Mechanical and Dielectric Properties of InTe Crystals

doi:10.4236/csta.2012.13015

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Crystal Structure Theory and Applications, 2012, 1, 79-83

http://dx.doi.org/10.4236/csta.2012.13015 Published Online December 2012 (http://www.SciRP.org/journal/csta)

Mechanical and Dielectric Properties of InTe Crystals

Teena Mathew1, Ayyappacharuparambil Gopalanachary Kunjomana1*, Keelapattu Munirathnam1,

Kunnath Appukuttan Chandrasekharan1,

Muthukrishnan Meena2, Chelliah Kamalakshiammal Mahadevan2

1Research Centre, Department of Physics, Christ University, Bangalore, India

2Physics Research Centre, S. T. Hindu College, Nagercoil, India

Email: *[email protected]

Received October 15, 2012; revised November 17, 2012; accepted November 26, 2012

ABSTRACT

The mechanical properties of indium telluride (InTe) crystals grown by the Bridgman technique were investigated at

room temperature using a Vickers hardness tester. The microhardness is observed to vary nonlinearly with the applied

load, 10 - 100 g. The cleaved ingots are found to have high value of microhardness (222.44 kg/mm2 at a load of 25 g),

which reflects an appreciable degree of strength due to their covalent bonding and homogeneity. The studies revealed

that the dislocations in the grown crystals offered a resistance to fresh dislocations due to interaction. At higher loads,

plastic deformation induces by slip, exhibiting a decrease in hardness from the peak value. The dielectric constant and

dielectric loss of indium telluride crystals were evaluated in the frequency range, 1 kHz - 1 MHz for different tempera-

tures (35˚C - 140˚C). The frequency dependence of AC conductivity was analyzed as a function of temperature. The

effect of temperature and frequency on the dielectric response of InTe crystals has been assessed on their cleavage faces

and the obtained results are discussed.

Keywords: Indium Telluride; Bridgman Technique; Microhardness; Dielectric Constant; Dielectric Loss; AC

Conductivity

1. Introduction

Indium telluride (InTe), a prominent semiconducting III-

VI compound, finds potential application in the fabrica-

tion of switching devices and has been used in semicon-

ductor hetero-structures [1,2]. Among all the mechanical

properties, hardness is a key factor governing the quality

of such structures. Hence, considerable literature [3-6]

exists on the microhardness studies of compound semi-

conducting crystals. The anisotropy of mechanical pro-

perties is associated with structural defects, chemical

bonding, plastic deformation and their tendency towards

crack formation and cleavage. Kunjomana and Chand-

rasekharan [3] have carried out the microindentation

analysis on the prism faces of GaTe whiskers. The effect

of annealing on the microhardness of zone-melted

InxBi2−xTe3 (x = 0.1 to 0.5 at% In) was studied by Pandya

et al. [4]. The mechanical properties of pure and doped

InP have also been investigated [5]. It is reported that

impurity hardening is much more pronounced at high

temperatures than at room temperature. Arivuoli et al. [6]

have described the growth and microhardness studies of

arsenic, antimony and bismuth chalcogenides. However,

the microindentation analysis of indium monotelluride

crystals has not been reported so far.

Under controlled conditions, InTe crystallizes in a

layer structure with the space group I4/mcm as described

by Chattopadhyay et al. [7]. There exists strong covalent

bonding within the layer planes with weak van der Waals

bonding perpendicular to them, resulting in easy clea-

vage. The studies on the dielectric behaviour of chalco-

genide materials are advantageous for understanding

their conduction mechanism and the origin of dielectric

losses [8]. The AC conductivity and dielectric properties

of Sb2Te3 thin films have been investigated in the fre-

quency range, 0.4 - 100 kHz as a function of temperature

[9]. Hegab et al. [10] have evaluated the dielectric pro-

perties and frequency dependence of AC conductivity of

amorphous Ge15Se60X25 (X = As or Sn) thin films depo-

sited by thermal evaporation. Bose and Purkayastha [11]

have determined the dielectric constants of In2Te3 crys-

tals grown by the Bridgman method. But, InTe, being a

member of III-VI family, is less investigated, as far as its

dielectric properties are concerned. In view of the above

considerations, the present report aims to investigate the

mechanical and dielectric properties of indium monotel-

luride crystals.

*Corresponding author.

T. MATHEW ET AL.

2. Experimental

2.1. Growth and Structural Analysis

Stoichiometric InTe crystals were grown from melt by

the Bridgman method, using a vertical single zone fur-

nace. The high pure (99.999%) indium and tellurium

were filled in a precleaned quartz ampoule of length 80

mm and inner diameter 10 mm, sealed under a vacuum of

~10–6 mbar and synthesized using a muffle furnace. The

temperature profile of the furnace was studied for per-

forming the growth experiments. The compound was

melted by raising the temperature above the melting

point (696˚C) in a tapered ampoule for a period of 48 h

and translated at a rate of 5 mm/h. The X-ray Powder

Diffraction (XRD) data of the grown crystals were re-

corded with a Philips X’pert diffractometer, subjecting

Cukα (λ = 1.5418 Å) radiation. Energy Dispersive

Analysis by X-rays (EDAX) was carried out to assess the

chemical homogeneity of the grown samples.

2.2. Mechanical Measurements

The mechanical strength of a grown crystal plays an im-

portant role in investigating the quality of a crystalline

surface for any desired application. It is essential to know

about the dislocation motion and stress relationships in-

volved in the crystal for studying the mechanical proper-

ties. Therefore, a Vickers projection microscope (MVH-I)

was employed to perform the indentations on the clea-

vage planes of the grown InTe crystals. The diamond in-

denter is in the form of a square pyramid, opposite faces

of which make an angle of 136˚ with one another. Sub-

sequent impressions were made after a time lapse of 30

min to allow for any elastic recovery. In order to avoid

mutual influence of the indentations, the process was

carried out at different sites such that the distance be-

tween consecutive indentation marks is greater than the

diagonal length (d). The microhardness was computed

for various loads using the “Quantimet software” coupled

with the tester.

2.3. Dielectric Measurements

The dielectric characteristics of InTe crystals were inves-

tigated by monitoring the capacitance (Ccrys) and dielec-

tric loss factor (tanδ) using a LCR meter (AGILENT

4284A) for different frequencies, viz. 100 Hz, 1 kHz, 100

kHz and 1 MHz. A good conductive surface layer was

prepared by coating the samples with silver paste. The

temperature was increased up to 140˚C and the electrical

parameters were recorded while cooling. The geometrical

dimensions of the crystals were measured using travel-

ling microscope and screw gauge (least count = 0.01

mm).

3. Results and Discussion

X-ray powder diffraction analysis of the sample con-

firmed the formation of InTe with tetragonal crystal

structure. The estimated cell parameters, a = b = 8.437 Å

and c = 7.139 Å are found to be quite consistent with the

JCPDS card 30 - 0636. The density of the grown crystals

(6.336 g/cm3) calculated from the powder diffraction data

supports the material property reported in the literature

[12,13]. The EDAX profile (Figure 1) revealed the ratio

of atomic percentages of In and Te as 49.98: 50.02 at%,

which shows reasonable agreement with the standard

value.

In order to study the mechanical properties of a mate-

rial, it is desirable to examine optically flat surfaces, free

from any microstructures or irregularities. The crystals

have been carefully cleaved at liquid nitrogen tempera-

ture and the polished slices were subjected to indentation.

The Vickers hardness number is computed using the for-

mula [14],

1.8544 kgmmHvP d2

(1)

where P is the applied load in kilograms and d is the

mean diagonal length in millimeters. Figure 2 represents

the results of microhardness measurements on the (001)

Figure 1. EDAX spectrum of InTe sample.

0.00 0.02 0.04 0.06 0.08 0.10

100

120

140

160

180

200

220

240

Microhardness Hv (kg/mm2)

Load P (kg)

Figure 2. Plot of microhardness with load for InTe crystals.

T. MATHEW ET AL. 81

plane of InTe crystals. The applied load was varied from

10 - 100 g, maintaining the dwell time at 15 s for all the

samples.

The nonlinear behavior of hardness depends on in-

ternal and applied stress, work hardening and intrinsic

plastic resistance of the material. The value of micro-

hardness increases with increase in load and is found to

be maximum (222. 44 kg/mm2) at 25 g. This is attributed

to the fact that one of the indium atoms has tetrahedral

coordination with four tellurium atoms and exhibits sp3

hybridization [13]. Moreover, the presence of covalent

bonding and the interaction between dislocations have a

pronounced effect on the hardening mechanism. It attains

a minimum value equal to 101.6 kg/mm2 at a load of 100

g. The decrease in hardness of InTe crystals is because of

the gliding between the layers on the cleavage plane of

InTe. Beyond 100 g, the hardness remains constant, due

to decrease in the resistance to the movement of disloca-

tions. However, it is found to be greater than that of other

class of semiconducting monotelluride compounds such

as ZnTe (82 kg/mm2), CdTe (56 kg/mm2), CuTe (19.2

kg/mm2) etc. [14]. Thus, a proper control on the growth

conditions ensures quite an appreciable strength and qua-

lity of InTe crystals, which makes them suitable for the

preparation of hetero-structures.

The study of dielectric behavior of chalcogenide

semiconducting crystals reveals structural information,

which helps to understand the conduction mechanism.

Hence in the present work, the dielectric constant of the

crystal was estimated in the frequency range 1 kHz to 1

MHz by applying the relation [15],









aircrys airair

air cryscrys1

rAACCA AC













(2)

where Acrys is the area of the crystal touching the elec-

trode and Aair is the area of the electrode. Since the crys-

tal area was smaller than the plate area of the cell, air

capacitance (Cair) was also measured [15]. Figure 3

shows the frequency dependence of dielectric constant of

indium telluride crystals at different temperatures. The

dielectric constant (εr) decreases with increase in fre-

quency and shows a steeper dependence at high fre-

quency region. Similar results were reported on the di-

electric properties of Sb2Te3 thin films [9]. At low fre-

quencies, εr depends on deformational (electronic and

ionic) and relaxation (orientational and interfacial) po-

larization. When the frequency is increased, the dipoles

will no longer be able to rotate rapidly and the oscilla-

tions begin to lag behind the field. As the frequency is

further increased, the dipoles will be randomly aligned

and the orientation is stopped. Hence, the dielectric con-

stant decreases at higher frequency, approaching a con-

stant value, corresponding only to the interfacial polari-

zation.

The dependence of dielectric constant on temperature

at various frequencies, 1 kHz, 10 kHz, 100 kHz and 1

MHz, is plotted in Figure 4. The dielectric constant in-

creases with increase in temperature and this behavior

becomes predominant at higher temperature and lower

frequency. The increase in dielectric constant with tem-

perature is due to the fact that, the orientational polariza-

tion is governed by the thermal motion of molecules. The

dipoles do not orient at low temperature, but as the tem-

perature increases, the orientation of dipoles is facilitated

and thus increases the value of orientational polarization,

which in turn increases εr [16].

Figures 5 and 6 indicate the variation of dielectric loss

with frequency and temperature respectively. It is found

that, the dielectric loss decreases with frequency and in-

creases with temperature. The origin of the dielectric

losses is associated with the relaxation phenomena,

which is divided into three parts: conduction losses, di-

pole losses and vibrational losses. As the temperature

increases, conductivity as well as electrical conduction

losses increase and hence the value of the dielectric loss

(tanδ) increases [9].

At high frequency, AC conductivity (σac) increases

with frequency, according to the equation [16],

91215

Dielectric constant

Ln()

35oC

50oC

70oC

90oC

100oC

120oC

140oC

Figure 3. Frequency dependence of dielectric constant at

different temperatures.

4080 120

Temperature (oC)

Dielectric constant

1 kHz

10 kHz

100 kHz

1 MHz

Figure 4. Temperature dependence of dielectric constant at

different frequencies.

T. MATHEW ET AL.

91215

0.07

0.14

0.21

300C

400C

500C

700C

1000C

1200C

Dissipation factor

Ln

Figure 5. Frequency dependence of dissipation factor at

different temperatures.

255075100 125

0.0

0.1

0.2

0.3

Dissipation facto

Temperature (oC)

1kHz

10kHz

100kHz

1MHz

Figure 6. Temperature dependence of dissipation factor at

different frequencies.







 (3)

where A is the constant, dependent on temperature and s

is the frequency exponent.

It is clear from Figure 7 that σac increases with fre-

quency, obeying Equation (3). The values of s calculated

from the slopes of the plot are shown in Table 1. The

frequency exponent decreases from 0.8059 to 0.7228

with increase in temperature from 35˚C to 100˚C. It was

found to be less than unity and slightly decreased with

temperature. This result proposes the conduction mecha-

nism of the grown crystals as due to Correlated Barrier

Hopping (CBH). According to this model, the hopping of

carriers between two sites over a barrier separating them

is responsible for the observed conductivity [10].

4. Conclusion

Good quality crystals of indium telluride (InTe) were

grown by the Bridgman technique. The stoichiometry of

the compound was confirmed by X-ray powder diffrac-

tion and chemical analysis. At a load of 25 g, the micro-

hardness is found to be 222.44 kg/mm2, whereas at

91215

-18

-15

-12

-9

Ln ac

Ln()

350C

500C

700C

900C

1000C

1200C

1400C

Figure 7. Logarithmic plots of AC conductivity against fre-

quency at different te mper atures.

Table 1. Values of frequency exponent at different tempe-

ratures.

Temperature (˚C) Frequency exponent (s)

35 0.8059

50 0.7952

70 0.7535

90 0.7304

100 0.7228

higher loads, a decrease in hardness was observed due to

slip mechanism. Further, the hardness remains constant

and exhibits comparatively larger value than that of other

telluride samples. The dielectric properties of the grown

InTe crystals were studied for different frequencies as a

function of temperature. The increase in dielectric con-

stant as well as dielectric loss with temperature is due to

the enhanced polarization of the system. The AC con-

ductivity was observed to vary as ωs in the chosen fre-

quency range. The decrease in the value of s with tem-

perature suggests that, the CBH model is the predomi-

nant mechanism responsible for conduction.

5. Acknowledgements

The authors would like to thank the University Grants

Commission, New Delhi for providing the facilities to

perform microindentation analysis of the samples. Thanks

are due to Prof. Guru Row, Department of Solid State

and Structural Chemistry Unit, IISc, Bangalore for struc-

tural characterization.

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