The invention relates to a radiation detector and a radiation image taking device to be used in a field of medical treatment, industry or atomic energy, more particularly, a structure of a radiation sensitive semiconductor film to be provided to the radiation detector of this type, wherein radiation is directly converted into a carrier formed of a pair of electron and hole (hereinafter referred to xe2x80x9ca pair of electron-holexe2x80x9d).
A conventional radiation detector is structured such that a semiconductor film for producing a carrier formed of a pair of electron-hole sensitive to radiation to be detected is sandwiched by a bias electrode and detecting electrodes, and by applying a bias voltage between these electrodes, and the carriers, specifically, small number of carriers, produced in the semiconductor film are taken out from the detecting electrodes. As the radiation sensitive semiconductor film, it is desirable to select a semiconductor film having a good radiation conversion efficiency as much as possible. As the semiconductor film of this type, a monocrystalline semiconductor film is preferable, and especially monocrystals of CdTe and CdZnTe are used since they operate at a room temperature and have a high sensitivity.
However, the conventional radiation detector having the structure as described above has the following problems.
Since the radiation detector is used in place of an image intensifier for detecting X-rays in the medical treatment field, a radiation detector having a large area has been desired. However, it is very difficult to obtain a monocrystalline semiconductor film of CdTe or CdZnTe necessary for the radiation detector having the large area. Thus, there has been proposed a radiation detector having a large area by bonding together crystals with small areas in a tile style or arrangement to thereby obtain the radiation detector with the large area. However, it is very complicated to produce the radiation sensitive layer of the large area by bonding the monocrystals of the small area in the tile style, which results in poor work efficiency and poor economy. Also, since the carriers are not accumulated on joint portions for bonding the crystals, when an image is outputted, vertical stripes and horizontal stripes appear on the joint portions for bonding the crystals to thereby lower the quality of the picture image.
Further, in order to raise a catching rate of radiations, such as X-rays, although it is desired to make a radiation sensitive layer thick, since a growing speed of the monocrystals of CdTe and CdZnTe is very slow, it takes a long time to obtain a necessary thickness of the radiation sensitive layer, which results in a poor productivity of the radiation detector.
The present invention has been made in view of the above problems, and an object of the invention is to provide a radiation detector and radiation image taking device having a large area.
Another object of the invention is to provide a radiation detector and radiation image taking device as stated above, wherein a semiconductor film having a thickness sufficient for catching radiation can be produced in a short time.
Further objects and advantages of the invention will be apparent from the following description of the invention.
In order to attain the above objects, the present invention has the structures as follows.
According to a first aspect of the invention, in a radiation detector, a semiconductor film for producing a carrier formed of a pair of electron-hole sensitive to radiation to be detected is disposed between a bias electrode and detecting electrodes, and when a bias voltage is applied between the bias electrode and the detecting electrodes, the carrier produced in the semiconductor film can be taken out from the detecting electrodes, wherein the semiconductor film is a polycrystalline film of CdTe (cadmium telluride).
According to a second aspect of the invention, in a radiation detector, a semiconductor film for producing a carrier formed of a pair of electron-hole sensitive to radiations to be detected is disposed between a bias electrode and detecting electrodes, and when a bias voltage is applied between the bias electrode and the detecting electrodes, the carrier produced in the semiconductor film can be taken out from the detecting electrodes, wherein the semiconductor film is a polycrystalline film of CdZnTe (cadmium zinc telluride).
According to a third aspect of the invention, in a radiation detector, a semiconductor film for producing a carrier formed of a pair of electron-hole sensitive to radiations to be detected is disposed between a bias electrode and detecting electrodes, and when a bias voltage is applied between the bias electrode and the detecting electrodes, the carrier produced in the semiconductor film can be taken out from the detecting electrodes, wherein the semiconductor film has a laminate structure of a polycrystalline film of CdTe (cadmium telluride) and a polycrystalline film of CdZnTe (cadmium zinc telluride).
According to a fourth aspect of the invention, in the radiation detector of the third aspect, the polycrystalline film of CdZnTe (cadmium zinc telluride) constituting the semiconductor film is disposed on an incident side of radiation.
According to a fifth aspect of the invention, in the radiation detector of the third aspect, the polycrystalline film of CdTe (cadmium telluride) constituting the semiconductor film is disposed on an incident side of radiation.
According to a sixth aspect of the invention, in the radiation detector of any one of the first, third through fifth aspects of the invention, the polycrystalline film of CdTe (cadmium telluride) is obtained by heating a sintered material of CdTe power under a reduced pressure to sublimate.
According to a seventh aspect of the invention, in the radiation detector of any one of the second through fifth aspects of the invention, the polycrystalline film of CdZnTe (cadmium zinc telluride) is obtained by heating a mixed sintered material of CdTe power and ZnTe powder under a reduced pressure to sublimate.
According to an eighth aspect of the invention, in the radiation detector of any one of the second through fifth aspects of the invention, the polycrystalline film of CdZnTe (cadmium zinc telluride) is obtained by heating a mixed sintered material of CdTe power and Zn powder under a reduced pressure to sublimate.
According to a ninth aspect of the invention, in the radiation detector of any one of the first through eighth aspects, a carrier injection preventing layer for preventing the carrier from being injected to the semiconductor film is formed between the bias electrode and the semiconductor film and/or between the semiconductor film and the detecting electrodes.
According to a tenth aspect of the invention, in the radiation detector of the ninth aspect, the carrier injection preventing layer is an n-type or p-type semiconductor layer for making a hetero-junction between the semiconductor film and itself.
According to an eleventh aspect of the invention, in the radiation detector of the ninth aspect, the carrier injection preventing layer is made of a material capable of forming a Schottky barrier on an interface between the semiconductor film and itself.
According to a twelfth aspect of the invention, in the radiation detector according to any one of the first to eleventh aspects, the detecting electrodes are separately disposed on a one-dimension or two-dimensions, and the bias electrode is a single common electrode.
A thirteenth aspect of the invention is a radiation image taking device including the radiation detector according to the twelfth aspect; and a switching matrix board having a plurality of charge storage capacitors connected separately to the respective detecting electrodes, and a plurality of charge read-out switching elements connected to the respective charge storage capacitors.
Next, functions of the radiation detectors and the radiation image taking device of the invention will be explained.
In the radiation detector according to the first aspect of the invention, between the bias electrode and detecting electrodes, the polycrystalline film of CdTe is used as the semiconductor film for producing the carrier formed of a pair of electron-hole sensitive to the radiations to be detected, so that the semiconductor film can be made with a large area, and at the same time, a film thickness sufficient to catch the radiations can be obtained.
In the radiation detector according to the second aspect of the invention, between the bias electrode and detecting electrodes, the polycrystalline film of CdZnTe is used as the semiconductor film for producing the carrier formed of a pair of electron-hole sensitive to the radiations to be detected, so that the semiconductor film can be made with a large area, and at the same time, a film thickness sufficient to catch the radiation can be obtained. Further, since the polycrystalline film of CdZnTe doped with Zn has a high resistance to thereby have a sufficient band gap, a current leak between the detecting electrodes and the bias electrode can be suppressed.
In the radiation detector according to the third aspect of the invention, between the bias electrode and detecting electrodes, the semiconductor film for producing the carrier formed of a pair of electron-hole sensitive to the radiation to be detected is formed of polycrystalline films of CdTe and CdZnTe. The polycrystalline film of CdTe has a high film forming ratio to thereby form a film of a large area with a film thickness sufficient to catch the radiation in a short time. Also, since the CdZnTe polycrystalline film doped with Zn has a high resistance and a sufficient band gap, a current leak can be suppressed. In other words, since the semiconductor film has a laminate structure of two kinds of polycrystalline films, the semiconductor film is provided with characteristics of both films to thereby obtain detecting results of a high sensitivity.
In the radiation detector according to the fourth aspect of the invention, in the structure of the semiconductor film in the third aspect, by providing the polycrystalline film of CdZnTe of the high resistance on an incident side of the radiation, a current leak in a vertical direction between the detecting electrodes and the bias electrode can be suppressed. As a result, an image information having a large concentration resolving ability with an enlarged dynamic range can be obtained by the radiation detector.
In the radiation detector according to the fifth aspect of the invention, in the structure of the semiconductor film of the third aspect, the polycrystalline film of CdZnTe having the high resistance is disposed on a side of the detecting electrodes, i.e. opposite side to the incident side of the radiation. In case the detecting electrodes of the radiation detector are disposed in one-dimension or two-dimensions, a current leak between the adjacent detecting electrodes can be suppressed. As a result, when the radiation detector is used as the radiation image taking device to output an image, an image information having a high resolution where a space resolving power is raised can be obtained.
In the radiation detector according to the sixth aspect of the invention, since a sintered material of a CdTe power is heated under a reduced pressure to sublimate, a polycrystalline film of CdTe having a large area and a sufficient film thickness can be formed in a short time.
In the radiation detector according to the seventh aspect of the invention, a mixed sintered material of a CdTe power and a ZnTe powder is heated under a reduced pressure to sublimate, so that a polycrystalline film of CdZnTe having a large area, a sufficient film thickness, a high resistance and a sufficient band gap can be easily obtained in a short time.
In the radiation detector according to the eighth aspect of the invention, a mixed sintered material of a CdTe power and a Zn powder is heated under a reduced pressure to sublimate, so that a polycrystalline film of CdZnTe having a large area, a sufficient film thickness, a high resistance and a sufficient band gap can be easily obtained in a short time.
In the radiation detector according to the ninth aspect of the invention, since a carrier injection preventing layer for preventing carriers from being injected to the semiconductor film is provided between the bias electrode and the semiconductor film and/or between the semiconductor and detecting electrodes, when a voltage is applied to the bias electrode, a detecting effect of a high S/N ratio, i.e. signal to noise, can be obtained because leak carriers which do not contribute to sensitivity are not injected into the semiconductor film. Also, in case the carrier injection preventing layers are provided on both bias electrode side and detecting electrode side, injection of the carriers from the outside is completely prevented, and only pure carriers produced in the semiconductor film can be detected to thereby further improve the S/N radio.
In the radiation detector according to the tenth aspect of the present invention, by providing an n-type or p-type semiconductor layer for making a hetero-junction between the carrier injection preventing layer and the semiconductor film of the ninth aspect, when a voltage is applied to the bias electrode, carriers other than those produced in the semiconductor film are prevented from being injected into the semiconductor film.
In the radiation detector according to the eleventh aspect of the present invention, a material for forming a Schottky barrier is allowed to contact the semiconductor film, so that a Schottky barrier can be formed on an interface between the material and the semiconductor film to thereby prevent the carriers from the outside from being injected into the semiconductor film.
In the radiation detector according to the twelfth aspect of the invention, since the detecting electrodes are separated and disposed in one-dimension or two-dimensions, a one-dimensional or two-dimensional radiation detector can be obtained.
Since the radiation image taking device according to the thirteenth aspect of the invention includes the switching matrix board having a plurality of charge storage capacitors connected to the respective detecting electrodes of the radiation detector according to the twelfth aspect, and a plurality of charge readout switching elements connected to the respective charge storage capacitors, the radiation image taking device in one dimension or two dimensions can be obtained.