FR2738080A1 - SEMICONDUCTOR-BASED X-RAY DETECTION DEVICE - Google Patents

Abstract

The invention provides an X ray detector comprising at least one high resistivity, type II-VI semiconductor material on which are arranged two or more electrical contacts, at least one of which is selected from the group of blocking contacts.

Description

BASED X-RAY DETECTION DEVICE
SEMICONDUCTORS
DESCRIPTION
Technical area
The present invention relates to an X-ray detection device based on semiconductors.

State of the art
Many types of detectors have been devised for the detection of X or y radiation. If the nature of the detector medium is very varied, solid, liquid or gaseous, the principles of detection, for their part, are generally based on the same processes of ionization or excitation of the detector medium by the passage of charged particles.

However, the number of charged particles created in the detector as well as the means of measuring the resulting signal are very different depending on whether it is to detect X-ray or y-radiation. (see reference [1]).

 The use of solid-state detectors based on semiconductors has been the main contribution of the last thirty years to X-ray or y-ray detection techniques, which mainly use gas or scintillation detectors.

 Detectors based on semiconductors directly convert energy into X or y radiation in the material without passing through intermediate steps such as emission of visible photons in the case of scintillators. This eliminates the problems of coupling synonymous with loss of yield. The energy required to create an electron-hole pair in a semiconductor is much lower than in a gas or in a scintillator (about 4eV in semiconductors against 30eV in gases and 300eV in a scintillator photomultiplier system) . As a result, the number of free charges created per photon detected is higher, which makes it possible to obtain high gains with low noise, and their high atomic number and density makes it possible to use significantly lower detection volumes. to those of gases or scintillators, while retaining the same quantum detection efficiency (see reference [2]).

All of these advantages have made it possible to use detectors based on semiconductors in the following three fields of application, which are given in chronological order of their study.
- nuclear detection whose objective is to measure the energy deposited by a photon there from a nuclear radiation source,
- scientific instrumentation, since it is necessary to detect brief X-ray pulses and measure their temporal evolution and intensity,
- X-ray detection, the objective of which is to produce the radiological image of an object irradiated by an X-ray generator.

 This latter field of application of X-ray detection by semiconductor-based detectors is very recent and therefore much less studied than that of y-radiation detection, which began in the 1960s.

 Among the semiconductor materials, Cadmium telluride (CdTe) is the best choice having regard to its electronic properties (see reference - [3]).

 However, other detectors, in particular based on type IV semiconductors (Si, Ge, ...), II-VI <ZnS, ...), III-V (GaAs, InP ...), or Il-Vil (HgI2, ...), can be used both in the X domain and in the y ray domain.

 The use of these semiconductor materials as X-ray detectors involves the deposition of two electrical contacts on the surface of the material, at the terminals of which a bias voltage is applied. The charge carriers, that is to say the electron-hole pairs created by the interaction of the photon X with the material, separate under the action of the electric field, the electrons migrating towards the positive electrode and the holes towards the negative electrode. The ability of these charge carriers to migrate towards the electrodes without being trapped by the faults present in the semiconductor material conditions the value of the measured signal.

This aptitude also called "transport property" of the charge carriers is all the higher as the electric field applied over the entire thickness of the detector is strong, because it limits their transit time in the detector.

 These transport properties of the charge carriers as well as the resistivity of the material, which impose the so-called dark current (detector current in the absence of radiation), and the useful detection area depend on the purity of the material. ie the presence of active faults in the prohibited band. These active defects appear systematically during the crystallogenesis of the material, regardless of the pulling method used (THM ("Traveling Heater Method" or mobile radiator method), HPBM ("High Pressure Brigman Method" or Br-igman high method). pressure) or BM ("Brigman Method" or Brigman method) for CdTe). The bibliography concerning the study of these defects using the pulling method is abundant and the latest developments show that it has not been possible to remove them (see reference [4]).

 The choice of the nature of the metallic contact deposited on the semiconductor material is dictated by the need to limit the dark current, to limit the contact resistance and to impose the electric field on the entire thickness of the detector, this which ensures a high useful detection area. Here again, the literature on the various possible detection structures such as ohmic structures (metallic deposit), junctions (implantations, diffusions), diode structures (heterojunctions, etc.) is abundant (see reference [5]).

 But in addition to the choice of the detector material according to its purity, and the nature of the contact in order to make the detector's performances optimal, the detection structure (contact - contact semiconductor) thus formed must meet specifications common to detection of X and y radiation, namely obtaining a high signal with a minimum of noise which is constant during the time of its acquisition.

 However, if these optimal detection structures of the junction, diode type make it possible to obtain a high signal with a minimum of noise, they unfortunately have a polarization effect which consists in an evolution over time of the spatial distribution of the electric field applied between the two electrodes (see reference [6]).

 Again many publications relate to the functioning of these ideal structures, but these are used exclusively for the detection of radiation there. These publications show that the polarization effect is linked to the presence of active defects in the semiconductor material (for example CdTe: Cl), which the optimal structures (type junctions, diodes) demonstrated. Today, only the use of a specific detection structure, a so-called "electroless" contact deposited on a previously chemically polished, so-called "ohmic" surface, makes it possible to make the measured signal constant over time. In return, the dark current is high (high noise), which limits the applied electric field and therefore the transport properties of the charge carriers (low efficiency) (see reference [7]).

 Such a non-optimal detection structure is the only one used for the detection of γ radiation. The interpretation of the polarization effect (evolution over time of the spatial distribution of the applied electric field) has led new users of X-ray detection systems to use such structures.

 The present invention relates to an X-ray detection device which overcomes these various drawbacks.

Statement of the invention
The present invention relates to an X-ray detector made of a high resistivity semiconductor material of type II-VI on which are arranged at least two electrical contacts, at least one of these being taken from the family of blocking contacts. .

 The nature and principle of X-ray detection, different from that of y-radiation detection, thus allow the use of optimal detection structures (Pin diodes, pn junction .... based on CdTe for X detection , while they do not work in y detection.

The polarization effect responsible for abandoning the optimal structures in y detection can be suppressed under certain conditions in X detection.

 Such a structure allows the application of a strong electric field while limiting the dark current by a factor of 3 to 10 and eliminating the polarization effect specific to the CdTe material.

 Such a structure, for which there is no stacking and no drag, makes it suitable for carrying out X-ray imaging.

 Advantageously, a detection structure of the blocking / CdTe / blocking diode head-to-tail diodes type can be deposited on any CdTe material.

Detection of X-rays at room temperature using optimal structures based on
CdTe therefore allows
- limit the current of darkness,
- allow a high electric field and therefore a high signal and a weak memory effect
- obtain a constant signal over time.

 As regards blocking contacts (such as aluminum, indium, silver), the present invention goes against what was done in the prior art.

 Indeed, such blocking contacts which are stable over time for X-ray radiation, make it possible to greatly improve the quality of X-ray detection. In addition, these blocking contacts (such as aluminum, silver, indium) were quickly abandoned in Gamma detection because they were not stable over time. Since X detection has developed on the basis of Gamma detection, those skilled in the art therefore use ohmic contacts.

Brief description of the drawings
FIGS. 1A, 1B and 1C illustrate a detection device y,
FIGS. 2A and 2B illustrate a detection device X according to the invention,
FIGS. 3A, 3B and 3C illustrate current-voltage characteristics for different structures according to the invention,
FIGS. 4A, 4B and 4C illustrate curves for detecting the radiation y,
FIGS. 5A, 5B and 5C illustrate X-ray detection curves,
FIGS. 6A, 6B and 6C illustrate a characterization by time of flight of the device of the invention with a source of radiation y,
- Figures 7A to 7C illustrate a characterization by time of flight of the device of the invention with an X-ray source.

Detailed description of embodiments
To better understand the invention, it is necessary to explain the differences between X and y radiation, the differences in the principle of detection of X and y radiation as well as the different physical criteria required for the X and y detectors.

 Both X and y radiation are made up of photons whose energies are roughly of the same order of magnitude. The differences lie in the sources of emissions and their control.

 The radiation there comes from radioactive sources whose emission of photons is random, therefore not controllable. The energy of each photon is quantified, because the photon comes from disintegrations of the atomic nucleus. The activity (number of disintegrations per second) is variable, but generally low.

 X-ray radiation comes from a generator whose emission of photons is controllable. We obtain an energy spectrum of photons which we can control the maximum energy (by the high voltage of the tube) and the number of photons per unit of time (by the intensity of the tube). The photon flow is generally quite high. The emission of X photons can be continuous or chopped in the form of repetitive pulses with the use of a chopper.

 Radiation is mainly used there in nuclear medicine. The objective is to perform spectrometry and photons from tracers which have been injected into the patient. This spectrometry consists in detecting all the photons emitted and measuring their energy.

 X-ray is mainly used in radiography. The objective is to produce the image of an object by subjecting it to a spectrum of photons, by measuring the signal from the transmitted photons which have not interacted with the object during the acquisition time.

 Unlike y spectrometry, we do not measure the energy of each photon, but the signal resulting from the interaction of photons transmitted during the acquisition time in the detector.

 The measurement of the energy of each photon produced in spectrometry is very different and more restrictive than that of the signal from a set of photons interacting in the detector produced in X-ray radiography.

 FIG. 1A illustrates a detection device y, with a source of rays y 10. FIGS. 1B and 1C respectively represent curves of the integrated current Q as a function of time and of the number of strokes as a function of the measured value Qmes.

 FIG. 2A illustrates a detection device X, with an X-ray generator 11. FIG. 2B illustrates the measured current I as a function of time, with integrated current values Q.

 The detection of γ radiation by a CdTe type semiconductor began in the 1960s. Many studies have been carried out in order to optimize the detection structure for better spectrometry. Today, only the ohmic structure with deposit of two electroless contacts (Gold or Platinum) on a surface previously chemically etched allows to obtain an acceptable (but not optimal) spectrometry with a constant signal over time. The other detection structures (junction type, diodes, etc.) display much better performance in terms of energy resolution, but this only within the first few -seconds or minutes of acquisition at the end of which no more signal is observed. This so-called polarization effect is due to the presence of defects in the material which progress in crystallogenesis has not yet made it possible to remove.

 The detection of X-rays by a CdTe semiconductor began in the 1990s and therefore much later than that of y-rays. This explains the reasons which pushed the many users of CdTe in detection X to use the only structure functioning in detection y. The polarization effect which causes an evolution of the spatial distribution of the electric field in the detector until its extension in a few seconds after its application, is highlighted in detection y and should in the same way be observed in detection X .

 The object of the present invention is to demonstrate that a certain optimal detection structure works in X detection, whereas it does not work in y detection.

The detection device of the invention consists of a semiconductor material of high resistivity of type II-VI: CdTe to C1, Cdl ~, Zn, Te, CdTe1 .Se ,, CdlxZnxTe: Cl CdTel-xsex: clt GaAs, HgIn on which is deposited a blocking contact by displacement of cations in solution thus giving the contact
Metal / Semiconductor with remarkable electrical properties. The blocking contact can be placed on one side, but better still on both sides.

Such a structure with two blocking contacts deposited on the opposite faces of a detector
CdTe has a resistivity 3 to 10 times higher than that of this same material provided with gold or platinum electrode contacts (so-called ohmic structure). Consequently, this blocking / CdTe / blocking structure is the seat of a dark current approximately 3 to 10 times weaker for the same polarization. It behaves like a head-to-tail diode structure.

Figures 3A, 3B and 3C illustrate the current-voltage characteristics respectively
- for an ohmic / CdTe / ohmic structure
- for a blocking / CdTe / blocking structure
- for a blocking / CdTe / ohmic structure.

 The contacts are, in fact, divided into two families: blocking contacts (such as aluminum, indium, silver) and ohmic contacts (such as gold or platinum).

 For the first structure (Figure 3A), a material resistivity of 109Q-cm is obtained; for the second (Figure 3B), we obtain an apparent resistivity of 101lQ-cm.

 In addition, the contact resistance turns out to be very low compared to that of CdTe, which allows the electric field to be applied over the entire volume of the CdTe detector and not to be consumed under the blocking electrodes.

 Such structures have already been studied in y detection (reference [8]), but nothing has been published on their use in X detection, because the reasons for abandoning them in y detection do not apply in X detection.

 The high resistivity CdTe-based detection structures used in detection are commonly called "ohmic structures". FIGS. 4A to 4C show the results of detection of radiation y obtained with the high resistivity CdTe semiconductor provided with ohmic contacts and diode contacts. The radiation source there is a cobalt 57 radioactive source for which the emitted photons have the following energies: 14 keV (9.1% of cases), 122 keV (85.7% of cases), 136 keV (10.7 % of cases).

 FIG. 4A presents the ideal theoretical spectrum incident to the CdTe detector.

 The Au / CdTe / Au ohmic structure (with a 3 × 3 × 3 min detector, a 150-volt polarization, a dark current -10 A) makes it possible to obtain a spectrometry of medium performance as shown in FIG. 4B, because the resolution in measured energy (between 5 and 8%) is far from the theoretical resolution (2%). The ohmic structure does not allow the application of a strong electric field which would certainly allow the charge carriers created in the volume of the CdTe detector to migrate towards the electrodes without being trapped by the active defects of the material, but which would generate a dark current too high. Thus, a high electric field and a low dark current are incompatible with an ohmic structure. Ohmic contacts mean that the dark current is not limited, but imposed by the resistivity of the material.

Thanks to this dark current, the ohmic detection structure does not polarize, that is to say the measured spectrum remains stable during the time of its acquisition (a few minutes).

 The blocking diode blocking / CdTe / blocking structure (with a 3x3x3 mm3 detector, a 300Volts polarization, a dark current 10-9A) does not make it possible to obtain a y spectrometry, as shown in FIG. 4C, no signal n 'being detected. The dark current being 3 to 10 times lower than that of the previous structure for the same bias voltage, a higher bias voltage can be applied. However, the absence of a spectrum shows that the electric field is not applied to the entire volume of the detector and that, subjected to a DC bias voltage, the blocking / CdTe / blocking detector polarizes.

 No detection structure combining the application of a strong electric field, a very low dark current and a constant signal over time has been proposed for the detection of y-radiation based on CdTe at room temperature.

 X-ray radiation is most often made up of a train of pulses of a few milliseconds at the frequency of a few tens of Hertz. The high voltage of generator X varies between 20 and 160 kV, the intensity between 2 and 40 mA. In FIG. 5A, there is a train of pulses of duration 2 ms, of frequency 50 Hz, with a voltage 120kV / 20mA.

The Au / CdTe / Au ohmic structure (with a 10 × 10 × 1 mm 3 detector, SOVolts polarization, 10-7A dark current) displays good sensitivity, but the presence of a trail 20 which appears at the end of each X pulse, as illustrated in FIG. 5B, causes a stack of the measured signal. This drag is linked to the trapping of the charge carriers which are trapped during the pulse X due to the presence of the defects of the
CdTe and the weak electric field applied.

The blocking / CdTe / blocking diode structure (with a - detector 10 x lOx lmm3, a 150Volts polarization, a dark current 10-9A) displays a sensitivity equivalent to the ohmic structure
Au / CdTe / Au and this without presenting the polarization effect (see FIG. 5C). This unexpected observation is remarkable, because it opens the way to the use of a structure allowing the application of a strong electric field for a weak dark current. The application of a strong electric field makes it possible to limit the trapping / trapping of the charge carriers and thus to limit the drag and consequently to suppress the stacking. These blocking / Cd / Te / blocking diode structures seem to follow perfectly the theoretical temporal evolution of the train of pulse X with a dynamic of attenuation at the cut of the radiation close to four decades.

 To better understand these phenomena, we study these structures using a characterization manipulation called "time of flight" (or "Time of flight" in English). It allows by the use of a very fast (500ps pulses) and repetitive (30Hz max.) Ultraviolet laser, as shown in FIGS. 6A and 7A., To observe the temporal evolution of the spatial distribution of the electric field ( see Figure 6B).

In y detection, as shown in FIG. 6A with a Cobalt 57 radioactive source, everything happens as if the detector is constantly in the dark since the photon y incident to the detector creates only very few charge carriers and in a space infinitely smaller than the volume of the detector. The flight time experiment confirms the presence of a constant electric field over time, higher on the cathode side for the structure
Au / CdTe / Au- (with a detector 1 Ox 1 Ox lmm3, a polarization 54V, a dark current 10-6A) (see figure 6C, curves 30 and 31 corresponding to use with and without filter. It confirms also the absence of an electric field for the blocking / Cd / Te / blocking structure (with a 10 X 10 X lmm3 detector, 90V polarization, 10-9A dark current). There is disappearance of the zoom signal after the power up.

 In X detection (see Figure 7A) with a 120kV, 20mA generator, one side of the detector is irradiated by the ultra-violet laser, the other side is irradiated by X photons from the generator; This time, the detection structures are subjected to a much higher flux of photons than in detection y, the X photons are absorbed throughout the volume and numerous charge carriers are created. The time-of-flight results show that the electric field of the Au / CdTe / Au structure (with a 10x10x1mm3 detector, 90V polarization, 10-oA dark current) is little changed unless the incident photon flow is too high. high, in which case the electric field becomes higher towards the opposite electrode to that which is irradiated by generator X (see figure 7B). The results concerning the blocking / CdTe / blocking structure (detector 10x10x1mm3, polarization 72V, current of darkness 10-9A) show the presence of an electric field which, under the presence of many charge carriers created, has regenerated when it should be absent (see Figure 7C). The presence of these numerous charge carriers, by their trapping, seems to be able to compensate for the effect of the defects responsible for the polarization effect.

REFERENCES [1] "Cadmium telluride and related materials as X and
gamma-ray detectors. A review of recent progress "by
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Claims (4)

 1. X-ray detection device based on semiconductors characterized in that it consists of a semiconductor material of high resistivity on which are arranged at least two electrical contacts of which at least one is taken from the family blocking contacts.  2. Device according to claim 1, characterized in that said semiconductor material is of type II-VI.  3. Device according to claim 1, characterized in that the material is chosen from the following materials: CdTe: Cl, CdlxZnxTe, CdTelxsex, Cdl-xZnxTe: cl, CdTe1-xSex: C1, GaAs, HgIn.  4. Device according to claim 1, characterized in that it comprises two metal contacts positioned on two opposite sides of the detection means.