JP2014145705A - Characteristic recovery method of silicon carbide radiation detector and operational method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an easy method for recovering the characteristics of a silicon carbide radiation detector whose detection signal intensity is decreased due to the exposure to radiation, and an operational method of the radiation detector using the method.SOLUTION: A point defect caused to silicon carbide crystal of a radiation detection part of a silicon carbide radiation detector due to the exposure to radiation is repaired by heating the silicon carbide crystal forming the detection part at a constant temperature in a range from 200°C to 350°C for 30 minutes or more, so that the characteristics of the silicon carbide radiation detector are recovered. The silicon carbide crystal is 4H-SiC.

Description

  The present invention relates to a method for recovering the characteristics of a silicon carbide (SiC) radiation detector whose charge collection efficiency (detection signal intensity) has deteriorated due to long-term use, and a method for continuously and stably operating a silicon carbide radiation detector. It relates to the driving method.

  In general, a radiation detector made of a semiconductor material, not limited to silicon carbide, is exposed to a large amount of radiation while being used for radiation measurement, and therefore, lattice defects are formed in the semiconductor crystal due to the burst damage effect. Lattice defects act as trapping centers and recombination centers for carriers (electrons and holes). Semiconductor radiation detectors such as silicon carbide radiation detectors operate by taking out the carriers induced in the detector by radiation as electrical signals to the outside, so that the intensity of the obtained signal decreases as the lattice defect density increases. Become. That is, the radiation detection characteristic is deteriorated. In addition to the radiation detector, it is known that lattice defects generated in a semiconductor device are improved by heating at a predetermined temperature for a predetermined time (Patent Documents 1 to 3, Non-Patent Document 1). ~ 3).

JP-A-6-244445 JP 2000-272994 A JP 2006-295061 A

Francesco Moscatelli, Nuclear Instruments and Methods in Physics Research A 583 (2007) 157-161 A. M. Ivanov et al., ISSN 1063-7826, Semiconductors, 2007, Vol.41, No.8, pp979-983 Naoya Iwamoto et al. IEEE Transactions on Nuclear Science, Vol.58, No.6, December 2011

  The above-mentioned patent documents and non-patent documents disclose that defects formed in various semiconductors by radiation are repaired by heat treatment. However, even in these documents, the heat treatment temperature for repairing a defect of a semiconductor having a certain composition is completely different from the heat treatment temperature for repairing a defect of a semiconductor having a different composition. In addition, in a specific semiconductor, the defect density may be further increased by heat treatment, leading to deterioration of characteristics of the semiconductor element. (For example, Reverse Annealing effect in silicon diodes: A. Holmes-Siedle and L. Adams, Handbook of radiation effects, 2nd edition, p. 85, Oxford press) Thus, certain semiconductor defects are actually recovered by heat treatment. However, it is no exaggeration to say that the number of temperatures suitable for heat treatment is not known until it is actually examined.

  As described above, it is known that lattice defects existing in a semiconductor are generally repaired by heat treatment. However, it is also known that the repair behavior varies depending on the composition of the semiconductor, and the characteristics of the silicon carbide radiation detector cannot be recovered by the methods known so far.

  Therefore, the main object of the present invention is to easily recover the characteristics of a silicon carbide radiation detector in which the detection signal intensity has dropped due to the influence of point defects generated in the silicon carbide crystal of the radiation detection unit when exposed to radiation. Is to provide a simple method. Another object of the present invention is to provide a method for continuously and stably operating a silicon carbide radiation detector using the above-described characteristic recovery method.

  According to one aspect of the present invention, there is provided a method for recovering characteristics of a silicon carbide radiation detector, wherein a silicon carbide crystal constituting a detection unit of the silicon carbide radiation detector is exposed to radiation by heating at a predetermined temperature for a predetermined time or more. Then, the point defect generated in the silicon carbide crystal of the radiation detection unit is repaired, and the characteristics of the silicon carbide radiation detector are restored. The predetermined temperature at this time is a constant temperature in the range of 200 ° C. to 350 ° C., and the predetermined time is 30 minutes or more.

  Preferably, the silicon carbide crystal is a 4H—SiC crystal, and more preferably, the predetermined temperature is 300 ° C.

  In the above-described method, a temperature adjustment mechanism including a heater that is integrally formed so that a predetermined temperature is in direct contact with the silicon carbide radiation detector and can be controlled in temperature according to the magnitude of the energization current. Thus, the characteristics can be recovered in situ without moving the silicon carbide radiation detector.

  The silicon carbide radiation detector operating method according to another aspect of the present invention, when the charge collection efficiency is 80% or less, 20% or more of the initial value by exposing the radiation detector to radiation, In this method, the radiation detector is continuously operated stably by repeatedly heating the radiation detector at a temperature in the range of 200 to 350 ° C. for 30 minutes or more to recover the charge collection efficiency.

  According to the present invention, the lattice defects (point defects) formed in the SiC semiconductor crystal can be repaired at an appropriate temperature that does not affect the constituent materials of the silicon carbide radiation detector by being used for radiation measurement, The silicon carbide radiation detector can be reused several times, that is, the life of the silicon carbide radiation detector can be extended.

(A), (b) and (c) is a figure which shows the typical example of the silicon carbide radiation detector which is the object of this invention. It is a graph which shows the relationship between the heat processing temperature of a silicon carbide radiation detector, and charge collection efficiency. It is a graph which shows the relationship between the heat processing temperature of a silicon carbide radiation detector, and charge collection efficiency. It is a schematic block diagram which shows an example of the heating apparatus of the silicon carbide radiation detector for enforcing the method of this invention.

  Reference is first made to FIG. FIG. 1 shows the configuration of a typical example of a diode-type silicon carbide radiation detector that is the subject of the present invention, and FIG. Here, reference numerals 11 and 13 are electrodes, and 12 is a silicon carbide crystal. (B) and (c) are sectional views of the silicon carbide radiation detector shown in (a), (b) shows a pn diode, and (c) shows an example of a Schottky diode. . Note that the p-type and n-type portions of the pn diode shown in FIG.

  The silicon carbide crystal 12 serving as a radiation detection unit receives radiation to generate electron-hole pairs, which are collected by a reverse bias applied between the electrodes 11 and 13, and the radiation (for example, a single alpha particle) The energy is measured as the magnitude of the charge collection amount (time integration value of the pulse current). However, since a large amount of lattice defects are generated in the silicon carbide crystal exposed to radiation for a long time, the charge collection efficiency (detection signal intensity) is lowered, and the sensitivity as a radiation detector is deteriorated.

  When exposed to radiation, the atoms gain large energy and are ejected from the crystal lattice points to become interstitial atoms, some of which form a pair of vacancies and interstitial atoms. As will be described later, the present inventors can heat and repair these silicon carbide crystals (4H-SiC) at a temperature in the range of 200 ° C. to 350 ° C. for 30 minutes or more, so that these point defects are moved and repaired by lattice vibration. Was found by experiments.

  FIG. 2 is a graph and a numerical diagram showing the relationship between the heat treatment temperature and the charge collection efficiency of the silicon carbide radiation detector, obtained by an experiment conducted on alpha particles having an energy of 5.5 MeV. In this graph, the horizontal axis indicates the heat treatment temperature, and the vertical axis indicates the charge collection efficiency, that is, the detection signal intensity. The heat treatment time at this time is 30 minutes. In addition, the big change was not seen even if it heat-processed for 30 minutes or more.

  The above-described heating was performed using an infrared lamp heat treatment apparatus manufactured by ULVAC-RIKO, which can set the heating temperature every 1 ° C. In the experiment, for example, the temperature was raised from room temperature to 100 ° C., heat treatment was performed, and then the temperature was once lowered to room temperature to measure the charge collection efficiency. Next, the temperature was raised from room temperature to 150 ° C. and heat-treated, and then the temperature was lowered again to room temperature to measure the charge collection efficiency. With such a method, the influence of heat treatment from 50 ° C. to 400 ° C. was measured.

  As apparent from the graph of FIG. 2A, this graph has bending points at 200 ° C. and 350 ° C., and in this range, the charge collection efficiency is improved to almost 90% or more of the initial value (100%). It was. The optimum heat treatment temperature was 300 ° C., and the charge collection efficiency at this time was improved to 92.43%.

  The above experimental data was carried out by bringing a silicon carbide crystal that has been exposed to alpha particles as radiation for a long time to an electric furnace at another location. However, in reality, radiation detection is achieved by using a temperature adjustment mechanism that is integrally formed so as to be in direct contact with the silicon carbide radiation detector and has a heater that can control the temperature according to the magnitude of the energization current. Heat treatment can be performed without removing the vessel from its installation location. An example of such a device is shown in FIG.

  In actual operation of a radiation detector made of 4H-SiC silicon carbide crystal, when the silicon carbide radiation detector is exposed to radiation and the charge collection efficiency is reduced to some extent, the silicon carbide radiation detector is changed from 200 By repeating heating for 30 minutes or more at a temperature in the range of 350 ° C. to recover the charge collection efficiency, the silicon carbide radiation detector can be stably operated continuously. At this time, if the charge collection efficiency is about 20% or less of the initial value before exposure to radiation, the operation of the silicon carbide radiation detector becomes difficult. From the actual operating state of the radiation detector and the experimental data shown in FIG. When the charge collection efficiency of the silicon carbide radiation detector reaches 80% or less and 20% or more of the initial value before exposure to radiation, the silicon carbide radiation detector is heated at a temperature in the range of 200 to 350 ° C for 30 minutes or more. It is preferable to do. Further, from the viewpoint of shortening the operation stop time of the silicon carbide radiation detector, the heating time is preferably set to 120 minutes or less.

  FIG. 3 does not show details of a temperature control device and a signal processing device, which are conventional devices. Actually, in the signal processing device, when the charge collection efficiency is 80% or less of the initial value (80%, 70%, etc., set the threshold appropriately), a signal is sent to the temperature control device, and the procedure described above To start heat treatment. Then, the temperature is raised from room temperature, for example, when the temperature of the SiC crystal reaches 300 ° C, for example, the temperature is maintained for 30 minutes, and then the temperature is lowered to room temperature with a signal from the timer, and then again. The use as a radiation detector is started.

DESCRIPTION OF SYMBOLS 11 ... Electrode 12 ... Silicon carbide crystal 13 ... Electrode

Claims (7)

Restores the characteristics of the silicon carbide radiation detector that repairs the point defects generated in the silicon carbide crystal of the radiation detection part by heating the silicon carbide crystal at a predetermined temperature for a predetermined time. A method of
The method for recovering characteristics of a silicon carbide radiation detector, wherein the predetermined temperature is a temperature in a range of 200 ° C to 350 ° C, and the predetermined time is 30 minutes or more.   2. The method for recovering characteristics of a silicon carbide radiation detector according to claim 1, wherein the silicon carbide crystal is a 4H-SiC crystal.   The method according to claim 1 or 2, wherein the predetermined temperature is 300 ° C.   The method according to any one of claims 1 to 4, wherein the predetermined temperature is integrally formed so as to be in direct contact with the silicon carbide radiation detector, and temperature control is performed according to the magnitude of the energization current. A method for recovering characteristics of a silicon carbide radiation detector, characterized in that the method is obtained by a temperature adjustment mechanism having a possible heater.   When the charge collection efficiency is 80% or less and 20% or more of the initial value due to exposure of the radiation detector, the radiation detector is heated at a temperature in the range of 200 to 350 ° C. for 30 minutes or more. The operation method of the silicon carbide radiation detector is characterized in that the radiation detector is stably operated continuously by repeating the recovery of the charge collection efficiency.   6. The method of operating a silicon carbide radiation detector according to claim 5, wherein the heating is performed for 30 minutes to 120 minutes.   The method according to claim 5 or 6, wherein the silicon carbide crystal is 4H-SiC.

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