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Journal of Crystal Growth 308 (2007) 218–222

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Molecular beam epitaxial growth of EuTe/SnTe strained superlattices

Beatriz Dıaz�, Paulo Henrique Oliveira Rappl, Eduardo Abramof

Laboratorio Associado de Sensores e Materiais - LAS, Instituto Nacional de Pesquisas Espaciais - INPE, CP 515, 12245-970,

Sao Jose dos Campos - SP, Brazil

Received 8 June 2007; received in revised form 20 July 2007; accepted 26 July 2007

Communicated by H. Asahi

Available online 6 August 2007

Abstract

In this work, we report on the growth of EuTe/SnTe superlattices (SLs) on BaF2ð1 1 1Þ substrates by molecular beam epitaxy.

Reflection high energy electron diffraction allowed to find the growth modes at each growth stage, while high resolution X-ray diffraction

(HRXRD) was employed to evaluate the heterostructure crystalline quality. In four subsequent stages, we studied the growth of SnTe

buffer layers, the growth of EuTe on SnTe, the growth of SnTe on EuTe, and finally undertook the SLs growth. EuTe grows in the

Stranski–Krastanov mode on SnTe, with a critical thickness for island formation that increases for lower temperatures and higher Te/Eu

flux ratios. On the other hand, SnTe starts growing in the Volmer–Weber mode on EuTe, with evolution to layer-by-layer growth after

the SnTe islands merge, which occurs faster for higher temperatures. As confirmed by HRXRD o=2y scans measured along the (2 2 2) SL

Bragg peak, high structural quality EuTe/SnTe SLs were grown with EuTe layers below the critical thickness for island formation and

SnTe layers thick enough for layer-by-layer growth to be restored. The X-ray spectra were fitted with those calculated using dynamical

diffraction theory in order to find accurately the individual layer thicknesses and strain status.

r 2007 Elsevier B.V. All rights reserved.

PACS: 81.15.Hi; 61.14.Hg; 61.10.Nz; 81.05.Hd; 68.65.Cd

Keywords: A1. High resolution X-ray diffraction; A1. Reflection high energy electron diffraction; A3. Molecular beam epitaxy; A3. Superlattices;

B2. Semiconducting materials

1. Introduction

The growth of semiconductor heterostructures bymolecular beam epitaxy (MBE) allows to obtain artificialstructures with new properties that can be used formicroelectronic devices. Lattice mismatch between com-posing materials is among the major obstacles to thegrowth of heterostructures, leading to defect formation andinterface roughening. A second important obstacle is theisland formation, which occurs when a higher surfaceenergy material is deposited on another with lower surfaceenergy. This means that in sandwiched structures A/B/Aeither A or B will grow in islands on the other [1,2].Nevertheless, many high quality strained heterostructures,

e front matter r 2007 Elsevier B.V. All rights reserved.

rysgro.2007.07.044

ing author. Tel.: +55 12 3945 6578; fax: +55 12 3945 6717.

ess: beatriz@las.inpe.br (B. Dıaz).

based on different semiconductor systems, have beengrown to date [2–5].The IV–VI semiconductors have narrow gaps and are

employed in the construction of lasers and detectorsin the thermal infrared region [6]. They crystallize inthe NaCl structure, and are usually combined withEuTe, a wide gap semiconductor with the same structureand close lattice parameter ðaEuTe ¼ 6:598 A; aSnTe ¼

6:326 A; aPbTe ¼ 6:462 AÞ. Several lattice mismatchedIV(Eu)–VI semiconductor systems, such as PbTe/SnTe[5], PbTe/EuTe [7], PbTe/PbEuTe [8] and SnTe/SnEuTe[9], have been studied to date. The inclusion of europiumallows to control the energy gap of the IVEuVI ternaryalloy, up to the higher Eu–VI gap. Sometimes, largedifferences in atom size hinder to obtain ternary alloys, andthe inclusion in a superlattice (SL) allows widening thereachable composition range, as for SnEuTe [9].

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Table 1

Full width at half maximum (FWHM) of the rocking curves around (2 2 2)

Bragg peak of SnTe buffer layers grown at different substrate

temperatures T sub

Sample T sub ð�CÞ FWHM (arcsec) Thickness ðmmÞ

7009 300 133 3.3

7012 310 110 3.2

7014 320 122 3.5

7015 330 131 3.3

B. Dıaz et al. / Journal of Crystal Growth 308 (2007) 218–222 219

In this work, we report on the MBE growth of EuTe/SnTe superlattices, with 4.1% lattice mismatch betweencomposing materials. To this goal, we first grew thickbuffer layers of SnTe on BaF2ð1 1 1Þ substrates. The BaF2

substrate is a common choice for IV–VI semiconductorstructures because of the close thermal expansion coeffi-cient and lattice parameter ðaBaF2 ¼ 6:200 AÞ [10]. Then,the growth dynamics of EuTe on SnTe and of SnTe onEuTe were investigated. Finally, MBE growth parameterswere finely tuned to obtain high structural qualityEuTe/SnTe SLs. The growths were monitored in situ

using reflection high energy electron diffraction (RHEED),and the structural quality of the buffer layers and SLswas evaluated through high resolution X-ray diffraction(HRXRD).

2. Experimental procedure

The samples were grown in a Riber 32P MBE systemequipped with Eu, SnTe, PbTe, and Te sources. The freshlycleaved BaF2ð1 1 1Þ substrates were preheated to 400 �C for15min. During the growths, the sample holder was keptrotating to ensure high film uniformity. The growth modesat each growth stage were studied by in situ RHEED.The beam equivalent pressure of each effusion cellwas measured with a Bayard–Alpert ionization gauge.HRXRD was used to evaluate the crystalline quality of thebuffer layers and the SLs. The X-ray spectra weremeasured with a X’Pert MRD high resolution diffract-ometer. For the SnTe buffer layers, with thickness from 2to 4mm, the rocking curves around the (2 2 2) Bragg peakwere measured, and the full width at half maximum(FWHM) was used to evaluate the crystalline quality. Forthe EuTe/SnTe superlattices, triple axis o=2y scans weremeasured around the (2 2 2) SL reflection, including thebuffer and substrate (2 2 2) Bragg peaks.

3. Results and discussion

3.1. Growth of SnTe on BaF2 substrates

The IV–VI semiconductors such as SnTe and PbTe beginto grow in the island mode on BaF2ð1 1 1Þ. This results notonly from the lattice mismatch but from the polarized,highly energetic (1 1 1) surfaces in IV–VI semiconductors,which make initial layer-by-layer growth unfavorable [10].After deposition of 8 or more monolayers (MLs), depend-ing on the growth conditions, the islands merge andthe growth proceeds layer-by-layer, as indicated by theevolution of the RHEED patterns.

Previous studies on the growth of SnTe on BaF2

substrates have focused on dislocation densities and filmscrystalline quality as a function of thickness [11,12]. Here,we studied the influence of substrate temperature on theSnTe structural quality. The films quality increased forhigher substrate temperatures up to 310 �C, and thendecreased again, as indicated by the FWHM of the SnTe

(2 2 2) Bragg reflection, shown in Table 1. The worsening ofthe films grown with substrate temperatures above 310 �Ccan be explained by the preferential desorption of Tespecies from the growing film surface. The minimumFWHM of 11000 obtained for the SnTe films indicates avery high structural quality compared to previouslyreported values of 19000 [11] and 16100 [12].

3.2. Growth of EuTe on SnTe

The growth of EuTe on SnTe films always started layer-by-layer, according to the RHEED patterns showingelongated streaks typical of reflection diffraction (Fig. 1(a)).After deposition of several EuTe monolayers, the RHEEDpatterns became spotty (Fig. 1(b)), which is characteristicof transmission diffraction and indicates the formationof islands. The observed growth evolution from layer-by-layer to island formation, known as Stranski–Krastanovgrowth mode, is typical of strained systems and is drivenby the strain energy, which above a certain thickness isreleased in the form of defects and ultimately leads to islandformation.The number of EuTe MLs deposited just before

transmission RHEED features appear, hereafter namedcritical thickness, was determined as a function of substratetemperature and Te/Eu flux ratio. The results, shown inFig. 2, indicate that the critical thickness of EuTe on SnTeincreases for lower substrate temperatures and higher Te/Euflux ratios. A higher Te/Eu flux ratio implies that Euadsorbed atoms hopping in the film surface will find a fasterTe atom to join, after which its mobility will be substantiallyreduced. Lower mobilities result in reduced island forma-tion, in agreement with the observations. The increase of thecritical thickness for lower substrate temperatures is alsoexplained by the reduced mobility, caused not only bythe lower temperatures, but also due to a higher ‘effective’Te/Eu flux ratio. In fact, Te re-evaporation rates are muchmore significant than Eu ones compared to the impingingfluxes, and cooling the substrate leads to a relative Teenrichment on the growing film surface.

3.3. Growth of SnTe on EuTe

The SnTe growth on atomically flat EuTe films showingclear RHEED reflection patterns was investigated for

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Fig. 1. RHEED patterns of EuTe grown on SnTe, in the ½110� azimuth,

during initial layer-by-layer growth (a) and after island formation (b).

MLs o

f E

uT

e

Temperature (°C) Te/Eu flu

x ratio

16

14

12

10

8

6

4

2

100125

150175

200225

250275 2

10

Fig. 2. Critical thickness for island formation during EuTe growth on

SnTe, as a function of substrate temperature and Te/Eu flux ratio.

Fig. 3. RHEED patterns of SnTe grown on EuTe, in the ½110� azimuth, at

initial growth stages indicating a Volmer–Weber island growth mode (a)

and after coalescence of the islands (b).

B. Dıaz et al. / Journal of Crystal Growth 308 (2007) 218–222220

different growth temperatures. The SnTe growth alwaysstarted in islands, as concluded from the RHEED patternsshowing transmission features typical of three-dimensional

island growth (Fig. 3(a)). This is not surprising, sinceEuTe was found to start growing layer-by-layer on SnTe(Section 3.2), and surface energy considerations in sand-wiched structures A/B/A indicate that if A grows layer-by-layer on B, then B will grow in islands on A [1,2]. Thepossible surfactant action of a previously deposited thin Telayer [13] to diminish SnTe islanding was tested withoutsuccess.As the SnTe growth continues, the transmission features

in the RHEED pattern give place to a streaky pattern, andfinally diffraction spots along the zero-order Laue zoneappear accompanied by Kikuchi lines, though the lattermight not appear clearly in the photo (Fig. 3(b)). Theobserved evolution of the RHEED pattern signals that theislands coalesce and the growth continues layer-by-layer.Comparison of SnTe and EuTe RHEED patterns duringlayer-by-layer growth (Fig. 3(b) and 1(a), respectively)indicate that the SnTe surface is smoother. The number ofSnTe MLs needed to restore the layer-by-layer growthdecreases for higher substrate temperatures, as shown inFig. 4. This is in accordance with the marked decrease ofthe films structural quality observed for substrate tempera-tures below �170 �C. Surprisingly, the same evolution tothe layer-by-layer growth mode was also observed for theSnTe growth on EuTe films that had already formedislands.

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102

101

100 125 150 175 200 225 250

Temperature (°C)

MLs o

f S

nT

e

Fig. 4. Number of SnTe monolayers, deposited on EuTe, needed to

recover the 2D layer-by-layer growth.

107

106

105

104

103

102

101

100

23.0 23.5 24.0 24.5 25.0 25.5

ω (degree)

Inte

nsity (

a.u

.)

experiment

best fit

SL-3

Sl-2

SL-1

SL0SL1

SnTe

BaF2

Cukα1

(222)SL1SL0

Fig. 5. o=2y scan along the (2 2 2) EuTe/SnTe superlattice Bragg

reflection, showing the presence of satellite peaks up to the 3rd order.

The (2 2 2) SnTe buffer and BaF2 substrate peaks can also be observed.

The inset magnifies the region including superlattice peaks of order zero

and one, showing the higher order oscillations due to beam interference in

the whole SL stack. The best fitting spectrum, calculated using dynamical

theory of X-ray diffraction, is also shown.

B. Dıaz et al. / Journal of Crystal Growth 308 (2007) 218–222 221

3.4. Growth of EuTe/SnTe superlattices

As follows from previous sections, to obtain highstructural quality EuTe/SnTe superlattices the EuTeindividual layer thickness must stay below the criticalthickness for island formation, for the chosen Te/Eu fluxratio and substrate temperature. Besides, the number ofSnTe MLs in each single layer must be above the limitfound for layer-by-layer growth to be restored, alsodependent on substrate temperature. These conditionsexplain why the higher structural quality SLs were thosewith higher SnTe to EuTe thickness ratio. The best SLgrowth parameters in our MBE system were found for asubstrate temperature of 180 �C and a Te/Eu flux ratioaround 10. Such a high Te/Eu flux ratio helps to increaseEuTe critical thickness (see Section 3.2), while excess Tereevaporates without affecting crystalline quality.

Fig. 5 shows the HRXRD o=2y scan along the (2 2 2)Bragg reflection for an EuTe/SnTe SL with nominally12�(3 ML EuTe/27 ML SnTe). Observe the presence of SLsatellites up to the third order, and of higher frequency SLoscillations, magnified in the inset, arising from beaminterference in the whole SL stack. Besides, no significantbroadening of higher order satellites is seen. These featuresindicate a high crystalline quality SL with abrupt interfacesand fairly equal periods [5,14]. The experimental spectrumwas fitted with those calculated using dynamical diffractiontheory of Takagi–Taupin [15]. The calculated spectrumthat better matched the measurement is shown in Fig. 5,and was obtained supposing a 12 repetitions SL with 26.9ML of SnTe and 2.8 ML of EuTe. The SnTe layers,including the buffer, were considered to be completelyrelaxed and the EuTe layers, fully strained. These assump-tions are supported by reciprocal space map measurementsaround (2 2 4) asymmetric reflection, where buffer layerand SL peaks appear at the same position in the qk½1 1 2�in-plane direction. The faster intensity decay for higher

order satellites in the experimental spectrum compared tothe calculated one (Fig. 5) is mainly due to mosaic spreadand, to a lesser extent, to interface roughness [16], none ofthem considered in the model.

4. Conclusions

MBE growth parameters were finely tuned to obtainhigh structural quality EuTe/SnTe superlattices. In situ

RHEED was used to find the growth mode at each growthstage, and high resolution X-ray diffraction allowed toevaluate the structural quality of the buffer layers and theSLs. High structural quality SnTe buffer layers were grownon BaF2 substrates at 310 �C. Above this temperature thefilm quality decreases due to a lack of tellurium. EuTegrows in the Stranski–Krastanov mode on SnTe, with acritical thickness for island formation that increasesfor lower temperatures and higher Te/Eu flux ratios. Onthe other hand, SnTe growth on EuTe starts in islands,which latter coalesce and the growth continues layer-by-layer. The number of SnTe MLs needed to restore thelayer-by-layer growth mode decreases with increasingsubstrate temperature. According to HRXRD analysis,

ARTICLE IN PRESSB. Dıaz et al. / Journal of Crystal Growth 308 (2007) 218–222222

high structural quality EuTe/SnTe superlattices wereobtained with EuTe layers below the critical thickness forisland formation and SnTe layers thick enough for layer-by-layer growth to be restored. The individual layerthicknesses and strain status of the SLs were accuratelyfound by fitting the measured X-ray spectra with thosecalculated with dynamical diffraction theory.

Acknowledgment

This work was partially supported by CNPq, Grant142292/2004-4.

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