Amorphous selenium and its alloys
from early xeroradiography to high
resolution X-ray image detectors and
ultrasensitive imaging tubes
solidi
status
pss
physica
Phys. Status Solidi B 246, No. 8, 1794–1805 (2009) / DOI 10.1002/pssb.200982007
b
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basic solid state physics
Feature Article
Safa Kasap* , 1 , Joel B. Frey1 , George Belev1 , Olivier Tousignant2 , Habib Mani2 , Luc Laperriere2 ,
Alla Reznik3 , 4 , and John A. Rowlands5
1
Department of Electrical and Computer Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon, SK S7N 5A9,
Canada
2
Anrad Corporation, 4950 rue Lévy, Saint-Laurent, QC H4R 2P1, Canada
3
Thunder Bay Regional Research Institute, 980 Oliver Road, Thunder Bay, ON P7B 6V4, Canada
4
Department of Physics, Lakehead University, 955 Oliver Road, Thunder Bay, ON P7B 5E1, Canada
5
Imaging Research, Sunnybrook Health Sciences Centre, University of Toronto, 2075 Bayview Avenue, Toronto, ON M4N 3M5,
Canada
Received 11 April 2009, revised 24 April 2009, accepted 24 April 2009
Published online 7 July 2009
PACS 72.20.Jv, 72.40.+w, 85.60.Gz
∗
Corresponding author: e-mail safa.kasap@usask.ca, Phone: 1-306-966-5390, Fax: 1-306-966-5407
We describe the progress in the science and technology of stabilized a-Se from its early use in xerography and xeroradiography
to its present use in commercial modern flat panel X-ray imagers
and ultrasensitive video tubes which utilize impact ionization
of drifting holes. Both electrons and holes can drift in stabilized
a-Se, which is a distinct advantage since X-ray photogeneration
of charge carriers occurs throughout the bulk of the photoconductive layer. An a-Se photoconductor has to be operated at
high fields to ensure that the photogeneration efficiency is sufficiently large to provide reasonable X-ray sensitivity. However,
at high fields, the dark current is unacceptably large in simple
metal/a-Se/metal devices, and special multilayer device structures need to be designed. The dark current decays with time
and increases with the nominal applied field. The reduction of
the dark current to a tolerable level was one of the key factors
that lead to the commercialization of a-Se X-ray detectors. We
discuss the origin of the dark current, and highlight some of the
current challenges in the design of next generation detectors.
We also discuss the origin of impact ionization in a-Se, and its
fruitful utilization in ultrasensitive imaging devices, including
the Harpicon, which are likely to lead to new high detective
quantum efficiency detectors.
© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction Amorphous selenium (a-Se), and in
particular stabilized a-Se (a-Se alloyed with about 0.2–0.5%
As to prevent crystallization, and doped with a few ppm Cl to
improve hole transport) is one of the best known photoconductors that served the photocopying industry for over three
decades until it was eventually replaced by modern inexpensive organic photoconductors in the late 1980s [1]. The first
automated commercial office copier, Xerox 914, marketed
in 1959, used an a-Se photoreceptor, a 50–60 ␮m thick a-Se
film coated onto an Al drum, and revolutionized document
reproduction. Xerox soon became a multibillion dollar com-
pany whose stock price eventually rose to $104 on 11 July
1962. What is less well known than xerography is the use
of a-Se photoreceptors in X-ray imaging in a process called
xeroradiography, which is the “photocopying” of a body part
by using X-rays. The X-ray photoconductivity of a-Se was
discovered during the early selenium photoreceptor development work at the Batelle Memorial Institute in the 1940s.
Xerox became involved in medical imaging by introducing a
commercial xeroradiographic system, the Xerox 125 Medical Imaging System, for medical imaging in the early 1970s
through Xerox Medical Systems, a small division of Xerox,
© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Phys. Status Solidi B 246, No. 8 (2009)
in Pasadena, California. The photoreceptor was stabilized
a-Se of thickness 125 ␮m vacuum coated onto oxidized Al
plates. For liquid development systems, introduced later, the
a-Se thickness was 320 ␮m. The surface of the a-Se layer
was coated by a thin organic layer (e.g., cellulose acetate) for
protection and for electrostatic charge retention. Xeroradiography produced high quality mammographic images, and
enjoyed a special attribute, called edge enhancement, which
amplified the contrast at object edges and thereby facilitated
diagnosis [2–4].
Xerox Medical Systems was eventually closed and
xeroradiography became obsolete by the mid-1990s. The
image development was cumbersome and made it unattractive. One approach to eliminate the toner development
process to modernize xeroradiography involved the introduction of an array of electrometers to read the electrostatic
image [5, 6]. Later, scanned electrometer readout was used
by Philips Medical Imaging Systems in their commercial
digital chest X-ray imaging system, called Thoravision; it
was based on the xeroradiographic process with the toner
development process replaced by an electronic readout technique, which enables the digitization of the X-ray image [7].
The photoconductor in the Thoravision system was a 500 ␮m
thick stabilized a-Se layer coated on a large Al drum, which
was rotated in the imaging system during charging, exposure, and readout. Much progress was also made in using
a laser scanning technique in which a laser beam through
a transparent electrode was used to locally discharge the aSe surface and read the induced voltage on the transparent
electrode [8]. There was an obvious need to modernize the
readout and eliminate the toner development technique if
xeroradiography was to survive as a viable X-ray imaging
method.
While the above readout techniques presented significant improvements and modernization in xeroradiography,
the fundamental xeroradiographic principle remained
unchanged, that is, the a-Se photoreceptor surface was first
charged positively, just as in the xerographic process, and
then selectively “photodischarged” by the incident X-rays
passing through the object. The charge distribution was
then suitably read out. It was not until the development
and broad commercialization of a-Si:H TFT (hydrogenated
amorphous silicon thin film transistor) based active matrix
arrays (AMAs) that the true modernization and transformation of xeroradiography occurred in the form of a digital
flat panel X-ray image detector or flat panel X-ray imager
(FPXI). An AMA is an array of pixels each of which can
be suitably addressed and read. Each pixel has a TFT switch
whose gate can be addressed as depicted in Fig. 1. When a
“read” signal is applied to the gate of the TFT, connected to
a particular address line, it conducts, and allows the charge
(the signal to be read out) stored at the pixel capacitance
(Cpx ) to flow to a data line and hence to an output amplifier, A/D converter and then into a computer as a pixel of
the image. The availability, usefulness and convenience of
such a readout technique inevitably lead to the development of a-Se based direct conversion flat panel detectors
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Figure 1 A schematic diagram of a pixel of an a-Se FPXI. The
X-rays absorbed in the a-Se layer generate charges that drift and
become collected and stored on the storage capacitor, Cpx . If a signal
is applied to the gate of the TFT, it conducts and the charge is
transferred to the data line and hence to the external electronics.
The Cpx and TFT structures are not inside the glass substrate but
on the surface of the glass substrate; the diagram is a simplified
schematic illustration to show the principle of operation.
[9–15] in which a layer of a-Se is coated on an AMA with
a top electrode to establish a field inside the photoconductor. The charges generated by the absorption of X-rays in
the a-Se are drifted in the presence of the applied field and
become collected on a storage capacitor placed at the pixel.
When the pixel is addressed externally by the scanning electronics, the charge, which is the signal of interest, is read
out. It is apparent that this system does not rely on the
xerographic principle that involves charging and photodischarging an a-Se photoreceptor and reading the remaining
charges. The a-Se layer in the FPXI no longer has a free surface as in xeroradiography but carries a top electrode that is
biased with a constant voltage during operation. Figure 2 is
a photograph of a-Se mammographic detectors outside their
cases; it clearly shows the flat a-Se layer and the peripheral
electronics.
The a-Se photoconductor that is used in FPXIs is
deposited as a thick layer (200–1000 ␮m) onto an AMA
where each pixel has an Al electrode, as part of the storage capacitance, and forms the bottom electrode of the a-Se
photoconductor. A top electrode is deposited to apply a bias
voltage. The problem with the metal/a-Se/metal photoconductive structure is that the dark current in such structures is
unacceptably large at the required operating fields.
One of the attractive advantages of the flat panel Xray image detectors is their very nature of being flat as a
convenient direct replacement of the film cassette used at
present. The TFT switches route the signals from the pixels
© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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S. Kasap et al.: Amorphous Se from early xeroradiography to high resolution X-ray image detectors
Figure 2 (online colour at: www.pss-b.com) The a-Se photoconductive layers on AMA substrates with peripheral electronics. These
units would each be inside a suitable case (cassette).
to the peripheral electronics where they are amplified and
converted to digital data and then fed into a computer for
display, storage, and image analysis. a-Se based FPXIs have
provided some of the best X-ray images in mammography
due to a number of advantages a-Se has in this particular
diagnostic range as discussed later in the paper. There are
several commercial manufacturers currently marketing a-Se
mammographic FPXIs. Figure 3 shows two X-ray images
of a human breast obtained with an LMAM detector, manufactured by Anrad, in tomosynthesis mode [16, 17]. These
images represent two slices of a three dimensional reconstruction of the breast based on several X-ray images taken
at multiple angles.
The FPXI we have described above refers to an a-Se photoconductor based detector, that is, the X-rays are directly
converted to charges by the a-Se photoconductive layer,
which are then collected as the image signal. These detectors
are called direct conversion as opposed to indirect conversion
detectors in which the X-rays are first converted to light by a
suitable scintillating phosphor over the AMA, and the emitted
light is detected by a photodiode integrated into each pixel.
The present paper in this special Kolomiets issue addresses
the recent progress in the science and technology of a-Se
photoconductors used in direct conversion FPXIs.
The work that was carried out by Kolomiets’s group at the
Ioffe Institute clearly showed that chalcogenide glass semiconductors could not be doped in the same way as normal
crystalline semiconductors could be. The Fermi level in these
chalcogenide glasses is pinned by the high concentration of
localized deep states and the additions of impurities do not
significantly modify the electrical properties. In most chalcogenide glasses, holes are the majority carriers, that is, they
are p-type. For example, if electrons and holes are photogenerated in As2 Se3 , electrons are trapped almost immediately
but holes contribute to photoconduction.
Amorphous selenium, like other chalcogenide glasses
is also p-type in the sense that holes are more mobile than
electrons. However, a-Se has two important attributes which
make it an exceptional case within the class of chalcogenide glasses. First, both holes and electrons can drift in
a-Se and both contribute to the photoconductivity, though
© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 3 An X-ray image of a breast from a mammographic a-Se
FPXI operating in tomosynthesis mode [16]. These images represent 2 views over 49 for the complete scan.
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Feature
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Phys. Status Solidi B 246, No. 8 (2009)
the contribution from holes is more significant inasmuch as
the hole range, µh τ h , is about a factor of 10 longer than the
electron range, µe τ e . Secondly, the electrical properties of
a-Se are particularly sensitive to small amounts of impurities even in the ppm range. For example, Cl addition to a-Se
even in amounts of a few ppm enhances the hole range enormously but almost totally eliminates the electron transport
(µe τ e ≈ 0). In contrast, a few ppm Na addition increases the
electron range but destroys the hole transport and a-Se:Na
behaves as if it is n-type.
In a paper published in 1966, Kolomiets and Lebedev
[18] mention how sensitive a-Se is to impurities, and report
the effects of adding S, Te, P, As, and Tl on charge transport. Their pure a-Se films had poor hole transport (which
may have been due to deposition onto unheated substrates)
so the conclusive part of the work was mainly on the effect
on electron transport rather than the hole transport. Nonetheless, they were able to show that alloying a-Se with 0.5%
As causes the hole transport to become trap limited whereas
the electron transport remains unaffected. The addition of Cl
in the ppm amounts to a-Se:0.5%As is sufficient to enhance
hole transport and thus render a-Se:0.5%As doped with Cl
as a stable and useful photoconductor composition. The control of charge transport in a-Se by compensation was one
of the key factors in its long history of use as a viable
photoconductor [19].
2 Stabilized a-Se The advantages of a-Se as a photoconductor in the visible range were well established by
the mid-1950s, as apparent by the number of patents that
were filed from the 1950s through to the 1970s by various organizations on a-Se based technologies. Bixby’s two
patents in 1956 and 1961 clearly demonstrated the importance of a-Se as a viable photoreceptor at the time for use
in xerography and xeroradiography. A few years later, aSe photoreceptor plates were being used in the first manual
xerographic copier, Xerox’s Model A, and later in Xerox’s
automated 914 office copiers. At the same time, there was
also interest in developing an a-Se photoconductive target
for TV pickup tubes at RCA and Westinghouse Electric Corporation. For example, a US Patent by Weimer in 1953 at
RCA [20] clearly describes the high photoconductivity of
vacuum deposited a-Se layers and their low dark current for
potential use in TV-pick-up tubes. Its X-ray photoconductivity was also recognized as an important attribute during
the 1960s and 1970s, which lead to the commercialization of a-Se X-ray medical imaging systems as mentioned
above.
It is important first to highlight some of the basic but
important reasons for amorphous selenium’s success as an
X-ray photoconductor, and also identify what are its shortcomings in search for better photoconductors; or simply
enhancing the properties of a-Se.
First, a reasonably thick a-Se layer is able to absorb Xrays and generate charge carriers that can be collected by
suitably electroding the a-Se layer and applying a field, that
is, a-Se exhibits good X-ray photoconductivity. While the
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atomic number (34) for Se is not as high as some of the other
X-ray photoconductors, it can, nonetheless, absorb X-rays,
especially in the mammographic range. In the general radiology range, it is obviously less efficient in X-ray absorption
than its high atomic number competitors such as HgI2 and
PbO. The X-ray photogeneration efficiency in a-Se depends
strongly on the field. Typically, a large field, 10 V/␮m or
higher, must be applied to achieve an acceptable X-ray photogeneration efficiency.
Secondly, a-Se can be readily coated by conventional
vacuum deposition over a large area with good uniformity
up to thicknesses of 1000 ␮m. X-ray image detectors need
to be larger than the body parts to be imaged since X-rays
cannot practically be focused. Although there are many crystalline semiconductors with superior properties, they cannot
easily be grown as a large area crystal and then fabricated
into a detector with suitable readout electronics. Polycrystalline semiconductors such as Znx Cd1−x Te, PbI2 , HgI2 , can
also be conveniently prepared in large areas (at least in
principle), but their main drawback is the adverse affect of
grain boundaries in limiting charge transport or causing a
high dark current. Further, the high substrate and annealing temperatures required to optimize the semiconductor
properties are normally incompatible with the a-Si:H AMA
substrates. The substrate temperature during the deposition
of the a-Se layer onto the AMA is typically between 60
and 70 ◦ C, above the glass transition temperature of a-Se,
and does not damage the underlying AMA on the glass
substrate.
The basic technology for the deposition of a-Se photoconductive layers has always been the thermal evaporation
of vitreous selenium pellets in a conventional stainless steel
vacuum coater. Vitreous Se pellets (shots obtained from
quenching liquid selenium) with the right composition are
loaded into a large, directly heated molybdenum or stainless
steel boat (or boats) and heated to a temperature above the
melting temperature of Se. The evaporating Se molecules
condense on a heated substrate to form an amorphous layer.
The deposition rates are typically 1–5 ␮m/min. Two distinct but key issues have always been recognized in the
fabrication of high quality a-Se layers: (a) the source and
grade of the starting Se material, even when it is alloyed or
requenched from the melt and (b) the conditions of vacuum
deposition.
Thirdly, both holes and electrons are mobile in a-Se,
which is a distinct advantage because X-rays are absorbed
throughout the bulk of the a-Se layer. Thus, both the electrons and holes generated by the absorption of an X-ray
photon can drift and become collected. Although electrons
move more slowly than the holes, µe is ∼30 times smaller
than µh , the signal is the collected charge (Q in Fig. 1) or
integrated X-ray photocurrent, and hence the mobility difference does not affect the signal as long as both carriers
can be collected. The key to the success of a-Se is the fact
that with an applied field of F, both the electron and hole
Schubwegs, µe τ e F and µh τ h F, are much longer than the photoconductor thickness, L, so that the trapping of carriers is
© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Table 1 Selected properties of undoped and stabilized a-Se photoconductive layers.
property
value
comment and reference
Eg (eV)
Eg (eV)
µh (cm2 V−1 s−1 )
τ h (␮s)
2.0
2.1–2.2
0.13–0.14a , b
10–100a
50–500b
5–7 × 10−3a
2–4 × 10−3b
10–100a
200–1000b
10−17 –10−14
optical transmission through thin films [22]
electrical and xerographic measurements
very reproducible, independent of thickness and source of a-Se. Thermally activated
good quality film. Depends on the substrate temperature, impurities and Se processing
µe (cm2 V−1 s−1 )
τ e (␮s)
σ dc (( cm)−1 )
a Undoped
slightly field dependent and depends on the source of a-Se and As content. Thermally activated
depends on impurities and Se processing. Independent of the substrate temperature. Increases
with As content
dark conductivity is thermally activated. Very sensitive to impurities. 10−17 is for deoxygenated
sample
a-Se; b stabilized a-Se.
negligible. The carrier Schubweg is defined as the distance
a carrier drifts before being captured by a deep trap. In the
case of mammographic detectors where L = 200 ␮m, nearly
98% of the X-ray generated charges are collected. a-As2 Se3
films were used as photoreceptors in xerography for many
years and were often preferred over a-Se photoreceptors due
to their longer machine lifetime, that is, the greater number
of copies they could generate, and their more panchromatic
spectral response. Unfortunately, a-As2 Se3 cannot be used
as an X-ray photoconductor because only holes are mobile
in this material. The trapping of photogenerated electrons in
this material would lead to a large sacrifice in the sensitivity. What is interesting is that a-As2 Se3 is still used in the
a-Se FPXI because of the special three layer photoconductive structure that is needed to suppress the dark current to a
tolerable level.
Fourthly, unlike many other amorphous solids, charge
transport in a-Se over the time scale of interest at room temperature is nondispersive for both holes and electrons. Both
hole and electron transport can be readily described by a set of
shallow traps that control the drift mobility and a distribution
of deep traps with well defined trapping times or lifetimes. A
well-defined lifetime does not refer to a well-defined discrete
level but refers to an effective trapping time into a distribution of deep localized states from which there is no release
over the time scale of interest. Further, the integrated concentrations of electron and hole deep traps are small, less
than 1014 cm−3 , and are relatively narrow bands near the middle of the bandgap. Stated differently, other localized states
between shallow and deep levels do not significantly affect
the transport. Thus, researchers and engineers have been able
to model and predict the behavior of a-Se based devices from
photoreceptors to X-ray photoconductors by simply using
shallow and deep sets of traps; again the emphasis is on
the fact that these need not be discrete levels as long as the
carriers equilibrate with the shallow traps and experience
no release from the deep traps over the experimental time
scale.
© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
The hole drift mobility, µh (≈0.13–0.14 cm2 V−1 s−1
from taking the transit time at half the transient photocurrent), is remarkably reproducible whereas the electron drift
mobility, µe (≈5–7 × 10−3 cm2 V−1 s−1 ), shows some tendency to depend on the source of a-Se. The most reliable
method for measuring the carrier lifetimes, τ h and τ e , has been
the interrupted field time-of-flight measurements, IFTOF, as
described in Ref. [21], which can also be used to examine
the variation of the lifetime throughout the sample thickness.
Typical lifetimes in high quality stabilized a-Se photoconductors are 50–500 ␮s for holes and 200–1000 ␮s for electrons
as summarized in Table 1. These lifetimes, however, depend
on a number of factors, the most important of which are
(a) the distillation and the quenching process that was used
to prepare the vitreous pellets that are used as the evaporant material, (b) the impurities or dopants that are in the
material either unintentionally or added to modify the electrical properties, and (c) the vacuum deposition conditions.
It would not be unusual to have a supply of stabilized a-Se in
which the carrier lifetimes are much poorer than the above
values and even trap limited. As apparent from Table 1, both
electron and hole Schubwegs are much longer than typical
photoconductor thicknesses, which means that the X-ray generated charges can be readily collected as discussed in more
detail below in Section 3 where we examine stabilized a-Se
in FPXIs.
A fifth important property is that the dark current in a-Se
photoconductors tends to be relatively small compared with
many other competing photoconductors. This is not unexpected since a-Se photoreceptors were well known for their
very small rate of dark discharge of the surface deposited
charge. Of course, in xerography one surface of the a-Se
was free to accept charges from a corotron. This xerographic
surface charge was dissipated in the dark quite slowly as a
result of bulk thermal generation. However, in contrast to
xerography, the a-Se in the FPXI carries metal electrodes,
and the dark currents in simple metal/a-Se/metal structures
at very high operating fields are unacceptably large. The dark
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current problem was solved by clever detector engineering
and appropriately doping a-Se as discussed in Section 5.
3 Stabilized a-Se photoconductors in X-ray
image detectors Consider a monoenergetic X-ray beam
with a photon energy Eph . The X-ray photons in a medium
are attenuated exponentially so that the number of photons
Nph (x) in the beam at a distance x from the radiation receiving electrode, as shown in Fig. 4, decays exponentially, that
is Nph (x) = Npho exp(−αx), where Npho is the incident number of photons, and α is the linear attenuation coefficient of
the medium, which depends on the photon energy and the
properties of the medium. The quantum efficiency, AQ , of an
X-ray sensitive material in medical physics represents the
fraction of incident photons that are attenuated inside the
photoconductor (not necessarily absorbed),
AQ (E) = 1 − exp(−αL) = 1 − exp
−L
δ
.
(1)
The reciprocal of α is the attenuation depth δ. a-Se has
an atomic number, Z, of 34 and its K-edge is at 12.7 keV
where δ is 21.6 ␮m. From the K-edge onwards, α decreases
with the photon energy, Eph , and empirically follows α ≈
(6.67 × 105 )E−2.7055
, in which α is in 1/m and Eph is in keV
ph
and 12.7 keV ≤ Eph ≤ 100 keV.
At a photon energy of 20 keV, in the mammographic
range, the attenuation depth, δ, is approximately 49 ␮m so
that a 200 ␮m thick a-Se layer absorbs most of the radiation (AQ = 98.3%). On the other hand, at 60 keV, in the
chest radiology range, δ is about 1 mm so that a 1000 ␮m
thick a-Se layer will only attenuate 63% of the radiation.
At thicknesses beyond 1000 ␮m, not only does it become
harder for the thick a-Se layer to maintain adhesion to the
substrate but also good charge collection efficiency becomes
Figure 4 X-rays are absorbed along the thickness, L, of the photoconductor so that charge generation occurs over the whole thickness
of the photoconductor. For an X-ray photon absorbed at a distance
x, the holes would have to drift (L − x) and electrons a distance x.
Their contributions to the total collected charge are different.
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somewhat compromised, which results in poorer sensitivity.
At present, a-Se based FPXIs for general radiology have an
a-Se thickness of 1000 ␮m. Thus, a-Se is particularly useful
in the mammographic range where a 200 ␮m photoconductor
is sufficient to absorb nearly all the X-rays and also exhibit
excellent charge collection efficiency.
The electronic quality of the a-Se alloy, and the quality
control of the material properties, are key issues in maintaining good X-ray sensitivity and reproducible performance.
Ideally all the charges generated by the absorption of Xrays must be collected. Consider an X-ray photon that is
absorbed at a certain distance x from the radiation receiving
electrode as shown in Fig. 4. The holes would have to drift
(L − x) and electrons a distance x before they can be collected. By Ramo’s theorem, their contributions to the total
collected charge are different. In this case, the hole contribution is much larger. There is a certain probability that these
holes and electrons may be deeply trapped during their drift.
The collection efficiency, ηCC , must account for the exponential distribution of photogenerated charge and is given
by [23]
ηCC = xh 1 −
+ xe
exp(1/∆ − 1/xh ) − 1
[1 − (∆/xh )][exp(1/∆) − 1]
1 − exp(−1/∆ − 1/xe )
1−
[1 + (∆/xe )][1 − exp(−1/∆)]
, (2)
where ∆ = δ/L, xh = µh τ h F/L (Schubweg per unit sample
thickness for holes), and xe = µe τ e F/L.
We can use the low-end values from Table 2 for the charge
transport parameters in Eq. (2) to calculate the worst ηCC . For
a detector of thickness 200 ␮m, operating at F = 10 V/␮m,
we find ηCC = 97.6% for X-rays with Eph = 20 keV. For a
1 mm thick a-Se detector (general radiology), ηCC = 82.2%
for Eph = 60 keV. It is important to emphasize what happens
if the quality of the a-Se material is “poor”, for example, the
electron transport is trap limited. Then the collection efficiencies 97.6 and 82.2% for the two detectors deteriorate to
76.5 and 55.4%, respectively. Good ambipolar charge transport is essential in X-ray photoconductors. The advantages
of ambipolar transport in a-Se for xeroradiography was well
recognized at Xerox, and even the methods of promoting
electron transport by adding small amounts of alkaline metals
without totally destroying hole transport have been described
in a US Patent [24].
The reader may wonder whether there is an optimum
photoconductor thickness for a given X-ray photon energy
since thicker samples absorb more radiation and have higher
AQ but the charge collection efficiency, ηCC , becomes poorer.
The optimum thickness depends on the radiation energy
and also on the charge transport parameters, µe τ e and
µh τ h , as shown previously [25]. There are experiments
reported in the past that clearly show that the X-ray sensitivity has a maximum as a function of photoconductor
thickness [2, 26].
© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Table 2 Selected properties of a-Se X-ray photoconductors for medical X-ray imaging.
property
value
comment and reference
K-edge (keV)
δ at 20 keV (␮m)
δ at 60 keV (␮m)
ionization energy W± (eV)
µh τ h F (mm)
µe τ e F (mm)
ηCC
sensitivity (pC cm−2 mR−1 )
dark current Id (pA cm−2 )
12.7
48.5
998
45–50
7–70
0.8–4
>97.0%
216
30–50
K-edge X-ray absorption
attenuation depth
attenuation depth
at F = 10 V/␮m, 20 keV depends on the field and the X-ray photon energy
at F = 10 V/␮m
at F = 10 V/␮m
L = 200 ␮m; F = 10 V/␮m; and Eph = 20 keV
charge generated per unit area per unit radiation, F = 10 V/␮m, Eph = 20 keV
multilayer n–i–p structures
1 R of exposure is 0.545 Gy of absorbed dose in a-Se at 20 keV.
The X-ray photogeneration efficiency is inversely proportional to the ionization energy, W± , which is the absorbed
X-ray energy needed to create one electron and one hole,
the so-called EHP (electron and hole pair) creation energy.
Experiments indicate that for a-Se W± strongly depends on
the field and weakly on the photon energy. Over typical Xray imaging ranges, one can write [23], W± (eV) ≈ W±o + B/F,
where W±o = 6 eV; B = 4.4 × 106 eV V cm−1 . Thus, the X-ray
photogeneration efficiency as gauged by W±o /W± increases
with the field and one must use as high fields as possible to
obtain the smallest W± .
It should be apparent that the overall conversion efficiency of incident radiation to collected charge relies on
three processes: (a) the attenuation of the X-rays in the photoconductor, determined by AQ , and the absorption of the
radiation energy, determined by (αen /α)Eph , where αen is the
energy absorption coefficient, per attenuated photon, (b) the
conversion of absorbed radiation to electron and hole pairs,
determined by W± , and (c) the collection of the charge carriers, determined by ηCC . If we define the X-ray sensitivity, Sx ,
as the charge collected per unit incident radiation (per unit
Roentgen), then at one specific photon energy Eph ,
Sx ∝
5.45 × 1013 e
(αen /ρ)air
× AQ ×
(αen /α)Eph
W±
× ηCC (3)
where the first term represents the effect of the incident
photon fluence per unit Roentgen, the second is the attenuated fraction, the third is the number of EHPs created per
absorbed radiation energy, and the fourth is the fraction of
those charges that are actually collected. All terms depend on
the photon energy but only the last two on the electric field.
4 Dark current and I–V characteristics of a-Se
Although during the 1960s and 1970s many researchers
reported I–V measurements on single a-Se films in metal/aSe/metal sandwich structures, there has been no general
conclusion on the behavior of metal/a-Se contacts and the
origin of the resulting I–V characteristics. Most researchers
have claimed to have observed steady-state dark currents
and some have interpreted the observed dark I–V charac© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
teristics in terms of space charge limited currents (SCLC)
(e.g., [27, 28]) and even extracted the energy distribution or
the density of the deep traps in a-Se films from the shape
of the SCLC I–V curve (e.g., [29]). SCLC claims have been
questioned by a number of researchers [30, 31], while others have put forth that I–V characteristics can be explained
in terms of carrier injection across a Schottky barrier [32].
Evidence against the dark currents being space charge limited comes from several sources. Pfister and Lakatos [30]
studied the I–V characteristics when one or both of the electrodes to a-Se were illuminated with an intense and strongly
absorbed light (λ = 399 nm). They found that the steady-state
photogenerated currents obey the SCLC theories, J ∝ V2 /L3
(Child’s Law) for one carrier and J ∝ V3 /L5 for two carrier injection, and scale in accordance with the scaling law
J/L = f(V/L2 ) for SCLCs [33]. Their dark current magnitudes
are much higher than typical dark currents in a-Se films.
At very high voltages, the photocurrents became emission
limited. Clearly, to obtain SCLCs, the authors had to generate a fully injecting contact (a reservoir of carriers at the
electrode) which is a prerequisite condition for observing
SCLCs.
Mort and Lakatos [34] have studied the metal to a-Se
contact properties by photoemission experiments in which
one of the electrodes is illuminated, and the resulting current
due to photoemission from the metal into the semiconductor is measured. They report barrier heights, Φh and Φe ,
for hole and electron emission, respectively, for Au, Cu,
and Al, and find that, as expected, Φh + Φe is close to the
bandgap energy. The most important conclusion from their
work was the clear existence of potential barriers against
carrier injection, and their dependence on the electrode
material.
Johanson et al. [35] studied the dark current in metal/aSe/ITO devices with electronic grade stabilized a-Se samples
(good hole and electron ranges). They found that the I–V
characteristics depend on the nature of the metal/a-Se contact
and do not follow well-established models, such as Schottky
emission. Most importantly, the dark current immediately
after the application of a bias voltage to a metal/a-Se/ITO
device has been observed to decay with time (in a nonexponential manner), with decay characteristics that depend on
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the type of metal. It has not been possible to simply correlate
the dark current to the metal work function, which would be
expected in the case of Schottky type of metal/a-Se contacts.
Based on their data, Johanson et al. suggested that the main
dominant conduction mechanism in these metal/a-Se/ITO
structures was due to the injection of holes from the positive
electrode rather than electrons from the negative electrode.
Since, usually, the hole range, µh τ h , in electronic grade a-Se
is much larger than the electron range, µe τ e , by almost an
order of magnitude (Tables 1 and 2), their argument seems
reasonable.
It is useful to estimate the lowest achievable dark current
that arises from the bulk thermal generation of charge carriers
within a-Se and not from injection of carriers from the electrodes. Xerographic dark discharge measurements involve
charging the surface of an a-Se photoreceptor positively and
then monitoring the surface potential as a function of time.
It has been shown that the surface potential decays mainly
by the bulk thermal generation of charge carriers [36]. For
a 100 ␮m thick a-Se film charged to 10 V/␮m, the highest
discharge rate in terms of the field, dF/dt, is about 0.02 V
␮m−1 s−1 from Fig. 2 in Ref. [36]. Taking the dark current as Jd ≈ εo εr (dF/dt) gives Jd = 0.1 nA cm−2 , which is
much smaller than typical dark currents observed at similar fields in electroded metal/a-Se/metal structures. In fact,
such low dark currents would correspond to a resistivity of
the order of 1015 cm (a dielectric relaxation time of several minutes). Thus, the evidence suggests that there must be
injection from the contacts that results in a substantial dark
current.
5 Dark current in a-Se X-ray detectors Although
the dark current in the simple metal/a-Se/metal structure is
relatively low compared to many other types of competing
photoconductors, it is still nonetheless not acceptable for Xray detector applications. There are several adverse effects
to a significant dark current in FPXIs. The dark current provides unwanted noise that is added to the signal, restricts
the dynamic range by accumulating unwanted charge on the
pixel capacitor, and the charges trapped in the bulk of a-Se
during the flow of the dark current modify the internal field
and hence the photogeneration efficiency across the thickness
of the layer.
To highlight the dark current in different a-Se photoconductor structures, we compare the dark current in three
different a-Se structures: (a) a simple i-layer, that is a metal/aSe/metal structure; (b) a n–i structure; (c) a n–i–p structure.
The i-layer and n–i–p structure are both roughly 200 ␮m
thick, closely approximating the photoconductive layer in
practical mammographic detectors. The n–i–p structure contains a 6 ␮m thick, alkaline doped n-layer and a 5 ␮m thick,
a-As2 Se3 p-layer while the 130 ␮m thick n–i structure uses
a 20 ␮m thick n-layer of undoped a-Se deposited on a 7 ◦ C
substrate. This cold-deposited n-layer has reduced hole transport with respect to intrinsic a-Se (it is able to effectively
trap injected holes), but has better electron transport than
alkaline doped n-layers and allows for vacuum deposition of
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Figure 5 Dark current density as a function of time for three different a-Se structures: a metal/a-Se/metal i-layer structure, a double
layer n–i structure with a cold-deposited n-layer and a n–i–p structure with an alkaline doped a-Se n-layer and an a-As2 Se3 p-layer.
All samples consist of a nonchlorinated Se:0.2%As alloy. The first
letter in the structure notation refers to the layer next to the radiation
receiving electrode.
an entire structure from a single composition of a-Se [31].
Figure 5 shows the time evolution of the dark current, Id , in
these three types of a-Se photoconductors. This dark current
has been observed to decrease with time after the application
of the bias voltage, sometimes by several orders of magnitude over several hours, and has a dependence on the applied
voltage, as seen in Fig. 6, which is clearly not ohmic (I ∝ V),
Figure 6 Dark current density as a function of nominal applied
electric field for the same three samples shown in Fig. 5 with a
power law regression shown for each plot. The R2 for the fit for
the i-layer, n–i and n–i–p samples are 0.9982, 0.9858, and 0.9925,
respectively. The samples were rested in short circuit in the dark
for 12 h between applications of voltage to allow for the release of
trapped charge carriers.
© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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S. Kasap et al.: Amorphous Se from early xeroradiography to high resolution X-ray image detectors
and does not imply space charge limited conduction in a
“trap-free solid” for one carrier injection (I ∝ V2 ) or two carrier injection (I ∝ V3 ). As previously reported [31], we have
found the dark I–V characteristics to be not symmetrical.
Indeed, in the latter work, the authors report that the I–V
characteristics are not symmetrical even if the structure has
been designed to be symmetrical, implying that the electronic
structure of even the most carefully deposited a-Se layer may
not be homogenous. It has also been shown that under certain biases the dark current does not decrease monotonically
with time and actually exhibits a transient with one or more
maxima [37].
Due to the lack of a complete understanding of the factors affecting these dark currents, it is very difficult to predict
the level of dark current that will flow through an a-Se layer
at a given time and bias. This makes any kind of correction for the charge built up on a pixel capacitance by the
flow of dark current in an FPXI quite difficult. It is therefore
important to reduce the dark current as much as possible.
Tolerable levels of dark current have been estimated to be
in the range of 0.1–0.5 nA cm−2 , depending on the criteria
used [31]. The a-Se photoconductor in the FPXI uses a special multilayer structure to achieve a dark current that is less
than this critical value. As shown in Fig. 7, there are two thin
blocking layers between the main a-Se layer (the i-layer)
and the electrodes. These blocking layers are essential for
reducing the dark current. In commercial detectors, the nand p-type blocking layers are made from alkaline doped aSe and a-As2 Se3 , respectively, and are typically a few ␮m
thick [38].
Figure 8 compares the dark currents in the three a-Se
structures described above. It can be seen that a single ilayer passes a dark current above acceptable levels but the
addition of the blocking layers reduces it, by two orders of
Figure 7 Example of an n–i–p multilayer structure with the electric
field distribution across the structure. At t = 0, the field will be
uniform, but as charge carriers are injected, holes will be trapped in
the n-layer and electrons in the p-layer, altering the electric field as
shown. For an a-Se based mammographic detector, the i-layer will
be roughly 200 ␮m thick while each of the blocking layers will be
only a few ␮m thick.
© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 8 Comparison of dark current density through all three
samples at 10 and 1000 s after the application of voltage. The hashed
area shows the acceptable limit of dark current for FPXIs.
magnitude in the case of the n–i–p structure, to well below
the tolerable limit.
A useful model for the dark current must be able to predict the observed time dependence of Id at different applied
bias voltages. It has been suggested that the basic time and
I–V characteristics of the dark current in multilayer a-Se photoconductive structures can be modeled by the injection of
charge carriers over a potential barrier into the bulk which
are then deeply trapped in the blocking layers, building up
a space charge which alters the electric field distribution
across the structure as shown in Fig. 7. The reduction of
the field at the electrodes with time reduces carrier injection
and hence the dark current. Recently, Kabir and coworkers
[39] have carried out numerical simulations for modeling
this change in the field distribution and the resulting dark
current for a n–i–p structure, which approximates the photoconductive layer in a FPXI used for general radiography.
The model, in essence, involves the following processes:
(a) the injection of holes over a potential barrier from the
positive electrode into a-Se, which includes the Schottky
effect (the field reduces the barrier height); (b) similar electron injection from the negative electrode over a potential
barrier with Schottky effect; (c) electron and hole deep trapping in the blocking p-like and n-like layers respectively;
(d) the modification of the field by the build-up of a net
space charge distribution in the bulk. By using commonly
accepted values for the material properties of a-Se such as
barrier heights and deep trap levels and concentrations, they
have found a reasonable agreement with experimental data at
different applied fields for samples in which the dark current
reached a quasi-steady-state level within 1000 s. While this
model includes the release of trapped carriers (detrapping), it
does not specifically involve bulk thermal generation. Dark
current in reverse biased a-Si:H p–i–n structures has been
attributed to Poole–Frenkel assisted bulk thermal generation
[40, 41] and although its contribution to dark current in a-Se
structures may be less due to its larger bandgap, the effect
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may not be negligible as it has been found to control xerographic dark discharge in a-Se as discussed in Section 4.
Further work in this area could prove to be very useful.
There is no doubt that the dark current in a-Se structures
is quite complicated and the explanation probably involves
an intricate play between contact effects and bulk properties.
As highlighted by Abkowitz and Scher [42] for metal/aAs2 Se3 /metal structures, the dark current must ultimately
reflect an equilibrium between rates of carrier extraction
and resupply in the interfacial region. Because the carrier
extraction rate is related to the bulk transport parameters, it
is not surprising to find steady-state currents reflect a dependence on these bulk parameters. A similar argument would
obviously be applicable to a-Se devices.
While models and simulations of the dark current will
help to understand the factors contributing to the dark current in a-Se based photoconductor structures, there is still
much debate about the basic properties of a-Se including
its physical structure and the nature, location and concentration of defect states in the bandgap. Models constructed
using commonly accepted values for these properties can
only be as accurate as their underlying assumptions and much
work is still needed to confirm these properties. Nonetheless,
even without a complete understanding of the nature of dark
currents, sound engineering principles have brought about
different multilayer structures which can reduce the dark current to acceptable levels for use in FPXIs. Xerography was
commercialized long before the physics of xerography was
fully understood.
6 Impact ionization and new devices Impact ionization in a-Se was first reported by Juska and Arlauskas
in 1980 [43]. They investigated the quantum efficiency of
charge photogeneration in a-Se at high electric fields and
discovered that at electric fields higher than about 80 V/␮m,
effective quantum efficiency, that is, the quantum yield or collected number of carriers per absorbed photon, exceeds unity,
and increases rapidly with the field. This effect was attributed
to impact ionization and avalanche multiplication triggered
by hot holes drifting at high electric fields. Electrons being
the slow carriers in a-Se, do not initiate avalanche multiplication; indeed their drift mobility is 30 times smaller than
that of holes. It was found that the effective quantum efficiency at electric fields above the avalanche multiplication
threshold depends exponentially on the thickness of a-Se,
which indicates that just one type of carrier, namely holes,
undergo avalanche multiplication. Further, densities of secondary electrons and holes were found to be equal, indicating
band-to-band impact ionization.
For their experiments, Juska and Arlauskas sandwiched
an a-Se layer (thickness, L = 4–33 ␮m) between two insulating polyethyleneteraphalate layers to avoid charge injection
from the contacts at high fields that are needed to initiate
and maintain the avalanche multiplication process. Although
these initial experiments unambiguously demonstrated the
existence of avalanche multiplication in a-Se, the special
metal/insulator/a-Se/insulator/metal structures used by Juska
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and Arlauskas were not convenient for practical applications in imaging devices or sensors with avalanche gain. The
first practical application of avalanche multiplication was
demonstrated by Tanioka et al. in the late 1980s in Japan
[44–46]. They developed a practical a-Se photoconductive
target, called a HARP, an acronym for High-gain Avalanche
Rushing Photoconductor, which they eventually used in commercial TV pick-up tubes or vidicons with avalanche gain.
The a-Se layer in the HARP was confined between two
specially designed blocking layers that prevented charge
injection while allowing the exit of photogenerated and multiplied carriers to the external electronic circuit. Figure 9
shows a simplified illustration of a HARP vidicon (or
Harpicon) with an a-Se photoconductive target that exhibits
avalanche multiplication at sufficiently large fields.
The a-Se HARP structure is deposited on a glass substrate
covered with indium tin oxide (ITO) which serves as a transparent anode. The entire target is typically about 25–35 ␮m
thick. The transparent ITO electrode is the signal electrode,
and is biased positively with respect to the cathode. The CeO2
and a-Se doped with LiF layers on the ITO side act as a
hole blocking contact for hole injection from the ITO into
the a-Se layer. The Sb2 S3 layer reduces electron injection
from the scanning electron beam and suppresses secondary
electron emission. The Sb2 S3 layer behaves somewhat reminiscently to a-As2 Se3 , a p-type semiconductor in which
electrons are deeply trapped, so that the injection of electrons into the a-Se layer is prevented. The electrons injected
into Sb2 S3 are deeply trapped in this layer and thereby form
a negative space-charge barrier which stops further electron
injection.
The incident light from the object is absorbed mainly in
the a-Se layer (or the photogeneration layer, a-Se:Te). The
electron and hole pairs photogenerated in the a-Se layer are
then separated by the applied electric field. The electrons
are neutralized quickly as they are very close to the positive electrode, whereas holes have to drift across the bulk
of the a-Se layer to reach the negative electrode. These
drifting holes constitute the signal current. As the photogenerated holes drift through the a-Se layer, as a result of
the large applied electric field (greater than 80 V ␮m−1 ), they
experience avalanche multiplication and hence yield an effective quantum efficiency (or gain) greater than unity. The
Figure 9 The HARP video tube with avalanche gain. Reproduced
with permission from Cambridge University Press [52].
© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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S. Kasap et al.: Amorphous Se from early xeroradiography to high resolution X-ray image detectors
Figure 10 Field dependence of the effective quantum efficiency
for an a-Se HARP structure for selected thicknesses. Clearly, as
expected, thicker samples allow higher avalanche gains at the same
field. High field data is extracted from Ref. [53] and combined with
low field data.
effective quantum efficiency resulting from avalanche multiplication depends on the field as well as the photoconductor
thickness, as shown in Fig. 10.
Even though the experimental evidence for avalanche
multiplication in a-Se has been clear from the early experiments of Juska and Arlauskas and the commercial application
of this phenomenon in the Harpicon, the understanding of the
nature of impact ionization in amorphous semiconductors has
remained, by and large, as an intriguing scientific problem
for almost 25 years since the mean free paths in this class
of semiconductors are short. Recently, it has been possible
to formulate an explanation for the avalanche multiplication
mechanism in this class of semiconductors in terms of a suitably modified “lucky-drift” (LD) model. [47–50]. The LD
model, as originally suggested by Ridley [51], allows carriers
to undergo scattering while drifting in an electric field but, at
the same time, the model allows the carriers to acquire energy,
because momentum and energy relax at different rates. At
sufficiently high fields, the carriers can build-up sufficient
energy to initiate impact ionization. In simple terms, the carrier motion in amorphous semiconductors is controlled by
elastic scattering from the disorder potential (potential fluctuations in the noncrystalline structure), and inelastic scattering
from optical phonons. There are thus two different rates for
the relaxation of momentum and energy. Elastic collisions are
much more frequent and not all the carrier energy is lost upon
scattering from an optical phonon. Thus, eventually, at high
fields, a carrier can gain sufficient energy during drift to cause
impact ionization. The LD model has been expanded and
applied to other noncrystalline media as recently discussed
by Baranovski and coworkers [50].
Currently, a-Se HARP photoconductive targets are
employed in electron-beam scanned Harpicon TV camera
tubes. Due to their very high effective quantum gain they are
© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
capable of producing images at extremely low levels of light.
Harpicons have therefore been used in many challenging
very low-illumination environments such as news gathering
at night (even under star light) and underwater videoing.
At present there is much research interest in solid-state
image sensors because they are simply more practical and
reliable due to their solid-state nature. Recently, Tanioka and
coworkers [54] have successfully incorporated a HARP layer,
operating with avalanche gain, onto a CMOS image sensor
to produce a prototype HARP–CMOS high sensitivity image
sensor. A conventional CMOS imaging array with electrodes
at each pixel is coated with a HARP that is biased to operate
in the avalanche multiplication mode. When the HARP at
a particular pixel receives signal photons, it photogenerates
holes that multiply as they drift towards the negative electrode. This multiplied photocurrent charges a capacitor that
stores the signal as charge. The signal charge is later read
out by appropriately addressing that particular pixel. Thus,
each pixel of the CMOS sensor receives an avalanche multiplied photocurrent signal and consequently the overall signal
to noise ratio becomes substantially improved; the signal is
multiplied at each pixel. The images from the HARP–CMOS
sensor have been very impressive and superior to those from
conventional imagers.
There is, of course, much current interest to use avalanche
multiplication in a-Se in all solid-state photoconductive
structures with electronic readout for various imaging applications, including applications in medical imaging. There
have been several studies that clearly show that such all
solid-state devices with intrinsic avalanche gain represent the
future of a-Se photodetectors in medical X-ray and functional
γ-ray imaging applications [48, 55]. For these applications,
avalanche a-Se photosensors are intended to replace vacuum photomultipliers or silicon (Si) avalanche photodiodes
(APDs) used to convert light emitted from a phosphor to collectable charges. For example, Reznik et al. were able to show
that a 35 ␮m thick a-Se HARP layer with metal electrodes
can exhibit a gain of 103 as shown in Fig. 10. By using this
HARP layer as an avalanche photodetector for light from a
scintillator, one should be able to develop high performance,
low-dose radiation detectors.
The future of a-Se today looks much brighter than it did
20 years ago when the writing on the wall signaled its end
as the king of all xerographic photoreceptors. The overall
use of the selenium material itself in these new technologies
however is unlikely to reach the high volume level that it did
during its heydays in xerography.
Acknowledgements We thank NSERC for financial support.
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