CZ307721B6 - Scintillation detector for the detection of ionizing radiation - Google Patents

Abstract

Scintillation detector for the detection of ionizing radiation comprising at least two nitride semiconductor layers (3, 4, 5, 6) arranged in a layered heterostructure, the structure of which contains at least one potential pit layer (5) for radiant electron and hole recombinations, and further comprising a heterostructure consisting of at least one active pair of nitride semiconductor layers (4, 5) consisting of a barrier layer (4) and a potential pit layer (5), wherein at least one upper nitride semiconductor layer (7) is arranged above the uppermost active pair of layers (4, 5). Further, at least one layer (6) with a graded composition that is adjacent to the outermost layer (5) of the potential pit is inserted on at least one side of the active region and / or into the active region to reduce the potential barrier and facilitate migration of electrons and holes into the active region of the heterostructure.

Description

Technical field

The invention relates to nitride heterostructure-based semiconductor scintillation detectors for the detection of ionizing radiation, in particular electron, X-ray and particle radiation.

BACKGROUND OF THE INVENTION

It is known that semiconductor materials with a wide band gap such as GaN or ZnO are suitable for use in ionizing radiation detectors. These semiconductor materials exhibit a short afterglow time of excited luminescence in the order of nanoseconds. The advantage of scintillators based on gallium nitride (GaN) and gallium nitride alloys with other metal nitrides is their radiation resistance and the possibility of their preparation with high crystallographic quality in the form of heterostructures consisting of monocrystalline epitaxial layers with different composition superimposed on large surfaces of monocrystalline substrates. These heterostructures exhibit low non-radiant losses and a narrow luminescence maximum. Nitride luminescent structures with InGaN quantum wells are the subject of a number of patents relating to emission diodes, as well as several patents relating to scintillator structures.

US 2002/0195606 (AI) deals with a luminescent nitride structure of an emission diode containing multiple InGaN / Al (In) GaN quantum wells surrounded by Al (In) GaN layers with n-type doping from one side of the active region and p-type doping from the other side of the active area.

Another well-known document is US 2014/0138617 A1, the invention of which is concerned with similar nitride luminescence heterostructure and grid matching of barrier and quantum pit layers, similar to the foregoing patent in a diode structure wherein the active region is surrounded by layers with p-type doping and with n-type doping or aluminum-containing layers. Although the lattice adaptation contemplated in this application reduces piezoelectric charge at the layer interface, the charge and barriers caused by the spontaneous polarization component in the structure continue to cause curvature of the band structure and the formation of barriers for electrons or holes at the edges of the active region.

U.S. Pat. No. 7053375 (B2) discloses a semiconductor scintillator for excitation by ionizing radiation in the form of a semiconductor consisting of a compound of elements III. groups of the periodic table in a nitrogen compound. The semiconductor compound is structured into a layer formed on a generally described substrate. Further, there may be a so-called backing layer between the semiconductor layer and the substrate for smoothing / improving the semiconductor structure. Various nitrogen compounds with element III. The groups and their alloys can be used in different layers superimposed to form heterostructures.

Another known patent document US 8 164 069 (B2) describes a fluorescent agent responsive to the impact of electrons by light emission, the so-called luminescence. The fluorescent agent comprises a carrier monocrystalline substrate, a nitride semiconductor sandwich structure in which barrier layers alternate with layers representing potential pits. The semiconductor layers form a heterostructure that is arranged on the surface of one side of the substrate. The potential pits are preferably formed by an alloy semiconductor In _x Gai_ _x N.

In the Czech national patent CZ 306026 (B6), a nitride scintillation heterostructure with fast luminescence persistence is achieved, which is achieved by increasing the overlap of electron and hole wave functions, either by balancing the polarization of the barrier layers with the layers representing

Or by embedding a thin inverse potential barrier into the quantum well. The backing layers of the structure may be doped with silicon. The invention solves more efficient excitation of the active region, but does not address facilitating the transport of charge carriers to the active region.

The disadvantages of the aforementioned patent documents are that they either relate directly to light emitting diodes (US 2002/0195606 (AI), US 2014/0138617 (AI)) or adopt the concept used in light emitting diodes except for the absence of pn transition in the structure and using a higher number of quantum wells (US 7053375 (B2), US 8164069 (B2)). The above documents do not take into account a sufficiently different way of exciting the structure and do not discuss the efficiency of excitation of the active scintillator layers.

In the case of emission diodes, the structure is excited by passing an electric current and the charge carriers (electrons and holes) are injected into the active region from their opposite sides, see Fig. 1 (a). However, when the heterostructure is excited by incident ionizing radiation, the charge carriers are generated at the same location of the heterostructure, where both charge carriers must then migrate to the active region in the same direction in order to achieve the desired luminescence. caused by the pn junction, see Fig. 1 (a), or by the actual piezoelectric field between the interfaces of the individual layers, see Fig. 1 (b).

Currently, n-type doping is used to equalize the electric field in the structure, most often using silicon atoms, see Figure 1 (c). The N-type endowment helps to align the electric field in the active layer, but at the edges of the active layer, more specifically at the first interface with the quantum well, there is a potential barrier that prevents one type of charge carrier from penetrating into the active area. In the case of scintillators, the active region can only be made wide enough for all ionizing radiation to be absorbed only in the active layer. For larger thicknesses of the active region, the crystallographic quality deteriorates due to the stresses in the structure.

Disadvantages are furthermore that the above patents do not consider the formation of this barrier which, when migrating the charge carriers in the same direction, prevents one of the charge carriers from penetrating into the active region, while the other type of charge carriers can penetrate into the active region without barrier. 3a. The barrier, which makes it difficult for one of the charge carriers to penetrate the active region, substantially reduces the intensity of the luminescence.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a scintillation detector for detecting ionizing radiation that removes the drawbacks of the known solutions, i.e. to allow both types of charge carriers to migrate from their generation to the active region with similar probabilities. ionizing radiation.

SUMMARY OF THE INVENTION

The object is solved by providing a scintillation detector for detecting ionizing radiation, in particular electron radiation, X-ray radiation, and particle radiation, according to the invention.

The scintillation detector comprises a single crystalline substrate on which at least one undercoat is deposited to bond at least one nitride semiconductor layer to the substrate by epitaxy. At least one nitride semiconductor layer is described by the general formula AlyIn _x Gai_ _x _yN, where 0 <x <1.0, 0 <y <la0 <x + y <1. At the same time, at least two nitride semiconductor layers are arranged in a layered heterostructure whose structure contains at least one layer of a potential well for radiant recombination of electrons and holes. Further, at least one n-type doped bottom layer is provided on the backing layer, over which an active heterostructure region consisting of at least one active pair of nitride semiconductor layers composed of AlybIn _X bGai_ _X b_ybN barrier layer and AlywIn _xw Gai potential pit layer is arranged. - _xw -ywN for which xb <xw and yb> yw. Doping with n-type

The dopant is used to partially equalize the piezoelectric field in the structure. Further, at least one upper nitride semiconductor layer is provided in the direction away from the substrate above the active pair of layers which is located at the top.

The principle of the invention is that on at least one side of the active region and / or within the active region at least one layer of Aly _g In _X gGa- _X gy _g N with a graded composition of less than 5 nm thickness is adjacent and adjacent to the potential layer. to reduce the potential barrier and facilitate the migration of electrons and holes to the active region of the heterostructure.

This layer of graded composition may be on only one edge of the active region according to the predominant absorption of ionizing radiation in the heterostructure, or it may be used on both sides of the active region. Alternatively, the graded layer can also be used within the active region, especially in the case of thick barrier layers. The purpose of this graded layer is to reduce the potential barrier that must be overcome by electrons or holes when penetrating into the active region from the backing or cover layers of the heterostructure, which also absorb ionizing radiation and generate an electron-hole pair. Thus, graded composition layers will facilitate the migration of electrons and holes into the active layer, thereby achieving a substantial increase in luminescence intensity upon detection of ionizing radiation, especially if the ionizing radiation is absorbed outside the active layer, as shown in Figures 4 and 5. the edge profile of the valence and conductive strip for graded layer structures is shown in Fig. 3 for the graded layer from the side of the underlayers.

In a preferred embodiment of the scintillation detector according to the invention, the graded composition layer has a continuously varying composition. The smooth composition improves the charge carrier transfer properties.

On the one hand, the graded layer has the same composition as the bottom layer, barrier layer or top layer, i.e., Aly _P In _X pGaI- _xp -ypN. In the direction of the potential pit, however, the composition of the graded composition layer changes by changing yp to yg, xp to xg, 1-xp-yp to 1-xg-yg, xg <xw and yw <yg. Content of individual elements III. groups of the Periodic Table of Elements in this layer gradually change from the values in the adjacent layer to the values of the potential pit layer, ie eg the proportion of aluminum changes in the layer with graded composition from yp to yg, indium is changed from xp to xg the percentage of gallium from 1-xp-yp to 1-xg-yg. In this case, xg <xw and yw <yg, where xp and yp express the proportion of indium and aluminum in the adjacent layer and xw and yw express the proportion of indium and aluminum in the potential pit layer.

In a preferred embodiment the composition of the graded composition layer changes from ₀ to In GaN, o3Gao, 97N.

Advantages of the scintillation detector produced in accordance with the present invention include the higher luminescence intensity achieved by more efficient filling of the active region with electrons and holes generated in the underlying or covering layers, by reducing the potential barrier for electrons or holes between the active region and the dominant electron-hole generation region. couples.

Clarification of drawings

The present invention will be explained in more detail in the following drawings, where:

Fig. 1a shows a schematic representation of the energy progress of the edge of a conductive and valence band of an emission diode nitride heterostructure with potential pits according to the prior art;

Fig. 1b shows a schematic representation of the energy profile of a conductive and valence strip-like heterostructure without the presence of surrounding layer doping, where the resulting electric field is caused by polarizing charge at the interfaces; heterostructures with n-type grants 5 to 10 ^{cm-1 X} 'in the layer below the active region, FIG. 2 shows a schematic representation of a heterostructure of layers with the layers in potential jam of the present invention with an interposed layer of graded composition below and above the active region, FIG. 3A shows calculated waveform edges valence and conduction bands for the structure of the prior art with the n-type grants ^lx 5-10 cm ³ in a layer below the active zone (with description of the highlighted layers and barrier for electrons at the lower edge of active region), Fig. 3b shows the calculated Fig. 4a shows a comparison of the cathodo-luminescence spectra of the sample with the usual structure of the ten potential wells and the sample with the graded barriers on the side of both the underlay and the cover layers according to Example 3, where the spectra was measured by excitation of electrons with kinetic energy of 2 keV for electron penetration into the 30 nm structure and generation of electron hole pairs above the active region, Fig. 4b shows comparison of cathode luminescence spectra of sample with usual structure of ten potential wells and sample with graded barriers of the cover layers according to Example 3, where the spectra were measured by exciting electrons with a kinetic energy of 4 keV for electron penetration into the 100 nm structure and generation of electron hole pairs above the active region, Fig. 4c shows a comparison of cathodo luminescence spectra in a spectra with the usual structure of ten potential wells and a sample with graded barriers from both the substrate and coating layers according to Example 3, where the spectra were measured by exciting electrons with a kinetic energy of 12 keV for electron penetration into 600 nm and generation of hole pairs above the active region; Fig. 5 illustrates the luminescence of the sample of Example 4 measured at 325 nm excitation at which most electron-hole pairs are generated outside the active region. Fig. 5b illustrates the photo luminescence of the sample of Example 4 measured at 375 wavelength excitation. nm, in which most electron-hole pairs are generated directly in the active region, where electrons and holes also recombine.

DETAILED DESCRIPTION OF THE INVENTION

It is understood that the specific embodiments of the invention described and illustrated below are presented by way of illustration and not by way of limitation of the invention to the examples given. Those skilled in the art will find or will be able to provide, by routine experimentation, more or less equivalents to the specific embodiments of the invention described herein. These equivalents will also be included within the scope of the following claims.

1a to 1c show the conductivity and valence band for heterostructures showing ten potential wells, and Fig. 1a is a schematic representation of the energy profile of the conductive and valence band edge of an emission diode nitride heterostructure with potential wells embedded in the p-n transition.

-4GB 307721 B6

Fig. 1b is a schematic representation of the energy profile of the edge of a conductive and valence band of a similar heterostructure in the absence of doping of surrounding layers, where the resulting electric field is caused by polarizing charge at the heterogeneous interfaces.

Fig. 1c is a schematic representation of the energy progress of the edge of a conductive and valence band of a similar heterostructure with a n-type doping of 5-10 ¹⁸ cm -1 according to the prior art.

Figure 2 schematically illustrates a layered heterostructure according to the present invention. There is shown a schematic representation of layers 3, 4, 5, 6 and 7 of the heterostructure with the potential well layers 5 according to the invention and with the intermediate layers 6 with gradual composition below and above the active region of the layered heterostructure. The layered heterostructure is formed on a monocrystalline substrate 1 with five layers 5 of potential wells of thickness di alternately interspersed with four barrier layers 4 of thickness ch forming the active region of the layered heterostructure of total height h. applying an epitaxial backing layer 3 with a layered thickness 3 of epitaxy to the monocrystalline substrate E A layer 6 with a graded composition <T less than 5 nm is inserted between the bottom layer 3 and the first layer 5 of the potential pit. The same is true between the top layer 7 of thickness and the last layer 6 of the potential pit.

In a non-illustrated embodiment of the invention, a graded composite layer 6 may be interposed between at least two active layer layers 4 and 5.

The composition of the graded composition layer 6 varies with increasing distance from the adjacent layer to the other side of the potential pit layer 5. The graded composite layer 6 fluently changes from one particular material of the adjacent layer to the material of the potential pit layer 5.

Fig. 3a shows in more detail the calculated course of the conductive and valence strip in a structure with ten layers 5 of potential pits for heterostructure prepared according to the prior art with n-type Si doping 5-10 ' ^x cni ³ in the bottom layer 3 below the active area with a barrier that prevents effective penetration of electrons into the active area.

Fig. 3b shows the calculated conductivity and valence band pattern in a structure with ten layers of 5 potential wells for heterostructure prepared according to the present invention with n-type Si doped 5- 1 () ^1x cni ³ in the layer below the active area and layer 6 of graded composition closely adjacent to the bottom layer 5 of the potential well in the active region. In comparison with FIG. 3a, there is a significant reduction in the electron barrier.

Example 1 - A layered heterostructure with a layer 6 of graded composition arranged on the side of layers 2 and 3 suitable for detecting X-rays.

The scintillator includes a multilayer semiconductor nitride heterostructure with 4N GaN backing layer 2 prepared by MOVPE technology. The nitride backsheet 3 is doped with silicon having a concentration of 5-10 ^{x 1} cm ³ and has a thickness of 300 nm. The barrier layers 4 are GaN and have a thickness of 7 nm in the active region. The layers of the 5 potential wells with a narrow band gap are made of Ino, o6Gao, 94N and have a thickness di 1.5 nm. Between the bottom layer 3 and the bottom potential layer 5 of the potential pit at the edge of the active region is a layer 6 with a graded composition of thickness 3 nm. The composition of this layer 6 fluently changes from GaN from the bottom layer 3 to Ino.coGao 4 N at the edge of the potential pit layer 5. The thickness of the topsheet 7 is 100 nm.

Fig. 3b shows the calculated energy profile of the edge of the conductive and valence bands for this structure with a significant reduction in the electron barrier.

-5GB 307721 B6

Example 2 - A layered heterostructure with a layer 6 of graded composition arranged on the side of the cover layer 7 suitable for detecting low energy UV, ions and electrons.

Scintillators include a multilayer semiconductor nitride heterostructure with 4 m thick GaN underlay 2 prepared by MOVPE technology. The nitride backsheet 3 is doped with 5-10 ¹⁸ cm ³ silicon and has a thickness of 300 nm. The barrier layers 4 are formed of GaN and have a thickness d? 7 nm. The layers of the 5 potential wells with a narrow band gap are made of Ino, o6Gao, 94N and have a thickness di 1.5 nm. Between the cover layer 7 and the uppermost layer 5 of the potential pit lying on the edge of the active area, a layer 6 with a graded composition d ₃ 3 nm is inserted. The composition of this layer 6 fluently changes from GaN from the top layer 7 to Ino, by ₃ Gao, 97N at the edge of the potential pit layer 5. The thickness of the topsheet 7 is 100 nm.

Example 3 - A layered heterostructure with a layer 6 of graded composition arranged on the side of the topsheet 7 and the backsheet 3 suitable for the detection of electrons with a wide range of energies.

The scintillators include a multilayer semiconductor nitride heterostructure with 4 Tm GaN backing layer 2 prepared by MOVPE technology. The nitride backsheet 3 is doped with 5-10 ¹⁸ cm ³ silicon and has a thickness of 300 nm. The barrier layers 4 are GaN and have a thickness of 7 nm in the active region. The layers of the 5 potential wells with a narrow band gap are made of Ino, o6Gao, 94N and have a thickness di 1.5 nm. Between the cover layer 7 and the top potential pit layer 5 at the edge of the active area and further between the bottom layer 3 and the bottom potential layer 5 at the edge of the active area, layers 6 with a graded composition d _{3 of} 3 nm are interposed. The composition of the layers 6 changes continuously from GaN to Ino, by ₃ Gao, 97N at the edge of the potential pit layer 5 by changing the epitaxial growth temperature from 730 to 800 ° C. The thickness of the topsheet 7 is 30 nm.

Figure 4 shows the effect of using layers 6 on increasing cathodoluminescence intensity for the structure of Example 3, with layers 6 of 3 nm thickness before and after the last InGaN layer 5 in the active region. The increase in luminescence is shown on cathodo luminescence spectra measured for the detection of electrons with different depths of penetration into the heterostructure. The most significant increase in luminescence intensity is achieved with electron absorption below and above the active region, see Figs. 4a and 4c.

Example 4 - Layered heterostructure with layers 6 between all InGaN layers 5 and GaN barrier layers 4 suitable for detection of a wide range of ionizing radiation

Scintillators include a multilayer semiconductor nitride heterostructure with GaN backing layer 2 with L 4 Im thickness prepared by MOVPE technology. The nitride backsheet 3 is doped with silicon having a concentration of ^{5-0 x 1} cm ³ and has a thickness t of 300 nm. The barrier layers 4 are GaN and have a thickness of 7 nm in the active region. The narrow band gap layers 5 consist of Ino, o6Gao, 94N and have a thickness di 1.5 nm. Between each layer 5 and the barrier layer 4 is a layer 6 of graded composition having a thickness d _{3 of} 2 nm. The composition of this layer 6 fluently changes from Ino, by ₃ Gao, 97N at the edge of layer 5 to GaN at the edge of the barrier layer 4, the change in composition is achieved by continuously changing the temperature during epitaxy of this layer from 720 to 800 ° C. The thickness of the topsheet 7 is 50 nm.

Figure 5a shows the effect of using layers 6 of d ₃ 2 nm thickness between each InGaN layer 5 and the GaN barrier layer 4 (structure according to Example 4) to increase the photoluminescence intensity when the exciting radiation is absorbed above the active region In this sample, layer 6 was applied at the upper boundary of each layer 5 with the barrier layer 4. Light excitation with a wavelength of 375 nm at which the hole electron is excited directly in the layers 5 was used to verify that the amplification luminescence was achieved through the insertion of layer 6 and more efficient penetration of charge carriers into the active region.

-6GB 307721 B6

In Fig. 5b it is shown that if the charge carriers are generated directly in the active region without the need for migration, no luminescence enhancement occurs in the same structure.

Industrial applicability

The scintillation detector for the detection of ionizing radiation, in particular electron, x-ray or particle radiation, according to the invention finds, inter alia, applications in medical fields working with ionizing radiation, in electron microscopes, in instruments requiring rapid detection for research or for analysis of materials and products. especially in applications of quality diagnostics of integrated circuits and other electronic components, further in micro-radiography, including fast high-resolution CT systems and in many other research fields such as astronomy, particle physics, etc. The proposed solution is particularly useful in scintillation detectors with different penetration depths of ionizing radiation.

PATENT CLAIMS

Claims (3)

Scintillation detector for detecting ionizing radiation, in particular electron, x-ray or particle radiation, comprising a single crystal substrate (1) on which at least one substrate layer (2) is deposited for bonding at least one nitride semiconductor layer (3, 4, 5, 6) ) on the substrate (1) by epitaxy, at least one nitride semiconductor layer (3, 4, 5, 6) is described by the general formula Al _y In _x Ga 1 _{x y y} where 0 <x <1.0, 0 <y <la0 <x + y <1, wherein at least two nitride semiconductor layers (3, 4, 5, 6) are arranged in a layered heterostructure, the structure of which comprises at least one potential well layer (5) for radiant electron and hole recombination, (2) there is arranged at least one bottom layer (3) with n-type doping, above the bottom layer (3) there is an active heterostructure region consisting of at least one active pair of nitride semiconductor layers v (4, 5) composed of a barrier layer (4) of the type Al _y bIn _x bGai_ _x b_ _y bN and a layer (5) of the potential pit of the type Al _yw In _X wGai- _{xw yw} N to which xb <xw and yb> yw, wherein at least one upper nitride semiconductor layer (7) is arranged in the direction away from the substrate (1) above the uppermost active pair of layers (4, 5), characterized in that on at least one side of the active region and / or inside the active region at least one layer (6) of graded composition is inserted to reduce the potential barrier and facilitate the migration of electrons and holes into the active region of the heterostructure adjacent to the potential pit layer (5), its thickness (c10) is less than 5 nm and its composition varies in the direction toward the layer (5) of the potential well of the general formula Al _{xp yp} in Gai_ _{xp yp} _ N in the general formula Al _X in _YG gGai- _X G _YG N, it being understood that XG <xw and yw <YG. Scintillation detector according to claim 1, characterized in that the graded composition layer (6) has a continuously varying composition. Scintillation detector according to claim 1 or 2, characterized in that the composition of the graded composition layer (6) varies from GaN to Ino, 13Gao, 97N.