JP6097854B2 - Method for producing compound semiconductor crystal for radiation detection element - Google Patents

Description

  The present invention relates to a method for producing a compound semiconductor crystal for a radiation detection element.

Development of hard X-ray and γ-ray detectors using compound semiconductors has been conducted in the past. Among these, cadmium telluride (CdTe) and zinc cadmium telluride (CdZnTe) are promising due to recent technological innovation in crystal development. It is attracting attention as a device.
Since CdTe and CdZnTe are made of an element having a relatively large atomic number, the radiation detection efficiency is high, and the semiconductor detection element can be made small and high performance.
In addition, CdTe and CdZnTe directly convert radiation into electric current, so that they are superior in detection efficiency and energy resolution compared to scintillator detectors with an indirect operating mechanism through luminescence represented by sodium iodide (NaI). Yes.

In addition, CdTe and CdZnTe have a large band gap and are not easily affected by heat, so they can be operated at room temperature and are smaller than silicon-germanium (Si · Ge) detectors that require a cooling device to operate. The field operability is excellent.
In particular, CdZnTe has a larger band gap than that of CdTe due to Zn doping, and a leakage current during operation is small. For this reason, the radiation detection element manufactured using CdZnTe can apply a higher bias current than that manufactured using CdTe to increase the energy resolution (see Patent Documents 1 to 3).

Japanese Patent Application Laid-Open No. 07-291784 Japanese Patent No. 2558218 Japanese Patent No. 3232461

However, although the radiation detection element using conventional CdZnTe has higher performance than the element using other materials, the charge collection efficiency is still low. That is, the ratio of carriers trapped during movement is high, and the energy resolution is not sufficient.
Moreover, although the radiation detection element using conventional CdZnTe is less than that using CdTe, the leakage current when a high voltage is applied is still large. Therefore, in order to reduce the leakage current, the structure of the element needs to be a Schottky type in which, for example, one main surface electrode is formed of platinum (Pt) and the other main surface electrode is formed of indium (In). was there. However, the Schottky type radiation detection element may cause a phenomenon in which the energy resolution decreases with the passage of time, called polarization, during use.

  The present invention has been made to solve the above problems, and an object of the present invention is to provide a compound semiconductor crystal for a radiation detection element capable of cutting out a substrate of the radiation detection element that continues to maintain high energy resolution.

In order to solve the above problems, the present invention provides:
Heat-treating a compound semiconductor crystal composed of cadmium zinc telluride doped with indium as an impurity at a first heat treatment temperature lower than a temperature at the end of crystal growth of the compound semiconductor;
Thereafter, in the method for producing a compound semiconductor crystal for a radiation detection element, wherein the compound semiconductor is heat-treated at a second heat treatment temperature lower than the first heat treatment temperature.
The second heat treatment temperature is 200 ° C. or more and 420 ° C. or less ,
The time for performing the heat treatment at the second heat treatment temperature is 24 hours or more and 96 hours or less .

Incidentally, in the above SL invention may be heat-treated were grown said compound semiconductor crystal from 0.2 wt ppm to 2.6 ppm by weight or less of the raw material melt which indium is added.
In the above invention, the compound semiconductor crystal whose composition is represented by Cd _1-X Zn _X Te and whose X value is 0.02 or more and 0.10 or less may be heat-treated.

  ADVANTAGE OF THE INVENTION According to this invention, the compound semiconductor crystal for radiation detection elements which can cut out the board | substrate of the radiation detection element which continues maintaining high energy resolution can be obtained.

It is a circuit diagram of a radiation detector concerning an embodiment of the present invention. It is a perspective view which shows the radiation detection element with which the radiation detector of FIG. 1 is equipped. FIG. 3 is a sectional view taken along line III-III in FIG. 2. It is a longitudinal cross-sectional view of the electric furnace for manufacturing the compound semiconductor crystal which forms the radiation detection element of FIG. It is a graph which shows the IV characteristic of the radiation detection element of the embodiment. It is a figure which shows the radiation spectrum of the radiation detection element of the embodiment. It is a figure which shows the radiation spectrum of the radiation detection element of the embodiment. It is a graph which shows the It characteristic of the radiation detection element of the embodiment.

<Embodiment>
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[Configuration of radiation detector]
First, a schematic configuration of the radiation detector of the present embodiment will be described.
As shown in FIG. 1, the radiation detector 1 of this embodiment includes a radiation detection element 2, a capacitor 3, an amplifier 4, a multichannel analyzer (MCA) 5, and the like. The radiation detection element 2 has one electrode (common electrode 7) connected to the ground (grounded) and the other electrode (pixel electrode 8) connected to a negative potential so that a predetermined bias voltage is applied. . The other electrode is connected to the MCA 5 via the capacitor 3 and the amplifier 4.

  The substrate 6 of the radiation detection element 2 is formed of a crystal of zinc cadmium telluride (CdZnTe (a part of Cd in CdTe is replaced with Zn)), which is a II-VI group compound semiconductor. When this CdZnTe receives radiation (hard X-rays or γ-rays), electrons are emitted, and these electrons become an ionization current due to a bias voltage. The radiation detector 1 of this embodiment converts the ionization current of the radiation detection element 2 into a pulse signal via the capacitor 3 and the amplifier 4. The MCA 5 analyzes the pulse signal to obtain a radiation spectrum.

[Configuration of radiation detection element]
Next, a specific configuration of the radiation detection element 2 will be described.
As shown in FIG. 2, the radiation detection element 2 of the present embodiment includes a substrate 6, a common electrode 7, a pixel electrode 8, and the like.
The substrate 6 is formed in a thin plate shape, and the main surface on which the common electrode 7 and the pixel electrode 8 are formed is a (111) plane. Since the crystal orientation [111] is a polar axis in CdZnTe, the surface composition on the one main surface (hereinafter referred to as A surface) 6a side of the surface of the substrate 6 has a high Cd ratio, and the other main surface (hereinafter referred to as B surface). ) The surface composition on the 6b side has a high Te ratio.

The common electrode 7 is formed so as to cover the entire B surface 6 b of the substrate 6. A plurality of pixel electrodes 8 are provided on the A surface 6a of the substrate 6 and are arranged in a matrix (in the figure, four columns in the vertical and horizontal directions). Both the common electrode 7 and the pixel electrode 8 are made of platinum (Pt) in a thin film shape. That is, both the substrate 6 and the common electrode 1 and the substrate 6 and the pixel electrode 8 are in ohmic contact. Hereinafter, when the common electrode 7 and the pixel electrode 8 are not distinguished from each other, the electrodes are collectively referred to as Pt electrodes 7 and 8.
Also, as shown in FIG. 3, the surface layer of the substrate 6 (here, the A plane side) where the Pt electrodes 7 and 8 (here, the pixel electrode 8) are formed on the surface, that is, the bulk of the substrate. Between the crystal 61 and the Pt electrodes 7 and 8, an intermediate layer 62 mainly made of an oxide of Te and Pt is formed.

[Configuration of electric furnace]
Next, the structure of the electric furnace for manufacturing the compound semiconductor crystal will be described.
As shown in FIG. 4, an electric furnace 9 for producing a compound semiconductor crystal used as a material for the substrate 6 of this embodiment is composed of a main body 91, heaters 92a to 92i, a quartz ampule 93, and the like.
The main body 91 includes a cylindrical main body upper portion 91a and a cylindrical main body lower portion 91b that is narrower than the main body upper portion 91a, and the internal space of the main body upper portion 91a and the internal space of the main body lower portion 91b communicate with each other.
The heaters 92 a to 92 i are disposed on the inner wall of the main body 91. The first to sixth heaters 92a to 92f are disposed on the inner wall of the main body upper portion 91a, and the seventh to ninth heaters 92g to 92i are provided in an annular shape on the inner wall of the main body lower portion 91b. Each heater 92a-92i can set heating temperature each independently.
The quartz ampule 93 includes a cylindrical reservoir portion 93b extending downward from a lower portion of the cylindrical crucible storage portion 93a and the crucible storage portion 93a. The quartz ampule 93 is disposed in the main body 91, and the reservoir portion 93b is inserted into the main body lower portion 91b. The crucible storage part 93a is surrounded by the first to fifth heaters 92a to 92e, and the reservoir part 93b is surrounded by the sixth to ninth heaters 92f to 92i.

[Method of producing compound semiconductor crystal]
Next, a method for producing the compound semiconductor crystal will be described.
A single crystal ingot of CdZnTe, which is a compound semiconductor crystal used as a material for the substrate 6 of the present embodiment, is manufactured through a crystal growth step and first and second heat treatment steps.
First, in the crystal growth step, a single crystal is grown from a melt of CdZnTe. Here, the vertical temperature gradient coagulation (VGF) method is used. Specifically, first, raw materials (Cd, Te, Zn, In) are put into a crucible. At this time, the Zn content in the entire raw material is 1 to 5 at%, that is, the composition of the produced single crystal ingot is Cd _1-X Zn _X Te (X = 0.02 to 0.1). Further, the respective amounts are adjusted so that the In concentration is 0.2 to 2.6 wtppm. Then, Cd is placed in the reservoir portion 93 b of the quartz ampule 93, and the crucible containing the raw material is placed in the quartz ampule 93. Then, the quartz ampule is vacuum-sealed, and the quartz ampule 93 is heated by the heaters 91a to 91i to melt the raw material contained in the crucible and volatilize Cd in the reservoir portion 93a. At this time, the vapor pressure of Cd is adjusted by adjusting the temperature of the seventh and eighth heaters 92g and 92h.

  When the crucible raw material is melted, the temperature of the first to sixth heaters 92a to 92f is adjusted so as to decrease from the sixth heater 92f to the upper heater so that a temperature gradient is generated from the upper end to the lower end of the melt. By doing so, a single crystal of CdZnTe is grown on the surface layer portion of the melt with the lowest temperature. Then, by gradually lowering the heating temperature of all the heaters with a temperature gradient, the single crystal is grown downward to a predetermined length.

  After the completion of the crystal growth process, the process proceeds to the first heat treatment process. In the first heat treatment step, first, the temperature of each of the heaters 92a to 92i is adjusted while maintaining the grown CdZnTe single crystal in the quartz ampule, so that the furnace temperature is 920 from the temperature at the end of the growth of the single crystal. ˜970 ° C. (first heat treatment temperature: preferably 930 to 960 ° C., more preferably 940 to 950 ° C.). In this state, the CdZnTe single crystal is heat-treated (annealed) for 15 to 25 hours (first heat treatment time: preferably 18 to 23 hours, more preferably 19 to 21 hours).

After the end of the first heat treatment step, the process proceeds to the second heat treatment step. In the second heat treatment step, the furnace temperature is lowered from the first heat treatment temperature to 200 to 420 ° C. (second heat treatment temperature: preferably 220 to 380 ° C., more preferably 250 to 350 ° C.). In this state, the CdZnTe single crystal is heat-treated for 24 to 120 hours (second heat treatment time: preferably 36 to 100 hours, more preferably 48 to 96 hours).
Through the above steps, the compound semiconductor crystal forming the substrate 6 of this embodiment is manufactured.

[Method of manufacturing radiation detection element]
Next, the manufacturing method of the said radiation detection element is demonstrated.
The radiation detection element 2 of this embodiment is manufactured through a substrate manufacturing process, an electrode forming process, and a dicing process.
The substrate manufacturing process performed first includes a cutting process and a polishing process.
In the cutting step, a CdZnTe single crystal ingot is cut along the crystal plane (111) to cut out a thin disk-shaped wafer (substrate 6).
After the cutting process, the process proceeds to the polishing process. In the polishing step, the cut surface of the cut wafer is physically mirror-polished using an abrasive such as alumina powder. The polishing process may be repeated a plurality of times.

After the substrate manufacturing process, the process proceeds to an electrode forming process. An electrode formation process consists of an electrode pattern formation process and a plating process.
In the electrode pattern forming process, first, the wafer manufactured in the substrate manufacturing process is immersed in methanol, and ultrasonic cleaning is performed at room temperature for 30 seconds to remove foreign matters attached to the wafer. Then, a photoresist is applied to the surface of the wafer, and the photoresist is exposed using a photomask on which a pixel electrode pattern is drawn. Then, the exposed photoresist is removed by development. Then, the wafer is immersed in methanol (brometa solution) mixed with 1% by volume of bromine, and the polished surface of the wafer is etched at room temperature for 3 minutes to remove the work-affected layer from the surface of the substrate 6. Then, the brometa solution is removed from the wafer using methanol, the methanol is removed from the wafer using pure water, and the electrode pattern forming step is completed.

  After the electrode pattern forming process, the process proceeds to the plating process. In the plating process, the wafer is immersed in a plating solution in which hydrochloric acid is mixed with an aqueous solution of chloroplatinic acid (IV) hexahydrate to deposit Pt on the polished surfaces 6a and 6b of the wafer where the photoresist has been removed. To form a Pt layer. The Pt layers 7 and 8 are formed by growing the Pt layer to a predetermined thickness. After the Pt electrodes 7 and 8 are formed, the unnecessary photoresist is removed and the wafer is washed with pure water. Then, nitrogen gas is sprayed onto the wafer and the Pt electrodes 7 and 8 to dry the wafer and the Pt electrodes 7 and 8, and the electrode forming process is completed.

After the electrode forming process, the process proceeds to a dicing process. In the dicing process, the wafer having the Pt electrodes 7 and 8 formed on the polished surfaces 6a and 6b is cut and divided into a plurality of substrates 6, and the individual radiation detection elements 2 are cut out from the wafer.
Through the above steps, the radiation detection element 2 is manufactured in which the Pt electrodes 7 and 8 are formed on the surface of the substrate 6 formed of CdZnTe crystal via the intermediate layer 62 containing Te oxide. .

Next, specific examples and characteristics of manufacturing the radiation detection element will be described.
Prior to the description, three radiation detection element samples (Examples 1 to 3) having different Zn ratios, In concentrations, heat treatment temperatures, and heat treatment times were manufactured, and the characteristics of each sample (IV characteristics, radiation) (Detection characteristics, μτ product, It characteristics).

<Example 1>
Here, a specific manufacturing method of each sample will be described.
First, a CdZnTe single crystal ingot (composition formula Cd _0.965 Zn _0.035 Te) containing 0.3 wt ppm In as an impurity was grown from a melt of CdZnTe containing 1.75 at% Zn. . Then, with the grown CdZnTe single crystal held in the quartz ampule, the furnace temperature was lowered from 1110 ° C. to 940 ° C., and the first heat treatment step was performed for 20 hours. Thereafter, the furnace temperature was lowered to 380 ° C., and the second heat treatment step was performed for 24 hours. The reservoir temperature was adjusted so that the Cd vapor pressure in the ampoule during the heat treatment was 1.3 atm in both the first and second heat treatment steps. And the Pt electrode was formed in the board | substrate cut out from the heat-processed single crystal ingot, and it diced so that a dimension might become 4 mm x 4 mm x 1.4 mmt, and the sample of Example 1 was manufactured.

<Example 2>
Also, a CdZnTe single crystal ingot (compositional formula Cd _0.95 Zn _0.05 Te) containing 2.0 wt ppm In as an impurity grown from a melt of CdZnTe containing 2.5 at% Zn is grown. I let you. Then, with the grown CdZnTe single crystal held in the quartz ampule, the furnace temperature was lowered from 1113 ° C. to 950 ° C., and the first heat treatment step was performed for 20 hours. Thereafter, the furnace temperature was lowered to 250 ° C., and the second heat treatment step was performed for 96 hours. The reservoir temperature was adjusted so that the Cd vapor pressure in the ampoule during the heat treatment was 1.3 atm. Then, in the same manner as in Example 1, the sample of Example 2 was created by cutting out the substrate, forming the Pt electrode, and dicing.

<Example 3>
Further, a CdZnTe single crystal ingot (composition formula Cd _0.95 Zn _0.05 Te) containing 2.0 wt ppm In as an impurity grown from a melt of CdZnTe containing 2.5 at% Zn is grown. I let you. Then, with the grown CdZnTe single crystal held in the quartz ampule, the furnace temperature was lowered from 1115 ° C. to 960 ° C., and the first heat treatment step was performed for 20 hours. Thereafter, the furnace temperature was lowered to 230 ° C., and the second heat treatment step was performed for 48 hours. The reservoir temperature was adjusted so that the Cd vapor pressure in the ampoule during the first and second heat treatment steps was 1.3 atm. Then, similarly to Examples 1 and 2, the substrate was cut out, the Pt electrode was formed, and dicing was performed to prepare the sample of Example 3.

〔Resistivity〕
Next, the characteristics of each sample manufactured as described above will be described.
First, the IV characteristics of each sample were examined. Specifically, a circuit in which a power source and an ammeter were connected for each sample was configured, and the leakage current when a voltage of 0 to 1000 V was applied to each sample was measured. FIG. 5 shows the results of Example 1. Then, the plotted points were connected to draw a curve, and the tangent slope (resistivity) at 100V was obtained. As a result, numerical values of 1.2E + 11 Ω · cm in Example 1, 3.1E + 11 Ω · cm in Example 2, and 5.5E + 11 Ω · cm in Example 3 were obtained. These numerical values indicate that the samples of Examples 1 to 3 can reduce the leakage current when the bias voltage is applied. If the leakage current is small, the bias voltage can be increased accordingly, so that the energy resolution of the radiation detector can be increased.

〔Half width〕
Next, the radiation detection characteristics of each sample were examined. Specifically, the radiation detector as shown in FIG. 1 is configured for each sample, and the radiation spectrum of cobalt (Co-57) when a bias voltage of 250 V is applied is measured using each radiation detector. . FIG. 6 shows the results of Example 1. And the half width H of the peak was measured from this spectrum. As a result, the numerical values of 5.5% in Example 1, 4.6% in Example 2, and 6.3% in Example 3 were obtained. These numerical values indicate that the samples of Examples 1 to 3 have high energy resolution.

[Μτ product]
Next, the μτ product of each sample was examined. Specifically, the radiation spectrum of cobalt (Co-57) when two different bias voltages (100 V, 200 V) were applied was measured using a radiation detector configured using each sample. FIG. 7 shows the results of Example 1. Then, the peak position of each spectrum was measured, and the μτ product of electrons was obtained using a predetermined calculation formula. As a result, the numerical values of 1.16E-03 cm ² / V in Example 1, 2.40E-03 cm ² / V in Example 2, and 7.85E-04 cm ² / V in Example 3 were obtained. These numerical values indicate that the samples of Examples 1 to 3 easily collect carriers with electrodes and have high energy resolution.

〔Stability〕
Next, the It characteristic of each sample was investigated. Specifically, a predetermined bias voltage was continuously applied to each sample, and the transition of the current value over time was measured. FIG. 8 shows the results of Example 1. Then, the amount of decrease (b) in current for a predetermined time (a: 60 seconds here) from the start of bias voltage application was measured, and the ratio (b / a: stability) was determined. As a result, 56% was obtained in Example 1, -4% in Example 2, and 5% in Example 3. These numerical values indicate that the samples of Examples 1 to 3 hardly cause a decrease in energy resolution with time.

  The excellent characteristics as described above were obtained in the sample of the example because the conditions for heat-treating the material ingot were optimized (the temperature was limited to 200 ° C. to 420 ° C., and the time was limited to 24 hours to 96 hours). This is probably because In, which is an impurity in the crystal, is activated, and the ratio of In that occupies the lattice sites of the crystal is increased compared to the conventional case.

Thus, in the compound semiconductor crystal for radiation detection element of this embodiment, the product μτ (e) of the electron mobility μ and the electron lifetime τ is 7.85E−04 cm ² / V or higher, which is higher than the conventional one. If a radiation detection element is manufactured using this crystal, the charge collection efficiency is increased, and the received radiation can be more accurately converted into an electrical signal. Moreover, since the compound semiconductor crystal for radiation detection elements of the present invention has high resistivity, there is little leakage current when a voltage is applied, and a high voltage can be applied without making the radiation detection element a Schottky structure.
Therefore, according to the compound semiconductor crystal for a radiation detection element of the present invention, a radiation detection element with high energy resolution can be produced.

Moreover, according to the manufacturing method of the compound semiconductor crystal for radiation detection elements of this embodiment, by setting the second heat treatment temperature to 200 ° C. or more and 420 ° C. or less, the μτ product of electrons is 7,85E−04 cm ² / V or more. Thus, a compound semiconductor crystal useful as a material for a radiation detection element can be produced.

As mentioned above, although the invention made by this inventor was concretely demonstrated based on embodiment, this invention is not limited to the said embodiment, It can change in the range which does not deviate from the summary.
For example, in this embodiment, the crystal is grown by the VGF method. However, the THM method, the Bridgman method, and other methods may be used as long as a single crystal can be grown. In this embodiment, the heat treatment is performed in an ingot state. However, the heat treatment may be performed after the ingot is sliced into a wafer.

  The embodiment disclosed this time should be considered as illustrative in all points and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

DESCRIPTION OF SYMBOLS 1 Radiation detector 2 Radiation detection element 6 Board | substrate (compound semiconductor for radiation detection elements)
7 Common electrode (metal electrode)
8 Pixel electrode (metal electrode)

Claims (3)

Heat-treating a compound semiconductor crystal composed of cadmium zinc telluride doped with indium as an impurity at a first heat treatment temperature lower than a temperature at the end of crystal growth of the compound semiconductor;
Thereafter, in the method for producing a compound semiconductor crystal for a radiation detection element, wherein the compound semiconductor is heat-treated at a second heat treatment temperature lower than the first heat treatment temperature.
The second heat treatment temperature is 200 ° C. or more and 420 ° C. or less ,
A method for producing a compound semiconductor crystal for a radiation detection element, wherein a time for performing the heat treatment at the second heat treatment temperature is 24 hours or more and 96 hours or less . 2. The compound semiconductor crystal for a radiation detection element according to claim 1, wherein the compound semiconductor crystal grown from a raw material melt to which 0.2 wt ppm or more and 2.6 wt ppm or less of indium is added is heat-treated. Manufacturing method. Composition represented by _Cd 1-X _Zn X Te, claim 1 or radiation detection according to 2 the value of X is characterized by heat-treating the compound semiconductor crystal is 0.02 or more 0.10 A method for producing a compound semiconductor crystal for a device.

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