JPS6314479A - Cdte radiation detecting element - Google Patents

Abstract

PURPOSE:To decrease leakage current for increasing voltage load and to improve energy resolution in detection of radiation, by utilizing the interrelation between plane orientations of CdTe crystals and leakage current and forming electrodes on the {111} planes. CONSTITUTION:CeTe has two {111} planes which are called A and B planes. Cd atoms only are present on the A plane while Te atoms only are present on the B plane. A potential positive to the {111} B plane 3 is applied to the {111} A plane 2. Electrodes 4 are formed on the A plane 2 and on the B plane 3, whereby electronhole pairs are formed in a depletion layer 5. The electron- hole pairs are subjected to a bias voltage to provide ionization current. After direct current components are removed by a capacitor C, the ionization current is amplified and taken out as an output signal. Comparing this case in which the electrodes are formed on the {111} planes with a case in which electrodes are formed on any other plane, the {110} plane for example, leaked current is decreased more in the former case than in the latter case. Thus, a higher voltage can be loaded in the case of the {111} plane than in the case of the {110} plane. As a result, improved energy resolution can be obtained by forming the electrodes on the {111} planes.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a CdTe radiation detection element, and more particularly to a CdTe radiation detection element in which the energy resolution of radiation is improved by forming electrodes in specific crystal plane orientations. The radiation detection element of the present invention is suitable for measurement fields that require high resolution, such as X1j ICT detector, γ
It can be suitably applied to various radiation detection devices such as a line spectrometer and a detector for an X-ray measuring instrument.

Background of the Invention In the past, three types of radiation detectors were used: ionization chambers that utilize the ionization effect in gas, proportional counter tubes, and GM tubes, and scintillators that use substances that emit light when radiation is incident, and thermal detectors. Thermoluminescence gravimeters that utilize the luminescence phenomenon have been used, but they have not always been satisfactory in terms of cost, sensitivity, etc. For this reason, semiconductor radiation detectors such as 81 and Ge have attracted attention since the 1960s, and today Si and G radiation detectors are used in a wide range of fields. However, although these St and Ge radiation detectors have better resolution than previous ones,
Both of these materials have a small bandgap, so they generate large noise due to thermal excitation at room temperature, and have the serious drawback that they cannot be used unless they are cooled to a low temperature.

Under these circumstances, CdT, a II-Vl group compound semiconductor,
Attention has been paid to e, and it has entered the stage of practical application. CdT
One of the characteristics of e is that it has a large bandgap of 155·V, which allows it to be used at room temperature. Furthermore, the average atomic number of CdTe is as large as 50, and therefore the radiation absorption coefficient is large, making it possible to obtain high sensitivity with a thin layer. As described above, radiation detection elements using CdTe single crystals are attracting a lot of attention because they have the advantages of high radiation detection efficiency and the possibility of miniaturizing the detector.

Conventional Technology and Problems Conventional CdTe radiation detection elements have been developed by cutting a single crystal ingot prepared by the Bridgman method or the like into arbitrary directions that are convenient for cutting out, such as the single crystal growth direction or a large-area removable surface. It is manufactured by cutting the electrode into pieces, polishing or etching its surface, and then forming electrodes.

There are still many issues that need to be improved in terms of the performance of these CdTe radiation detection elements, but one of the important ones is improving the energy resolution during radiation detection ◇ Summary of the Invention Under these circumstances, the present inventors is cdT.

We worked on the problem of improving the energy resolution of radiation detection elements. The energy resolution in radiation detection is highly dependent on the average confession path λ of the carrier. λ is expressed as follows; λ = μτE where μ = mobility of carriers in the crystal τ = average lifetime of carriers E = electric field strength applied to the crystal μ and τ are constants determined by the type of crystal, so the λ value In order to increase the electric field strength E, the electric field strength E can be increased.

However, if the electric field strength E is increased, the leakage current increases when the voltage is loaded, so it has not been possible to increase the electric field strength E in the past.

Therefore, if it is possible to reduce the leakage current during voltage load, the voltage load can be increased, and as a result, it should be possible to improve the energy resolution during radiation detection.

Based on these considerations, the present inventors have conducted repeated research on methods for reducing leakage current during voltage loads.

Through repeated research, it was discovered that there is a correlation between the plane orientation of CdTe crystal and the leakage current, and that the 'rL pole formation plane is (1111
We have obtained new knowledge that leakage current can be reduced by making it a flat surface. To date, there have been few studies in which the polar formation of 1 was performed with particular attention to the plane orientation in the crystal, and the correlation with radiation characteristics was investigated, and this knowledge is extremely significant in this field.

Based on the above findings, the present invention provides a CdTe radiation detection element in which electrodes are formed on opposing surfaces of a CdTe single crystal, in which electrodes are formed on the (111) plane of the CdTs single crystal. A radiation detection element is provided.

FIG. 4 is a diagram showing the operating principle of the CdTe radiation detection element according to the present invention. When a semiconductor material is irradiated with radiation, bound electrons in the valence band are emitted across the energy gap into the conduction band due to effects such as the photoelectric effect, forming electron-hole pairs. In the case of CdTe, the generation energy of electron-hole pairs is 4.65·V, which is one order of magnitude smaller than that in the case of gas (~S Oe V ). Some of the generated carriers are neutralized and disappear due to the recombination effect, but when a voltage load is present, most of them are accelerated and reach the electrodes, resulting in a 'wL separation current.

The (111) plane of CdTe has an A plane and a B plane.
Only Cd atoms exist on the surface, and only Te atoms exist on the B surface. Note that CdTe single crystal ('1' 1
1) When cutting along a plane, plane A and plane B appear crystallographically, and identification of plane A or plane B can be determined by, for example, plane A when etching with a hydrogen fluoride etchant. This is done because a pit can only be formed in the area. That is, cdT'
@(111) ff1l is A-B-person-B-A-B,
! : Side A and side B are arranged alternately. Therefore, (111
) surface is considered to be a surface with strong comparative polarity compared to other surfaces.

(111)3 to (111}A-plane 2 of CdTe single crystal 1
Apply a positive potential to surface 3, and apply it to A surface 2 and 3 surface 3.
, 8ii4, electron-hole pairs are created in the depletion layer 5, which are accelerated by the bias voltage,
This becomes an ionizing current, which is passed through a capacitor CTr to remove the DC component, amplified, and taken out as an output signal. A major factor determining charge collection characteristics is that the mean free path λ of carriers is larger than the thickness of the depletion layer, and in the present invention, this factor is achieved by the physical properties of CdTe and the ability to increase the bias voltage.

Here, as a representative example of cases where electrodes are formed on the (111) plane and planes other than the (111) plane, 1! on the (110) plane! FIG. 2 shows the t-V characteristics when a pole is formed. From this result <11
It can be seen that when the electrodes are formed on the 11th surface, the leakage current is reduced. Therefore, the (111) plane can be loaded with a higher voltage than the (110) plane. As described above, the energy resolution in radiation detection largely depends on the carrier mean free path λ (λ=μτE), and the energy resolution improves as the λ value increases. As the leakage current is reduced, the electric field strength E can be increased, and as a result, the λ value can be increased, and the energy resolution can be improved.

As mentioned above, the A plane (Cd plane) is different from the B plane (Te plane).
The leakage current can be reduced if a voltage is applied so that the potential is high, but the leakage current cannot be reduced if the polarity is reversed.

When the resistivity of the crystal used is the same, the leakage current during a bias load depends on the junction state between the electrode metal and the semiconductor. The reason why the leakage current is reduced when the A-plane is set to a positive potential is considered to be because a barrier is formed between the B-plane and the metal due to the difference in bonding state.

A CdT-e single crystal can be grown by a method such as the vertical Bridgman method. For example, by cutting a single crystal grown in the (111) direction thinly in the growth direction,
11) A wafer is cut out with the faces facing each other. After the wafer is processed into a predetermined device size, it is polished through lapping and boring steps. The wrapping is
For example, it is carried out using alumina particles having a particle size of 5 to 12 μm, and the bolling is carried out using alumina particles having a particle size of 1 μm or less.

However, the polishing method is not limited to this (111
}Any method can be used as long as it is capable of polishing the A surface (Cd surface) and the (111)B surface (Te surface).

Etching may be performed using an etchant such as a bromine-methanol mixture;
1} Easily disrupts the atomic arrangement of the A-plane and (111)B-plane,
Caution is required. Etching is usually not necessary and can be harmful for the reasons mentioned above. Although the reason is not clear, when etching was performed, only Cd was aligned (111}
The A side tends to be a mixed surface of Cd and T.
It is thought that the (111) B plane in which only T. and Cd are arranged is also a mixed plane of T. and Cd, which tends to lead to a result that destroys the concept of the present invention of utilizing the (111') plane orientation.

CdTs single crystals can be either p-type or n-type depending on the Cd:T ratio, growth method, heat treatment, addition of dopant, etc., and can also be of high resistance or low resistance. set to target.

The (111) plane orientation generally has a tolerance range of within ±10°, preferably within ±3°. Within this range, virtually no characteristic changes occur.0 Electrodes are formed using a polished CdT* single crystal (11
1) This is done by attaching a metal such as Au5PtXAl or In to the surface by vacuum evaporation or plating.

Effects of the Invention According to the present invention, by forming an electrode on the (111) plane of CdT@single crystal, it is possible to reduce IJ + current during voltage load, and it is possible to improve energy resolution. .

Example 2 grown in the (111) direction by the vertical Bridgman method
A CdTe single crystal with an inch diameter was cut perpendicularly to the (111) direction to produce a CdTe single crystal with a (111) plane having a size of 101111×10 fins and a thickness of 1w.

After lapping and bollising, gold was vacuum-deposited on the (111) plane. For comparison purposes, a (110) plane was also produced in the same manner.

Figure 3 shows the radiation detection spectrum for @VC at ``Ca: 662.The half width is @V at 80 when the electrode is formed on the (-,,111) plane, and on the other side (110).
When electrodes are formed on the surface, the voltage is 120 V.

It can be seen that the energy resolution is improved by forming electrodes on the (111) plane. This is because, as shown in Fig. 2, when electrodes are formed on the (111) plane,
This is because the maximum load voltage can be up to 5oov, so the λ value becomes large.

The highest load voltage on the (110) plane is SOV.

Noise occurs when the leakage current reaches 10-'A, and it cannot be increased any further.

[Brief explanation of drawings]

@Figure 1 is an explanatory diagram showing the operating principle of the radiation detection element, Figure 2 is a graph showing the relationship between voltage and leakage current under voltage load, and Figure 3 shows the measurement results of energy resolution. It is a graph. 1: CdTe single crystal 2: (111} A-plane 3: (111) B-plane 4: [Pole 5: Depletion layer Fig. 4, 1 Fig. 2-i! (V)

Claims (1)

[Claims] 1) C in which electrodes are formed on opposing surfaces of a CdTe single crystal
In a dTe radiation detection element, an electrode is formed on the {111} plane of the CdTe single crystal.
e-radiation detection element. 2) {111}A plane (Cd plane) is {111}B plane (Te
5. The CdTe radiation detection element according to claim 4, which has a high potential with respect to the CdTe radiation detection element. 3) The CdTe radiation detection element according to claim 1 or 2, wherein the electrodes are formed by vacuum evaporation or plating of a metal such as Au, Pt, Al, In or the like.