JPH09124310A - Method for producing CdTe crystal - Google Patents

Abstract

(57) Abstract: A method for producing a CdTe crystal is provided, which greatly improves the energy resolution when applied to a radiation detector. A method for producing a high-resistance CdTe crystal by heat-treating a chlorine-added CdTe crystal, wherein the chlorine-added CdTe crystal is heated in a temperature range of 400 ° C. to 440 ° C. inclusive. After performing the first heat treatment in, once lower the temperature below the second heat treatment temperature below, then,
The second heat treatment is performed in a temperature range of 360 ° C. or higher and 400 ° C. or lower. By this heat treatment, the resistance of the CdTe crystal can be increased and the lifetime of the carrier can be improved. When applied to a radiation detector, the energy resolution can be greatly improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high-resistance CdTe crystal used as a material for a radiation detector such as a radiation dosimeter or a medical radiation diagnostic apparatus, and in particular, the energy resolution of the radiation detector is significantly increased. CdTe to improve
The present invention relates to a method for producing crystals.

[0002]

2. Description of the Related Art In general, a cadmium telluride (CdTe) crystal, which is one of II-VI group compound semiconductors, has a large absorption coefficient and is capable of operating at room temperature, so it is a promising element material for an excellent radiation detector. Is.

This radiation detector uses Cd as a detection element.
It is manufactured by forming a pair of electrodes on opposite surfaces of a Te crystal substrate. Then, in this radiation detector, when a bias voltage is applied between the electrodes and radiation is incident on the CdTe crystal, an electron-hole pair is formed in the crystal and the generated carriers (that is, electrons and holes) are formed. ) Can reach each electrode and obtain a radiation detection signal.

By the way, in order to obtain a high energy resolution of the radiation detector, the CdTe crystal used for the detection element has a high resistance and the lifetime of the carrier generated in the crystal is long (that is, the carrier is It is required to be difficult to be trapped). This is because if the specific resistance value of the crystal is low, the leak noise due to the high bias voltage increases, and it becomes impossible to obtain high energy resolution.

In order to obtain a high energy resolution, a specific resistance value of 1 × 10 ⁹ (Ω · cm) or more is required.

[0006]

Here, such a CdTe crystal having a high resistance is conventionally manufactured by adding chlorine to this crystal. The phenomenon of increasing the resistance of the CdTe crystal is caused by the action of a compound defect in which the added chlorine atom replaces a part of Te and the substituted chlorine atom and the vacancy of Cd. It is speculated.

Conventionally, a CdTe crystal added with chlorine is heat-treated in vacuum to make the resistance in the crystal uniform, and thus provided as an element material for a radiation detector.

By the way, the CdTe crystal had such characteristics that the specific resistance value of the crystal decreased and the above-mentioned lifetime also decreased as the amount of chlorine added increased.

Therefore, the radiation detector using the CdTe crystal as the element material has a problem that the energy resolution thereof has a certain limit.

Therefore, conventionally, studies have been made such that the amount of chlorine to be added is set to an appropriate amount and the heat treatment of the CdTe crystal in vacuum is performed in multiple stages while gradually lowering the temperature range. However, the effect of this method has a certain limit.

The present invention has been made by paying attention to such a problem, and the problem is that a CdTe crystal that significantly improves the energy resolution when applied to a device material of a radiation detector is used. It is to provide a manufacturing method.

[0012]

The inventors of the present invention have made extensive studies in order to solve the above problems, and have obtained the following technical knowledge. That is, when a two-step heat treatment is performed on a CdTe crystal to which chlorine is added, the first heat treatment is performed under a certain temperature condition, and once the temperature is lowered below the second heat treatment temperature, preferably near room temperature. It was found that a CdTe crystal that enables a high energy resolution of the radiation detector is obtained when the second heat treatment at a temperature lower than the first heat treatment temperature is performed. Furthermore, regarding this heat treatment through the temperature lowering process, it is possible to use 3 steps instead of 2 steps.
When the same experiment was attempted by setting in multiple stages such as stages, no significant improvement was confirmed as compared with the two-stage heating. The present invention has been completed based on such technical knowledge.

That is, the invention according to claim 1 heats a CdTe crystal to which chlorine is added to obtain a high resistance CdTe crystal.
Based on the method of producing crystals, Cd added with chlorine
The Te crystal is subjected to a first heat treatment in a temperature range of 400 ° C. or higher and 440 ° C. or lower in a vacuum, and then temporarily cooled to a temperature lower than the following second heat treatment temperature, and then 360 ° C. or higher and 400 ° C. or higher.
It is characterized in that the secondary heat treatment is performed in a temperature range of ℃ or less.

The CdTe crystal added with chlorine by such technical means may be either a single crystal or a polycrystal, but the single crystal is preferable in order to obtain a higher energy resolution of the radiation detector.

Further, the temperature lowering treatment temperature after the first heat treatment is optional as long as it is lower than the second heat treatment temperature, but it is preferable to lower the temperature to near room temperature, and
It is desirable to hold the CdTe crystal for 1 hour or more under this temperature condition (claim 2).

Next, the temperature range of the first heat treatment is 400.
It is necessary that the temperature is from ℃ to 440 ℃. 440 ° C
If the temperature exceeds 400 ° C or is lower than 400 ° C, the effect of improving the energy resolution when applied as a radiation detection element cannot be obtained. In addition, the processing time of the first heat treatment is preferably 1 hour or more.

On the other hand, the temperature range of the second heat treatment must be 360 ° C. or higher and 400 ° C. or lower. When the temperature exceeds 400 ° C. or lower than 360 ° C., the effect of improving the energy resolution when applied as a radiation detecting element cannot be obtained as in the first heat treatment. In addition, the processing time of the secondary heat treatment is preferably 1 hour or more.

The purity of each raw material applied to the synthesis of CdTe crystals is preferably 99.9999% or more in order to avoid the behavior of carriers due to other impurities.

Here, the reason why the CdTe crystal which enables high energy resolution of the radiation detector is obtained when the above-mentioned heat treatment method is applied to the CdTe crystal to which chlorine is added is as follows. I guess.

First, as shown in the heat treatment temperature profile of FIG. 1, the CdTe crystal added with chlorine is subjected to the first heat treatment in a temperature range of 400 ° C. to 440 ° C. in a vacuum, similar to the conventional heat treatment. By the action of, it becomes possible to make the resistance in the CdTe crystal uniform.

Next, after the above-mentioned primary heat treatment, the temperature is once lowered to below the secondary heat treatment temperature, and then 360.
When the second heat treatment is performed in the temperature range of ℃ to 400 ℃,
The cause is unknown at present, but the trap level in the crystal is lowered and the above-mentioned carrier lifetime can be improved. That is, as is clear from the data of Examples and Comparative Examples described below, the specific resistance value of each CdTe crystal is substantially the same. However, the energy resolution (half-width of peak spectrum) of the radiation detector incorporating each crystal as an element material is significantly superior to that of the comparative example.
As described above, the energy resolution of the radiation detector according to the embodiment is greatly improved even though the specific resistance values of the CdTe crystals are substantially the same.
It is assumed that the subsequent heat treatment lowers the trap level in the crystal and improves the above-mentioned lifetime.

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail below with reference to specific examples.

[Example 1] Purity is 99.9999 in all cases
% Cd and Te were stored in a quartz container having an inner diameter of 100 mm so that the molar composition ratio (Te / Cd) was 0.999, and cadmium chloride was added so that the chlorine concentration was 300 ppm. Next, place this quartz container at 1.0 ×
After enclosing at a vacuum degree of 10 ⁻⁶ Torr or less and installing in an electric furnace, crystal growth was performed by a gradient freeze method. That is, the raw material was melted at 1100 ° C. for about 10 hours, cooled to 1050 ° C. at a rate of 0.3 ° C./hour and crystallized, and then cooled to room temperature at a rate of 60 ° C./hour, and CdTe containing chlorine was added. Crystals were obtained. Then, a 20 × 20 mm wafer was cut out from the CdTe crystal thus produced.

Next, the cut-out wafer was subjected to a heat treatment (primary heat treatment) at a temperature of 420 ° C. for 10 hours in a vacuum, then cooled once to room temperature, and kept at room temperature for 1 hour. After that, a heat treatment (that is, a second heat treatment) was performed at a temperature of 380 ° C. for 10 hours. Cd after heat treatment
The resistivity of Te crystal wafer is 1.4 × 10 ⁹ (Ω · c
m).

Then, a radiation detector was produced from this CdTe crystal wafer, and its radiation detection characteristics were evaluated.

For the radiation detection characteristic evaluation, ²⁴¹ Am (americium) was used as a radiation source. As a result, it was possible to obtain a high energy resolution of 10.1% in which the half-value width of the peak spectrum was better than the target value of 12 to 13% with respect to the bias voltage of 30V.

[Example 2] CdTe produced in Example 1
The crystal wafer is subjected to heat treatment (primary heat treatment) at a temperature of 440 ° C. for 5 hours in vacuum, then once cooled to 360 ° C., and then held at this temperature for 3 hours, and then 400
Heat treatment for 10 hours at a temperature of ℃ (ie secondary heat treatment)
Was given. The specific resistance value of the CdTe crystal wafer after the heat treatment was 1.5 × 10 ⁹ (Ω · cm).

Then, from this CdTe crystal wafer, a radiation detector similar to that of Example 1 was prepared, and a radiation source of ²⁴¹ A was used.
Radiation detection characteristics were evaluated using m (americium).

As a result, with respect to the bias voltage of 30 V, the high energy resolution of 11.2%, which is better than the target 12 to 13%, was obtained.

[Example 3] CdTe prepared in Example 1
The crystal wafer is subjected to heat treatment (primary heat treatment) in vacuum at a temperature of 400 ° C. for 10 hours, then once cooled to room temperature and kept at room temperature for 1 hour, and then at a temperature of 360 ° C. for 10 hours. A heat treatment (that is, a secondary heat treatment) was performed for a time. The specific resistance value of the CdTe crystal wafer after heat treatment is 1.5
It was × 10 ⁹ (Ω · cm).

Then, from this CdTe crystal wafer, a radiation detector similar to that of Example 1 was prepared, and a radiation source of ²⁴¹ A was used.
Radiation detection characteristics were evaluated using m (americium).

As a result, it was possible to obtain a high energy resolution of 10.3%, which is better than the target 12 to 13% in the half width of the peak spectrum with respect to the bias voltage of 30V.

[Comparative Example 1] CdTe produced in Example 1
A radiation detector similar to that of Example 1 was produced from this CdTe crystal wafer without heat-treating the crystal wafer. This CdT
e The specific resistance value of the crystal wafer is 1.6 × 10 ⁹ (Ω · cm)
Met.

When radiation detection characteristics were evaluated using ²⁴¹ Am (americium) as a radiation source, the half-value width of the peak spectrum was 5 at a bias voltage of 30V.
It was 2.6%, and it was difficult to use it as a radiation detector.

[Comparative Example 2] CdTe produced in Example 1
The crystal wafer was heat-treated in vacuum at a temperature of 420 ° C. for 10 hours. The specific resistance value of the CdTe crystal wafer after this heat treatment was 1.4 × 10 ⁹ (Ω · cm).

Then, a radiation detector similar to that of Example 1 was prepared from this CdTe crystal wafer and used as a radiation source of ²⁴¹ A.
Radiation detection characteristics were evaluated using m (americium).

As a result, the full width at half maximum of the peak spectrum was 19.4% with respect to the bias voltage of 30 V, and the target high energy resolution could not be obtained.

[0038]

EFFECTS OF THE INVENTION CdTe according to the invention of claims 1 and 2
According to the method for producing a crystal, a CdTe crystal to which chlorine is added is subjected to a first heat treatment in a temperature range of 400 ° C. or higher and 440 ° C. or lower in a vacuum, and then once less than the following second heat treatment temperature. Since the second heat treatment is performed in the temperature range of 360 ° C. or higher and 400 ° C. or lower, there is an effect that a CdTe crystal that significantly improves the energy resolution when applied to a radiation detector can be obtained. doing.

[Brief description of the drawings]

FIG. 1 is a graph showing a heat treatment temperature profile according to the present invention.

Claims (2)

[Claims] 1. A method for producing a high resistance CdTe crystal by heat-treating a chlorine-added CdTe crystal, wherein the chlorine-added CdTe crystal is heated at 400 ° C. in vacuum.
After performing the first heat treatment in the temperature range of 440 ° C. or lower,
A method for producing a CdTe crystal, which comprises once lowering the temperature to below the second heat treatment temperature and then performing the second heat treatment in a temperature range of 360 ° C. or higher and 400 ° C. or lower. 2. The method for producing a CdTe crystal according to claim 1, wherein after the first heat treatment, the temperature is lowered to near room temperature, and the CdTe crystal is held under this temperature condition for 1 hour or more.