A new heterostructures fabrication technique and propertiesof produced SbSI/Sb2S3 heterostructures

Bartłomiej Toroń a,n, Marian Nowak a, Mirosława Kępińska a, Andrzej Grabowski a,Janusz Szala b, Piotr Szperlich a, Iwona Malka c, Tomasz Rzychoń b

a Institute of Physics – Center of Science and Education, Silesian University of Technology, Krzywoustego 2, PL-44100 Gliwice, Polandb Department of Materials Science, Silesian University of Technology, Krasińskiego 8, 40-019 Katowice, Polandc Institute of High Pressure Physics, Polish Academy of Sciences, Sokołowska 29/37, 01-142 Warsaw, Poland

a r t i c l e i n f o

Article history:Received 19 August 2013Received in revised form12 November 2013Accepted 22 November 2013Available online 14 December 2013

Keywords:HeterostructuresHeterojunctionsAntimony sulfoiodideLaser processing

a b s t r a c t

A new technique for heterostructure fabrication in semiconducting compounds has been proposed.Heterojunctions have been made by applying CO2 laser beam. In contradistinction to currently appliedmethods (MBE – Molecular Beam Epitaxy and CVD – Chemical Vapor Deposition) the presentedtechnique is simple, of low-cost and very efficient. The new technique has been tested on antimonysulfoiodide (SbSI) single crystals, i.e., a semiconducting, ferroelectric material characterized by manyvaluable properties. The most important structural, optical and electrical properties of the obtainedheterostructures have been presented. Studies have shown that in the CO2 laser treated samples thejunction exists between SbSI single crystal and amorphous phase of antimony sulfide (Sb2S3). At roomtemperature difference between energy gaps of both parts of SbSI/Sb2S3 heterostructure is equal to0.3 eV. This leads to many interesting phenomena and potential applications.

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1. Introduction

There is a great demand for semiconductor heterostructureswhich are applied in electronic and optoelectronic devices. Insidethese structures at least one junction is created between materialshaving different values of energy gap. The first theoretical articleon physical phenomena in heterojunction was published in theearly 1950s by A.I. Gubanov [1]. Practical application of this kind ofjunction in transistor was proposed by W. Shokley [2] and later byH. Kroemer [3]. The first heterojunctions with quality appropriatefor scientific research were produced by Anderson in 1960 [4,5].Nowadays heterojunctions are commonly used in diodes, transis-tors, semiconductor lasers, and many other devices [6–9]. How-ever, their properties are still investigated.

There are many ways to produce heterojunctions [10], e.g., MBE(Molecular Beam Epitaxy) and CVD (Chemical Vapor Deposition).Unfortunately, they need relatively sophisticated and expensive equip-ment. In this article we present a new technique to manufactureheterostructures. It is based on laser treatment of a semiconductingcompound, e.g., antimony sulfoiodide (SbSI). Absorbed laser radiationevokes melting and thermal decomposition of this material. Thesolidified substance, after laser treatment, has chemical composition

and energy gap different from those of the primary semiconductordue to vaporization of more volatile component. This approach isconsistent with the results of thermogravimetric (TG) analysis andX-ray diffraction (XRD) crystallography reported by Cho et al. [11]: SbSIdecomposes to Sb2S3 residue at around 597 K:

3SbSI-Sb2S3ðresidueÞþSbI3 ðvaporÞ ð1ÞPresented experiments have been performed on single crystals

of SbSI, the most outstanding representative of the ternaryand quaternary compounds formed from the group 15–16–17elements [12]. This material is a ferroelectric semiconductor (withthe Curie temperature TCE292 K), which shows plenty of inter-esting properties, e.g., pyroelectricity, piezoelectricity, electro-optic and pyro-optic effects. It is influenced by light leading tophotoferroelectricity, photorefractive effect, and photodesorptionof adsorbates. The needle morphology is found to be predominantin SbSI crystals due to the growth rate anisotropy. The direction ofspontaneous polarization is along their longitudinal c-axes. Themain features of SbSI have been reviewed in a few monographs[13–16]. However, to the best of our knowledge, despite the factthat many reports on SbSI have been published, no informationabout heterojunction devices made from this material can befound. The published data on laser treatment of SbSI are alsorather rare. Recently, articles about a laser-write method to formSbSI single crystals in chalcogenide glasses (used in infrareddevices) have been published [17–20] but no information aboutlaser irradiation decomposition of SbSI has been presented.

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n Correspondence to: Solid State Physics Section, Institute of Physics, SilesianUniversity of Technology, Krasińskiego 8, PL-40-019 Katowice, Poland.

E-mail address: bartlomiej.toron@polsl.pl (B. Toroń).

Optics and Lasers in Engineering 55 (2014) 232–236

The main aim of this work is to develop a new heterojunctionproduction technique and to investigate properties of the newtype of heterostructures based on single crystals of ferroelectricand semiconducting SbSI.

2. Experiment

Single crystals of SbSI used in our experiment have been grownfrom vapor phase of SbSI xerogel. This xerogel consists of crystal-line nanowires with lateral dimensions of about 10–50 nm andlengths reaching up to several micrometers. At sufficiently smalldimensions, the sublimation rate increases as predicted fromclassical sublimation theory. One should notice that the sublima-tion temperature of small-molecule semiconductors can bereduced by sonocrystallization [21]. Different from the other,commonly used procedures [22], usage of SbSI nanocrystals as asource product for vapor growth makes this process free of anyexplosion hazard. These nanocrystals are prepared sonochemicallyfrom the constituents (the elements: antimony, sulfur and iodine)in ethanol at 323 K. We used the same experimental setup and thesame procedure as those for sonochemical preparation of SbSIethanogel [23].

To grow SbSI single crystals from the vapor phase, theprocessing has been performed in closed ampoules with con-stant temperature gradient. It has been done in a two-zonevertical furnace with zone temperatures controlled indepen-dently. Sonochemically prepared material has been put into athermisil ampoule of length 13.5 cm and diameter 2.8 cm. Noseed crystal has been used. The ampoule has been sealed afterpumping out (p¼100 Pa). Lower part of the ampoule has beencoiled with a sheet of aluminum foil to obtain a homogeneoustemperature distribution in the source zone. This restrictsnucleation position and allows the growth of large crystals atthe top of ampoule.

The source temperature has been established T1¼633(1) K andthe seed temperature has been T2¼593(1) K. Time of SbSI singlecrystal growth has been 48 h. Typical size of the grown crystalshas been about 0.3�0.3�10 mm3. Their surfaces have had goodquality. No hollow crystals have been observed.

Thermogravimetric (TG) analysis of SbSI crystals has beencarried out using a STA 449 F1 Jupiter analyser (Netzsch) at 5 K/min temperature ramp rate under He atmosphere. SbSI begins todrastically loss weight at around 545 K and stops around 609 K(Fig. A1 in the Appendix). About 40% of the initial mass survivesthe experiment. Gradual weight loss occurs above 609 K.

To perform spatially selective thermal decomposition of theSbSI crystals we have treated them with laser in air. Continuouswave CO2 laser emitting λ¼10.6 μm radiation of power P¼30 Whas been used. The laser has been equipped with a ZnSe lens tofocus the beam on a sample mounted in a vertical position in 3Dstage. Applied radiation has been absorbed by SbSI. The irradiatedsection of the crystal has been melted in a short time (tE3 s).The sample obtained by laser treatment of SbSI single crystal ispresented in Fig. 1. Characteristics of the produced structureshave been obtained using different techniques, such as powderX-ray diffraction (XRD), scanning electron microscopy (SEM),energy dispersive X-ray analysis (EDAX), or optical transmittancespectroscopy.

The powder XRD measurements have been performed on aJEOL JDX-7S X-ray diffractometer with graphite-monochromatizedCuKα radiation (λ¼0.154056 nm). 40 kV acceleration voltage witha 20 mA current flux has been used. A scan rate of 0.05 s�1 hasbeen used to record the patterns in the 2θ range of 10–801. Thephase identification has been done using the PCSIWIN computerprogram and the data from JCPDS-International Centre for

Diffraction Data 2000. Orthorombic phase (with the cell constantsa¼0.858 nm, b¼1.017 nm, and c¼0.414 nm) and glass structurehave been recognized for untreated and laser treated sections ofthe SbSI sample, respectively (Fig. A2 in the Appendix). Peaksintensities and positions in diffraction patterns of not irradiatedsections are in good agreement with literature values for crystal-line SbSI [24].

Scanning electron micrograph and EDAX patterns have beentaken on a Hitachi S-4200 scanning electron microscope with aNoran Instruments EDS Voyager 3500 spectrometer. SEM image ofthe CO2 laser treated SbSI single crystal with curves representingthe elements distribution along the heterostructure is presented inFig. 2. Relative composition in the not irradiated section of SbSIsingle crystal is equal to about 33% for sulfur, iodine, and antimony.In the case of laser melted material a significant decrease of iodinecontent and the small relative increase of antimony content areobserved (Fig. 2). An atomic ratio (0.37:0.57:0.06 for Sb, S and I) hasbeen established by EDAX. So determined chemical composition oflaser irradiated sections of the samples is close to the stoichiometryantimony(III) sulfide (Sb2S3).

During optical transmittance measurements the sample hasbeen mounted on an optical D2209 chamber (MMR Technolo-gies, Inc.) connected with a TSH 071E turbomolecular drag

Fig. 1. Photo image and energy scheme of a double heterostructure obtained bylaser treatment of SbSI single crystal (EV and EC – top of valence band and bottom ofconduction band, respectively; Eg1¼1.62 eV and Eg2¼1.85 eV – optical energy gaps;EF1¼0.66 eV and EF2¼0.62 eV – Fermi level positions; indices 1 and 2 correspondto laser treated and not treated sections of the sample, i.e., amorphous Sb2S3 andcrystalline SbSI, respectively; T¼288 K).

Fig. 2. SEM image of the CO2 laser treated SbSI single crystal with curvesrepresenting the elements distribution along the heterostructure.

B. Toroń et al. / Optics and Lasers in Engineering 55 (2014) 232–236 233

pumping station (Pfeiffer). The chamber has been equipped witha R2205 Cryogenic Microminiature Refrigeration II-B System andK20 temperature controller (MMR Technologies, Inc.). Investiga-tions have been performed in vacuum (p¼10�2 Pa) in tempera-ture range from 80 K to 333 K. Optical transmittance has beenmeasured using a PC2000 (Ocean Optics Inc.) spectrophot-ometer with master card of 600 lines grating (blazed at500 nm). The spectrophotometer has been equipped withappropriate optical fibers and a deuterium–halogen light source

DH2000-FHS from Ocean Optics Inc. Incoming radiation hasbeen linearly polarized using a Glan–Thompson polarizer (LOT-Oriel) with the electric vector parallel or perpendicular to the c-axis of SbSI single crystal. The multiple averaged spectralcharacteristics containing 2048 data points for various wave-lengths have been registered using a PC computer and the OOI-Base program from Ocean Optics Inc. The presented measure-ments have been carried out in spectral range from 380 nm to1050 nm for normal illumination of the (010) plane of SbSIsingle crystals. Optical transmittance data have been trans-formed [25] into absorbance αw (where α is the absorptioncoefficient and w is the absorption path length). Spectraldependences of absorbance determined at different tempera-tures (Fig. A3 in the Appendix) have been least-square fittedusing the method presented in [25]. In both cases of lasertreated and not treated sections of SbSI single crystal, the bestfitting has been obtained for the sum of indirect forbiddenabsorption (without excitons and phonon statistics), Urbachruled absorption and constant absorption term (see the appro-priate formula in [25]). Temperature dependences of indirectforbidden energy gap determined for CO2 laser treated and nottreated sections of SbSI sample are presented in Fig. 3. In thefirst case the energy gap is typical for amorphous antimony(III)sulfide (Sb2S3) [26] and is 0.3 eV smaller (in room temperature)than that of SbSI single crystal.

The most important electrical feature of produced heterostruc-tures, their current–voltage (I–U) characteristics (Fig. 4) has beendetermined using a Keithley 6517A electrometer. The sample hasbeen mounted in the same chamber as for the optical transmit-tance measurements. Investigations reported in this paper havebeen performed near the Curie temperature of SbSI (in tempera-ture range from 253 K to 333 K). Data have been recorded using aPC computer with IEEE 488 bus and appropriate programs inLabView (National Instruments). One can recognize the forwardand reverse directions of junction polarization in Figs. 4 and 5.Forward current flows through the sample with higher electricpotential applied to the Sb2S3 side of the junction. The influence ofunpolarized light on the I–U characteristics (Fig. 5b), leading tophotoconductivity, has been investigated using a laser diode withλ¼652 nm wavelength. The influence of sample repolarization ontypical DC current–voltage characteristics of SbSI/Sb2S3 singleheterojunction at temperature lower than TC of SbSI crystal ispresented in Fig. 4.

3. Discussion of the results

Thermogravimetric analysis (Fig. A1 in the Appendix) con-firmed the results reported by Cho et al. [11] regarding

Fig. 3. Temperature dependences of indirect forbidden energy gap determined forCO2 laser treated (▄) and not treated ( ) sections of SbSI sample ( –

plane polarized radiation with electric vector parallel and perpendicular to thec-axis, respectively, of the single crystal; ▄ – unpolarized radiation).

Fig. 4. Typical DC current–voltage characteristics of SbSI/Sb2S3 single heterojunc-tion recorded at temperature T¼253 K with visible ferroelectric effects. Arrowsshow directions of bias changes.

Fig. 5. Influence of temperature (a) and illumination (b) on DC current–voltage characteristics of single heterostructures (T1¼253 K; T2¼293 K; T3¼333 K; λ¼652 nm;Ii1¼0; and Ii2¼1022 photon/(m2 s); the bias voltage changes from þ50 V to �50 V).

B. Toroń et al. / Optics and Lasers in Engineering 55 (2014) 232–236234

decomposition of SbSI around 597 K. This decomposition startsaround 545 K (Fig. A1 in the Appendix). Above 609 K the residuematerial is Sb2S3.

Absorption of radiation emitted by CO2 laser evokes heating ofSbSI single crystals and causes melting of the sample. Tempera-ture of our sample melted in air rose above 609 K but did notexceed 673 K. When laser beam stopped illuminating the SbSIcrystal, the melted part of the sample solidified. The performedEDAX (Fig. 2) and optical transmittance measurements (Fig. A3 inthe Appendix) prove that this solid material is Sb2S3. The XRDmeasurements(Fig. A2 in the Appendix) show that it is of amorphous character.According to [11] SbSI may react with oxygen in air at 673 K toform yellow-colored Sb2O4 and Sb2O3. We did not observe anytraces of Sb2O4 oxides in our samples.

It should be underlined that heterostructures mainly consist oftwo crystalline materials. Both materials must have similar crystallattices constant at every temperature. There is no such need inthe case of amorphous–crystalline junction. Laser fabricated SbSI–Sb2S3 heterostructures belong to a small group of amorphous–crystalline junctions [27].

Due to crystal anisotropy the energy gaps of SbSI single crystalsare slightly different for plane polarized radiation with electricvector parallel and normal to the c-axis (Fig. 3). Similar opticalanisotropy could not be observed in amorphous Sb2S3. The 0.3 eV(at room temperature) difference between energy gaps of crystal-line SbSI and its laser treated amorphous section (Sb2S3) causesthe heterojunction.

The value of the Curie temperature (TC¼292 K) can bedetermined from temperature dependences of SbSI opticalenergy gap (Fig. 3). Below TC the energy gap is affected bythe temperature dependence of spontaneous electric polariz-ability of ferroelectric material. The energy gap of SbSI inparaelectric phase decreases linearly with temperatureincrease. No phase transition can be deduced from opticalinvestigations of the laser treated section of SbSI sample, i.e.,amorphous Sb2S3.

The increase of electric conductivity with temperature increase(Fig. 5a) as well as with illumination (Fig. 5b) is caused by theincrease of carrier concentration in the semiconducting material.

Different slopes of the current–voltage characteristics foropposite polarization of bias voltage (Figs. 4 and 5) are typicalfor heterojunctions. Applying more sophisticated analysis [9,27] ofthese characteristics one can recognize that the laser fabricatedSbSI–Sb2S3 heterojunction is an isotype (materials with the sametype of conductivity are on both sides of the junction). Therefore,when using p-type SbSI crystals, the produced heterostructuresare of p–p type.

Below TC displacements of I–U characteristics are observed(Fig. 4) due to opposite initial polarization of SbSI by a DC voltage.Namely, current at a given U is larger for decreasing than forincreasing voltage. A similar effect has been observed in ferro-electric (Pb,La)(Zr,Ti)O3/SrTiO3:Nb diodes [28]. The influence ofSbSI ferroelectric phase on I–U characteristics of SbSI/Sb2S3 het-erostructures can be recognized also in noticeable discontinuitiesappearing at U¼725 V after repolarization (Fig. 4). These dis-continuities correspond to coercive field in crystalline SbSI(Eco¼10 kV/m [29]). This effect disappears at temperatures greaterthan the Curie temperature of SbSI (Fig. 5).

Due to anisotropy of SbSI the best piezoelectric properties areobserved along the longitudinal axes of these needle crystals.However, due to easy cleavage parallel to longitudinal axes, SbSIcrystals split at the ends. It limits practical applications of SbSIcrystals. Presented laser treatment of the ends of SbSI singlecrystals (Fig. 1) increases their mechanical resistance.

4. Conclusions

The presented laser technique is a convenient, fast, and low-cost route for producing heterostructures in compound semi-conductors in a single step. However, additional efforts areneeded to evaluate the presented fabrication technique and scaleit up from a prototype stage to an established, reliable, repea-table, high-volume, industrialized production method. In parti-cular, the laser treatment will be performed in controlledenvironment (e.g., in nitrogen) to avoid the possible oxidationand contamination.

Devices with one heterojunction (diodes), two heterojunctions(transistors) and multi-heterojunctions (thyristors) can be pro-duced using the presented fabrication technique. The CO2 lasertreatment of ferroelectric, semiconducting SbSI single crystalevokes melting and chemical decomposition of one or a fewselected crystal sections. Produced heterostructures contain amor-phous, semiconducting Sb2S3. The laser produced SbSI–Sb2S3heterostructures are examples of the small group of amorphous–crystalline junctions.

Coexistence of many phenomena (e.g., semiconductivity, fer-roelectricity, and amorphous–crystalline junction) makes theSbSI–Sb2S3 structures interesting for further investigations andapplications. Due to high photosensitivity, these structures can beused as light sensors. There is also one more important advantageof laser treatment of SbSI single crystals. It can be applied to increasethe mechanical resistance of their tips, e.g., in piezoelectric actuatorsand sensors.

The presented laser technology can also be extended to theheterostructure production of some other semiconductorcompounds.

Acknowledgments

This paper has been partially supported by the NCN (Poland)under Contract no. DEC-2011/01/B/ST5/06273.

One of the author (B.T.) is a grant holder in “SWIFT” project(Scholarships Supporting Innovative Technology Forum) POKL.08.02.01-24-005/10 co-funded by the European Union under theEuropean Social Fund.

Appendix A

See Figs. A1–A3.

Fig. A1. TG thermal curve for the decomposition of SbSI single crystal (descriptionin the text).

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Fig. A2. Diffraction patterns of SbSI single crystal sections irradiated ( ) and notirradiated (─) with CO2 laser beam.

Fig. A3. Spectra of optical absorbance of the CO2 laser treated SbSI single crystal(T1¼78 K; T2¼333 K; ) and untreated SbSI single crystal illuminated withplane polarized light with electric field parallel and perpendicular to the c axis ofthe crystal, respectively; ▄ – laser melted section of SbSI single crystal; solid curvesrepresent the best fitted theoretical dependences (description in the text).

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