Photoconductivity in Tl6SI4: A Novel Semiconductor for Hard

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Photoconductivity in Tl6SI4: A Novel Semiconductor for Hard Radiation Detection Sandy L. Nguyen,† Christos D. Malliakas,† John A. Peters,‡ Zhifu Liu,‡ Jino Im,§ Li-Dong Zhao,† Maria Sebastian,‡ Hosub Jin,§ Hao Li,† Simon Johnsen,† Bruce W. Wessels,‡ Arthur J. Freeman,§ and Mercouri G. Kanatzidis*,† †

Department of Chemistry, ‡Department of Materials Science and Engineering, and §Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60208, United States S Supporting Information *

ABSTRACT: The chemical concept of lattice hybridization was applied to identify new chalcohalide compounds as candidates for X-ray and γ-ray detection. Per this approach, compound semiconductor materials with high density and wide band gaps can be produced that can absorb and detect hard radiation. Here, we show that the mixed chalcogenide−halide compound Tl6SI4 is a congruently melting, mechanically robust chalcohalide material with strong photoconductivity response and an impressive room-temperature figure of merit. Tl6SI4 crystallizes in the tetragonal P4/mnc space group, with a = 9.1758(13) Å, c = 9.5879(19) Å, V = 807.3(2) Å3, and a calculated density of 7.265 g·cm−3. The new material requires a more simplified crystal growth compared to the leading system Cd0.9Zn0.1Te, which is the benchmark room-temperature hard radiation detector material. We successfully synthesized Tl6SI4 crystals to produce detector-grade wafers with high resistivity values (∼1010 Ω·cm) and high-resolution detection of X-ray spectra from an Ag (22 keV) source. KEYWORDS: chalcogenide, wide-gap semiconductors, crystal growth, radiation detection

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INTRODUCTION Hard radiation (e.g., X-ray, γ-ray) detection has many uses in defense, research, medical, and industrial applications.1 The best γ-ray detection material is Ge, but it is limited in practicality because operation requires cooling with liquid nitrogen to prevent thermal noise. Effective room-temperature nuclear detection materials are needed. These require large resistivity values (ρ ≥ 108 Ω·cm) and band gaps wide enough (1.5 eV ≤ Eg ≤ 2.4 eV) so that only excitations caused by ionizing radiation, and not thermal energy, are detected. Dark current generated by thermal ionization results in a low signalto-noise ratio and limited detector resolution. At the same time, a high mass density (≥6 g·cm−3) must be engineered into the materials to increase the stopping power of the detector. These attributes are often mutually exclusive, as increased density of the elements in a material is generally correlated with large band widths and consequently smaller band gaps. The figure of merit for γ-ray detector materials is the μτ product, where μ is the mobility of the carriers and τ is the carrier lifetime. For CdxZn1−xTe (CZT), the commercial benchmark, the μτ products for electron and holes are (μτ)e ≈ 2 × 10−2 cm2 V−1 and (μτ)h ≈ 1 × 10−5 cm2 V−1, respectively, and these respective values are rather low for other well-studied materials such as HgI2 (3 × 10−4 and 4 × 10−5 cm2 V−1), PbI2 (1 × 10−5 and 3 × 10−7 cm2 V−1), TlBr (6.5 × 10−3 and 4 × 10−4 cm2 V−1), etc.1,2 Current leading compound semiconductor γ-ray detector materials are generally restricted to binary or © XXXX American Chemical Society

pseudobinary compounds, such as CZT, TlBr, HgI2, or PbI2, which are limited in terms of crystal quality, mechanical stability, polarization effects, and μτ product.1,3 The metal chalcogenides such as CZT have high densities but feature narrow band gaps. CZT, in particular, suffers from Te precipitates and a low thermal conductivity, which hinder large-scale crystal growth.4 In contrast, heavy metal halides can have high densities and large band gaps (on the larger end of spectrum of desirable band gaps) resulting from the higher ionicity of the compounds but have serious limitations as well. For example, TlBr suffers from polarization effects attributed to ionic conduction as well as difficulties in device processing as a result of its extremely low hardness.1,5 HgI2 has poor charge transport properties and is highly susceptible to plastic deformation.6 Lastly, PbI2 has low resolution at higher γ-ray energies resulting from its low μτ product.7 In general, band gaps > 2.6 eV may lead to lower mobility of photoexcited carriers.8 Previously, we reported on how the concept of “dimensional reduction” can identify new, functional alkali metal-based chalcogenide semiconductor materials.9 Per this approach, binary semiconductors with high mass density and narrow band gaps can be converted to ternary materials with wider band Received: April 29, 2013 Revised: June 6, 2013

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dx.doi.org/10.1021/cm401406j | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials

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Figure 1. Lattice hybridization: addition of thallium chalcogenides to halide materials results in wide band gap semiconductors combining the best elements of both types. Growth of Tl6SI4 was performed in tapered, carbon-coated, vacuumsealed 12−15 mm diameter fused silica tubes using a two-zone Bridgman furnace equipped with a motor-controlled platform. Crystal growth proceeded with a lowering rate of 0.5 mm·h−1 in a temperature gradient of 15−40 °C·cm−1. The temperature of the hot zone was about 600−650 °C to ensure complete melting of the compound, while the cold zone was set at about 100 °C to allow for in situ annealing to increase crystal quality and electronic properties. Several samples were annealed at ∼250 °C for 1.5−2 days to increase crystal quality. Crystal Processing and Characterization. Samples were cut with a Struers Accutom-50 waferizing saw with a 300 μm wide diamond-impregnated blade and polished with silicon carbide sand paper and alumina slurries (0.05−1 μm particle size). Further processing using HI as an etchant was performed to decrease surface damage caused by polishing. All manipulations were performed in a hood with adequate ventilation, and proper protective equipment (face masks, labcoats, and two layers of gloves) was used. The quality of the synthesized wafers was checked using transmitted polarized light, and analysis of the surface of the polished and etched crystals was performed using a Hitachi S3400N-II scanning electron microscope (SEM) equipped with a backscattered electron (BSE) detector operating with an accelerating voltage of 20 kV. The presence of large single-crystalline domains was verified using Laue backreflection from a Mo X-ray source. Powder X-ray diffraction (XRD) for verification of phase purity was performed on a Si-calibrated Inel CPS 120 diffractometer with a position-sensitive detector and graphite-monochromated Cu Kα radiation operating at 40 kV and 20 mA. The XRD powder pattern was recorded using the Windif data acquisition program. To verify cell parameters, single-crystal X-ray diffraction was performed at 298(2) K on Tl6SI4 with a Stoe image plate diffraction system (IPDS) II diffractometer using graphitemonochromated Mo Kα radiation (λ = 0.71073). Data reduction and numerical absorption corrections were done on the structures using Stoe X-Area software.14 Structures were solved by direct methods and refined by full-matrix least-squares on F2 (all data) using the SHELXTL software suite.15 Thermal displacement parameters were refined anisotropically for all atomic positions. Thermal Analysis. To verify the melting point of Tl6SI4, differential thermal analysis was performed using a Shimadzu DTA50 thermogravimetric analyzer. Ground crystalline material (∼30 mg) was flame sealed in a silica ampule evacuated to 10−3 Torr. As a reference, a similarly sealed ampule of ∼30 mg of Al2O3 was used. Samples were heated to 550 °C at 5 °C/min and then cooled at 5 °C/ min to 100 °C. Samples were cycled for a second time at 5 °C/min to 550 °C and then cooled at −5 °C/min to 50 °C. Products of the DTA experiments were examined by X-ray powder diffraction and compared to patterns taken before DTA to verify that Tl6SI4 is congruently melting. The reproducibility of DTA measurements was monitored by running multiple heating/cooling cycles. Melting and crystallization points were measured at a minimum endothermic peak and a maximum exothermic peak. Thermal Conductivity. The obtained Tl6SI4 ingot was ground into powder and then densified at 573 K for 2 min in a 12.7 mm

gaps and high density. This is accomplished by addition of a salt unit to break up the 3D structure of the binary compound and effectively reduce the dimensionality of the lattice.9c,10 Recently, we modified this approach to use highly dense Tl as the cation in the salt added, resulting in the promising detector materials Tl2Hg3Q4 (Q = S, Se, Te).11 Incorporation of Tl has the added benefit of higher covalency in the materials produced relative to alkali metal cations12 as a result of larger atomic orbitals and serves to increase band dispersion and mobility of the electron carriers generated by nuclear radiation. An alternative concept to “dimensional reduction” is that of lattice hybridization where, for example, a metal chalcogenide (typically of low band gap) can be combined with a metal halide (typically of high band gap) to produce a chalcohalide exhibiting an intermediate band gap.8 Thus, in principle, addition of two materials with undesirable properties for γ-ray detection can result in a hybrid material with superior performance relative to its component parts (Figure 1). We suggest that the heavy metal halides and chalcogenides be combined to produce chalcohalides, a new class of roomtemperature γ-ray detection materials such as Tl6SeI4. Tl6SeI4 is a very promising γ-ray detector material, with higher mechanical stability compared to TlI and an excellent band gap energy (1.86 eV) compared to Tl2Se (Eg ≈ 0.6 eV).8 In this report, we combined TlI (Eg = 2.75 eV) and Tl2S (Eg = 1.12 eV)13 to produce Tl6SI4. Compared to Tl6SeI4, Tl6SI4 has the added benefit of easier bulk single-crystal growth, a larger band gap, and superior mechanical properties. We show that Tl6SI4 is another promising member of the class of Tl-based chalcohalides that needs to be studied in detail for use in hard radiation detection.

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EXPERIMENTAL SECTION

Synthesis and Crystal Growth. Warning: Tl is very toxic, and great care should be exerted with appropriate protective equipment in both the synthesis and handling of the Tl6SI4 crystals. Synthesis of Tl6SI4 was performed via combination of Tl2S and TlI in the appropriate stoichiometry. Tl2S was generated by combining Tl (99.99% metals basis, Alfa Aesar) and S (99.98%, 5N Plus Inc.) in a tapered, carboncoated fused silica tube under 10−4 mbar vacuum for 12 h at 550 °C. Subsequently, this material was recrystallized and purified in a dualzone Bridgman furnace with the top zone set at 550 °C and the bottom zone at 150 °C. The top and bottom ends of the resulting ingot were removed and discarded to ensure purity of the resulting starting material. To make Tl6SI4, Tl2S and TlI (99.999% CERAC) were combined in a stoichiometric ratio in a glovebox under inert N2 atmosphere. These starting materials were sealed in a fused silica tube under 10−4 mbar vacuum and reacted at 550 °C for 24 h. Purification of Tl6SI4 proceeded with the use of a program-controlled traveling heater furnace with a set temperature of 500 °C. B



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Figure 2. Crystal structure of Tl6SI4 in (a) the c direction and (b) the a direction. (c) Distorted square pyramidal coordination environment of Tl, and (d) inert lone pair effect of Tl exhibited in voids of crystal structure. Coordination environments of (e) octahedral S and (f) distorted trigonal antiprismatic I. diameter graphite die under an axial compressive stress of 40 MPa using the spark plasma sintering (SPS) system method (model SPS211Lx). The high-density SPS processed pellet was polished into coins of Ø ≈ 8 and 2 mm thickness for thermal diffusivity measurements. Our highly dense sample achieved 95.9% of the theoretical density for Tl6SI4. Samples were coated with a thin layer of graphite to minimize errors from the emissivity of the material. Total thermal conductivity was calculated using the equation κ = αCpd

multiplier (PMT) tube R928. An optical chopper and lock-in amplifier were used to increase the signal-to-noise ratio: the PMT tube is equipped with a photon-counting system coupled to a preamplifier linked to the chopper frequency. Electronic Transport and Detector Property Measurements. Direct current conductivity was measured using a Keithley 617 electrometer. Photoconductivity was measured in a custom setup featuring a chopped He−Cd laser (325 nm) for sample illumination. Gold electrodes of ∼80 nm thickness were evaporated on the front and back surfaces of the samples in a parallel plate configuration; the chopper frequency was 51 Hz, and the load resistance was 5 kΩ. The time-dependent photocurrent was recorded with bias voltages to the illuminated electrode of up to 250 V. The photocurrent response curves were then analyzed using Many’s equation to obtain the μτ values for holes and electrons as a function of positive and negative bias voltages, respectively.17 For strongly absorbed light, the relation is

(1)

where is κ the total thermal conductivity, α is the thermal diffusivity coefficient, Cp is the specific heat, and d is the density. The thermal diffusivity coefficient (α) was measured using the laser flash diffusivity method in a Netzsch LFA457, the specific heat capacity (Cp) was indirectly derived using a representative sample (Pyroceram 9606) in the range of 300-573 K, and the density (d) was determined using the dimensions and mass of the sample. Thermal diffusivity data were analyzed using a Cowan model with pulse correction, and heating and cooling cycles gave reproducible values for each sample. The uncertainty of the thermal conductivity is estimated to be within 5%, considering the uncertainties for α, Cp, and d. Optical Characterization. Room-temperature optical diffuse reflectance measurements on powdered crystalline samples were performed using a computer-controlled Shimadzu UV-3101PC double-beam, double-monochromator spectrophotometer equipped with an integrating sphere. Samples were ground to a fine powder and spread on a compacted surface of powdered BaSO4, used as 100% reflectance standard material, preloaded into a sample holder. Reflectance data was converted to absorption data as described earlier,16 and the band gap was determined as the intersection point between the energy axis and the line extrapolated from the linear portion of the absorption edge in a (a/S)2 vs E plot, where a is the absorption coefficient and S is the scattering coefficient. Lowtemperature photoluminescence (PL) measurements were performed on polished samples in a He vapor cryostat. Individual samples are illuminated by a chopped 325 nm line from a He−Cd laser at 2.6 mW as an excitation source, and the PL signal is dispersed by a 0.75 m SPEX grating monochromator attached to a Hamamatsu photo-

I(U ) =

⎛ ⎜1 − exp I0μτU ⎝

( )⎠⎞

L2

1+

−L2 μτU

Ls Uμ

⎟

(2)

Here, I is the photocurrent, I0 is the saturation current, L is the sample thickness, and U is the applied voltage. Our photoconductivity measurements have been shown to yield reliable μτ values using CZT measurements as a reference.8,9f,10c,11,18 The surface recombination parameter s/μ is related to the quality of the processed sample surface; s/μ lowers the charge collection efficiency and should be minimal for optimal detector performance. In this photoconductivity measurement setup, the incident laser light (325 nm) is strongly absorbed because the photon energy is higher than the band gap energy of the material. Consequently, photoconductivity measurements using light with energies above the band gap are inherently sensitive to surface recombination, as photon absorption occurs near the surface. Detector properties were analyzed using white radiation from a Ag X-ray tube source (ATPS XRD 1000) with a 40 kV accelerating voltage and 20 mA current. A polished Tl6SI4 wafer with dimensions of 1.1 mm × 4.3 mm × 5.2 mm was processed for detector application. Ag contacts were used for the electrodes of the detector sample. C



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Sample was placed in an eV-480 test fixture attached to an eV-550 preamplifier box. The bias voltage was 500 V, corresponding to an electric field of 4.55 × 105 V/m. A reference detector, the SPEAR detector with 5 × 5 × 5 mm3 CZT, was operated at a bias voltage of 1000 V corresponding to an electric field of 2 × 105 V/m. An ORTEC 572A amplifier with a gain of 1000 and 0.5 μs shaping time was used to amplify the signal before inputting the latter into a dual 16 K input ASPEC-927 multichannel analyzer. Data was then recorded using MAESTRO-32 software. A schematic of the setup has been reported previously.18 Mechanical Property Assessment. Analysis of the mechanical strength of Tl6SI4 was performed via nanoindentation measurements using a Hysitron Triboindenter equipped with a Berkovic tip (triangular cross section). Five indents were performed on each sample. Each indent consisted of a trapezoidal load function with loading (10 s), dwelling (30 s), and unloading (10 s) phases. Indents were performed in load-controlled feedback mode with a maximum load of 400−600 μN for the dwelling phase, resulting in a loading rate of 100 μN·s−1. Modulus and hardness values were computed using the TriboIndenter software for each indentation trace. Theoretical Calculations. Band structure calculations were performed using the full-potential linearized augmented plane wave (FLAPW) method19 with the screened-exchange local density approximation (sX-LDA).20 Full and scalar relativistic treatments were applied for the core and valence states, respectively. Experimental lattice parameters and atomic coordinates used for the calculations were obtained in the single-crystal refinement shown below. The interstitial plane-wave basis and star functions energy cutoffs were 12.3 and 144 Ry, respectively.

Table 1. Crystallographic Refinement Data for Tl6SI4 at 298(2) Ka fw temperature wavelength cryst syst space group unit cell dimens

volume Z density (calcd) abs coeff F(000) cryst size θ range for data collection index ranges no. of reflns collected no. of independent reflns completeness to θ = 25.66° refinement method data/restraints/parameters goodness-of-fit final R indices [>2σ(I)] R indices [all data] extinction coefficient largest diff. peak and hole

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RESULTS AND DISCUSSION Previously, we have shown that Tl6SeI4 is a very promising γray detector material with higher mechanical stability compared to TlI and optimized band gap compared to Tl2Se.8 Its structural analog described here, Tl6SI4, has the added benefit of easier bulk single-crystal growth, a larger band gap, and superior mechanical properties. In addition, we show that Tl6SI4, too, is a viable candidate for room-temperature radiation detection. Single-Crystal Characterization. Tl6SI4 crystallizes in the tetragonal P4/mnc space group, with a = 9.1758(13) Å, c = 9.5879(19) Å, V = 807.3(2) Å3, and a calculated density of Dcalcd = 7.265 g·cm−3 (Figure 2a and 2b). Four atomic positions are found in the structure of Tl6SI4. The Tl(1) atom resides at a 8h Wyckoff position, while Tl(2), I, and S atoms reside at 4e, 8g, and 2b positions, respectively. Tl atoms are found in a coordination that is a slightly distorted square pyramid, where the top of the pyramid is the S atom and the bottom of the pyramid is a square plane of 4 I atoms (Figure 2c). Tl is found in the middle of the square, slightly below the plane formed by the 4 I atoms. The directional nature of Tl bonding in this structure and the void spaces between these distorted square pyramidal polyhedra point to the symmetry-directing effect of the Tl atom’s inert lone pair (Figure 2d and 2e). The S atom is octahedrally coordinated by Tl atoms, while the I atom is coordinated by 6 Tl atoms in an extremely distorted trigonal antiprism (Figure 2f). Details of crystallographic data collection and refinement of Tl6SI4 are shown in Tables 1−3. The threedimensional tetragonal crystal structure of Tl6SI4 is likely to result in enhanced mechanical properties relative to layered van der Waals gapped semiconductor materials, making it suitable for processing and device fabrication. Single-phase purity was verified with powder XRD, and uniformity of the ingots was verified by elemental mapping using SEM/EDS (see Supporting Information). Single crystallinity of crystal domains was verified using Laue

1765.88 298(2) K 0.71073 Å tetragonal P4/mnc a = 9.1758(13) Å, α = 90° b = 9.1758(13) Å, β = 90° c = 9.5879(19) Å, γ = 90° 807.3(2) Å3 2 7.265 g/cm3 67.402 mm−1 1428 0.024 × 0.019 × 0.009 mm3 3.14−25.66° −11 ≤ h ≤ 6, −11 ≤ k ≤ 10, −10 ≤ l ≤ 11 1870 406 [Rint = 0.0579] 98.5% full-matrix least-squares on F2 406/0/18 1.388 Robs = 0.0480, wRobs = 0.0891 Rall = 0.0584, wRall = 0.0912 0.0018(5) 1.476 and −1.435 e·Å−3

R = Σ∥Fo| − |Fc∥/Σ|Fo|, wR = {Σ[w(|Fo|2 − |Fc|2)2]/Σ[w(|Fo|4)]}1/2, and calcd w = 1/[σ2(Fo2) + (0.0305P)2 + 14.1721P], where P = (Fo2 + 2Fc2)/3.

a

backscattering diffraction, with the presence of large singlecrystalline domains seen in the measured samples. To assess the mechanical properties of Tl6SI4, hardness measurements were performed with loads experimentally applied by a pyramidal nanoindenter. At a load of 0.4−0.6 N for 30 s, the Knoop hardness of Tl6SI4 is >98.0 kg·mm−2, which is higher than Tl6SeI4, CdTe, and ZnTe (∼60−80 kg·mm−2)1,8,21 and significantly greater than TlBr (∼17 kg·mm−2).22 This higher hardness value implies increased mechanical robustness and potential for wider applicability compared to conventional room-temperature detector materials. Optical Characterization. Crystals of Tl6SI4 are transparent red (Figure 3), with crystal colors varying between batches. UV−vis electronic absorption spectroscopy revealed a consistently sharp band gap of 2.1 eV. A representative spectrum (Figure 4a) indicated high purity of the resulting crystals. PL measurements performed on samples of Tl6SI4 do not show any peaks at ∼2.1 eV. Instead, a defect peak below the band gap is seen at 1.65 eV (Figure 5a), exhibited consistently between different samples of Tl6SI4. Preliminary DFT calculations suggest that it is the result of transitions between shallow levels caused by S and Tl vacancies (discussed below). One exception in the PL results was found for sample SN22471, with an extra peak at 2.4 eV (Figure 5b). Interestingly, the peak at 2.4 eV is too high in energy to be attributed to the band gap even though PL peaks often blue shift at low temperatures. The band gap is only 2.06 eV at room temperature, and a shift of nearly 0.4 eV is highly unlikely. The presence of this additional peak at 2.4 eV suggests some compositional variation in the sample. We considered the possibility that it may be a D



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Table 2. Tl6SI4 crystal growth, optical, and charge transport properties sample number

band gap (eV)

SN2172-1 SN2198-2 SN2218-3

2.10 2.26 2.04

SN2247-1

2.07

SN2247-2

2.07

SN2260-3

2.05

resistivity (Ω·cm)

(μτ)e (cm2·V−1)

(s/μ)e (V·cm−1)

(μτ)e (cm2·V−1)

(s/μ)h (V·cm−1)

0.5

1.0 × 1010 2.2 × 1010 1.8 × 1010

2.1 × 10−3 3.50 × 10−4 5.61 × 10−5

1175 1000 11

2.3 × 10−5 5.76 × 10−5 2.11 × 10−5

5740 760 64

0.5

5.7 × 109

1.04 × 10−4

1302

2.29 × 10−5

525

0.5

3.2 × 1010

2.51 × 10−6

525

0.5

2.6 × 1010

growth parameters multiple fast runs (3) lowered 1.2 mm/h gradient 19 °C/cm; lowered mm/h + annealing gradient 28 °C/cm; lowered mm/h + annealing gradient 37 °C/cm; lowered mm/h + annealing gradient 30 °C/cm; lowered mm/h + annealing

processing parameters polished with 0.3 μm grit polished with 0.3 μm grit polished with 0.05 μm grit + HI etch polished with 0.05 μm grit + HI etch polished with 0.05 μm grit + HI etch polished with 0.3 μm grit

Table 3. Property Comparison of Leading Hard Radiation Semiconductor Detector Materials compound Tl6SI4 Tl6SeI4 Cd0.9Zn0.1Te TlBr HgI2 PbI2

crystal system tetragonal tetragonal cubic cubic tetragonal hexagonal

density (g·cm−3) 7.3 7.4 5.8 7.5 6.4 6.2

band gap (eV) 2.04 1.87 1.52 2.75 2.13 2.55

congruent melting yes yes no yes yes no

(μτ)e (cm2·V−1) 2.1 7 2 6.5 3 1

× × × × × ×

−3

10 10−3 10−2 10−3 10−4 10−5

(μτ)h (cm2·V−1) 2.3 6 5 4 4 3

× × × × × ×

−5

10 10−4 10−5 10−4 10−5 10−7

ρRT (Ω·cm) 10

10 4 × 1012 (3−5) × 1010 >1.0 × 1010 1013−1014 1013

ref this paper 8, 32 2, 33 30 1, 34 1, 34a, 35

Figure 3. Crystals of Tl6SI4. (a) Crystals with naturally cleaved faces in the vertical direction (SN2218-3). (b) Crystal chunk was removed from the tube and broken off from ingot (SN2172-1). (c) Crystal cut perpendicular to the direction of growth and polished with alumina slurry (SN2247-1). (d−f) Polished crystal cut perpendicular to the growth direction viewed under a polarized microscope (SN2247-1). Colors of the crystals varied between batches, with the most transparent red crystals (SN2247-1) containing an excess of TlI.

Figure 4. Optical band gap and differential thermal analysis measurements of Tl6SI4, a congruently melting compound, indicate that it has a band gap of Eg ≈ 2.1 eV and a melting point of 427 °C.

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Figure 5. Photoluminescence (PL) of Tl6SI4 samples: SN2172-1 at 55 K and SN2247-1 at 53 K. Absorption measurements performed on Tl6SI4 show the band gap energy to be 2.06 eV. PL measurements do not show any peak at this energy. Instead, a defect peak below the band gap at 1.65 eV is seen in SN2172-1 (a). PL of the sample SN2247-1 shows an additional peak at 2.4 eV (b). Even though PL peaks often blue shift at low temperatures, the peak at 2.4 eV is too high in energy to be attributed to the band gap; its presence suggests some compositional variation in the sample and may be attributed to an excess of TlI.

result of precipitation of one of the starting materials, TlI. Tl2S has a band gap of 0.98 eV, which is too low to be possibility,23 but TlI has a band gap of 2.8 eV, which matches closely with the high-energy edge of the PL peak. There was no other change in slope seen in the UV−vis spectrum of sample SN2247-1, indicating that the level of excess TlI in the material is relatively low. Great care was taken when handling, cutting, and processing the Tl6SI4 crystals owing to the high level of toxicity of Tl-based materials. Carbon coating was found to be essential for maintaining a high degree of single crystallinity as reaction of Tl2O impurities in the starting material with the glass resulted in etching of the tube and polycrystalline growth resulting from excess nucleation points. Future attempts will be made to purify the elemental Tl starting material to remove oxide impurities. In general, crystals were somewhat brittle and poorer quality samples tended to cleave or fall apart during polishing. Greater robustness of the crystals was seen with annealing, as well as during the crystal growth step when the lowering speed of the samples was reduced from 1.2 to 0.5 mm/h and the furnace gradient was increased from 15 to ∼30 °C/cm. Thermal Properties. On the basis of DTA measurements of Tl6SI4, we find that it is a congruently melting material with a melting point of 427 °C (Figure 4b), in reasonable agreement with literature reports of 444 °C.24 Thermal properties of Tl6SI4 were assessed to determine the growth parameters necessary for optimization of crystal growth. Higher thermal conductivity correlates with the ease of scaling up single-crystal growth, ideally allowing for synthesis of larger crystals as a result of a low horizontal gradient in a vertical Bridgman growth setup. Thermal conductivity measured to determine and optimize crystal growth parameters of Tl6SI4 crystals yielded a value of 1.20(6) W·m−1·K−1 at room temperature, decreasing to 0.99(5) W·m−1·K−1 at 300 °C (Figure 6), comparable to that of molten and solid CZT (2 and 1 W·m−1·K−1, respectively)25 and higher than that of TlBr (0.52 W·m−1·K−1).26 This higher thermal conductivity, added to the fact that Tl6SI4 is a stoichiometric compound, indicates that its use in radiation detection should reduce the time and cost of detector production compared to CZT, a solid solution material known for suffering from Te precipitates and compositional inhomogeneity. Band Structure Calculations. The overall electronic structure of Tl6SI4 is shown in Figure 7. Relative to thallium halide materials, which feature wide indirect band gaps,27 Tl6SI4

Figure 6. Thermal conductivity (left axis, black) and diffusivity (right axis, blue) of Tl6SI4.

exhibits a more narrow direct gap reflecting the increase in covalent character brought about by addition of Tl2S (Eg ≈ 1.12 eV)13 into the structure of TlI (Eg = 2.75 eV).28 This is exhibited in the 2.36 eV band gap of Tl6SI4 calculated by the sX-LDA, which is 0.48 eV larger than Tl6SeI48 and relatively close to the experimentally derived value of 2.1 eV. The larger band gap of Tl6SI4 relative to its Se analog originates from stronger Tl−S bonding, designated by the smaller Tl−S distances (a 2.3−2.7% reduction). Thus, the low-energy spectra and many other electronic properties are determined by S−Tl6 octahedra. Strong Tl−S bonding results in wide band dispersion and hence small effective masses of both n (0.124 in the Γ to Z direction and 0.515 in the Γ to X/Y direction) and p-type carriers (0.185 in the Γ to Z direction and 0.600 in the Γ to X/Y direction). Specifically, the band structure along the Γ to Z direction of the Brillouin zone is more disperse compared to the in-plane directions; this is reflected in the effective masses as well. In the projected density of states (Figure 7), the conduction and valence bands are mainly composed of Tl 6p and S 3p, respectively. The out-of-plane Tl(1) atoms have a slightly larger contribution to the band edges (valence band maximum and conduction band minimum) than that of the in-plane Tl(2) atoms, reflected in the differences in Tl−S and Tl−I bond lengths. The Tl(1)−S bonds are 2.86 Å compared to 3.01 Å for the Tl(2)−S bonds, reflecting a higher degree of Tl−S hybridization in the out-of-plane direction. Conversely, the in-plane Tl atoms feature shorter Tl−I bonds (3.46 Å) compared to the out-of-plane Tl(1)−I bonds (3.47 Å). These F



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Figure 7. (a) Electronic band structure of Tl6SI4 within the sX-LDA scheme and (b) projected density of states.

Figure 8. Resistivity measurements of Tl6SI4 (a) parallel and (b) perpendicular to the direction of crystal growth indicate that it is a highly resistive (ρ∥ = 2.2 × 1010 Ω·cm; ρ⊥ = 5.7 × 109 Ω·cm) semiconductor.

Figure 9. Photocurrent response of Tl6SI4 measured using a He−Cd chopped laser with a wavelength of 633 nm. Mobility−lifetime (μτ) product values for electron and hole carriers in Tl6SI4 are comparable to that of commercially used CZT.

wafers cut both parallel and perpendicular to the growth direction (Figure 8). This value is comparable to CZT, which can achieve resistivity values typically on the order 1010−1011 Ω·cm,1,3a,29 and it is well above the requirement of >108−109 Ω·cm for effective γ-ray detector materials. High resistivity values result in minimal dark current, a requisite for efficient nuclear detection. Higher electric fields can then be used to increase detector resolution and decrease readout time. Using the Many equation to fit the photoconductivity data measured on wafers of Tl6SI4, the mobility−lifetime products were in the range of (μτ)e = 2.1 × 10−3 cm2·V−1 for electrons

differences illustrate the anisotropy in band dispersion and can explain the higher mobilities of carriers in the Γ to Z direction (see below). Relative to Tl6SeI4 effective masses for electrons (0.097 in the Γ to Z direction and 0.485 in the Γ to X/Y direction) and holes (0.134 in the Γ to Z direction and 0.719 in the Γ to X/Y direction), bonding between S and Tl results in a comparable dispersion of the valence and conduction bands and similar effective masses and mobilities of the carriers. Electronic Transport and Detector Property Measurements. The wide band gap of the material is also reflected in its high resistivity, which was measured to be ∼1010 Ω·cm in G



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Figure 10. Backscattered scanning electron microscope images of Tl6SI4 before and after etching with concentrated HI for (a) 10, (b) 40, and (c) 60 s. More uniform surface with decreased roughness and number and depth of scratches from polishing is observed with increased etching time.

and (μτ)h = 2.3 × 10−5 cm2·V−1 for holes (Figure 9). Notably, the (μτ)e of Tl6SI4 is comparable to both Tl6SeI4 [(μτ)e = 7 × 10−3 cm2·V−1 and (μτ)h = 6 × 10−4 cm2·V−1] and recently reported traveling heater method-grown commercially available TlBr [Radiation Monitoring Devices; (μτ)e = 6.5 × 10−3 cm2·V−1 and (μτ)h = 4 × 10−4 cm2·V−1].30 Though lower than the (μτ)h of TlBr, the (μτ)h of Tl6SI4 is also on the order of that found for optimized commercially grown Bridgman CZT [Redlen Products, (μτ)h ≈ (0.5−5) × 10−5 cm2·V−1 and (μτ)e ≈ 2 × 10−2 cm2·V−1].2 Via fitting of the photoconductivity data shown in Figure 9, the surface quality of the samples can be assessed in the form of calculated s/μ values; lower values correlate with higher sample surface quality. For Tl6SI4, it was found that use of HI as an etchant decreased the surface roughness and depth of scratches from polishing, as seen under a scanning electron microscope (Figure 10). With improved polishing and etching techniques, calculated s/μ parameters were found to decrease by as much as 2 orders of magnitude from ∼5700 to ∼50 V·cm−1 between samples. Optimization of etchant composition should lead to a substantially lowered s/μ.31 Radiation detection response to a Ag X-ray source (22 keV) in the form of a pulse height spectrum was measured (Figure 11). The spectrum shows the presence of two peaks at 22.0 and

the given Ag X-ray energy source. Hence, sample thickness is likely to play a large role in the efficiency of X-ray detection and will have to be more closely studied for optimization of detector setups. Ag X-ray detection by Tl6SI4 shows a calculated energy resolution based on the larger Kα peak of 0.59 keV at fwhm, or 2.6%, at 22.2 keV compared to 3.2 keV (14%) at 22.1 keV for a commercial SPEAR CZT detector that we used for comparison. For any given time interval of measurement (180, 300, and/or 660 s), CZT showed at least 1 order of magnitude higher signal intensity. However, the resolution of our Tl6SI4 crystal was significantly higher than the CZT samples used as references (both SPEAR and research grade CZT). This is a very promising result, as there is a great potential for improvement of radiation detection resolution and signal strength with optimization of Tl6SI4 crystal growth parameters in the future. Continuing development of crystal growth techniques for chalcohalides (e.g., improved starting material purification, optimized growth parameters, and enhanced surface treatment) should result in significant improvement of detector material properties, as previously seen for CZT and TlBr.

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CONCLUSION

We applied a simple chemical concept as a design tool for discovering new X- and γ-ray detector materials. The so-called lattice hybridization approach led to identification of Tl6SeI48 and Tl6SI4. The latter has better crystal growth properties. High-quality single crystals of Tl6SI4 can be grown using the Bridgman method, and they are promising as a potential radiation detector material. This material exhibits a high figure of merit (μτ) and energy resolution comparable to CZT and TlBr. With a favorable combination of high crystal density and wide band gaps in the visible region, Tl6SI4 is another compound semiconductor member of the important class of thallium chalcohalides showing capacity as effective roomtemperature X- and γ-ray detector materials. The stoichiometric nature of Tl6SI4 relative to conventional binary and solid solution detector materials (e.g., CZT) makes it a good candidate for bulk crystal growth in high yield. Its higher hardness contributes to the ease of sample processing and should allow facile device fabrication. These factors are of major importance if high-quality detector-sized wafers are to be pursued for commercial applications. Continued improvements in crystal growth and sample processing of Tl6SI4 could result in new room-temperature detectors that can outperform currently used devices.

Figure 11. Pulse-height spectrum of Ag X-ray detection response of Tl6SI4. Increase in signal intensity is seen with increased exposure time. Resolution of the major Kα peak with an exposure time of 600 s is 2.6% at fwhm, an improvement on the CZT sample used as a standard reference.

22.2 keV, which can be attributed to the two Kα peaks of Ag based on their proximity and peak positions. The Kβ peaks, which are higher in energy (24.9 keV), were not seen, indicating that the thickness of the sample is not high enough to detect the portion of the Ag X-ray spectrum above 22.2 keV. Samples above 2 mm in thickness did not demonstrate detection, indicating that this thickness may be too high for H



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ASSOCIATED CONTENT

S Supporting Information *

CIF with fractional atomic coordinates; displacement parameters for Tl6SI4. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Department of Homeland Security with grant 2010-DN-077-ARI042-02. SEM/EDS analyses were performed at the EPIC facility of the NUANCE Center at Northwestern University (NU), Laue X-ray diffraction was performed at the Jerome B. Cohen X-ray Diffraction facility, and hardness measurements were performed at the Nanoscale Integrated Fabrication, Testing and Instrumentation (NIFTI). Support for these facilities is granted by NSF-NSEC, NSF-MRSEC, Keck Foundation, the State of Illinois, and NU.

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Photoconductivity in Tl6SI4: A Novel Semiconductor for Hard

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