Growth, morphology, and microindentation analysis of Bi 2Se 3, Bi 1.8In 0.2Se 3, and Bi 2Se 2.8Te...

Growth, morphology, and microindentation analysis ofBi2Se3, Bi1.8In0.2Se3, and Bi2Se2.8Te0.2 single crystals

Saji Augustine*, Elizabeth Mathai

Crystal Growth Laboratory, Department of Physics, Cochin University of Science and Technology, Cochin,682 022, Kerala, India

(Refereed)Received 20 September 2000; accepted 20 May 2001

Abstract

Bismuth Selenide single crystals have been grown by the vertical normal freezing technique.Confirmation of the compound formation and the lattice parameters are calculated from the X-raydiffractogram. SEM studies show the parallel cleavage lines in Te-doped samples. Triangular etch pitsare obtained on the (111) planes of Bi2Se3. Microindentation studies are carried out on the same planeto understand the mechanical behavior. Annealing and quenching effects of microhardness areevaluated. © 2001 Elsevier Science Ltd. All rights reserved.

Keywords: A. Semiconductor; B. Crystal growth; D. Defects; D. Mechanical properties

1. Introduction

Bismuth Selenide single crystals have received considerable attention in the last decadesbecause of their important applications in the fabrication of ideal Hall effect magnetometers[1], hyperfrequency power sensors, thermopiles, wide-band radiation detectors, etc. [2].Review articles and data sheets are available on the electrical [3], optical [4], and galvano-magnetic [5] properties of Bi2Se3 crystals by various researchers. Horak et al. reportedvapour solid phase equilibrium of Bi2Se3 and its mixed crystals [6]. Thermoelectric powerand scattering of carriers [7] as well as pulsed laser induced a “transient thermoelectriceffect” [8] of V2VI3 compounds were reported by Kulbachinski et al. Bi2Se3 crystallizes into

*Corresponding author.E-mail address: [email protected] (S. Augustine).

Pergamon Materials Research Bulletin 36 (2001) 2251–2261

0025-5408/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved.PII: S0025-5408(01)00717-6

hexagonal structure with a direct band gap of 0.35 eV and its melting point is about 710°C.Bi2Se3 single crystals belong to the family of hexagonal system with the space group D5

3d

(R3̂m) and lattice parameters a � 4.1385 Å, c � 28.623 Å, and V � 424.6 (Å)3. Because ofits layered structure, transport phenomena such as the resistivity, Hall coefficient, magnetoresistivity, and thermal conductivity are strongly anisotropic.

Hardness measurements are commonly carried out to estimate the mechanical strength ofthe material, as it is a measure of resistance against plastic deformation. However, hardnessis not a single property but rather a whole complex of mechanical properties and at the sametime a measure of the intrinsic bonding of the material. The significance of the present work

Fig. 1. X-ray powder diffraction pattern of (a) Bi2Se3, (b) Bi1.8In0.2Se3, (c) Bi2Se2.8Te0.2.

Table 1Lattice parameters, work-hardening index, Quench-hardening index, and hardness of Bi2Se3, Bi1.8 In0.2Se3,Bi2 Se2.8 Te0.2

Samples Calculated latticeparameters

Work-hardeningindex (n)

Quench-hardeningindex (k)

Average hardness (kg/mm2)

Quenching Annealing

a (A°) c (A°) V (A°)3 27°C 350°C 400°C 450°C 350°C 400°C 450°C

Bi2Se3 4.1572 28.7712 430.60 2.439 2.684 108.8 165.8 70.8 86.1 91.4 45.1 76.6Bi1.8In2Se3 4.1213 28.7998 423.62 2.542 .62 80.6 82.6 69.5 71.08 125.8 72.7 73.1Bi2Se2.8Te0.2 4.1572 28.7712 430.60 1.824 �2.798 88.2 57.08 68.56 116.3 77.7 57.2 82.2

2252 S. Augustine, E. Mathai / Materials Research Bulletin 36 (2001) 2251–2261

is to evaluate the effects of doping as well as the annealing and quenching effects of hardnesson Bi2Se3 single crystals because no data is available in this regard.

In this letter an account is presented for the growth, morphology, and microhardness ofBi2Se3, Bi1.8In0.2Se3, and Bi2Se2.8Te0.2 single crystals by vertical normal freezing (VNF).X-ray analysis was carried out to determine the structure, lattice constants, and the stackingfaults probabilities. Hardness measurements were made on the (111) planes of these crystals,wherein the hardness was found to be a function of indentor load.

2. Experimental

Single crystals of Bi2Se3 and its In, Te-doped samples were prepared from 5N purity Bi,Se, Te, and In. The synthesis of the compounds were carried out in tapered quartz ampoulesevacuated to a pressure of 10�5 Torr. The charge in the sealed ampoules were kept in amuffle furnace having flat temperature profile at 600°C for 24 h. The ampoule was rotated

Fig. 2. EDAX profile of Bi1.8In0.2Se3.

2253S. Augustine, E. Mathai / Materials Research Bulletin 36 (2001) 2251–2261

and rocked periodically to ensure complete mixing and reaction. The identification of thecompound was made using a Rigaku X-ray diffractometer with monochromatic nickelfiltered CuK� radiation as the X-ray source. The powder diffractogram revealed that Bi2Se3,Bi1.8In0.2Se3, and Bi2Se2.8Te0.2 had been formed. The crystals were grown using the VNFmethod. Single crystals of good quality can be obtained by a similar method [9]. Beforelowering, the ampoule containing the melt was heat treated at 720°C for 24 h and thenlowered at the rate of 4 mm/h through a temperature gradient of about 10°C/cm. Singlecrystals of 50 mm long and 8 mm in diameter were obtained in this method. EDAX weretaken to find out the doping and compositional analyses, and found that the elements areproperly doped.

In the present study, microhardness measurements were carried out with Vicker’s dia-mond pyramidal indentor. All the measurements were made on the (111) plane at roomtemperature, after quenching and annealing at three different temperatures for comparativestudies, and the indentation time was kept at 15 s. The applied indentor load was varied from5 � 10�3 to 100 � 10�3 kg, and the diagonal of the impression was measured using a

Fig. 3. EDAX profile of Bi2Se2.8Te0.2.


microscope calibrated ocular at magnification �500. Hardness of the crystals were calcu-lated using the relation,

Hv � 1.8544 P/d2

where Hv is the hardness value in kg/mm2; P is the indentor load in kg; and d is the diagonallength of the impression in mm. At each surface, several trials of indentation are taken andthe average value of hardness is found.

3. Results and discussion

Bi2 Se3 crystallizes in the hexagonal structure and consists of layers of atoms stacked inthe order of [Se–Bi–Se–Bi–Se]–[Se–Bi–Se–Bi–Se]–[Se….] parallel to the (0001) direction.The bonding between Bi–Se is predominantly covalent in character, while between Se–Se,it is of weak Van Der Walls in nature [10]. Fig. 1 shows the X-ray diffraction pattern forBi2Se3, Bi1.8In0.2Se3 and Bi2Se2.8Te0.2 crystals. The X-ray diffractogram with peak charac-teristics indicates the crystalinity of these materials. The calculated values of the latticeparameters are given in Table 1. Fig. 2 and 3 (EDAX) verifies the doping and compositionalanalysis of In and Te in Bi2Se3 crystals.

Fig. 4 gives the SEM images of the river patterns obtained on the as grown crystalsBi2Se2.8Te0.2, respectively. Parallel cleavage lines are always found only in the Te-dopedsamples of the (111) plane. Fig. 5 shows the triangular etch pits obtained on the (111) plane

Fig. 4. SEM photograph of river patterns on (111) planes of Bi2Se2.8Te0.2.


of Bi2Se3 by using methanol, bromine, and iodine as the etchant. Fig. 6 depicts the gratingstructured pits obtained on a highly strained region of Bi2Se3.

Structural defects in the material under test must affect the results of microhardnessmeasurements because they resist the deformation of the material. So microhardness anddislocation density in the case of semiconducting crystals are interrelated. The results ofmicrohardness measurements on the (111) faces of Ge, Si, and In2Sb3 [11–13] by variousgroups shows that as the dislocation density both in the case of shallow (fine) and deep (edgedislocation) etch pits increases, microhardness increases to a greater extent. The same resultsare obtained in these crystals also.

Figures 7, 8, and 9 show the variation of hardness with load for bismuth selenide, its Inand Te-doped samples. Hardness increases to a maximum value, then decreases and saturatesfor Bi2Se3 and Bi2Se2.8Te0.2 samples, while in the case of In-doped samples, hardnessdecreases to a minimum and thereafter increases.

This nonlinear behavior of hardness depends on the cohesive shear strength, strainhardening, and intrinsic plastic resistance of the material. For covalent compound, theintrinsic strength of the chemical bonds determine its hardness. Because the bonding in

Fig. 5. Perfectly triangular pits on (111) planes of Bi2Se3.


covalent crystals are highly stereospecific, dislocation energy strongly depends on its posi-tion. So the plastic characteristics will be closely connected with the electronic characteris-tics of the covalent crystals. Moreover, in covalent crystals, the flow rate is limited by theinteractions between dislocations and chemical bonds of the compound [14,15]. Thus, theincrease of hardness is due to the more intensive saturation of the covalent bonds betweenatoms in the chain with the presence of tellurium. The Bi–Te and Bi–Se bonds may bestronger than the Bi–In and In–Se bonds. In addition, selenium is replaced by tellurium withlarger ionic radius while indium, with a smaller ionic radius, replaces the bismuth atom.Consequently, generation of unfilled spaces is comparatively higher in Bi1.8In0.2Se3 than inBi2 Se2.8Te0.2. Hence, doping of indium causes slip to occur at the time of applying lowerloads and hardness decreases to a minimum value. Thereafter, due to pinning of dislocations,hardness increases and saturates at a particular value.

While in the case of Bi2Se3 and Bi2Se2.8Te0.2, deformation is accompanied by rosette ofdislocations creating interacting jogs and dislocation dipoles with the progress of penetrationas shown in Fig. 10. Similar results are also obtained in the case of Bi2Te3 [16]. This complexnetwork of immobile dislocations serves as the effective barrier to the motion of newdislocations and the plastic flow gets limited. Hence, hardness exhibits a high value at the

Fig. 6. Grating structured pits obtained on the cleavage planes of Bi2Se3.


Fig. 7. Variation of microhardness with load for Bi2Se3 ■ at room temperature, F Q—350°C, Œ Q—400°C, �

Q—450°C, � A—350°C, � A—400°C, � A—450°C (Q—quenched; A—annealed).

Fig. 8. Variation of microhardness with load for Bi1.8In2Se3 ■ at room temperature, Q—350°C, Œ Q—400°C,� Q—450°C, � A—350°C, � A—400°C, � A—450°C (Q—quenched; A—annealed).


Fig. 9. Variation of microhardness with load for Bi2Se2.8Te0.2 ■ at room temperature, Q—350°C, Œ Q—400°C,� Q—450°C, � A—350°C, � A—400°C, � A—450°C (Q—quenched; A—annealed).

Fig. 10. “Rosette” of dislocations formed around an indentation.


intermediate load range. At higher loads, pinning of dislocation breaks and slip occurs. Sothe hardness decreases and saturates at higher loads.

Crystal surfaces of these samples are quenched and annealed at three different tempera-tures. Table 1 gives the comparative values of average hardness, which shows that mechan-ical properties of materials can be changed either by quenching or annealing. Materialhardening occurs because of the retention of a high concentration of vacancies in thesolution.

Preserved or precipitated vacancies will then act as barriers to dislocation movement, andwill increase the general hardness of the material during rapid or slow cooling. However, inthe case of Bi2Se3 and Bi1.8In0.2Se3 quenching and annealing gives maximum hardnessvalues at 350°C and Bi2Se2.8Te0.2 at 450°C.Quenched-in as well as annealed defects are thereasons for the increase of hardness. The quench hardening indices of these crystal samplesare calculated from the power law [17]

H Tqk � A

and are given in Table 1.The plot of log P against log d yields a straight line for all the samples with a slope greater

than two as well as in some cases less than two (Fig. 11). This supports the concept ofOnitsch [18] that if the slope is less than 2, the hardness number increases with decreasingload and vise versa.

Fig. 11. Mayer plot for �—Bi2Se3, E—Bi1.8In0.2Se3, ‚—Bi2Se2.8Te0.2.


4. Conclusion

X-ray topographic investigations established that the crystals of Bi2Se3 grown are hex-agonal with lattice constants a � 4.1572 and c � 28.7712 Å. Doping of Te does not haveany impact on the unit cell edges of Bi2Se3, while that of In causes slight changes in thesevalues. Anisotropic variations of microhardness are observed.

Acknowledgments

We thank Prof. and Head M. A. Ittyachan and Dr. Cyriac, Mahatma Gandhi University,for their help in microhardness studies. Thanks are also due to Prof. Vikram Jayaram, IISc,Bangalore, for helping with SEM and EDAX studies. Thanks to UGC for funding to ourdepartment through DSA/COSIST scheme.

References

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Growth, morphology, and microindentation analysis of Bi 2Se 3, Bi 1.8In 0.2Se 3, and Bi 2Se 2.8Te...

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