d atom exists, and conversely, only Te atom exists on B plane. When a CdTe single crystal is cut along the {111} plane, crystallographically the cut surfaces appear as the A plane and the B plane,
The identification of the A surface or the B surface is made because, for example, when etching is performed with a hydrogen fluoride based etchant, only the A surface can be bitten. That is, the {111} plane of CdTe is ABAB
-A-B and planes A and B are arranged alternately. Therefore,
The {111} plane is considered to have a stronger polarity than the other planes.

By applying a positive potential to the {111} A plane 2 of the CdTe single crystal body 1 with respect to the {111} B plane 3 and forming an electrode 4 on the A plane 2 and the B plane 3, an electron-hole pair is formed. When the depletion layer 5 is formed, this is accelerated by the bias voltage to become an ionization current, which is amplified after removing the direct current component through the capacitor C and then taken out as an output signal. A major factor that determines the charge collection characteristic is that the mean free path λ of the carrier is larger than the thickness of the depletion layer. In the present invention, the fact that the physical properties of CdTe and the bias voltage can be made large can satisfy this factor.

In the case where an electrode is formed on the {111} plane and as a representative other than the {111} plane, here, I-
The V characteristic is shown in FIG. From this result, it is found that the leakage current is reduced when the electrode is formed on the {111} plane. Therefore, the {111} plane can be loaded with a higher voltage than the {110} plane. As described above, the energy resolution in radiation detection is the mean free path λ (λ = μ) of the carrier.
It greatly depends on τE), and the energy resolution improves as the λ value increases. As the leak current is reduced, the electric field strength E can be increased, and as a result, the λ value can be increased and the energy resolution can be improved.

As described above, the leakage current can be reduced when a voltage is applied so that the A surface (Cd surface) has a higher potential than the B surface (Te surface), but when the polarities are opposite, Can not reduce the leakage current. Leakage current under bias load is
When the resistivity of the crystals used is the same, it depends on the bonding state of the electrode metal and the semiconductor. It is considered that the reason why the leakage current is reduced when the A surface is set to a positive potential is that a barrier is formed between the B surface and the metal due to the difference in the bonding state.

The CdTe single crystal can be grown by a method such as the vertical Britzmann method. For example, by cutting a single crystal grown in the {111} direction into thin pieces in the growth direction, a wafer having the {111} plane as an opposing surface is cut out. After the wafer is processed into a predetermined element size, it is polished through a lapping and polishing process. Lapping is performed, for example, by using alumina particles having a particle size of 5 to 12 μm, and polishing is performed by using alumina particles having a particle size of 1 μm or less. However, the polishing method is not limited to this, and the {111} A plane (Cd plane) and the {11} A plane (Cd plane)
1} Any method can be adopted as long as it can polish the B surface (Te surface).

Etching may be carried out using an etchant such as a bromine-methanol mixture, but
Caution is required because the atomic arrangements on the 1} A and {111} B planes are easily destroyed. Etching is usually not necessary and is rather harmful for the above reasons. I'm not sure why,
When etching is performed, the {111} A plane where only Cd is arranged is Cd
And Te are likely to be a mixed surface, and the {111} B surface in which only Te is arranged is also likely to be a mixed surface of Te and Cd, and the concept of the present invention that the {111} plane orientation is used. It is easy to cause the result of breaking.

CdTe single crystals can be either p-type or n-type depending on the Cd: Te ratio, growth method, heat treatment, addition of a dopant, etc., and both high resistance and low resistance can be obtained. And

{111} plane orientation is generally within ± 10 °, preferably ± 3
The allowable error range is within °. In this range, the characteristic change does not substantially occur.

The electrodes are formed by depositing a metal such as Au, Pt, Al or In on the {111} surface of the polished CdTe single crystal by vacuum vapor deposition or plating.

EFFECTS OF THE INVENTION According to the present invention, by forming an electrode on the {111} plane of a CdTe single crystal, it is possible to reduce the leak current under voltage load and improve the energy resolution.

Example A 2-inch diameter CdTe single crystal grown in the {111} direction by the vertical Britzmann method was cut perpendicularly to the {111} direction to obtain {1}.
11} surface has a size of 10 mm x 10 mm and has a thickness of 1 mm C
A dTe single crystal was prepared. After lapping and polishing this, gold was vacuum-deposited on the {111} plane. For the purpose of comparison, a {111} plane was also prepared in the same manner.

FIG. 3 shows the radiation detection spectrum for ¹³⁷ Cs: 662 KeV. Full width at half maximum is 80K when electrodes are formed on the {111} plane
eV, on the other hand, 1 if the electrode is formed on the {110} plane
It is 20 KeV. It can be seen that the energy resolution is improved by forming the electrode on the {111} plane. this is,
As shown in FIG. 2, when the electrode is formed on the {111} plane, the maximum load voltage can be up to 300V, so that the λ value becomes large.

The maximum load voltage on the {110} plane is 50V. Leakage current
Noise is generated at 10 ^-7 A and cannot be further increased.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing the operation principle of the radiation detecting element, FIG. 2 is a graph showing the relationship between the voltage and the leak current under voltage load, and FIG. 3 shows the measurement result of energy resolution. It is a graph. 1: CdTe single crystal body 2: {111} A plane 3: {111} B plane 4: Electrode 5: Depletion layer

Claims (3)

[Claims] 1. A CdTe radiation detecting element in which electrodes are formed on opposite surfaces of a CdTe single crystal, wherein the CdTe single crystal {11
A CdTe radiation detection element characterized in that electrodes are formed on the 1} plane. 2. The {111} A plane (Cd plane) is the {111} B plane (Te
The CdTe radiation detecting element according to claim 1, wherein the CdTe radiation detecting element has a high potential with respect to the surface. 3. The CdTe radiation detecting element according to claim 1 or 2, wherein the electrodes are formed by vacuum vapor deposition or plating of a metal such as Au, Pt, Al, In or the like.