Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
  • H01L31/08—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof in which radiation controls flow of current through the device, e.g. photoresistors
  • H01L31/10—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof in which radiation controls flow of current through the device, e.g. photoresistors characterised by potential barriers, e.g. phototransistors
  • H01L31/115—Devices sensitive to very short wavelength, e.g. X-rays, gamma-rays or corpuscular radiation

Definitions

• the present invention relates to an X-ray detection device based on semiconductors.
• detectors have been devised for the detection of X or ⁇ 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.
• Semiconductor-based detectors directly convert energy into X or ⁇ radiation in matter without going through intermediate steps such as emission of photons visible 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 (around 4eV in semiconductors versus 30eV in gases and 300eV in a scintillator photomultiplier system). Consequently, the number of free charges created per photon detected is greater, which makes it possible to obtain high gains with low noise.
• their high atomic number and density makes it possible to use significantly lower detection volumes than those of gases or scintillators, while retaining the same quantum detection efficiency (see reference [2]).
• detectors based on semiconductors in the following three fields of application, which are given in chronological order of their study: - nuclear detection, the objective of which is to measure the energy deposited by a ⁇ photon from a source of nuclear radiation,
• 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.
• the detection structure (contact - semiconductor - contact) thus formed must meet a common specification. to the detection of X and ⁇ radiation, namely obtaining a high signal with a minimum of noise which is constant during the time of its acquisition.
• Such a non-optimal detection structure is the only one used for the detection of ⁇ radiation.
• the interpretation of the polarization effect 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.
• 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. .
• 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.
• a detection structure of the head-to-tail diode type blocking / CdTe / blocking can be deposited on any CdTe material.
• 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.
• 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.
• FIGS. IA, IB and 1C illustrate a detection device ⁇
• 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 of detection of ⁇ radiation
• 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
• FIGS. 7A to 7C illustrate a characterization by time of flight of the device of the invention with a source of X-ray radiation. Detailed description of embodiments
• Both X and ⁇ 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 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 in nuclear medicine.
• the objective is to perform ⁇ spectrometry of photons from tracers that 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.
• the measurement of the energy of each photon produced by ⁇ 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 ⁇ , with a source of rays ⁇ 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 Q mes .
• 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 object of the present invention is to demonstrate that a certain optimal detection structure works in X detection, while it does not work in ⁇ detection.
• the detection device of the invention consists of a semiconductor material of high resistivity of type II-VI: CdTe to Cl, CdTei- x Se x , Cd ⁇ _ x Zn x Te: Cl, CdTe ⁇ _ x Se x : Cl, GaAs, Hgln on which is deposited a blocking contact by displacement of cations in solution thus conferring on the Metal / Semiconductor contact properties remarkable electrics.
• 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 CdTe detector has a resistivity 3 to 10 times greater 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.
• FIG. 3A, 3B and 3C illustrate the current-voltage characteristics respectively:
• the contacts are, in fact, divided into two families: blocking contacts (such as aluminum, indium, silver) and ohmic contacts (such as gold or platinum).
• the source of ⁇ radiation is a radioactive source of cobalt 57 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 3x3x3 mm detector, a 150-volt polarization, a dark current. -10 " A) makes it possible to obtain ⁇ spectrometry with average 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 too high dark current. Thus high electric field and weak dark current are incompatible with an ohmic structure.
• the 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, i.e. the spectrum measured remains stable during the time of its acquisition (a few minutes).
• the blocking diode blocking / CdTe / blocking structure (with a 3x3x3 mm 3 detector, 300Volts polarization, 10 "9 A dark current) does not allow to obtain a ⁇ spectrometry, as shown in FIG. 4C, no signal not 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.
• the absence of spectrum shows that the field electric is not applied to the entire volume of the detector and that, subjected to a DC bias voltage, the blocking / CdTe / blocking detector polarizes.
• 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.
• 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 ⁇ 10 mm detector, 50Volts polarization, 10 " A dark current) displays good sensitivity, but the presence of a trail 20 which appears at the end of each pulse X- , as illustrated in FIG. 5B, causes a stacking of the measured signal. This drag is linked to the trapping of the charge carriers which have been trapped during pulse X due to the presence of CdTe faults and the weak electric field applied.
• the blocking / CdTe / blocking diode structure (with a 10 x 10 ⁇ 1mm detector, 150Volts polarization, 10 "9 A dark current) displays a sensitivity equivalent to the Au / CdTe / Au ohmic structure without presenting the polarization effect (see Figure 5C).
• This unexpected finding 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. strong electric field makes it possible to limit the trapping / trapping of the charge carriers and thus to limit the drag and consequently to eliminate the stacking.
• These blocking / Cd / Te / blocking diode structures seem to perfectly follow the theoretical temporal evolution of the train d pulse X with a dynamic attenuation at the cutoff of the radiation close to four decades.
• the flight time experiment confirms the presence of a constant electric field over time, higher on the cathode side for the Au / CdTe / Au structure (with a 10 ⁇ 10 ⁇ 10 mm detector, 54V polarization, 10 " dark current 6 A) (see FIG. 6C, curves 30 and 31 corresponding to use with and without filter. It also confirms the absence of an electric field for the blocking / Cd / Te / blocking structure (with a detector 10 X 10 X 1mm , 90V polarization, 10 " dark current A). The 100ms signal disappears after switching on.
• 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, the X photons are absorbed throughout the volume and numerous charge carriers are created.

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• Physics & Mathematics (AREA)
• Condensed Matter Physics & Semiconductors (AREA)
• Electromagnetism (AREA)
• General Physics & Mathematics (AREA)
• Engineering & Computer Science (AREA)
• Computer Hardware Design (AREA)
• Microelectronics & Electronic Packaging (AREA)
• Power Engineering (AREA)
• Measurement Of Radiation (AREA)
• Light Receiving Elements (AREA)

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• 1995
  • 1995-08-24 FR FR9510046A patent/FR2738080B1/fr not_active Expired - Fee Related
• 1996
  • 1996-08-23 CA CA 2203413 patent/CA2203413A1/fr not_active Abandoned
  • 1996-08-23 JP JP9509915A patent/JPH10512398A/ja active Pending
  • 1996-08-23 WO PCT/FR1996/001313 patent/WO1997008758A1/fr not_active Application Discontinuation
  • 1996-08-23 EP EP96929365A patent/EP0788662A1/de not_active Withdrawn

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