RU2797929C2 - X-ray and gamma-ray photodiode - Google Patents

Abstract

FIELD: detecting systems.

SUBSTANCE: group of inventions relates, in general, to photodiodes for the detection of photons of X-ray and gamma-radiation. A photodiode contains InGaP located and made with the possibility of absorption of X-ray radiation and/or gamma-radiation supplied to this photodiode, and generation of charge carriers in response to it.

EFFECT: increase in energy resolution of a detector.

20 cl, 4 tbl, 20 dwg

Description

FIELD OF TECHNOLOGY

The present invention relates generally to photodiodes for detecting photons, and in particular to a photodiode that has improved performance.

BACKGROUND OF THE INVENTION

Photodiodes are commonly used as detectors for x-rays, gamma rays (γ rays), and other types of radiation. Most photodiodes for X-ray and gamma ray applications are made from silicon. Recently, significant efforts have been expended in trying to develop alternative materials that, unlike silicon, would be able to operate at high temperatures and in environments with high levels of radiation and that would be more efficient (i.e., would detect a higher percentage of incident radiation). Many materials have been explored including GaAs, AlGaAs, AlInP, SiC, CdTe and CdZnTe as some of the most promising. However, all of these materials have been found to be subject to significant problems and limitations, and have both advantages and disadvantages. For example, while SiC is the most "state of the art" material for these photodiodes, capable of operating at relatively high temperatures and high radiation environments, it is no more efficient than silicon.

It is desirable to use photodiodes, for example, in X-ray and gamma-ray photon counting spectrometers. Such spectrometers are required in many applications in science and industry. For example, there is currently a strong commercial demand for the development of detectors to prevent the smuggling of nuclear materials (eg nuclear weapons, dirty bombs, etc.). However, the requirements for such photon counting spectrometers place extremely high demands on the materials used in photodiodes.

It would be desirable to create improved photodiodes.

DISCLOSURE OF THE INVENTION

The present invention provides a photodiode for use in detecting x-rays and/or gamma rays, comprising indium gallium phosphide InGaP, positioned and configured to absorb x-rays and/or gamma rays incident on the photodiode and generate charge carriers in response to This.

The inventors have found that the use of InGaP (also known as GaInP) as an active material in a photodiode allows the photodiode to be used to detect x-rays and gamma rays. This is quite surprising since InGaP is a ternary compound of indium phosphide InP and gallium phosphide GaP, both of which are known to be quite unsuitable as active materials for X-ray and gamma-ray photodiodes. For example, GaP was previously found not to be spectroscopic at X-ray energies, and InP was previously found to be spectroscopic at X-ray energies only at low temperatures (≤-60°C). Because InGaP is a ternary compound of InP and GaP, it has been assumed that InGaP performance will be even worse than either of its binary equivalents.

The use of InGaP photodiodes is particularly useful in X-ray spectroscopy and in X-ray photon counting spectroscopy. InGaP detectors have been found to perform significantly better than the corresponding GaP and InP binary compounds, and have also been found to have sufficiently high energy resolution to enable room temperature X-ray ion counting spectroscopy, which is not possible in GaP based devices. and InP.

In addition, InGaP has a low leakage current, due to which the photodiodes according to the embodiments described herein have the ability to operate at room temperature and above (ie ≥20°C) without the use of cooling systems. Thus, it is possible to create an X-ray and gamma radiation detection system having a relatively small mass, volume and power consumption. Therefore, said system can be relatively inexpensive, compact and undemanding in terms of temperature and thus particularly useful in applications such as aerospace or terrestrial non-laboratory environments (eg underwater reconnaissance). Appliances containing cooling systems are also contemplated so as to allow operation of the LED in very high temperature environments. Due to the use of InGaP, the required level of cooling can be zero or relatively low, for example compared to a silicon-based detector.

The use of InGaP in the photodetector instead of other conventional aluminum-containing semiconductor materials (such as (AlInP) has the advantage of being able to eliminate aluminum from the detector. This is useful because aluminum, like silicon, is often a material of interest in planetary and geological X-ray fluorescence spectroscopy (XRF), and therefore it is desirable to exclude it from the detector in order to reduce the complexity of the spectral analysis by removing the corresponding lines from the detector's own fluorescence spectrum.

Due to the relatively high linear X-ray attenuation coefficients in InGaP (for example, In _0.5 Ga _0.5 P) compared to some other wide bandgap materials (for example, GaAs, AlGaAs and AlInP), it is possible to manufacture relatively thin InGaP detectors and to achieve improved high temperature performance due not only to a wide bandgap, but also to a reduced volume of semiconductor material.

InGaP photodiodes also provide an alternative to GaAs, CdTe and CdZnTe in hard X-ray and gamma ray detection.

In addition, InGaP may be a structure compatible with commercially available substrates such as GaAs substrates and may be processed using commercially available technologies.

As a result of the absorption of X-rays and/or gamma-rays by the InGaP material, photogeneration of electron-hole pairs occurs.

The photodiode may include a housing or screen positioned and configured to prevent photons other than X-ray and/or gamma radiation photons from reaching the InGaP, and/or positioned and configured to prevent radioactive beta and/or alpha particles from entering. on InGaP.

The housing or shield may be configured to substantially prevent photons having a frequency lower than that of x-ray or gamma ray photons from passing through it. For example, the housing or screen may be positioned and configured to prevent blue light from reaching the InGaP.

The housing or screen may be made of metal, in particular metal foil. For example, the foil may be aluminum or beryllium foil. However, it is contemplated that other metals or materials may be used.

The photodiode may comprise a PIN junction or a p-n junction formed by said InGaP.

A PIN junction (eg, p ⁺ -in ⁺ -junction) can be formed by an undoped InGaP layer located directly between the p-doped InGaP layer and the n-doped InGaP layer.

The inclusion of an internal i-layer in the PIN junction can be used to provide a relatively large depletion zone and therefore a relatively large volume for absorbing x-rays and/or gamma rays and generating charge carriers.

However, it is also contemplated that InGaP can be implemented as a p-n junction. The p-n junction may be formed by a p-doped InGaP layer in contact with an n-doped InGaP layer.

The I-layer of the PIN junction can have the following thickness: ≥ 5 µm, ≥ 10 µm, ≥ 15 µm, ≥ 20 µm, ≥ 25 µm, ≥ 30 µm, ≥ 35 µm, ≥ 40 µm, ≥ 45 µm or ≥ 50 µm . By having such a relatively thick i-layer, it is possible to absorb a relatively high percentage of incident X-rays and/or gamma rays, so that a relatively large number of charge carriers are generated, and the photodiode is relatively efficient at generating current based on X-rays and/or gamma radiation.

The p-layer and/or the n-layer of the PIN junction may have the following thickness: ≤ 0.5 μm, ≤ 0.4 μm, ≤ 0.3 μm, ≤ 0.2 μm, or ≤ 0.1 μm. Due to the presence of such relatively thin layers, the absorption of x-rays and/or gamma rays by these layers is limited. For example, if the p-layer (or n-layer) is located on the opposite side of the junction that faces the X-ray or gamma-ray source, then due to the use of a relatively thin p-layer (or n-layer), a relatively small percentage of photons will be absorbed, especially lower energy photons, so that a higher percentage of incident x-rays and/or gamma rays can pass into the i-layer of the PIN junction to be absorbed therein and generate charge carriers (although it is assumed in the present invention that part p -layer can also actively contribute to the generation of useful charge carriers). If the n-layer (or p-layer) is located on the opposite side of the transition, which is facing away from the X-ray or gamma-ray source, then using a relatively thin n-layer (or p-layer) will reduce the absorption of photons and, therefore, accumulation of partial charge from this layer. As a result, it is possible, for example, to improve the spectral response of the instrument.

The photodiode may include electrodes on both sides of the PIN junction or p-n junction for applying voltage to the junction and/or for measuring photogenerated charge carriers generated in the junction, wherein at least one of the electrodes does not cover a portion of that side of the junction on which it is located so that it is possible for X-rays or gamma rays to pass inside the transition through said side without passing through said at least one electrode.

Said at least one junction may be annular, have a hole, have a notch, or be interdigitated so as not to completely cover said side and allow X-rays and/or gamma rays to pass into the junction without passing through the material forming said electrode.

The photodiode may include a voltage source configured and configured to supply a reverse bias voltage to the specified PIN junction or p-n junction, and the specified voltage is ≥ 2 V, ≥ 3 V, ≥ 4 V, ≥ 4.5 V or ≥ 5 V By applying such a reverse bias voltage, it is possible to create a relatively large depletion depth as it displaces free charge carriers from the i-layer. In this way, it is also possible to achieve relatively low noise for some noise sources and to achieve relatively high resolution, for example when using a photodiode in an X-ray and/or gamma-ray spectrometer. However, it is also contemplated that there may be no reverse bias applied.

InGaP may be a crystal structure with a structural composition of In _0.5 Ga _0.5 P. This composition of InGaP allows the material to be structurally compatible with and grow on conventional substrates such as GaAs. These proportions also make it possible to grow InGaP with high crystal quality and/or relatively large thicknesses. This is particularly useful because, as described above, it may be desirable to create a relatively thick layer of InGaP in order to increase the likelihood that each of the photons of a given x-ray or gamma ray will be absorbed in a given material.

However, it is contemplated that the InGaP material may be a crystal structure with a structural composition of In _x Ga _1-x P, where x is a value other than 0.5.

InGaP can be made on a substrate, preferably a semiconductor substrate such as GaAs.

The photodiode may have a mesadiode structure.

The photodiode may include one or more layers to form electrical contact on each side of the InGaP material. For example, said one or more layers may include at least one metal layer and/or at least one semiconductor layer. Examples of metal layers include gold and titanium. Examples of the semiconductor layer include InGe.

Between said one or more electrical contact layers and the InGaP layer, a tie layer may be provided to enable good communication of the electrical contacts with the InGaP layer. The bonding layer may be a semiconductor such as GaAs. One or more tie layers may be a substrate on which the InGaP is made.

The InGaP material may be provided within a substantially planar structure, and the planar structure may contain Schottky contacts, or ion implantation into the device to form electrical resistance regions is possible.

Ion implantation can be carried out in such a way that regions of electrical resistance are formed which electrically isolate portions of the semiconductor wafer. For example, if a plurality of diode structures are provided, then ion implantation is possible to form regions that prevent the voltage applied to one diode structure from dissipating across adjacent diodes.

The photodiode may be an avalanche diode or a non-avalanche diode.

The photodiode is desirably a single crystal InGaP detector, i. The InGaP is desirably present as a single InGaP structure. However, it is also contemplated that the InGaP material may be polycrystalline, i. contain a plurality of crystals in the material, possibly having arbitrary orientations.

The photodiode may contain a heterostructure formed from layers or regions of different semiconductors and, therefore, containing heterojunctions. As described above, the InGaP material absorbs x-rays and/or photons to generate charge carriers, although other semiconductor layers for other functions may also be included. For example, a layer of another semiconductor (eg, AlInP) may be included to receive charge carriers from the InGaP material and generate secondary charge carriers, such as to increase the number of charge carriers and form an avalanche photodiode. However, it is also contemplated that the InGaP photodiode may contain one or more homojunctions so that an avalanche effect is provided.

The present invention also provides an X-ray or gamma-ray detector comprising a photodiode as described herein and an electronic circuit for processing an electrical signal generated by said photodiode to determine if said electrical signal is due at least in part to the generation of said photogenerated charge carriers. .

The photodiode forms part of the circuitry in the detector and said electronic circuitry can be tuned and calibrated to detect the signal due to the generation of said charge carriers. For example, in the current measurement mode, it is possible to determine that the part of the signal that exceeds the dark current is due to X-rays or gamma rays received by the detector.

The present invention also provides an X-ray and/or gamma-ray photon counting spectrometer comprising the detector described herein and a processor configured and configured to determine the energies of individual X-ray and/or gamma-ray photons detected by the detector, based on said electrical signal, and/or with the ability to determine the number of x-ray and/or gamma ray photons detected by the detector based on said signal.

In photon counting spectroscopy, the detector may be coupled to an electronic circuit, typically consisting of a charge-sensitive preamplifier, a shaping amplifier, a multichannel analyzer, and a computer. When X-ray or gamma-ray photons are absorbed by an InGaP material, a certain charge (a certain number of electrons and holes) is created in this material. The amount of charge created is proportional to the photon energy. The charges arrive at the detector contacts, and during their migration, under the action of their movement, a charge is induced on the detector contacts. One of the contacts is connected to a charge-sensitive amplifier, and thus this charge-sensitive amplifier detects the charge on its input and converts it into a voltage pulse with a long decay time, which is proportional to the charge it receives. The shaping amplifier takes the specified pulse with a long decay time (which has a high rise rate and a low fall rate) and changes the shape of this pulse in such a way as to make it easier to measure in a multichannel analyzer. The multichannel analyzer takes the output of the shaping amplifier, measures the amplitude of the pulse (which is proportional to the photon energy), and constructs a histogram consisting of the amplitude of that pulse and other received pulses, resulting in a spectrum that can be observed on a computer.

The present invention also provides a system comprising an x-ray and/or gamma source and a photodiode, detector or spectrometer as described herein for detecting x-rays and/or gamma rays from said source.

The source of x-rays and/or gamma rays referred to in this document may be a primary source of x-rays and/or gamma rays, such as a source of radioactive radiation. Alternatively, the x-ray and/or gamma source referred to in this document may be a fluorescent x-ray and/or gamma source. Thus, embodiments of the present invention cover X-ray and/or gamma-ray spectroscopy of photons directly from a primary source, as well as X-ray and/or gamma-ray fluorescence spectroscopy.

The system may be a nuclear or radioisotope battery containing the specified x-ray and/or gamma radiation source and the specified photodiode for converting the specified x-ray and/or gamma radiation into electric current.

The photodiode is configured to operate in the photoelectric mode.

The nuclear or radioisotope battery may be a nuclear or radioisotope microbattery.

The battery may be configured such that most or substantially all of the generated electrical current (i.e., non-dark current) comes from a photodiode that converts x-rays and/or gamma rays from said source into electrical current, and not was due, for example, to alpha and beta particles.

The source may be radioactive material.

The source and the photodiode can be placed inside the housing, and the specified housing, if necessary, is located and is configured to essentially prevent the exit of the specified x-ray radiation and/or gamma radiation from the source.

While the x-ray and/or gamma source has been described as being in the same package as the photodiode, it is alternatively contemplated that one or more such sources may not be located in the same package as the photodiode. For example, the photodiode may receive x-rays and/or gamma rays from the environment in which the photodiode is located. Embodiments are contemplated where the source is nuclear waste emitting x-rays and/or gamma rays and a photodiode in the battery converts this radiation into an electrical quantity.

The photodiode may have any of the features described elsewhere in this document. However, a PIN junction or a p-n junction can be zero-biased or forward-biased rather than reverse-biased.

The present invention also provides a method for detecting x-rays and/or gamma rays, comprising the steps of:

subjecting the x-ray and/or gamma ray detector described herein to an x-ray and/or gamma ray source; And

based on the signal generated by the photodiode, the detector's reception of x-rays and/or gamma rays is determined.

Said method steps may be preceded by steps in which said x-ray and/or gamma radiation source is selected and the detector is moved in the direction of said source. For example, the method may be intentionally used to analyze the particular source that has been selected.

The method may also include determining the presence and/or location of said x-ray and/or gamma source using a detector. For example, the method can be used to detect nuclear or radiological material.

The present invention also provides a method for counting X-ray and/or gamma-ray photons, comprising the steps of:

subjecting the X-ray and/or gamma-ray spectrometer described herein to an X-ray and/or gamma-ray source; And

based on the electrical signal generated by the photodiode, the energies of individual x-ray and/or gamma-ray photons detected by the detector are determined and/or the number of x-ray and/or gamma-ray photons detected by the detector is determined.

The various devices and methods described herein may be used in a wide range of different applications, including scientific research, medicine, defense, security, food processing, aerospace, and the like.

The photodiode described herein can be used in the specified detector for X-ray spectroscopy/spectrometry (for example, X-ray fluorescence spectroscopy/spectrometry).

Although the photodiode, detector, spectrometer, system and methods are described as being used to detect x-rays and/or gamma rays and/or used in conjunction with a source of such x-rays and/or gamma rays, it is contemplated that these devices or methods provide the ability to detect other types of photons or detection of particles such as electrons, ions, alpha particles or beta particles.

Accordingly, the present invention also provides a device for detecting photons or particles, comprising:

a diode containing InGaP located and configured to absorb and/or interact with said photons or particles and generate charge carriers in response thereto; And

an electronic circuit for processing the electrical signal generated by the diode to determine whether said electrical signal is due at least in part to the generation of said charge carriers.

The diode may have the features of a photodiode as described herein, except that the diode may interact with particles or photons to generate charge carriers.

The instrument may include a user interface, such as a display or a speaker, and a controller for operating the interface to inform the user that said photons or particles have been detected.

The present invention also provides a photon or particle counting spectrometer comprising the instrument described herein and a processor configured and configured to determine the energies of individual photons or particles detected by said instrument based on said electrical signal and/or capable of determining the number of photons or particles detected by said instrument based on said signal.

The spectrometer can output specified energies or quantities to a specified user interface.

The present invention also provides a system comprising a photon or particle source and an instrument or spectrometer as described herein for detecting photons or particles from said source.

The present invention also provides a method for detecting photons or particles, comprising the steps of:

subjecting the device described herein to a source of said photons or particles; And

based on the signal generated by the diode, it is determined that photons or particles have been received by the instrument.

The method may include transmitting output to an electronic user interface, such as a display or speaker, to inform the user that said photons or particles have been detected.

The present invention also provides a method for counting photons or particles, comprising the steps of:

subjecting the spectrometer described herein to a source of photons or particles; And

based on the electrical signal generated by the diode, the energies of individual photons or particles detected by the instrument are determined and/or the number of photons or particles detected by the instrument is determined. Although a nuclear battery has been described that uses a photodiode to convert x-rays and/or gamma rays directly into an electrical quantity (using InGaP), it is contemplated that the battery may first convert x-rays and/or gamma rays into photons or other particles. types, and these photons or other types of particles can act on the InGaP material to generate charge carriers.

Accordingly, the present invention also provides a nuclear or radioisotope battery comprising:

a transducer for absorbing or interacting with x-rays and/or gamma rays and generating photons or other types of particles in response; And

a diode containing InGaP disposed to receive said photons or particles and convert them into electric current.

The battery may contain an x-ray and/or gamma source, for example located in the same housing as the diode, or, alternatively, it may receive x-rays and/or gamma rays from the local environment.

BRIEF DESCRIPTION OF THE DRAWINGS

The various embodiments will now be described by way of example only, with reference to the accompanying drawings, in which:

in fig. 1A-1B show how the currents generated by two differently sized photodiodes vary with applied reverse bias;

in fig. 2A-2B show how the capacitances of two different sized photodiodes vary with applied reverse bias;

in fig. 3A-3B show how the depletion depth of two photodiodes of different sizes varies with applied reverse bias;

in fig. 4 shows how the impurity concentration in a photodiode varies with the depletion depth;

in fig. 5A-5B show X-ray spectra taken with two photodiodes of different sizes;

in fig. 6A-6B show parallel white noise, series white noise, and 1/f noise as a function of applied reverse bias for two different sized photodiodes;

fig. 7 shows the equivalent noise charge versus applied reverse bias for two photodiodes of different sizes; And

in fig. 8A-8B show how the detection efficiency in PIN structures having layers of different thicknesses varies with the energy of photons incident on these structures.

IMPLEMENTATION OF THE INVENTION

Hereinafter, an exemplary embodiment of an InGaP photodiode will be described by way of example only to facilitate understanding of the present invention. It should be borne in mind that the present invention is not limited to the inclusion of all of the described layers or the specific composition of the various layers in this example.

With reference to Table 1 below, an example of a photodiode containing nine layers is shown. In _0.5 Ga _0.5 P wafer (layers 4-6 in Table 1) was grown by metal organic vapor phase epitaxy (MOVPE) on an n ⁺ -doped GaAs substrate (layer 7 in Table 1) such that p ⁺ -in ⁺ -structure. Layers of In _0.5 Ga _0.5 P wafers were successively grown on a GaAs substrate in such a way that a Si-doped Ino _0.5 Ga _0.5 P n ⁺ -layer having an impurity concentration of 2×10 ¹⁸ cm ^-3 and a thickness of 0.1 μm was formed (layer 6 in Table 1), then formed an inner In _0.5 Ga _0.5 P layer containing no impurities and having a thickness of 5 μm (layer 5 in Table 1), and then formed a zinc-doped p ⁺ -layer In _0.5 Ga _0.5 P having an impurity concentration 2×10 ¹⁸ cm ^-3 and a thickness of 0.2 μm (layer 4 in table 1). On the upper surface of the doped p ⁺ -layer In _0.5 Ga _0.5 P (layer 4 in table 1) formed heavily doped with zinc p ⁺ -layer GaAs, having an impurity concentration of 1×10 ¹⁹ cm ^-3 and a thickness of 0.01 μm (layer 3 in the table 1) to promote ohmic contact.

Then, wet chemical etching techniques were applied to these layers to fabricate circular mesa photodiodes having a diameter of 200 µm and 400 µm. To etch these circular mesa photodiodes, a solution of 1:1:1 K ₂ Cr ₂ O ₇ :HBr:CH ₃ COOH was used and then for 10 sec. solution 1:8:80 H ₂ SO ₄ :H ₂ O ₂ :H ₂ O. In _0.5 Ca _0.5 P devices were depassivated.

The top ohmic contact was made on a heavily doped GaAs layer (Layer 3 in Table 1) by depositing a gold layer having a thickness of 0.2 µm (Layer 2 in Table 1) and then a layer of titanium having a thickness of 0.02 µm (Layer 1 in Table 1). 1). The top ohmic contact had an annular shape so that photons could pass through the hole in the ohmic contact. The ring ohmic contact covered 33% and 45% of the upper end in a photodiode with a diameter of 400 µm and 200 µm, respectively. A rear ohmic contact was made on the back surface of the GaAs substrate (layer 7 in Table 1) by depositing an InGe layer having a thickness of 0.02 µm (Layer 8 in Table 1) and then a gold layer having a thickness of 0.2 µm (layer 9 in table 1).

It was determined that for the areas of the photodiode not covered by the upper ring ohmic contact, the X-ray quantum efficiencies were 53% and 44% at energies of 5.9 keV and 6.49 keV, respectively, which was calculated using the Bouguer-Lambert-Beer law and under the assumption of full charge accumulation in p- and i-layers. For areas covered by an annular top contact, these values dropped to 44% and 38%, respectively. The linear attenuation coefficients used in calculating the quantum efficiency were 0.132 µm ^-1 and 0.102 µm ^-1 at 5.9 keV and 6.49 keV, respectively. These values are higher than other semiconductors such as Si, GaAs and Al _0.52 In _0.48 R.

The electrical characteristics of InGaP photodiodes 200 µm and 400 µm in diameter were studied.

The currents generated by InGaP photodiodes were investigated as a function of the reverse bias applied to the photodiodes under darkening conditions (i.e., in the absence of an X-ray and gamma radiation source) and when irradiated with a ⁵⁵ Fe radioisotope X-ray source (Mn Kα=5.9 keV, Mn Kβ=6.49 keV). The X-ray source was placed 6 mm above the top surface of each mesa photodiode. Photodiodes were tested at room temperature in a dry nitrogen atmosphere (relative humidity < 5%). A Keithley 6487 picoammeter/voltage source was used during the experiment. The error associated with the instrument's current readings was 0.3% of their readings plus 400 fA, while the error associated with applied biases was 0.1% of their readings. values plus 1 mV.

In FIG. 1A shows current versus reverse bias curves for a photodiode with a diameter of 200 µm. The lower open-square curve shows the current versus reverse bias when the photodiode is under shading conditions, and the upper solid-square trace shows the current versus reverse bias when the photodiode is irradiated with an X-ray source. In FIG. 1B shows current versus reverse bias curves for a 400 µm photodiode. The lower open-square curve shows the current versus reverse bias when the photodiode is under shading conditions, and the upper solid-square trace shows the current versus reverse bias when the photodiode is irradiated with an X-ray source. From FIG. 1A-1B, it can be seen that for photodiodes of both diameters, the dark current values were less than 0.22 pA (corresponding to current densities of 6.7×10 ^-10 A/cm ² and 1.7×10 ^-10 A/cm ² for photodiodes with diameters of 200 µm and 400 µm, respectively). At a reverse bias of 30 V, currents in the irradiated state were observed equal to 3.5 pA and 7 pA for photodiodes with a diameter of 200 μm and 400 μm, respectively. By subtracting the currents in the irradiated state from the dark currents, it can be determined that the photocurrents at 30 V reverse bias were 3.3 pA and 6.5 pA for devices with a diameter of 200 µm and 400 µm, respectively.

The capacitance of the photodiodes was also studied as a function of reverse bias from 0 V to 30 V using an HP 4275A Multi Frequency LCR meter (4275A multi-frequency LCR meter from Hewlett Packard). The test signal was sinusoidal, cf. square amplitude 50 mV and frequency 1 MHz. The error associated with each capacitance reading was approximately 0.12% plus experimental reproducibility error (±0.07 pF). The error associated with the supplied offsets was 0.1% of their values plus 1 mV. The capacitance of an identical empty package was also measured and subtracted from the measured capacitance of each packaged photodiode to determine the capacitances of the devices themselves.

In FIG. 2A shows capacitance versus applied reverse bias for a 200 µm diameter photodiode, and FIG. 2B shows capacitance versus applied reverse bias for a 400 µm diameter photodiode.

Then, the depletion depth (W) of each photodiode was calculated by the formula W=(ε ₀ ε _r A)/C, where ε ₀ is the vacuum dielectric constant, ε _r is the dielectric constant of In _0.5 Ga _0.5 P, A is the area of the device, and C is capacity.

In FIG. 3A shows the depletion depth versus applied reverse bias for a 200 µm diameter photodiode, and FIG. 2B shows the depletion depth versus applied reverse bias for a 400 µm diameter photodiode. From FIG. 3A-3B, it can be seen that at low reverse biases, the depletion depth increases with increasing reverse bias. When the reverse bias exceeds approximately 5 V, the depletion depth remains essentially constant as the reverse bias increases (due to the fact that the i-layer of the PIN structure is completely free of charge carriers at these biases). At a reverse bias voltage of 30 V, the measured depletion depth was 4.0 µm ± 0.5 µm and 4.6 µm ± 0.2 µm for devices with a diameter of 200 µm and 400 µm, respectively.

The impurity concentration (N) below the p ⁺ -i junction (i.e., the junction between layers 4 and 5 in Table 1 above) as a function of the depletion depth (W) was calculated by the formula:

where q is the electron charge, ε ₀ is the permittivity of the vacuum, ε _r is the relative permittivity of In _0.5 Ga _0.5 P, A is the area of the device, and C is the capacitance.

In FIG. 4 shows the measured impurity concentration N(W) for an In _0.5 Ga _0.5 P device with a diameter of 400 µm.

Mesa photodiodes (without avalanche breakdown) having a diameter of 200 µm and 400 µm were connected to a low-noise charge-sensitive preamplifier electronics to create an X-ray spectrometer. For example, this instrument was able to achieve an energy resolution of 900 eV of the system at 5.9 keV for a 200 µm photodiode operating at a reverse bias of over 5 V.

Using devices with a diameter of 200 µm and 400 µm, X-ray spectra were collected from ^{a 55} Fe radioisotope X-ray source. The distance between the upper surface of the InGaP photodiodes and the X-ray source was 6 mm. Connected to each InGaP diode was a low-noise, charge-sensitive preamplifier having a resistorless feedback design (similar to that described by Bertuccio, P. Rehak, and D. Xi, Nucl. Instrum. Meth. Phys. Res. (Nuclear Devices and Methods). in physical research”), A 326, 71 (1993)). The signal from the preamplifier was amplified and formed using an Ortec 572a shaping amplifier, the output of which was connected to a multichannel analyzer. The spectra were collected at reverse biases of 0 V, 5 V, 10 V, and 15 V on diodes. For each spectrum, a formation time of 10 μs and a limiting lifetime of 100 s were used.

In FIG. 5A and 5B show X-ray spectra taken at 5 V reverse bias for devices with a diameter of 200 µm and 400 µm, respectively. In each spectrum, the observed ⁵⁵ Fe photopeak was a combination of the Mn Kα and Mn Kβ lines at 5.9 keV and 6.49 keV, respectively. Using Gaussian distributions, the combined peaks were approximated taking into account the relative X-ray intensity values of the ⁵⁵ Fe radioisotope X-ray source at 5.9 keV and 6.49 keV in the appropriate ratio [U. Shotzig, Applied Radiation and Isotopes (Applied application of radiation and isotopes) 53, 469 (2000), 53, 469 (2000)], and also taking into account the difference in the values of the relative efficiency of the detector at these energies of X-ray radiation.

The energy resolution of the InGaP spectrometer, quantified in terms of full width at half maximum (FWHM) at 5.9 keV, was examined as a function of the detector's reverse bias. At a reverse bias of 0 V, the worst FWHM was obtained (the FWHM observed at 5.9 keV was 1 keV and 1.4 keV for devices with a diameter of 200 µm and 400 µm, respectively). This was due to the increased contribution of noise due to incomplete charge accumulation, which decreased at higher reverse biases. At reverse biases of 5 V and above, charge storage efficiency increased (noise due to incomplete charge accumulation decreased), FWHM at 5.9 keV improved and remained constant with further increase in reverse bias within the studied range. At a reverse bias of 5 V, the FWHM observed at 5.9 keV was 0.9 keV and 1.2 keV for devices with a diameter of 200 µm and 400 µm, respectively.

In order to identify the values of the contribution of different noises to the FWHM expansion, a noise analysis was carried out. The spectral resolution of an X-ray spectrometer with a photodiode without avalanche breakdown is determined by the formula:

where ΔE - FWHM, ω - energy of creating electron-hole pairs, F - Fano coefficient, E - energy of the absorbed X-ray photon, R and A - electronic noise and noise due to incomplete charge accumulation, respectively [G. Lioliou, A.M. Barnett, Nucl. Instrum. Meth. Phys. Res. (Journal "Nuclear Instruments and Methods in Physical Research"), A 801, 63 (2015)]. The basic "Fano-limited" energy resolution (i.e. at R=0 and A=0) for In _0.5 Ga _0.5 P was estimated to be 137 eV, assuming that the electron-hole pair creation energy in In _0.5 Ga _0.5 P is 4.8 eV (2.5 times the band gap) and the Fano coefficient is 0.12. This value of the noise contribution takes into account the statistical nature of the ionization process in a semiconductor X-ray detector. Since the measured FWHM was greater than 137 eV, it was necessary to take into account the contributions of other noise sources. The system electronic noise consists of parallel white noise, series white noise, induced gate current noise, 1/f noise, and dielectric noise. The leakage currents of the detector and preamp input FET cause parallel white noise, while the capacitances of the detector and preamp input FET cause series white noise and 1/f noise. The specified serial white noise has been adjusted for the induced gate current noise.

In FIG. 6A and 6B show the calculated parallel white noise, serial white noise, and 1/f noise as a function of detector reverse bias for a 200 µm diameter device and a 400 µm diameter device, respectively. Parallel white noise contributions are shown as circles, serial white noise contributions are shown as triangles, and 1/f noise contributions are shown as squares. It can be seen that the values of the parallel white noise contribution were similar for the 200 µm and 400 µm devices at each reverse bias. This was due to similar dark currents in both device sizes, as shown in FIG. 1A-1B.

In contrast, serial white noise and 1/f noise were higher in the 400 µm device compared to the 200 µm device. This was due to the higher measured capacitance in the larger diameter device as shown in FIG. 2A-2B. The increased FWHM observed in devices with a diameter of 400 µm can be partly explained by the increased values of the contribution of serial white noise and 1/f noise.

The contributions from Fano noise, parallel white noise, serial white noise, and 1/f noise were subtracted from the measured FWHM at 5.9 keV to calculate the sum of the contributions from dielectric noise and incomplete charge accumulation noise at 5.9 keV.

In FIG. 7 shows the equivalent noise charge of dielectric noise and partial charge storage noise versus reverse bias for a spectrometric device with a diameter of 200 µm (plot shown with crosses) and a diameter of 400 µm (plot shown with diamonds). The total value of the contribution of the dielectric noise charge and noise due to incomplete charge accumulation turned out to be larger in devices with a diameter of 400 μm than in devices with a diameter of 200 μm for all reverse biases. At a reverse bias of 0 V, the total value of the contribution, expressed in terms of the equivalent noise charge, was 123 e- (rms) and 87 e- (rms) for a device with a diameter of 400 μm and for a device with a diameter of 200 μm, respectively. At reverse biases above 5 V, the calculated equivalent noise charges were 105 e- (rms) and 78 e- (rms) for devices with a diameter of 400 µm and 200 µm, respectively. Since the dielectric noise was independent of the detector bias, the difference in the total equivalent noise charge due to the dielectric noise and the noise due to incomplete charge accumulation at 0 V from the same charge at ≥ 5 V can be attributed to the noise due to incomplete charge accumulation. at 0 V. Thus, at 0 V, the amount of noise due to incomplete charge accumulation was 18 e- (rms) and 9 e- (rms) using devices with a diameter of 400 μm and 200 µm, respectively, and that noise due to incomplete charge storage was negligible at reverse biases ≥ 5 V.

In FIG. 7 the equivalent noise charge at reverse biases ≥ 5 V is due to the dielectric contribution. The dielectric equivalent noise charge (ENC _D ) is calculated from:

where q is an electric charge, A ₂ is a constant (1.18), depending on the type of signal formation [E. Gatti, PF Manfredi, M. Sampietro, and V. Speziali, Nucl. Instrum. Meth. Phys. Res., A 297, 467 (1990)], k is Boltzmann's constant, T is temperature, D is loss factor, and C is capacitance [G. Lioliou, and A.M. Barnett, Nucl. Instrum. Meth. Phys. Res. A 801, 63 (2015)]. Using the above equation for ENC _D and the experimental data from FIG. 7, the effective dielectric loss factor was determined to be up to (4.2±0.4)×10 ^-3 . It should be noted that it does not directly correspond to the loss factor in In _0.5 Ga _0.5 P, but characterizes the effective total loss factor for all dielectrics contributing to this noise, as analyzed in this document.

While examples of energy resolution (FWHM) are given in this document, higher resolutions are achievable, for example, using preamplifiers having lower electronic noise.

We also studied the influence on the detection efficiency of the thickness of the inner layer in the p ⁺ -in ⁺ -structure ^In _0.5 Ga _0.5 P. It was assumed that the p ⁺ -side of the p ⁺ -in ⁺ that photons are incident on this side of the device. It was also assumed that only the inner layer of the p ⁺ -in ⁺ -structure was active.

In FIG. 8A shows the detection efficiency in the p ⁺ -in ⁺ -structure depending on the thickness of the inner layer, which is 5 μm, 10 μm, 30 μm and 50 μm. The thickness of the p ⁺ -layer in each p ⁺ -in ⁺ -structure was 0.2 μm. It can be seen that at comparatively low photon energies, the detection efficiency is essentially the same for all structures, even though they have inner layers of different thicknesses. However, as the photon energy increases, structures having thinner inner layers show lower detection efficiencies.

In FIG. 8B shows detection efficiencies in p ⁺ -in ⁺ -structures corresponding to those shown in FIG. 8A, except that the thickness of the i-layer in the p ⁺ -in ⁺ -structure is 0.1 µm (rather than 0.2 µm). The graphs in Fig. 8B have the same shape as the graphs in FIG. 8A, except that in FIG. 8B, detection efficiencies are higher at lower photon energies.

From FIG. 8A-8B, it can be seen that at low photon energies, the detection efficiency is determined predominantly by the thickness of the p ⁺ layer in the p ⁺ -in ⁺ structure. In other words, at low photon energies, the thicker the p ⁺ -layer in the p ⁺ -in ⁺ -structure, the lower the detection efficiency. However, at higher photon energies, the detection efficiency is determined mainly by the thickness of the inner layer in the p ⁺ -in ⁺ -structure. In other words, at higher photon energies, the thicker the inner layer, the higher the detection efficiency.

Although not shown in FIG. 8A-8B, the detection efficiency at a photon energy of 59.5 keV is of particular interest because it is the gamma ray energy from ²⁴¹ Am. At such high energies, both the p ⁺ layer in the GaAs side cap and the p ⁺ layer in the p ⁺ -in ⁺ structure are more or less transparent to photons, and hence the detection efficiency is limited by the thickness of the inner layer at p ⁺ -in ⁺ -structure. For example, for photons having an energy of 59.5 keV, the detection efficiency using a 5 µm inner layer would be 0.005, while the detection efficiency using a 50 µm inner layer would be 0.05.

X-ray astronomy and high-resolution X-ray fluorescence spectroscopy have been made possible by the use of photon-counting X-ray spectrometers. The ability to determine the energy of individual X-ray photons and the number of detected X-ray photons at a particular energy can be important, for example, in astronautics. These parameters can be especially useful for studying the surfaces of planets and magnetospheres and in solar physics, as well as for terrestrial applications such as industrial monitoring and non-destructive testing. As described earlier in this document, the use of wide bandgap materials in such spectrometers is promising because such materials can have low thermally generated leakage currents and therefore are able to operate at high temperatures without cooling systems, allowing for more compact and lightweight devices with lower power consumption.

Photon-counting X-ray spectrometers, characterized by high energy resolution and low temperature requirements, using detectors based on various wide-gap semiconductors coupled with low-noise preamplifier electronics, have been reported. However, embodiments of the present invention have provided improvements to such devices.

According to Table 2 below, the In _0.5 Ga _0.5 P p+-i-n+ epitaxial layer was grown on a heavily doped n+ GaAs substrate by low-pressure metal-organic vapor phase epitaxy (150 Torr) using trimethylgallium, trimethylindium, arsine, and phosphine as precursors, and hydrogen as a carrier gas. Disilane and dimethylzinc:triethylamine were used for n- and p-doping, respectively. The epitaxial surface of the substrate had the (100) orientation with a substrate cut angle of 10° in the <111>A direction. The unintentionally doped i-layer (thickness 5 μm) was located between the upper p+ layer (thickness 0.2 μm; impurity concentration 2×10 ¹⁸ cm ^-3 ) and the lower n+ layer (thickness 0.1 μm; impurity concentration 2×10 ¹⁸ cm ^-3 ). It should be noted that the thicknesses of the p+ and n+ layers were set as small as possible in order to reduce absorption in these layers. The thicknesses of the p+ layer (0.2 µm) and the n+ layer (0.1 µm) were chosen based on the experience of growing high quality In _0.5 Ga _0.5 P. increase the quantum efficiency in this layer. It should be emphasized that the specified In _0.5 Ga ₀ . _{The 5P} device is an In _0.5 Ca _0.5 P photodiode with the thinnest i-layer of all photodiodes reported to date, although i-layers greater than 5 µm thick can be provided. On the top surface of the In _0.5 Ga _0.5 P p+-i-n+ epitaxial layer, a thin p+ GaAs layer (thickness 0.01 µm; impurity concentration 1×10 ¹⁹ cm ^-3 ) was grown to help achieve good top ohmic contact. n-type GaAs, n-type In _0.5 Ga _0.5 P, and unintentionally doped In _0.5 Ga _0.5 P were grown at 700°C, and subsequent p-doped layers were grown at 660°C. At room temperature, the photoluminescence peak energy of the grown In _0.5 Ga _0.5 P was 1.89 eV. This energy is in good agreement with the band gap of a material with suppressed spontaneous long-range order of the structure in the group III sublattice. The ohmic contact on the upper surface of the p+ GaAs layer was made of Ti (20 nm thick) and Au (200 nm thick). The rear ohmic contact deposited on the rear surface of the n+ GaAs substrate was made of InGe (20 nm thick) and Au (200 nm thick). In _0.5 Ca _0.5 P photodiode was not passivated. For fabrication of the In _0.5 Ca _0.5 P device with a mesa structure 200 µm in diameter used in the research, wet chemical etching techniques were used (solution 1:1:1 K ₂ Cr ₂ O ₇ :HBr:CH3COOH and then finishing for 10 sec in a solution of 1:8:80 H2SO4:H2O2:H2O). The layers of said device, their relative thicknesses and materials are summarized in Table 2 below:

192 MBq ⁵⁵ Fe radioisotope X-ray source (Mn Kα=5.9 keV, Mn Kβ=6.49 keV) was placed at a distance of 5 mm from the upper surface of the In _0.5 Ga _0.5 P-photodiode with a mesa structure 200 µm in diameter with in order to investigate the characteristics of the detector under irradiation.

The values of the X-ray quantum efficiency (QE) of In _0.5 Ga _0.5 P through the optical window of the device (the region not covered by contacts) were calculated using the Bouguer-Lambert-Beer law and assuming full charge accumulation in p- and i-layers.

In FIG. 10 shows the X-ray quantum efficiency of In _0.5 Ga _0.5 P as a function of photon energy up to 10 keV. The quantum efficiency values calculated for this structure were 53% at 5.9 keV and 44% at 6.49 keV. Table 3 below shows the attenuation coefficients at 5.9 keV and 6.49 keV for In _0.5 Ga _0.5 P as well as various other materials. The attenuation coefficients for binary and ternary connections have been estimated based on the appropriately weighted attenuation coefficients of their single elements.

An In _0.5 Ca _0.5 P device was installed inside the TAS Micro MT climate chamber for temperature control. First, the temperature was set at 100°C and decreased to 20°C in 20°C increments. Before making any measurements at each temperature, the device was left for 30 minutes to ensure stabilization. Leakage current In _0.5 Ga _0.5 P versus reverse bias was measured using a Keithley 6487 picoammeter/voltage source. with applied biases was 0.1% of their values plus 1 mV. Capacitance In _0.5 Ga _0.5 P versus reverse bias was measured using an HP 4275A Multi Frequency LCR meter (4275A multi-frequency LCR meter from Hewlett-Packard). The error associated with each capacitance reading was 0.12%, while the error associated with applied biases was 0.1% of their values plus 1 mV. The test signal was sinusoidal, cf. square amplitude 50 mV and frequency 1 MHz. When measuring both leakage current and capacitance, the reverse bias was raised from 0 V to 15 V (in 1 V steps).

X-ray spectra were obtained using ^{a 55} Fe radioisotope X-ray source to irradiate an In _0.5 Ca _0.5 P device with a diameter of 200 μm at temperatures from 100°C to 20°C. The experimental setup used a custom-made charge-sensitive preamplifier with a resistorless feedback design. The preamplifier was operated at the same temperature as the photodiode. The signal from the preamplifier was formed using an Ortec 572a shaping amplifier and digitized using a multichannel analyzer (Ortec Easy-MCA-8K). Spectra were collected and analyzed at 0.5 µs, 1 µs, 2 µs, 3 µs, 6 µs, and 10 µs shaping times. In each case, the In _0.5 Ga _0.5 P device was reverse biased at 0 V, 5 V, 10 V, and 15 V. The lifetime for each spectrum was 200 s. The experiments were carried out in a dry nitrogen atmosphere (relative humidity < 5%).

The measured leakage currents of the packaged devices at 100°C and 80°C are shown in FIG. eleven; leakage currents at temperatures below 80°C are not shown because they were below the intrinsic noise level of the picometer. Measurements of leakage currents versus reverse bias for an empty package showed that the diode package made a significant contribution to the measured leakage current. At 100°C and 80°C, the packaged device (defined as the combined semiconductor and package) had leakage currents of 1.5 pA and 0.5 pA, respectively, at a reverse bias of 10 V. Under the same temperature and reverse bias conditions, the empty package had leakage currents 1.1 pA and 0.2 pA respectively. By increasing the reverse bias to 15 V in each case, the leakage currents measured for the packaged device and the empty package became indistinguishable at both temperatures. Taking into account the errors associated with measuring leakage currents, the leakage current from the diode itself can be considered negligible compared to the leakage current from the package.

At different temperatures, the capacitance values of the packaged In _0.5 Ca _0.5 P detector were measured depending on the reverse bias. The capacitance of an empty package of the same type was also measured at different temperatures, and this capacitance was subtracted from the measured capacitance of the packaged In _0.5 Ga _0.5 P photodiode. At each temperature, the capacitance values were repeatedly measured and the mean and its relative standard deviation were determined. In the temperature range under study, it was found that the capacitance (C) of the In _0.5 Ga _0.5 P detector itself is temperature invariant. 1/C ² versus reverse bias at 100°C and at 80°C is shown in FIG. 12 and similar results were obtained at temperatures ≤ 60°C. A relationship between 1/C ² and reverse bias has been found at reverse biases below 3 V; 1/C ² was constant at reverse biases above 3 V.

X-ray spectra were obtained using a ⁵⁵ Fe radioisotope X-ray source. Although temperatures above 100° C. may be used, such results are not described in this document. At 100°С, the diode was stable during the entire spectrum acquisition time. The diode did not deteriorate after being used at the indicated temperatures.

An improvement in energy resolution (quantified with FWHM at 5.9 keV) was observed as the applied reverse bias was increased from 0 V to 5 V. This result can be explained by the reduction in detector capacitance and possibly improved charge storage. No further change in FWHM was observed during detector operation at reverse biases >5 V. This latter behavior can be explained by considering that the In _0.5 Ca _0.5 P photodiode is completely depleted above 5 V.

The optimal shaping time (ie, the time at which the lowest FWHM is achieved) varies with temperature, as shown in FIG. 13. FWHM decreased at lower temperatures due to lower leakage currents in the In _0.5 Ga _0.5 P photodiode and Silicon FET at these temperatures. In FIG. 13 shows the smallest observed FWHM of the 5.9 keV peak as a function of temperature at the optimum shaping time when the detector was reverse biased at 5 V.

The spectra with the best energy resolution (lowest FWHM) at 100°C and 20°C and with 5V reverse bias applied to the photodiode are shown in FIG. 14A and 14B, respectively. In addition, the Mn Kα (dashed line) and Mn Kβ (dashed-dotted line) peaks reconstructed from the convolution are also shown in each spectrum. The observed ⁵⁵ Fe photopeaks were a combination of the characteristic lines of Mn Kα (5.9 keV) and Mn Kβ (6.49 keV) of the ⁵⁵ Fe radioisotope X-ray source. To determine the FWHM of the 5.9 keV peaks in FIG. 13 and 14, a Gaussian approximation of the photo peaks was performed: the Mn Kα and Mn Kβ peaks were reconstructed from the detected combined photo peak. The approximation was made taking into account the relative intensities of the radioisotope X-ray source at 5.9 keV and 6.49 keV in the appropriate ratio, as well as taking into account the relative efficiency of the detector at these X-ray energies.

The energy resolution (FWHM) of an X-ray photodiode spectrometer without avalanche breakdown was degraded by Fano noise, charge trapping noise, and electronic noise. Fano noise is due to the statistical nature of the ionization process. At each temperature examined, the observed FWHM was larger than the expected Fano-limited energy resolution, indicating the presence of more powerful noise sources than the statistical charge generation process. In a photodiode X-ray spectrometer, electronic noise is caused by five different components: parallel white noise, series white noise, noise due to induced gate current, 1/f noise, and dielectric noise. Leakage currents from the detector and the preamplifier's silicon input FET (which ran uncooled at each temperature) affected the parallel white noise. The capacitances of the detector and input FET in the preamplifier affected the serial white noise and 1/f noise. Parallel white noise and series white noise are directly and inversely proportional to shaping time, respectively, while 1/f noise and dielectric noise are independent of shaping time.

The calculated parallel white noise, series white noise (adjusted for noise due to induced gate current), and 1/f noise at shaping times of 0.5 µs, 1 µs, and 10 µs with 5 V reverse bias applied to the photodiode are shown. in fig. 15A, 15B, and 15C, respectively. The graphs show the contributions of parallel white noise (open circles), serial white noise (solid squares), and 1/f noise (open triangles). The high parallel white noise observed at elevated temperatures and extended shaping times was not due to high detector leakage current, but to the higher current of the preamplifier's uncooled silicon input FET.

As described above, (1) the FWHM of the photopeak did not decrease with reverse biases >5 V, (2) the number of photopeak center of gravity channels did not increase with increasing reverse bias above 5 V, and (3) the spectra shown in FIG. 14 fit well with Gaussian distributions without steps or other distortions beyond the expected low energy smearing. Thus, it can be assumed that the squared difference between the FWHM at 5.9 keV and the calculated contributions (Fano noise, parallel white noise, serial white noise, and 1/f noise) can be attributed to dielectric noise (i.e., at under these conditions, the noise due to charge capture was negligible).

The temperature dependence of dielectric noise is shown in Fig. 16. FIG. 16 shows the equivalent noise charge of dielectric noise at 5.9 keV as a function of temperature with 5 V reverse bias applied to the photodiode. Over the temperature range of 100°C to 20°C, the contribution of dielectric noise at 5.9 keV decreased linearly with increasing temperature: at 100°C the calculated value was 94 e- (rms) ± 15 e- (rms), while at 20°C the calculated value was 68 e- (rms) ± 15 e - (compare square). The dependence of the squared equivalent charge of dielectric noise on temperature was approximated by the linear least squares method. Based on this, the total loss factor estimated from this, relating to all dielectrics in the spectrometer, was (8.5±0.8)×10 ^-3 . Comparison of standard deviations of the approximation with experimental errors showed the acceptability of the linear approximation, and thus the correctness of the calculated value of the total loss factor was confirmed.

In FIG. 17 shows the equivalent noise charge squared (ENCD2) of dielectric noise at 5.9 keV as a function of temperature. Also shown is a line of best fit calculated by a linear least squares fit.

The In _0.5 Ga _0.5 P spectrometer made it possible to operate at high temperatures (up to the investigated maximum of 100°C). It showed a better FWHM compared to that achieved with Al _0.52 In _0.48 P- and Al ₀ . ₈ Ga _0.2 As spectrometers. The ability to operate at such high temperatures (100°C), combined with their higher X-ray attenuation coefficients, makes In _0.5 Ga _0.5 P spectrometers preferred over the GaAs spectrometers reported in recent publications, which have the highest operating temperature 60°C. In addition, the In _0.5 Ga _0.5 P X-ray spectrometer performs better at 100°C compared to the Al _0.52 In _0.48 P X-ray spectrometers previously reported in publications. The FWHM at 5.9 keV for the In _0.5 Ga _0.5 P device was 1.27 keV at 100°C, compared to 1.57 keV for the Al _0.52 In _0.48 P device using a similar readout electronics in the device. In _0.5 Ga _0.5 P also has higher linear attenuation coefficients than Al _0.52 In _0.48 P.

Since the readout electronics used to characterize these materials were largely similar, the differences in the obtained FWHMs for these materials (GaAs, AlInP, InGaP) can be explained by taking into account the difference in their electron-hole pair creation energies and the values of the noise contribution to reading electronic circuits at high temperature. For example, a total noise of 86 e- (rms) at the output of the preamplifier corresponds to 840 eV in GaAs, 1.00 keV in In _0.5 Ga _0.5 P, and 1.08 keV in Al _0.52 In _0.48 P. The observed FWHM is 1.02 keV at 5.9 keV at 60°C for In _0.5 Ca _0.5 P-spectrometer was very close to the expected value. Therefore, the overall noise in the e-(rms) was similar in the GaAs and In _0.5 Ga _0.5 P spectrometers, since the preamplifier was limited by noises different from those due to detector leakage current at these temperatures. It should also be noted that the X-ray attenuation coefficients in In _0.5 Ga _0.5 P are much higher than in other materials such as SiC. Thus, even if the maximum achievable energy resolution with In _0.5 Ga _0.5 P is lower than with other materials, In _0.5 Ga _0.5 P may still be preferred for applications with low fluxes and high energies.

Fano-limited energy resolution refers to the process of generating charge by absorbing an X-ray photon and is the statistically limited energy resolution of an X-ray photodiode spectrometer without avalanche breakdown. The Fano-limited energy resolution (FWHM in eV) can be calculated using the formula (1) below:

where ε is the energy of creating electron-hole pairs in a semiconductor, F is the Fano coefficient, and E is the energy of X-ray photons. Different semiconductors have different Fano-limited energy resolutions for the same X-ray photon energy. This is because the Fano-limited energy resolution at each energy depends on the physical properties of the material (average energy of creation of electron-hole pairs and the Fano coefficient).

When measuring the creation energy of electron-hole pairs at room temperature (20°C), the charge created as a result of absorption of X-rays from ^{a 55} Fe radioisotope X-ray source in an In _0.5 Ga _0.5 P photodiode was measured relative to the charge created in the reference GaAs -photodiode with a mesa structure with a diameter of 200 μm. The structure of the GaAs device is shown in Table 4 below. In _0.5 Ga _0.5 P and GaAs detectors were connected in parallel with a custom low noise charge sensitive preamplifier.

Both In _0.5 Ga _0.5 P and GaAs photodiodes were independently biased with a reverse bias of 10 V. In turn, the spectra were collected using a radioisotope X-ray source, which separately irradiated the In _0.5 Ga _0.5 P device and the GaAs device. Using Gaussian distributions, the detected peaks of Mn Kα (5.9 keV) and Mn Kβ (6.49 keV) of the collected spectra were approximated; collected ⁵⁵ Fe X-ray spectra and fit peaks at 5.9 keV for the In _0.5 Ga _0.5 P detector and reference GaAs photodiode are shown in FIG. 18.

In FIG. 18 shows collected X-ray spectra at a reverse bias of 10 V obtained with an ln _0.5 Ga _0.5 P device and a reference GaAs photodetector, irradiated with ^{a 55} Fe radioisotope X-ray source. Also shown are the approximated 5.9 keV lines for the ln _0.5 Ga _0.5 P device (dotted line) and the reference GaAs photodetector (dashed line). For simplicity, the approximate Mn Kβ peaks at 6.49 keV are not shown, but they have been properly included in the approximation.

The amount of charge corresponding to each channel of the multichannel analyzer was calculated using the location of the zero energy peak of the preamplifier and the 5.9 keV peak detected using a reference GaAs photodiode. In this calculation, the energy of creation of electron-hole pairs in GaAs (4.184 eV ± 0.025 eV) was also used. Next, the energy (εInGaP) of creating electron-hole pairs in In _0.5 Ga _0.5 P was determined by the following formula (2):

where εGaAs is the energy of creating electron-hole pairs in GaAs, NGaAs, and NInGaP is the number of charges created in the reference GaAs detector and ln _0.5 Ga _0.5 P detector, respectively. The experimental value measured for εInGaP at room temperature (20°C) was 4.94 eV ± 0.06 eV. To investigate the performance of the In _0.5 Ga _0.5 P detector at higher reverse biases, the reverse bias was increased to 15 V and the experiment was repeated. In this case, the measured energy of creating electron-hole pairs was 4.90 eV ± 0.04 eV. The similarity of these values further confirms that the charge capture was negligible. If the charge capture were significant, then there would be a significant decrease in the apparent energy of creating electron-hole pairs at higher reverse biases due to increased charge transfer at a higher electric field.

The dependence of the energy of creation of electron-hole pairs in In _0.5 Ga _0.5 P on temperature was studied in the temperature range from 100°С to 20°С. For this set of measurements, the detector was separately (ie without a reference GaAs detector) connected to a custom low noise charge sensitive preamplifier and irradiated with a ⁵⁵ Fe radioisotope X-ray source. The temperature variation of the preamplifier itself with temperature was measured over the specified temperature range by connecting a stabilized pulse generator (Model BH-1 from Berkeley Nucleonics Corporation) to the test input of the preamplifier. Changing the location of the center of gravity of the peak of the pulse generator made it possible to separate the change in the characteristics of the preamplifier depending on temperature from the change in the energy of creating electron-hole pairs in the photodiode. The change in the location of the center of gravity of the pulse generator peak has been appropriately corrected for the change in test capacitance with temperature. Spectra were collected and photopeaks and peaks from the pulse generator were approximated using Gaussian distributions in order to determine the locations of their centers of gravity relative to the zero noise peak. The charge created in the In _0.5 Ga _0.5 P photodiode under the action of X-ray photons was correlated with the relative change in the location of the photopeak on the charge scale of the multichannel analyzer. The latter has been corrected for the change in conversion factor with temperature (determined from the peak of the pulser). The different amounts of charge created at different temperatures were due to the change in the energy (εInGaP) of creating electron-hole pairs in In _0.5 Ga _0.5 P. Then, at each temperature, the absolute value of εInGaP was calculated using the previously determined value of εInGaP at room temperature.

The energy of creating electron-hole pairs in In _0.5 Ga _0.5 P as a function of temperature is shown in FIG. 19. An undoubted weak trend was found confirming that the energy of creating electron-hole pairs in In _0.5 Ga _0.5 P increases with increasing temperature: at 100 ° C εInGaP = 5.02 eV ± 0.07 eV, while at 20 °С εInGaP=4.94 eV ± 0.06 eV. This trend is unexpected. It was usually assumed that the average energy of creating electron-hole pairs decreases linearly with increasing temperature. Such a decrease can be understood taking into account the dependence of the energy of creation of electron-hole pairs on the band gap of the material. According to previous publications, the empirical relationship between the creation energy of electron-hole pairs and the band gap in a semiconductor is linear. Since the band gap increases with decreasing temperature, a similar behavior is expected for the creation energy of electron-hole pairs, at least in part, due to a change in the band gap. Theoretical Monte Carlo calculations performed earlier for silicon predicted a decrease in the energy of creating electron-hole pairs in silicon as a function of temperature. Such a model took into account physical mechanisms such as X-ray absorption, atomic relaxation, and electron energy losses.

The calculated expected Fano-limited energy resolution (FWHM) at 5.9 keV in X-ray detectors made of In _0.5 Ga _0.5 P was 139 eV at 20°C. It was estimated using formula (1), determined values for the creation energy of electron-hole pairs, and assuming that the Fano coefficient is equal to 0.12. There was a negligible change in the temperature range from 20°C to 100°C.

The energy of creation of electron-hole pairs obtained at 27°C (300 K) (4.95 eV ± 0.03 eV) is in good agreement with the value predicted for In _0.5 Ga _0.5 P based on the Bertuccio-Maiocchi-Barnett relation (BMW ) (4.83 eV ± 0.21 eV).

In FIG. Figure 20 shows the average energy of creation of electron-hole pairs for Ge, Si, GaAs, Al _0.2 Ga _0.8 As, Al _0.8 Ga _0.2 As (solid circles) and In _0.5 Ga _0.5 P (solid squares) depending on their band gap at temperature 300K.

Approximation of these data by the linear least squares method showed that the previously published WMW dependence between the energy of creation of electron-hole pairs and the band gap can be refined using new data for In _0.5 Ga _0.5 P. The new ratio has the form: ε=AEg+В , where A=(1.62±0.08) and B=(1.79±0.13) eV.

Initially, the authors of the present invention demonstrated the use of an X-ray spectrometer with an InGaP detector in the temperature range from 100°C to 20°C. The characteristics of the spectrometer were determined for different values of the formation time and reverse biases of the detector. The best energy resolution (lowest FWHM) at 5.9 keV was 1.27 keV at 100°C using a shaping time of 0.5 µm, and it improved to 840 eV at 20°C (using a shaping time of 10 µs) when 5 V reverse bias was applied to the InGaP detector. An improvement in energy resolution (quantified by FWHM by 5.9 keV) was observed when the applied reverse bias was increased from 0 V to 5 V. The best results obtained at 5 V can be explained with taking into account improved charge storage at higher electric field strengths. FWHMs similar to those measured at 5 V were observed at 10 V and 15 V, confirming that charge-trapping noise at 5 V and above was negligible. System noise analysis showed that the observed FWHM was better than the likely statistically limited energy resolution (ie, Fano-limited energy resolution). Parallel white noise, series white noise, 1/f noise, and dielectric noise were calculated. The higher parallel white noise observed at elevated temperatures was due to the preamplifier's silicon input FET rather than the photodetector.

While circular photodiodes (when viewed from top to bottom) have been described in this document, other photodiode geometries are contemplated. Circular devices may be preferred for use in single pixel detectors because they maximize the ratio of top surface area to side wall area. For pixel array detectors containing a plurality of such photodiodes arranged in a one- or two-dimensional array, circular or other shaped devices may be desirable. For example, it may be desirable to use devices that are tiled, such as square or polygonal devices.

Various embodiments have been described in which a photodiode was used for spectroscopy and/or for locating an x-ray and/or gamma ray source. However, it is assumed that the photodiode can be used in other devices. For example, a photodiode can be used in a radioisotope microbattery (also known as a nuclear microbattery).

Nuclear microbatteries contain a radioactive source for emitting radioactive particles (ie alpha or beta particles) or photons and a detector that receives these particles or photons and converts them into electrical current. Such devices are desirable because they have a relatively long service life (eg > 10 years), high energy density and small size. These devices can be used in applications such as microelectromechanical systems (MEMS), implantable medical devices, and military equipment. It is desirable that nuclear microbatteries have the ability to operate in extreme environments (including high and low temperatures, extreme pressures, mechanical shock and vibration) as this may be required for certain applications such as aeronautics, astronautics, deep sea operations and equipment for hot and cold climatic conditions, for example for deserts.

The effect of operating temperature on the performance of a microvoltaic photovoltaic cell is important in many target applications because temperature can greatly influence the performance of a photovoltaic cell.

The present inventors have found that an InGaP-containing photodiode is particularly useful in a nuclear microbattery because, as described above, InGaP is surprisingly effective in converting X-rays and gamma rays into electrical charge carriers. In addition, the use of InGaP makes it possible to operate the microbattery in a relatively wide range of temperatures and at high doses of radiation with relatively little degradation in performance. These characteristics, combined with relatively high conversion efficiency and relatively low manufacturing costs (for example, it can be grown on commercially available GaAs substrates using conventional growth methods), make InGaP particularly suitable for use in X-ray and/or gamma-ray nuclear microbatteries.

It has also been found that the use of a nuclear microbattery containing a radioisotope X-ray source reduces the risk associated with damage to the device, compared to, for example, radioisotope sources of beta particles, since soft X-ray sources (for example, with photon energies < 10 keV) can be relatively easily shielded to provide a safe environment.

Hereinafter, by way of example only, an illustrative embodiment of a nuclear microbattery will be described to assist in understanding the present invention. It should be understood that the present invention is not limited to the inclusion of all of the above layers or the specific composition of the various layers in this example.

A microbattery was fabricated containing a 400 μm diameter mesa photodiode as described above for Table 1 and a 206 MBq ⁵⁵ Fe radioisotope X-ray source (Mn Kα=5.9 keV, Mn Kβ=6.49 keV) for irradiating said photodiode. . The X-ray source was placed at a distance of 5 mm from the upper surface of the detector (ie, with the PIN structure between the X-ray source and the substrate (layer 7 in Table 1)).

The microbattery was placed inside a TAS Micro MT climate chamber in a dry nitrogen atmosphere (relative humidity < 5%). Current characteristics versus forward bias applied (0V to 1V in 0.01V steps) were measured with a Keithley 6487 picoammeter/voltage source over a temperature range of -20°C to 100°C.

In FIG. 9 shows the electric current generated by the microbattery as a function of the forward bias on the photodiode for various temperature conditions.

Although the present invention has been described with reference to preferred embodiments, those skilled in the art will appreciate that various changes in form and detail may be made without departing from the scope of the present invention as set forth in the appended claims.

Claims (26)

1. A photodiode for use in detecting x-rays and/or gamma rays, comprising PIN junction formed by an undoped InGaP layer located directly between a p-doped InGaP layer and an n-doped InGaP layer; or a p-n junction formed by a p-doped InGaP layer in contact with an n-doped InGaP layer; wherein said PIN junction or p-n junction is positioned and configured to absorb x-rays and/or gamma rays incident on said photodiode and generate charge carriers in response thereto. 2. The photodiode according to claim 1, containing a housing or screen located and configured to prevent photons other than photons of x-rays and / or gamma radiation from entering the InGaP, and / or located and configured to prevent the entry of radioactive beta- and/or alpha particles on InGaP. 3. Photodiode according to claim. 2, in which the housing or screen is made of metal, in particular of metal foil. 4. Photodiode according to any one of paragraphs. 1-3, in which the i-layer of the PIN junction has the following thickness: ≥ 5 μm, ≥ 10 μm, ≥ 15 μm, ≥ 20 μm, ≥ 25 μm, ≥ 30 μm, ≥ 35 μm, ≥ 40 μm, ≥ 45 µm or ≥ 50 µm. 5. Photodiode according to any one of paragraphs. 1-4, in which the p-layer and/or n-layer of the PIN junction have the following thickness: ≤ 0.5 μm, ≤ 0.4 μm, ≤ 0.3 μm, ≤ 0.2 μm, or ≤ 0.1 µm. 6. Photodiode according to any one of paragraphs. 1-5, containing an electrode on each side of a PIN junction or a p-n junction for applying voltage to said junction and/or for measuring photogenerated charge carriers generated in said junction, wherein at least one of said electrodes does not cover a portion of a side of said junction , on which it is located, so that X-rays and/or gamma rays can pass to the specified transition through the specified side without passing through the specified at least one electrode. 7. A photodiode according to claim 6, wherein said at least one electrode is annular, has a hole, has a notch, or is interdigitated such that it does not completely cover said side and allows said x-rays and/or gamma radiation to pass through. radiation to the specified transition without passing through the material forming the specified electrode. 8. Photodiode according to any one of paragraphs. 1, 4-7, containing a voltage source configured and configured to supply a reverse bias voltage to the specified PIN junction or p-n junction, and the specified voltage is ≥ 2 V, 10 ≥ 3 V, ≥ 4 V, ≥ 4.5 V or ≥ 5 V. 9. A photodiode according to any one of the preceding claims, wherein InGaP is a crystalline structure having the structural composition In _0.5 Ga _0.5 P. 10. A photodiode according to any one of the preceding claims, having a mesadiode structure. 11. Photodiode according to any one of paragraphs. 1-9, in which the InGaP material is provided within a substantially planar structure containing Schottky contacts, and/or in which ions have been implanted into the device to form electrical resistance regions. 12. An X-ray and/or gamma-ray detector, comprising a photodiode according to any one of the preceding claims, and also comprising an electronic circuit for processing an electrical signal generated by the photodiode to determine whether said electrical signal is due at least in part to the generation of said photogenerated carriers charge. 13. X-ray and/or gamma-ray photon counting spectrometer, comprising a detector according to claim 12 and a processor configured and configured to determine the energies of individual x-ray and/or gamma-ray photons detected by a PIN junction or a p-n junction in said detector, based on said electrical signal, and/or with the ability to determine the number of X-ray and/or gamma-ray photons detected by the PIN junction or p-n junction in said detector, based on said electrical signal. 14. System for detecting x-rays and/or gamma rays, containing a source of x-rays and/or gamma rays and a photodiode according to any one of paragraphs. 1-11, a detector according to claim 12 or a spectrometer according to claim 13 for detecting x-rays and/or gamma rays from said source. 15. The system according to claim 14, which is a nuclear or radioisotope battery containing the specified x-ray and / or gamma radiation source, and the specified photodiode for converting the specified x-ray and / or gamma radiation into electric current. 16. The system of claim 15, wherein said source is radioactive material. 17. The system according to claim 15 or 16, in which said source and photodiode are placed inside the housing, and said housing, if necessary, is located and configured to substantially prevent the exit of the specified x-ray radiation and/or gamma radiation of the specified source from the specified housing. 18. A method for detecting x-rays and/or gamma rays, comprising the steps of: exposing the x-ray and/or gamma ray detector according to claim 12 to an x-ray and/or gamma ray source; And on the basis of the signal generated by the photodiode, the reception of x-rays and/or gamma radiation by said detector is determined. 19. The method of claim 18, comprising the preliminary steps of selecting said X-ray and/or gamma radiation source and moving the detector in the direction of said source. 20. The method of claim 18, further comprising determining the presence and/or location of said x-ray and/or gamma source using a detector.

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