Direct measurement of surface stress during Bi-mediated Ge growth on Si

Surface Science 609 (2013) 157–160

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Surface Science

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Direct measurement of surface stress during Bi-mediated Ge growth on Si

Hidehito Asaoka a,⁎, Tatsuya Yamazaki a, Kenji Yamaguchi a, Shin-ichi Shamoto a,Sergey Filimonov b, Maki Suemitsu c

a Quantum Beam Science Directorate, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japanb Department of Physics, Tomsk State University, Tomsk 634050, Russiac Research Institute of Electrical Communication, Tohoku University, Aoba-ku, Sendai 980-8577, Japan

⁎ Corresponding author. Tel.: +81 29 282 5479; fax:E-mail address: asaoka.hidehito@jaea.go.jp (H. Asao

0039-6028/$ – see front matter © 2012 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.susc.2012.12.002

a b s t r a c t

Article history:Received 9 September 2012Accepted 1 December 2012Available online 19 December 2012

Keywords:StressesSurface structureMolecular beam epitaxySurfactant-mediated epitaxy

We have focused on stress measurements during Bi termination of Si (111) and Ge growth on thisBi-mediated Si (111). In order to obtain information on both the surface stress and the surface structure si-multaneously, we have combined the surface-curvature and the reflection-high-electron-energy-diffractioninstrumentations in an identical ultrahigh vacuum system. We find the Bi-terminated Si (111) √3×√3-β sur-face releases 1.8 N/m (=J/m2), or (1.4 eV/(1×1 unit cell)), of the surface energy from the strong tensile Si(111) 7×7 reconstruction. Subsequent Ge deposition on the Bi-terminated Si surface develops a compressivestress, which oscillates with a period corresponding to the growth of a single bilayer. The real-time stressmeasurement provides a direct evidence for this oscillatory stress relaxation in the layer-by-layer growth.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Control of intrinsic stress in heteroepitaxial layers is one of themost important challenges in modern technology. The intrinsic stressarises mainly from the surface morphology and lattice mismatch be-tween the growth layer and the substrate. The interplay between thesurface energy including surface stress and the lattice strain plays acritical role in determining the growthmode of heteroepitaxial layers.As a result, the surface morphology is sensitive to presence of a thirdspecies on the surface, so-called surfactant, which affects the surfaceenergy due to the termination of the surface or the surface reconstruc-tion accompanied by change in the surface stress. For instance, it iswell known that the Stranski–Krastanow (SK) mode of the Ge growthon Si [1–3] is suppressed by a surfactant such as As or Sb [4,5]. In such asurfactant-mediated epitaxy, a flat Ge layer grows on Si while the sur-factant floats up to the growth front and always covers the Ge surface.

Among the surfactants for Ge/Si (111) heteroepitaxy, Bi has attracteda lot of attention due to its ability to provide a convenient chemical con-trast between Ge and Si in scanning-tunneling microscopy (STM).Moreover, Bi allows fabrication of self-organized Ge/Si nanostructureson the Si (111) surface, which is easily detected by this chemistry-sensitive STM measurement [6,7]. The low incorporation of Bi in theGe layer [8] allows us to remove most of the Bi atoms easily from theflat Ge surface even after the Bi-mediated growth [9].

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Despite the aforementioned importance of Bi as a surfactant duringGe growth on Si, knowledge on the impacts of Bi on the surface stressduring Ge/Si epitaxy has been quite limited. In this work, we havemeasured the surface stress in real-time during Bi deposition onto aclean Si (111) 7×7 surface as well as during Bi-mediated growth ofGe on Si (111) both conducted at 500 °C. The stress evolutions are ob-served by measuring the curvature of free-standing substrates whichallows ideally unconstrained bending. In clarifying the role of surfac-tant on the surface stress during heteroepitaxy, however, we shouldalso care about the surface reconstruction. Although rearrangementof the surface atoms largely reduces the electronic surface energy byreducing the number of surface dangling bonds, the surface recon-struction increases the surface stress and the surface energy as well.A typical example is the Si (111) 7×7 reconstructed surface. Complexarrangements of the surface atoms, such as adatoms, dimers, and stack-ing faults are formed on Si (111) 7×7, which significantly affects thesurface stress by introducing both tensile and compressive stressesdepending on the site, although partial compensation between themcoexists as a whole [10,11]. To obtain information on both the surfacestress and the surface reconstruction simultaneously, we have com-bined the surface-curvature and the reflection-high-electron-energy-diffraction (RHEED) instrumentations in an identical ultrahigh vacuum(UHV) system (base pressure: 1×10−8 Pa). As a result, we find a dis-tinct difference in the surface stress between the clean Si (111) 7×7surface and the Si (111) √3×√3-β surface terminated with 1 ML ofBi (1 ML=7.8×1014 atoms/cm2). Subsequent Ge deposition on theBi-terminated Si surface develops a strong compressive stress accompa-nied by a stress oscillationwith a period corresponding to the growth ofa single bilayer (1 BL=2 ML).

158 H. Asaoka et al. / Surface Science 609 (2013) 157–160

2. Experimental procedures

Si (111) wafers with a thickness of 0.1 mm and a diameter of25 mm were used. Both sides of each wafer were mirror polished.We boiled the substrate in a solution of H2SO4 and H2O2 (H2SO4:H2O2=3:1) for 10 min and etched the wet oxide in a 1% HF solutionfor 5 min. After this treatment, the substrate was dipped in a hot 40%NH4F solution for 30 s for hydrogen-termination of the surface [12].The substrate was then set, without any clamping, on a substrateholder in the UHV system. This free-standing holding of the substrateallows ideally unconstrained bending of the substrate required for thestress measurement. We used Auger electron spectroscopy to confirmthat the surface was devoid of contamination. The thickness of the Biand the Ge layers as well as their growth rates were monitored by aquartz crystal oscillator that was placed close to the substrate. Thequartz thickness monitor was calibrated ex situ by using X-ray reflec-tometry. Solid sources of Bi (99.99%) and Ge (99.999%) were used todeposit them on the Si (111) surfaces at 500 °C.

Stress behavior during deposition was monitored by means of asubstrate curvature, which was measured using a multiple parallellaser beam technique [13,14]. The relationship between the film stressper unit area σf, and the radius R of the curvature is given by the fol-lowing equation developed originally by Stoney [15]:

σ fhf ¼ Mshs2

� �= 6Rð Þ ð1Þ

where Ms and hs are the substrate biaxial modulus and the thickness,respectively, and hf is the film thickness.

(b) (c)

(a)

Fig. 1. (a) Evolution of surface stress (red line), RHEED intensity of √3×√3 streaks (blue linSi (111) 7×7 surface at 500 °C. Evolution of the RHEED patterns during Bi termination of Si (the incident electron is parallel to the 11�2

� �azimuth of the Si (111) surface.

3. Results and discussion

The RHEED pattern of the H-terminated Si (111) surface shows asharp and bright 1×1 pattern with a low background. After heatingthe sample up to 600 °C, the pattern changes into a one indicating theformation of a clean 7×7 reconstructed Si (111) surface (Fig. 1(b)).Upon depositing Bi onto this Si (111) 7×7 surface at 500 °C, theRHEED pattern begins to show a √3×√3-α structure at around the Bicoverage of 1/3 ML (Fig. 1(c)) and a subsequent √3×√3-β structureat around 1 ML (Fig. 1(d)). The √3×√3-α and β structures are knownto have Bi atoms and Bi trimers on T4 sites above the second-layerSi atoms respectively [16,17]. A step-by-step increase of the √3×√3streak intensity at marked streaks in Fig. 1(c) and (d) corresponds toa formation of the √3×√3-α and the √3×√3-β structures (blue linein Fig. 1(a)). Simultaneously, the specular beam intensities decrease,start to recover, and eventually saturate at around 1 ML (green line inFig. 1(a)). The saturation of both the √3×√3 streak and the specularbeam intensities corresponds to a completion of a √3×√3-β structureswith saturated coverage of Bi on Si. Any Bi atoms excess of 1 ML desorbsfrom the Si surface at 500 °C [18,19].

In situ stress measurement shows that the substrate immediatelystarts to bend at the beginning of the Bi deposition. Before the Bi de-position, both sides of the substrate have the same surface structureof Si (111) 7×7 and the same surface stresses thereof. Therefore, the“surface stress” evolution (i.e., onset of the curvature) actually resultsfrom the surface stress difference between the front Bi-terminated Si(111) √3×√3-β and the rear Si (111) 7×7 surface. The surface stress(σf hf in Eq. (1)) thus derived from the bending (red line in Fig. 1(a))shows negative values (compressive stresses) and saturates as in the

(d) specular beam

√3x√3 streak

e) and RHEED intensity of specular beam (green line) during Bi termination of a clean111) 7×7 (b) through √3×√3-α surface (c) to√3×√3-β surface (d). The orientation of

159H. Asaoka et al. / Surface Science 609 (2013) 157–160

√3×√3 streak and the specular beam intensities of RHEED at the com-pletion of a 1 ML Bi coverage. Analysis of Fig. 1(a) reveals that theBi-terminated √3×√3-β reconstruction releases 1.8 N/m (=J/m2),or (1.4 eV/(1×1 unit cell)), of the surface energy caused by thestrong tensile Si (111)7×7 reconstruction. The released stress of theBi-terminated Si (111) √3×√3 surface turns out to be larger thanthat of a Sb-terminated Si (111) √3×√3 surface (0.9 eV/(1×1 unitcell)) [20]. The element of Sb is in the same group V semiconductoras Bi and the Sb-terminated Si (111)√3×√3 also has the same trimerson T4 sites at 1 ML coverage [21] as Bi-terminated Si (111) √3×√3.The fact that Bi has a larger covalent radius than Sb is consistentwith this larger compressive stress as compared to Sb termination.

Subsequent Ge and Bi co-deposition on the substrate of aBi-terminated Si (111) √3×√3-β surface provides further bending,but with several stages as shown in Fig. 2. The magnitude of the com-pressive surface stress increases nearly linearly with the Ge coverage.The stress gradient matches with the calculated stress vs. Ge coveragerelationship of −2.30 (N/m)/BL, obtained assuming an ideal pseudo-morphic Ge layer on Si (111) at 500 °C. The stress evolution indicatesformation of a compressive two-dimensional (2D) Ge layer on Si.When the Ge coverage approaches the critical thickness of around2 BL, the stress starts to be relieved drastically until it follows a stressgradient of −0.36 (N/m)/BL at around 15 BL Ge. This change of thestress gradient indicates that 84% of the initial stress established bythe pseudomorphic wetting layer is now relaxed. Onset of the disloca-tion introduction is most likely the cause, consistent with the resultsof STM measurement [18,22]. The undulated surface with buried dis-location network observed in the STM images indicates introductionof misfit dislocations into the Ge/Si interface at or above the criticalcoverage of 2 BL. STM image shows, for Ge coverages between 2 and15 BL, that the height of the initial islands develops in the presenceof dislocations, which results in the deepening of the trenches be-tween islands and increase of the surface roughness.When theGe cov-erage reaches 15 BL, the islands are coalesced and the surface becomesflat. The formation of a flat surface is consistent with the gradual in-crease of the stress gradient after the stress relief and the constant stress

Ge15BLGe2BL

Fig. 2. Evolution of surface stress (red line) and RHEED patterns during Bi-mediated Geheteroepitaxial growth at 500 °C. The stress gradient matches with−2.30 (N/m)/BL upto 2 BL and with −0.36 (N/m)/BL from 15 BL. The RHEED √3×√3 streak patterns aremaintained due to the Ge surface covered with 1 ML Bi. The orientation of the incidentelectron is parallel to the 11�2

� �azimuth of the Si (111) surface.

gradient after 15 BL. For thicker Ge layers, namely from 15 to 90 BL, thestress relaxation increases slightly from 84% to 88%. It should be notedthat the Bi deposition is unrelated to the stress gradient during theGe growth.When theGe deposition is stopped in the co-depositing con-dition, on the other hand, the stress development also stops, and pre-serves the same stress under subsequent Bi deposition. The √3×√3streak pattern during the co-deposition indicates that the Ge surface iscovered with a saturated 1 ML Bi at 500 °C [18,19]. The Bi depositionbalances the Bi desorption from the Ge growth front.

During growth of the pseudomorphic layer before the critical cov-erage, in situ RHEED measurement shows an oscillation in the specu-lar beam intensity (green line in Fig. 3). The period of the oscillationcorresponds to the growth of a single BL of Ge. The oscillation disap-pears when dislocations are introduced at the critical coverage of2 BL. For Ge coverages from 5 BL to 15 BL, the intensity recovers(green line in insert of Fig. 3) and this behavior is consistent withthe island coalescence observed by STM [18]. The dislocation networkcovers the whole Ge/Si interface and relaxed Ge layer grows in alayer-by-layer mode. Besides the oscillation in the RHEED specularbeam intensity we also observe oscillations in the in-plane latticeconstant of the Ge layer from the RHEED streak interval originatingfrom Ge as well as in the stress deviation from the constant develop-ment of the stress at a rate of −2.30 (N/m)/BL calculated for an ide-ally psuedomorphic Ge layer. The blue line in Fig. 3 indicates thein-plane lattice constant of Ge, which is normalized with the initialvalue. The relative lattice constant for sub-BL layers is only slightlylarger than that of the Si surface and shows an oscillation with aperiod corresponding to the growth of a single BL of Ge. After the crit-ical coverage of 2 BL, the relative lattice constant ceases to oscillateand rapidly increases towards its relaxed value. Simultaneously, thestress deviation (red line in Fig. 3) from the constant accumulationof stress also shows an oscillation. The initial stress deviation in thesub-BL shows negative (relaxed) stress. The stress oscillation is con-current with those of RHEED specular beam intensity and in-planelattice constant. It is well known that the RHEED intensity oscillationis due to a cyclic process of the 2D island nucleation, growth and co-alescence. Therefore the oscillations reveal that the step edge of the2D seed islands allows relaxation of the stress and of the strain during

Fig. 3. Evolution of RHEED specular beam intensity (green line), relative in-plane Gelattice constant from Si lattice of surface (blue line) and surface stress deviation fromcalculated stress of −2.30 (N/m)/BL (red line) during Bi-mediated Ge heteroepitaxialgrowth at 500 °C.

160 H. Asaoka et al. / Surface Science 609 (2013) 157–160

the layer-by-layer growth. It should be noted that there are subtlediscrepancies in the period of the oscillations. The behaviors resultfrom deviation from ideal layer-by-layer growth, i.e. Ge nucleates inthe second layer before the first layer completed. The maximum ofthe specular beam intensity appears before the surface roughnessstarts to increase at slightly lower coverage than 1 BL, while themaximum of the relative lattice constant and the minimum of thestress deviation appear at slightly higher coverage than 1 BL. Whilethe in-plane lattice constant oscillations have been reported for thegrowth of Gewetting layer on Si (001) and Si (111) substrateswithoutsurfactant [23], our stress measurements provides a direct evidencefor the oscillatory stress relaxation in the layer-by-layer growth.

4. Conclusion

We have measured in real-time the stress and the in-plane latticeconstant (strain) during Bi-mediated growth of Ge on Si (111). Thestress measurement reveals that the Si (111) √3×√3-β terminatedwith 1 ML of Bi has a surface-energy relaxation of 1.8 N/m (1.4 eV/(1×1 unit cell)) as compared with the strong tensile Si (111) 7×7surface. Subsequent Ge deposition on this Bi-terminated Si surfaceincreases the compressive stress, which oscillates with a period corre-sponding to the growth of a single BL. The stress oscillation is accom-panied by concurrent oscillations of RHEED specular beam intensityand in-plane lattice constant. At the critical coverage of 2 BL, the stressdevelopment starts to decrease, due to introduction of misfit disloca-tions and increasing surface roughness. Meanwhile, only a slight stressrelaxation is found for coverages greater than 15 BL. This indicatesthat defect introduction into thicker layers is suppressed in thisBi-mediated Ge growth and a flat surface morphology is maintainedas a result. These observed stress evolutions reveal details of thesurfactant-mediated growth with a variety of stress relief processes,i.e. elastic relaxation at the step edge of the 2D seed islands up to2 BL, dislocations at the critical coverage of 2 BL, trench formation

up to 15 BL and dislocation network covering the whole Ge/Si inter-face and relaxed Ge layer growth in the layer-by-layer mode after15 BL. The observed stress relief processes reveal a nature of eachgrowth stage during the Bi-mediated Ge heteroepitaxial growth. Thestress controlled Bi-mediated growth has thus opened up a way tofabricate nanostructureswith sharp interfaces to be free from incorpo-ration of surfactant for future Si-based technology.

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