JP2008151795A - X-ray detector - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a large-area X-ray detector which minimizes the structural defects that are potentially generated at the formation of an amorphous selenium (a-Se) semiconductor thick film, minimizes the changes in defects, as nearing the charge trapping center, and the sensitivity of which will not deteriorate. <P>SOLUTION: As a semiconductor layer sensitive to X-rays, an amorphous selenium (a-Se) semiconductor thick film 4 doped with alkali metal in a range of 0.01 ppm to 10 ppm is used. Accordingly, this makes it possible to obtain an X-ray detector which minimizes structural defects and changes in defects, as nearing the charge trapping center and which hardly deteriorates in the sensitivity. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

The present invention relates to an X-ray detector for measuring X-rays in the medical field, industrial field, nuclear field, and the like.

A semiconductor used in an X-ray detector that forms electrodes on both sides of a semiconductor, applies a predetermined bias voltage between the electrodes, and detects electric charges generated inside the semiconductor by X-ray incidence as an electric signal. Various materials are properly used depending on the application, and the production methods thereof are also various. Generally, high-purity single crystal semiconductors such as silicon (Si) tend to be used for X-ray detector applications that require energy resolution.

Among these, an X-ray detector using amorphous selenium (a-Se) can easily obtain a high resistance thick film having a size of 1000 cm ² or more by utilizing a thin film forming technique such as a vacuum deposition method. It is ideal for applications in fields that require large area X-ray measurements.

However, the amorphous selenium (a-Se) film formed by such a method contains many structural defects. Therefore, in order to improve performance, it is common practice to add (doping) an appropriate amount of impurities ( see, for example, Patent Document 1).
European Patent Publication EP1009038

However, the conventional example having such a configuration has the following problems. That is, unlike the single crystal semiconductor, the conventional device described above has a large number of potential structural defects. For this reason, since the electron / hole charge transfer medium (carrier) generated in the semiconductor layer by X-ray incidence is captured by these, the captured carrier cannot be taken out as an electric signal, and the sensitivity of the X-ray detector is reduced. The phenomenon of deterioration occurs.

Hereinafter, the above phenomenon will be specifically described with reference to FIG. FIG. 2 is a diagram for explaining the internal structure of the X-ray detector.

As shown in FIG. 2B, structural defects in amorphous selenium (a-Se) include D0 which is a recombination center, D + (electron capture center) and D− (hole capture) in which D0 is ionized. It exists at a certain ratio. The initial sensitivity value of the X-ray detector is determined by the density of D + and D− at this time. This state is expressed by the following equation.
2D0 → D + + D-

In this state, when X-rays are incident and a charge transfer medium (carrier) of electrons (e−) and holes (h +) is generated in amorphous selenium (a-Se), these are first captured by the recombination center D0. Are changed to D- and D +, respectively. As the density of D + and D− increases in this way, sensitivity is deteriorated. This relationship is expressed by the following two expressions.
D0 + e- → D-D0 + h + → D +

It is an object of the present invention to provide an X-ray detector that compensates for structural defects in amorphous selenium (a-Se) and does not deteriorate sensitivity.

In order to achieve such an object, the present invention has the following configuration. That is, the invention according to claim 1 is made of amorphous selenium doped with an alkali metal , and a semiconductor layer that generates electron-hole pairs therein by X-ray incidence;
A bias voltage applying electrode provided on the X-ray incident surface side of the semiconductor layer, to which a positive bias voltage is applied; a carrier collecting electrode provided on the opposite side of the X-ray incident surface side of the semiconductor layer; A capacitor for accumulating the charge collected from the carrier collecting electrode and a switching element for reading out the electric charge accumulated in the capacitor, wherein the sodium doping amount is in the range of 0.01 to 10 ppm; This is an X-ray detector characterized by the following.

(Operation / Effect) As shown in FIG. 2A, in the amorphous selenium (a-Se) which is a semiconductor layer sensitive to X-rays, an alkali metal M having a large ionization tendency causes structural defects D ⁰ . Since it is added (doping) equal to the amount to be compensated, the structural defects in amorphous selenium (a-Se) are D ⁰ which is a recombination center and D ⁻ (hole trapping center where D ⁰ is negatively ionized). ) Only. This state is represented by the following formula (1).

2D ⁰ + M → M ⁺ + D ⁻ + D ⁰ (1)

In this state, when X-rays are incident and a charge transfer medium (carrier) of electrons (e ⁻ ) and holes (h ⁺ ) is generated in a-Se, the electrons are captured by the recombination center D ⁰ , and D ⁻ The hole is captured by D ⁻ and changed to D ⁰ . This relationship is expressed by the following two expressions (2) and (3).

D ⁰ + e ⁻ → D ⁻ (2)
^{D - + h + → D 0} ··· (3)

As can be seen from these equations (2) and (3), if the electron capture probability and the hole capture probability are exactly the same, the density of D ⁻ does not increase, and the sensitivity does not deteriorate. Even if the electron capture probability is higher than the hole capture probability and the density of D ⁻ may increase, only the holes increase the capture amount, and the increase in the electron capture amount Does not occur, so the amount of sensitivity degradation can be halved.

In addition, since the electrode on the side on which X-rays are incident is positive, that is, a bias voltage is applied so as to increase the potential, as shown in FIG. In addition, the holes move to the opposite side. In addition, as a feature of the interaction between X-rays and substances, since the reaction is more likely to occur on the surface of the substance, most of the electrons generated by the incidence of X-rays are generated near the X-ray incident surface and move. Since the direction is the electrode direction on the X-ray incident side, the moving distance is short.

Therefore, the probability that the electrons reach the electrode without being captured by the recombination center D ⁰ is increased, and the increase in D ⁻ is minimized. In this way, an increase in the amount of trapped electrons can be suppressed, and an increase in the amount of trapped holes can also be suppressed, thereby obtaining an X-ray detector with little deterioration in sensitivity.

Further, since the amount of the alkali metal added (doping) is in the range of 0.01 ppm to 10 ppm, it is almost equal to the amount that compensates for the structural defect D ⁰ of a-Se, and the reaction of the formula (1) is ensured. It happens and sensitivity degradation is suppressed.

If the content is less than 0.01 ppm, the effect of the alkali metal is diminished and the sensitivity is deteriorated. If the content is more than 10 ppm, a phenomenon such as precipitation of the alkali metal alone occurs and the dark current increases. Or a sudden drop in sensitivity occurs.

Further, X-ray detector configured as described above, the amount of doped alkali metal is in the range of 2ppm from 0.05 ppm (claim 2).

Naturally, even within this range, there is an optimum value depending on the type of alkali metal and the film forming conditions such as the deposition temperature and the substrate temperature. For example, the range is from 0.05 ppm to 2 ppm.

The alkali metal is, for example, sodium (Claim 3).

A plurality of the carrier collection electrodes, the capacitors, and the switching elements may be arranged in a two-dimensional array with respect to a single bias voltage application electrode. In the case of a conventional X-ray detector having such a configuration, sensitivity deterioration occurs according to the incident intensity of X-rays, so that local sensitivity unevenness occurs, and the influence on the image quality to be captured after that appears remarkably. . On the other hand, in the X-ray detector according to claim 4, since the amorphous selenium (a-Se) semiconductor layer is doped with an alkali metal and applied with a positive bias applied to the bias voltage application electrode, the sensitivity deteriorates. There is almost no image quality deterioration such as uneven sensitivity.

As is apparent from the above description, according to the present invention, amorphous selenium (a-Se) in which an alkali metal is added (doped) in a range of 0.01 ppm to 10 ppm to a semiconductor layer sensitive to X-rays. Since the semiconductor thick film is used, it is possible to obtain an X-ray detector in which the structural defect and the change of the defect to the charge trapping center are suppressed to the minimum and the sensitivity is hardly deteriorated.

Such an effect appears remarkably when the two-dimensional array configuration is adopted, and a large-area X-ray image detector that does not cause sensitivity unevenness due to local sensitivity deterioration can be obtained.

An embodiment of the present invention will be described below with reference to the drawings. 1 to 4 relate to one embodiment of the present invention, FIG. 1 is a schematic sectional view of an X-ray detector according to the embodiment, and FIG. 2 is a diagram for explaining the internal structure of the X-ray detector according to the embodiment.
none '> FIG. 3 is a diagram for explaining the operation of the X-ray detector, and FIG. 4 is a schematic sectional view showing a modified embodiment of the X-ray detector.

As shown in FIG. 1, the X-ray detector according to this embodiment is formed on an insulating substrate 3 such as a glass substrate on which a carrier collecting electrode 1 and a lower carrier selection layer 2 are formed. An amorphous selenium (a-Se) semiconductor thick film 4 to which an alkali metal is added (doping) is formed so as to be in the range of 0.05 ppm to 2 ppm, and the a-Se semiconductor thick film 1 is formed. A voltage application electrode 6 is formed on the upper surface of the first electrode via an upper carrier selection layer 5.

The lower and upper carrier selection layers 2 and 5 are for suppressing dark current. When a positive bias is applied to the voltage application electrode 6, the upper carrier selection layer 5 is restricted from injecting holes. For this purpose, for example, an n-type semiconductor layer such as CdS or a semi-insulating layer such as Sb ₂ S ₃ is used. For the lower carrier selection layer 2, for example, a p-type semiconductor layer such as AsSe or a semi-insulating layer such as Sb ₂ S ₃ is used in order to limit electron injection.

The X-ray detector according to this embodiment is used by applying a bias voltage to the voltage application electrode 6, and charges generated in the amorphous selenium (a-Se) semiconductor thick film 4 by the X-ray incidence (electron / positive). The holes) detect the electric charges induced by moving in the directions of both electrodes, respectively, from the carrier collecting electrode 1 as electric signals.

Inside the amorphous selenium (a-Se) semiconductor thick film 4, as shown in FIG. 2 (b), and ^{D 0} is the recombination centers, ^{D +} (electron trapping centers) ^{D 0} is ionized and D ^- three structural defects (hole trapping centers) potentially exists. However, in the case of the X-ray detector according to this embodiment, since the alkali metal M is added (doping) to the amorphous selenium (a-Se) semiconductor thick film 4, as shown in FIG. Only D ⁰ and D ⁻ exist. The value of sensitivity is determined by the density of D ⁺ and D ⁻ , but D ⁻ increases by X-ray irradiation according to the above equation (2), but there is no increase of D ⁺ , so the sensitivity of the X-ray detector deteriorates. Can be reduced to half.

Further, when a positive bias voltage is applied so that the potential of the X-ray incident side electrode, that is, the potential of the voltage application electrode 6 in FIG. 3 is higher than the potential of the carrier collection electrode 1, as shown in FIG. Electrons generated by X-ray incidence move to the X-ray incidence side, and holes move to the opposite side. In addition, the characteristic of the interaction between X-rays and substances is that the surface of the substance is more likely to react, so most of the electrons generated by X-ray incidence are generated near the X-ray incidence plane and move. Since the direction is the electrode direction on the X-ray incident side, the movement distance can be shortened. Therefore, electrons probability of reaching the voltage application electrode 6 without being captured in the recombination centers D ⁰ is increased, D ^- increase is minimized in. As a result, there is almost no sensitivity deterioration of the X-ray detector.

The X-ray detector according to this embodiment is expanded to a plurality of channels in a two-dimensional matrix, and each carrier collecting electrode 11 is provided with a charge storage capacitor 12 and a charge readout switch element 13 to form a two-dimensional array configuration. A schematic cross-sectional view of a modified embodiment is shown in FIG. In addition, detailed description is abbreviate | omitted by attaching | subjecting the same code | symbol as the X-ray detector mentioned above depending on the structure.

In the X-ray detector according to this modification, when X-rays are applied in a state in which a bias voltage is applied to the voltage application electrode 14 formed entirely as a common electrode, charges (electrons) generated accordingly・ Hole) moves in both electrode directions, and induced charges are accumulated in the charge accumulating capacitor 12 connected via the carrier collecting electrode 11 corresponding to the X-ray incident location, and the readout timing is reached. At this time, an ON signal is sent from the gate driver 15 through the gate line 16 and the switch element 13 is turned ON (connected), so that the accumulated charge is a radiation detection signal from the readout line 17 as a charge-voltage converter group 18 and a multiplexer 19. Are sequentially sent out as a digital signal to obtain a two-dimensional X-ray image.

In the case of such a two-dimensional array configuration, the features of the X-ray detector according to the present invention remarkably appear. That is, in the case of a conventional X-ray detector, sensitivity deterioration occurs according to the incident intensity of X-rays, so that local sensitivity unevenness occurs, and the influence on the image quality to be taken after that appears remarkably. On the other hand, in the X-ray detector according to this modified embodiment, an alkali metal is added (doping) to the amorphous selenium (a-Se) semiconductor thick film 20 and a positive bias is applied to the voltage application electrode 14. Since it is used, there is almost no sensitivity deterioration, and image quality deterioration such as sensitivity unevenness does not occur.

In the above-described X-ray detector and its modified embodiments, Li, Na, and K are typical examples of alkali metals. However, as explained in the operation of the present invention, the ionization tendency is large and the reduction effect is high. For certain elements, similar effects can be obtained by doping an alkaline earth metal such as Ca or a non-metallic element such as H.

<Comparison with Measurement Data of the Invention Product and Conventional Example> Next, it is actually confirmed that the sensitivity deterioration is improved in the X-ray detector according to this example.

As shown in Table 1 below, the samples were obtained by adding 0.01 ppm, 0.1 ppm, 0.5 ppm, 1.0 ppm, and 0.5 ppm of alkali metal Na into the amorphous selenium (a-Se) semiconductor thick film 20, respectively. 0 ppm, 10.0 ppm added (doping) test 1, test 2, test 3, test 4, test 5 and test 6 X-ray detectors and 20.0 ppm alkali metal Na doped These are an X-ray detector for comparison 1 and eight X-ray detectors for comparison 2 in which a non-doped amorphous selenium (a-Se) semiconductor thick film is formed. Note that the thickness of the amorphous selenium (a-Se) semiconductor thick film 4 of each X-ray detector is 1 mm.

For both the test and the comparison, a bias voltage of ± 10 kV was applied to the voltage application electrode 6 and an ammeter was connected to the carrier collection electrode 1 so that the signal current could be read out. In this state, X-rays that passed through a 1 mm aluminum filter were continuously irradiated for 15 minutes under the conditions of a tube voltage of 80 kV and a tube current of 2.2 mA, and the change in signal current was recorded. An example of the change in the signal current at this time is shown in FIG.

The signal current of the X-ray detector for comparison 2 decreases exponentially with both positive and negative biases. However, the signal current of the X-ray detector for test 3 hardly changes with positive bias, and even with negative bias. It can be seen that the downward trend is small. The signal current degradation amount ΔI in Table 1 represents the difference between the signal current immediately after X-ray irradiation and the signal current after 15 minutes, that is, the sensitivity degradation amount.

From the above results, it is understood that the signal current deterioration amount ΔI is small and the sensitivity deterioration is small when the alkali metal Na doping amount is in the range of 0.01 to 10.0 ppm. It can also be seen that the sensitivity deterioration of the positive bias is smaller than that of the negative bias.

Similarly, a test 7 X-ray detector doped with 0.5 ppm of potassium (K) was prepared, and the change in signal current was examined. The results are shown in Table 2 below. It can be seen that the amount of signal current deterioration is clearly smaller than that of the second detector for comparison not doped with potassium (K).

Further, an X-ray detector for test 8 doped with 0.1 ppm of lithium (Li) was prepared, and the change in signal current was similarly examined. The results are shown in Table 3 below. It can be seen that the amount of signal current deterioration is clearly smaller than that of the second detector for comparison not doped with lithium (Li).

From these results, it can be seen that the same results can be obtained even if an alkali metal other than sodium (Na) is doped, and the sensitivity deterioration can be suppressed.

In addition, this invention is not limited to embodiment mentioned above, A various deformation | transformation implementation is possible.

It is a schematic sectional drawing which shows the structure of the X-ray detector which is one Embodiment of this invention. It is a figure explaining the internal structure of the X-ray detector which is one Embodiment of this invention. It is a figure explaining the effect | action of the X-ray detector which is one Embodiment of this invention. It is a schematic sectional drawing which shows the structure of the X-ray detector which is a modified example of this invention. It is a graph which shows the change of the signal current of two X-ray detectors for test 3 and comparison 2.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1,11 ... Carrier collection electrode 2 ... Lower carrier selection layer 3 ... Substrate 5 ... Upper carrier selection layer 6 ... Voltage application electrode 12 ... Capacitor 13 ... Switch element 14 ... Voltage application electrode 15 ... Gate driver 16 ... Gate line 17 ... Read-out Line 18 ... Charge-voltage converter group 19 ... Multiplexers 4, 20 ... Amorphous selenium (a-Se) semiconductor thick film

Claims (4)

A semiconductor layer made of amorphous selenium doped with an alkali metal and generating electron / hole carriers therein by X-ray incidence ;
Provided X-ray incident surface side of the semiconductor layer, and the bias voltage application electrode to which a positive bias voltage is applied,
A carrier collecting electrode provided on the opposite side to the X-ray incident surface side of the semiconductor layer, and for collecting the carriers;
A capacitor for accumulating carriers collected from the carrier collection electrode;
A switching element for reading out the electric charge accumulated in the capacitor,
The alkali metal doping amount is in a range of 0.01 to 10 ppm. The X-ray detector according to claim 1, wherein the doping amount of the alkali metal is in a range of 0.05 to 2 ppm. The X-ray detector according to claim 1, wherein the alkali metal is sodium. The carrier collection electrode, the capacitor, and a plurality of the switching elements are arranged in a two-dimensional array with respect to a single bias voltage application electrode. X-ray detector.

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