DIAMOND RADIATION DETECTOR

Abstract: A radiation detector comprises a boron-doped diamond substrate (10) having an overlayer (12) of diamond epitaxially grown on surface (14) of the substrate (10). The top surface (16) of the layer (12) is provided with an interdigitated electrode array (18) in electrical contact therewith.

Description: DIAMOND RADIATION DETECTOR BACKGROUND OF THE INVENTION This invention relates to a radiation detector Diamond can be used to respond electrically to radiation which penetrates only a small distance (e g less than 10μm) into diamond such as alpha particles and electromagnetic radiation with a wavelength of less than approximately 220nm e g ultra-violet and soft X-rays Current diamond detectors for such radiation consist of a thin layer of diamond generally about 1 to 200μm in thickness of as-grown diamond either free-standing or a non-diamond substrate such as a silicon substrate Typically the growth surface of the thin layer will be patterned with an mterdigitated electrode array Diamond is a wide band gap semiconductor and at room temperature it is under normal circumstances an electrical insulator For a pure diamond to conduct electrons must be promoted from the normally full valence band to the normally empty conduction band creating electron-hole (e-h) pairs this occurs when radiation such as gamma-rays X-rays ultra-violet light alpha particles and beta particles impinges on the diamond If there is an electric field across the diamond the carriers will move and a current the photocurrent will flow The size of the photocurrent for a particular diamond will depend on the type and intensity of the radiation and it will flow until the e-h pairs recombine The charge carriers which are produced by the radiation are typically collected by the mterdigitated electrode array on the growth surface of the layer US Patent 5,216 249 describes a neutron detector comprising a layer of polycrystallme diamond material deposited by chemical vapour deposition the diamond material containing sufficient amounts of 10 B as a dopant to optimise the neutron detection characteristics of the detector SUMMARY OF THE INVENTION According to the present invention a radiation detector particularly for radiation such as gamma-rays X-rays, ultra-violet light alpha particles and beta particles, comprises an overlayer of diamond grown on a surface of a boron-doped diamond substrate The substrate is boron-doped diamond and may be polycrystallme or single crystal in nature The diamond may be natural or synthetic diamond in which the boron doping is achieved by ion implantation by introduction into the growth capsule in a high pressure/high temperature synthesis or naturally The diamond may also be produced by chemical vapour deposition (CVD) in which event the boron doping will generally be achieved during synthesis of the diamond The boron atoms may be in substitutional or interstitial positions in the diamond lattice The boron content of the boron-doped diamond will typically be in the range 10 1 ' to 10 21 boron atoms per cm 3 The boron-doped diamond substrate will typically have a thickness of 0 1 to 2mm The radiation detector has an overlayer of diamond which is produced epitaxially on a surface of the boron-doped diamond substrate The diamond of the overlayer will have a grain size comparable with that of the substrate Thus, if the boron-doped diamond of the substrate has a grain size of 20 to 50μm the gram size of the diamond of the overlayer will be 20 to 50μm The thickness of the diamond overlayer will typically be in the range 1 urn to 500μm, preferably in the range 3μm to 50μm The overgrown diamond layer may also preserve some of the crystalline features of the substrate and thus be, for instance, of larger grain size than a layer of the same thickness grown on a non-diamond substrate The overgrown diamond will preferably be grown by CVD Methods of depositing diamond on a substrate by CVD are now well established and have been described extensively in the patent and other literature Where diamond is being deposited on a substrate, the method generally involves providing a gas mixture which, on dissociation can provide hydrogen or a halogen (e g F,CI) in atomic form and C or carbon containing radicals and other reactive species, e g CH X , CF X wherein x can be 1 to 4 In addition oxygen containing sources may be present, as may sources for nitrogen, and for boron In many processes inert gases such as helium, neon or argon are also present Thus, a typical source gas mixture will contain hydrocarbons C x H y , wherein x and y can each be 1 to 10 halocarbons C x Y y Hal z wherein x, y and z can each be 1 to 10, optionally one or more of the following CO,C0 2 0 2 H 2 N 2 NH 3 , B 2 H b and an inert gas Each gas may be present in its natural isotopic ratio, or the relative isotopic ratios may be artificially controlled, for example hydrogen may be present as deuterium or tritium and carbon may be present as 2 C or 13 C Dissociation of the source gas mixture is brought about by an energy source such as microwaves lasers RF energy a flame or a hot filament and the reactive gas species so produced are allowed to deposit onto a substrate and form diamond In one preferred form of the invention, the surface on which the CVD diamond overlayer is grown is a polished surface The surface may be polished to a low roughness, for example an RA (or centre line average) of less than 30nm With such a polished surface the overlayer will have a much lower as-grown roughness than one grown on a rough surface In use, the radiation detector will typically include a first electrical contact applied to the overlayer, and a second electrical contact applied to or in electrical contact with the substrate Further according to the invention a method of detecting or measuring radiation includes the steps of providing a radiation detector of the type described above and exposing a surface of the diamond overlayer to the radiation BRIEF DESCRIPTION OF THE DRAWING The drawing illustrates a schematic view of an embodiment of a radiation detector of the invention DESCRIPTION OF EMBODIMENTS An embodiment of the invention will now be described with reference to the attached drawing which is a schematic view of a radiation detector Referring to this drawing a boron-doped CVD diamond substrate 10 has a thin high quality, overlayer 12 of diamond epitaxially grown using CVD on surface 14 of the substrate 10 The surface 14 may be a polished surface The boron content of the substrate 10 will typically be in the range 10 1 to 10 2 ' boron atoms per cm 3 The overlayer 12 will have a grain size comparable with that of the substrate The top surface 16 of the layer 12 is provided with an mterdigitated electrode array 18 in electrical contact therewith The array 18 comprises a plurality of electrodes 20 A back surface contact 22 is provided on the substrate 10 and is connected to earth The mterdigitated electrode array 16 is connected to a current or charge-measuring system through a suitable isolation circuit (represented schematically in the drawing as a capacitor 24) 26 indicates the bias voltage In use, radiation to be detected impinges on the thin, high quality layer 12 E-h pairs are produced and these separate under the influence of the external bias voltage A current is induced in the external circuit which is measured by the current or charge-measuring system The magnitude of the current/charge gives a measure of the radiation intensity In a second embodiment (not illustrated), an mterdigitated electron array is fabricated on the surface of the high quality layer 12 One set of electrodes is biased to a voltage of between -1000 V and +1000 V and the second set is connected to earth In use radiation to be detected impinges on the thin high quality layer 12 E-h pairs are produced and these separate under the influence of the external bias voltage A current is induced in the external circuit which is measured by the current or charge-measuring system The magnitude of the current/charge gives a measure of the radiation intensity The radiation detector described and illustrated has several advantages over prior art diamond detectors First the large gram size of the overgrown layer gives better performance Second the number of single grains bridging the gap between adjacent electrodes in the mterdigitated electrode array is increased This leads to an increase in the signal amplitude for a given radiation intensity Third, the substrate, being electrically conductive can be used as the back electrode Fourth the defect density in the overgrown layer is lower due to the large gram size of that layer Fifth the detector is considerably more robust The invention is further illustrated by the following example Example A boron-doped diamond layer with dimensions 4,5 x 4,5 x 0,8mm and a measured surface roughness R a of less than 30nm was shown by SIMS to contain a uniform distribution of boron at a level of 10 19 atoms/cm 3 This was used as a substrate for the overgrowth, by means of CVD of a high purity diamond layer with thickness 80μm The substrate, with overgrown diamond layer, was processed to produce a product with a thin high quality layer with thickness 10μm and R a < 30nm on top of the boron-doped substrate SIMS profiling showed that no boron could be detected on the high purity side of the interface The final dimensions of the product were 4,5 x 4,5 x 0,81 mm The structure was found to be useful as a radiation detector in an arrangement as illustrated by Figure 1

Claims: CLAIMS 1 A radiation detector comprises an overlayer of diamond grown on a surface of a boron-doped diamond substrate 2 A radiation detector for use in detecting radiation selected from gamma rays X-rays ultra-violet light alpha particles and beta particles 3 A radiation detector according to claim 1 or claim 2 wherein the boron- doped diamond is polycrystallme or single crystal in nature 4 A radiation detector according to any one of the preceding claims wherein the boron-doped diamond is CVD diamond 5 A radiation detector according to any one of the preceding claims wherein the boron content of the boron-doped diamond is in the range 10 1 ' to 10 21 boron atoms per cm 3 6 A radiation detector according to any one of the preceding claims wherein the diamond of the overlayer has a grain size comparable with that of the substrate 7 A method according to any one of the preceding claims wherein the boron-doped diamond of the substrate has a grain size of 20 to 50μm and the diamond of the overlayer has a grain size of 20 to 50μm 8 A radiation detector according to any one of the preceding claims wherein the boron-doped diamond substrate has a thickness of 0 1 to 2mm A radiation detector according to any one of the preceding claims wherein the thickness of the diamond overlayer is 1 μm to 500μm A radiation detector according to any one of the preceding claims wherein the thickness of the diamond overlayer is 3 to 50um A radiation detector according to any one of the preceding claims wherein the diamond of the overgrown layer is grown by a CVD A radiation detector according to claim 1 1 wherein the CVD diamond overlayer is grown on a polished surface A radiation detector according to claim 12 wherein the polished surface has an RA of less than 30nm A method of detecting or measuring radiation includes the steps of providing a diamond detector according to any one of the preceding claims and exposing a surface of the overlayer to the radiation A radiation detector substantially as herein described with reference to the accompanying drawing A method of detecting or measuring radiation substantially as herein described with reference to the drawing

Inventors: WHITEHEAD ANDREW JOHN, WHITEHEAD, ANDREW, JOHN
Applicants: DE BEERS IND DIAMOND, WHITEHEAD ANDREW JOHN, DE BEERS INDUSTRIAL DIAMONDS (PROPRIETARY) LIMITED, WHITEHEAD, ANDREW, JOHN
Application year: 2001
Main classification: G01T1
IPC codes: G01T1, H01L31