The role of stoichiometry in the growth of large Pb2P2Se6 crystals for

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The role of stoichiometry in the growth of large Pb2P2Se6 crystals for nuclear radiation detection Yadong Xu, Xu Fu, Hongjian Zheng, Yihui He, Wenwen Lin, Kyle M McCall, Zhifu Liu, Sanjib Das, Bruce W. Wessels, and Mercouri G. Kanatzidis ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b01119 • Publication Date (Web): 17 Nov 2017 Downloaded from http://pubs.acs.org on November 20, 2017

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The role of stoichiometry in the growth of large Pb2P2Se6 crystals for nuclear radiation detection Yadong Xu†, ‡, Xu Fu†, Hongjian Zheng†, Yihui He‡, Wenwen Lin‡, Kyle M. McCall‡, § , Zhifu Liu§, Sanjib Das§, Bruce W. Wessels§, Mercouri G. Kanatzidis‡, § †

State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, China, ‡ Department of Chemistry, Northwestern University, Evanston, IL 60208 § Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208

ABSTRACT: Pb2P2Se6 as a heavy element, chemically robust semiconductor, has been identified as a promising material for cost-effective room temperature X/γ-ray detection. Here, we report the properties of Pb2P2Se6 crystals grown by a vertical Bridgman method under off-stoichiometric Se-rich and Pb-rich conditions. Regardless of the conditions the resulting single crystals exhibited high bulk resistivity on the order of 1011 Ω·cm. However, the photoconductivity and charge transport properties varied based on growth condition indicating the different dominant defects associated with the type of stoichiometric deviation. The formation and nature of intrinsic defects in Pb2P2Se6 crystals were also studied by first-principles density functional theory (DFT) calculations as well as thermally stimulated current (TSC) spectroscopy. The TSC results indicated that four traps were common to both Se-rich and Pb-rich Pb2P2Se6, while a higher density of shallow defects were observed in Se-rich Pb2P2Se6. DFT calculations predict that the anti-site defects PPb+, PSe- and PbP- are the dominant deep donors and acceptors in Se-rich and Pb-rich Pb2P2Se6, respectively, which leads to the degradation of mobility lifetime product (µτ) on the order of 10-5 cm2·V-1 as measured under 241Am (5.48 MeV) alpha particles irradiation. Nevertheless, Pb2P2Se6 detectors with a thickness of 2 mm show reliable linear response under a series of radiation sources, including

241

Am and

57

Co γ-ray sources. A high X-ray sensitivity

comparable to that of amorphous Se for Pb-rich Pb2P2Se6 detectors was realized, with the value of 68.3 µC·Gyair-1cm-2 under 40 kVp Ag X-rays at an electrical field of 50 V·cm-1. Keywords: nuclear radiation detector, semiconductor crystal, photoconductivity, X-ray sensitivity, photon detection

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The recent rapid demand for large-volume X-ray and γ-ray spectrometers and imaging arrays have triggered tremendous opportunities in the field of astronomy and high energy physics, nuclear medicine, industrial on-line monitoring, and national security.1-5 Cadmium zinc telluride (CZT) is undeniably the leading material for room-temperature semiconductor radiation detector, however, its widespread deployment is impaired by high cost as well as structural imperfections.6-8 Recently, there has been considerable interest in identifying new low-cost, heavy element, chemically robust compounds as detector materials for room-temperature radiation detection. Among new materials, the organic-inorganic hybrid perovskites9-12 and full inorganic13,14 lead-halide perovskites are receiving attention as potential radiation detection materials. However, serious hurdles remain for these perovskite materials, such as poor chemical stability and phase stability, poor crystal growth characteristics, and so on. In particular, the polarization effects found in the hybrid halide due to ion migration cannot be ignored.15 The metal chalcogenides, however, indicate excellent prospects for lack of ion migration, long-term phase and environmental stability because of their relatively strong covalent bonds. On the basis of the new concepts of “dimensional reduction” and “lattice hybridization”, a number of heavy element, high density semiconductor compounds with suitable bandgaps, have been identified and recognized as potential X-ray and gamma-ray detection materials by our group.16-18 Recently, we proposed Pb2P2Se6 as a potential semiconductor for low cost bulk production and detector development.19 Although there is a congruent region for growing Pb2P2Se6 crystals from the melt, a stoichiometric deviation inevitably occurs because of the loss of volatile P and Se above the melt. Consequently, secondary phase particles (precipitates/inclusions) tend to be generated within these materials, which have been studied intensively in other metal chalcogenides.20,21 More importantly, native point defects which show thermodynamic stability cannot be completely eliminated by improved crystal growth and in high concentrations they could dominate the crystal's resistivity and charge transport properties. Therefore, to optimize the detector-grade Pb2P2Se6 crystals, the effects of grown-in defects on the electrical and optical behaviors should be investigated. The relationship between grown-in defects and stoichiometry needs to be understood. In this article, we report on large Pb2P2Se6 crystals grown with a vertical 2


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Bridgman method under slightly different stoichiometries of the starting materials. We focus on the native grown-in defects, which are responsible for the variation of optical and electrical properties between Se-rich Pb2P2Se6 and Pb-rich Pb2P2Se6 crystals. To better determine the nature of native point defects in Pb2P2Se6, the formation energies were calculated by first-principles theory methods. Based on the experimental results and theory calculations, we predict that the anti-site defects PPb+, PSe- and PbP- are the dominant deep donors and acceptors in Se-rich and Pb-rich Pb2P2Se6, respectively. These observations impact the strategies for further improvement of Pb2P2Se6 crystals by targeted doping and post-growth annealing under a given atmosphere.

RESULTS Crack-free Pb2P2Se6 ingots, 10 mm in diameter and 60 mm in length, were obtained from the growth procedure mentioned above, Figure 1a. Wafers reproducibly free from twins and grain boundaries were sliced perpendicular to the growth direction using a diamond wire saw into sizable single crystals, as seen in Figure 1b. Then the samples were cut into 5×5×2 mm3 wafers, which subsequently mechanically lapped and chemo-mechanically polished prior to performing the measurements.

Figure 1. (a) Sealed silica ampoule with carbon coating (top image), as-grown Pb2P2Se6 ingot (bottom). (b) Pb2P2Se6 wafer before and after polishing. (c) The completed Pb2P2Se6 planar devices. (d) Diagram of charge carrier transport induced by alpha particles. (e) The corresponding PXRD patterns. (f) UV-Vis optical transmittance experiments at RT measuring the band gap of Pb2P2Se6. 3


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Ingots 0729, 1102S and 1102P were grown under excess Se with amount of 1%, 0.5%, and excess Pb with the amount of 0.5%, respectively. The phase structure of these three individually grown crystal batches were investigated by powder X-ray diffraction (PXRD) spectra, Figure 1e. The patterns reveal that all grown samples are pure phases after growth regardless of the small stoichiometric deviations used, and this is consistent with the congruent melting behavior of the material.19 UV-Vis transmission spectroscopy of the as-grown Pb2P2Se6 single crystals show a red shift of the absorption edge giving an optical band gap of 1.972 eV, 1.954 eV and 1.936 eV for 0.5% Pb-rich, 0.5% Se-rich and 1% Se-rich Pb2P2Se6, respectively, Figure 1f. The variation of the fitting band gap edge is mainly attributed to the degree of incorporation of shallow level defects in Pb2P2Se6 samples. Secondary Phase Particles

Generally, for crystals grown from a non-stoichiometric melt, the constitutional supercooling that is usually generated from the solute piled up at the growth interface, causes secondary phase (SP) inclusions to form by trapping extra solute in the unstable liquid/solid interface. To examine this possibility, the as-grown Pb2P2Se6 crystals were characterized by both IR and optical transmission imaging (Figure S1 in the Supporting Information). Well-defined SP particles with elliptical and bar shapes were observed. The typical sizes of the SP inclusions are 10-60 µm. The total concentration of SP inclusions was ~(1-2)×104 cm-3 in 1% Se-rich Pb2P2Se6, while for 0.5% Pb-rich Pb2P2Se6, ~102 cm-3. SEM/EDS analysis indicates that the inclusions in Se-rich Pb2P2Se6 are a Se-rich phosphorous binary compound (Figure S2(a) in the Supporting Information), while the smaller size SP inclusions are almost pure Se, as shown by EDS in Figure 2a-2d. We attribute the size and density variations of inclusions in melt grown Pb2P2Se6 mostly to the deviation from the stoichiometric composition. SEM/EDS analysis demonstrates that the main inclusions in Pb-rich Pb2P2Se6 are PbSe, (Figure S2(b) in the Supporting Information). PbSe is very stable with a much higher melting point than Pb2P2Se6. Therefore, the growth rate is faster for PbSe due to the higher supercooling, which results in the cellular grains, as seen in Figure 2e-2i. The formation conditions for secondary phase inclusions in Pb2P2Se6 were studied theoretically by first-principles total energy calculations of thermodynamic stability, as shown in the supporting information. Pure and precipitate-free Pb2P2Se6 can only 4


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be grown in a limited range of chemical potentials of its components. The results indicate that the Se-rich and Pb-rich conditions promote the formation of P-Se and Pb-Se binary phases, respectively, which is in agreement with the EDS measurements. On the other hand, the formation conditions of native point defects are closely related to the resulting SP-inclusions, which affect the optical and electrical properties of Pb2P2Se6.

Figure 2. Analysis of the SP particles. (a) SEM image of SP particle embedded in Pb2P2Se6 crystal grown from Se-rich conditions. (b), (c) and (d) Component elemental mapping derived from EDS spectra. (e) SEM image of SP particle embedded in Pb2P2Se6 crystal grown from Pb-rich conditions. (f) Enlarged area shown in (e). (g), (h) and (i) Component mapping derived from EDS spectra, the dotted line is guide for the eye. All scale bars correspond to 20 µm.

Electrical and Charge Transport Properties

The bulk resistivity is on the order of 2-5×1011 Ω·cm for both Se-rich and Pb-rich grown Pb2P2Se6 crystals, and this is one order of magnitude higher than the state-of-the-art CdZnTe crystals.22 Typical I-V plots are shown in Figure 3a. The good linear behavior observed even when subjected to a higher bias of 300 V for the 2 mm thick sample, indicated the absence of polarization effects. The high resistivity was attained without introducing any doping elements. The visible photoresponse differs by a factor of two between Se-rich and Pb-rich grown Pb2P2Se6 samples.

Figure 3. (a) Typical I-V curve for Au/Pb2P2Se6/Au device base on a Pb-rich sample. (b) Photo (ambient light) response of Pb2P2Se6 detector under bias of 10 V at RT. 5


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Both types of Pb2P2Se6 samples show photoconductivity when exposed to the ambient room light (~0.8 mW·cm-2). The ON-OFF state photocurrent response under a 10 V bias voltage recorded as a function of time is shown in Figure 3b. The ON-OFF current response is instantaneous without much exponential decay. The ratios of photocurrent to dark current (σphoto/σdark) were 30-120 and 400-500 for Se-rich and Pb-rich Pb2P2Se6, respectively. The σphoto/σdark ratio reflects the difference in the dominant defect levels and densities in the as-grown Pb2P2Se6 crystals. The higher photoconductivity response of Pb-rich grown Pb2P2Se6 samples indicates a higher concentration of photo-induced carriers and larger carrier mobility. To further understand the effects of Se-rich and Pb-rich conditions on the as-grown Pb2P2Se6 crystals, we studied the charge transport behaviors using alpha particles and recorded the induced pulse height spectra as a function of applied bias, seen in Figure 4a. The charge collection efficiency (CCE) increased with applied bias, as reflected by the shift of the corresponding curves toward higher channel number. The mobility-lifetime product (µτ) for both electrons and holes was estimated from the maximum channel number by using the single charge carrier approximation Hecht equation,23,24 CCE ≈

µτ V  d

2

d2  ) 1 − exp( µτ V  

(1)

where, V is the bias voltage. d is the sample thickness. The estimated (µτ)e products are on the order of 10-5 cm2·V-1 for both Pb-rich and Se-rich Pb2P2Se6 samples, Figure. 4b. These µτ values are comparable to that of the chalcohalides,18 and the heavy halides,25 measured by alpha particles. The electron (µτ)e is higher than hole (µτ)h for Pb-rich Pb2P2Se6 crystals, but for Se-rich Pb2P2Se6, this is reverse, as seen in Table 1. This difference is mainly ascribed to the variation of the dominant grown-in defects. These native point defects as electron or hole traps are determined by the stoichiometry of the starting materials.

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Figure 4. Carrier transport behaviors: (a) α particle pulse height spectra as a function of applied bias for a Pb-rich Pb2P2Se6 sample. (b) (µτ)e of Pb-rich Pb2P2Se6 fitted using the single carrier Hecht equation. (c) Induced alpha pulse shapes at various applied biases. (d) Electrons drift velocity as a function of the electrical field strengths of Pb-rich Pb2P2Se6.

According to the carrier transport scheme shown in Figure 1d, the charge loss introduced by the SP-inclusions severely degrades the CCE with increasing bias due to the growing size of the created electron cloud. Bolotnikov et. al.26 revealed that the large SP particles of >10 µm are detrimental to the device’s responses at concentrations of 104 cm-3. In addition, the uniform carrier trapping caused by native point defects also plays a major role in CCE degradation. The carrier drift mobility of Pb2P2Se6 samples was evaluated using a time of flight (TOF) technique.24 The carrier drift velocity (vdr) was acquired by analyzing the rise time (tdr) of transient waveforms from the preamplifier with the sample irradiated by a 241Am α particle source, as seen in Figure 4c. The mobility µ was extracted using the expression: µ=

vdr d 2 = E Vtdr

(2)

Due to the shallow penetration depth of α particles (few micrometers), we assume the transit distance approximately equals the sample thickness of 2 mm. The photogenerated carrier mobility can be obtained by the linear fitting of vdr vs the electrical field (E), Figure 4d. The resulting µe and µh from the data were 40.5 7


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cm2·V-1s-1 and 10 cm2·V-1s-1, respectively, for Pb-rich Pb2P2Se6 crystals. In the case of Se-rich Pb2P2Se6 crystals, µe and µh were 9.5 cm2·V-1s-1 and 35 cm2·V-1s-1, respectively. Generally, the low level vacancies act as recombination centers, thus reducing the carrier lifetime. While the deep level anti-site defects as the trapping center are responsible for the mobility deterioration. The deep donors or acceptors in as-grown Pb2P2Se6 crystals are determined by the formation energy and stoichiometric deviation during growth. Table 1. Electrical and carrier transport properties of Pb2P2Se6 crystals. Samples

ρ /Ω·cm

σphoto/σdark

(µτ)e / cm2·V-1

µe / cm2·V-1s-1

(µτ)h cm2·V-1

µh / cm2·V-1s-1

Se-rich

(2.1-3.7)×1011

30-120

1.5×10-5

9.5

2.6×10-5

35

Pb-rich

11

-6

10

(2.6-4.8)×10

400-500

3.8×10

-5

40.5

4.4×10

Thermally Stimulated Current (TSC) Measurements

Typical TSC measurements in the temperature range of 50-310 K for as-grown Se-rich and Pb-rich Pb2P2Se6 samples are shown in Figure 5. The simultaneous multiple peak analysis (SIMPA) method was adopted to determine trap signatures in the TSC spectra.27 The trap-related parameters, e.g., activation energy and trap density, were obtained (Table 2) based on the “first-order kinetics” approximation;28 more details are given in the supporting information. We identified 8 and 9 trap peaks in Se-rich and Pb-rich Pb2P2Se6 samples, respectively. Four current peaks are in common in both Se-rich (TSe4, TSe5, TSe7, TSe8) and Pb-rich (TPb3, TPb4, TPb6, TPb7) samples, pointing by the blue arrows, which are possibly attributed to impurities, oxygen and P related defects. Among which, the O interstitial (Oi) was reported as a deep level recombination center.29 The total concentration of shallow level traps in Se-rich (TSe1, TSe2, TSe3) is higher than that in Pb-rich (TPb1, TPb2), which could be attributed to the high density of dislocations30,31 and vacancies. The high density of SP inclusions in Se-rich Pb2P2Se6 samples result in more extended dislocations. The total concentration of traps with high activation energy (TSe6 and TSe9 in Se-rich, TPb5 and TPb8 in Pb-rich) is on the order of 1011-1012 cm-3. These deep level traps, which are possibly attributed to the anti-site defects (SePb

and PPb in Se-rich, PSe and PbP in Pb-rich), pin the Fermi level near the midgap and are responsible for the high resistivity of the Pb2P2Se6 samples.

8


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Figure 5. Typical TSC curves of Pb2P2Se6 crystals grown under Se-rich and Pb-rich conditions, respectively. Table 2. Trap activation energies and densities in Pb2P2Se6 crystals obtained by the SIMPA method. Sample condition Trap level Peak maximum /K Activation energy /eV T 82 0.138 Se1 T

86

0.145

1.9E11

Se3

110

0.195

1.2E11

Se4

131

0.239

3.1E11

Se5

156

0.295

9.1E10

Se6

202

0.398

2.5E10

Se7

241

0.490

2.2E11

Se8

294

0.619

7.0E11

Se9

304

0.642

12.5E11

Pb1

57

0.086

8.6E10

Pb2

66

0.106

8.2E10

Pb3

135

0.246

7.9E10

Pb4

157

0.296

3.6E10

Pb5

213

0.425

1.1E11

Pb6

245

0.501

6.8E11

Pb7

290

0.609

7.6E11

Pb8

320

0.683

5.4E11

T T T T T T T T T T Pb-rich

T T T T

1.8E11

Se2

T

Se-rich

Concentration /cm-3

First Principles calculation

Pb2P2Se6 is a selenophosphate compound and adopts the P21/c space group with 2+

Pb cations bonded to ethane-like [P2Se6]4- anions,32 Figure 6a. Based on the crystal structure, we have calculated the formation energies of all native point defects in 9


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Pb2P2Se6 by first principles methods using the supercell approach. Considering the chemical potential and the stoichiometric deviation, PbSe-Se and PbSe-P systems were adopted in Se- and Pb-saturated conditions, respectively, which gave the formation energy results in Figure 6b and 6c. The very high resistivity of Pb2P2Se6 suggests the Fermi level is located near the midgap region.

Figure 6. (a) Pb2P2Se6 crystal structure. Formation energies of native point defect in Pb2P2Se6 under, (b) Se- and (c) Pb- saturated conditions.

Thermodynamic stability calculations show that VPb2-, VP4- and VSe0, VP4- have the lowest formation energy (Table. 3), and therefore are easily created in Se-rich and Pb-rich Pb2P2Se6, respectively. These vacancies and the grown-in dislocations act as recombination centers and therefore reduce the carrier lifetime.33 Our calculations indicate that VSe0 vacancies in Pb-rich Pb2P2Se6 are electronically inactive, Figure 6c. Deep level anti-site defects are preferred over other defects as the trapping center for mobility deterioration. Our calculations indicate that SePb0 and PPb+ are the most dominant anti-site defects in Se-rich Pb2P2Se6. Although SePb0 is electronically inactive, PPb+ acts as an electron trap which may responsible for the low µe of Se-rich Pb2P2Se6. However, in the case of Pb-rich Pb2P2Se6, PSe- and PbP- are the dominant anti-site defects, act as hole traps result in low µh. 10


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Table 3. Main point defects in high resistivity Pb2P2Se6 based on First Principles calculation and growth situation. Samples

Vacancies

Anti-sites

Se-rich

VPb2- and VP4-

SePb0 and PPb+

Pb-rich

VSe0 and VP4-

PSe- and PbP-

X/γ-ray Response and the Sensitivity

Typical γ-ray spectroscopic response of a 2 mm Pb-rich Pb2P2Se6 planar device irradiated by 57Co @ 122keV and 241Am @ 59.5keV source is shown in Figure 7. The maximum channel number (MCN) clearly shifts to higher values as the bias increases without saturation even over the voltage near 300 V, demonstrating a steady increase in CCE at higher field strength. In addition, the MCN ratio of Pb2P2Se6 from 57Co and 241

Am γ-ray (520/328) is close to the photon energy ratio (122/59.5), which indicates

a reliable linear response that is important for energy-discrimination devices. However, the photopeaks were not resolved in the spectra, this is mainly attributed to the limited carriers mobility lifetime products.

Figure 7. γ-Ray response of a Pb-rich Pb2P2Se6 sample under

57

Co @ 122keV or

241

Am @

59.5keV by cathode irradiation. The shaping time is 2 µs, and the counting time was 300 s.

We further assessed the Pb-rich Pb2P2Se6 sample’s response to a low dose Ag X-ray beam, with dose rates varying in the range 0.2-2.5 R·min-1 under cathode irradiation. The electrical signal was recorded as a function of applied bias from -10 to 10 V, showed a linear response both in the dark and under an irradiation of 40 kVp, 11


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and tube current range of 5 µA~100 µA (Figure. S5 in the Supporting Information). The time dependent response of the Pb2P2Se6 devices to X-rays (40 kVp and 30 kVp) was also recorded by changing the incident X-ray flux under a fixed applied bias of 10 V, as shown in Figure 8a. In the figure, the numbers indicate the tube current (µA) and the solid lines plot the averaged photocurrent for a Pb-rich Pb2P2Se6 sample.

Figure 8. X-ray sensitivity, (a) Time response of Pb-rich Pb2P2Se6 sample under X-ray beam bias of 40 kVp, and 30 kVp, and current in the range of 10 µA~100 µA. (b) X-ray photocurrent as a function of exposure dose rate.

The device sensitivity, defined as S=(ION-IOFF)/dose rate, was determined by normalizing the averaged photocurrent as a function of the dose rate. Figure 8b shows the linear response in the dose rate range of 0.25-2.25 R·min-1. The observed slope change indicates a decrease in X-ray sensitivity as the photon energy decreases, which is consistent with the quantum efficiency variation.34 A sensitivity of 683 nC·R-1cm-2 (equivalent to 68.3 µC·Gyair-1cm-2) was derived at 40 kVp, which is three times higher than that of reported α-Se X-ray detectors (20 µC·Gyair-1cm-2).35 By comparison, the sensitivity of Pb2P2Se6 is similar to that of methylammonium lead tribromide perovskite single crystals (80 µC·Gyair-1cm-2),10 however, unlike Pb2P2Se6 the hybrid perovskite material suffers from strong polarization under high bias. The superior X-ray sensitivity suggests that Pb2P2Se6 crystals could be used as active components in energy-discrimination X-ray detection applications.

CONCLUSIONS Detector-grade high resistivity Pb2P2Se6 (~ 1011 Ω·cm) crystals were prepared by a vertical Bridgman method under different stoichiometric conditions and evaluated experimentally. Under Se-rich and Pb-rich conditions, elliptical and bar shaped binary inclusions of P-Se and Pb-Se were observed to exist in the crystals, respectively. 12


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These are in agreement with the thermodynamic stability calculations. Se-rich Pb2P2Se6 presents a lower photoconductivity than Pb-rich Pb2P2Se6 sample, which is attributed to a higher density of shallow defects as determined by TSC spectra. The shallow level defects (with activation energy 10 µm) should be avoid by increasing the liquid convection during growth. Moreover, the grown-in point defects can be carefully engineered by the post-growth annealing under the given atmosphere.

METHODS Crystal Growth Polycrystalline Pb2P2Se6 was synthesized by a combination of lead metal (rod, Alfa Aesar, 99.9999% wt.), selenium (pellets, Stanford Advanced Materials, 99.9999% wt.) and red phosphorous (pieces, Alfa Aesar, 99.999% wt.) with a stoichiometric 2:2:6 ratio. To compare the role of the stoichiometry of the starting materials, excess Se with amounts of 1%, 0.5% and excess Pb with the amount of 0.5%, respectively, were charged in ingots labeled 0729, 1102S and 1102P. All the raw materials (~23 g) were loaded into a carbon-coated fused silica ampoule (13 mm OD, 1 mm wall thickness and 120 mm in length), and sealed under a vacuum of 2×10-3 mbar, as seen in Figure 1a. More details on the initial material synthesis can be found elsewhere.29 The polycrystalline Pb2P2Se6 samples were directly used for crystal growth in a modified vertical two-zone Bridgman furnace equipped with a computer controlled 13


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linear stage. The temperature of the hot zone was 860 °C, while the cold zone was 660 °C. The temperature gradient in the two-zone Bridgman furnace was estimated to be ~8 °C·cm-1. The crystal growth was carried out with a lowering rate of 0.5 mm per hour. Once the liquid/solid interface was completely in the cold zone, the silica tube was annealed in-situ at 660 °C for 48 hrs. Finally, the silica tube containing the grown crystal was cooled to 420°C at 10°C·h-1, and then quenched to room temperature.

Structure Characterization and Optical Measurements Powder X-ray diffraction data (XRD) were collected on a Rigaku MiniFlex600 X-ray diffractometer (Cu Kα, 1.5406 Å) operating at 40 kV and 20 mA. Diffuse reflectance spectra of the Pb2P2Se6 powders were recorded from 300 to 1200 nm at room

temperature

using

a

Shimadzu

UV-3600

PC

double-beam,

double-monochromator spectrophotometer equipped with an integrating sphere. The Pb2P2Se6 crystal surface was observed using a Hitachi 8030 FE-SEM at a bias of 15 kV. Simultaneously, the chemical compositions of local areas in Pb2P2Se6 samples were obtained using energy dispersive spectrometer (EDS) analysis. IR transmission microscopy (IRTM) system and a Nikon ECLIPSE LV100ND optical microscopy were employed to locate any secondary phase particles in the crystals.

Electrical Measurements Au contacts (60 nm thick) deposited by thermal evaporation or alternatively C contact deposited by carbon paint were used for making Pb2P2Se6 devices. Current-voltage (I-V) curves were measured at room temperature using a Keithley 6517b picoammeter/voltage supply under both low and high bias. The photoconductivity was conducted using a custom-made setup. The Pb2P2Se6 device was placed inside a guarded dark box. Then the device was exposed to ambient visible light (~0.8 mW·cm-2) with an ON-OFF state. The current- time (I-t) curve was obtained with a Keithley 6517b. The defect levels in Pb2P2Se6 crystals were probed and identified using thermally stimulated current (TSC) spectroscopy in the temperature range of 25-310 K. In TSC measurements, the sample with strip structure electrodes was placed in a closed-cycle cryostat with liquid helium and cooled in darkness to 25 K, where defect traps were filled with 500 nm light at low temperature. Then the temperature was increased to 310 K in darkness with the heating rate of 0.2 K·s-1 under the bias voltage of 100 V, more details can be found in previous work.36 14


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Radiation Detection Measurements To calculate the mobility lifetime products, the pulse height spectra as a function of bias voltages were obtained from a sample irradiated by an un-collimated

241

Am

5.48 MeV α particle source with an activity of ~1.0 µCi. Pulse shape information was simultaneously recorded by connecting the pre-amplifier (eV-550) output to a high-speed waveform digitizer card. The rise time distribution of the Pb2P2Se6 detector was determined by analyzing the pulse shapes using a homemade interface based on NI LabVIEW software. The signals from the preamplifier were further amplified and shaped by the ORTEC amplifier (model 572A) with a gain of 100-500 and shaping time of 1-6 µs, before they were acquired by a dual 16 K input multichannel analyzer (model ASPEC-927) and read into the MAESTRO-32 software. For X-ray response measurements, Pb2P2Se6 detectors were exposed to an X-ray source (Amptek Mini-X X-ray tube system with a silver target) with a low dose rate of 0.2-2.5 R·min-1. X-rays from the source were collimated using a brass cylinder with a 2-mm-diameter circular central bore. The source-to-detector distance was 5 cm. The X-ray intensity was modulated by adjusting the tube bias and current. The induced photocurrent signal of the detector was recorded as a function of time using a Keithley 6517b.

First-principles calculations First-principles total energy calculations were employed to investigate the thermodynamic stability, electronic structure, and defect properties of Pb2P2Se6. The calculations employed the Vienna Ab-initio Simulation Package (VASP) implementation of the density functional theory (DFT) in conjunction with the projector-augmented-wave (PAW) formalism.37,38 The Perdew-Burke-Ernzerhof generalized

gradient

approximation

(PBE-GGA)

was

used

for

the

exchange-correction potentials.39 The related electronic configuration is P 3s23p3, Pb 6s26p2 and Se 4s24p4. The energy convergence criterion for the structural relaxation is 1×10-6 and the force convergence criterion is set to 0.01 eV/Å. The electronic wave functions are expanded in plane waves using an energy cutoff of 520 eV. The Brillouin zones of the 64-atom orthorhombic supercells are sampled with 2×2×4 Monkhorst-Pack k-point meshes. More calculation details are in Supplementary material. 15


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AUTHOR INFORMATION Corresponding Author: *[email protected]

ORCID Yadong Xu: 0000-0002-1017-9337 Kyle M. McCall: 0000-0001-8628-3811 Bruce W. Wessels: 0000-0002-8957-7097 Mercouri G. Kanatzidis: 0000-0003-2037-4168

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work is supported by a Department of Homeland Security ARI program with grant 2014-DN-077-ARI086-01. This work made use of the (EPIC, Keck-II, and/or SPID) facility of the NUANCE Center at Northwestern University. Project was also supported by the National Natural Science Foundations of China (No. U1631116), and the National Key Research and Development Program of China (2016YFE0115200). Yadong Xu thanks the Top International University Visiting Program for Outstanding Young Scholars of Northwestern Polytechnical University.

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For Table of Contents Use Only

The role of stoichiometry in the growth of large Pb2P2Se6 crystals for nuclear radiation detection Yadong Xu, Xu Fu, Hongjian Zheng, Yihui He, Wenwen Lin, Kyle M. McCall, Zhifu Liu, Sanjib Das, Bruce W. Wessels, Mercouri G. Kanatzidis

Pb2P2Se6 crystals grown by vertical Bridgman method under off-stoichiometric conditions exhibited high resistivity of 1011 Ω·cm. The formation and nature of grown-in defects in Pb2P2Se6 crystals have been studied. The resulting Pb2P2Se6 detectors show a linear response to X/γ-rays. An X-ray sensitivity of 68.3 µC·Gyair-1cm-2 was achieved, which is three times higher than that of the state-of-the-art amorphous Se detector.

19


The role of stoichiometry in the growth of large Pb2P2Se6 crystals for

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