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Structural examination of the interface between Au catalysts and Ge(111)

H. Zitouni,a,c A. Mehdaoui,a A. Spiesser,b,1 K. Driss Khodja,c L. Josien,a V. Le Thanhb and C. Pirria,⇑aIS2M, Universite de Haute Alsace, CNRS-UMR7361, F-68057 Mulhouse, France

bAix-Marseille Universite, CINaM-CNRS, Campus de Luminy, Case 913, F-13288 Marseille Cedex 9, FrancecLaboratoire de Physique des Couches Minces et des Materiaux pour l’Electronique (LPCM2E), Universite d’Oran,

BP 1524 El’Mnaouer, Oran 31100, Algeria

Received 24 October 2014; revised 3 March 2015; accepted 3 March 2015

Abstract—We investigate the interface between Au catalysts and a Ge(111) substrate. This system is achieved by dewetting an Au layer above theAu–Ge eutectic temperature. We show by high resolution transmission electron microscopy that a large amount of Ge can be moved over a surface ofGe(111) by using assistance of eutectic Au–Ge droplets. This localization is achieved thanks to the ability of nano Au–Ge droplets to incorporate alarge amount of Ge and to release it by cooling down the sample at room temperature. This makes the localization process irreversible with respect toannealing at a very high temperature. The extra Ge supplied by precipitation is in epitaxy with the Ge(111) substrate. This reflects in macroscopicI(V) measurements.� 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Eutectic; Catalysts; Germanium; Interface; Electrical measurements

1. Introduction

The interaction of liquid phases with surfaces is a verybroad subject, with numerous fundamental and practicalinterests. The catalytic property of melt alloy clusters (ordroplets) is one of these remarkable properties. These alloysare also of interest in the overall metallurgy and microelec-tronic areas as solder materials and in all technological areain which low temperature and corrosion resistance arerequired, such as space technology, gas sensor and medicaldevices. The ability of some alloys to form liquid eutecticsat a temperature well below the melting temperature ofeach component separately, such as Au and Si but also ofAu and Ge has thus been exploited. Their bulk phase dia-gram exhibits deep eutectics, at 18% Si–82% Au (363 �C)and 28% Ge–72% Au (360 �C�), respectively. A very niceissue of the use of these catalysts is the growth of Si andGe nanowires in epitaxy on Si(111) and Ge(111), bydecomposition of germane or silane gas on liquid Au–Geand Au–Si catalysts in the VLS (Vapor–Liquid–Solid)mechanism [1–3]. The formation of Si and Ge nanowireswith a diameter in the nanometer scale and a length as highas few microns has opened the way to the fabrication ofnanowire-based electronic devices [4,5]. The growth

http://dx.doi.org/10.1016/j.actamat.2015.03.0061359-6462/� 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights

⇑Corresponding author; e-mail: carmelo.pirri@uha.fr1 Present address: Spintronics Research Center, National Institute of

Advanced Industrial Science and Technology (AIST), Central 2, 1-1-1Umezono, Tsukuba, Ibaraki 305-8568, Japan.

mechanism and the crystallographic orientation of thesenanowires on Si and Ge have been studied, either experi-mentally or theoretically [6–10]. An effort has been doneto control also the size of these nanowires and to organizethem on a surface [11–16]. In that respect, it has beenshown the dependence of the growth rate and the nanowirediameter [11,14,17–20]. Functionalization of the catalystshas also been reported to grow high nanowire density[21–24]. In the meantime, a lot of effort has been made tocharacterize the catalysts themselves [25–50]. For the Au–Ge catalysts, it has been shown that Ge nanowires can begrown by germane or di-germane decomposition on Au–Ge catalysts below the eutectic temperature TE. For thissystem, the Ge nanowire growth is a mixture of the VLSand a VSS (Vapor–Solid–Solid) process [25–28]. Also inter-esting is the ability of the liquid Au–Ge catalysts to etch thegermanium surface at the places of the catalysts [29,30]. Allthese effects are intimately related to the Au–Ge phase dia-gram at the nanoscale and then the ability of Au–Ge liquiddroplets to incorporate Ge. The phase diagram, thecomposition and related properties of several binarynano-alloys have been studied in the past decade for theAu–Ge system [25–27,31–33,38,41,44,48,49]. Two maineffects have been explored, namely the modification of theeutectic temperature versus crystallite size and the phaseseparation upon solidification when the temperature isdecreased below TE.

As to the Au–Ge system, most of the measurementshave been done on Au–Ge catalysts located on top of the

reserved.

H. Zitouni et al. / Acta Materialia 90 (2015) 310–317 311

nanowires, after or during the VLS or VSS process[25,27,31–33,41,43]. Several works have been done on (orin) non-interacting substrates, which was the better wayto study free-standing Au–Ge nano-alloys. They wereeither embedded in a silica matrix [38] or on a silica sub-strate [26]. In all experiments reported in the literature, itwas shown that upon decreasing the alloy temperaturebelow TE, Ge precipitation occurs and bi-lobed crystallitesare formed. Then, Ge nucleation occurs via this process.Some very interesting applications of the formation of bi-lobed droplets and of the decrease of the eutectic tempera-ture TE for droplets in the nanoscale have been reportedrecently. A sophisticated method for producing Au–Ge liq-uid nano-alloys on sapphire with a very nice control of thedroplets size has recently been reported by Sundar et al.[49]. The immiscible couple Au–Ge is used to synthesizeperiodic arrays of heterodimers (bi-lobed Au–Ge droplets).Kim et al. reported a nice work on Au–Ge nano-alloysformed by germane decomposition on Au droplets depos-ited on a non-interacting substrate [48]. It is shown thatGe crystallizes at a temperature as low as 150 �C, whichopen the way to the formation of crystalline semiconduc-tors on low temperature processed systems. Nevertheless,only few works have been devoted to the atomic scaleanalysis of the interface between catalysts and Gemonocrystal, on Ge(111) [30] and on Ge(001) [50].

Different Ge configurations are expected if the Au–Gealloy is formed on non-interacting substrates, on one-dimensional nanowires or on two-dimensional Ge substrates.On clean Ge substrates, precipitated Ge is in epitaxy. Oneinteresting issue is that the system formed by the Au crys-tallite, the precipitated Ge and Ge substrate results in theformation of ohmic contacts, as shown below [51].

In this paper, we investigate the interface between an Aucatalysts, used for the growth of Ge nanowires, and theGe(111) substrate, by using high resolution transmissionelectron microscopy (HRTEM), secondary electron micro-scopy (SEM) and reflection high energy electron diffraction(RHEED). When an Au particle interacts with a Ge(111)surface, an alloyed droplet is formed when the system isheated at a temperature above a critical temperature TE

(eutectic temperature). This droplet incorporates Ge fromthe Ge(111) surface thanks to a high solubility of Ge inthe liquid phase of the Au-Ge alloy. When the dropletreturns at room temperature, it precipitates pure Ge butin a localized region around the Au seeds, whatever theannealing temperature above TE. We have shown in a pre-vious work that Ge precipitates at the Au/Ge(111) inter-face with a deep penetration of the Au-Ge liquid dropletsand precipitation of Ge in this hole [30]. In this work, adetailed examination of the epitaxial Ge released at theAu/Ge(111) interface at room temperature. Some electricalmeasurements are also given.

Fig. 1. SEM image collected at room temperature for a 1.2 nm Audeposit after anneal at 360 �C for 2 h. The temperature is decreasedslowly at a rate 0.5 �C/min. The image is tilted of 45� to enhance theobservation of the pedestal. The beam energy is 12 keV.

2. Experimental

The sample preparation was performed in an ultrahighvacuum setup with a base pressure below 1 � 10�10 mbar.The Au catalysts were grown on a clean Ge(111) substrate,as described in Ref. [30]. They were formed by annealingAu layers evaporated on such a Ge(111) substrate keptat room temperature. An effusion cell was used with adeposition rate of about 0.05 nm/min. The Au layer thick-ness was set 1.2 nm, which is the Au amount generally used

for the Ge nanowire growth. The annealing time is largeenough (2 h) to achieve an equilibrium Au–Ge concentra-tion within the liquid droplets. HRTEM measurementswere performed with a JEOL 3010 microscope operatingat 300 keV with a spatial resolution of about 0.2 nm.SEM images were acquired with a XL30-FEG Philipsmicroscope.

3. Results and discussion

Room temperature Au deposition on clean Ge(111)results in a rather flat layer [30]. Surface dewetting startsat an annealing temperature below 300 �C, thus belowTE, and results in flat, pure Au crystallized islands, in epi-taxy on Ge(111) [30,52]. The “bare” surface exposes a wet-ting layer (Au-Ge d layer) with a

p3 � p3 R30� surface

periodicity. This delta layer extends over the entire surface.It involves one Au monolayer in the topmost Ge(111)planes but extends vertically over more than 1 atomicplane. The

p3 � p3 R30� superstructure induces a distor-

tion in the deeper Ge layers, with most notably a bucklingin the third and fourth Ge layers, thus more than 1 nmbelow the top-most layers [53–55]. The d layer reflects thusas a darker line on the TEM images, as shown below. Uponincreasing the annealing temperature above the bulk Au–Ge eutectic temperature TE (360 �C), dome-like islands ordroplets are observed when the sample is returned at roomtemperature. Their shape is now associated with their crys-tallization after melt.

Fig. 1 shows droplets formed upon annealing the sampleat 360 �C for 2 h and observed at room temperature. Thebright and dark contrast reveals Au and Ge contribution,respectively [30,38]. These droplets are crystallized but ran-domly oriented each other after cooling down at room tem-perature. Upon increasing the annealing temperature, themean droplet size increases due to the coalescence of smal-ler eutectic droplets (formed at lower annealing tempera-ture). This is evidenced in Fig. 2. This figure shows SEMimages acquired for annealing temperatures of 400 �C,550 �C and 650 �C, with a very low temperature decrease

Fig. 2. SEM images collected at room temperature for a 1.2 nm Au deposit after anneal at 400 �C (a, b and c), 550 �C (d, e and f) and 650 �C (g, hand i) for 12 h. The temperature is decreased slowly at a rate 0.5 �C/min. These images are tilted of 45� to enhance the observation of the pedestal.The electron beam energy is 20 keV. (See also Ref. [30]).

Fig. 3. Cross-section TEM image of a droplet for a 1.2 nm Au depositannealed at 400 �C. The electron beam energy is 300 keV.

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(at a rate of 0.5 �C/min) down to room temperature. Theeffect of the cooling rate has been highlighted previouslyand is important [30]. This image shows that the mean dro-plet diameter increases with annealing temperature due tothe high liquid Au–Ge mobility on Ge(111). At any anneal-ing temperature, the bright part of the droplets exposeswell-defined facets, randomly oriented from one dropletto the other, associated with the crystallization of the topof the droplets. Fig. 2 also shows that for the highestannealing temperatures (650 �C and above), the droplet sizebecomes as large as 500 nm. They then exhibit cracks,which are associated with the limited Ge diffusion lengthin Au during temperature decrease and Au and Ge phaseseparation (Fig. 2g and h). These cracks are also observedfor Au–Si droplets [39,45].

Most interesting is the pedestal or puddle formed belowthe bright part of the droplets. This pedestal is observed forboth binary Au–Si and Au–Ge droplets formed by dewet-ting a thin Au layer [30,39,45], whatever the annealing tem-perature is and thus we believe that the following discussionworks for both types of droplets, with or without cracks.Note also that the presence of this pedestal could havesome consequences on the measurement of contact angles,as sometimes used to characterize the wetting properties.The pedestal is attributed to Ge precipitation during tem-perature decrease. A close attention is paid here to this ped-estal. Fig. 2 shows that the pedestal overall shape does notchange significantly upon increasing the annealing tempera-ture or the droplet size. This holds also for the ratiobetween the bright part of the image (Au) and the darkestone (Ge pedestal) of the droplets.

Fig. 3 shows a TEM image of a single droplet, which isalso observed at a higher resolution in Fig. 4. This dropletis formed by annealing an Au layer at 400 �C for 2 h andcooled down slowly (at a rate of 0.5 �C/min) to room

temperature. The droplet sits on a pedestal, in agreementwith SEM images. A dark line separates the pedestal andthe substrate and is associated with the wetting layerformed during the dewetting process, a d-layer, as statedpreviously. Thus, Fig. 3 clearly shows the d-layer (darkline), the pedestal and the droplet itself. The pedestalextends laterally over more than 100 nm for a dropletdiameter of about 40 nm. Fig. 4 shows the high-resolutioncross-section TEM image, with the electron beam alignedwith the [1–10] plane. This reveals that the pedestal is crys-tallized and in epitaxy, with a crystal orientation parallel tothe substrate crystal. A detail in inset shows that the lattice

Fig. 4. A high-resolution cross-section TEM image of the droplet shown in Fig. 3, with the electron beam aligned with the [1–10] plane. The electronbeam energy is 300 keV.

H. Zitouni et al. / Acta Materialia 90 (2015) 310–317 313

parameter along the [111] direction is that of the Ge(111),within HRTEM resolution. This is very important since itshows that the pedestal is of Ge. Indeed, the solubility ofAu in Ge, which is less than 10�6 at.%, is very low[35,40]. This low solubility prevents from the formationof Au–Ge alloys with the same crystallographic structureas diamond Ge, except for the d-layer formed at theGeepitaxial droplet/Gesubstrate. The scales labeled A and Bin Fig. 4 also show that the pedestal lattice is in registrywith that of the Ge(111) substrate. This is quite surprisingsince it is located on top of the d-layer, which is supposed tohave a distorted lattice, due to a small Au incorporation(one Au monolayer). This suggests that the d-layer isordered enough to allow Ge epitaxy on top of it. Also sur-prising is that the d-layer still persists below the Ge pedes-tal. Indeed, the d-layer is formed on top of a clean Ge(111)to lower the surface energy, the Au solubility in Ge beingvery low [52]. It was found that the

p3 � p3 R30� surface

superstructure, responsible for the d-layer, has a remark-able stability versus annealing temperature, up to the melt-ing point of Ge [52]. Note that this d-layer could have an

important role in the dewetting process itself since it isformed at low temperature, well below TE and that Audewetting occurs even below TE. In other words, one couldexpect that the Au monolayer segregates on top of thepedestal. The persistence of the d-layer at the pedestal/substrate interface makes that it acts as a marker todiscriminate the substrate and the pedestal contributionto the TEM image. Alternatively, this spatial configurationcould be stabilized by a slight Au incorporation in the ped-estal, in the form of a dilute Ge(Au) metastable alloy, asrecently observed at the Au seed/Ge nanowire interfaceby Sutter et al. [41]. Note although the difference betweenthe present data and that of Ref. [41]. In the present experi-ments, the Au droplets sit on a large 2D (Ge(111) surface)instead of 1D (Ge nanowire) Ge reservoir. The incorpo-rated (climbing the liquidus–solidus line) and thus thereleased (going down the liquidus–solidus line) Ge amountis not limited by Ge diffusion through a nanowire. At anyrate, the Au concentration in Ge would be very small.

Also interesting is the image of the droplet itself. Indeed,as suggested by a SEM image in Fig. 2b, it is composed of a

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vertically-oriented base and polycrystalline Au on top of it.A gray-scale sketch of this droplet is shown as an insert inFig. 2b. The vertically-oriented base is a small nanowire.Nevertheless, it is hard to clearly identify the interfacebetween the droplet base and the pedestal due to the depthof field of the TEM in Fig. 4. This seems more visible inFig. 3, as a darkest thin line indicated by the arrow L. Inparticular, the lower-right part of the droplet, between dot-ted lines F and G in Fig. 4, seems hidden by the pedestal (inregistry with the substrate as shown by the scale labeled B).The lower-left part of the droplet seems visible and it is alsocrystallized (zone C), with [111] orientation along that ofthe substrate. The lattice parameter along the [111] direc-tion is quite the same as that in Ge(111) and in the pedestal.This small zone is out of phase, as evidenced by the lines Dand E. As argued for the pedestal, this small nanowire is ofGe. It is out of registry with the Ge(111) crystal but the lat-tice parameter is the same as that of the substrate, within theTEM resolution. This is a direct evidence of the formationof an epitaxial Ge nanowire precursor formed without sup-ply of external Ge material, by using solid or gas sources.These observations suggest that two types of nanocrystalsare formed: a small Ge nanocrystal just below the Au dotand a Ge puddle or pedestal which extends laterally onthe substrate, more exactly on the d-layer, both inducedby precipitation from the melt Au–Ge seed.

These measurements show that the dewetting process ofa pure Au layer seems rather complex. The Ge below theAu droplet arises from two concomitant phenomena, atleast. One is the Ge incorporation in the Au melt droplet,with a Ge precipitation upon decreasing the annealingtemperature below TE, and the other is the wetting of thed-layer by pure Ge which results in the formation of a ped-estal. In contrast with what occurs at the Ge nanowire/Audroplet interface [31,33,38], a competition between wettingproperties and one dimensional growth has to be consid-ered. The d wetting layer has a dominant role here, proba-bly since it acts as a passivation layer, owing to the highstability of the

p3 � p3 R30� superstructure [52]. In the

Ge dissolution/Ge precipitation process, the excess Geserves not only to elongate the nanowire precursor but alsoto lower the interface energy between the nanowire and thesubstrate. The dark d-line in the TEM image in Fig. 3 is areminiscence of the bare Ge surface after dewetting. Itindicates then the location of the substrate/droplet inter-face. The Ge diffuses through it to form the pedestal, as evi-denced by the depression under the Au–Ge droplet. At alarger scale, this large Ge atom displacement makes theinterface rough, as drawn by the dark d-line in TEM image.

Furthermore, we can see that the amount of precipitatedGe seems large compared to that expected from Au–Gephase separation (bulk phase diagram). Indeed, the Geamount would be about 30% at. Ge of the Au + Ge vol-ume, at odd with that is observed in Figs. 3 and 4. Anestimation of the Ge to Au ratio, by using SEM andTEM images, is: the volume of Au dot (assuming that isof pure Au) represents 38% of the total volume, the volumeof the small Ge nanowire (assuming that is of pure Ge)represents 8% of the total volume and the volume of thepedestal (assuming that is of pure Ge) represents 54% ofthe total volume. Thus, the Ge amount is at least 62% ofthe total volume. Note that if there is still Ge in the Audot, this amount would be larger. This is more than twicethat is expected from the bulk Au-Ge phase diagram (27at.% Ge). Although this conclusion could be moderated

by an experimental error by determining a compositionfrom SEM and HRTEM images, the composition is solarge that it could reflect a real physical effect. The mod-ification of the Au–Ge composition for melt Au–Ge dro-plets has already been studied by Sutter et al. [31,32]. Intheir works, Sutter et al. have nicely demonstrated thatAu droplets on top of Ge nanowires can incorporate morethan 27 at.% Ge and have proposed a modified experimen-tal phase diagram. This phase diagram is sensitive to thenanowire diameter, and then to the Au–Ge catalyst volumeon top of it. No significant variation of the Au–Gecomposition is observed for nanowires with a diameter lar-ger than 50 nm. For smaller Ge nanowire diameters, theAu–Ge composition increases up to 40 at.% Ge for a nano-wire diameter of 32 nm. Moreover, Sutter et al. have shownthat the overall liquidus line is modified versus nanowirediameter. Similar conclusions have been drawn from theo-retical predictions for nano-sized binary alloys [35,42].Nevertheless, a quantitative comparison between whatoccurs on top of the nanowires and on a two-dimensionalGe surface is impossible. As stated above, the Au dropletssit on a large 2D (Ge(111) surface) instead of 1D (Ge nano-wire) Ge reservoir in Refs. [31,32].

These measurements suggest that the bulk phase dia-gram could be modified for that nanoscaled system in inter-action with a substrate (non equilibrium configuration).Furthermore, the occurrence of a specific phase diagramat a nanoscale could also reflect in the formation of cracksfor Au–Ge droplets wider than 200 nm. These cracks couldalso be attributed to the agglomeration or coalescence ofsmall droplets which can incorporate more Ge than thatexpected from bulk phase diagram. The droplets becomelarge enough to reach a size for which the bulk phase dia-gram works. Note also that these cracks appear at a lowerdroplet size for Au–Ge than for Au–Si, for which theyappear at a few microns on Si(111) [39,45]. This makes thatprocess highly spatially localized, even at annealing tem-peratures as high as 650 �C.

The Au-Ge interaction at the interface results in aninteresting counterpart in macroscopic electrical behavior.Indeed, a quite linear I(V) characteristic is observed atthe Au droplet/Ge(111) interface. The electric propertiesof this Au/Ge/Ge(111) interface are investigated by con-ductive AFM as well as by 4-points I–V measurements.The purpose of the following paragraph is to take a macro-scopic characterization of the Aucrystal/Gepedestal/Ge(111)system. Upon using macroscopic tips for I–V measure-ments on such dispersed Au crystallites, we had to be surethat the current passes through them and not directly fromGe(111) to the tips. Thus, we have done conductive AFMmeasurements on that surface. These measurements consistin the measurement of a current I induced through an AFMtip in “contact” with the surface at a given bias voltage V.These measurements have been done on a sample exposedto air after growth, before macroscopic I(V) measurements.Within this procedure, one would expect that the “bare”surface between the droplet is oxidized enough and thennon-conductive. In conductive AFM, the tip radius is wellbelow the mean Au crystallite diameter and then we canestimate the conductive or non conductive areas of the sur-face. The tip is scanned across the surface, as shown inFig. 5a and b, and we measure the surface topography aswell as a current image, at a given bias voltage. Fig. 5ashows a topographic z (x,y) image and Fig. 5b shows acurrent I (x,y) image.

Fig. 5. (a) Topography AFM image of Au-Ge crystallites and (b) conductive AFM current image.

Fig. 6. Room temperature measurement of 4-points I–V curves for Audroplets formed at 500 �C.

H. Zitouni et al. / Acta Materialia 90 (2015) 310–317 315

Fig. 5b clearly shows that, upon applying a bias voltageof 100 mV, the current through the Au crystallites is a feworders of magnitude larger than that through the “bare”surface. Thus, one can expect that the I–V curves measuredwith macroscopic tips also reflect the contribution of theAucrystal/Gepedestal/Ge(111) interface only. I(V) measure-ments performed on the Au droplets are shown in Fig. 6.This figure shows raw data in the voltage gap [�1V; 1V].The current is shown to vary linearly with bias voltage,as expected for a perfect ohmic contact. The linear varia-tion of I versus V for pure deposited Au is in line withrecent measurements performed by Kishore et al. [51].

4. Conclusion

In conclusion, we have shown that a large amount of Gecan be moved and can be located on selected places of asurface of Ge(111), over a very large temperature range,from 360 �C up to 650 �C. Upon decreasing the sampletemperature down to the room temperature, even at alow rate, the extra Ge does not spread again over the wholeGe(111) surface, each Au dot and nanowire precursor act-ing as a nucleation center. This pins the extra Ge at the Audroplet place. The main result suggested by these experi-ments is that unusual and interesting binary (and probablyternary) phase diagrams can be drawn at a nanoscale. This

opens the way to a new metallurgy at such a scale[29,34,48,49,56]. The universality of the occurrence of thisprocess has to be tested and could be encouraged by thepresent results.

Acknowledgment

This work was supported by the French National ResearchAgency Contract No. ANR Blanc 2008 Mag2Wires. C.P thanksDr. J.L. Bubendorff for AFM measurements.

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