Mercury Chalcohalide Semiconductor Hg3Se2Br2 for Hard Radiation Detection

Article abstract of DOI:10.1021/acs.cgd.6b01118

Hg₃Se₂Br₂ is a wide band gap semiconductor (2.22 eV) with high density (7.598 g/cm³) and crystallizes in the monoclinic space group C2/m with cell parameters of a = 17.496 (4) ?, b = 9.3991 (19) ?, c = 9.776(2) ?, β = 90.46(3)°, V = 1607.6(6) ?³. It melts congruently at a low temperature, 566 °C, which allows for an easy single crystal growth directly from the stoichiometric melt. Single crystals of Hg₃Se₂Br₂ up to 1 cm long have been grown using the Bridgman method. Hg₃Se₂Br₂ single crystals exhibit a strong photocurrent response when exposed to Ag X-ray and blue diode laser. The resistivity of Hg₃Se₂Br₂ measured by the two probe method is on the order of 10¹¹ cm, and the mobility-lifetime product (μτ) of the electron and hole carriers estimated from the energy spectroscopy under Ag X-ray radiation are (μτ)_e ≈ 1.4 × 10^-4 cm²/V and (μτ)_h ≈ 9.2 × 10^-5 cm²/V. Electronic structure calculations at the density functional theory level indicate a direct band gap and a relatively small effective mass for carriers. On the basis of the photoconductivity and hard X-ray spectrum, Hg₃Se₂Br₂ is a promising candidate for X-ray and γ-ray radiation detection at room temperature.

Full text of DOI:10.1021/acs.cgd.6b01118

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Article
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Mercury Chalcohalide Semiconductor HgSeBr for Hard Radiation Detection
Hao Li, Fang Meng, Christos D. Malliakas, Zhifu Liu, Duck
Young Chung, Bruce W. Wessels, and Mercouri G. Kanatzidis
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b01118 • Publication Date (Web): 28 Sep 2016
Downloaded from http://pubs.acs.org on October 4, 2016
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Mercury Chalcohalide Semiconductor Hg Se Br for Hard
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Radiation Detection
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Hao Li, Fang Meng , Christos D. Malliakas, Zhifu Liu, Duck Young Chung, Bruce Wessels
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and Mercouri G. Kanatzidis*
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Materials Science Division, Argonne National Laboratory, Argonne, Illinois, 60439, USA
Department of Chemistry, Northwestern University, Evanston, IL, 60208, USA
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Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois
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0208, United States
Department of Electrical Engineering and Computer Science, Northwestern University,
Evanston, Illinois 60208, United States
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Abstract
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Hg Se Br is a wide band gap semiconductor (2.22 eV) with high density (7.598 g/cm ), and
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crystallizes in the monoclinic space group C2/m with cell parameters of a = 17.496 (4) Å, b =
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.3991 (19) Å, c = 9.776(2) Å, β = 90.46(3) , V = 1607.6(6) Å . It melts congruently at a low
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temperature, 566 C, which allows for an easy single crystal growth directly from the
stoichiometric melt. Single crystals of Hg Se Br up to 1 cm long have been grown using the
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Bridgman method. Hg Se Br single crystals exhibit strong photocurrent response when exposed
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to Ag X-ray and blue diode laser. The resistivity of Hg Se Br measured by the two probe
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method is in the order of 10 ∙cm and the mobility-lifetime product (μτ) of the electron and
hole carriers estimated from the energy spectroscopy under Ag X-ray radiation are (μτ) ~ 1.4
e
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10 cm /V and (μτ) ~ 9.2×10 cm /V. Electronic structure calculations at the Density
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Functional Theory level indicate a direct band gap and a relatively small effective mass for
carriers. Based on the photoconductivity and hard X-ray spectrum, Hg Se Br is a promising
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candidate for X-ray and γ-ray radiation detection at room temperature.
Keywords: chalcohalide, crystal growth, photoconductivity, mobility-lifetime product, X-ray
detector, γ-ray detector.
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Introduction
Solid state semiconductors for hard radiation detection at room temperature are needed in a
number of applications such as medical imaging, cosmic ray astronomy, national security, and
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for several scientific research, they generally provide better energy resolution than scintillator
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detectors. The semiconductor CdZnTe (CZT) is the current benchmark material for room
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temperature X-ray and γ-ray detection.
However, due to some of its intrinsic physical
properties, such as high Cd vapor pressure, high solubility of Te in CdTe at the crystal growth
temperature, etc, CZT is prone to form macroscopic defects such as grains, cracks, twins and
tellurium precipitates, which lead to low yield and high cost of the detector-grade single
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crystals. Therefore, there is a great demand for superior and low cost semiconductor materials
for radiation detection to meet the technological requirements.
Generally, among the required properties of a good semiconducting material for hard radiation
detection at room temperature are: 1) band gap (E ) in the range of 1.5 eV < Eg < 2.5 eV, which
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is large enough for high resistivity and small enough for small electron-hole ionization energy; 2)
high average atomic number (Z) for high stopping power to ensure high absorption; 3) high
mobility-lifetime products (μτ) for rapid signal readout; and 4) cost effective and efficient growth
of high quality single crystals. In the search of new detector materials, a variety of heavy metal
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chalcogenides (GaTe, Cs Hg S , TlGaSe etc.) and halides (PbI , HgI , Hg Br , BiI ,
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TlBr, CsPbBr etc.) have been investigated. Generally most binary heavy metal chalcogenides,
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such as HgQ (Q = Se and Te) possess high density, high average Z, and high carrier mobility but
have very low or zero energy gap. On the other hand, heavy metal halides such as HgX (X = Br,
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I) and TlBr have a suitable band gap for detector applications but they suffer from mechanical
softness, polarization effects, and low carrier mobility to be good detector materials. One
approach to obtain favorable properties in candidate materials is to combine attributes from both
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metal chalcogenides and halides by applying a lattice hybridization approach. This approach is
based on the fact that heavy metal chalcogenides generally have band gaps that are too low for
room temperature detection (giving high dark currents) while heavy metal halides tend to have
band gaps that are too high (giving low carrier mobilities). On the basis of this concept, we have
explored several chalcohalides (SbSeI, β-Hg S Cl , Hg Q Bi Cl (Q = S, Se, Te), TlHg Q Br (Q
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=
S, Se) )
and identified the thallium chalcohalides Tl SeI and Tl SI , to be promising
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materials for hard radiation detection
In this work, we extend the lattice hybridization concept to the mercury chalcohalide. Mercury is
a heavy element that is chemically stable. High purity elemental mercury can be easily obtained
and starting materials of mercury compounds such as mercury metal and mercury halides are
also relatively inexpensive. Therefore, mercury chalcohalides could be low cost materials for X-
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ray and γ-ray radiation detectors. Hg Se Br was reported more than 50 years ago,
but single
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crystals of Hg Se Br have not been grown and consequently the physical properties are
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unknown. Here, we report the synthesis, purification, and physical characterizations of large
Hg Se Br single crystals grown for the first time, which proposed as a very promising wide
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band gap semiconductor for hard radiation detection.
Experimental Section
Syntheses and Crystal Growth. The following reagents were used: (i) Mercury metal, 99.999%,
Alfa Aesar, Ward Hill, MA; (ii) Selenium shot, 99.999+%, Alfa Aesar, Ward Hill, MA; (iii)
Mercury Bromide, 99.99%, Alfa Aesar, Ward Hill, MA. HgSe was prepared as a stoichiometric
mixture of mercury metal and selenium shot. Liquid mercury was added by pipetting into a
ϕ13mm and ~30 cm long fused silica tube and then selenium was loaded. The tube was flame
-4
sealed under ~ 10 mbar (the bottom part of the tube was cooled in liquid N to prevent
2
evaporation of Hg metal). One end of the reaction tube was extended to the outside of the tube
o
furnace which was then set to 500 C and stayed for three days. HgSe formed and condensed at
the cool end of the tube. The “as synthesized” HgSe was transferred to a vial and stored in glove
box for further use. The commercially purchased HgBr was purified twice using vapor transport
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method. HgBr was loaded into a long quartz tube and sealed under vacuum. The tube was
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placed in a box furnace vertically, with one end extended to the outside of the furnace, and the
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temperature was set at 300 C. HgBr was transported and condensed at the top part of the tube,
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while black non-volatile impurities were left at the bottom of the tube.
HgSe and HgBr in the molar ratio of 2:1.05 were weighed and sealed in an evacuated thick wall
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quartz tube (O.D. 13mm x I.D. 7 mm) where the material mixture was loaded up to 80% of the
inner volume. The crystal growth was processed without a seed crystal. Over a period of 12 h,
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the tube was heated to 650 C and then kept there for 24 h to ensure a homogeneous melt. The
tube was then cooled to room temperature over 8 h. Then the tube was transferred to a three zone
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Bridgman furnace. Three zones in the furnace were set at 650 C - 650 C - 200 C from the top.
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The temperature gradient in the growth zone was about 38 C/cm, as shown in Figure 1(a). The
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growth speed was 1 cm to 2.4 cm per day. The as grown Hg Se Br ingots and crystals are
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shown in Figure 1(b).
Semi-quantitative elemental analysis by Energy Dispersive Spectroscopy (EDS) on several
Hg Se Br crystals cleaved from the ingot was performed using Hitachi S-3400 scanning
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electron microscope (SEM) equipped with a PGT energy dispersive X-ray analyzer with an
accelerating voltage of 25 kV and a 60 s accumulation time for data acquisition.
Thermal Analysis. In order to investigate thermal behavior of Hg Se Br , Differential Thermal
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Analysis (DTA) was performed using a Shimadzu DTA-50 Differential Thermal analyzer.
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Crystalline materials (~36 mg) were flame sealed in a silica ampoule evacuated at 10 Torr. A
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similarly sealed ampoule of Al O was used as a reference. Samples were heated to 650 C at a
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rate of 5 C/min and then cooled at 5 C/min to 50 C. This thermal cycle was repeated at the
same condition to verify the thermal behavior. The residue was then examined by X-ray powder
diffraction and compared to the diffraction patterns taken before DTA to verify that Hg Se Br is
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congruently melting. Melting and crystallization points were measured at a minimum
endothermic peak and a maximum exothermic peak.
X-ray Diffraction. Powder X-ray diffraction (XRD) for the samples randomly collected from
the grown crystals to examine the phase purity was performed using a PanAlytical X’Pert Pro
o
powder diffractometer (Cu Kα radiation, λ = 1.5418 Å) over the 2 θ range of 10 - 70 with a step
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size of 0.017 . The diffractometer was operated at 45 kV/40 mA.
Single crystal X-ray Diffraction data were collected at 293K on a STOE 2T image plate
diffractometer with Mo Kα radiation (λ = 0.71073 Å). Data reduction and numerical absorption
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corrections were done on the structures using STOE X-Area software package. The crystal
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structure was solved by direct methods and refined by full-matrix least square on F (all data)
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using the SHELXTL software suite. Thermal displacement parameters were refined
anisotropically for all atomic positions. The parameters for data collection and the details of the
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structure refinement are given in Table 1. Atomic coordinates and equivalent isotropic thermal
parameters are given in Table 2.
Optical Spectroscopy. The band gap energy of Hg Se Br was determined using optical diffuse
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reflectance spectroscopy. The measurement was performed at room temperature using a
Shimadzu Model UV-3101PC double-beam, double-monochromator spectrophotometer. BaSO4
was used as a 100% reflectance standard. The absorption data were calculated based on the
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obtained reflectance spectra data using the Kubelka-Munk equation.
Electronic Band Structure Calculations. The electronic band structure of Hg Se Br was
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investigated by first-principles electronic structure calculations within the Density Functional
Theory (DFT) framework. We employed the plane-wave basis set and pseudo-potential scheme
using the projector augmented wave method implemented in the Vienna Atomic Simulation
Package (VASP) code. The cut-off energy for the plane-wave basis was set to 450 eV and a
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x7x7 k-point mesh was used for Brillouin zone sampling. For exchange-correlation functional,
generalized gradient approximation (GGA) method is used with Perdew-Burke-Ernzerhof
formalism.
Electrical Resistivity and Photoconductivity. Single crystals of Hg Se Br were selected for
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the DC resistivity measurements by a two probe method using a Keithley 617 electrometer. Ti
5nm) and Pt (20nm) were evaporated on the front and back surfaces of the samples. The
(
resistance measurement was conducted perpendicular to the (001) plane of the plate crystal. The
electrical current from the two electrodes on the crystal sample was measured while voltage was
applied from -300 V to 300 V.
Photoconductivity measurement was carried out using a home-built setup on two single crystal
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plates of Hg Se Br with similar dimensions (3.0 × 2.1 × 0.5 mm and 3.9 × 2.5 × 0.6 mm )
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using the Amptek Mini-X Ag X-ray (50 kV, 40 μA) white radiation. The crystal was irradiated
perpendicular to the (001) plane by the X-ray beam. The bias voltage was maintained constant at
each value while the shutter on the X-ray source was switched ON and OFF manually. The
resulting current was recorded as a function of time using a Keithley 6517B Electrometer. The
X-ray induced photocurrent under different bias voltage was calculated by subtracting the X-ray
OFF current from the X-ray ON current. The carrier mobility-lifetime (μτ) products for electrons
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and holes were obtained by fitting the photocurrent curves versus bias voltage using the Hecht
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equation:
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L
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where I is saturation current, L is the sample thickness, and V is the applied voltage. A
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qualitative measurement of laser response was conducted. A blue diode laser (473 nm, 50 mW)
was focus on the crystal sample and turned ON and OFF every 20 seconds. The bias voltage was
set constant at 100V. The resulting current was recorded as a function time.
X-ray spectroscopy measurement was performed by the same X-ray source (50 kV, 40 uA).
Hg Se Br single crystal was used to measure the X-ray radiation spectrum. One Hg Se Br plate
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single crystal (3.9 × 2.5 × 0.6 mm ) was cleaved from the as grown ingot. The sample with large
cleaved surfaces was not polished or etched. Ti and Pt metal contact were evaporated onto the
two sides of the crystal. The single crystal was placed in an eV-480 test fixture attached to an
eV-550 preamplifier box (eV Products Inc.). An ORTEC 572A amplifier (Ametek Inc.) with a
gain of 1000 and 10 μs shaping time was used to amplify the signal before it is evaluated by a
dual 16 K input multichannel analyzer (Model ASPEC-927), the spectrum data was recorded
13,22
using MAESTRO-32 software.
Results and discussion
Synthesis and crystal growth. We synthesized Hg Se Br directly using a stoichiometric
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combination of HgSe and HgBr2.
The thermal behavior of the product obtained from the
synthesis was examined by DTA. In the DTA curves (Figure 2(a)), one endothermic peak at 566
o
o
C on heating and one exothermic peak at 543 C during cooling process and no change in a
repeated cycle were observed. The sample after the DTA measurement was examined by PXRD
to confirm that the sample remains as a single phase with no phase transitions or decomposition
after two cycles of heating and cooling. The experimental X-ray diffraction pattern of the sample
matches well with the calculated pattern from the crystal structure, Figure 3(a). This result
suggests that single crystals of Hg Se Br can be grown directly from stoichiometric melt by the
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Bridgman method. When the synthesis was scaled up to ~10 g, however, the binary phase HgSe
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as an impurity was observed in PXRD of the final product. The presence of HgSe is caused by
partial dissociation of Hg Se Br due to high vapor pressure of HgBr , which was not noticed in a
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small scale synthesis. Accordingly, 5 mol% excess of HgBr was added in the reaction mixture
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to compensate for the evaporation of HgBr during synthesis and also the starting polycrystalline
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mixture of HgSe and HgBr was filled up to > 80 % of total volume in the quartz tube to
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suppress evaporation of HgBr . A thick wall quartz tube (3 mm thick wall) was used to avoid
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explosion by excessive pressure of HgBr vapor. After the crystal growth process by the
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Bridgman method, some white and light yellow polycrystalline coating (Figure S1) was observed
on the surface of the Hg Se Br ingots, which was identified as excess HgBr by PXRD.
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The obtained Hg Se Br ingots are red in color and transparent to light as shown in Figure 1(b).
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Because of the layered structure, the ingots are apt to cleave along the (001) plane, which was
determined from XRD as shown in Figure 3(b). The color of the Hg Se Br crystals ranges from
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yellow (thin crystals ~ 0.3 mm) to red (thick crystals >1 mm) (Figure 1(b)). The crystal shows no
noticeable change in ambient atmosphere for several weeks. EDS/SEM analysis on the crystals
shows an average composition of Hg Se Br and no HgBr inclusions were found in the SEM
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micrograph, although an excess of HgBr was added in synthesis and crystal growth.
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Crystal Structure The cell parameters of Hg Se Br were previously determined by powder X-
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ray diffraction as a = 9.42 Å, b = 9.74 Å, c = 8.78 Å. Later, Yu. V. Minets et al. investigated
the structure by single crystal X-ray diffraction and found Hg Se Br crystallizes in the
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monoclinic space group C2/m with the cell parameters a = 17.529(6) Å, b = 9.408(4) Å, c =
1
7
9
.775(4) Å, and β = 89.51(3)°. However the single crystal X-ray diffraction data had only 684
reflections, which is not sufficient to obtain an accurate structure for monoclinic space group.
From our structural solution, the cell parameters a = 17.496(4) Å, b = 9.3991(19) Å, c = 9.776(2)
Å, and β = 90.46(3)° were refined based on 7859 reflections (2294 independent reflections)
collected. The data refinement resulted in very accurate atomic positions and consequently
significantly more precise atom distances and angles, Table 2 and 3.
As shown in Figure 4, the structure of Hg Se Br is built of [Hg Se ] layers spreading along the
3
2
2
3
2
ab plane, which are separated by Br atoms. There are also Br atoms embedded in the grooves of
the [Hg Se ] layers. The [Hg Se ] layers are formed by [Hg Se ] zig-zag chains (Figure 5(a))
3
2
3
2
2
2
connected by Hg-Se bonds between them (Figure 5(b)). The [Hg Se ] chains run parallel to the
2
2
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crystallographic a-axis. This chain is composed of Hg atoms linearly coordinated with two Se
atoms. The asymmetric unit of Hg Se Br contains five Hg atoms, four Br atoms, and two Se
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atoms. Selected distances and angles for Hg Se Br are summarized in Table 3. The Hg-Se
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range found in Hg/Se and Hg/Br compounds . The Hg-Se bond distances within the [Hg Se ]
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2
layer are in good agreement with the sum of the Hg and Se covalent radii and the relatively long
Hg-Br distances indicate weak metal-halogen interactions between the layers. This explains why
the as grown crystals cleave easily along the (001) plane. The asymmetric unit of Hg Se Br
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contains five Hg atoms, four Br atoms, and two Se atoms. Selected atom distances and angles for
Hg Se Br are summarized in Table 3. As shown in Figure 4, Hg Se Br is built of infinite
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2
[
Hg Se ] layers along the ab plane loosely bound by the interlayer Br atoms along the c-axis.
3 2
The [Hg Se ] layers are formed by [Hg Se ] zig-zag chains (Figure 5(a)) running parallel to the
3
2
2
2
crystallographic a-axis. The linear Hg-Se bond distances in [Hg Se ] (Figure 5(b)) are ~2.5 Å
2
2
which is in a good agreement with those reported in literatures.(ref. 4e, find one more from
online) The structure of Hg Se Br involves long interactions between Br and Hg atoms beyond
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2
4
the normal Hg-Br covalent distance which is in the range of 2.4 – 2.7 Å. Br(1) and Br(4)
enclosed inside the [Hg Se ] layers are weakly bonded by Hg(2) and Hg(3) in a square planar
3
2
geometry at the distance of 3.1448(14) - 3.3006(12) Å. Br(2) and Br(3) are bridging the [Hg Se ]
3
2
layers by long interactions with Hg(1), Hg (4), and Hg(5) at the distance of 2.8438(13) -
.1789(21) Å. The Hg-Br distances in these two circumstances are significantly longer than the
sum (2.5 Å) of covalent radii of Hg and Br, implying that Hg Se Br has a significant ionic
3
3
2
2
character in the structure. All Hg atoms are surrounded by Br and Se atoms in distorted
octahedral environments at the distance ranging from 2.844(1) to 3.741(1) Å and their detailed
local geometries are shown in Figure S2. The structure of ternary Hg Se Br retains motifs of
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2
2
2
5
linear Se-Hg-Se and octahedral HgBr units observed in the parent binary compounds, HgSe
6
2
6
and HgBr2 respectively. The weak Br-Hg interactions between and inside the [Hg Se ] layers
,
3
2
accounts for easy cleavage of the Hg Se Br crystals along the (001) plane as well as the partial
3
2
2
decomposition into two binary HgBr and HgSe during the Bridgman crystal growth process at
2
o
6
50 C that we observed.
Band Gap Energy. The UV/Vis/NIR absorption spectrum of Hg Se Br measured using ground
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2
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crystalline powder is shown in Figure 2(b). From the well-defined absorption edges, the optical
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band gap of Hg Se Br is estimated as 2.22 eV, which is in the range of 1.5 to 2.5 eV suitable for
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hard radiation detectors at room temperature. The band gap of Hg Se Br is between those of its
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parent binary compounds HgSe (0.061 eV at 300 K) and HgBr (3.65 eV), which directly
2
supports the feasibility of our chalcogenide-halide hybridization approach for band gap
engineering.
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Band Structure Calculations. To investigate the electronic properties related to the -ray
detection mechanisms of Hg Se Br , first-principles calculations were performed. The
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2
calculations were carried with the lattice constants obtained by our single crystal analysis. The
electronic band structure of Hg Se Br is shown in Figure 6(a). The band gap is direct in nature
3
2
2
and appears at the Г point. The gap energy 1.25 eV is underestimated from the optical band gap
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2
.22 eV measured as it is typical for this kind of calculations. The conduction bands of
Hg Se Br as depicted in Figure 6(a) are quite dispersive along Г - X, Г - Y and B - D directions,
3
2
2
indicating small effective masses for electron carriers. The valence bands, on the other hand, are
relatively flat, especially along the S – Y direction. As seen in the Projected-Density-of-States
plot in Figure 6(b), the conduction bands are mainly composed of Hg s- and Se p-states and the
valence bands are mixture of Br p-, Se p- , and Hg s-states.
Resistivity and Photoconductivity. The resistivity of a Hg Se Br crystal was measured using a
3
2
2
two probe method at room temperature. The voltage-current characteristics of the single crystal
sample are shown in Figure 7(a). The I-V curve deviates from linearity at positive bias, which is
a normal behavior for Schottky contact barriers and has often been observed in commercial CZT
3
0
crystals. The resistivity calculated from the slope of the I-V curve (-300 – 300 V) based on the
11
dimensions of the crystal was 5.19 x 10 Ω∙cm and is sufficiently high to suppress dark current
at room temperature, and also allowing for higher bias voltage be applied to increase detection
resolution.
Hg Se Br single crystals exhibit strong photoconductivity response when exposed to low flux
3
2
2
Ag X-rays. Figure 8 shows the current as a function of time while the X-ray beam was turned
ON-OFF. The X-ray induced current is ~3 times higher than dark current under 100 V bias. The
photoconductivity response curve as a function of positive and negative bias voltage is shown in
Figure 9, and the mobility-lifetime (μτ) products for electrons and holes were fitted as described
-4
in the Experimental Section. The mobility-lifetime products of Hg Se Br are (μτ) = 1.13 × 10
3
2
2
e
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cm /V for electrons and (μτ) = 1.26 × 10 cm /V for holes. These values are promising for a
h
relatively unexplored compound, grown here for the first time, and are comparable to (μτ) = 8 ×
e
-4
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-5
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31
10 cm /V and (μτ) = 3 × 10 cm /V of α-HgI a widely studied semiconductor for detector
h
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1
e,9a,17,32
applications.
Furthermore, the μτ of Hg Se Br samples is higher than other layered
3 2 2
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materials studied for X-ray and γ-ray detectors, such as PbI ((μτ) = 8 × 10 cm /V, (μτ) = 9 ×
2
e
h
-7
2
1e
-5
2
-5
2
33
1
0 cm /V), CsHgInS ((μτ) = 3.6 × 10 cm /V, (μτ) = 2.9 × 10 cm /V), CsCdInSe ((μτ) =
3
e
h
3
e
-5
2
-6
2
34
-5
2
-
1.18 × 10 cm /V, (μτ) = 2.74 × 10 cm /V), TlGaSe ((μτ) = 6 × 10 cm /V, (μτ) = 9.2 × 10
h
2
e
h
6
2
7
-6
2
-7
2
35
cm /V) and GaSe ((μτ) = 6.8 × 10 cm /V, (μτ) = 5.8 × 10 cm /V). The μτ products of
e
h
-2
Hg Se Br are lower than the leading room temperature detector material, CZT ((μτ) = 4.5 × 10
3
2
2
e
2
-4
2
31
cm /V, (μτ) = 1.0 × 10 cm /V) . Further purification of starting materials, reduction of trap
h
defects in Hg Se Br crystals, and wafer production with suitable chemical surface treatment are
3
2
2
expected to produce the material with lower trap densities and higher μτ products.
Hg Se Br single crystal has also shown strong photocurrent response when illuminated by a
3
2
2
blue diode laser (473 nm). The dark current and laser induced photocurrent under 100 V bias
voltage was recorded when the laser was turned ON-OFF (Figure S2S3). The dark current under
100 V bias was ~ 0.09 nA. Upon laser illumination of 50 mW, the photocurrent increased to a
peak value of ~7000 nA, indicating the photocurrent is >77,000 times larger than the dark
current. The laser photocurrent was much higher than the Ag X-ray induced because the photon
flux of the laser is thousands of times higher than that of the latter source.
Detector Property Hg Se Br is composed of elements with high atomic numbers (Z =80,
3
2
2
Hg
3
Z =34 and Z =35) and has a high density of 7.598 g/cm , giving the material a high stopping
Se
Br
power for X-ray and -ray radiation. The attenuation length of the material for hard radiation as a
function of photon energy was calculated using the atomic attenuation coefficient tabulated by
3
6
NIST as shown in Figure 7(b). The compound Hg Se Br has a much higher stopping power
3
2
2
than CZT, especially at high photon energy range of 0.1 to 10 MeV. For example, at the energy
1
37
of Cs (i.e., 662 keV), the attenuation length of CZT is 2.28 cm, while Hg Se Br is 1.39 cm,
3
2
2
confirming the much higher absorption coefficient compared to CZT at this energy. Therefore,
thinner crystals of the Hg Se Br can be used to absorb the hard radiation. This property helps to
3
2
2
relax some of the requirements for the high carrier mobility-lifetime products.
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To assess the detection performance of the Hg Se Br crystals for X-ray radiation, we performed
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Ag X-ray irradiation experiments at room temperature. The pulse height spectra obtained from
the Hg Se Br crystal are shown in Figure 10 (a) and Figure 11 (a) for electron and hole carriers,
3
2
2
respectively. The sample was 0.6 mm thick, the bias voltage was set in the range of 30/40 –
00/900 V, and the counting time was 200 seconds. The recorded spectra clearly show responses
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over X-ray irradiation compared to the dark conditions (without X-ray) for both electron and
hole carriers. The electron response exhibits well resolved two peaks, while the hole response
shows a broad unresolved peak. The peak position shifts to the high channel numbers as the
voltage increases, and then nearly becomes constant above 200 V. To get the mobility-lifetime
(μτ) products, the average peak positions are plotted as a function of voltage applied for electron
and hole responses in Figure 10 (b) and Figure 11 (b), respectively. The μτ values for both
carriers were estimated using the maximal charge collection efficiency (CCE) under each bias
3
7
voltage applied to the sample. The details of this method were presented in our previous report.
The estimated mobility-lifetime (μτ) products of Hg Se Br from these X-ray energy spectra are
3
2
2
-
4
2
-5
2
(
μτ) = 1.4 × 10 cm /V for electrons and (μτ) = 9.2 × 10 cm /V for holes, which are in the
e
h
same order of magnitudes as the vales obtained from photocurrent measurements.
The X-ray energy spectrum for electron response of a Hg Se Br single crystal is shown in
3
2
2
Figure 12(a) comparing with that of a commercial CZT crystal measured under 200 V bias
voltage. The counting times for CZT and Hg Se Br are set as the same of 200 seconds. Similar
3
2
2
to the CZT, the characteristic lines of Ag K 22.2 keV and K 24.9 keV are well resolved in the
α
β
spectrum detected by Hg Se Br crystal. The count of K line from the Hg Se Br crystal is
3
2
2
α
3
2
2
comparable to that of CZT, while the signal for K is weaker. CZT has a low μτ for hole carriers
β
and thus as expected did not show a hole response on X-ray irradiation at the same measurement
5
7
condition. Furthermore, Hg Se Br crystal has also exhibited counter response under Co γ-ray
3
2
2
irradiation. The pulse height energy spectra in Figure 12(b) shows obvious counting comparing
to the dark condition under positive bias 500 V, 600 V and 700 V, respectively. This result from
the Hg Se Br crystal suggests comparable γ-ray spectroscopy capabilities found in Bi doped
3
2
2
3
8
CZT. Such results are encouraging considering the Hg Se Br crystal is still at the early stage
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2
2
of development. There should be some impurities and defects in our crystal samples which act as
traps for photo induced electrons and holes, better spectroscopy performance from Hg Se Br
2
3
2
can be achieved using improved crystal with better quality synthesized in the future.
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Compared to other mercury chalcohalides β-Hg S Cl in the same chemical series which we
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reported earlier, Hg Se Br crystal exhibit much stronger photo-induced response. The μτ
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-4
2
product for electron carriers obtained under X-ray is 1.4 × 10 cm /V for Hg Se Br crystal
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which is ~ 4 times higher than 3.2 × 10 cm /V of β-Hg S Cl . This high μτ of Hg Se Br is
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confirmed by the pulse height spectra of Ag X-ray detected using Hg Se Br crystal, while there
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was no Ag X-ray spectra reported using β-Hg S Cl crystal. One of the main reasons could be
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2
the high quality with lower defect concentration in Hg Se Br crystals that can be grown directly
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1
5a
from stoichiometric melt, while β-Hg S Cl crystal is formed by decomposition process of
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quaternary Hg Bi S Cl melt involving other phases in the product. One of the main reasons
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8
could be the high quality with lower defect concentration in Hg Se Br crystals that were grown
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directly from stoichiometric melt, while β-Hg S Cl crystals were formed by decomposition
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2
process of quaternary Hg Bi S Cl in which other phases may present in the product.
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2
8
Conclusion
The mercury chalcohalide, Hg Se Br , is a promising candidate material for X-ray and
3
2
2
possibly γ-ray radiation detection. It has a very high stopping power, suitable optical band gap
and high resistivity for the detector applications. Large single crystals of Hg Se Br up to 1 cm in
3
2
2
length were grown by Bridgman method for the first time. Hg Se Br crystallizes in a layered
3
2
2
structure with 2-D layers weakly bonded by Br atoms. The single crystal exhibits strong
photocurrent response to both Ag X-ray and blue laser radiation. Pulse height spectra of Ag X-
ray were obtained from Hg Se Br crystal, where two peaks corresponding to the characteristic
3
2
2
lines of Ag K 22.2 and K 24.9 keV were well resolved. The µτ products of HgSe Br
2
α
β
2
Hg Se Br are comparable or superior to other layered materials such as PbI , CsHgInS ,
3
2
2
2
3
CsCdInSe etc. Hg Se Br is a low cost and promising material for X-ray and γ-ray detection,
3
,
3
2
2
suggesting that further investigation and development to improve the quality of single crystals
are warranted.
Acknowledgment
This work was supported by the Office of Nonproliferation and Verification Research and
Development under National Nuclear Security Administration of the U.S. Department of Energy
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under contract No. DE-AC02-06CH11357. Use of the Center for Nanoscale Materials, an Office
of Science user facility, was supported by the U. S. Department of Energy, Office of Science,
Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.
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Corresponding Author
*To whom correspondence should be addressed. E-mail: m-kanatzidis@northwestern.edu.
Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval
to the final version of the manuscript.
Notes
The authors declare no competing financial interest.
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Supporting Information. The Supporting Information includes the following information:
Figure S1, The HgBr coating formed on the surface of as grown Hg Se Br ingot. Figure S2, the
2
3
2
2
coordination environment of Br and Hg atoms. Figure S2S3, Photocurrent response of Hg Se Br
2
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single crystal to the blue laser radiation. Also, the crystallographic information file (CIF). This
material is available free of charge via the Internet at http://pubs.acs.org.
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Physik 1932, 77, 235; (g) Puff, H.; Küster, J. Naturwissenschaften 1962, 49, 299.
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G. Ieee T Nucl Sci 2011, 58, 1981.
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(a) Liu, Z.; Peters, J. A.; Li, H.; Kanatzidis, M. G.; Im, J.; Jin, H.; Freeman, A. J.; Wessels, B.
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(b) Li, H.; Malliakas, C. D.; Liu, Z. F.; Peters, J. A.; Sebastian, M.;
(c) Im,
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Wessels, B. W.; Freeman, A. J.; Kanatzidis, M. G. Chem Mater 2013, 25, 2868; (c) Wibowo, A. C.;
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(a) Johnsen, S.; Liu, Z. F.; Peters, J. A.; Song, J. H.; Nguyen, S.; Malliakas, C. D.; Jin, H.;
(b) Nguyen, S.
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16) Liu, Z. F.; Peters, J. A.; Nguyen, S.; Sebastian, M.; Wessels, B. W.; Wang, S. C.; Jin, H.; Im,
J.; Freeman, A. J.; Kanatzidis, M. G. Proc Spie 2012, 8507.
(a) Wibowo, A. C.; Malliakas, C. D.; Li, H.; Stoumpos, C. C.; Chung, D. Y.; Wessels, B. W.;
(b) Wibowo, A. C.; Malliakas, C.
(c) Wibowo, A.
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Minets, Y. V.; Voroshilov, Y. V.; Pan'ko, V. V. J Alloy Compd 2004, 367, 109.
X-AREA, X-SHAPE, and X-RED; STOE & Cie GmbH: Darmstadt, Germany, 2004.
Sheldrick, G. M. SHELXTL version 5.1; Univeristy of Göttingen: Göttingen
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(b) Tandon, S. P.; Gupta, J. P. physica status solidi (b)
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Uxa, S.; Grill, R.; Belas, E. J Appl Phys 2013, 114.
Liu, Z.; Peters, J. A.; Li, H.; Kanatzidis, M. G.; Wessels, B. W. Semicond Sci Tech 2013, 28.
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b) Hostettler, M.; Schwarzenbach, D.; Helbing, J.; Dmitriev, V.; Weber, H. P. Solid State Communications
004, 129, 359.
(
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Hostettler, M.; Schwarzenbach, D.; Helbing, J.; Dmitriev, V.; Weber, H.-P. Solid State
Communications 2004, 129, 359.
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L.; broerman, J. G.; Nelson, D. A.; Whitsett, C. R. Phys. Rev. B 1974, 9, 1598
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(b) Mattila, T.; Zunger,
A. Phys Rev B 1998, 58, 1367; (c) Wu, X. F.; Walter, E. J.; Rappe, A. M.; Car, R.; Selloni, A. Phys Rev B
009, 80; (d) Cohen, A. J.; Mori-Sánchez, P.; Yang, W. Science 2008, 321, 792; (e) Cohen, A. J.;
Mori-Sánchez, P.; Yang, W. Phys Rev B 2008, 77, 115123.
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Bilikiss, A.; Wright, G. W.; Li, L.; Fu, F.; James, R. B. J Electron Mater 2003, 32, 756;
M.; Szeles, C. J Appl Phys 2006, 100, 014503.
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Piechotka, M.; Magerl, A.; Steichele, E.; Wetzel, G.; Kaldis, E. J. Cryst. Growth 1995, 146, 112;
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September 7 - 12, 2009, Munich, Germany: Vol. 25/2 Diagnostic Imaging; Dössel, O., Schlegel, W. C., Eds.;
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Wessels, B. W.; Freeman, A. J.; Kanatzidis, M. G. Chem Mater 2013, 25, 2089.
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(a) Rodnyi, P. A. Physical Processes in Inorganic Scintillators; Taylor & Francis, 1997;
(
(
(
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Table 1. Crystal data and structure refinement for Hg Se Br at 293(2) K.
3
2
2
Empirical formula
Formula weight
Temperature
Hg Se Br
3 2 2
919.51
293(2) K
0
1
2
3
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9
0
1
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9
0
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8
9
0
1
2
3
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5
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9
0
1
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9
0
Wavelength
0.71073 Å
Monoclinic
C 2/m
Crystal system
Space group
a = 17.496(4) Å, α = 90.00°
b = 9.3991(19) Å, β = 90.46(3)°
c = 9.776(2) Å, γ = 90.00°
Unit cell dimensions
3
Volume
1607.6(6) Å
Z
8
3
Density (calculated)
Absorption coefficient
F(000)
7.598 g/cm
-1
76.049 mm
3024
3
Crystal size
0.1 x 0.09 x 0.02 mm
4.11 to 29.21°
θ range for data collection
-
23<=h<=23, -12<=k<=12, -
Index ranges
13<=l<=13
Reflections collected
Independent reflections
Completeness to θ = 29.21°
Refinement method
7859
2294 [R = 0.0729]
int
99.2%
2
Full-matrix least-squares on F
2294 / 0 / 76
Data / restraints / parameters
Goodness-of-fit
0.989
Final R indices [>2σ(I)]
R indices [all data]
Robs = 0.0339, wRobs = 0.0563
R = 0.0613, wR = 0.0597
all
all
Extinction coefficient
Largest diff. peak and hole
0.000052(9)
-
3
3.700 and -2.080 e·Å
2
2 2
4
1/2
R = Σ||F |-|F || / Σ|F |, wR = {Σ[w(|F | - |F | ) ] / Σ[w(|F | )]} and calc
o
2
c
2
o
o
c
o
2
2
2
w=1/[σ (Fo )+(0.0109P) ] where P=(Fo +2Fc )/3
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Table 2. Atomic coordinates (x10 ) and equivalent isotropic displacement parameters (Å x10 )
for Hg Se Br at 293(2) K with estimated standard deviations in parentheses.
3
2
2
*
Label
Hg(1)
Hg(2)
Hg(3)
Hg(4)
Hg(5)
Br(1)
Br(2)
Br(3)
Br(4)
Se(1)
X
Y
z
Occupancy
Ueq
0.1239(1)
1/4
0.2245(1)
0.2017(1)
1/2
1
1
1
1
1
1
1
1
1
1
1
40(1)
41(1)
40(1)
40(1)
43(1)
30(1)
32(1)
35(1)
32(1)
28(1)
27(1)
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
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1/4
0
0.7641(1)
1/2
0.2722(1)
-0.0249(1)
0.1236(1)
0.1184(1)
0.1249(1)
0.1267(1)
0.2657(1)
-0.0179(1)
1/2
0.2154(1)
0.2297(1)
0.4779(2)
0.0069(2)
0.0229(2)
0.4770(2)
0.2459(2)
0.2464(2)
1/2
1/2
0
1/2
0
0.2363(2)
0.2359(2)
Se(2)
*
U is defined as one third of the trace of the orthogonalized U tensor.
eq
ij
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Table 3. Selected Bond
lengths atom distances
[
[
Å] and Angles angles
deg] for Hg Se Br at
3
2
2
293(2) K with
2.5177(11)
Hg(4)-Br(2)
2.9070(19)
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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7
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9
0
1
2
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4
5
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7
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9
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4
5
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1
2
3
4
5
6
7
8
9
0
estimated standard
deviations in
parentheses. Hg(1)-
Se(1)
Hg(1)-Se(2)
2.5247(12)
2.5045(12)
2.4971(12)
2.4994(13)
2.4903(13)
2.8438(13)
3.1243(13)
3.4195(16)
3.2326(11)
3.1965(12)
3.3006(12)
3.1448(14)
Hg(4)-Br(3)
Hg(4)-Br(4)
Hg(5)-Br(1)
Hg(5)-Br(1)
Hg(5)-Br(3)
3.1789(21)
3.4765(22)
3.5410(21)
3.3517(21)
3.0152(21)
Hg(2)-Se(1) × 2
Hg(3)-Se(2) × 2
Hg(4)-Se(1) × 2
Hg(5)-Se(2) × 2
Hg(1)-Br(2)
Hg(1)-Br(3)
Se(1)-Hg(1)-Se(2)
Se(1)-Hg(2)-Se(1)
Se(2)-Hg(3)-Se(2)
Se(1)-Hg(4)-Se(1)
Se(2)-Hg(5)-Se(2)
Br(2)-Hg(1)-Br(3)
159.55(4)
180.0
Hg(1)-Br(4)
Hg(2)-Br(1)
179.99(7)
165.28(6)
170.68(6)
103.92(4)
Hg(2)-Br(4)
Hg(3)-Br(1)
Hg(3)-Br(4)
Hg(1)-Se(1)
2.5177(11)
2.5247(12)
2.5045(12)
2.4971(12)
2.4994(13)
2.4903(13)
2.8438(13)
Hg(4)-Br(2)
2.9070(19)
3.015(2)
159.55(4)
180.0
Hg(1)-Se(2)
Hg(5)-Br(3)
Hg(2)-Se(1) × 2
Hg(3)-Se(2) × 2
Hg(4)-Se(1) × 2
Hg(5)-Se(2) × 2
Hg(1)-Br(2)
Se(1)-Hg(1)-Se(2)
Se(1)-Hg(2)-Se(1)
Se(2)-Hg(3)-Se(2)
Se(1)-Hg(4)-Se(1)
Se(2)-Hg(5)-Se(2)
179.99(7)
165.28(6)
170.68(6)
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Hg(1)-Br(3)
3.1243(13)
Br(2)-Hg(1)-Br(3)
103.92(4)
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Figure 1. (a) Temperature profile of the Bridgman furnace, (b) As grown Hg Se Br ingot and
3
2
2
single crystals, the crystal cleaves along (001) plane.
o
Figure 2. (a) DTA curves of crystalline Hg Se Br , showing melting at 566 C and crystallization
3
2
2
o
o
at 541543 C. The heating rate is 5 C/min; (b) Optical absorption spectrum of Hg Se Br ground
3
2
2
powder showing a band gap of 2.22 eV.
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Figure 3. (a) Powder X-ray diffraction patterns of ground Hg Se Br crystals and the residue sample of
3
2
2
the DTA experiments comparing to the simulated pattern; (b) X-ray diffraction pattern of a
Hg Se Br single crystal.
3
2
2
Figure 4. The structure of Hg Se Br view down the b-axis.
3
2
2
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0
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9
0
Figure 5. (a) Infinite one dimensional [Hg Se ] zigzag chain as viewed down b-axis; (b) [Hg Se ]
2
2
3
2
layer formed by [Hg Se ] chains connected by Hg-Se bonds.
2
2
Figure 6. (a) Calculated electronic band structure for Hg Se Br ; (b) Element orbital
3
2
2
contributions to the electronic density of states (per eV unit cell), as a function of the energy
relative to the Fermi level at 0 eV.
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Figure 7. (a) DC voltage-current curve from a Hg Se Br single crystal; (b) Comparison of the
3
2
2
attenuation length between Hg Se Br and CZT.
3
2
2
-
-
15V
30V
-20V
-35V
-60V
-90V
-25V
-40V
-70V
-100V
0
.5
.4
0V
15V
5V
10V
25V
50V
90V
0
.1
.0
20V
40V
70V
(b)
(a)
-50V
80V
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0V
60V
00V
-
0
0
1
125V
0.3
0.2
-0.1
-0.2
0.1
0.0
1
00 200 300
Time (s)
1
00 200 300
Time (s)
3
Figure 8. Photocurrent response of on a Hg Se Br plate crystal (3.0  2.1  0.5 mm ) along c-
3
2
2
axis, (a) under positive bias and (b) under negative bias.
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Electron
Holes
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.3
0.2
.1
0.0
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0
-
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2
(
(
) =1.13X10 cm /V
e
-5
2
) =1.26X10 cm /V
h
0
20 40 60 80 100
Bias Voltage (V)
Figure 9. Ag X-ray induced photocurrent as a function of the bias voltage applied for electron
3
and hole carriers measured on a Hg Se Br plate crystal (3.0  2.1  0.5 mm ) along c-axis.
3
2
2
-4
2
-5
Mobility-lifetime (µτ) products for electron and hole carriers are 1.13×10 cm /V and 1.26×10
2
cm /V, respectively.
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Figure 10. (a) The pulse height energy spectra of unfiltered Ag X-ray for electron response from
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Hg Se Br crystal (3.9 × 2.5 × 0.6 mm ) under bias voltage from 30 V to 800 V, (b) The average
3
2
2
peak positions obtained as a function of bias voltage. The (μτ) value was estimated from the
e
fitted data.
Figure 11. (a) The pulse height energy spectra for hole response from Hg Se Br crystal (3.9 ×
3
2
2
3
2
.5 × 0.6 mm ) under bias voltage from 40 V to 900 V, (b) The average peak positions obtained
as a function of bias voltage. The (μτ) value was estimated from the fitted data.
h
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