C Gorecki, T Gorecki, A Kozdras - Kinetics of phase transitions in vitreous chalcogenide semiconductors studied by differential thermal analysis (dta) and exoelectron emi - страница 1
ВІСНИК ЛЬВІВ. УН-ТУ VISNYK LVIV UNIV.
Серія хім. 2007. Bun. 48. Ч. I. С. 105-110 Ser. Khim. 2007. No 48. Part I. P. 105-110
PACS 74.70.Ad; 79.75.+g; 71.23.Cq
KINETICS OF PHASE TRANSITIONS IN VITREOUS CHALCOGENIDE SEMICONDUCTORS As*Se100-;c AS STUDIED BY DIFFERENTIAL THERMAL ANALYSIS (DTA) AND EXOELECTRON EMISSION (EEE) METHODS
Cz. Gorecki 1, T. Gorecki 2, A. Kozdras 1, R. Golovchak 3
1 Physical Laboratory, Opole University of Technology, ul. Ozimska, 75, 45-370 Opole, Poland e-mail: email@example.com
2 Institute of Physics, Opole University, ul. Oleska, 48, 45-052 Opole, Poland
3 Lviv Scientific Research Institute of Materials of SRC "Carat", Stryjska Str., 202, 79031 Lviv, Ukraine
Temperature dependencies of the DTA signal and intensity of EEE from y-irradiated and non-irradiated chalcogenide glasses AsxSe100-x (x = 0, 10, 20, 30, 40 and 50) have been investigated. Activation energies for the retrification process in investigated materials have been determined by the Ozawa method. It has been found that the activation energy for retrification process in the surface layers is smaller than that for retrification in the volume. Gamma-irradiation improves the thermal stability of investigated materials.
Key words: chalcogenide glasses, DTA, exoelectron emission, activation energy, retrification, y-irradiated.
Chalcogenide glasses, obtained by melting together the elements from the IV and V group of the periodic table of the elements with chalcogens (S, Se, Te) remain to be a subject of intensive investigations because of unusual combination of their physical properties. The most stimulating factor for these studies steams from potential applications of these materials, e.g. in optoelectronics . These promising applications of chalcogenide glasses are, however, limited by their structural instability, resulting from the production process, in which the liquid phase is quenched to the ambient temperature . The process of thermal ageing of the as-quenched material (e.g. long-lasting isothermal ageing at proper temperature) leads to the achievement of the metastable state characteristic of the overcooled liquid .
The ageing process of chalcogenide glasses can be stimulated by y-irradiation [3-5]. The effect of irradiation is an additional factor complicating the control of production of materials with desired (prognozed) properties.
Amorphous materials tend to crystallize (and to loose their unique properties) at a proper combination of temperature and time. The stability of the chalcogenide glasses may be characterized by the activation energies and temperature of the thermally activated retrification and crystallization processes. There is an additional complicating factor resulting from the fact, that the parameters characterizing the thermal stability of the volume
© Gorecki Cz., Gorecki T., Kozdras A. et al., 2007
and of the surface layer of amorphous materials may differ among themselves. About 20 years ago we elaborated a method  for determination of the thermal stability for both the surface and volume of amorphous materials by parallel measurements of the temperature dependencies of the DTA signal and the intensity of photostimulated EEE. This method has been already successfully applied in investigations of the kinetics of phase transformations in some chalcogenide glasses .
The aim of the present communication is to report the results of investigations of the kinetics of phase transformations and of the thermal stability of chalcogenide glasses AsxSe100-x (x = 0, 10, 20, 30, 40 and 50) as well as the effect of y-irradiation on these processes and their parameters.
Measurements of the temperature dependencies of photostimulated exoelectron emission (EEE) intensity were carried out by means of the arrangement described in . An open point counter with saturated ethanol quenching vapor was used for detecting the exoelectrons. The sample temperature, controlled using an Pt-PtRh thermocouple with an accuracy of about 5 K, was changed at four constant heating rates (2, 5, 10 and 20 K/min). The sample surface was irradiated during the measurements by unfiltered radiation from a quartz lamp with a Q-400 burner. All the EEE measurements were performed in an air atmosphere under ambient pressure.
The calorimetric investigations were accomplished using the NETZSCH DSC 404/3/F differential scanning calorimeter with E-type thermocouple using an empty crucible made of high density Al2O3 as reference. All the DTA measurements were performed in air, using five different heating rates (1, 2, 5, 10 and 20 K/min).
The vitreous samples of AsxSe100-x (x = 0, 10, 20, 30, 40 and 50) chalcogenide glasses were prepared by the melt quenching method using a mixture of high purity precursors sealed in evacuated quartz ampoules (~10-3 Pa). The furnace was rocked to obtain the most homogeneous melt. The ingot was quenched at the ambient temperature and then annealed additionally near Tg point to remove the residual mechanical stresses. The amorphous state of chalcogenide glasses was controlled by a visible character of cinchlike fracture, data of x-ray diffraction analysis and transmission IR microscopy. The measurements were performed for two different types of samples: a - for samples not submitted to any excitation (irradiation) prior to measurements, and b - for y-irradiated samples (Co60 source, energy of y quanta 1.25 MeV, dose 2.06 MGy).
Temperature dependencies of the intensity of photostimulated exoelectron emission (EEE) and of the differential thermal analysis signal (DTA), for investigated chalcogenide glasses, all measured at the same heating rate of 2 K/min. are shown in Fig. 1. The parameter of the curves presented in this figure is the arsenic content (x) in the sample. The temperature dependencies of the intensity presented in Fig. 1 display a maximum coinciding with the endothermal effect occurring on the DTA curves. The temperature of these anomalies occurring on both the EEE and DTA curves systematically increase with the increase of the arsenic content in investigated samples. The process responsible for the effects displayed by EEE (surface sensitive method) and DTA (volume sensitive method) is the retrification process occurring in the surface layer and in the volume of investigated sample, correspondingly.
Fig. 2 represents the temperature dependencies of the DTA signal and of the intensity of photostimulated exoelectron emission (EEE) from non-irradiated (a) and y-irradiated (b) chalcogenide glass As20Se80, measured at different heating rates. The parameter of the curves shown in Fig. 2 is the heating rate in K/min. The temperature of the anomalies occurring on the DTA and EEE curves presented in Fig. 2 systematically shift toward higher
EEE - 2K/min
о "З с
Fig. 1. Temperature dependencies of the DTA signal and exoelectron emission intensity for chalcogenide glasses AsxSe100-x determined at a heating rate of 2 K/min. Parameter - As-content (x) in the sample
values with increasing the heating rate. This confirms that the process responsible for the occurrence of these anomalies (retrification) is a thermally activated process.
Fig. 2. Temperature dependencies of the DTA signal and the intensity of EEE from non-irradiated (a) and y-irradiated (b) As20Se80 samples. Parameter - heating rate in K/min
Measurements of the temperature dependencies of EEE intensity at four different heating rates, and the DTA curves at five heating rates, enabled the determination of the activation energy for the retrification process on the surface and in the volume of the samples, responsible for the occurrence of the maxima on the EEE and DTA curves, correspondingly. Activation energy was determined using the well-known Ozawa relation :
where V is the heating rate; A - constant; E - activation energy; k - Boltzmann's constant and T - transformation temperature.
The dependencies of the transformation (retrification) temperatures T, determined from the temperature dependencies of the EEE intensity and DTA signal, on the heating rate V, represented in the Ozawa coordinates lnV = f(1/T) (so called Ozawa plots) for non-irradiated and y-irradiated As20Se80 samples are shown on Fig. 3. The retrification temperature T has been determined from the position of the maximum on the EEE curves , and as the deflection point (from the base line) on the DTA curves .
Fig. 3. Ozawa's plots for the surface (EEE) and volume (DTA) retrification process for non-irradiated and y-irradiated As20Se80 samples
Activation energy of the retrification process is a parameter characterizing well the thermal stability of amorphous materials. The values of the activation energy for the retrification process in the surface layer and in the volume of non-irradiated and y-irradiated chalcogenide glass As20Se80, determined in the present study on the basis of systematic EEE and DTA investigations by the Ozawa method, are given on Fig. 3. For both the non-irradiated and y-irradiated As20Se80 samples the activation energy for the surface retrification is smaller than that for volume retrification. Thermal stability of the y-irradiated samples is greater than that of the non-irradiated material (activation energy for retrification process in irradiated samples is greater than that for non-irradiated material).
lnV = A - E/kT
From the results of the present study it follows out that:
- retrification temperature of the AsxSe100-x (x < 50) glasses increases with increasing arsenic content (x);
- retrification processes in the surface layer occur with a smaller activation energy than in the volume;
- y-irradiation causes an increase in the value of the activation energy for retrification process in both the surface layer and in the volume, thus enhancing the thermal stability of chalcogenide glasses AsxSe100-x (see also );
- the combination of the EEE and DTA methods would be very useful and efficient in investigations of the crystallization kinetics and thermal stability of chalcogenide glasses.
Further studies are in progress.
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ДОСЛІДЖЕННЯ КІНЕТИКИ ФАЗОВОГО ПЕРЕХОДУ У НАПІВПРОВІДНИКОВОМУ ХАЛЬКОГЕНГДІ As*Se100-;c МЕТОДОМ ДИФЕРЕНЦІЙНОГО ТЕРМПНОГО АНАЛІЗУ (ДТА) ТА ЕКЗОЕЛЕКТРОННОГО ЕМІСІЙНОГО (ЕЕЕ) МЕТОДУ
Ч. Гурецький 1, Т. Турецький 2, А. Коздрась 1, Р. Головчак 3
1 Лабораторія фізики, Технологічний університет м. Ополе, вул. Озімська, 75, 45-370 Ополе, Польща e-mail: firstname.lastname@example.org
2 Інститут фізики, Університет м. Ополе, вул. Олеська, 48, 45-052 Ополе, Польща
3 Львівський НДІматеріалів НВО "Карат", вул. Стрийська, 202, 79031 Львів, Україна
Вивчено температурні залежності ДТА сигналу та інтенсивності ЕЕЕ для у-опромі-нених та неопромінених халькогенідних стекол AsxSe100-x, де x = 0, 10, 20, 30, 40 та 50. Для цих процесів визначено енергії активації за допомогою методу Озава.
З'ясовано, що у-опромінення приводить до поліпшення термічної стабільності вивчених матеріалів.
Ключові слова: халькогенідне скло, ДТА, екзоелектронна емігія, енергія активації, у-опромінення.
Стаття надійшла до редколегії 28.05.2006 Прийнята додруку 05.10.2006