RAPID PULSE ANNEALING OF CDZNTE DETECTORS FOR REDUCING ELECTRONIC NOISE
A combination of doping, rapid pulsed optical and/or thermal annealing, and unique detector structure reduces or eliminates sources of electronic noise in a CdZnTe (CZT) detector. According to several embodiments, methods of forming a detector exhibiting minimal electronic noise include: pulse-annealing at least one surface of a detector comprising CZT for one or more pulses, each pulse having a duration of ˜0.1 seconds or less. The at least one surface may optionally be ion-implanted. In another embodiment, a CZT detector includes a detector surface with two or more electrodes operating at different electric potentials and coupled to the detector surface; and one or more ion-implanted CZT surfaces on or in the detector surface, each of the one or more ion-implanted CZT surfaces being independently connected to one of the two or more electrodes and the surface of the detector. At least two of the ion-implanted surfaces are in electrical contact.
The United States Government has rights in this invention pursuant to Contract No. DE-AC52-07NA27344 between the United States Department of Energy and Lawrence Livermore National Security, LLC for the operation of Lawrence Livermore National Laboratory.
FIELD OF THE INVENTIONThe present invention relates to CdZnTe (CZT) detectors, and more particularly, this invention relates to fabrication techniques and corresponding structures for reducing electronic noise in CZT detectors by reducing bulk and surface leakage current.
BACKGROUNDHigh resolution, room temperature spectroscopy of gamma rays requires semiconductor gamma detectors such as CdZnTe or TlBr. In order to achieve the best performance, these detectors utilize advanced electrical contact and readout schemes including pixilation, co-planar grids, and hemispherical contacts.
These schemes require an electric field both through the bulk of the device and often across a given surface. This results in electronic noise from leakage current, which can be defined as either bulk or surface leakage current. Electronic noise arises from current injected into a CdZnTe (CZT) detector that flows along the surface and/or through the bulk thereof; current generated by defects along the surface of the CZT, bursts of anomalous noise, and/or buildup of charge at non-Ohmic contacts.
The current state of the art for CdZnTe gamma detectors uses contacts deposited on an as-polished surface, such as the detector 100 shown in
Notably, the doped portions 106, 108 are not in physical contact, nor are they electrically coupled/contacting. A doped layer 110 is formed, again optionally, on a surface of the bulk portion 102 opposite the surface onto which contacts 104 are formed; and a final Ohmic contact layer 112 is formed on a surface of the doped layer 110 opposite the bulk portion 102. One or more surfaces of the bulk portion 102 and/or doped portions/layer 106, 108, 110 may be treated via etching and oxidation to generate passivating layers 114 on surface(s) of the corresponding portions, as shown in
Conventional detector configurations such as shown in
Accordingly, it would be useful to provide systems and techniques that minimize or eliminate injected current and defect generated noise, enabling operation of detectors at higher field strengths to improve signal collection ability.
SUMMARYAccording to one embodiment, a method of forming a detector exhibiting minimal electronic noise includes: pulse-annealing at least one surface of a detector comprising CdZnTe (CZT) for one or more pulses, each pulse being characterized by a duration of approximately 0.1 seconds or less.
According to another embodiment, a method of forming a detector exhibiting minimal electronic noise includes: pulse-annealing at least one surface of a detector comprising ion-implanted CdZnTe (CZT) for one or more pulses, each pulse being characterized by a duration of approximately 0.1 seconds or less.
According to yet another embodiment, a CdZnTe (CZT) detector includes a detector surface with two or more electrodes operating at different electric potentials and coupled to the detector surface.
Other aspects and advantages of the present invention will become apparent from the following detailed description, which, when taken in conjunction with the drawings, illustrate by way of example the principles of the invention.
For a fuller understanding of the nature and advantages of the present invention, as well as the preferred mode of use, reference should be made to the following detailed description read in conjunction with the accompanying drawings.
The following description is made for the purpose of illustrating the general principles of the present invention and is not meant to limit the inventive concepts claimed herein. Further, particular features described herein can be used in combination with other described features in each of the various possible combinations and permutations.
Unless otherwise specifically defined herein, all terms are to be given their broadest possible interpretation including meanings implied from the specification as well as meanings understood by those skilled in the art and/or as defined in dictionaries, treatises, etc.
It must also be noted that, as used in the specification and the appended claims, the singular forms “a,” “an” and “the” include plural referents unless otherwise specified.
The following description discloses several preferred embodiments of CdZnTe (CZT) detectors, and more particularly, this invention relates to fabrication techniques and corresponding structures for reducing electronic noise in CZT detectors by reducing bulk and surface leakage current and minimizing charge buildup at contacts.
Electronic noise as described herein generally refers to two sources: dark current and anomalous burst noise.
Dark current includes electrical current flowing through a photosensitive detector even in the absence of any photons entering the detector. Dark current may arise from random generation of electrons and holes in the detector structure. Sources of dark current of primary interest in the context of the present disclosure includes current that flows along the surface of the detector (i.e. horizontally between contacts 104 as shown in
The inventors have found that novel fabrication techniques using a combination of ion implantation, pulsed optical and/or thermal annealing, and employing unique detector structures (e.g. utilizing P-I-N homojunctions as described in further detail below), can reduce noise within the detector, particularly noise generated by one or more of the foregoing mechanisms. Particularly, pulsed annealing and unique detector structures (such as shown in
In particular, rapid, pulsed annealing with characteristic times of <100 ms is required in order to activate the dopants without damaging the bulk transport properties of the crystal. Annealing at temperatures >150° C. detrimentally affects the transport properties and thus the spectroscopic characteristics of the crystal. By using rapid pulses, the thermal load is minimized and only the near surface region is heated. Further, it has been shown that rapid, pulsed annealing of conventional detector structures without doping, an example of which is shown in
For instance, conventional structures such as shown in
Accordingly, the inventive concepts presented herein, in several embodiments, involve methods and corresponding structures that reduce dark current (including bulk and surface leakage current) by a factor of two or more, as well as the anomalous bursting noise in CZT detectors. In this manner, the electronic noise component, particularly of the gamma spectrum, is reduced for a given field and, if desirable, higher strength fields can be applied to the detector to improve signal collection. Without wishing to be bound to any particular theory, the inventors postulate the dark current reduction is a result of activation of intentional dopants to form p and n type regions and thus junctions, improved crystallinity and removal of defects from surfaces, and activation of intentional dopants between electrodes to pin the Fermi level near the middle of the band gap.
According to one general embodiment, a method of forming a detector exhibiting minimal electronic noise includes: pulse-annealing at least one surface of a detector comprising CdZnTe (CZT) for one or more pulses, each pulse being characterized by a duration of approximately 0.1 seconds or less.
According to another general embodiment, a method of forming a detector exhibiting minimal electronic noise includes: pulse-annealing at least one surface of a detector comprising ion-implanted CdZnTe (CZT) for one or more pulses, each pulse being characterized by a duration of approximately 0.1 seconds or less.
According to yet another general embodiment, a CdZnTe (CZT) detector includes a detector surface with two or more electrodes operating at different electric potentials and coupled to the detector surface.
Referring now to
The detector 200 includes a bulk portion 202 comprising, preferably consisting of, a suitable detector material for detecting target radiation including but not limited to gamma rays. For example, the bulk portion 202 may comprise an undoped, intrinsic semiconductor or a semiconductor with intentional dopants that pin the Fermi level in the mid gap in order to increase resistivity, in various approaches. According to one approach, bulk portion 202 comprises or consists of a CdZnTe composition. The respective mole fraction of the cadmium, zinc, and tellurium may be varied according to knowledge generally available in the art to optimize the detector 200 for detecting the target radiation without departing from the scope of the presently disclosed inventive concepts.
On opposite surfaces of the bulk portion 202 are formed contacts 204a, 204b, 212, with the contacts 204a, 204b, 212 preferably being formed in, on, or adjacent to doped regions 206 and 210, respectively (doped regions may be equivalently referred to as “ion-implanted surfaces” in accordance with the presently disclosed inventive concepts). In various embodiments in accordance with
In one approach in accordance with the embodiment shown in
In other embodiments (e.g. as shown in
In all cases, the area between contacts, including top and/or side surfaces of the detector, may preferably be passivated to reduce leakage current through the use of rapid pulse annealing, optionally with an additional dopant species selected to increase the resistivity of the material 202 either through reduced mobility or reduced carrier concentration. Passivation may be accomplished using etching and chemical oxidation, implantation of a particular dopant species, and/or rapid pulse annealing of the corresponding region, with pulse annealing being preferred. For instance, and with reference to
In some approaches (e.g. as shown in
In additional embodiments, different metals may be used for the contacts 204a, 204b, and/or 212, e.g. in order to reduce leakage current, with metals selected depending on the dopant species and Ohmic contacts being preferred to reduce leakage current and reduce charge buildup under the contacts, which can adversely affect performance.
For example, for a contact coupled to a p-type region, a metal with a high work function (Au, Pt, Ni, etc.) would be preferred to form an Ohmic contact. For a contact coupled to an n-type region, a metal with a low work function (e.g. Al, In, etc.) would be preferred to form an Ohmic contact.
As noted above, contacts 204a, 204b, and/or 212 in various embodiments may be provided in the form of metal (preferably Ohmic) contacts, pixels, steering grids, coplanar grids, etc. in any combination. According to preferred embodiments of the respective structures shown in
In various approaches, the doped regions (particularly 206 and 208) may be in contact with each other (e.g. as shown in
Accordingly, in various embodiments such as shown in
Preferably, the dopants employed in the regions 206, 208, 210 are chosen so as to create a diode structure such as an N-P-N, a P-N-P, an N-I-N, a P-I-N, a P-I-P, or a PN type diode, with P-I-N structures being particularly preferred. Most preferably, the detector 200 includes both surface and bulk diodes, e.g. a surface P-I-N diode between an n-doped region 206 and a p-doped region 208, and a bulk P-I-N diode between n-doped region 206 and p-doped region 210.
Again with reference to
For certain embodiments of detector 210 as shown in
In accordance with preferred embodiments of detector 220 as shown in
In preferred variants of detector 230 as shown in
Now with reference to preferred embodiments of detector 240 as shown in
Regarding detector 250 as shown in
Accordingly, the contacts 204a, 204b on, in or adjacent to regions 206, 208 operate at different electric potentials, and are preferably electrically coupled (whether or not regions 206, 208 are in physical contact, as shown according to the embodiment of
Critically, during formation of the detector 200, doped regions 206, 208 and/or 210 (preferably all doped regions) are subjected to heating in the form of rapid, pulsed annealing in order to incorporate the doping species into the lattice of the CdZnTe and to repair any damage resulting from ion implantation. It has also been observed that the use of rapid, pulsed annealing on conventional detectors, such as shown in
Finally, according to any of the embodiments shown in
It will be understood by skilled artisans reading the present descriptions that the various features set forth above individually with respect to
The inventive structure of detectors such as shown in
All of the foregoing advantages serve, particularly in combination, to significantly reduce the amount of noise generated by the detector, reducing false positive events and enabling improved signal detection, e.g. via the use of higher magnitude electric fields within the detector. Moreover still, investigators may have greater confidence in the magnitude of detector response being due to presence of target radiation rather than noise generated by the detector, and may derive more accurate information regarding the amount of radiation present in the detection environment.
Of course, in various embodiments different detector element configurations, may be employed.
For example, a pixelated CZT detector structure 300 is shown from a top-down view in
Preferably, such structures are characterized by including contacts (e.g. 204a, 204b) which operate at different electric potentials. In various embodiments, any number of contacts included in complex detector arrangements may operate at the same, or at different potentials, without departing from the scope of the presently disclosed inventive concepts.
For instance, in one approach a plurality of contact pairs (again, e.g. 204a, 204b) may be incorporated into the detector structure, where each contact pair includes a first contact (e.g. 204a) operating at a first electric potential and a second contact (e.g. 204b) operating at a second electric potential. In more approaches, different contact pairs may operate at different electric potentials, e.g. a first contact pair having a first contact operating at a first electric potential and a second contact operating at a second electric potential, while a second contact pair includes a third contact operating at a third electric potential and a fourth contact operating at a fourth electric potential.
Contact pairs, according to some embodiments, may “overlap” such that one contact may be shared between two or more contact pairs (e.g. a central contact may be a member of four contact pairs, each including one of the vertically or horizontally adjacent contacts of a grid-like structure such). In other more complex embodiments, contact “groups” may include any number or arrangement of contacts such as lines, square/rectangular blocks, etc. as would be understood by persons having ordinary skill in the art upon reading the present descriptions. According to these more complex arrangements, any number of contacts may be shared among overlapping contact groups.
Turning now to
In still more embodiments, a detector may be configured as a coplanar grid 320 such as shown in
As mentioned briefly above,
As noted above, the inclusion of doped junction structures within detectors in accordance with the presently disclosed inventive concepts advantageously contributes to reduction of noise originating from the detector and not associated with a photon interacting with the detector.
The presently disclosed inventive concepts include the notion that rapid, pulsed annealing is required to activate the implanted dopants without heating the bulk of the detector, which can result in degraded performance. In addition, it includes the concept that this rapid, pulsed annealing can also reduce electronic noise when performed on unimplanted, conventional detectors such as shown in
In various embodiments, pulse-annealing may employ an optical source, e.g. using optical energy sources such as flash lamps, light emitting diodes (LEDs), lasers, or other suitable high intensity optical sources capable of emitting photons with energies at or above a band gap of the material(s) to be annealed. In one particular embodiment, a laser was employed to anneal B and/or N dopants, incorporated into a CZT detector via implantation at a concentration in a range from approximately 1×1016-1×1019 atoms/cm3. The experimental results in
In more embodiments, pulse-annealing may employ chemical sources, such as by leveraging intermetallic or thermitic reaction schemes. In embodiments where a chemical energy source is employed, preferably the reactants necessary to perform the intermetallic or thermitic reaction are incorporated into portions of the detector structure to be annealed, e.g. at least in the boundaries (i.e. near surfaces) of regions to be annealed. In preferred embodiments, reactants are present in or near adjacent surfaces of doped regions 206, 208 and more preferably throughout doped regions 206, 208. Reactants may be preferably separated from the surface of the CdZnTe by a thin film, e.g. SiO2 or Si3N4, which acts as a protective and sacrificial layer such that the CdZnTe is not damaged by the chemical reaction. The thin film is chosen such that it can easily be removed by chemical or plasma etchants which do not etch the CdZnTe surface.
While some approaches recognize the use of thermal annealing (e.g. by incubating the detector at ˜>300° C.) to activate dopants, the presently disclosed inventive fabrication techniques are distinct in the use of pulse-annealing, in which surface(s) of a detector are exposed to an appropriate energy source for a short duration (e.g. in a range from approximately 1 ns to approximately 100 ms) for a plurality of pulses (e.g. in a range from 1-1,000,000 pulses, in various embodiments). The pulse source is designed to deliver sufficient energy over a short time period such that the doped region is heated to the desired annealing temperature, but with a small total thermal load such that when the heat diffuses into the bulk of the CZT the average temperature is <150° C.
Accordingly, in various approaches detector structures exhibiting reduced electronic noise in accordance with the presently disclosed inventive concepts preferably include a CZT bulk detector having at least one surface with two or more electrodes (contacts) formed thereon. At least two of the two or more electrodes operate at different electric potentials, which generally results in increased dark current, but in accordance with the inventive structures disclosed herein the detector structure is characterized by a reduction of dark current by a factor of two or more relative to conventional CZT detector structures.
The reduction in the case of undoped structures is likely due to annealing of surface defects and for doped structures is due to activation of dopants, but generally is observed for structures fabricated using techniques as described herein, preferably including at least pulse-annealing of detector surfaces.
The exemplary inventive detector structure may optionally but preferably include ion-implanted and/or ion-doped surfaces or regions, which are preferably each connected to one or more of the electrodes formed on the surface of the detector. More preferably, each ion-implanted and/or ion-doped surface/region is independently coupled to one of the electrodes. In various embodiments, ion-implanted and/or ion-doped surfaces/regions may form junctions (preferably heterojunctions), semi-insulating regions, charge steering regions, and/or high-conductivity implants. In more embodiments, high-conductivity implants may include any suitable conductive material, whether ionic or otherwise.
In various embodiments, the ion-implanted CdZnTe surfaces may include one or more ionic species of one or more materials selected from: Group III, Group V, and Group VII.
Turning now to particular methods of fabricating inventive detector structures as described herein,
As shown in
As noted above, and in further embodiments of method 800, the pulse-annealing may involve optical annealing, thermal annealing, or combinations thereof. Optical annealing may be performed using one or more of a flash lamp, light emitting diode, laser, or other high intensity optical source with photon energies at or above the band gap of the material.
Thermal annealing may include initiating a thermitic reaction on the at least one surface of the detector or in a thin film in close proximity to the surface(s) of the detector. Optionally, the detector may be protected or separated from the thermite material by a thin protective barrier of suitable composition and configuration, which may selected based on knowledge generally available in the art. Thermal annealing may additionally or alternatively include an exothermic intermetallic reaction, for example Ni—Al forming NiAl. As with thermitic reactions, the intermetallic reaction is preferably initiated in a film in close proximity to the CZT, either touching the surface thereof or separated therefrom by a thin protective barrier.
In accordance with another embodiment, a method 900 of forming a detector exhibiting minimal electronic noise includes operation 902, in which at least one surface of a detector comprising ion-implanted CdZnTe is pulse-annealed for one or more pulses, each pulse being characterized by a duration of approximately 0.1 seconds or less. A notable distinction between methods 800 and 900 is that method 800 requires pulse annealing surface(s) of a CZT detector, while method 900 specifies the pulse-annealed surfaces include ion-implanted CZT.
As such, a person having ordinary skill in the art will appreciate that electronic noise inherent to detectors (particularly diodes) may be reduced by pulse-annealing ion-implanted as well as non-ion-implanted surfaces of the detector. As prior work has focused exclusively on annealing ion-implanted surfaces for extended durations (e.g. 30 seconds or more), the presently disclosed inventive techniques represent a novel development and application of thermal annealing in a pulsed manner to reduce noise rather than activate implanted ions.
Skilled artisans will further appreciate that any of the additional or alternative features, operations, etc. set forth above with respect to
The presently disclosed inventive techniques and structures formed thereby are characterized by minimal electronic noise, and therefore are particularly well suited for application as radiation detectors. More specifically, the inventive concepts presented herein are useful in applications such as detecting very low amounts of gamma radiation. The added sensitivity achieved using the inventive concepts described herein is a result of plasma etching, chemical oxidation, and rapid pulse-annealing surfaces of the detector structures, which enables higher operational field strength and improves detection capability.
The inventive concepts disclosed herein have been presented by way of example to illustrate the myriad features thereof in a plurality of illustrative scenarios, embodiments, and/or implementations. It should be appreciated that the concepts generally disclosed are to be considered as modular, and may be implemented in any combination, permutation, or synthesis thereof. In addition, any modification, alteration, or equivalent of the presently disclosed features, functions, and concepts that would be appreciated by a person having ordinary skill in the art upon reading the instant descriptions should also be considered within the scope of this disclosure.
While various embodiments have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of an embodiment of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.
Claims
1. A method of forming a detector exhibiting minimal electronic noise, the method comprising:
- pulse-annealing at least one surface of a detector comprising CdZnTe (CZT) for one or more pulses, each pulse being characterized by a duration of approximately 0.1 seconds or less.
2. The method as recited in claim 1, further comprising chemically oxidating some or all portions of the at least one surface of the detector.
3. The method as recited in claim 1, wherein the pulse-annealing comprises optical annealing; and
- wherein the optical annealing is performed using at least one optical source selected from: a flash lamp, and a light emitting diode.
4. The method as recited in claim 1, further comprising plasma etching some or all portions of the at least one surface of the detector from: a flash lamp, a light emitting diode, and a laser.
5. The method as recited in claim 1, wherein the pulse-annealing comprises thermal annealing using a chemical energy source.
6. The method as recited in claim 5, wherein the thermal annealing comprises initiating a thermitic reaction on the at least one surface of the detector.
7. The method as recited in claim 5, wherein the thermal annealing comprises: an exothermic intermetallic reaction.
8. The method as recited in claim 5, wherein pulse-annealing the detector reduces dark current generated by the detector by a factor of at least 2 relative to a dark current generated by the detector prior to the pulse-annealing.
9. A method of forming a detector exhibiting minimal electronic noise, the method comprising:
- pulse-annealing at least one surface of a detector comprising ion-implanted CdZnTe (CZT) for one or more pulses, each pulse being characterized by a duration of approximately 0.1 seconds or less.
10. The method as recited in claim 9, further comprising chemically oxidating some or all portions of the at least one surface of the detector.
11. The method as recited in claim 9, wherein the pulse-annealing comprises optical annealing; and
- wherein the optical annealing is performed using at least one optical source selected from: a flash lamp, and a light emitting diode.
12. The method as recited in claim 9, further comprising plasma etching some or all portions of the at least one surface of the detector.
13. The method as recited in claim 9, wherein the pulse-annealing comprises thermal annealing using a chemical energy source.
14. The method as recited in claim 13, wherein the thermal annealing comprises initiating a thermitic reaction on the at least one surface of the detector.
15. The method as recited in claim 13, wherein the thermal annealing comprises: an exothermic intermetallic reaction initiated in a film in close proximity to the CZT.
16. The method as recited in claim 13, wherein pulse-annealing the detector reduces dark current generated by the detector by a factor of at least 2 relative to a dark current generated by the detector prior to the pulse-annealing.
17. A CdZnTe (CZT) detector comprising:
- a detector surface with two or more electrodes operating at different electric potentials and coupled to the detector surface; and
- one or more ion-implanted CZT surfaces on or in the detector surface, each of the one or more ion-implanted CZT surfaces being independently connected to one of the two or more electrodes and the surface of the detector, wherein at least two of the ion-implanted surfaces are in electrical contact.
18. The detector of claim 17, wherein the two or more electrodes are arranged in a coplanar grid.
19. The detector as recited in claim 17, wherein the ion-implanted CZT surfaces comprise one or more ionic species of one or more materials selected from: Group III, Group V, and Group VII.
20. The detector as recited in claim 17, comprising a passivating layer formed on the surface of the detector in a region between the electrodes.
21. The detector as recited in claim 17, comprising one or more additional ion-implanted CZT surfaces on or in the detector surface, each of the one or more additional ion-implanted CZT surfaces being located in a region of the detector surface that is not connected to any of the two or more electrodes.
22. The detector as recited in claim 21, wherein the one or more ion-implanted CZT surfaces comprise one or more ionic species of one or more materials selected from Group VII.
Type: Application
Filed: Nov 3, 2016
Publication Date: May 3, 2018
Inventors: Lars Voss (Livermore, CA), Adam Conway (Livermore, CA), Art Nelson (Livermore, CA), Rebecca J. Nikolic (Oakland, CA), Stephen A. Payne (Castro Valley, CA), Erik Lars Swanberg, Jr. (Livermore, CA)
Application Number: 15/343,081