Electrical resistivity and photoluminescence of lead

c 2007 Institute of Chemistry, Slovak Academy of Sciences DOI: 10.2478/s11696-006-0092-y

Electrical Resistivity and Photoluminescence of Lead Iodide Crystals M. MATUCHOVÁ*, K. ŽĎÁNSKÝ, M. SVATUŠKA, J. ZAVADIL, and O. PROCHÁZKOVÁ

Institute of Radio Engineering and Electronics, Academy of Sciences of the Czech Republic, Chaberská 57, 182 51 Prague, Czech Republic e-mail: [email protected] Received 8 January 2006; Revised 2 August 2006; Accepted 10 August 2006

Direct synthesis of lead iodide, a promising material for X-ray and γ detectors operating at room temperature, was developed and optimized. The influence of admixture of rare earth elements Ce, Ho, Gd, Yb, Er, and Tb in concentrations 0.05—0.5 at. % on the quality of prepared PbI2 was investigated. Zone melting was employed in order to increase the lead iodide purity. Electrical and optical properties of PbI2 samples were assessed on the basis of the measurement of electrical resistivity and low-temperature photoluminescence. The electrical resistivity of synthesized samples varied from 109 Ω cm to 1011 Ω cm and occasionally it was increased up to 1013 Ω cm. Keywords: zone melting, PbI2 , electrical resistivity, photoluminescence, rare elements

INTRODUCTION

surement of electrical resistivity and low-temperature photoluminescence (PL).

Lead iodide is one of the promising candidates for the high-efficiency room-temperature solid-state radiation detectors for use in the medium energy range of 1 keV to 1 MeV [1—3]. It can be used in devices in a laboratory or outdoors, e.g. for ecological measurements or for improved diagnostics methods. PbI2 offers some favorable properties, as low vapor pressure and high chemical stability at room temperature and normal pressure. No degradation of this compound has been observed in a laboratory within six months. Crystals of PbI2 have a hexagonal layered structure and can be grown from solutions, melts, vapors, and gels. More polytypes have been reported [4] and the most common one is 2H. The aim of this work was the development and optimization of the method of direct synthesis [5] of PbI2 crystals. Similarly to the recently published results reporting on the influence of rare earth (RE) elements on the quality of materials for radiation detectors [6—9], the effect of Ce, Ho, Gd, Yb, Er, and Tb admixture on the lead iodide synthesis was investigated. Some of the prepared samples were purified by zone melting and the effect of purification was evaluated. Assessment of the quality of prepared crystals was based on the mea-

EXPERIMENTAL The PbI2 crystals were prepared by direct synthesis from the elements in a 50 cm long ampoule containing lead with traces of the RE elements (Fig. 1). Furthermore, smaller sealed ampoule filled with iodine was placed inside. Then, the large ampoule used for synthesis was evacuated, heat-sealed, and the smaller inner ampoule was carefully broken. According to the previous experiments [5], the temperature for iodine sublimation was set to 200 ◦C. The other section of the evacuated ampoule was heated to 700 ◦C in order to melt Pb with added RE elements. At given conditions, iodine was diffusing into the lead melt forming PbI2 . The prepared lead iodide was further purified by zone melting at 420 ◦C (Fig. 2). The purified ingot of PbI2 was usually 30 cm long. About two thirds of the unused part of the ampoule was used for further crystal growth. Single crystals were grown in the Bridgman—Stockbarger vertical growth apparatus. Table 1 presents the conditions, at which individual PbI2 samples were prepared. The hexagonal layered structure of the prepared

*The author to whom the correspondence should be addressed.

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Chem. Pap. 61 (1) 36—40 (2007)

PROPERTIES OF LEAD IODIDE CRYSTALS

Table 1. Conditions during the Preparation of PbI2 Crystals Sample

RE (content/at. %)

Zone melting repetitions

6zm 13zm 14zm 35 35zm 52a5zm 55/1zm 19 21 22 29 37 39 51

– – – – – – – Ce (0.5) Ho (0.05) Gd (0.1) Yb (0.4) Er (0.05) Tb (0.2) Tm (0.05)

20 20 20 None 20 5 20 None None None None None None None

Resistivity · 109 /(Ω cm) 2400 2800 2000 15 2900 9300 10000 10 980 12 6.2 43 3.1 700

Fig. 1. Ampoule for the PbI2 synthesis (length of the ampoule about 50 cm).

Fig. 2. Ingot of lead iodide obtained after the zone melting (length about 30 cm). About two thirds of pure part was used for further crystal growth.

PbI2 samples was visualized employing TESCAN scanning electron microscope. Fig. 3 presents 60 µm × 70 µm large digital microscopy image of the lead iodide sample. The impurities present in the prepared samples were analyzed by fluorescence spectroscopy (XRF). After synthesis and zone melting the content of impurities and added RE elements was below the detection limit (20 ppm) of this method. Thin single crystal PbI2 wafers were cleaved by a razor blade. Thickness of each wafer sample was homogeneous ranging from 0.1 mm to 0.3 mm as estimated by a micrometer. Circular metal contacts (1—3 mm) were attached symmetrically on both sides of the cleaved wafers using air-drying of silver paste. Electrical characteristics of the lead iodide samples were measured at room temperature in the range from 0 V to 110 V using KEITHLEY 236 Source-Measure Unit. The characterization of 2H-PbI2 single crystals using near band-gap PL together with the microwave and thermally modulated PL was studied in [10]. In the present study low-temperature PL spectra, excited by the Ar ion laser at 457.9 nm were recorded. RESULTS AND DISCUSSION The results of the measurement of electric proper-

Chem. Pap. 61 (1) 36—40 (2007)

Fig. 3. Image of the PbI2 layered structure.

ties of selected PbI2 samples are presented in Figs. 4 and 5 in the form of current density vs. electric field intensity plot. The effect of zone melting purification on the resistivity of PbI2 samples prepared in the absence of rare earth metals is demonstrated in Fig. 4 by a decrease of the corresponding value of current density. In the upper zone is found the curve obtained for the sample 35 prior to its purification, while those


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M. MATUCHOVÁ, K. ŽĎÁNSKÝ, M. SVATUŠKA, J. ZAVADIL, O. PROCHÁZKOVÁ

1000

D-A

20

PL intensity/a.u.

-2

Current density/(nA cm )

100

10

15 -LO -2LO

10 5

1 0

3 2 1 2350

2500

2550

Fig. 6. Near band edge low-temperature PL spectrum of 2HPbI2 crystals measured at T = 4 K and Pex = 150 mW cm−2 . Spectra of the RE-doped lead iodide samples 21 (solid line 1) and 37 (dashed line 2) and the PbI2 sample prepared without the rare earth admixture 35 (dotted line 3).

100

1000

10000 -1

Electric field/(V cm )

Fig. 4. Current density vs. electric field intensity of the synthesized and purified PbI2 samples 6zm ( ), 13zm ( ), 14zm ( ), 35 (), 35zm ( ), 52a5zm ( ), and 55/1zm ().

•

◦

Energy/eV 2.4

PL intensity/a.u.

1000

2.2

1.8

2.0

1.6

D-A

200

-2

2450 Energy/meV

0.01

Current density/(nA cm )

2400

0.1

0.001

BE

150 100 BE

deep level

× 10 deep level

50

×2

100 0 500

10

550

600

650

700

750

800

Wavelength/nm

Fig. 7. Low-temperature PL spectrum of the sample 21 measured at T = 4 K and Pex = 150 mW cm−2 . Deep level transition bands with the maximum at about 600 nm and 735 nm were magnified 10 and 2 times, respectively.

1

0.1 100

1000

10000

Electric field/(V cm -1)

Fig. 5. Current density vs. electric field intensity of the PbI2 samples 19 ( ), 21 (), 22 ( ), 29 ( ), 37 ( ), 39 (), and 51 ( ) synthesized in the presence of REs.

•

◦

in the lower zone represent the electric properties of samples that undergone multiple zone melting. Fig. 5 collects the results of the measurement of resistivity of PbI2 samples prepared in the presence of different RE metals. Resistivity of individual samples is summarized in Table 1. Zone melting of the lead iodide samples prepared without the admixture of RE dramatically improved their resistivity up to 1013 Ω cm (samples 6zm, 13zm, 14zm, 35zm, 52a5zm, and 55/1zm). Similar PbI2 resistivity was published only once [11]. Interesting resistivity improvement among the lead iodide crystals synthesized in the presence of different REs 38

was observed in the case of samples 21 and 51 doped with Ho and Tm, respectively. Figs. 6—9 present typical low-temperature PL spectra of PbI2 crystals prepared by the direct synthesis with or without the addition of RE3+ ions. Within the studied range of wavelengths 400—1700 nm (3.0— 0.73 eV) no sharp radiative 4f—4f inner shell transitions, characteristic of RE ions were observed. Thus, it can be concluded that RE elements introduced during the direct synthesis of PbI2 were not present in the crystal lattice in sufficient amount or RE were not easily excited in PbI2 host by Ar ion laser. Three different radiative transitions could be observed, namely band-edge (BE) transitions caused by the decay of excitons at 2.49 eV, somewhat broader shallow donor—acceptor (D-A) recombination band at 2.43 eV associated with the lattice imperfections, and broad bands at 2.0 eV, 1.72 eV, 0.99 eV, and 0.8 eV corresponding to deep levels. The excitonic as well as shallow donor-related transitions were rapidly


Chem. Pap. 61 (1) 36—40 (2007)

PROPERTIES OF LEAD IODIDE CRYSTALS

2.37 eV 200

2 1.72 eV 2.41 eV

Current/nA

150

3

100 2.05 eV

1

50

4 0 1.5

5

2.5

Energy/eV

Fig. 8. Shallow donor and deep level transitions in the lowtemperature PL spectra of the samples 6 (dashed line 1), 22 (solid line 2), 26 (dotted-dashed line 3), 40 (dotted line 4) measured at T = 4 K and Pex = 64 mW cm−2 .

25

0.99 eV

20 0.79 eV Potential/mV

1 15

2 3

10

4

{

5 absorption of water vapor 0

0.8

0.9

1.0

1.1

Energy/eV

The other two samples exhibit much broader bands reflecting higher concentration of impurities, which correlates well with the measurement of current density (Figs. 4 and 5). Additional broad bands at 600 nm (2.06 eV) and 730 nm (1.7 eV) related to the deep level transitions could be found e.g. in the spectrum of the PbI2 sample 21 (Fig. 7). It is seen from Figs. 8 and 9 that samples 6 and 40 (Ho, 0.25%) exhibit four broad bands due to deep levels while the bands at 1.72 and 0.79 eV are missing in the samples 22 (Gd, 0.1%) and 26 (Ho, 0.05%). Thus observed deep levels are probably inherent to PbI2 crystal and do not depend on RE doping. The appearance of deep level bands does not show strict correlation with the addition of RE elements. Therefore, it was suggested that at least two of the observed broad bands (at 2.0 eV and 0.99 eV) are inherent to the samples prepared by direct synthesis and related to the crystal lattice perturbations and/or presence of localized complexes. Radiative transitions due to deep level energy states have not been reported previously for PbI2 . Phonon-assisted transitions associated with shallow acceptor level and involving 5—8 meV LO phonons were observed in the PL spectra of several samples characterized by sufficiently narrow highenergy (BE, D-A) transitions. To conclude, further experiments are needed in order to correlate the effect of RE addition and the observed broad PL bands due to deep energy levels in the band gap. It is assumed that the reactivity of RE, particularly to elements that could act as shallow impurities in PbI2 , is responsible for the purification (gettering effect) of samples synthesized with the addition of RE elements. This effect might be responsible for the creation of complexes not influencing the lead iodide resistivity. Zone melting efficiently accumulates these complexes outside the useful part of the PbI2 crystal. Acknowledgements. The authors wish to thank J. Maixner, V. Hervert, V. Lískovec, R. Král, and V. Bierhanzl for the technical assistance. This work was supported by the Grant Agency of the Czech Republic, Grants No. 102/04/0959 and 102/03/0379.

Fig. 9. Deep layer PL bands in the spectra of the samples shown in Fig. 8.

REFERENCES

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Chem. Pap. 61 (1) 36—40 (2007)