CharacteristicofZnSe__省略_conductorScinti

Characteristic of ZnSe(Al,O,Te) Semiconductor Scintillator W.G. LEE1, Y.K. KIM2?, J.K. KIM2, N. STRAZHINSKIY3, V. RYZHIKOV3, and B. GRINYOV3

1)Innovative Technology Center for Radiation Safety, Hanyang University, Seoul, Korea

2)Department of Nuclear Engineering, Hanyang University, Seoul, Korea

3)Institute for Scintillation Material of Science-Technology Concern “Institute for Single Crystals” National Academy of Science

of Ukraine, Kharkov, Ukraine

Abstract: Zinc selenide crystals were grown in graphite crucibles by Bridgman–Stockbarger method in vertical compression furnace under argon pressure of 5x106 Pa. The grown ZnSe(Al,Te,O) single crystals were annealed in a Zn vapor at 1290 K for 24 hrs. A X-ray diffractometer (XRD) was used to investigate the structural properties of the zinc selenide single crystals. The lattice constants of the ZnSe single crystals were obtained from XRD data. After annealing it in a Zn vapor at 1290 K, it is found that the lattice constant decreases. From the absorption spectra, the band gap energies of the ZnSe single crystals were calculated by a linear fitting process. The band gap energy of the ZnSe(Al,O,Te) single crystal decreases more than that of the ZnSe single crystal. The maximum wavelength of the radioluminescence of the ZnSe(Al,O,Te) scintillator excited by X ray was 606 nm, which was well matched with the response wavelength of the Si photodiode. The energy resolution of the annealed ZnSe(Al,O,Te) scintillator was 13.9 % when it was exposed to 137Cs γ-ray. Its size was 10x10x1 mm3. The afterglow level of the annealed ZnSe(Al,O,Te) scintillator after 5 ms was 0.014%. The luminescence decay time of the annealed ZnSe(Al,O,Te) scintillator has two exponential components with 4 and 12 μs time constants. The charged particle and the low energy gamma ray can be detected successfully with the annealed ZnSe(Al,O,Te) scintillator. It was investigated that the ZnSe(Al,O,Te) scintillator can be used for the security inspection system.

KEYWORDS: ZnSe, scintillator, XRD, pulse height spectrum, afterglow

1. Introduction

II-VI compound semiconductor has various physical properties such as piezoelectricity, photo-electricity, luminescence, and scintillation. But single crystals of the II-VI compound semiconductor were not used as scintillator and radiation sensor because they have low transmittance and a recrystallization. But recently they can be used as the radiation sensor due to the advance of the single crystal growth technique[1, 2].

Scintillators based on ZnSe crystals are not hygroscopic. Their light output is about 1.5 times higher and afterglow level after 10 ms is about three orders lower than that of CsI(Tl)[3]. Scintillators based on ZnSe crystals are quite acceptable for their use in X-ray introscopy detectors because their radiation stability is relatively good and their light output is about 4 times higher than the oxide scintillators(CdWO4, Bi4Ge3O12, Gd2SiO5) used on similar purposes[4,5]. The ZnSe scintillators based on the II-VI compound semiconductor doped with various activators were investigated[6].

In our work, the ZnSe single crystals based on the II-VI compound semiconductors were grown by a modified Bridgman–Stockbarger technique. The doping and annealing effects of the ZnSe single crystal were observed and discussed. It was also investigated the physical and the scintillation properties of the ZnSe single crystals such as X-Ray Diffraction (XRD), transmittance, radioluminescence, afterglow, light output, pulse height spectrum, and decay time.

2. Experiment

Doped and un-doped ZnSe single crystals were grown by a modified Bridgman –Stockbarger technique with a narrow zone heater in a vertical compression furnace under argon atmosphere. Graphite crucible was used. During growth, the melt overheating value was varied in the range of 30~200 K, the inert gas pressure P of Ar was ~5×106 Pa and the crystallization rate was 2~5 mm/h. The raw materials were ZnSe polycrystals of 5N offered by ELMA Inc. After growth, thermal treatment on the ZnSe crystal was carried out at 1290 K for 24 hrs in sealed quartz tube containing Zn vapor. In our study, the size of the ZnSe specimen was 10x10x1 mm3.

The crystal structures and lattice parameters of the single crystals were characterized by the XRD method using Cu Kα radiation (Rigaku, D/MAX 2100H, Japan).

The pulse height spectra were measured with the gamma energy radioactive isotopes, 241Am(59.6 keV) and 137Cs(662 keV). The PMT R1307(Hamamatsu) was used to measure the pulse height spectra of the ZnSe(Al,O,Te) scintillator.

In order to obtain the decay time of the ZnSe(Al,O,Te) scintillator at room temperature, a pulsed Nd:YAG laser was employed as an excitation source. The repetition rate of the pulsed laser was 10 Hz with a 6 ns duration.

? Tel : +82-2-2220-2354; Fax. : +82-2-2296-8154; Email: ykkim4@hanyang.ac.kr

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3. Result and discussion

Figure 1 shows XRD patterns of un-doped and doped zinc selenide crystals. It is found that ZnSe cross-section shows preferred orientation of (220) direction.

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(331)

(400)

(311)

(220)

(111)

I n t e n s i t y [a r b . u n i t ]

Degrees [2θ]

(a)

Fig. 1. XRD patterns of ZnSe single crystals (a) as grown ZnSe(pure),

(b) as grown ZnSe(Al,O,Te), (c) annealed ZnSe(pure), and (d) annealed ZnSe(Al,O,Te).

There are no new observable phases even when doping and annealing in Zn condition, this means that doping and annealing do not change the cubic structure of ZnSe.

However, the doping and annealing in Zn condition are shifted the position of (220) peak. The lattice constant a of cubic structure ZnSe were calculated according to Bragg’s Law λθm d =sin 2 (1) For cubic structure of ZnSe, lattice constant a is given by

2

22221a

l k h d hkl ++= (2) where d is plane-spacing, λ is x-ray wavelength of 1.5405 ?. The calculated lattice constants a are listed in Table 1.

Table 1. The lattice parameter a of as grown and annealed ZnSe single crystals.

As grown

Annealed

(220) peak [2θ]

a [?]

(220) peak [2θ]

a [?]

ZnSe(pure) 45.258 5.659 45.271 5.654 ZnSe(Al,O,Te)

45.244 5.662 45.305 5.651

With the doped impurities (Al, O, Te), the lattice constant a was changed from 5.659 ? to 5.662 ?. With the annealing in Zn condition, the lattice constant a was changed little. In general, when impurity is doped to ZnSe, impurities replaced the vacancies of Se, Zn ion or remained as an interstitial. When impurities are placed as an interstitial ion, the atoms can be rearranged and placed on the equilibrium state due to the Coulomb interaction between Zn 2+ and Se 2- ions, so the lattice constant a will increase. However, after annealing, lattice constant a was decreased. Because the ion radius of O is 0.66 ?, which is smaller than that of Se ion, 1.14 ?, therefore the inter-atomic distance of Zn-O will become shorter compared to the bonding length of Zn-Se. Al impurities follow the same mechanism. As a result, the lattice constant will decrease after annealing.

Figure 2 shows the absorption spectra of the ZnSe and ZnSe(Al,O,Te) crystals by annealing. Assuming the absorption coefficient α corresponding to the direct band gap of the zinc blend structure, a plot of [α*(h ν)]2 versus the photon energy h ν yields in the sharp absorption edge by linear fit. In these results, the optical band gap energy of pure ZnSe crystal was not changed by annealing, but the band gap energy of ZnSe(Al,O,Te) crystals has decreased weakly from 2.61 eV to 2.44 eV and 2.46 eV. The decrease of band gap energy is assumed to result from the impurities defect level in ZnSe(Al,O,Te) crystals.

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2.42 2.44 2.46 2.48 2.50 2.52 2.54 2.56 2.58 2.60 2.62 2.64 2.66

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[a r b . u n i t ]Photon Energy [eV]2.607 [eV]

2.459 [eV]

2.437 [eV]

(a)

(d)

(c)(b)

I n t e n s i t y [a r b . u n i t ]

Wavelength [nm]

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Fig. 2. Absorption secptra of ZnSe single crystals (a) as grown ZnSe(pure), (b) annealed ZnSe(pure),

(c) as grown ZnSe(Al,O,Te), and (d) annealed ZnSe(Al,O,Te).

The transmittance curves of the ZnSe crystals were measured from 400nm to 900nm with a double-beam spectrophotometer (Cary-5, Varian, Australia). The transmittance coefficient of the annealed ZnSe(Al,O,Te) crystal was more than 60 % above the 600 nm.

The radioluminescence provides the exact wavelength of the scintillation when the radiation energy is absorbed. The X-ray generator was the radiation source. Figure 3 shows the radioluminescence spectra of the annealed ZnSe crystal and the annealed ZnSe(Al,O,Te) crystal. The maximum wavelengths of the annealed ZnSe crystal is 625 nm, and that of the annealed ZnSe(Al,O,Te) crystal is 606 nm, which coincides with the response wavelength of the Si photodiode.

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(a) anneled ZnSe

(b) anneled ZnSe(Al,O,Te)

Wavelength [nm]

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Fig. 3. Radioluminescence spectra of the annealed ZnSe and annealed ZnSe(Al,O,Te) single crystals at the room temperature.

The pulse height spectrum of the annealed ZnSe(Al,O,Te) scintillator is shown in Fig. 4. The energy resolutions of the annealed ZnSe(Al,O,Te) scintillator was 13.9 % when exposed to 137Cs γ-ray . The gamma peak of the 59.6 keV appeared distinctly.

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E γ=662 keV

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E γ=59.6 keV

Fig. 4. Pulse height spectrum of the annealed ZnSe(Al,O,Te) scintillator to the 241Am and 137Cs gamma radiations.

The emission wavelengths (600 nm - 630 nm) of the scintillators are not coincidence with the response wavelength of the PMT but those are matched well with the photodiode. The light output of the annealed ZnSe(Al,O,Te) scintillator was measured with a Si photodiode (S3590, Hamamatsu) under a X ray (100 kV , 1 mA). CsI(Tl) scintillator was also measured with the same condition. The relative light output of the annealed ZnSe(Al,O,Te) scintillator is about 1.3 times higher than that of the CsI(Tl) scintillator.

The afterglow is one of the important physical properties. The measurement method of the afterglow was that the scintillation from the ZnSe(Al,O,Te) scintillator was measured with a photodiode as a function the time as soon as the X-ray generator was off. The afterglow levels of the ZnSe(Al,O,Te) scintillators are shown in Fig. 5. The afterglow levels of the as grown ZnSe(Al,O,Te) and the annealed ZnSe(Al,O,Te) scintillators are 23.9% and 0.014% (5 ms), respectively.

This means that the afterglow level of the annealed ZnSe(Al,O,Te) scintillator is less than that of the CsI(Tl) scintillator.

As it has been realized, after annealing in Zn condition, the lattice constant a decreased little. Because the interstitial impurities (Al, O, Te) replaced the vacancies of Se, Zn ions, the annealed ZnSe(Al,O,Te) crystal reduced the traps which were caused by impurities. The afterglow concerns the traps of the crystal, and the afterglow level of the annealed ZnSe(Al,O,Te) scintillator is less than that of the as grown ZnSe(Al,O,Te) scintillator.

In order to obtain the decay time of the ZnSe(Al,O,Te) scintillator at room temperature, the data were acquired in 1,000 times.

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(b) annealed ZnSe(Al,O,Te)

I n t e n s i t y [a r b . u n i t ]

Time [ms]

(a) as grown ZnSe(Al,O,Te)

Fig. 5. Afterglow curves of the ZnSe(Al,O,Te) scintillaors (a) as grown ZnSe(Al,O,Te) and (b) after annealing ZnSe(Al,O,Te).

The decay time of the as grown ZnSe(Al,O,Te) scintillator has three components, its values are 5, 23, and 128 μs. But the decay time of the annealed ZnSe(Al,O,Te) scintillator has only two components, its values are 4 and 12 μs.

The annealing is the proper method to improve the structural and scintillator properties of ZnSe(Al,O,Te) crystal. 4. Conclusion

The good structure crystals of the ZnSe based on the II-VI semiconductor were grown by a modified Bridgman–Stockbarger technique. The physical properties and scintillation properties of the ZnSe(Al,O,Te) single crystals were investigated. The maximum wavelength of the radioluminescence of the annealed ZnSe(Al,O,Te) single crystal was 606 nm, which is coincidence with the response wavelength of the Si photodiode.

The ZnSe scintillators can detect the alpha, beta particles, and the low energy gamma ray. The light output of the annealed ZnSe(Al,O,Te) scintillator is more than that of the CsI(Tl) scintillator. The afterglow level of the annealed ZnSe(Al,O,Te) scintillator is less than that of the CsI(Tl) scintillator. Especially the time properties of the annealed ZnSe(Al,O,Te) scintillator are quite suitable for the detectors of the introscopy system.

It is expect that the annealed ZnSe(Al,O,Te) scintillator based on the II-VI compound semiconductor can be used for the X-ray detector for a security inspection system.

Acknowledgment

This work has been supported financially by the Innovative Technology Center for Radiation Safety (iTRS) at Hanyang University, Korea.

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