JPH085749A - Semiconductor radiation detector and manufacture thereof - Google Patents

Abstract

PURPOSE:To improve an S/N ratio by a method wherein an insulation film with an even thickness comprising cadmium tellurate of a uniform composition is provided on an interface between a cathode electrode and a compound semiconductor and checks the injection of electrons from the cathode electrode evenly to obtain a dark current low sufficiently in a stable manner. CONSTITUTION:A substrate 1 comprising a high resistance CdTe semiconductor crystal doped with chlorine is ground on both sides thereof and a modified layer caused by working is etched away. An In metal layer is formed as anode electrode 2 on one main surface of the substrate 1 by vacuum evaporation film method. The electrode 2 employs a metal element with electric barrier thereof high against holes, for example, Ga or Al. A uniform insulation film 3 comprising CdTeO3 is formed on the surface of the substrate 1 except for the portion where the electrode 2 is formed by thermal oxidation in an atmosphere of oxygen. Then, an Au layer with electric barrier high against electrons is formed on the insulation film 3 of the other main surfaces of the substrate 1 facing the electrode 2 as cathode electrode 4 by vacuum evaporation method. The substrate 1 containing the electrodes 2 and 4 and the insulation film 3 is cut off to complete a radiation detector 5.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement of a cathode electrode of a semiconductor radiation detector which detects radiation by a current flowing between electrodes.

[0002]

2. Description of the Related Art A semiconductor radiation detector measures a photocurrent generated in a semiconductor due to radiation by an electrode provided on its surface. When a compound semiconductor containing CdTe as a main component such as CdTe or CdZnTe is used, it can operate at room temperature because of its wide band gap, and the absorption coefficient of X-rays and γ-rays is large because the atomic numbers of constituent elements are large. Large and high sensitivity is obtained. Such detectors are used for monitors of radiation facilities, spectrum survey meters, and the like. Further, the detectors can be downsized and arrayed, and the arrayed detectors are beginning to be applied to medical diagnostic equipment, industrial nondestructive inspection devices, and the like.

Conventionally, as electrodes of a semiconductor radiation detector using CdTe, Pt formed by electroless plating,
A metal electrode containing Au or the like as a main component has been mainly used. (BTAMckee et al .; Nucl. Instr. And Method,
A272, p.825 (1988)) Further, it has been proposed to use In as an anode electrode in order to reduce dark current. (JP-A-3-248578, MRSquilantte et a
l.; Nucl.Instr. Since the voltage (bias voltage) can be increased, high carrier collection efficiency and high speed operation are possible.

Further, the cathode electrode containing Pt, Au or the like as a main component can enhance the electrical barrier against electrons and can reduce the dark current. Such a cathode electrode is
It is formed on the compound semiconductor surface by electroless plating in an aqueous solution. At this time, an oxide layer is formed at the interface between the compound semiconductor and the cathode electrode, and the injection of electrons is suppressed, so it is considered that the dark current is reduced.

[0005]

However, according to the above-described manufacturing method, the value of the dark current varies widely. Especially when the applied voltage is increased, the characteristics of the detection element may become unstable. An object of the present invention is to provide an electrode structure capable of stably obtaining a sufficiently low dark current value and a method for forming the electrode structure.

[0006]

A semiconductor radiation detector according to the present invention comprises a compound semiconductor containing CdTe as a main component and CdTeO ₃ provided on the surface of the compound semiconductor.
Made of a metal, a cathode electrode formed on the insulating film and made of a metal that serves as a high barrier against electrons, and a hole provided on the surface of the compound semiconductor facing the cathode electrode and having high holes. It includes an anode electrode made of a metal that serves as a barrier. In, Ga, Al, etc. as the anode electrode, Pt as the cathode electrode,
A metal such as Au or an alloy containing it as a main component can be used. The thickness of the insulating film is 50n.
It is desirable that it is less than m.

Further, the method for manufacturing a semiconductor radiation detector according to the present invention comprises the step of etching the surface of a compound semiconductor containing CdTe as a main component, and the surface of the compound semiconductor is made of a metal which becomes a high barrier against holes. CdT is formed by forming an anode electrode of the compound semiconductor and thermally oxidizing the surface of the compound semiconductor.
An insulating film made of eO ₃ is formed, and a cathode electrode made of a metal that serves as a high barrier against electrons is formed on the insulating film in a region facing the anode electrode. It is preferable that the thermal oxidation is performed at a temperature of 300 ° C. or higher in an oxidizing atmosphere, and the cathode electrode is formed by a forming means such as a vacuum evaporation method that does not affect the characteristics of the insulating film.

The inventor of the present invention has studied the mechanism of dark current in detail. The injection of electrons from the cathode electrode is dominant in the dark current, and the oxide layer at the cathode electrode and the semiconductor interface has a different composition and thickness. We came to the conclusion that the dark current fluctuates due to non-uniformity in the size. Based on this, it was conceived to use an insulating film of CdTeO _{3 having} a uniform composition and a uniform thickness as the oxide layer at the interface between the cathode electrode and the compound semiconductor. This makes it possible to suppress the injection of electrons from the cathode electrode uniformly,
A small dark current can be stably obtained.

[0009]

EXAMPLE A manufacturing process of a CdTe radiation detector which is an example of the present invention will be described below with reference to FIG.

As shown in FIG. 1A, a substrate 1 (15 mm square) made of a chlorine-doped high-resistance CdTe semiconductor single crystal is polished on both sides using alumina abrasive grains or the like to have a thickness of 2 mm. The work-affected layer by polishing is removed by etching using an etching solution such as a bromine-methanol solution. Then, the substrate 1 is heat-treated in vacuum at 200 ° C. for 20 minutes in a resistance heating furnace. As a result, the surface of the substrate 1 is cleaned. This heat treatment may be performed at 100 ° C. or higher and lower than 250 ° C. for 5 minutes to 60 minutes. If it is below this range, it is not sufficiently cleaned, and if it is above this range, the components of the substrate compound are separated and decomposed, which is not preferable.

As shown in FIG. 1B, indium (I) having a thickness of 150 nm is formed on one main surface of the substrate 1 by a vacuum deposition method.
n) A metal layer is formed as the anode electrode 2. As the metal forming the anode electrode 2, a metal element such as gallium (Ga) or aluminum (Al) having a high electric barrier against holes can be used. As the formation method, it is desirable to use a formation method such as a vacuum vapor deposition method in which an oxide is unlikely to be generated at the interface with the substrate 1, and a plating method or the like is not desirable because an oxide or the like is newly formed at the interface.
In addition, after forming the anode electrode 2, heat treatment (150
C. or higher and lower than 250.degree. C., 1 minute to 20 minutes).

Then, as shown in FIG. 1C, an insulating film 3 made of CdTeO ₃ (cadmium tellurate) having a thickness of about 7 nm is formed on the surface of the substrate 1 other than the anode electrode 2 formed thereon.
To form. The insulating film 3 was formed by thermal oxidation of the substrate 1 in an oxygen gas atmosphere at 350 ° C. for 70 hours. The thermal oxidation may be performed at 300 ° C. or higher and lower than 400 ° C. for 20 to 80 hours, but in order to make the insulating film 3 uniform, conditions of 370 ° C. or lower and 65 hours or longer are desirable. The insulating film 3 may be made of CdTeO ₃ having a thickness of 5 to 50 nm formed by a CVD method or the like, but heat treatment in an oxidizing atmosphere can uniformly cover the entire surface in a simpler process. If the thickness of the insulating film 3 is less than 5 nm, the dark current cannot be sufficiently reduced, and if the thickness exceeds 50 nm, the electric field is not sufficiently applied to the CdTe semiconductor portion.

As shown in FIG. 1D, a gold (Au) layer having a thickness of 200 nm is formed as a cathode electrode 4 on the insulating film 3 on the other main surface of the substrate 1 facing the anode electrode 2 by a vacuum deposition method. To do. As the metal forming the cathode electrode 4, a metal element having a high electric barrier against electrons such as platinum (Pt) can be used. As the forming method, it is desirable to use a forming method such as a vacuum vapor deposition method that does not damage the insulating film 3, and a plating method or the like is not desirable because the oxide at the interface is altered or changed. Although the anode electrode 2 is formed first, the insulating film 3 may be formed first and then the insulating film 3 may be partially removed to form the anode electrode 2 and the cathode electrode 4.

Thereafter, as shown in FIG. 1E, the radiation detecting element 5 is completed by cutting the substrate 1 including the anode electrode 2, the insulating film 3 and the cathode electrode 4 into a 2 mm square with a dicing saw. This radiation detecting element 5 was evaluated by the radiation measuring circuit shown in FIG. In this evaluation circuit, a bias voltage of up to 1000 V is applied to the radiation detection element 5 by the bias power supply 6, and the current flowing in proportion to the dose of the incident X-ray is measured by the ammeter 7. The current flowing in proportion to the dose of the incident X-ray can be counted as a pulse.

The current when the radiation detecting element 5 was not irradiated with X-ray was measured as a dark current. The results are shown in FIG. 3 as an example using a solid line. In addition, after forming the anode electrode 2 on the substrate 1, the cathode electrode 4 is directly formed without forming the insulating film 3, and the measurement results of the radiation detecting element manufactured in the same manner as in the other example are shown as a broken line as a comparative example. Are also shown together. As is apparent from this figure, the dark current of this example is about half that of the comparative example. Moreover, when X-rays were incident in both the examples and comparative examples, the current increased in proportion to the dose. Further, a radiation detecting element was repeatedly manufactured and evaluated in the same manner as in the example, but almost the same detection characteristics could be stably obtained every time.

[0016]

As described above, according to the present invention, C is formed as a uniform insulating film at the interface between the cathode electrode and the compound semiconductor.
Since dTeO ₃ is used, the dark current can be suppressed to a low level, and a semiconductor radiation detector capable of measuring radiation with a high S / N ratio can be obtained with good reproducibility.

[Brief description of drawings]

FIG. 1 is a cross-sectional view for explaining a manufacturing process of a CdTe radiation detector according to this example.

FIG. 2 is a radiation measuring circuit using a CdTe radiation detector.

FIG. 3 is a characteristic diagram showing dark current according to an example and a comparative example.

[Explanation of symbols]

 1 substrate (compound semiconductor) 2 anode electrode 3 insulating film 4 cathode electrode 5 radiation detection element 6 bias power supply 7 ammeter

Claims (2)

[Claims] 1. A compound semiconductor containing CdTe as a main component, an insulating film made of CdTeO ₃ provided on the surface of the compound semiconductor, and a metal provided on the insulating film as a high barrier against electrons. A semiconductor radiation detector, comprising: a cathode electrode, and an anode electrode made of a metal, which is provided on the surface of the compound semiconductor facing the cathode electrode and serves as a high barrier against holes. 2. A surface of a compound semiconductor containing CdTe as a main component is etched to form an anode electrode made of a metal that serves as a high barrier against holes on the surface of the compound semiconductor. By thermal oxidation of CdTeO ₃
And a cathode electrode made of a metal that is a high barrier against electrons is formed on the insulating film in a region facing the anode electrode. Method.