Microstructural evolution of Ge/Si(1 0 0) nanoscale islands

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Journal of Crystal Growth 259 (2003) 232–244

*Corresp

Departmen

versity, Te

fax: +1-48

E-mail1Curren

Blvd., Cha2Curren

Phoenix, A

0022-0248/

doi:10.101

Microstructural evolution of Ge/Si(1 0 0) nanoscale islands

David J. Smitha,b,*, D. Chandrasekharb,1, S.A. Chaparroc,2, P.A. Crozierb,J. Druckera,c, M. Floydb, M.R. McCartneyb, Y. Zhanga,c

aDepartment of Physics and Astronomy, Arizona State University, Tempe, AZ 85287-1504, USAbCenter for Solid State Science, Department of Physics and Astronomy, Arizona State University, Tempe, AZ 85287-1704, USA

cMaterials Research Institute, University of Texas at El Paso, El Paso, TX 79968-0515, USA

Received 9 May 2003; accepted 14 July 2003

Communicated by K.H. Ploog

Abstract

This paper reports our detailed investigation of the microstructural evolution of Ge/Si(1 0 0) nanoscale islands grown

by deposition of pure Ge onto Si(1 0 0) substrates using molecular beam epitaxy. Substrate temperatures during growth

were varied in the range 400pTp700�C, although we have mostly concentrated our attention here on studying samples

grown at T ¼ 550�C, 650�C and 700�C. Atomic-force microscopy was first used ex situ to document the shape and size

evolution of the Ge/Si(1 0 0) islands as a function of the growth conditions (Ge coverage, substrate temperature). A

range of transmission electron microscopy techniques including energy-dispersive X-ray spectroscopy and electron

energy-loss spectroscopy was then used to further investigate the local microstructure and composition of the islands.

Substantial Si diffusion into the Ge islands was measured, and trenches extending well into the Si substrate were also

observed at the bases of larger clusters grown with TX550�C. Plan-view imaging of selected samples using both bright-

field and dark-field imaging modes identified the critical size for dislocation formation. Cross-sectional high-resolution

imaging enabled the strain-relieving dislocations to be identified, and also confirmed the multi-facetted and often

asymmetrical shapes of larger islands.

r 2003 Elsevier B.V. All rights reserved.

Keywords: A1. Characterization; A1. Defects; A1. Diffusion; A1. Nanostructures; A3. Molecular beam epitaxy; B1. Germanium

silicon alloys

onding author. Center for Solid State Science,

t of Physics and Astronomy, Arizona State Uni-

mpe, AZ 85287-1704, USA. Tel.: +1-480-965-4540;

0-965-9004.

address: [email protected] (D.J. Smith).

t address: Intel Corporation, 5000 W. Chandler

ndler, AZ 85226, USA.

t address: Lytek Corporation, 4717 E. Hilton Ave.,

Z 85034, USA.

$ - see front matter r 2003 Elsevier B.V. All rights reserve

6/j.jcrysgro.2003.07.025

1. Introduction

The initial stages of Ge/Si heteroepitaxy con-tinue to attract much interest, both from afundamental growth viewpoint and also becauseof potential device applications for Ge/Si self-assembled quantum dots (SAQDs) of uniform sizeand shape. Ge initially wets Si because of its lowersurface energy, but due to the Ge/Si latticemismatch of 4.2%, a layer-to-island transition

d.

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D.J. Smith et al. / Journal of Crystal Growth 259 (2003) 232–244 233

(Stranski–Krastanov growth mode) occurs afterthe deposition of B3 Ge monolayers (where 1monolayer (ML) corresponds to one atomic layerwith 6.78� 1014 Ge atoms/cm2 on the Si(1 0 0)surface). Early studies have concentrated on theGe/Si(1 0 0) system [1–3], and further work (see,for example, Refs. [4–10]) has led to a progres-sively better knowledge and understanding of thecomplex evolution of island morphology as afunction of Ge coverage, yGe; growth temperature,T ; and annealing treatment. Dislocation forma-tion, as well as complicated shape evolution as afunction of island size, provide effective pathwaysfor relieving mismatch strain [8–10]. It is alsorecognized that slow Ge deposition rates allowGe/Si islands to adopt the height, width anddistribution of dislocations required to minimizetheir energy [4]. However, Ge/Si intermixing[9,11,12] and trench formation around islandperimeters [9,12–15] have also been shown torepresent alternative strain-relief mechanisms thatare activated at higher growth temperatures.Transmission electron microscopy (TEM) meth-ods have been indispensable in the process ofdocumenting these various microstructural fea-tures. We describe here our TEM investigationsinto the microstructural evolution of Ge/Si(1 0 0)nanoscale islands, grown by molecular beamepitaxy (MBE), as a function of Ge coverage anddeposition temperature, with particular emphasisbeing given to comparing sample sets deposited at550�C, 650�C, and 700�C.

2. Experimental details

The samples were grown in a standard MBEchamber, equipped with facilities for Augerelectron spectroscopy (AES), reflection high-en-ergy electron diffraction (RHEED) and residualgas analysis. The base pressure of the systemwas in the low 10�10 Torr range, rising to thelow 10�9 Torr range during Ge deposition. AKnudsen-type effusion cell was used for depositionof pure Ge. The samples were grown on /1 0 0Sp-type B-doped Si wafers. These substrates werefirst degassed in situ, followed by flash-heatingseveral times to B1200�C for oxide desorption

until AES spectra confirmed the absence of oxygenand carbon contaminants on the Si surface, andsharp 2� 1 RHEED spots indicative of surfacereconstruction were obtained. All samples re-ported here were grown at Ge deposition rates of1.4ML/min, with Ge coverages ranging from 3.5to 14ML in 1.5ML increments.

The sample morphology was analyzed bytapping-mode atomic-force microscopy (AFM)using a Digital Instruments Nanoscope IIIascanning probe microscope. Cross-sectional ana-lysis of the Ge islands was performed by AFM todetermine facet planes and facet contact angles,based on the geometrical alignment of the islandsrelative to the known crystallography of theSi(1 0 0) substrate. Island size distributions werequantified by analyzing sufficiently large sampleareas to ensure adequate statistics. Samples forplan-view TEM imaging were prepared by me-chanical polishing and dimpling from the back-side of the Si wafer, followed by ion-milling at2–3 keV with low ion-beam incidence angles in therange of 4–6� until electron transparency wasreached. Cross-sectional samples were usuallyprepared by slicing thin sections that were thenglued together face-to-face before mechanicalpolishing, dimpling and ion-milling to achieveperforation. Some samples grown at T ¼ 400�Cwere prepared using wedge polishing to avoid anypossibility that Si re-deposition on the islandsurfaces could cause any systematic error incomposition measurement. The evolution of islandmicrostructure was followed using a 400-keVJEM-4000EX high-resolution TEM having aninterpretable image resolution of 0.17 nm. A 200-keV Philips CM200 field-emission-gun (FEG)TEM was used for energy-dispersive X-ray spec-troscopy (EDXS), and a 100-keV Vacuum Gen-erators HB501 STEM was used for electronenergy-loss spectroscopy (EELS).

3. Results

3.1. Size and shape distribution

The initial step in our comprehensive analysis ofthe island evolution was to establish the size and

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D.J. Smith et al. / Journal of Crystal Growth 259 (2003) 232–244234

shape distributions as a function of Ge coverageand substrate temperature during growth usingAFM: many of these measurements are describedin detail elsewhere [10]. As an example of theinformation available from AFM, Fig. 1 showstwo typical AFM micrographs from a set ofsamples that was deposited at a substrate tem-perature of 600�C. The first image shows the highconcentration of small islands present in a samplehaving an equivalent Ge coverage of 8.0ML. Thesecond lower magnification micrograph, takenfrom the sample with a Ge coverage of 12.5ML,demonstrates the substantial increase in thenumber density of larger islands that occurs asthe coverage is increased. Analysis of AFM imagessimilar to those in Fig. 1 produced the island sizedistributions.

0.25µm

1µm

(a)

(b)

Fig. 1. Amplitude-mode AFM micrographs showing the

typical distribution of Ge/Si(1 0 0) islands grown at 600�C with

a growth rate of 1.4ML/min: (a) 8ML of Ge; (b) 12.5ML of

Ge. Note high density of small hut clusters visible in (a).

Fig. 2 compares the island size distributions forGe coverages over the range of 5–12.5ML and forsubstrate temperatures during growth of 400�Cand 650�C. For each deposition temperature, theleft-most peak is populated by what are commonlyreferred to as ‘‘hut clusters’’ [3]. These arerectangular-based islands which are bound by{1 0 5} facets that are inclined at an angle ofB11� to the substrate surface. The right-mostpeak is populated by octagonal-based domeclusters which are generally bound by steeper{2 1 0} and {3 1 1} facets, although even steeperfacets are observed at the bases of the larger domes[10]. Note that the position of the peak diameterfor the smaller huts tends to remain unchangedindependent of growth temperature, whereas thelarger dome peak shifts considerably to muchlarger diameters as the growth temperature isincreased. The lateral shift of this dome peakdiameter is associated with a trend towardsincreased Ge–Si alloying within islands grown athigher temperatures [16]. This Si interdiffusion hasbeen confirmed using EDXS [11] and EELS [16].We also observe a tail in the size distribution

Fig. 2. Island size distributions for indicated growth tempera-

tures and coverages.

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0

50

100

150

200

250

300

-5051015

GeSi

Inte

gra

ted

Inte

nsi

ty (

cou

nts

)

distance (nm)

(b)

25 nm

(a)

Fig. 3. (a) Annular-dark-field STEM image of Ge/Si dome

cluster grown at 600�C and (b) corresponding Si-K and Ge-L

EDXS profiles from along indicated line. Note continuation of

Si-K signal into Ge cluster indicative of Si diffusion.


extending to very large diameters. Plan-view TEManalysis, as described further below, confirmedthat all islands having diameters greater than thelarger diameter edge of the dome peak containedgrowth defects such as dislocations and {1 1 1}-type stacking faults, whereas the smaller islandsgenerally remained coherent. Thus, the island sizeat which it becomes energetically favorable for theisland to dislocate increases with the growthtemperature, which we have interpreted as evi-dence for Si/Ge intermixing in islands grown athigher temperature [16]. At higher growth tem-peratures, we also note the appearance of a distinctthird peak located in between the hut and domepeaks. Islands from within this peak are similar tothe smaller huts in that they are usually boundby {1 0 5} facets. The existence of these largerhut clusters has also been attributed to Ge/Sialloying [11].

3.2. Compositional analysis

The techniques of EDXS and EELS, whencoupled with the sub-nanometer probe size of aFEG-TEM, offer the possibility of obtainingdirect quantification of elemental compositionand distribution within individual islands atspatial resolutions closely approaching the atomicscale. Fig. 3(a) is an annular-dark-field electronmicrograph showing part of the cross-section of alarge dome cluster grown at 600�C. The almostvertical line of white contrast corresponds to theapproximate position of the Ge wetting layer onthe Si substrate surface. Energy-dispersive X-rayspectroscopy measurements of the Si-K and Ge-Ledges were acquired along the indicated line, andthe corresponding counts as a function of probeposition are shown plotted in Fig. 3(b). The Ge-Lsignal drops rapidly to background noise levels assoon as the probe position crosses the Ge/Siinterface into the Si substrate. Conversely, it issignificant that the Si-K signal remains well abovebackground levels across the entire island. TheEDX spectra of this study established the im-portant result that substantial amounts of Si musthave diffused into the Ge island [11]. This Siintermixing was attributed to enhancement ofinterfacial strain energy at the cluster edges.

Because of difficulties associated with EDXsignal calibration for the edge-on sample geome-try, the EELS technique is preferable for extract-ing quantitative compositional information [17]. Acomprehensive EELS analysis of islands that weregrown at temperatures over the range from 400�C

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20nm

L C R

Nominal Si/Geinterface

RCL

0

0.2

0.4

0.6

0.8

1.0

-5 0 5 10 15

x (G

e)

d (nm)

(a)

(b)

Fig. 5. (a) Annular-dark-field STEM image of Ge/Si dome

cluster grown at 650�C, where lines labelled L (left edge), C

(center) and R (right edge) indicate positions of probe scans

used for EELS analysis; (b) Corresponding EELS profiles

showing uniformity of Ge–Si composition within the island

independent of lateral probe position.


to 700�C has recently been completed [16]. The useof a Si/Ge alloy layer of known compositionenabled accurate quantification of the local islandcomposition to be achieved. Considerable broad-ening of the Ge/Si interfacial transition regionfrom B1 nm up to B5 nm was measured as thegrowth temperature was increased. As shown inFig. 4, the Si concentration within the islandsincreased continuously as the temperature wasincreased, to reach levels of greater than 50% atour highest growth temperature of 700�C.Furthermore, as shown by the example in Fig. 5,our EELS measurements also revealed the surpris-ing result that the compositions within coherentislands were vertically and laterally homogeneousaway from an interdiffused interfacial region, atleast to within the experimental error of ourmeasurements. This vertical composition homo-geneity has recently been confirmed by grazing-incidence X-ray scattering investigations [18,19].Similar trends of enhanced intermixing at highergrowth temperature have also been documentedusing other non-localized techniques such as X-rayphotoemission spectroscopy [20]. This alloying ofthe Ge/Si islands serves to reduce the substrate/island misfit, hence lowering the island strainenergy, and it provides a rationale for explainingthe changes in island size and morphology that areobserved at higher deposition temperatures. Con-versely, however, the likelihood of this intermixingoccurring during Ge/Si island growth must always

Fig. 4. Plot showing variation of Ge island concentration with

growth temperature, as derived from quantitative EELS

analysis.

be taken into account when future optical andelectronic applications of Ge/Si SAQDs are beingconsidered.

Our studies of island composition have involvedGe/Si samples grown by MBE over a range ofdeposition temperatures and Ge coverages. How-ever, the growth rate of 1.4ML/min is relativelyslow compared with that used by some otherworkers [20,21], thus possibly accounting for thedifferent results that have been obtained. Forexample, using the technique of ultrahigh-vacuumchemical-vapor deposition (UHV-CVD) with agrowth rate of 10–20ML/min, Ge/Si intermixingwas only initiated at growth temperatures above550�C [20], unlike our studies where the onset ofintermixing was found to occur at close to 450�C.Similarly, incoherent islands grown by MBE at700�C with a growth rate of 0.02 nm/s (i.e., 1.2 nm/min) showed many perfect dislocations at theGe/Si interface, as well as the formation of 30�

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partial misfit dislocations at the island surface [21].These results again differ from our findings, andthe observations of other workers [6], where thegrowth rates used were substantially less.

3.3. Microstructure

Plan-view TEM images of Ge/Si(1 0 0) islandsare usually obtained with the [1 0 0] zone axis ofthe substrate aligned closely parallel to the incidentelectron beam direction. As a representativeexample, Fig. 6 shows a plan-view, bright-field(BF) electron micrograph recorded from thesample with 11ML of Ge deposited at a substratetemperature of 550�C. A small objective aperturehas been used to emphasize diffraction contrasteffects. Note the highly characteristic ‘‘double-cross’’ feature which is a strain-contrast effect thatis caused by local bending of lattice planes in orderto relax strain within the coherent Ge islands[1,7,22]. Variations in the separation of thecoupled bars of the cross-like contrast, reportedmost recently in coherent Ge islands with typicalsizes of B90–100 nm grown at 700�C, have beeninterpreted in terms of Ge–Si composition gradi-ents [23]. However, it has also been demonstratedthat strain will have a major influence on this typeof Ge island contrast [24]. Any variations in strainacross the islands are likely to seriously affect theappearance and interpretation of the cross-likecontrast. It has been suggested that alternative

10nm

Fig. 6. Plan-view BF electron micrograph showing typical

island contrast for 11ML of Ge grown at 550�C. Note bimodal

size distribution.

imaging approaches will be required to differenti-ate reliably between these competing effects ofcomposition and strain [25].

Dislocation introduction drastically lowers theisland chemical potential, causing growth accel-eration at the coherent-to-incoherent transition[2,26]. This growth acceleration significantlybroadens the distribution of island sizes. Inaddition to this deleterious effect on islandensemble morphology, these defects may produceunwanted optical and electronic effects. TEM hasproven indispensable in establishing the sizes atwhich it is energetically favorable for coherentislands to dislocate as well as identifying the natureof these defects [4,6]. Diffraction-contrast, dark-field (DF), imaging of plan-view samples isparticularly useful for imaging dislocations inthese islands. Fig. 7 shows an [0 2 2] DF imagefrom a Ge/Si(1 0 0) sample that was deposited at asubstrate temperature of 450�C with a nominal Gecoverage of 11ML. A high density of smallpyramid clusters (B20–30 nm in diameter) isclearly visible, in addition to a much lower densityof considerably larger domes. Note the character-istic black-white lobe contrast of the smallerpyramids, and the much more complex appearanceof the larger domes shown at higher magnificationin Fig. 7(b), where the finely spaced lines of brightcontrast are indicative of the presence of strain-relieving 90� edge dislocations [27]. A furtherperpendicular set of edge dislocations would bevisible using an orthogonal reflection.

Cross-sectional TEM imaging with the electronbeam aligned parallel to the plane of the substrate/epilayer interface, is widely used for defectcharacterization. For the Ge/Si system, thisgeometry is valuable for confirming island shapesas measured by AFM, which are often uncertaindue to the finite tip-radius convolution effects.Cross-sectional TEM images of Ge/Si(1 0 0) is-lands are primarily obtained using the [0 1 1]projection of the Si substrate as a referenceorientation, thereby ensuring that the Ge/Si inter-face normal is aligned perpendicular to theelectron beam direction. Although the numberdensity of islands is not very high in this viewinggeometry because of the limited sampling area,island profiles are easily visible, and it is usually

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Fig. 7. (a) Plan-view DF electron micrograph showing island

distribution in sample grown at 450�C with nominal Ge

coverage of 11ML. (b) Line contrast visible in larger islands

at higher magnification is indicative of strain-relieving

dislocations.

5nm

10nm

(a)

(b)

Fig. 8. Cross-sectional high-resolution electron micrographs of

Ge/Si islands from sample with 14ML Ge grown at 400�C.

Note {1 1 1}-type stacking defects shown arrowed in (b).


possible to document the types of defects that arepresent in the Ge islands. In some cases, however,because of the nature of the sample cross-section-ing procedure, the substrate is locally much thickerthan the island, and dynamical diffraction effectscan then make it difficult to interpret reliably the

image contrast in the region of the substrate/islandinterface. Fig. 8 shows a pair of high-resolutioncross-sectional images from the sample with14ML of Ge deposited at 400�C. The very small(diameter dB25 nm) island shown in Fig. 8(a) hasno visible defects, which is what would be expectedfor an island having a size less than the larger-diameter edge of the coherent dome peak. Anexample of a dislocated dome cluster is shown inFig. 8(b). This larger (dB65 nm) island has several{1 1 1} stacking faults extending through to theisland surface.

As the growth temperature for Ge depositionwas increased, the maximum size for defect-freeisland formation increased markedly. The micro-graphs in Fig. 9 show a striking example of a large(dB125 nm) coherent island from the samplegrown with 12.5ML of Ge at 700�C. In this case,there are no visible defects either at the substrate/island interface or within the island itself. Theseresults are consistent with an interpretation ofincreased Si interdiffusion and island alloyingreducing the island-substrate misfit.

The formation of trenches at the bases of largeGe islands becomes more prevalent at highergrowth temperatures [12–15,28,29], and this typeof morphology has been attributed to diffusion ofhighly strained material towards regions of lowerstrain. Detailed profile analysis using tapping-

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Fig. 9. (a) Cross-sectional TEM micrograph of large Ge/Si

island from sample with 12.5ML Ge grown at 700�C. No

defects are visible within the island nor at the nominal Ge/Si

interface (arrowed) shown enlarged in (b).

5nm

Fig. 10. Cross-sectional TEM micrograph showing part of a

large dome grown at 600�C with 9.5ML Ge coverage. Note

growth defects within dome and the extensive trench into the Si

substrate at the dome edge.

10nm

20nm

(a)

(b)

Fig. 11. Cross-sectional high-resolution electron micrographs

of Ge/Si islands from sample with 8.0ML Ge grown at 550�C.

The smaller island is coherent with the substrate and defect-

free, whereas the larger island shows several {1 1 1}-type

stacking defects originating at the Ge/Si interface. Also note

the strain beneath the island, including a short {1 1 1} defect.


mode AFM has been used to establish a lineardependence of the trench depth on the island basewidth [13]. Fig. 10 is a BF electron micrographshowing part of the cross-section of a large domecluster grown at 600�C with 9.5ML of Ge. Inaddition to growth defects visible within theisland, the deep trench at the island edge is clearlyevident.

3.3.1. Growth at 550�C

The island size and shape distributions forgrowth at 550�C, again determined by AFMprofile analysis, are summarized elsewhere [10].Deposition at 550�C represents an intermediarygrowth regime where both sessile Lomer edgedislocations and glissile 60� partial dislocationsmight be expected to accommodate misfit strain atthe Ge/Si interface [6,27,30]. Our [0 1 1] cross-section observations indicate that the latter type ofdefect, which is not as effective as the Lomer-edge

dislocation at relieving misfit strain, is far moreprevalent for MBE growth at this temperature.Fig. 11 compares high-resolution profile images oftwo islands grown with a Ge coverage of 8.0ML.The smaller coherent island in Fig. 11(a), with alateral extent ofB40 nm, shows no growth defects,whereas the larger island in Fig. 11(b), measuring

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50nm

10nm(a)

(b)

Fig. 12. Cross-sectional high-resolution electron micrographs

of Ge/Si islands from sample with 14ML Ge grown at 550�C.

(a) small island with {1 1 1}-type stacking defects (arrowed); (b)

Larger island with complex profile and trench at right-hand

edge of island.


about 75 nm across, contains several {1 1 1} stack-ing faults that appear to be initiated at thesubstrate/island interface. Strain beneath the latterisland is also evident, including a short {1 1 1}stacking fault (arrowed).

Fig. 12 compares two islands grown with a Gecoverage of 14ML. The smaller island in Fig.12(a), with a lateral extent of B65 nm, shows three{1 1 1}-type stacking-fault defects (arrowed) origi-nating from the nominal Ge/Si interface. Thelarger dome cluster shown in Fig. 12(b) has a morecomplicated profile, with very steep boundingfacets, and many internal {1 1 1} stacking defectsagain extending all the way from the substrate/island interface to the island surface. Note also thetrench evident at the right-hand edge of the island.Our cross-section observations of other largedome clusters indicated that asymmetric profilesin the /1 1 0S projection were a common occur-rence. These large islands are highly defective, andthe presence of these defects is likely to alter thesymmetry of their strain distribution and affecttheir morphology.

3.3.2. Growth at 700�C

An extensive set of samples was grown at 700�C,and size distributions have been determined as afunction of Ge coverages ranging from 5.0 to14.0ML [31]. Qualitatively, we find that the islandevolution is different from growth at lower

temperature. Small clusters are still observed, butwith contact angles shallower than 11�. IncreasedGe coverage leads to a marked lateral shift tolarger sizes and broadening of the size distributionpeak populated by dome clusters. Fig. 13 sum-marizes the overall facet evolution as a function ofdiameter for Ge islands grown at 700�C. Transi-tion structures [10,14,32,33] having shapes inter-mediate between pyramids and domes areobserved for diameters, d; in the range90odo145 nm, mostly coherent dome clustersexist in the approximate range 145odo210 nm,and only dislocated dome clusters are found atlarger diameters. Using EELS analysis, we alsodiscovered that the coherent dome clusters had anaverage composition of Si0.59Ge0.41, independentof their size [16].

Our plan-view and cross-section TEM observa-tions were consistent with these findings. Inparticular, the coherent dome clusters were gen-erally defect-free and considerably larger thanthose deposited at lower temperature with similarGe coverage. Growth defects were rarely seen,except in the very largest islands (dB210 nm),which is to be expected considering the reducedlattice mismatch between the Si substrate and thealloyed Ge-Si islands. Fig. 14 is a BF plan-viewimage which shows several of the larger islandsfrom the sample with a Ge coverage of 6.5ML,where the increased island size, both laterally andvertically, results in a far more complex imageappearance than normally observed in the smallerislands deposited at lower deposition tempera-tures. Fig. 15 is a BF diffraction contrast imagefrom the sample with 9.5-ML Ge coverage,showing the cross section of an even largerdislocated island. This cluster has a markedlyasymmetrical profile with several {1 1 1} stackingfaults, and a steeper facet contact angle, corre-sponding to {1 1 1}, at the left-hand point ofcontact with the substrate. AFM analyses ofsamples grown at 700�C has also established theappearance of this type of steep faceting for verylarge dislocated islands [31].

3.3.3. Growth and annealing at 650�C

Significant changes in island morphology resultfrom in situ annealing at the growth temperature

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Fig. 13. Summary of facet evolution for Ge islands grown at 700�C.

Fig. 14. Plan-view BF image of islands in sample grown at

700�C with Ge coverage of 6.5ML.

50nm

Fig. 15. Cross-section of Ge-Si island from sample with Ge

coverage of 9.5ML grown at 700�C.

5nm

10nm

(a)

(b)

Fig. 16. High-resolution cross-sectional electron micrographs

from samples grown at 650�C: (a) small hut cluster from 6.5-

ML sample showing B11� contact angle with Si substrate

surface; (b) dome cluster from 8.0-ML sample showing steeper

faceting and asymmetrical profile. Also note infrequent example

of short {1 1 1} stacking defect in substrate underneath island.


[9], and our detailed AFM analysis of islandevolution upon annealing for temperatures in therange of 450–650�C is reported elsewhere [34].Here we concentrate our attention briefly on

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Fig. 17. Samples grown at 650�C with Ge coverages of 9.5-ML:

(a) plan-view BF image from sample without annealing; (b)

cross-section BF image from sample annealed for 30min; (c)

plan-view [2 2 0] DF image from sample annealed for 120min.


samples grown and annealed at 650�C, and havingGe coverages of 6.5, 8.0 and 9.5ML. Fig. 16 showshigh-resolution cross-section images from sampleswithout any annealing. The small (dB40 nm)alloyed pyramid cluster in Fig. 16(a) is from the6.5-ML sample and shows the typical B11�

contact angle expected for this small cluster size[3,35]. The dome cluster in Fig. 16(b) originatesfrom the 8.0-ML sample, and shows the steeper

faceting and asymmetrical profile often observedin larger islands. Also note the short {1 1 1}stacking-fault defects (arrowed) extending down-wards into the Si substrate. These type of defectwere only observed infrequently, as reportedpreviously [28].

Fig. 17 shows representative islands fromsamples with Ge coverage of 9.5ML. Fig. 17(a)is a plan-view BF image from the sample withoutannealing, showing small coherent pyramids andlarger dislocated domes. Fig. 17(b) is a BFdiffraction contrast image in profile geometryfrom the sample annealed for 30min, which alsodisplays strain contrast beneath the island. Finally,Fig. 17(c) is a plan-view [2 2 0] DF image showingseveral large, dislocated islands from the sampleannealed for 120min. These observations of islandmorphology after annealing are consistent with theresults of AFM studies (see, for example, Refs.[9,34]), which showed a significant island coarsen-ing (Ostwald ripening) due to annealing at thegrowth temperature T ¼ 650�C.

4. Discussion

This comprehensive microstructural study basedon electron microscopy has revealed many differ-ent aspects of Ge/Si(1 0 0) nanoscale island mor-phology and evolution. Plan-view observationsconfirmed that the critical cluster size for disloca-tion formation was temperature-dependent due tothe increased Si/Ge alloying that occurred athigher growth temperature. Cross-sectional TEMimaging enabled the nature of strain-relievingdislocations to be identified as well as demonstrat-ing that larger dislocated islands had complicatedmulti-facetted shapes. Novel strain-relief mechan-isms were found to be activated at higher growthtemperatures. Trenches were observed around thebases of the larger dome clusters extending into theSi substrate. Substantial Si diffusion into the Geislands, especially at higher growth temperatures,was also documented using small-probe micro-analysis. Si interdiffusion and trench formationserves as alternative pathways for misfit strainrelief that are activated at higher growth tempera-tures [11].

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Identifying growth conditions for the Ge/SiSAQD system that are best suited for ensuringsize and composition uniformity throughout thedot ensemble is of paramount importance. Evolu-tion of island morphology during molecular beamepitaxy is influenced via the following experimen-tally accessible parameters: substrate temperature,deposition rate, Ge coverage and post-growthannealing. Our detailed investigations usingTEM and other techniques suggest the followingavenues for obtaining the highly uniform islandensembles that are required for technologicalapplications. First, to ensure formation of themost uniform ensembles, the areal density ofdislocated clusters must be minimized. It is well-known that the reduction in chemical potential ofa cluster subsequent to dislocation formationcauses its growth rate to accelerate [2,26]. Ourobservations indicate that the density of dislocatedclusters can be minimized by limiting the Gecoverage for any substrate temperature/depositionrate pair. We have also observed that highersubstrate temperatures result in narrower islandsize distributions [10,11,36], an effect which hasbeen attributed to the existence of a kineticattachment barrier that increases with island size[37,38]. Growth at these higher temperatures isaccompanied by increased Si interdiffusion as wellas the formation of trenches at some of the clusterbases. Either of these effects will alter the opticaland electronic properties of the dots. It may bepossible to exploit this kinetic attachment barrierwhile avoiding Si interdiffusion or trench forma-tion by growing at lower substrate temperatureswith reduced deposition rates. Whether or not thisstrategy will be effective in producing more uniformisland ensembles without altering the optical andelectronic properties will depend on the relativerates of deposition, surface diffusion and Siinterdiffusion through their activation energies.

Several authors have suggested that there maybe a minimum in the island free energy whichcould lead to very uniform island ensembles[39–41]. Post-growth annealing of the islandensemble should cause the islands to seek this freeenergy minimum if it existed. The results presentedabove in Section 3.3.3, and recently discussedelsewhere in more detail [34], seem to rule out this

possiblity. Our results indicate that regardless of thegrowth conditions, the island ensembles will con-tinuously coarsen during post-growth annealing.

Perhaps the most promising avenue for obtain-ing island ensembles which simultaneously satisfythe stringent periodicity and uniformity require-ments is to self-assemble the islands on alithographically patterned substrate. Several stra-tegies have been demonstrated that could achievesuch ordered arrays [42–45]. For example, oxidepatterning plus etching to the Si(1 0 0) substratesurface could produce regularly spaced and well-aligned Ge island arrays located along the ridgesof Si mesa stripes [43]. Alternatively, carefulcontrol over the lithographic patterning couldlead to well-aligned arrays of coherent Ge islandshaving sizes in the range of a few tens ofnanometers [44]. For periodic island arrays, eachisland would collect diffusing Ge adatoms fromidentical substrate areas, which could then lead todramatic reduction in island size dispersion.Further optimization of growth parameters isrequired to fully exploit self-assembly on patternedsubstrates. Detailed TEM microstructural investi-gations should contribute significantly to a betterunderstanding of the ordering process.

Finally, it is well recognized that kinetic effectsplay an important role in determining islandevolution, and thus growth rate has receivedincreased attention recently as an importantadditional parameter that can be used to affect is-land size, shape and spatial distribution [20,46–48].For example, growth rates of up to 110ML/minusing UHV-CVD growth at 600�C led to sub-stantially increased island density and a markednarrowing of island size distribution [47]. Highgrowth rates using MBE at 600�C, but withcarefully controlled Ge coverages, resulted in pro-nounced island ordering along /100S directions[48]. Microstructural studies as well as compositionalanalysis as a function of growth rate are still neededbut have yet to be reported in any detail.

Acknowledgements

This work was supported by NSF Grants DMR9496296, DMR-9724305, DMR-9804310 and

ARTICLE IN PRESS


DMR-0196018. The electron microscopy wasperformed at the Center for High ResolutionElectron Microscopy at Arizona State University.

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Documents

Microstructural evolution of Ge/Si(1 0 0) nanoscale islands