Interfacial abruptness in axial Si/SiGeheterostructures in nanowires probedby scanning capacitance microscopy

P. Periwal1, F. Bassani*,1, G. Patriarche2, L. Latu-Romain1, V. Brouzet1, B. Salem1, and T. Baron1

1 Laboratoire des Technologies de la Microélectronique (LTM), UMR 5129 CNRS – UJF, CEA Grenoble, 17 rue des Martyrs,38054 Grenoble, France

2 Laboratoire de Photonique et de Nanostructures (LPN)-CNRS, Route de Nozay, 91460 Marcoussis, France

Received 14 June 2013, revised 14 October 2013, accepted 20 December 2013Published online 3 February 2014

Keywords germanium, heterostructured nanowires, interface abruptness, scanning capacitance microscopy, silicon

* Corresponding author: e-mail franck.bassani@cea.fr, Phone: þ33 4 38 78 65 11, Fax: þ33 4 38 78 50 19

Si/SiGe heterostructured nanowires (hNWs) were grown bycatalyst mediated vapor–liquid–solid mechanism via chemicalvapor deposition. As-grown hNWs were characterized byscanning electron microscopy, transmission electron micro-scopy, and scanning capacitance microscopy (SCM). All these

techniques allow us to spatially delineate Si/SiGe and SiGe/Siheterojunctions and show an asymmetry in terms ofabruptness between these two interfaces. Material SCMcontrast in Si/Ge system is explained by a difference of nativeoxide quality.

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1 Introduction Semiconductor heterostructurednanowires (hNWs) offer promising capabilities for theirpotential applications in high performance electronics [1],optoelectronics [2], sensors [3], and thermoelectrics [4].Among them, axial Si/SiGe heterostructures [5] with well-defined, controlled interfaces and controlled doping betweendifferent nanomaterials have recently emerged as aninteresting subject of research. In particular Si/SiGe hNWsare a good candidate from technological compatibility forlarge-scale integration into devices. Moreover, high carriermobility exhibited by SiGe promotes the performance ofdevices. Various methods employing laser ablation [6],plasma enhanced-CVD [7], and thermal CVD [8] have beenillustrated in the literature for the elaboration of suchheterostructures. In case of catalyzed growth of hNWs, whileswitching the fluxes of gases, the catalyst acts as a reservoirleading to a graded transition region from residual atomspresent in the catalyst, over a certain length. This transitionregion width will affect the performance of electronicdevices.

Theoretical predictions [9] suggest that this interfaceabruptness should be proportional toNWdiameter. Therefore,the characterization of transition region widths is significant tounderstand device characteristics. Up to now, conventionalmicroscopic techniques such as scanning electron microscope

(SEM), and scanning transmission electron microscopy(STEM) measurements have been mainly used for thecharacterization of Si/SiGe hNWs. The transition regionwidths are commonly evaluated by STEM-energy dispersiveX-ray spectroscopy (EDX) [10]. However, in this work, alongwith STEM studies we present the capability of scanningcapacitance microscopy (SCM) to acquire the informationabout the spatial location and the interfacial abruptness ofheterojunctions in an individual NW.

SCM is based on the properties of MOS capacitor. Itinvolves measurement of local capacitance between themetallized AFM probe and the semiconducting sample.In this technique, the changes in capacitance with lowfrequency AC bias were monitored using highly sensitivefrequency resonant circuit. It is a powerful and non-destructive technique for imaging dopant variations insemiconductor devices at nanometer scale and hence,extensively used for studying dopant profiling in twodimensions with high spatial resolution [11]. Various groupshave demonstrated carrier density measurement usingSCM [12–14]. Vallett et al. [15] extended this techniqueto one-dimensional nanomaterials and illustrated the dopingprofile in thermally oxidized p–n–nþ Si NW showing abruptn–nþ junction by SCM. The sensitivity of this tool is of theorder of attofarads but lateral resolution is dependent on the

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tip-sample surface contact. Apart from imaging the dopantnature and concentration, SCM can also detect failure insemiconductor devices by characterizing the quality of oxidelayer [16]. To the best of our knowledge, this technique hasnever been applied to characterize axial heterojunctions inunintentionally doped NWs.

In this work, we report on the characterization ofintrinsic SiGe/Si/SiGe hNWs using SCM. Throughout thispaper, an A/B interface will refer to the growth of B on A.This paper primarily focuses on the characterization of SiGe/Si/SiGe heterojunctions grown by Au catalyzed VLS-CVDby SCM and conventional microscopic techniques (SEM,STEM, STEM-EDX). SCM allows us to locate preciselySiGe/Si and Si/SiGe heterojunctions and quantify thetransition region width of these interfaces. We have studiedthe interfacial abruptness of Si/SiGe and SiGe/Si with thehelp of STEM and SCM. The influence of DC sample bias onSCM contrast of this SiGe/Si/SiGe hNW is reported.

2 Experimental details Intrinsic axial SiGe/Si/SiGehNWs were synthesized by Au catalyzed vapor–liquid–solid(VLS) mechanism on Si (111) substrates in a hot-walledreduced pressure chemical vapor deposition (Easy tube 3000First Nano, a division of CVD equipment corporation).Twenty ångströms of thick Au layer was deposited on oxidefree Si (111) substrates. Prior to the growth, the substrateswere subjected to 10% liquid HF and then introduced to thegrowth chamber where the substrates were annealed at650 8C and cooled down to the deposition temperature,4508C. Three segments of SiGe/Si/SiGe were grown at450 8C (schematic illustration shown in Fig. 1a). Ninetystandard cubic centimeters per minute of silane and 45 sccmof germane (�10% in H2) gas precursors were used assources of Si and Ge, respectively and H2 as a carrier gas. Siand SiGe segments were obtained by alternatively changingthe fluxes of these precursor gases. Throughout the growth,40 sccm of HCl was maintained in order to prevent theuncatalyzed growth and improve the structural quality of theNW [17]. A total pressure of 4.5 Torr was kept in the reactorduring the growth. The growth rates of Si and SiGe segmentswere 175 and 100 nmmin�1, respectively. The growth time

was adjusted such as to obtain the length typically 2.5–1–2.5mm for the SiGe/Si/SiGe structure shown in Fig. 1a. Thefluxes of GeH4 and SiH4 were adjusted so as to obtain a Gecomposition of 30%. It was measured to be 28% by Ramanand XRD. The obtained NWs were characterized by SEM(ZEISS Ultra) operating at 5 kV using InLens detector. Thecrystallinity and transition region widths were analyzed byaberration corrected STEM microscope Jeol 2200 FSequipped with EDX analysis operating at 200 kV. For that,the as grown NWs were dispersed in ethanol by sonicationand were transferred to the Cu grid supported by carbonmembrane.

SCM measurements were performed using the Dimen-sion 3100 microscope with Pt–Ir coated metallic probewith spring constant 0.2Nm�1. SCM works in contactmode AFM. So, it is mandatory to fix the NW in betweenthe electrodes, to avoid its movement during AFM scanning.A single NW was fixed on pþ doped substrate by usingstandard technological steps including deposition of resin,optical lithography, metal deposition, and lift off [18].The idea of using highly doped substrate is to prevent anyadditional capacitance coupling between the NW and thesubstrate. Figure 1b illustrates a schematic representationof the localized NW in between the metal electrodes.Simultaneously, along with a topographic image, thecapacitance variations of the sample are acquired togenerate the capacitance image. So, when the SCM tip isbrought into the proximity of the semiconductor samplesurface, a MOS capacitor is formed between them, where Mis the metal probe, S is the semiconductor material, and O is athin native oxide formed on the semiconductor surface (inour case). The ac bias applied between the sample andthe tip produces a corresponding capacitance variation.The capacitance signal is a function of the contact areabetween the tip and the oxide, the properties of the oxide,and the type and concentration of the charge carriers insidethe semiconductor. In case of doping, the derivative ofthe capacitance–voltage curve has different signs for p- andn-type semiconductors, which allows the determination ofthe type of the material being measured. The amplitudeof this signal yields information about the relative level of

Figure 1 (a) Schematic illustration of SiGe/Si/SiGe hNW. (b) Schematic representation of SCM set up with a single SiGe/Si/SiGe NWfixed in between the metal electrodes on pþ Si substrate. A Pt–Ir coated metallized probe is used for AFM scanning.

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dopant just beneath the tip. In general, SCM is used toprovide qualitative information on doping of semiconduc-tors. Additionally, with the help of modeling, it is alsopossible to attain quantitative information [19].

3 Results and discussions3.1 Characterization by SEM and STEM Figure 2

shows a 5 kV SEM image of SiGe/Si/SiGe hNW where thethree segments can be clearly seen. The darker segment inthe middle corresponds to the Si part and the two brightersegments at the ends correspond to SiGe parts. However,SEM is not the appropriate technique to get completeinformation about crystallinity and interfacial abruptness.The same sample was thus analyzed using STEM. FromTEM observations, NWs are crystalline and defect free.Figure 3a and b shows the bright field and dark field STEMimages of SiGe–Si–SiGe hNWs, respectively. In bright fieldSTEM image, the darker part represents SiGe and lighterrepresents Si, on the contrary, in the dark field STEM imagethe brighter part corresponds to SiGe and darker in themiddle corresponds to Si. Figure 3c illustrates the zoomedSTEM image of the SiGe/Si/SiGe heterojunction. One canclearly see that Si/SiGe interface appears to be more abruptthan SiGe/Si interface. Thanks to HCl flow during thereaction, no diffusion of Au was observed [20]. To confirmthat, the interfacial abruptness was verified using EDXline scan at Si/SiGe interface and SiGe/Si interface. Theabruptness is defined as decrease in the composition ofmaterial from 90% to 10%. Figure 3e shows the EDX lineprofile taken along the NW axis. The transition region widthis found to be 30� 5 nm for Si/SiGe interface and 60� 5 nmfor SiGe/Si interface for this NW of 65 nm in diameter. This

asymmetry between the SiGe/Si and Si/SiGe interfaces canbe explained by the difference of solubility of Si and Geelements in the catalyst droplet. We observe that the growthrate of SiGe is lower than pure Si. In case of SiGe/Si

Figure 2 SEM image of as-grown Si0.7Ge0.3: 2.5mm/Si: 1mm/Si0.7Ge0.3:2.5mm hNW at 5 kV.

Figure 3 (a) Bright field and (b) dark field STEM images ofSi0.7Ge0.3/Si/Si0.7Ge0.3 hNWwith Au catalyst on top. (c) Dark fieldSTEM image showing zoomed view of Si0.7Ge0.3/Si/Si0.7Ge0.3NW with Au catalyst in the right. Si-part appears in brightfor bright field and dark for dark field STEM images. (d) EDXprofile of Si0.7Ge0.3/Si (left) and Si/Si0.7Ge0.3 (right) interfaces. Theinterfacial abruptness for Si0.7Ge0.3/Si and Si/Si0.7Ge0.3 are foundto be 60� 5 and 30� 5 nm, respectively.

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interface, when Ge supply is turned off, because of reservoireffect exhibited by Au, there still exists a small amount ofresidual Ge in Au catalyst. As a consequence, an exponentialdecay in Ge content is observed. Moreover, this incorpo-ration of the residual Ge in the NW is a slow processresulting in a broader interface. On the contrary, in case ofSi/SiGe interface, Au droplet contains residual Si atoms. Dueto high growth rate it incorporates quickly. Additionally,when GeH4 is switched on, the high solubility of Ge in Aucatalyst enables to cause rapid supersaturation. There is asharp increase in Ge content resulting in the formation ofrelatively abrupt Si/SiGe interface.

3.2 Characterization by SCM Figure 4a showstopographic image of an individual SiGe/Si/SiGe hNWfixed by the metal contacts. No tapering is seen in the NW.Figure 4b–f shows simultaneous SCM images of this hNWfor different DC sample bias voltages ranging from 1 to�1V. All these images were taken by applying 1V ACmodulation voltage with 80 kHz modulating frequency.Note that the bias was applied to the substrate. Figure 4gshows corresponding SCM signal profiles averaged acrossthe width of the NW. As it can be seen, the SCM contrastdepends on the DC bias applied. The maximum contrastis observed for VDC¼ 0.5V. As shown in SCM profilerepresented by red line in Fig. 4g, the amplitude of SCMsignal is maximum in the Si part and close to zero for bothSiGe segments at this polarization. Note that the SCM signalis negative indicating that the unintentional doping in Si partis of n-type character. As seen in Fig. 4g the amplitude ofSCM signal in the Si part changes drastically with theapplied voltage while in the SiGe parts it does not vary.This effect can be certainly explained by the quality ofthe native oxide, which is different on Si and SiGe parts.For Si, we have a real MOS capacitor and the derivative ofthe capacitance versus voltage reach a maximum for aparticular polarization. In this case it occurs at 0.5V. On thecontrary, for SiGe, due to the poor quality of native oxidethe C–V curve is rather flat, and this is the reason whywe observe no dependence of the capacitance with theapplied bias.

From SCM profiles, we can estimate the transitionregion width for both SiGe/Si and Si/SiGe interfaces.According to the definition of abruptness stated above, wefound 90� 10 nm for SiGe/Si interface and 50� 10 nm forSi/SiGe interface. This asymmetry in interfacial abruptness

Figure 4

Figure 4 (a) AFM topographic image of an individual Si0.7Ge0.3/Si/Si0.7Ge0.3 hNW fixed by metallic electrodes. SCM images of aSi0.7Ge0.3/Si/Si0.7Ge0.3 hNW with 1V AC voltage for different DCbias applied to the substrate: (b) 1V, (c) 0.5V, (d) 0V, (e) �0.5V,(f) �1V, and (g) corresponding SCM signal profiles. The profileswere obtained by averaging across the width of the hNW. The SCMcontrast is maximum at VDC bias¼ 0.5V. SiGe/Si interface (on theleft) is broader than Si/SiGe (on the right). The transition regionwidth of SiGe/Si and Si/SiGe interfaces are found to be 90� 10and 50� 10 nm, respectively. Au catalyst is on the right side.

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is consistent with what we previously observed using STEM,i.e. Si/SiGe is more abrupt than SiGe/Si interface. Note thatthese values are slightly larger than the ones obtained bySTEM-EDX line scan as measured above. This is notsurprising as SCM is sensitive to free carriers underneath theAFM tip and in the case of unintentionally doped NW thedepletion region can be rather large of the order of a fewtens or hundreds of nanometers depending on the dopinglevel. So we think that for this undoped hNW, the lateralresolution of SCM is limited by the depletion depth ratherthan the tip radius. Therefore the actual abruptness ofthese two interfaces is enlarged by this effect and explainsqualitatively why they are higher than those measured usingSTEM-EDX.

4 Conclusions In conclusion, we report on thecharacterization of VLS SiGe/Si/SiGe hNW using differenttechniques. We have demonstrated that SCM can be a goodtechnique to localize and quantify the transition regionwidths of heterojunctions in 1D structure. In SiGe/Si/SiGehNWs, SCM contrast arises from the difference of thenative oxide formed on Si and SiGe. For this unintentionallydoped hNW, SCM resolution is limited by the depletiondepth and thus gives larger values than the ones obtained bySTEM-EDX. All these techniques evidence that SiGe/Si ismuch broader than Si/SiGe interface.

Acknowledgements SCM experiments were performed atCEA LETI-Minatec Campus at the platform of PFNC underguidance of D.Mariolle and N. Chevalier. This work was supportedby European Community’s Seventh Framework Program (FP7/2007-2013) under grant agreement NANOFUNCTION No.257375, the French Research National Agency for NAHDEVIproject (ANR-11-IS09-0008), and Grenoble Innovation Recherche(AGIR).

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