277978-1-4799-5296-0/14/$31.00 © 2014 IEEE

PROC. 29th INTERNATIONAL CONFERENCE ON MICROELECTRONICS (MIEL 2014), BELGRADE, SERBIA, 12-14 MAY, 2014

Crystallization Processes in Ge-Ga-Se Glasses Studied with Positron Annihilation Technique

H. Klym, A. Ingram, O. Shpotyuk

Abstract – The positron annihilation lifetime spectroscopy was applied to study controlled crystallization processes in 80GeSe2-20Ga2Se3 chalcogenide glass, which was thermally treated at 380°C for 10, 50 and 100 h. It is shown that the structural changes caused by crystallization can be adequately described by positron trapping modes determined within two-state model. The observed changes in defect-related component in the fit of experimental positron lifetime spectra for annealed glasses testifies in a favour of structural fragmentation of larger free volume entities into smaller ones with preceding nucleation in the initial stage of thermal annealing.

I. INTRODUCTION

GeSe2–Ga2Se3 chalcogenide glasses (ChG) have shown many advantages for potential applications of optical modulator or frequency converter, efficient laser host materials, and fiber-optical amplifier in the IR spectral region [1]. Their rare-earth ions solubility is greatly increased through the incorporation of gallium, which makes some structural modifications of the GeSe2 network. Atomic arrangement in such ChG can be studied with numerous techniques (XRD, SEM, XPS, NMR, etc etc.), while number of probes available to study atomic-deficient distribution is rather limited, especially at a sub-nanometer scale. One of the best techniques capable to probe such fine free volumes is the positron annihilation lifetime (PAL) spectroscopy [2]. In the present paper, we imply the PAL method to study of crystallization processes in 80GeSe2-20Ga2Se3 ChG.

II. EXPERIMENTAL

The crystallization of the 80GeSe2-20Ga2Se3 was performed with a single step of heat treatment at Tg+10 oC [3,4]. This temperature has been chosen as an optimal temperature of ceramization as it permits to control the generation by simultaneous nucleation and growth of nanoparticles within the glassy matrix according to the heat

treatment time. Thus, glass samples were placed in a ventilated furnace where the accuracy of temperature is 2 oC for various time varying from 10 to 50 and 100 hours.

The PAL spectra were recorded with conventional fast-fast coincidence system (ORTEC) of 230 ps resolution (full width at half maximum FWHM of a single Gaussian, determined by 60Co isotope measuring) at the temperature T = 22 oC and relative humidity RH = 35 %, provided by special climatic installation. Contribution intensity of source is 18 %. Two identical ChG samples were used to build a character sandwich arrangement needed for PAL measurements. Isotope 22Na of slow activity (~50 kBq) sandwiched between two identical tested samples was used as a source of positrons. The measured PAL spectra of ChG were processed with standard LT 9.0 computer program [5]. The positron trapping modes in the studied ChG, e.g. average positron lifetimes av, positron lifetime in defect-free bulk b, positron trapping rate in defects d were calculated using a formalism of two-states trapping model.

Assuming two-state positron trapping model like as for typical ChG systems [2], two components in the fit of experimental PAL spectra can be provided with reduced bulk positron lifetime 1 which itself has no physical meaning, positron lifetime in free-volume entities (positron traps) 2 and corresponding intensities I1 and I2. Within formalism of this model, the open volume holes free of electron density are treated as specific positron trapping “defects”, while structural domains without them is related to intrinsic “defect-free” bulk (represented by b value).

III. RESULTS AND DISCUSSION

Typical spectrum obtained by PAL technique is shown in Fig. 1. They are characterized by a narrow peak and region of long fluent decaying of coincidence counts in a time. The mathematical decay of such curve can be represented by a sum of decreasing exponents with different values of power-like indexes inversed to positron lifetimes [2,4]. The PAL spectrum of 80GeSe2-20Ga2Se3 base glasses is decomposed into two components with lifetimes 1, 2 (usual case of PAL spectra fitting for ChG, if we let this sentence, we should add the references at this place). According to the mathematical decomposition proposed in [2], tangent to the sites of PAS spectrum correspond to lifetimes 1, 2 and the area under each of these curves is proportional to the intensities I1, I2. Source contribution intensity is near 18 % (τ1s=372 ps τ2s≈2 ns).

H. Klym is with Lviv Polytechnic National University, Bandera 12, 79013 Lviv, Ukraine, E-mail: klymha@yahoo.com

A. Ingram is with Opole University of Technology, Ozimska 75, 45370 Opole, Poland, E-mail: a.ingram@po.opole.pl

O. Shpotyuk is with Lviv Institute of Materials of SRC “Carat”, Stryjska 202, 79031 Lviv, Ukraine and Institute of Physics of Jan Dlugosz University, Al. Armii Krajowej 13/15, 42201 Czestochowa, Poland, E-mail: shpotyuk@novas.lviv.ua

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0 2 4 6100

101

102

103

104

source contribution1s=372 ps

source contribution1s= 2 ns

component

component

Cou

nts

Time, ns

experimentalcomponent

component

source contributionsource contribution

Figure 1. Typical PAL spectrum decomposed into two components for 80GeSe2-20Ga2Se3 base glass.

It is shown that with increase of annealing duration

from base 80GeSe2-20Ga2Se3 glass to samples annealed for 10 h, the lifetimes 2 decreases and I2 intensities rather rises, testifying growing behaviour in the number of corresponding free-volume entities (Table I).

TABLE I

FITTING PARAMETERS FOR PAL SPECTRA OF 80GESE2-20GA2SE3 GLASSES BEFORE AND AFTER THERMAL ANNEALING

Conditions Fitting parameters 1, ns I1, a.u. 2, ns I2, a.u.

Untreated, 0 h 0.214 0.69 0.439 0.31 380 °C, 10 h 0.218 0.71 0.453 0.29 380 °C, 50 h 0.216 0.65 0.426 0.35 380 °C, 100 h 0.206 0.57 0.416 0.43 Positron trapping modes

Conditions av., ns

b, ns

d, ns-1

τ2 - b, ns

τ2/b

Untreated, 0 h 0.283 0.253 0.74 0.19 1.73 380 °C, 10 h 0.287 0.257 0.70 0.20 1.76 380 °C, 50 h 0.281 0.252 0.83 0.16 1.65 380 °C, 100 h 0.296 0.263 1.05 0.15 1.58

It was shown that coupling of open volumes created by

bond free solid angles, belonging to different chalcogen chains or coordination polyhedrons, can be effective traps for positrons in ChG. During crystallization, the glass structure relaxes towards more thermodynamically favourable state (crystallization shrinkage or densification), eliminating the excess of free volume of neighbouring voids. It means that existing free volume voids either disappear or convert into a greater number of smaller ones. In other words, we can argue that crystallization in 80GeSe2-20Ga2Se3 glass induced by annealing causes fragmentation of larger free-volume entities into smaller ones [4]. Such process is accompanied by essential decrease in 2 and corresponding increase in I2 in full agreement with above scenario.

These changes cause the regular extension of the lifetime av. But under increase of annealing to 50 h and

100 h, the I2 intensity ceases to increase, while lifetime 2 appreciably decreases to 0.416 ns. This trend leads to a significant rises of av and d. Other positron trapping parameters (for example, 2/b) behave is in harmony with similar ones. But (2 - b) difference, accepted as a size measure for extended free-volume defects where positrons are trapped, naturally decreases with annealing duration.

In contrast to above fitting PAL parameters (2,I1,2,I2), the changes in the positron trapping rate of free-volume defects d are more pronounced, especially at long annealing durations, when a specific fragmentation behaviour revealed a decrease in dimensions of free-volume entities accompanied by simultaneous increase in their amount. In principle, these changes in d can be caused by charge state of trapping centres too. However, the constant τ2/b ratio close to 1.7 for all ChG samples despite their treatment duration, testified that positron-trapping centres are rather of the same type, being most probably as large as di- or tri-atomic vacancies.

IV. CONCLUSION

Crystallization behavior of 80GeSe2-20Ga2Se3 glasses

during annealing at 380°C for 10, 50 and 100 h indicates on the possibility of formation of GeGa4Se8 and Ga2Se3 crystals [4]. These crystallites modify free-volume structure of ChG leading to specific fragmentation of larger free-volume entities into greater number of smaller ones with nucleation in the initial stage of annealing. Such effect reveals an increase of the second lifetime component in the initial stage of annealing (10 h) because of preliminary void expansion and further decrease of 2 lifetimes with simultaneous I2 increase during more prolonged annealing (50 and 100 h) testifying in a favour of increased number of smaller free volumes.

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[2] R. Krause-Rehberg, and H.S. Leipner, “Positron annihilation in semiconductors. Defect Studies”, Springer-Verlag, Berlin-Heidelberg-New York, 1999, p. 378.

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