The effective thermal conductivity (λe), effective thermal diffusivity (χe) and specific heat per unit volume (ρCp) of twin pellets of Se80In20-xPbx (x = 0, 5, 10 and 15) glasses have been measured at room temperature by  using transient plane source (TPS) technique. All three parameters have  been varied with Pb content and show reversal trend for 5 at. wt.% of  Pb. This reversal trend can be explained on the basis of carrier type  reversal as a consequence of unpinning of the Fermi level due to  incorporation of Pb in Se-In glassy system

RAM, INDRA  SEN

RAM, Jagjiwan  SEN

Singh, Kedar 

English

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Indian Journal of Pure & Applied Physics Vol. 56, June 2018, pp. 440-443       Carrier type reversal in Pb modified Se80In20 semiconducting glassy alloys Indra Sen Rama, Jagjiwan Rama & Kedar Singhb,c* aDepartment of Physics, Dyal Singh College, University of Delhi, New Delhi 110 003, India bDepartment of Physics, Ban aras Hindu University, Varanasi 221 005, India cSchool of Physical Science, Jawaharlal Nehru University, New Delhi 110 067, India Received 10 May 2017; accepted 26 December 2017 The effective thermal conductivity (λe), effective thermal diffusivity (χe) and specific heat per unit volume (ρCp) of twin pellets of Se80In20-xPbx (x = 0, 5, 10 and 15) glasses have been measured at room temperature by using transient plane source (TPS) technique. All three parameters have been varied with Pb content and show reversal trend for 5 at. wt.% of Pb. This reversal trend can be explained on the basis of carrier type reversal as a consequence of unpinning of the Fermi level due to incorporation of Pb in Se-In glassy system.  Keywords: Thermal conductivity, Thermal diffusivity, Fermi level, Carrier type reversal 1 Introduction Chalcogenide glasses are vitreous material consist of one or more chalcogen elements of group VI of the period table. Their physical properties are varying with chemical compositions1. These glasses have many useful properties and have recently drawn a great attention due to their use in solid state and optoelectronics devices2,3. The thermal transport properties of a material in bulk form provide useful consequence from structural point of view4. Thermal transport properties such as thermal conductivity, thermal diffusivity and specific heat of materials depend on the scattering behavior of phonons with lattice defects, impurities and dislocations present in them5. Thermal transport properties of materials in the crystalline and liquid state have been well investigated and known for long time, whereas these investigations in amorphous materials are not so well studied yet. These properties are significant to understand the dissipation of heat in the opto-electronic and solid state devices, as well as they can provide information about the thermal degradation of chalcogenide materials.  In general, chalcogenide glasses are known to be amorphous p-type semiconductors6,7 owing to the number of holes excited below the valence band mobility edge and their lifetime are larger than the number of electrons excited above the conduction band mobility edge and their lifetime. The earlier studies show that the addition of certain heavy elemental metallic impurities8,9 like Bi and Pb in certain chalcogenide glassy systems, at a particular concentration of impurity, the conduction mechanism begins to change from p to n-type. This phenomenon is called carrier type reversal (CTR), this occurs due to unpinning of the Fermi level and consequent enhancement in electron density in the system. This property may be helpful in designing p-n junctions with higher rectification proportion.  Nowadays, our laboratory is involved in study of thermal transport properties along with other physical properties of various chalcogenide glassy systems. Some works on these properties already have reported by our group5,10-14. In our recent studies, we have reported the crystallization kinetics15, glass transition kinetics16 and dielectric relaxation17 in Se80In20-xPbx glassy systems.   In the present work an attempt has been made to study the variation of thermal transport properties like thermal conductivity, thermal diffusivity and specific heat per unit volume of Se80In20-xPbx glassy systems at room temperature. The purpose of present investigation is to determine the effect of Pb content on the thermal transport properties of Se-In glassy system. The thermal conductivity, thermal diffusivity, specific heat per unit volume is measured simultaneously by using transient plane source technique (TPS), as discussed by Gustafsson18.  ————— *Corresponding author (E-mail: kedarbhp@rediffmail.com) RAM et al.: CARRIER TYPE REVERSAL IN Pb MODIFIED Se80In20   4412 Experimental Details The bulk Se80In20-xPbx (x = 0, 5, 10 and 15) chalcogenide glasses were prepared from high purity (99.999%) Se, In and Pb elements by the melt quenched technique as described in our earlier work15. The amorphous nature of prepared samples is confirmed from XRD patterns in our earlier work15. The modification of chemical bonds due to incorporation of Pb in Se80In20 glassy system is confirmed from Raman spectra in our previous work17. Thermal transport properties of as prepared samples were measured with transient plane source (TPS) (Model: TPS 2500 S). For this, samples were pressed into cylindrical pellet forms having diameter 10 mm and thickness around 1.0 mm under uniform load of 5 tons using hydraulic press. The TPS sensor is sandwiched between two pellets of same material in sample holder and surfaces of these pellets were made smooth in order to ensure good thermal contact between sample and sensor. The power output to the sample is adjusted according to the nature of materials and it is kept in range 0.01-0.02 W/cm2 for chalcogenide glasses. The TPS sensor is made up of a 10 μm thick nickel foil (resistance~ 5.286 Ω at room temperature and a temperature coefficient of resistance (TCR) ~ 4.6×10-3 K-1) with an insulating layer of 50 μm thick kapton, on each side of metal pattern.   3 Results and Discussion The effective thermal transport properties (thermal conductivity, thermal diffusivity and specific heat per unit volume) of Se80In20-xPbx (x = 0, 5, 10 and 15) glasses were measured at room temperature by using TPS technique. The variations of thermal conductivity and thermal diffusivity with Pb concentration are shown in Figs 1 and 2, respectively. Both thermal conductivity and thermal diffusivity show a reversal trend at 5 at. wt.% of Pb and each has a minimum value for that composition. This reversal trend can be explained on the basis of carrier type reversal due to incorporation of Pb in Se80In20 glassy system. The carrier type reversal in Se80In20-xPbx glassy systems can be explained as follows; The total density of valence alternation pairs (N) is calculated by the following relation19:  N = [Seଷା] + [Seଵି ] … (1)  Where, Se is the chalcogen atom used in present study. The superscript denotes the formal charge on the Se and subscript represents the number of covalent bonds by which Se is linked to other atoms. Without a metallic additive, the positively and negatively charged native defects are equal in number, i.e., ([Seଷା] = [Seଵି ]) and N = 2N0, where N0 is the concentration of individual native charged defects.  Also, the density of native VAPs equilibrating at the quenching or glass transition temperature (Tg) is estimated as follows:  N02 = [Seଷା] [Seଵି ] = NA2 exp (- EVAP / kTg) … (2)  Where, NA is the density of Se and EVAP is the amount of energy needed to create a valence alternation defect pair from a normally bonded Se. If n and n0, are electron density in the presence and absence of Pb additives, respectively, two are related by following relation20:  Fig. 1 — Effective thermal conductivity versus Pb concentration.  Fig. 2 — Effective thermal diffusivity versus Pb concentration.INDIAN J PURE & APPL PHYS, VOL. 56, JUNE 2018   442n2[Seଷା] / [Seଵି ] = Nc2exp(- 2εn/ kT) = n02  … (3)  Where Nc is the effective density of conduction band states and εn is the electron activation energy. This is related with the mobility gap (Eg) as εn ≈ Eg/2. It is considered that metallic additives like Pb equilibrate at Tg and create positive centers of concentration [Pb+]. In order to estimate the thermal activation energy for various concentrations of [Pb+], the total density of valence alternation centers (N0) that equilibrate with [Pb+] throughout the glass transition, which remains unaltered in the range of interest (T < Tg), should be determined. Further assuming that n0 << N0, the defects density after the addition of Pb atoms into the chalcogenide glass can be estimated as follows20:  [Seଷା]* = -1/2 [Pb+] + (N02 + 1/4 [Pb+]2)1/2 … (4)  [Seଵି ]* = 1/2 [Pb+] + (N02 + 1/4 [Pb+]2)1/2 … (5)  Total density of VAPs after addition of Pb atoms, Nm is given by:  Nm = [Seଷା]* + [Seଵି ]* = 2 (N02 + 1/4 [Pb+]2)1/2 … (6)  It is clear from Eq. (6) that the density of Pb additives has to approach or exceed N0 to facilitate any appreciable effect. The value of n, after incorporation of the additive Pb can be estimated as follows:  n2 (2 N02/ [Pb+] + n) = 2 [Pb+] n02 … (7)  For low Pb concentrations, Eq. (7) can take the form:  n = [Pb+] n0 / N0 … (8)  Whereas, for high Pb concentrations, Eq. (7) can take the form:   n = (2[Pb+]) 1/3 n02/3 … (9)  These expressions clearly, indicate that electron density increases due to incorporation of Pb in present chalcogenide glassy system. In the Se80In20-xPbx glassy systems, the addition of Pb in Se-In glassy system converts some of Seଷା centers into Seଵି  centers. Consequently, the number (concentration) of Seଷା centers that can undergo thermal excitation as per model of Kolobov et al.21 decreases. Thereby a decrease in the number of free holes formed by the conversion of Seଷା centers into Seଵି  centers. Also, the number of traps that can capture electrons excited into the conduction band decreases with the decrease in the number of Seଷା centers. This causes an overall increase in the electron density in the system. Besides, the number of shallow acceptors that capture holes from the valence band enhances due to increase in the number of Seଵି  charged defect states. These two effects collectively shift the Fermi level towards the conduction band resulting in p to n transition at a particular concentration of Pb.  The carrier type reversal (CTR) in the Se80In20-xPbx glassy systems could also be explained on the basis of charged dangling bond model, when Pb is added to the Se-In network, the equilibrium between positively and negatively charged dangling bonds which pin the Fermi level near the middle of the gap gets affected22. As a result, the Fermi level gets unpinned and moves towards the conduction bands which results p to n transition at a particular concentration of Pb.  Further, the carrier type reversal (CTR) could be explained on the basis of formation of ionic Pb-Se bonds instead of covalent bonds at higher concentration23 of Pb. The formation of ionic bonds disturbs the equilibrium between charge defects states, owing to shift in the Fermi level towards the conduction band, consequently p to n-type transition occur at a particular concentration of Pb.   Generally, thermal conductivity and thermal diffusivity show a decrease as system undergoes carrier type reversal19. Effective thermal conductivity and thermal diffusivity in chalcogenide glasses are subjugated by transport of heat by vibrational (phonon) modes of the glassy matrix (network). The mean free paths of the propagating phonon are limited by molecular vibrations and density of localized states in structure. On the other hand, electronic contribution to thermal resistance is rather small due to the low concentration of electrons in medium. Further, addition of Pb (> 5 at. wt. %) in the Se–In system causes increase in density of localized states in system, which is responsible for increase in values of thermal conductivity and thermal diffusivity for higher concentration of Pb. This type of reversal trends in electrical conductivity and dielectric parameters at 5 at. wt.% of Pb is already reported in our previous work17.  Figure 3 shows the variation of specific heat per unit volume with Pb concentration. It is observed that the variation in specific heat per unit volume also RAM et al.: CARRIER TYPE REVERSAL IN Pb MODIFIED Se80In20   443shows a reversal trend at 5 at. wt. % of Pb and has a maximum value for that composition. It suggests that glass containing 5 at. wt. % of Pb has availability of the large number of degrees of freedom. The decrease in value of specific heat per unit volume for higher concentration of Pb may be due to the non availability of the large number of degrees of freedom in these alloys, which could absorb heat energy13.  4 Conclusions Effective thermal conductivity and thermal diffusivity show reversal trend at 5 at. wt. % of Pb, this reversal trend could be explained on the basis of carrier type reversal due incorporation of Pb in Se80In20 glassy system. The occurrence of carrier type reversal in present systems has been explained on basis of electron density and unpinning of Fermi level. This type of reversal trend for electrical conductivity and dielectric parameters is also confirmed by dielectric relaxation study. The variation of specific heat per unit volume can be explained on the basis of availability of the large number of degrees of freedom in the systems.  Acknowledgement We are thankful to DST, New Delhi for providing financial assistance under research Project no. SR/FTP/PS- 200/2012 to carry out research work. We are also thankful to UGC, New Delhi for providing financial assistance for Thermal Constant Analyzer under UGC Networking Scheme and financial assistance under research Project no. 42-112/2013SR.   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Fig. 3 — Specific heat per unit volume versus Pb concentration.  

Carrier type reversal in Pb modified Se80In20 semiconducting glassy alloys

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Carrier type reversal in Pb modified Se80In20 semiconducting glassy alloys

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