U.S. patent application number 15/343081 was filed with the patent office on 2018-05-03 for rapid pulse annealing of cdznte detectors for reducing electronic noise.
The applicant listed for this patent is Lawrence Livermore National Security, LLC. Invention is credited to Adam Conway, Art Nelson, Rebecca J. Nikolic, Stephen A. Payne, Erik Lars Swanberg, Jr., Lars Voss.
Application Number | 20180122977 15/343081 |
Document ID | / |
Family ID | 62013856 |
Filed Date | 2018-05-03 |
United States Patent
Application |
20180122977 |
Kind Code |
A1 |
Voss; Lars ; et al. |
May 3, 2018 |
RAPID PULSE ANNEALING OF CDZNTE DETECTORS FOR REDUCING ELECTRONIC
NOISE
Abstract
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 .about.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.
Inventors: |
Voss; Lars; (Livermore,
CA) ; Conway; Adam; (Livermore, CA) ; Nelson;
Art; (Livermore, CA) ; Nikolic; Rebecca J.;
(Oakland, CA) ; Payne; Stephen A.; (Castro Valley,
CA) ; Swanberg, Jr.; Erik Lars; (Livermore,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Lawrence Livermore National Security, LLC |
Livermore |
CA |
US |
|
|
Family ID: |
62013856 |
Appl. No.: |
15/343081 |
Filed: |
November 3, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 31/02966 20130101;
G01T 1/24 20130101; H01L 31/117 20130101; H01L 31/1864 20130101;
H01L 31/115 20130101; Y02P 70/50 20151101; H01L 31/1832 20130101;
H01L 31/02161 20130101; H01L 31/022408 20130101; Y02E 10/50
20130101 |
International
Class: |
H01L 31/115 20060101
H01L031/115; G01T 1/24 20060101 G01T001/24; H01L 31/18 20060101
H01L031/18; H01L 31/0296 20060101 H01L031/0296; H01L 31/0224
20060101 H01L031/0224; H01L 31/0216 20060101 H01L031/0216 |
Goverment Interests
[0001] 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.
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.
Description
FIELD OF THE INVENTION
[0002] The 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.
BACKGROUND
[0003] High 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.
[0004] 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.
[0005] 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 FIG. 1. The detector 100 includes a bulk portion 102
composed of a suitable detector material for the particular target
to be detected (e.g. cadmium-zinc-tellurium (CdZnTe, or CZT) for
detecting gamma radiation). The bulk portion 102 has formed on a
surface thereof metal contacts 104, comprising a material suitable
for use as an electrode, e.g. gold or aluminum (which act as Ohmic
or Schottky contacts), and the contacts 104 may optionally be
formed on doped portions 106, 108 of the surface of the bulk
portion 102. Ohmic contacts typically display higher leakage
current while Schottky contacts may display charge buildup at the
interface, distorting the electric field. The optional doped
portions 106, 108, which are notably excluded from most
conventional CZT detector structures, may be doped with a suitable
dopant species such as aluminum or phosphorous. Alternatively, a
layer of amorphous silicon or selenium (not shown) may be formed in
place of the doped portions 106, 108, and may extend across the
entire width of the structure as shown in FIG. 1.
[0006] 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 FIG. 1.
[0007] Conventional detector configurations such as shown in FIG. 1
can achieve low leakage current but still suffer from injected
current at the contacts which acts as a source of electronic noise.
In addition to the leakage current, anomalous bursting noise is
commonly observed at high biasing fields. Finally, buildup of
charge at non-Ohmic contacts can distort the applied electric
field. Because of this, detectors are operated at lower fields than
may be optimal.
[0008] 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.
SUMMARY
[0009] According 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.
[0010] 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.
[0011] 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.
[0012] 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.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] 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.
[0014] FIG. 1 is a simplified schematic of a conventional CZT
detector structure, shown from a side view.
[0015] FIG. 2A is a simplified schematic side-view of an improved
CZT detector structure exhibiting reduced electronic noise relative
to the conventional CZT detector structure of FIG. 1, according to
one embodiment of the presently disclosed inventive concepts.
[0016] FIGS. 2B-2F depict simplified schematic side views of
several additional exemplary embodiments of improved CZT detector
structures exhibiting reduced electronic noise relative to the
conventional CZT detector structure of FIG. 1.
[0017] FIG. 3A is a simplified schematic top-down view of a
pixelated CZT detector, according to one embodiment of the
presently disclosed inventive concepts.
[0018] FIG. 3B is a simplified schematic top-down view of a
pixelated CZT detector including a steering grid, according to one
embodiment of the presently disclosed inventive concepts.
[0019] FIG. 3C is a simplified schematic top-down view of a
coplanar grid CZT detector, according to one embodiment of the
presently disclosed inventive concepts.
[0020] FIG. 4 is a chart depicting reduced surface leakage current
of a conventional CZT detector after pulsed flash lamp annealing,
According to one embodiment.
[0021] FIG. 5 is a chart showing activation of B and N dopants in
CZT after implantation and laser annealing
[0022] FIGS. 6A and 6B are charts depicting impact of presence of a
doped junction in a CZT detector structure on detector current and
voltage both on the surface and through the bulk, according to one
embodiment of the presently disclosed inventive concepts.
[0023] FIGS. 7A and 7B shows images of a CZT surface exposed to
laser fluence below and above the damage threshold, respectively,
according to one embodiment.
[0024] FIG. 8 is a flowchart of a method, according to one
embodiment of the presently disclosed inventive concepts.
[0025] FIG. 9 is a flowchart of a method, according to one
embodiment of the presently disclosed inventive concepts.
DETAILED DESCRIPTION
[0026] 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.
[0027] 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.
[0028] 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.
[0029] 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.
[0030] Electronic noise as described herein generally refers to two
sources: dark current and anomalous burst noise.
[0031] 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 FIG.
1, also referred to as "surface current") and/or through its bulk
(i.e. vertically through the bulk portion 102 as shown in FIG. 1,
also referred to as "bulk current," exemplified by current
traveling from contact 104 to contact 112, or vise-versa); as well
as current generated by defects along the surface of the detector
(although such defects are generally associated with anomalous
burst noise). In addition, the use of non-Ohmic contacts at 104,
and/or 112 may lead to build up of charge that distorts the applied
electric field and degrades performance.
[0032] 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
FIGS. 2A-2F and described in greater detail below) convey
advantageous reductions in detector noise relative to conventional
structures (e.g. as shown in FIG. 1) and techniques for fabrication
thereof.
[0033] 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.degree. 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
FIG. 1, can decrease the electronic noise and improve their
performance.
[0034] For instance, conventional structures such as shown in FIG.
1 exhibit significant bulk leakage current due to the presence of
surface and interface defects and lack of junctions such as a P-I-N
structure to minimize such sources of noise; in addition, the use
of non-Ohmic contacts to reduce leakage current may result in
charge buildup underneath the contacts 104.
[0035] 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.
[0036] 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.
[0037] 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.
[0038] 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.
[0039] FIGS. 2A-F represent simplified schematic side-views of
various embodiments of a CZT detector in accordance with the
presently disclosed inventive concepts.
[0040] Referring now to FIG. 2A, a detector 200 in accordance with
a preferred embodiment of the presently disclosed inventive
concepts may have a structure substantially identical to the
conventional detector 100 shown in FIG. 1, but exhibits reduced
electronic noise as a result of using pulse-annealing to form the
dopant regions 206 and/or 210 with improved crystallinity and
reduced incidence of defects on the surfaces of the detector 200.
Data demonstrating the advantages conveyed by pulse annealing an
undoped conventional detector structure such as shown in FIG. 1
(but lacking doped portions 106, 108 and 110, as is the case for
most conventional CZT detectors) are presented in FIG. 4, and
described further below.
[0041] 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.
[0042] 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 FIGS. 2A-2F, contacts 204a, 204b, and/or 212 (but especially
204a and 204b) may be in the form of a metal electrode, a pixel, a
steering grid, a coplanar grid. Exemplary embodiments of these
arrangements will be discussed in greater detail below regarding
FIGS. 3A-3C. In the particular embodiment shown in FIG. 2A, where
the detector 200 exhibits a bulk P-I-N configuration, the contacts
204a, 204b, and 212 are metal contacts, preferably Ohmic metal
contacts.
[0043] In one approach in accordance with the embodiment shown in
FIG. 2A, the upper (e.g. anode) surface of the detector is doped
only with one (preferably p-type) species in both regions 206,
while a different (preferably n-type) species is included in doped
region 210 near the lower (e.g. cathode) surface. The dopants,
according to preferred embodiments, are chosen to form a bulk P-I-N
diode, however other diode types described herein may also be
employed without departing from the scope of the present
disclosure.
[0044] In other embodiments (e.g. as shown in FIGS. 2B-2D and 2F),
regions 206 and 208 may be doped with different species to create a
suitable surface diode, preferably a P-I-N diode, while the doped
region 210 near the lower surface is doped with a suitable dopant
species to create bulk diode(s) between the contacts. Preferably,
the bulk diode(s) include a first bulk diode between one of the
contacts 204a, 204b and contact 212, and a second diode between the
other of contacts 204a, 204b and contact 212. Most preferably, the
first bulk diode comprises a P-I-N diode while the second bulk
diode comprises an N-I-N or a P-I-P diode. As shown in FIG. 2D, in
some approaches the doped regions 206, 208 may be in electrical
contact, causing the electrodes to stably operate at different
potentials. Electrical contact does not necessarily require
physical contact as depicted in FIG. 2D, but does require the doped
regions extend to at least some extent into the region of the
detector surface between contacts, preferably contact pairs.
[0045] 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 FIGS. 2E and 2F, a damage region 216 may be
created between contacts 204a, 204b, e.g. via ion implantation of
the bulk portion 202 near the upper surface thereof.
[0046] In some approaches (e.g. as shown in FIG. 2E), no dopant
regions exist under the contacts and the only modification is to
passivating layer 214, for instance pulsed annealing in the region
where passivating layer 214 is formed, to reduce or eliminate
surface defects. In other embodiments, additional dopant regions
may be created in regions between the contacts, and the regions
beneath the contacts may be doped as described hereinabove (e.g.
additional dopant regions 216 according to the embodiment as shown
in FIG. 2F). Preferably, the additional dopant regions comprise
dopant(s) in the form of one or more ionic species of one or more
materials selected from Group VII.
[0047] 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.
[0048] 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.
[0049] 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 FIGS. 2A-2F, contacts 204a and 204b may each
independently comprise or consist of: a metal Ohmic contact in the
case of detector 200 (FIG. 2A); coplanar grids in the case of
detectors 210, 230, and 250 (FIGS. 2B and 2D-2F, respectively);
while in the case of detector 220 (FIG. 2C) contacts 204a each
preferably comprise a pixel, while contacts 204b form a steering
grid (this configuration is also shown from a top-down view in FIG.
3B). In more embodiments, particularly regarding detectors 210,
230, 240 and 250, contacts 204a and 204b may cooperatively define a
coplanar grid (e.g. as shown from a top-down view in FIG. 3C).
[0050] In various approaches, the doped regions (particularly 206
and 208) may be in contact with each other (e.g. as shown in FIG.
2D) to create junctions such as PN or NP junctions. In more
approaches the doped regions may be separated by a defined spacing
to create a junction (e.g. as shown in FIGS. 2A, 2B, 2C, and 2F),
preferably a P-I-N junction. The contacts may be positioned
relative to the detector surface so as to be confined within a
plane defined by the doped region. In other embodiments, dopants
may be introduced into region(s) between the contacts in order to
increase resistivity of this area (e.g. as shown in FIGS. 2E and
2F).
[0051] Accordingly, in various embodiments such as shown in FIGS.
2A-2F, the doped regions 206, 208, 210 are each independently doped
with one or more appropriate dopants, such as one or more species
of material(s) selected from Group III (e.g. B, Al, Ga, In), Group
V (e.g. N, P, As, Sb), or Group VII (e.g. F, Cl, Br, I). Dopants
may be introduced using any known technique, but preferably are
incorporated using ion implantation.
[0052] 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.
[0053] Again with reference to FIGS. 2A-2F, and preferred
embodiments of the corresponding structures, the dopant regions may
be doped so as to create particular functional structures as
follows. In accordance with detector 200 of FIG. 2A, preferably
doped regions 206 comprise n-type dopants, while doped region 210
comprises a p-type dopant.
[0054] For certain embodiments of detector 210 as shown in FIG. 2B,
preferably the doped region 206 comprises an n-type dopant, while
doped regions 208, 210 each independently comprise a p-type dopant
(which may be the same or different in each respective region).
This configuration results in bulk and surface P-I-N diodes between
the contacts and is particularly advantageous where at least some
of the contacts are a coplanar grid arrangement.
[0055] In accordance with preferred embodiments of detector 220 as
shown in FIG. 2C, doped regions 206 preferably comprise an n-type
dopant, while doped regions 208 and 210 preferably comprise p-type
dopants. This configuration is particularly advantageous in
embodiments where the detector structure includes pixels surrounded
by a steering grid, e.g. according to the top-down view of FIG.
3B.
[0056] In preferred variants of detector 230 as shown in FIG. 2D,
doped region 206 preferably comprises an n-type dopant, while doped
regions 208, 210 preferably comprise p-type dopants. This results
in bulk and surface P-I-N diodes between the contacts, and is
particularly advantageous where at least some of the contacts
comprise coplanar grids.
[0057] Now with reference to preferred embodiments of detector 240
as shown in FIG. 2E, the damage region 216 may comprise
ion-implanted CZT (e.g. an "i-type" dopant) with the dopant species
being chosen so as to provide higher resistivity within the damage
region 216. Additionally and/or alternatively, damage region 216
may be formed via exposing surface(s) of the detector 240 to rapid,
pulse annealing in an amount greater than a damage threshold of the
material (e.g. as shown in FIG. 7B).
[0058] Regarding detector 250 as shown in FIG. 2F, the doped
regions 206, 208 and 210 preferably follow the same scheme as set
forth above regarding detector 210 shown in FIG. 2B.
[0059] 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
FIG. 2D). In some embodiments, contacts 204a, 204b may be
considered individual "grids" of a detector 200. In more
embodiments, a detector may include a plurality of pixels, pixels
with steering grid, or a co-planar grid, such as shown in FIGS.
3A-3C, described in further detail below.
[0060] 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 FIG. 1, can also be used
to reduce electronic noise. FIG. 4 shows reduced leakage current of
a conventional, undoped detector before and after sequentially
greater rapid, pulse annealing. This is postulated to be due to
removal of surface and interfacial defects, annealing of the
contacts, and modification of the crystal structure. In various
embodiments, rapid pulse annealing may be performed in accordance
with methods 800 and/or 900, as described in further detail below,
to accomplish the aforementioned advantageous reductions in
electronic noise.
[0061] Finally, according to any of the embodiments shown in FIGS.
2A-2F, side and top surfaces of the detector 200 may be
functionalized by plasma etching and chemical oxidation, preferably
after forming contacts 204a, 204b on the top surface of the
detector, to form passivating layers or films 214 on the top and
side surfaces of the detector. Preferably, the passivating layers
or films 214 are formed on portions/surfaces of the detector
adjacent and/or including the doped regions 206, 208. On the top
surface of the detector, a first passivating layer or film 214 may
be formed between the contacts 204a, 204b, while additional
passivating layers or films 214 may be formed on side surfaces of
the detector in or adjacent to the doped regions 206, 208, 210
and/or bulk portion 202.
[0062] It will be understood by skilled artisans reading the
present descriptions that the various features set forth above
individually with respect to FIGS. 2A-2F and the corresponding
embodiments depicted therein may be combined in any suitable manner
without departing from the scope of the present disclosures.
Accordingly, any combination of junction types, configurations,
etc. e.g. as conveyed by composition of the doped regions, may be
employed. Similarly, different contact arrangements (described in
further detail below regarding FIGS. 3A-3C) may be employed in
various combinations without departing from the scope of the
instant description. Passivation layers may be present on various
surfaces of the detector, and various regions thereof in any
combination as set forth herein.
[0063] The inventive structure of detectors such as shown in FIGS.
2A-2F advantageously minimize surface leakage current through a
combination of the doping profile of doped regions 206, 208, 210,
as well as the formation of passivation layers or films 214.
Moreover, bulk leakage current is reduced via the side passivation
layers or films 214 and may be further reduced in embodiments where
the doped regions 206, 208, 210 form a homojunction such as a P-I-N
structure. Further still, since surface defects (associated with
damage typically present in the absence of treatment including
plasma etching and chemical oxidation to form passivating layers as
described herein) are associated with generation of anomalous burst
noise, detectors such as shown in FIGS. 2A-2F advantageously
exhibit reduced burst noise.
[0064] 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.
[0065] Of course, in various embodiments different detector element
configurations, may be employed.
[0066] For example, a pixelated CZT detector structure 300 is shown
from a top-down view in FIG. 3A, according to one embodiment. The
pixelated detector 300 includes a plurality of contacts (e.g. 204a,
204b), preferably pixels, each coupled to an upper surface of the
detector. The upper surface of the detector preferably comprises a
passivating layer 214, which may be formed via pulse annealing of
at least the regions of the upper surface positioned between
pixels. Although not shown in FIG. 3A, each pixel is preferably
positioned above a doped region, e.g. doped region 206 and/or 208
depending on the detector structure employed. Moreover, pixels are
preferably characterized by an inter-pixel distance d (also
referred to as a contact pitch) between the center of individual
pixels, which may be approximately 1.75 mm in one embodiment, and
generally falls within a range from approximately 100 .mu.m to
approximately 5 mm.
[0067] 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.
[0068] 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.
[0069] 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.
[0070] Turning now to FIG. 3B, a detector structure 310 is shown
according to one embodiment. The detector structure 310 is also a
pixelated detector, but distinct from the embodiment shown in FIG.
3A the detector structure 310 includes a steering grid positioned
between pixels 204a, the steering grid being formed of a different
contact material 204b and portions of the steering grid being
positioned above dopant regions 208, e.g. as shown in FIG. 2C.
Regions not located beneath pixels 204a and grid 204b preferably
comprise a passivating layer 214 (again as shown in FIG. 2C).
[0071] In still more embodiments, a detector may be configured as a
coplanar grid 320 such as shown in FIG. 3C. The coplanar grid
comprises contacts 204a and 204b arranged in a complementary grid
configuration and separated by passivating layer regions 214
between the contacts 204a and 204b. Although not shown in FIG. 3C,
preferably the contact 204a is positioned above a doped region 206
of the detector, while the contact 204b is positioned above a doped
region 208 of the detector, such that the respective doped regions
are positioned under the contacts but do not necessarily extend
into the regions therebetween (i.e. regions below the passivating
layer 214 as shown in FIG. 3C). See, for example, the side-views
shown in FIGS. 2B and 2D, respectively.
[0072] As mentioned briefly above, FIG. 4 shows reduced leakage
current of a conventional detector before and after sequentially
greater rapid, pulse annealing. Notably, with increasing amounts of
annealing, the amount of intergrid current flowing through a
coplanar grid arrangement is reduced as a function of field
strength. Accordingly, treating detector surfaces via rapid pulse
annealing is associated with reduced current at a given voltage,
allowing operation of the detector at higher field strength and
improving the signal detection capability of the detector.
[0073] FIG. 5 is a chart depicting amount of current at a given
voltage for a detector structure after implantation, both before
and after laser annealing. The data represented in FIG. 5 will be
discussed further below, but generally note the amount of current
after laser annealing increases, corresponding to a desirable
increase in conductivity after doping, and further increase after
laser annealing. Notably, this is indicative of increased
activation of B and N dopants.
[0074] FIGS. 6A and 6B are charts depicting impact of presence of a
doped junction (particularly an N-P-N type) on the resulting
surface and bulk detector current and voltage respectively,
according to one embodiment of the presently disclosed inventive
concepts. Importantly, the junction is preferably formed via ion
implantation followed by rapid, pulsed thermal annealing.
[0075] 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.
[0076] 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 FIG. 1. For example, see data shown in
FIG. 4. Accordingly, the presently disclosed inventive detectors,
and methods of making the same, are characterized by inclusion of
pulse-annealing surfaces of a detector, preferably ion-implanted
surfaces to generate junctions within the detector structure.
[0077] 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.times.10.sup.16-1.times.10.sup.19 atoms/cm.sup.3. The
experimental results in FIG. 5 revealed that the laser effectively
activated the B and N dopant at energies in a range from
approximately 16 mJ/cm.sup.2 to approximately 24 mJ/cm.sup.2. It
should be noted that the energy delivered by the optical source
should be carefully tuned to avoid structural damage to the
detector. For instance, as in FIGS. 7A-7B, laser energies of
approximately 75 mJ/cm.sup.2 or above were observed to cause
exemplary CZT detector structures to crack when pulsed at 10 Hz
with a 15 ns pulse width.
[0078] 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. SiO.sub.2 or Si.sub.3N.sub.4, 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.
[0079] While some approaches recognize the use of thermal annealing
(e.g. by incubating the detector at .about.>300.degree. 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.degree. C.
[0080] 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.
[0081] 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.
[0082] 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.
[0083] 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.
[0084] Turning now to particular methods of fabricating inventive
detector structures as described herein, FIG. 8 shows a method 800
of making a detector exhibiting reduced electronic noise, in
accordance with one embodiment. The method 800 as presented herein
may be carried out in any desired environment that would be
appreciated as suitable by a person having ordinary skill in the
art upon reading the present disclosure. Moreover, more or less
operations than those shown in FIG. 8 may be included in method
800, according to various embodiments. It should also be noted that
any of the aforementioned features may be used in any of the
embodiments described in accordance with the various methods.
[0085] As shown in FIG. 8, method 800 includes at least operation
802, where at least one surface of a detector comprising CdZnTe
(CZT) is pulse-annealed for one or more pulses, each pulse being
characterized by a duration of approximately 0.1 seconds or less,
but may preferably include pulse durations in a range from
approximately 1.0 ns to approximately 10 ms in various embodiments.
More preferably, the pulse-annealing comprises a plurality of
pulses, e.g. in a range of 2-1,000,000 pulses. The detector surface
may be pulse-annealed with or without contacts 204a, 204b, 212
being formed/present on the surface, and/or in regions to which the
contacts are coupled or regions lacking any contacts coupled
thereto (again, whether or not the contacts have been formed on the
surface at the time of annealing).
[0086] 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.
[0087] 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.
[0088] 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.
[0089] 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.
[0090] Skilled artisans will further appreciate that any of the
additional or alternative features, operations, etc. set forth
above with respect to FIG. 8 and method 800 are equally applicable
to, and may be used in the context of, FIG. 9 and method 900,
according to various embodiments.
[0091] 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.
[0092] 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.
[0093] 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.
* * * * *