Compositional trends in the native open-volume microvoids concept for vitreous arsenic selenides icon

Compositional trends in the native open-volume microvoids concept for vitreous arsenic selenides




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НазваCompositional trends in the native open-volume microvoids concept for vitreous arsenic selenides
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ВІСНИК ЛЬВІВ. УН-ТУ VISNYK LVIV UNIV.

Серія фізична. 2002. Вип.35. С.150-154 Ser.Physic. 2002. № 35. P.150-154


PACS number(s): 61.43.Fs +78.70.Bj

COMPOSITIONAL TRENDS

IN THE NATIVE OPEN-VOLUME MICROVOIDS CONCEPT FOR VITREOUS ARSENIC SELENIDES




A. Kozdras



Physics Laboratory, Opole Technical University, 75, ul. Ozimska, Opole, PL-45370, Poland, e-mail: koan@po.opole.pl


The modified concept of the native open-volume microvoids (the effective positron trapping sites) has been checked at the example of As-Se chemical system of chalcogenide vitreous semiconductors (ChVS). It have been shown that the native open-volume microvoids concept is quite meaningful one for quantitative and qualitative interpretation of the well-known experimental results on positron lifetime measurements in ChVS.

^ Key words: chalcogenide glasses, positron annihilation, open volume, microvoids


The positron annihilation method is one of the most sensitive research tools that allow observing the current state of material structure as well as of its changes. One of its main uses is the observation of the material defect state in a crystalline phase due to the fact that the defects of the like-point type present in the examined materials are potentially attractive volumes for annihilating positrons. A separate branch of this measurements is a use of the positron annihilation method for the materials with a long distance lack disorder e.g. glasses or ceramic. In the materials of this type the thermalized positron lifetime may be not only determined by a free state annihilation in a charged point defects (bulk positron lifetime), but this value may also be prolonged thanks to the presence of the electrically neutral microvoids with the sizes bigger than point-type defects.

Obviously, the interpretation of the positron lifetime spectra in topologically disordered materials is very complicated. Numerous researchers, who have published the data obtained by this method, have presented on many occasions only the numerical values, leaving the result interpretation in a form of delicate suggestions. Therefore, there appears a need for systematizing the data that have been accumulated so far in order to check the possible existence of correlations between annihilation parameters and basis physicochemical properties.

This paper presents an interpretation possibility of the experimentally obtained positron annihilation parameters on the example of As-Se system chalcogenide glasses. The relative changes of the annihilation parameters have been explained as a result of an existence of technological frozen native open free volumes.

The structure defects – vacancies, vacancy clusters (bubbles, microvoids) and dislocations – are very attractive volumes for positrons whose lifetimes are longer in comparison with the positrons annihilating from the free state. Jensen et al. [1] have presented the results of the theoretical calculations of the positron lifetime in a crystalline As2Se3 based on the model of Puska et al. well described in [2], simulating the presence of free volumes by removing from the crystal matrix atoms or the whole groups of atoms. The obtained relationship presents a simple linear relation between the positron lifetime 2 and the simulated free volumes V02:

2 [ns]= 0,240 + 0,0013V023] (1)

where: 0,240 = 1 – the positron lifetime in a crystal without open free volumes – bulk lifetime.

The works that have been published for many years on positron annihilation research in chalcogenides glasses have shown the existence of the two-component positron annihilation lifetime spectra. The first component (1) is associated with the bulk annihilation (bulk lifetime). According to Jensen, the second one (2) is associated with positron annihilation in the volumes exceeding like-point defect values. The observed changes of the lifetime second component value and of the relative intensity (proportional to the number of annihilation) registered gamma quanta for the materials treated with a radiation damages (a creation of additional volumes) shows that the positron annihilation method may become a reliable tool allowing to estimate the number and sizes of open volumes in chalcogenides glasses [3].



Fig. 1. Compositional dependencies of positron annihilation lifetime components in amorphous systems As-Se (^ Z – average coordination number)


The Figure 1 shows dependencies 1, 2 of the positron lifetimes in amorphous systems As-Se as a function of Z – an average coordinational number calculated as the number of covalent chemical bonds – per one glass-formed atom. It may be observed that lifetime component value 1 considering the experimental errors is constant and amounts to 0,22 ns. This component is related to the characteristic free state positron annihilation (bulk lifetime) independent from the glass-formed atom configuration existing in the glass. In the second case (2 component) one can see a considerable growth of 2 value for the numbers Z = 2,4 and Z = 2,1. The anomalies should be associated with the topological phase transition for the given values Z. In the As-Se systems for the number Z = 2,4 (well known topological Thorpe transition) there is a change of the soft 2D matrix into a 3D hard matrix. Saiter [4] has proved that the changes of activation energy for relaxation, viscosity, optical trapping and crystallization ability show anomalies for Z = 2,1. He has connected it with the topological modifications of the glass structures. With the use of (1), the open free volumes have been calculated. The calculation results have been presented in Table 1. On the basis of the obtained results, it is also possible to determine the relative changes of the native open-volume microvoids concentrations:

(2)

n0max denote the maximum NOVmV concentration (in vitreous selenium).

The compositional dependencies of open free volumes and of the relative changes of their concentration for amorphous arsenic selenides are presented in Figure 2.




Fig. 2. Trends of the compositional changes of the native open-volume microvoids V02 and their relative concentration in vitreous As-Se system


According to the conception of the native open free volumes model (NOVmV,
O. Shpotyuk et al., 2001), the second parameter I2 obtained on the basis of the positron life time spectra is informative. The component value depends on the number of positron annihilations from the trapped state and it is proportional to the free open volume concentrations in the examined material. The relationship of the I2 intensity versus coordinational number Z is presented in Figure 3. One may see a very good agreement between the shapes of the curve in Figure 3 and the curve presenting relative concentration changes in Figure 2. It is an evidence for an internal integration of the native free open volume model.



Fig. 3. Compositional changes of the second intensity component of the lifetime positron annihilation spectra in As-Se system glasses

Table 1

Physical-chemical properties, positron lifetime characteristics and calculated parameters of associated native open-volume microvoids in chalcogenide vitreous semiconductors of As-Se system.


Physical-chemical parameters

Positron lifetime characteristics

Parameters of native open volume microvoids


Glass

composition


Z

Va,

cm3/mol

[5]

V0

cm3/mol

1,

ns

[6]

2,

ns

[6]

I2,

a.u.

[6]

V02,

Å3

[1], (1)

n0/n0max,

a.u.

(2)

Se

2,00

18,50

2,07

0,22  0,01

0,31  0,01

0,74  0,05

54

1,00

As3Se97

2,03

18,32

2,00

0,18  0,02

0,33  0,01

0,60  0,04

70

0,80

As5Se95

2,05

18,22

1,96

0,21  0,01

0,32  0,01

0,44  0,04

62

0,85

As7Se93

2,07

18,12

1,93

0,19  0,02

0,36  0,01

0,39  0,04

92

0,60

As15Se85

2,15

17,80

1,88

0,23  0,02

0,36  0,01

0,30  0,02

92

0,55

As20Se80

2,20

17,56

1,80

0,21  0,01

0,36  0,01

0,25  0,02

92

0,55

As25Se75

2,25

17,31

1,72

0,22  0,01

0,38  0,01

0,38  0,02

108

0,45

As28,5Se71,5

2,285

17,18

1,71

0,21  0,01

0,36  0,01

0,37  0,02

92

0,55

As37Se63

2,37

16,82

1,63

0,21  0,01

0,36  0,01

0,35  0,02

92

0,55

As40Se60

2,40

16,70

1,62

0,20  0,01

0,36  0,01

0,38  0,02

92

0,55

As43Se57

2,43

16,80

1,81

0,21  0,01

0,37  0,01

0,29  0,02

100

0,55

As45Se55

2,45

16,85

1,93

0,23  0,01

0,39  0,02

0,30  0,03

115

0,50

As47Se53

2,47

16,92

2,07

0,22  0,01

0,39  0,02

0,31  0,03

112

0,55

As50Se50

2,50

17,00

2,25

0,24  0,01

0,36  0,01

0,48  0,04

92

0,70

As53Se47

2,53

17,05

2,40

0,21  0,01

0,38  0,01

0,45  0,04

108

0,65

As55Se45

2,55

17,12

2,54

0,23  0,01

0,39  0,02

0,22  0,02

115

0,65

As60Se40

2,60

17,29

2,83

0,21  0,01

0,39  0,01

0,46  0,04

115

0,70


Notes:

Va – the average molar volume of a glass (measured experimentally) calculated as a ratio of the average molecular weight M to the measured density ,

Va /NA = Va0 – the average molar volume per one atom of a glass (NA = 6.0221023 mol-1),

– the sum of volumes of all glass components (theoretically calculated),

– the total free volume in glass [7],

where xi, i, i – the atomic fraction, atomic weight and density of i-th element in a glass-forming compound, respectively (Ge = 5,35 g/cm3, As = 5,73 g/cm3, S = 2,01 g/cm3 [5]),

V02 – (the mean open volume of native microvoid) is calculated owing to positron lifetimes dependence on open volumes in crystalline As2Se3, (1)

The concept of the free open volumes model is an interesting supplement to the existing chalcogenides glasses models because of the possibility of more thorough study of the phenomena connected with the microvoids presence in amorphous structure. However, it requires an additional verification for other systems of such glasses.

Acknowledgment. The author gratefully acknowledges helpful discussions with Professor Oleh Shpotyuk from Scientific Research Company “Carat”, Ukraine.


_______________


  1. Jensen K. O., Salmon P. S., Penfold I. T., Coleman P. G. Microvoids in Chalcogenide Glasses Studied by Positron Annihilation. // J. Non-Cryst. Sol. 1994. Vol.170.
    P. 57-61.

  2. Krause-Rehberg R. and Leipner H. S. Positron Annihilation in Semiconductors. Defect Studies, Springer-Verlag. Berlin-Heidelberg-New York. 1999.

  3. Shpotyuk O., Filipecki J. Radiation-Induced Defects in Vitreous Chalcogenide Semiconductors Studied by Positron Annihilation Method. // Mater. Sci. Eng. B. 2002. Vol. 91. P. 537-540.

  4. Saiter J. M. Physical Ageing in Chalcogenide Glasses. // J. Opt. Adv. Mater. Vol. 3. N 3. 2001. P. 685-694.

  5. Feltz A. Amorphous and Vitreous Inorganic Solids. Moscow. 1986.

  6. Alekseeva O. K., Mikhailov V. I., Shantarovich V. P. Positron annihilation in point defects of the glassy As-Se system. // Phys. Stat. Sol. A. 1978. Vol. 48. P. K169-K173.

  7. Saffarini G., Matthiensen J., Blachnik R. The Influence of Mechanical and Chemical Thresholds on the Free Volume Percentage in Ge-Se-(Fe,In) Chalcogenide Glasses. // Physica B. 2001. Vol. 305 P. 293-297.



^ КОМПОЗИЦІЙНІ ТЕНДЕНЦІЇ У КОНЦЕПЦІЇ ПРИРОДНІХ МІКРОПУСТОТ У ВІЛЬНОМУ ОБ’ЄМІ ДЛЯ СКЛОПОДІБНИХ СЕЛЕНІДІВ МИШЯКУ


А. Коздрась


Фізична лабораторія, Опольський технічний університет

вул. Озімська, 75, 45370 Ополє, Республіка Польща


Перевірено модифіковану концепцію природних мікропустот у вільному об’ємі (які є ефективними пастками для захоплення позитронів) на прикладі системи As-Se, що належить до халькогенідних склоподібних напівпровідників (ХСН). Показано, що в рамках цієї концепції достатньо добре інтерпретуються відомі результати експериментальних вимірювання часу життя позитронів.

Ключові слова: халькогенідні скла, позитронна анігіляція, відкритий об’єм, мікропустоти.


Стаття надійшла до редколегії   20.06.2002

Прийнята до друку   17.10.2002

 Kozdras A., 2002



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