An Effective Purification Process for the Nuclear Radiation Detector

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An Effective Purification Process for the Nuclear Radiation Detector Tl6SeI4 Wenwen Lin, Oleg Y. Kontsevoi, Zhifu Liu, Sanjib Das, Yihui He, Yadong Xu, Constantinos C. Stoumpos, Kyle M McCall, Alexander JE Rettie, Duck Young Chung, Bruce W. Wessels, and Mercouri G. Kanatzidis Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00242 • Publication Date (Web): 18 Apr 2018 Downloaded from http://pubs.acs.org on April 18, 2018

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Crystal Growth & Design

An Effective Purification Process for the Nuclear Radiation Detector Tl6SeI4 Wenwen Lin,† Oleg Y. Kontsevoi,§,⊥Zhifu Liu,‡ Sanjib Das,‡ Yihui He,† Yadong Xu,† Constantinos C. Stoumpos, † Kyle M. McCall,

†, ‡

Alexander J.E. Rettie,※ Duck Young Chung,※ Bruce W.

Wessels,‡ and Mercouri G. Kanatzidis†,*

†

Department of Chemistry, §Department of Physics and Astronomy,

⊥Northwestern-Argonne

Institute of Science and Engineering, ‡Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, United States, ※Materials Science Division, Argonne National Laboratory, Lemont, IL 60439

ABSTRACT: The semiconductor Tl6SeI4 was previously identified as a promising semiconductor for room temperature nuclear radiation detection. As the detection performance and carrier transport strongly depend on concentration of impurity energy levels acting as scattering centers and carrier trapping, material purification is a crucial prerequisite step to obtain spectroscopic-grade detector performance. In this contribution, we present a highly efficient purification method using a bent ampoule for evaporating Se, Tl2Se and TlI precursors for Tl6SeI4. Based on impurity analysis performed by glow discharge mass spectroscopy, the main impurities in Tl2Se were identified to be Pb, Bi and Al, while in TlI the main impurities are Al and Sn. The bent-ampoule method successfully reduces or removes the Cl, Pb, and Te impurities from the Se precursor, the Pb, Bi and Al impurities from the Tl2Se precursor, and removes Sn from TlI. Informed by the analysis results, density functional theory calculations were performed to study the identified impurities and related defects. The calculation results show that Bi and Al act as deep defect levels, which can be detrimental to the detector performance of Tl6SeI4. If the growth condition of Tl6SeI4 is Tl-rich/Se-poor, impurity of Si can introduce deep donors. However, it becomes electrically benign if growth conditions are Tl-poor/Se-rich, while Sn and Pb impurities are shallow donors. Centimeter-size Tl6SeI4 crystals were grown by the two-zone vertical Bridgman method using the purified precursors. The detector made of Tl6SeI4 crystal maintains the high resistivity on the order of 1011 Ω·cm after purification, ideal for suppressing leakage 1

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current. The detector exhibits both full-energy and Tl escape photopeaks upon 122 keV γ-ray from 57

Co radiation source. The electron mobility-lifetime product µeτe for Tl6SeI4 detector is 8.1 × 10-5

cm2·V-1. Based on the carrier rise time measured from output pulses induced by 5.5 MeV α-particles from 241Am, the electron and hole mobilities were estimated to be 112 ± 22 and 81 ± 16 cm2·V-1·s-1, respectively, comparable to those of the leading detector materials HgI2 and TlBr. These results validate the potential of this compound for hard radiation detection, and the impurity analysis presented here allows future efforts to focus on reducing the concentration of the identified impurities.

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INTRODUCTION

There are rapidly growing demands for developing low-cost and high-performance semiconductor compounds for room temperature nuclear radiation detection in diverse fields including homeland security, medical imaging, nonproliferation of nuclear materials and petroleum well logging.1-3 CdxZn1-xTe (CZT) is currently the leading semiconductor utilized in room temperature radiation detection, with a high resistivity (≥ 109 Ω·cm) and high carrier mobility lifetime product µτ for electrons (above 10-3 cm2·V-1).4-6 However, CZT still suffers from intrinsic defects such as Te precipitates and poor compositional uniformity,3, 7 leading to a low yield and high production cost. TlBr, another leading high performance detection material with a µτ product comparable to CZT, suffers from weak mechanical properties and severe polarization effects.8-11 Other studied semiconductor materials include HgI2, PbI2, and BiI3,12-18 however, these materials are even more ionic and have undesirable two-dimensional structures and softer lattices compared to CZT and TlBr. Therefore, it is imperative to find alternative high-quality semiconductor detector materials with low production cost and simple crystal growth technology. At present, ternary semiconductor compounds for nuclear detection are much less explored. However, we have been studying on alkali metal-based chalcogenide and halide ternary compounds to identify and screen new semiconductor detector materials.19-24 Based on the concepts of lattice hybridization,20,21 the generally narrow bandgaps of heavy chalcogenides can be widened by combining the features of chalcogenides and halides to produce hybrid chalcohalide compounds with suitable bandgaps (~1.6 - ~2.6 eV). In this context, we previously identified the Tl-based ternary semiconductor Tl6SeI4 as a promising material for high-performance room temperature hard radiation detection.25-26 This material has an appropriate bandgap of 1.86 eV, a very high density of 7.38 g·cm-3 , high average atomic number (Z = 66) and high resistivity of 1010 Ω·cm.25 The low melting point (417 oC) and congruent melting of Tl6SeI4 makes single crystal growth easier and possibly allows a lower-concentration thermally activated intrinsic defects. This compound crystallizes in a 3D tetragonal structure (P4/mnc space group),27 and has a high static dielectric constant of 18.28 The high dielectric constant strongly screens charged defects and impurities, which in turn reduces carrier scattering and trapping effects. 3



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However, the detector performance of Tl6SeI4 is still poorer than those of CZT and TlBr, so further investigation is required to reach its full potential for nuclear radiation detection. The long history of development of the leading detection materials CZT and TlBr suggests that the detection performance strongly depends on the concentration of deep levels with large capture cross sections, which act as scattering, trapping and non-radiation recombination centers for carriers.29-30 Therefore, to obtain high performance, it is crucial to minimize the concentration of deep levels. These deep levels arise from native (intrinsic) defects or impurity-induced (extrinsic) defects. Theoretical calculations on native defects of Tl6SeI4 revealed that vacancies of VI and VSe are both deep donors, which may exist in appreciable concentrations and subsequently cause carrier trapping.28 The concentrations of these two deep levels can be lowered by appropriately adjusting experimental conditions of synthesis, crystal growth and annealing. On the other hand, the concentration of extrinsic defects, such as impurity-induced deep levels, is much harder to control but cannot be neglected. Given the severely detrimental effects of impurity-induced deep levels on detection performance, material purification is crucial for obtaining higher performance. In this contribution, we present the purification of the precursor reagents of Tl6SeI4 (i.e., Se, Tl2Se and TlI) by a highly efficient evaporation method in a bent ampoule. The effectiveness of this purification process was assessed by trace elemental analysis of the precursors before and after purification. Informed by the elemental analysis results, density functional theory calculations were performed to identify potential deep levels induced by the main impurities identified. A centimeter-size Tl6SeI4 single crystal was grown with purified precursors using a Bridgman method to evaluate the effectiveness of the purification methods in improving the charge transport properties. The effective purification process on the precursors leads to a greatly increased photoresponse for Ag X-rays (22.4 keV) and occurrence of full-energy photopeak for 122 keV γ-rays from

57

Co radiation source. Based on a time-of-flight technique using 5.5 MeV

α-particles, the drift mobilities of electrons and holes were determined to be 112 ± 22 and 81 ± 16 cm2·V-1·s-1, respectively, which are comparable to those of the leading detector materials TlBr and HgI2.13, 31 The impact of purification on detection performance and potential of this material is discussed.

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EXPERIMENTAL SECTION Purification of Precursors by Evaporation in a Bent Tube. Tl6SeI4 raw material was

synthesized by a direct reaction of 0.01 mol (0.7896 g) Se, 0.02 mol Tl (4.0876 g) and 0.04 mol (13.2516 g) TlI precursors with a stoichiometric combination. Se and TlI are volatile enough to be purified by evaporation at elevated temperatures. Because of low vapor pressure32 of Tl metal, we prepared and purified the volatile Tl2Se binary precursor instead of Tl metal. Tl2Se was synthesized by chemical stoichiometric reaction of 0.2 mol Tl (40.8760 g) metal and 0.1 mol purified Se (7.896 g). The conventional purification by evaporation involves a straight ampoule by virtue of vapor transport driven by temperature gradient, as shown in Figure 1a. However, due to the reflux of droplets condensing on the inner wall of ampoule, this method is not sufficiently efficient for purifying Se, Tl2Se and TlI. In this work, we invented a new evaporation method for purification. A silica ampoule is bent in the middle using flame, and one half of the ampoule is loaded with material to be purified. The tube is sealed under vacuum and mounted into a two-zone furnace in horizontal configuration, as shown in Figure 1b. The half ampoule loaded with material is located in the hot zone of furnace, while the other half ampoule is located in the cold zone of finance. In order to evaporate the material, the hot zone is set to a temperature above the melting point or boiling point of material. In order to create a desirable temperature gradient, the cold zone is set to a temperature lower than hot zone. Once melted, the material begins to vaporize and condenses along the temperature gradient as droplets. Driven by gravity, these droplets flow and accumulate at the cold end of ampoule. In this way, the reflux of droplets can be avoided, and thereby a much higher efficiency of purification and higher transfer rates can be achieved. The purification by evaporation in a bent tube was applied to purify Se, Tl2Se and TlI at a 20 g scale for each precursor sealed under 10-4 mbar pressure. TlI was preheated under vacuum of 10-2 mbar in dynamic vacuum at 76 oC for 24 h in order to remove surface moisture prior to purification. Note that the commercial Tl metal has a black oxide layer, as Tl is very air sensitive. Therefore, before synthesis of Tl2Se, this black oxide layer on Tl precursor was manually scraped off with a blade. Silica ampoules with a length of 300 mm, inner diameter of 9 mm and outer diameter of 13 mm were used. The ampoules were bent in the middle using a flame to an approximate angle of 120o. The specific hot and cold zone temperatures used are shown in Table 1. 5



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After complete material transport, purified precursors were collected in the cold zone. Composition and Impurity Analysis: Stoichiometry of purified Tl2Se and TlI were analyzed using a Hitachi S4800-II electron microscope (SEM) equipped with an energy-dispersive spectroscopy (EDS) detector. Impurity analysis was performed on Se, Tl2Se and TlI precursors before and after purification by using Glow Discharge Mass Spectrometry (GDMS) in EAG Laboratories. Positive Ar+ ions are accelerated onto the sample to generate erosion and atomization of the surface of sample. The sputtered species from sample surface are ionized by plasma, and then extracted into a mass spectrometer where they are identified and measured. Since all of our samples are electrically semi-insulating, the initiation of plasma is difficult. In order to increase the conductivity of sample, the sample was crushed into small pieces and embedded into high-purity indium metal, which is very conductive. The accuracy of GDMS measurements is in the range of 1 part per million (ppm) to 0.1 part per billion (ppb) depending on the host material and the specific impurity. Electronic Band Structure Calculations. The electronic structure of impurities in Tl6SeI4 was investigated by employing first-principles total energy calculations within the density functional theory (DFT) framework. We employed the Projector Augmented Wave method33 that was implemented in Vienna Ab-initio Simulation Package34-35. The energy cut off for the plane wave basis was set to 350 eV. The exchange-correlation contribution to the potential was included by employing the generalized gradient approximation (GGA) within the Perdew-Burke-Ernzerhof (PBE) functional.36 In order to describe the isolated impurities and defects, 2 × 2 × 2 supercells containing 176 atoms and a 3 × 3 × 3 k-point mesh were utilized. The internal atomic positions of the defect structures were fully relaxed until the residual forces on atoms were less than 0.01 eV·Å-1, while the lattice parameters of the supercells remained fixed. Synthesis and Crystal Growth. Tl6SeI4 polycrystalline raw material was synthesized by the direct chemical reaction of 0.01 mol (4.8773 g) purified Tl2Se and 0.04 mol (13.2516 g) TlI binary precursors in an evacuated silica ampoule at 520 oC for 12 h in a rocking furnace, followed by slow cooling with a rate of -50 oC·h-1. Fine graphite powder with a weight ratio of 0.1% (purity of 99.9995%, 200 mesh, Alfa Aesar) was added to Tl2Se and TlI precursors to remove oxide impurity.37 The soaking temperature of 520 oC for synthesis ensured complete reaction and 6


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melting of Tl6SeI4. After complete reaction, the ternary raw material was loaded into a conical-tipped fused silica ampoule (10 mm ID and 14 mm OD), which was then sealed under continuous pumping. A single-crystalline ingot of Tl6SeI4 was grown by the two-zone vertical Bridgman method. Before crystal growth, the ampoule was loaded in the hot zone (600 oC) of a Bridgman furnace for 12 h to achieve complete melting. After melting, the ampoule was then descended to the cold zone at a translation speed of 0.5 mm·h-1.The temperature for the cold zone was set at 200 oC to generate a temperature gradient of 30 oC·cm-1. After crystal growth, the ingot was annealed in-situ at 200 oC for 24 h without translation of travelling stage. In order to evaluate the effectiveness of purification on the impurity concentration and detection performance, another batch of Tl6SeI4 raw material was also synthesized with the same Tl, Se and TlI precursors but without purification. The unpurified Tl6SeI4 raw material was also subject to crystal growth under identical conditions for comparison. Detector

Device

Fabrication

and

Charge

Transport

Measurements.

The

Bridgman-grown Tl6SeI4 single crystalline ingots were sliced perpendicular to the growth direction. One wafer was selected from the middle section of each ingot for characterization. The sample grown with purified precursors was labeled to “Sample A”, while the reference sample grown with raw materials without purification was labeled to “Sample B”. The surfaces of Samples A and B wafers were polished with 800 and 1200 grit silicon carbide papers and Al2O3 slurries with a particle size of 0.05-1 µm. The sample was mounted on 1-square inch electrically glass substrate. The electrodes were fabricated by applying Ag paint ( TED Pella, fast-drying). The diameter of the top is around 2 mm, while the whole area of the bottom surface was covered by the Ag paint for the bottom electrode. Cu wires were attached to the contacts, and then bonded to Cu foil strips on the glass substrate. The thicknesses of sample is around 1.0 mm, and the sample area is about 3 mm × 5 mm for both samples. The direct current (DC) current-voltage (I-V) measurements were performed to assess the leakage current in the dark. DC conductivity was measured by using a Keithley 6517B electrometer equipped with a Keithley 6105 resistivity adapter Photocurrent measurements upon X-rays were carried out using 22.4 keV Ag X-rays generated from a CPS 120 INEL diffractometer operating at a tube current of 2 mA and an accelerating voltage of 40 kV. Room temperature Seebeck coefficient measurements were 7



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conducted on a polycrystalline pellet (approximate dimensions: 5 × 5 × 1 mm3) with graphite paste (Ted Pella) contacts using the thermal transport option on a Dynacool Physical Property Measurement System (Quantum Design). The pellet was sandwiched between two thin copper discs with leads in a symmetric Cu|graphite paste|Tl6SeI4|graphite paste|Cu configuration and held with a clip while the contact dried. Values were the average of three measurements. Detector Performance Characterization. Detector performance was probed using a 0.2 mCi

57

Co source generating a characteristic γ-ray photo peak at 122 keV. The fabricated device

was connected to an eV Product-550 preamplifier box. The signals were transferred to an ORTEC amplifier of the Model 572A at a shaping time of 1.0 µs, and gain of 200 × 5, and counting time of 60 s, and then evaluated by a dual 16 K input multichannel analyzer (Model ASPEC-927) and read by the MAESTRO-32 software. The detector performance measurements were performed in ambient atmosphere. The distance between detector and height spectra without

57

57

Co radiation is around 5 cm. Pulse

Co radiation source were also recorded to define the background noise

under the same experimental settings. Carrier Drift Mobility Estimation. An 241Am a-particle source without collimator was used for carrier mobility estimation. The activity of the

241

Am source was around 0.9 µCi. The

measurements were performed in atmosphere with a detector- source- distance of ~2 mm. The electron and hole mobility of Tl6SeI4 were estimated by measuring the carrier drift time (tdrift) from an interaction that creates holes/electrons near the cathode/anode and assuming that the carriers drift through the whole thickness of the crystal.

38

The µ can be estimated by the following

equation: ߤ=

஽

(1),

ா௧೏ೝ೔೑೟

where E and D are the applied electric field and the detector thickness (drift distance) , respectively. The carrier drift time tdrift is determined by recording the carrier rise time from output pulse from preamplifier. In order to decrease statistical fluctuation, 50 measurements of carrier rise time were carrier out and averaged. For carrier mobility estimation, the complete transient waveforms from the preamplifier were recorded by applying a custom interface based on National Instruments software.

8


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RESULTS AND DISCUSSION Purification Effectiveness and Efficiency. The evaporation method in a bent tube was

employed to purify Se, Tl2Se and TlI. The hot zone for Se was set to 540 oC, which is between its melting point of 221 oC and boiling point of 685 oC. Se melts at this temperature without generating too high saturated vapor pressure. The cold zone for Se was set to 300 oC, which can facilitate the flowing of condensing dropplets in the cold zone. Since Se is very volatile, the temperature gradient between hot zone and cold zone is merely 12 oC·cm-1. Compared to Se and TlI, Tl2Se is much less volatile, the temperature of hot zone was set to 1000 oC, which is much higher than its melting point 380 oC. The temperature of cold zone was set to 500 oC, which can also facilitate the collection of droplets in the cold end. The temperature of hot zone for TlI was set to 840 oC, which is higher than its boiling point 823 oC. Therefore, this purification process is actually distillation in a bent tube. Figure S1a in Supporting Information (SI) shows the purified Se after evaporation. The transport of 20 g Se was completed in less than 1 h. Traces of black impurities were observed in the hot end of ampoule. Table 2 shows the concentration of impurities present in elemental Se before and after purification. It shows that the evaporation method is highly effective in eliminating Cl, Ca, Te, Pb and Bi. The concentrations of Na and Si increase after purification, which may be due to Na and Si contamination from the silica ampoule. The absence of the purification effectiveness on S is attributed to the high volatility of S impurity. The transport of 20 g Tl2Se only took 5 hours. Figure S1b in SI shows the purified Tl2Se after evaporation. Compositional analysis by EDS reveals that the purified Tl2Se is stoichiometric,

as shown in Figure S2a and S2b. From the result of impurity analysis, evaporation is quite effective in removing Na, Mg, Al, Cl, Ca, Fe, Ni, Ag, Pb and Bi, as shown in Table 3. The increase in Si concentration is likely caused by contaminants from the silica ampoule. A similar purification cycle on a TlI ingot was performed as well. The transport of 20 g TlI only took 20 min,. Figure S1c shows the purified TlI after evaporation. Compositional analysis by

EDS reveals that the purified TlI is stoichiometric, as shown in Figure S2c and S2d. Small black spots are decorating the ampoule wall, particularly where the periphery of the molten pool of TlI was located. These contaminant residues are a signature of purification. In order to assess the purity of the evaporated TlI, a sample from the tip of the TlI ingot was selected for impurity 9



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analysis by GDMS. Table 4 shows the impurity concentrations measured by GDMS for the TlI purified. From these results, we found that the distillation/evaporation method is very effective for removing Cl, Al, Fe, Br, and Sn. Since TlBr and TlI have almost the same chemical and physical properties, the complete removal of Br proved difficult.

Electronic Structure of Defects. Based on the impurity analysis, the main impurities in raw materials were identified to be Si, Sn, Al, Pb and Bi. Additionally, Cl, Br, S and Te were observed in significant amounts. In order to understand the influence of these impurities on the electronic properties of Tl6SeI4, first-principles electronic structure calculations of were performed in the supercell approach. The main goal is to determine the character of the defect electronic states introduced by the impurities and to identify those impurities introducing deep energy levels in the bandgap of Tl6SeI4. Tl6SeI4 is a direct gap semiconductor with experimental bandgap of 1.86 eV. The calculations with the PBE functional yield a direct band gap of 1.43 eV at Γ point. Compared to the experimental value, the calculated value is underestimated, sincethe underestimation of bandgaps is a known issue of DFT calculations employing exchange-correlation functionals based on the local density approximation, and it also makes it difficult to determine energy level positions of deep levels in the bandgap. To correct the positions of the defect levels due to band gap underestimation, an approach proposed in was employed. 39 The calculations for the primitive cell of Tl6SeI4 were first performed both with the PBE functionals and the hybrid functional of Heyd-Scuseria-Ernzerhof (HSE).40 The HSE calculations allow correcting for the band gap underestimation, but high computational cost makes them prohibitively expensive if applied directly to defect calculations. From alignment of the HSE band-edge positions relative to those from the PBE calculation, the required relative shifts of the valence band maximum (VBM) and the conduction band minimum (CBM) were then determined. To correct for the defect levels position, shallow defect levels were shifted together with their respective band edges, whereas the localized deep levels were not shifted with the band edge corrections. As the first step of the calculations, the preferred sites that the impurities occupy after being introduced in the Tl6SeI4 crystal were determined based on the formation energies of the corresponding defect configurations. From the crystal structure of Tl6SeI4 shown in Figure 2, there are 4 non-equivalent lattice sites in the structure (1 Se, 1 I, and 2 Tl) which can be substituted by 10


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the impurity atoms, as well as the interstitial site. The formation energies of intrinsic and extrinsic defects depend on the crystal growth conditions, which dictate the possible values of chemical potentials of the constituents (Tl, Se, I). Changes in growth conditions may affect the preference cite of impurities. The formation energies of charge neutral defects can be calculated as follows:

(

)

∆H D = ( ED − EHost ) + ∑ ni µi0 + ∆µi ,

(2)

i

where ED and EHost are the total energies of the defect-containing and the defect-free host supercells, ni is the number of atoms of species i participating in the exchange, ߤ௜଴ is the bulk chemical potential and ∆ߤ௜ is the relative chemical potential of the ith atomic species

participating in the atom exchange. The available range of relative chemical potentials of the constituents ∆ߤ௜ (i = Tl, Se, I) were determined from the thermodynamic stability phase diagram

of Tl6SeI4 presented in previous work.37 For the impurities, ∆ߤ௜ was taken as 0 (∆ߤ௜ terms will

cancel out in determination of site preference energy). The relative chemical potentials for the two extreme growth conditions are as follows: Tl-poor/Se-rich – ∆ߤ ்௟ = −0.67 eV, ∆ߤௌ௘ = 0, ∆ߤூ = −0.66 eV, and Tl-rich/Se-poor – ∆ߤ ்௟ = 0, ∆ߤௌ௘ = −1.34 eV, ∆ߤூ = −1.32 eV.37

The calculated formation energies of impurities occupying different lattice sites are presented in Table 4. The formation energies for each impurity are given relative to the most stable defect, for which the formation energy is assigned to 0 and is shown in bold. It can be seen from Table 4 that in Tl-poor/Se-rich growth conditions Al, Sn, Pb and Si impurities favor Tl1 sites and strongly dislike occupying S and I sites and the interstitial site. For Sn and Pb impurities the preference for Tl1 site over Tl2 is very small, but it is larger for Al. The situation is different for Bi impurity, which prefers to occupy Tl2 site over Tl1. Among metallic impurities these preferences directly correlate with the covalent radii of the impurities: the covalent radii increase in the sequence Al < Sn < Pb < Bi, and so does the preference to occupy site Tl2 over site Tl1. Though sites Tl1 and Tl2 have the same character of chemical bonding and the same nearest neighbors (1 Se, 4 I), the distances to the neighbors are smaller for Tl1 (Tl-Se: 2.955 Å, Tl-I: 3.477 Å) than for Tl2 (Tl-Se: 3.078 Å, Tl-I: 3.484 Å and 3.494 Å). Thus, it is energetically favorable for larger impurity atoms to replace Tl2 and for smaller atoms to replace Tl1 to minimize the energy cost associated with lattice relaxation around the impurity. In Tl-rich/Se-poor growth conditions Al, Sn, and Pb impurities continue to occupy Tl1 sites, 11



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however Bi and Si impurities change their preference site: Bi occupies Se site, while the Si impurity now prefers to occupy the interstitial position in the lattice. From the calculated formation energies of the neutral and charged defects the charge transition levels can be determined. The energy position of these levels with respect to the band edges describes the character of the defect (donor or acceptor, shallow or deep), and for strongly localized deep levels corresponds to the actual position of such a level with the band gap. According to the calculations, Al impurity is a deep donor with the charge transition levels located at -0.70 eV below the CBM with the charge transition state of 2+/0. The calculated band 176-atom supercell of Tl6SeI4 with one Al atom in the 1+ charge state substituting Tl1 atom is shown in Figure 3a. It can be seen that Al impurity produces a deep electronic level within the gap which is mostly comprised of non-interacting Al s states and is capable of electron trapping. It should be noted that the positions of the defect levels obtained in band structure calculations (one-electron Kohn-Sham levels) are not equal to the energy positions of the charge transition levels because one-electron levels are calculated for the system in the fixed charge state, whereas charge transition levels represent the Fermi energy at which two different charge states are at equilibrium. Also, no band gap corrections to the position of the one-electron levels are applied. Nevertheless, calculated band structures provide a good qualitative illustration of the defect transition levels and allow understanding of their character and physical origins. Sn and Pb are electronic analogues, and their impurities in Tl6SeI4 exhibit similar behavior. Sn and Pb p states interact with the electronic states of Tl6SeI4 at and near CBM resulting in resonance states that push CBM -0.03 eV below its original position. Hence, Sn and Pb can be characterized as very shallow donors in Tl6SeI4. Figure 3b illustrates the changes to the Tl6SeI4 band structure due to PbTl1 impurity in the 1+ charge state. Bi impurity occupies Tl2 site in Tl-poor/Se-rich growth conditions, and it is a deep donor which introduces 2 defect levels at -0.45 eV and -0.25 eV relative to the CBM (Figure 3c). The electronic levels created by BiTl2 have a character of Bi p states and are capable of capturing of up to 2 electrons. In Tl-rich/Se-poor growth conditions Bi impurity prefers to occupy Se site, and acts as a deep acceptor: it introduces 2 Bi p defect levels very deep into the band gap at +0.80 eV and +0.90 eV relative to the VBM. The calculated band structure of Tl6SeI4 with one Bi atom in the 1- charge 12


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state substituting Se atom is shown in Figure 3d. Being located near the center of the band gap, these BiSe defect levels are capable of trapping and recombination of both holes and electrons and therefore are particularly detrimental to the detector properties of Tl6SeI4. Si impurity can occupy two different sites depending on growth conditions. In Tl-poor/Se-rich growth conditions Si occupies the Tl1 site. From the band structure of Tl6SeI4 with the Si atom in the 1+ charge state substituting Tl1 atom presented in Figure 3e, it can be seen that Si p states interact with the bulk electronic states of Tl6SeI4 at approximately 0.5 eV above CBM. SiTl1 does not introduce in-gap or near-gap states and does not affect band gap width. Therefore, Si impurities occupying Tl1 positions are electronically inactive and not harmful to the detector properties of Tl6SeI4. However, in Tl-rich/Se-poor growth conditions Si prefers to occupy interstitial site in Tl6SeI4, and its electronic behavior is completely different: Siint is a deep donor defect which introduces a charge transition level located 0.70 eV below the CBM. Figure 3f shows the formation of defect level caused by Si interstitial; the main character of this level is Si p states. Cl and Br impurities are chemical and electronic analogues of I, and S and Te impurities are electronic analogues of Se. The calculation of defect formation energies showed that, independent of the crystal growth conditions, Cl, Br, S and Te impurities prefer to occupy exclusively the lattice sites of their corresponding chemical analogues, where they participate in chemical bonding in place of the original atoms. ClI, BrI, SSe and TeSe defects do not introduce defect transition levels in the band gap and do not change band gap width, and therefore are electrically inactive. The results of the calculations are summarized in Table 5. Among all the impurities considered, Al and Bi introduce deep levels in the band gap in all growth conditions which can act as recombination centers impeding carrier collection and having detrimental effect on carrier transport, therefore a particular attention should be taken to eliminate these impurities. Si impurity also introduces deep electron trapping levels, but only in Tl-rich/Se-poor growth conditions, hence its adverse effect can be mitigated by performing synthesis in Tl-poor/Se-rich conditions. Under these conditions Si impurities will change their occupation position to SiTl1 and become electrically benign, their detrimental effect will be eliminated, and deep removal of Si may not be necessary. Note that the preference for Si to occupy the interstitial site is small (0.05 eV, see Table 4) even in extreme Tl-rich/Se-poor conditions. A slight deviation to more Tl-poor/Se-rich growth 13



Page 14 of 29

conditions will be sufficient to change the occupation position of Si and make it electrically inactive. Shallow donors Sn and Pb are not as harmful to the charge carrier collection performance of Tl6SeI4 but are n-type dopants that can decrease resistivity and increase dark current in Tl6SeI4-based detector devices, so the reduction of their concentration should have a beneficial effect. The halides Cl, Br, S and chalcogenide Te impurities are electrically inactive and generally harmless. However, in high concentrations they can lead to lattice distortion, band broadening and the increase in hole effective masses resulting in the reduction of carrier mobility, therefore lowering Cl, Br, S and Te impurities concentration is still advised, but it is not as critical to the performance of Tl6SeI4-based detector devices. Crystal Growth and Charge Transport. Tl6SeI4 raw material was synthesized by a stoichiometric direct chemical reaction of the purified TlI and Tl2Se precursors, and then used for crystal growth in a vertical two-zone Bridgman furnace. Figure 4a shows the 1 cm-size pristine boule of Tl6SeI4 under ambient light. Figure 4b shows the I-V characteristics of the Tl6SeI4 sample A detector (grown with purified raw materials) and sample B detector (grown with unpurified raw materials). The inset in Figure 4b shows a typical 1 mm thick planar detector fabricated with Ag electrodes. Sample A shows a linear I-V characteristic in the voltage range of -100 V to 100 V with a high resistivity of 3.1 × 1011 Ω·cm, indicating this material is a good candidate for radiation detectors. Sample B also shows liner I-V behavior with a high resistivity of 6.3 × 1011 Ω·cm. In order to compare photoconductivity response, photocurrents as a function of time under the irradiation of the 22.4 keV Ag X-rays were measured by switching ON-OFF states. As shown in Figure 4c, both of these two samples show obvious photoconductivity response with high contrast ratios of photocurrent to dark current. However, the photocurrent for sample A (σphoto = 62 nA) is almost two times higher than that for sample B (σphoto = 21 nA). The significant increase in photosensitivity after purification implies that purification is highly effective in improving Tl6SeI4 crystal quality. The determination of conducting type of Tl6SeI4 crystal is very crucial for future optimization on electrode materials and configurations. Figure 4d shows the Seebeck coefficients for sample A at room temperature 300 K. The average Seebeck coefficient is S = + 403 ± 6 µV⋅K-1, indicating p conducting type. Detector Performance and Carrier Mobility. Figure 5a and b shows pulse height spectra 14


Page 15 of 29 1 2 3 4 5 6 7 8 9 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 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


obtained sample A detector with

57

Co γ-rays under both cathode (electron-collection) and anode

(hole-collection) irradiation configurations, respectively. As shown in Figure 5a, Sample A shows the full-energy (122 keV) photopeak shifting with increasing bias. The other photopeak (~49 keV) at lower channel number can be ascribed to the escape of Tl X-rays from Tl6SeI4 detector volume.41 In contrast, no photopeak can be observed in the spectra for sample B detector (Figure 5c), although the detector’s spectral response can be distinguished from the background. The dramatic improvement in detection performance suggests that purification is a crucial step to improve crystal quality and a prerequisite for obtaining higher detection performance. Sample A also shows reasonable photoresponse for 122 keV γ-rays under anode irradiation configuration, however, no photopeak can be observed under cathode irradiation configuration, indicating that the electron transport in Tl6SeI4 detector is much better than hole transport. The carrier mobility-lifetime product µτ is the figure of merit to estimate the performance of detector materials in their radiation detection efficiency. 42-43 The µτ for electrons can be obtained from the analysis of charge collection efficiency (CCE) at each bias applied to the the detector. The Hecht equation for a single charge carrier can be expressed as ொ

‫ = )ܸ(ܧܥܥ‬ொ = బ

ఓఛ௏ ௅మ

ቆ1 − ݁

ಽమ ഋഓೇ

ି

ቇ

(3),

where Q is the measured channel number of full-energy photopeak at bias V, L (1.0 mm) is the thickness of fabricated detector, and Q0 refers to saturated channel number of the full-energy photopeak in theory. The µeτe)and Q0 can be derived from the experimental data of CCE(V) and Q. Figure 5d shows the datasets of CCE(V) and Q , and the µτ for electron carriers was estimated to be 8.1 × 10-5 cm2·V-1 by fitting of the experimental results using equation (3). We further measured the pulse height spectra from sample A Tl6SeI4 detector in response to 59.5 keV γ-rays from

241

Am and 662 keV γ-rays from

137

Cs. A full-energy photopeak can be

clearly distinguished from the background noise in Figure 6a. Compared to the maximum channel number in the energy spectra for 122 keV γ-rays from

57

Co, the maximum channel number

decreases to ~450 due to a lower photon energy of 59.5 keV. Sample A exhibits reasonable photoresponse for 662 keV γ-rays from 137Cs as well. However, no photopeak can be observed due to insufficient carrier collection efficiency. The carrier mobility of Tl6SeI4 was experimentally determined using a time-of-flight analysis of 15



the pulse induced by 5.5 MeV α-particles from an

241

Page 16 of 29

Am source.44 Figure 7a illustrates a typical

electron output pulse of the sample A Tl6SeI4 detector under α-particle irradiation from 241Am. As shown in Figure 7b, an average value of electron rise time of ~1.77 µs was measured at an applied electric field of 500 V·cm-1 from a detector with a thickness of 0.10 cm. Since the spatial decay length (~20 µm) of 5.5 MeV α-particles is much short than the detector thickness (0.10 cm), the electron drift time is essentially identical to the electron rise time. Using these values with equation 3, the electron mobility of the detector is estimated to be 112 ± 22 cm2·V-1·s-1. This electron mobility is comparable to those of the detection materials TlBr (30-50 cm2·V-1·s-1) and HgI2 (100 cm2·V-1·s-1).13, 31 Similarly, the hole mobility is estimated to be 81 ± 22 cm2·V-1·s-1. The hole mobility is much higher than those of TlBr (4 cm2·V-1·s-1), PbI2 (2 cm2·V-1·s-1) and HgI2 (4 cm2·V-1·s-1), and almost comparable to that of CZT (120 cm2·V-1·s-1).13 The high carrier mobilities suggest great potential of Tl6SeI4 as a detector material.

CONCLUSION

In this work, a new efficient purification method was developed to purify Se, Tl2Se and TlI precursors by evaporation in a bent tube. The effectiveness of purification in removing impurities was confirmed by GDMS impurity analysis. DFT calculations were performed to study the electrical behaviors of the main impurities (Al, Pb,Si, Bi, Sn, S Cl, , Te and Br) identified experimentally. Among these impurities, calculation results suggest that Bi and Al introduce deep energy levels which are detrimental to the charge transport, while Si impurity can introduce deep donors if growth conditions are Tl-rich/Se-poor. Sn and Pb act as shallow donors, while Br,Cl, Te and S impurities are inactive electrically. This purification process effectively reduced the concentrations of Bi and Al which could introduce deep level trap states. Large-size Tl6SeI4 crystal was grown by the vertical Bridgman method using the purified precursors. The detector made of Tl6SeI4 crystal has a high resistivity of 1011 Ω·cm, suppressing dark current. The reduction of impurity concentrations by purification substantially improves photoconductivity under 22.4 keV Ag X-rays, and leads to higher photosensitivity of γ-rays. The detector shows full-energy and Tl escape photopeaks upon

57

Co γ-ray irradiation. The detector also exhibits reasonable

photoresponse for 59.5 keV γ-ray from 241Am and 662 keV γ-ray from 137Cs. The µeτe for Tl6SeI4 16


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detector is achieved as 8.1 × 10-5 cm2·V-1 based on γ-ray photoresponse. Based on the carrier rise time estimated from carrier output pulses induced by 5.5 MeV α-particles from 241Am, the electron and hole mobilities were estimated to be 112 ± 22 and 81 ± 16 cm2·V-1·s-1, respectively. These values further validate the promise of Tl6SeI4 as a room temperature radiation detector material. Future work will be development on purification technology to reduce the Al and Bi impurities which can introduce deep levels.

ASSOCIATED CONTENTS

Supporting Information The Supporting Information is available. Figure S1. (a) Purified Se precursor. (b) Purified Tl2Se precursor. (c) Purified TlI precursor. Figure S2. (a) SEM image of purified Tl2Se. (b) Compositional analysis on purified Tl2Se by EDS (Tl atomic% is 67.13%, Se atomic% is 32.87%). (c) SEM image of purified TlI. (d) Compositional analysis on purified TlI by EDS (Tl atomic% is 49.94%, I atomic% is 50.06%).

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected]. ORCID Wenwen Lin: 0000-0002-1627-9558 Sanjib Das: 0000-0002-5281-4458 Constantinos C. Stoumpos: 0000-0001-8396-9578 Kyle M. McCall: 0000-0001-8628-3811 Bruce W. Wessels: 0000-0002-8957-7097 Mercouri G. Kanatzidis: 0000-0003-2037-4168

ACKNOWLEDGMENT

This work is supported by a Department of Homeland Security grant (HSHQDC-13-C-B0039) and DHS-ARI grant 2014-DN-077-ARI086-01. Seebeck coefficient measurements were performed at 17



Page 18 of 29

Argonne National Laboratory supported by the Department of Energy, National Nuclear Security Administration, Office of Defense Nuclear Nonproliferation Research and Development under contract No DE-AC02-06CH11357. This work made use of the EPIC facility the NUANCE Center at Northwestern University. We thank Alexei Churilov, Paul Bennett, Alireza Kargar, Leonard Cirignano from Radiation Monitoring Devices Inc. for fruitful discussion and suggestions.

REFERENCES

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Figure 1. (a) Scheme of conventional purification by evaporation in a straight ampoule. (b) Scheme of purification by evaporation in a bent tube.

Table 1. The temperatures set for hot and cold zones in two-zone furnace and melting and boiling points of Se, Tl2Se and TlI precursors. Precursor

Purity from manufacturer

Temperature for hot zone (oC)

Temperature for cold zone (oC)

Temperature gradient estimated (oC·cm-1)

Melting point (oC)

Boiling point (oC)

Se

99.999%, Alfa Aesar Tl: 99.999%, Alfa Aesar 99.999%, Alfa Aesar

540

300

12

221

685

1000

500

25

380

N/A

840

520

17

442

823

Tl2Se TlI

21



Page 22 of 29

Table 2. Comparison of the analyzed major impurities in the Se precursor before and after purification. Impurities

Concentration before purification (ppm, wt)

Concentration after purification (ppm, wt)

Na Si S Cl Ca Te Pb Bi

0.09 0.24 0.1 18 0.16 0.6 0.59 0.03

0.19 0.35 0.1 0.31

An Effective Purification Process for the Nuclear Radiation Detector

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