Correlation of some opto-electrical properties of Se–Te–Sn glassy semiconductors with the average single bond energy and the average electronegativity

Journal of Alloys and Compounds 660 (2016) 503e508

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Correlation of some opto-electrical properties of SeeTeeSn glassy semiconductors with the average single bond energy and the average electronegativity Omar A. Lafi Department of Physics, Faculty of Science, Al-Balqa Applied University, Al-Salt 19117, Jordan

a r t i c l e i n f o

a b s t r a c t

Article history: Received 28 August 2014 Received in revised form 28 October 2015 Accepted 22 November 2015 Available online 2 December 2015

Studies of some electrical and optical properties of Se90Te10
xSnx (x ¼ 2, 4, 6 and 8) glassy alloys, in bulk and thin film forms, were performed at room temperature. Pellets of a diameter ~1.3 cm and different thicknesses ranging from 0.04 cm to 0.06 cm of these compositions were obtained. Measurements of IeV characteristics of these pellets have been carried out at room temperature. Ohmic behavior was observed in low voltage range (0e5 V) while a deviation from ohmic towards non-ohmic behavior was observed at higher voltage range (6e20 V). This deviation can be interpreted in terms of space charge limited conduction (SCLC) mechanism whereas the plots of ln (I/V) vs. V were found to be straight lines. The room temperature DC electrical conductivity sDC has been calculated from the linear region of the IeV characteristic curves. In addition, thin films of a thickness ~1100 Å of the alloys under investigation were prepared and the optical band gap Eopt of these thin films is obtained from Tauc plot (ahn)2 ¼ B (hn
Eopt) after the determination of the absorption coefficient a over a wavelengths range of 440e1100 nm. Analysis of the experimental data shows that sDC increases while Eopt decreases with increasing Sn content. These results are explained in terms of the decrease in the values of average single bond energy HS/ and average electronegativitycm. © 2015 Published by Elsevier B.V.

Keywords: Chalcogenide glasses IeV characteristics Electrical conductivity Optical band gap Single bond energy Electronegativity

1. Introduction Electrical and optical properties of chalcogenide glasses have attracted a great deal of scientific attention since the discovery of the unique phenomenon of reversible switching in certain types of these glasses in 1968 [1]. Several efforts have been made, from that time, to develop suitable chalcogenide materials for electronic and optoelectronic applications. Chalcogenide glasses are generally semiconductors with band gab energies of 1e3 eV, opaque through the visible spectrum and to begin transmission from 2 to 14 mm. These properties make these materials suitable in several applications such as CO2 laser (10.6 mm) radiation delivery for photovoltaics, infrared sensors, optical fiber communication systems over super long distances, industrial cutting/welding applications and also for microsurgery [2e5]. In addition, chalcogenide alloys have several applications in very exciting fields like x-ray imaging, photonics, acousto-optic devices, solid state optical limiters, solid state switching devices, optical memory devices, inorganic photo-

E-mail addresses: olafi[email protected], olafi[email protected]. http://dx.doi.org/10.1016/j.jallcom.2015.11.161 0925-8388/© 2015 Published by Elsevier B.V.

receptors, solar cells, bio-chemical sensing and holographic recording systems [6e9]. Wide window for improvement of chalcogenide glasses is still open and nowadays research focus has been shifted towards nano-sized chalcogenide materials. Undoped a-Se usually suffers from thermal instability, aging effects, and low electrical conductivity, due to phase transformation and structural relaxations even in normal ambient conditions as it has a low glass transition temperature [10]. The addition of Te to Se overcomes some of these disadvantages and causes structural changes in the material which in turn modify band structure and hence electrical properties [11]. Moreover, addition of metallic or semi metallic elements (like Sn in this study) is used to improve the properties of pure chalcogen elements or chalcogenide compounds. The effect of an impurity in an amorphous semiconductor may be widely different, depending upon the conduction mechanism and the structure of the material. In crystalline semiconductors the effect of a suitable impurity is always to provide a new donor or acceptor state and this is not essential in amorphous semiconductors. Instead of providing a localized impurity level in the forbidden gap, an impurity may merely alter the mobility of the charge carriers or may introduce structural changes

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in amorphous material with or without modification of the localized states in the forbidden gap [12]. The currentevoltage characteristic is an important tool for analyzing the different conduction processes operative in chalcogenide glasses. Several models were proposed to explain the nonlinearity in the IeV curves of these materials such as PooleeFrenkel effect [13,14], small polaron conduction [15], tunneling conduction [16], hopping conduction [17] and space charge limited conduction [18,19]. Chalcogenide glasses generally exhibit p-type electrical conduction owing to the fact that the number of electrons excited above the conduction band mobility edge is smaller than the number of holes below the valence band mobility edge [20]. It has been observed [21e26] that when certain heavy metallic additives (like Bi or Pb) are added to SeeGe or SeeIn glasses, the number of holes decreases in these systems, while the number of electrons increases. These two effects together shift the Fermi level towards the conduction band and a remarkable change from p-to n-type conduction results. The study of the optical absorption spectra in chalcogenide glasses provides essential information about band structure and band gap energy [27]. The optical band gap (Eopt) of amorphous semiconductors is usually determined using the Tauc plot ahn ¼ B (hn
Eopt)m [28,29]. In addition, conductivity (DC or AC) measurements have been widely used to investigate the nature of defect centers in disordered systems since it is assumed that they are responsible for the type of conduction. This in turn determines the field in which a particular glass can be used. It is well known [20] that the dependence of band tail on the absorption spectra of amorphous materials is usually related to the distribution of localized states in the valance band tail which is sensitive to the structure and disorder level in the material. In addition, the electrical properties of amorphous materials are sensitive to charge defects with energy levels inside the forbidden gap which can be affected by doping. Therefore, the dependence of the optical band gap and the electrical conductivity on the structure of chalcogenide glasses is very important for better understanding of transport mechanisms. The average coordination number and the heat of atomization have been conducted to characterize the glassy network. There were lots of investigations [30e37] that related the optical band gap and the electrical conductivity with chemical composition using these two parameters. Moreover, the optical band gap and the electrical conductivity are found to be correlated with the average electronegativity of the glassy alloy. In the present work, the DC conductivity at room temperature and the optical band gap (Eopt) of some SeeTeeSn glassy alloys were measured. The composition dependence of these parameters was investigated through the correlation with the average coordination number, the average heat of atomization, and the average electronegativity. 2. Experimental details Glassy alloys of Se90Te10
xSnx (x ¼ 2, 4, 6 and 8) were prepared by the conventional melt quenching technique that described elsewhere [38]. To ensure the glassy nature, X-ray diffraction of the prepared samples were done and given in Fig. 1. Disc shaped samples (pellets) of a diameter ~1.3 cm and different thicknesses ranging from 0.04 to 0.06 cm were obtained by grinding the alloy to a fine powder and then compressing it in a die at a load of about 5 tons. For measurement of the DC electrical conductivity, pellets were mounted in between a specially designed sample holder consists of two copper electrodes of identical size and shape. The pellets were coated with silver paint to ensure good electrical contact between the samples and the electrodes. DC voltage, ranging from 0 to 20 V, was applied across the samples at room temperature and the resultant current was measured by a

Fig. 1. XRD patterns of Se90Te10
xSnx (x ¼ 2, 4, 6 and 8) glasses.

digital electrometer (Keithley 6430) in order to obtain the IeV characteristic curves. For optical measurements, thin films of a thickness ~1100 Å of glassy alloys of bulk Se90Te10
xSnx (x ¼ 2, 4, 6 and 8) were deposited on a well cleaned glass substrate, at a rate of 4 nm/s, by vacuum evaporation technique, keeping the substrate at room temperature and in a vacuum of 2  10
5 Torr using a molybdenum boat. The film thickness was accurately controlled using a single crystal thickness monitor. A double beam (Cintra-10) UVeVIS spectrophotometer, coupled with personal computer, was used to find the variation of the reflectance and transmittance with wavelength, which in turn were used to calculate the absorption coefficient and the optical band gap of the prepared films. The measurements were carried out at room temperature, with wavelength in the range 440e1100 nm.

3. Results and discussion 3.1. Room temperature IeV characteristics and DC electrical conductivity The resultant currentevoltage (IeV) curves for Se90Te10
xSnx (x ¼ 2, 4, 6 and 8) glassy samples in the voltage range (0e20 V), recorded at room temperature (T ¼ 300 K) are shown in Fig. 2 and Fig. 3. It is observed, from these figures, that the studied samples showed linear (ohmic) behavior in low voltage range (0e5 V). However, at higher voltage range (6e20 V), they deviate from linearity (i.e. a non-ohmic behavior is observed). In order to understand qualitatively this trend, the whole mechanism will be divided into two parts. First one is related to the linear region from which the room temperature DC electrical conductivity of the studied samples can be calculated. The second one is related to the non-linear region of the IeV characteristics at higher applied voltages (higher electric field). Here, this non-linearity will be explained according to a suitable model. DC electrical conductivity can be found, at a given temperature, using the following well known relation:

sDC ¼

1 L ¼ rDC RA

(1)

where R is the resistance, L is the thickness, A is the cross-sectional area and rDC is the resistivity of the studied sample. The room temperature (R) for all samples was calculated from the slopes of

O.A. Lafi / Journal of Alloys and Compounds 660 (2016) 503e508

be interpreted in terms of space charge limited conduction (SCLC) model. According to theory of this model, in the case of uniform distribution of localized states, the current (I) is related to voltage (V) by the equation [18,19]:

0.12 0.10

Se90Te4Sn6 Se90Te6Sn4 Se90Te8Sn2

I (A)

0.08 0.06

I¼

0.04

S¼

0.00 -2

0

2

4

6

(2)

8

2εr εo qe NðEF ÞkB Td2

(3)

where εr is the static value of the dielectric constant of the sample, εo is the permittivity of free space, N (EF) is the density of traps (localized state) near the Fermi level and kB is Boltzmann constant (¼ 1.38  10
23 J/K). It is worth to mention that Eq. (2) is not the exact solution of SCLC equation, but it is a very good approximation of the one carrier space charge limited current under the condition of uniform distribution of traps [39]. Also, it is evident from Eq. (2) that the ln (I/V) vs. V curve, at a given temperature, should be a straight line. The slope S of this line should decrease with increasing temperature as clear from Eq. (3). Such a plot is drawn in Fig. 4 at room temperature, for Se90Te10
xSnx (x ¼ 2, 4, 6 and 8) samples, in the high voltage (high electric field) regions. The observed linearity of ln (I/V) vs. V curves in Fig. 3 indicates the presence of SCLC in the samples of the present study. It is mentioned [18,40,41] that, in the SCLC model, the applying of high voltage across the sample causes an electrodes to inject a non-

10 12 14 16 18 20 22

V (V) Fig. 2. IeV characteristics for Se90Te10
xSnx (x ¼ 2, 4, and 6) glasses at room temperature (T ¼ 300 K).

1.2 1.0

Se90Te2Sn8

0.8

qe Amno V SV e d

where qe is the charge of electron, A is the cross sectional area of the sample (thin pellet), m is the mobility, no is the density of the thermally generated charge carriers, d is the electrode spacing which is the sample thickness, and S is given by:

0.02

I (A)

505

0.6 0.4

-3.0 -3.5

0.2

Se90Te2Sn8 Se90Te4Sn6 Se90Te6Sn4 Se90Te8Sn2

-4.0

0.0 0

2

4

6

8

ln (I / V)

-4.5

-2

10 12 14 16 18 20 22

V (V) Fig. 3. IeV characteristics for Se90Te2Sn8 glass at room temperature (T ¼ 300 K).

-5.0 -5.5 -6.0

the linear parts (ohmic regions) of Figs. 2 and 3. To ensure the results, direct method was used to find the conductivity, that is the resistance (R) is measured directly from Keithley electrometer after applying a fixed voltage of 3 V around the samples. The DC conductivity at room temperature is calculated using Eq. (1) and is listed in Table 1. In amorphous materials, the deviation from ohmic towards nonohmic behavior in the IeV characteristics, at high electric field, can

-6.5 -7.0

4

6

8

10

12

14

16

18

20

22

V (V) Fig. 4. Plots of ln I/V vs. V for Se90Te10
xSnx (x ¼ 2, 4, 6 and 8) glasses IeV at room temperature (T ¼ 300 K).

Table 1 The room temperature DC electrical conductivity sDC, the optical band gap Eopt, the average coordination number , the heat of atomization Hs, the average single bond energy Hs/, and the average electronegativity cm for Se90Te10
xSnx (x ¼ 2, 4, 6 and 8) glasses. Composition

sDC (U cm)
1

Eopt (eV)

[38]

Hs kJ/mole

Hs/ kJ/mole

cm

Se90Te8Sn2 Se90Te6Sn4 Se90Te4Sn6 Se90Te2Sn8

0.487  10
4 0.921  10
4 1.62  10
4 10.8  10
4

1.45 1.33 1.13 1.04

2.04 2.08 2.12 2.16

226.1 228.2 230.3 232.4

110.8 109.7 108.5 107.6

2.498 2.494 2.491 2.487

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O.A. Lafi / Journal of Alloys and Compounds 660 (2016) 503e508

equilibrium density of electronic charges. The injected charges are populated in the empty gap above Fermi level and will be larger than the thermally generated free carriers causing the IeV characteristics to deviate from linearity. At low voltage, the injected charge carrier density is lower than the thermally generated charge carrier density leads to ohmic behavior.

3.2. Optical band gap The optical band gap can be estimated from the determination of the absorption coefficient (a) of the films under investigation over a wide range of wavelengths. The absorption coefficient (a) of the films under investigation is calculated using the following relation [42]:

a¼

" # 1 ð1
FÞ2 ln d T

(4)

where d is the thickness of the thin film, F is the reflectance and T is the transmittance. The absorption coefficient a has been calculated from the optical reflection and transmission spectra of thin films of the studied SeeTeeSn glasses. Both of F and T have been determined, over a wide range of wavelength, from spectrophotometer measurements. These measurements were carried out, of the films under investigation, at room temperature. Electronic transitions in many amorphous semiconductors give rise to an optical absorption edge described by an S-shaped curve [43]. The high absorption part of this curve is caused by band-to-band transition and is followed by the exponential Urbach tail [44] and then finally, a weak absorption tail is observed. It is well known that the weak absorption tail originate from defects and impurities and the exponential region is strongly related to the characteristic structural randomness of the amorphous materials, while the high absorption region determine the optical energy gap. In the high absorption region (where a  104 cm
1) which involves optical transition between extended states in both valence and conduction bands, the values of a can be fitted according to the model proposed by Tauc through the relation [28,29]:

 m ahn ¼ B hn
Eopt

(5)

where h is Planck's constant, n is the frequency of the incident photon, B is a parameter depends on the transition probability and Eopt is the optical band gap of the investigated film while m is a parameter depending on both the type of transition (direct or indirect) and the profile of electron density in valence and conduction bands. In the above equation, m ¼ 1/2 for allowed direct transition, m ¼ 2 for allowed indirect transition, m ¼ 3/2 for forbidden direct transition and m ¼ 3 for forbidden indirect transition. In order to determine the nature of the transition occurring, the value of m must be determined by plotting the relations (ahn) 1/2, (ahn)2, (ahn) 2/3 , and (ahn) 1/3 as a function of the incident photon energy hn. The relation (ahn)2 vs. hn for Se90Te10
xSnx (x ¼ 2, 4, 6 and 8) thin films, as shown in Fig. 5, gives a will fit to equation (5) indicating that the absorption mechanism in these glasses occurs via an allowed direct transition. The optical band gap (Eopt) is obtained from the extrapolation of the linear portion to the photon energy hn axis (at a ¼ 0) as shown in the same figure. The resulting values of the optical band gap of the studied films are listed in Table 1. The optical band gap decreased from 1.45 eV to 1.04 eV as Sn content increased from 2 at % to 8 at % and these results agree fairly well with the results reported in other studies of chalcogenide glasses contained small amount of Sn [45e47]. The variation of the optical band gap and the electrical conductivity of the present glassy samples can be

explained according to the dependence of these two parameters on the average coordination number , the average heat of atomization HS and the average electronegativity. 3.3. The average heat of atomization, the average coordination number and the average electronegativity The heat of atomization HS is the quantity of heat required, at standards conditions (at room temperature and atmospheric pressure), for total separation of all atoms in the chemical compound such that the compound bonds are broken and the atoms in the compound reduced to individual atoms [35]. The average heat of atomization provides a direct measure of the cohesive energy and thus the relative bond strengths among isostructural materials [48]. As proposed by Pauling [49,50], the heat of atomization HS(AB), at standard conditions, in the case of binary glassy semiconductor formed from atoms A and B is the sum of the heat of formation, DH, and the average of heat of atomization HSA and HSB that corresponds to the average non - polar bond energy of the two atoms:

HS ðA
BÞ ¼ DH þ

 1 A HS þ HSB 2

(6)

The heat of formation, DH, in this equation is proportional to the square of the difference between the electronegativities cA and cB of the two atoms. In most cases, the heat of formation of chalcogenide glasses is unknown. Even for those few glasses, for which the heat of formation is known, its value does not exceed 10% of the heat of atomization and thus it can be neglected [51]. For a ternary system AaBbCg, the average heat of atomization HS can be calculated as [48]:

HS ¼

aHSA þ bHSB þ gHSC aþbþg

(7)

Using HSSe ¼ 227 kJ/mol, HSTe ¼ 197 kJ/mol and HSSn ¼ 302 kJ/mol [52], the average heat of atomization HS of the SeeTeeSn glassy alloys under investigation has been calculated and listed in Table 1. The coordination number Nc of the covalent atoms in a glass is given by the Nc ¼ 8 e N rule [53], where N is the number of valence electrons in an atom. The average coordination number , which is useful in describing the crosslinking, for binary, ternary and multi-component chalcogenide glassy system is defined simply as the atom-averaged covalent coordination of the constituents. Phillips, Thorpe [54,55] and Tanaka [56] proposed the idea of studying the physical properties of chalcogenide glasses in terms of the average coordination number . According to them, the glass has a critical compositions at ¼ 2.40 where the glassy network changes from an elastically floppy type to a rigid type and is attributed to transition from one to two-dimensional structure, and at ¼ 2.67 where a change from two dimensional layer-like structure to a three dimensional cross linked arrangement is observed. The average coordination number of the present Se90Te10
xSnx (x ¼ 2, 4, 6 and 8) glassy alloys were calculated as suggested in Ref. [38] and listed in Table 1. It is known that [30], the optical band gap (Eopt) is sensitive to the average single bond energy in the alloy which is defined as the ratio of the heat of atomization HS to the average coordination number . The calculated values of HS/ for the studied SeeTeeSn glassy alloys are given in Table 1. From this table it is clear that HS/ values decrease with Sn content which is responsible for valance band broadening. This causes a reduction in the band gap and an increasing in the corresponding electrical conductivity. Moreover, the average electronegativity of the glassy alloy is another factor that affects on the optical band gap and the electrical

O.A. Lafi / Journal of Alloys and Compounds 660 (2016) 503e508

507

Fig. 5. (aed) Plot of (ahn)2 vs. hn for Se90Te10
xSnx (x ¼ 2, 4, 6 and 8) thin films.

conductivity [57e59]. Electronegativity is a measure of the tendency of an atom (or a functional group of atoms) to attract electrons. For single atoms, the Pauling scale [49] is the most commonly used where fluorine (the most electronegative element) is assigned a value of 3.98 while francium has the least value of electronegativity of 0.7. In addition, electronegativity is related to the average energy of the valence electrons in a free atom (ionization energy) [60]. Elements with low ionization energies have low electronegativities because their nuclei do not exert a strong attractive force on electrons and this increases the electrical conductivity and also supports the decrease in the corresponding band gap. According to Sanderson [61], when an element combines with other element, they come together at an intermediate value of electronegativity which is the geometric mean of the electronegativity of combining atoms. For the glassy alloy AaBbCg, the average electronegativity can be determined as [62]:

 1  aþbþg cm ¼ caA cbB cgC

(8)

where cA, cB and cC are the electronegativity values of the elements A, B and C respectively. Using the values of the electronegativity of Se, Te and Sn as cSe ¼ 2.55, cTe ¼ 2.10 and cSn ¼ 1.96, the average electronegativities cm of Se90Te10
xSnx (x ¼ 2, 4, 6 and 8) glasses are calculated and listed in Table 1. It is quite evident from Table 1 that as one increases Sn content in the system, cm decreases. This decrease thought to be related with the increase in the electrical

conductivity and the corresponding decrease in the optical band gap. The decrease in the optical band gap and the corresponding increase in the electrical conductivity with the decrease in the average single bond energy and the average electronegativity is observed and reported in several researches [57e59,62e64]. The results of the present work are in good agreement with these researches.

4. Conclusions The IeV characteristic curves, the electrical conductivity sDC and the optical band gap Eopt of Se90Te10
xSnx (x ¼ 2, 4, 6 and 8) glassy alloys were obtained at room temperature. Compositional trends were investigated through the correlation of these properties with the average electronegativity cm and the average single bond energy HS/. The following conclusions were drawn:  The analysis of the IeV characteristic curves shows the existence of space charge limited conduction (SCLC) mechanism in the studied alloys.  The room temperature DC electrical conductivity sDC, which was calculated from the linear region of the IeV characteristic curves, is found to increase with an increase in Sn content.  The optical band gap Eopt of thin films of the studied alloys, which was obtained from Tauc plot, is found to decrease with an increase in Sn content.

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