Journal ofMaterials Chemistry C

PAPER

Publ

ishe

d on

10

Oct

ober

201

3. D

ownl

oade

d by

Uni

vers

ity o

f Il

linoi

s at

Chi

cago

on

22/1

0/20

14 0

1:11

:08.

View Article OnlineView Journal | View Issue

State Key Laboratory of Information Photon

University of Posts and Telecommunica

xzhang@bupt.edu.cn

Cite this: J. Mater. Chem. C, 2013, 1,7914

Received 6th September 2013Accepted 9th October 2013

DOI: 10.1039/c3tc31758g

www.rsc.org/MaterialsC

7914 | J. Mater. Chem. C, 2013, 1, 79

Realization of Stranski–Krastanow InAs quantum dotson nanowire-based InGaAs nanoshells

Xin Yan, Xia Zhang,* Xiaomin Ren, Junshuai Li, Jiangong Cui, Liang Li, Sijia Wang,Qi Wang and Yongqing Huang

We investigate the formation and optical properties of InAs quantum dots on an InGaAs nanosubstrate.

We find that Stranski–Krastanow InAs quantum dots are hardly formed on Au-catalyzed InGaAs

nanowires due to the phase separation as well as stacking faults. High-quality Stranski–Krastanow InAs

quantum dots are realized on a pure zinc blende InGaAs shell radially grown on a GaAs nanowire core.

The quantum dots have a large size and regular shape, residing on a wetting layer of several

nanometers. For optical characterization, we fabricate a “dot-in-well” structure by capping the quantum

dots with InGaAs/GaAs double layers. Photoluminescence from the quantum dots is observed at 77 K,

with a peak wavelength of 954 nm, which is distinctly redshifted compared with that of InAs quantum

dots directly grown on GaAs nanowires. This work shows the potential of growing Stranski–Krastanow

QDs on more types of NWs and obtaining longer wavelengths for wider applications.

Introduction

Semiconductor nanowires (NWs) have shown great potentialas building blocks for future optoelectronic devices such aslasers, light emitting diodes (LEDs) and solar cells.1–3 Thanksto the one-dimensional geometry, NWs can be used to fabri-cate various heterostructures, which are attractive forachieving devices with improved functionality. In recent years,a hybrid structure fabricated by growing quantum dots (QDs)on the surface of NWs has attracted much attention.4 Due tothe excellent optical properties of QDs, the structure isexpected to have enhanced performance and more functionscompared with homogeneous NWs. To date, this structurehas been realized in several materials.5–11 Some of themhave exhibited excellent properties, showing a promisingprospect in single photon sources and intermediate-bandsolar cells.11,12

Stranski–Krastanow (S–K) QDs have a number of advantagessuch as high quality, high density and good optical properties.13

High-performance lasers and solar cells based on planar S–KQDs have been widely reported.14,15 In the hybrid structurementioned above, the S–K growth has been only realized inInGaAs QD–GaAs NW and Ge QD–Si NW systems, and both theInGaAs and Si QDs have high quality and regularmorphology.5,7,10 Particularly, the S–K InGaAs QDs on GaAs NWsexhibit strong photoluminescence.5,7 However, the wavelengthis limited due to a large strain between GaAs and InAs. To

ics and Optical Communications, Beijing

tions, Beijing 100876, China. E-mail:

14–7919

obtain longer emission wavelength for wider applications, aneffective way is to use a smaller lattice-mismatched substrate,e.g. InGaAs or InP. However, there have been no reports of S–KInGaAs QDs on InGaAs or InP NWs to date. Kawaguchi et al.have grown InAs QDs on wurtzite (WZ) InP NWs by introducingstacking faults (SFs).8 Heiss et al. have reported the formation ofInGaAs QDs on catalyst-free InGaAs NWs.9 These QDs typicallyhave a lot of stacking faults (SFs) and irregular morphology,which are considered to be formed by inhomogeneous accu-mulation of adatoms rather than strain-driven S–K mode. TheSFs and irregular morphology are considered to have a badeffect on the optical performance of QDs such as spectralbroadening.8

Another difficulty in achieving InAs QDs on InGaAs NWs isthe phase separation. As has been reported recently, a sponta-neous core–shell structure is formed during the growth of Au-catalyzed InGaAs NWs, with an In-poor core and an In-richshell, which is attributed to competitive alloying of differentgroup-III elements with Au catalysts.16 The In-rich shell isunfavorable for the QD formation due to relatively small latticemismatch.

In this work, we investigate the formation of InAs QDson InGaAs nanosubstrates. We nd that S–K InAs quantumdots are hardly formed on Au-catalyzed InGaAs NWs. Plentyof S–K InAs quantum dots are obtained on a GaAs NW-based InGaAs nanoshell, with a large size, regular shape andmainly zinc blende (ZB) structure. For optical characteriza-tion, we fabricate a NW-based “dot-in-well (DWELL)” struc-ture. Photoluminescence from the QDs is obtained at 77 K,with an emission wavelength of 954 nm and linewidth of46 meV.

This journal is ª The Royal Society of Chemistry 2013

Paper Journal of Materials Chemistry C

Publ

ishe

d on

10

Oct

ober

201

3. D

ownl

oade

d by

Uni

vers

ity o

f Il

linoi

s at

Chi

cago

on

22/1

0/20

14 0

1:11

:08.

View Article Online

Experimental detail and characterization

The growth was performed by using a Thomas Swan CCS-MOCVD system at a pressure of 100 Torr. Trimethylgallium(TMGa), trimethylindium (TMIn) and arsine (AsH3) were usedas precursors. The carrier was hydrogen. For InAs QDs onInGaAs NWs, Au-catalyzed InGaAs NWs were rstly grown on aGaAs (111)B substrate at 420 �C for 600 s, and the ow rates ofTMIn, TMGa and AsH3 were 5.44 mmol min�1, 72.4 mmolmin�1 and 2232 mmol min�1. Then InAs was deposited at475 �C for 70 s, with a TMIn ow rate of 10.7 mmol min�1 andTMGa ow rate of 111.6 mmol min�1. For InAs QDs on GaAsNW-based InGaAs shells, Au-catalyzed GaAs NW cores wererstly grown on a GaAs (111)B substrate at 440 �C for 600 s. AnInGaAs shell was subsequently grown at 500 �C for 100 s. Theow rates of TMIn, TMGa and AsH3 were set to 1.35 mmolmin�1, 5.40 mmol min�1 and 625 mmol min�1, respectively.Then InAs QDs were grown on the InGaAs shell at 475 �C for140 s, with a TMIn ow rate of 10.7 mmol min�1 and AsH3 owrate of 111.6 mmol min�1.

The morphological, structural, and compositional charac-teristics of as-grown NWs were characterized by eld emissionscanning electron microscopy (FESEM) and transmissionelectron microscopy (TEM) equipped with X-ray energydispersive spectroscopy (EDS). Individual NWs for TEM obser-vations were prepared by ultrasonicating the samples inethanol for 5 min, followed by spreading drops from thesuspension onto a holey carbon–Cu grid. The remaining NWroots on the ultrasonicated substrate were used for top-viewSEM characterization. For optical characterization, as-grownNWs were mechanically cut down and dispersed onto a SiO2-coated Si substrate. The microphotoluminescence (mPL)measurements were carried out using a 632.8 nm continuous-wave He–Ne laser for excitation. The excitation beam wasfocused onto �2 mm in diameter with a � 50 microscopeobjective on the sample placed in a cryostat. The emissionthrough the same microscope objective was detected by thecombination of a grating spectrometer and a silicon charge-coupled device (CCD).

Fig. 1 (a) TEM image of a single InGaAs NW. (b) TEM image of a smooth part of the(a)–(c) are 100 nm.

This journal is ª The Royal Society of Chemistry 2013

InAs QDs on InGaAs NWs

Fig. 1(a) shows the TEM image of a pure InGaAs NW. The NW istapered, which is commonly observed in Au-catalyzed InGaAsNWs.17 High-density of SFs are observed along the entire NW,which may be attributed to an enhanced uctuation in thesupersaturation when two group-III elements are incorporatedinto the droplets.18 The lower part of the NW shows a saw-toothfaceting, while the upper part is relatively smooth, as shown inFig. 1(b) and (c). A EDS line scan along the NW width shows anincrease of the In/Ga ratio at the edge, indicating an inhomo-geneous In distribution. This is consistent with a recent reportthat a spontaneous core–shell structure is formed during thegrowth of Au-catalyzed InGaAs NWs, with an In-poor core andan In-rich shell, which is attributed to competitive alloying ofdifferent group-III elements with Au catalysts.16 Due to thediffusion mechanism, the shell decreases along the NW,resulting in a tapered morphology and a relatively smoothupper part.17 The saw-tooth structures on the lower part are alsoobserved in GaAs NWs, which are considered to be related to therandomly formed rotation twins.19

Fig. 2 shows the characterization results of a single InGaAsNW aer depositing InAs. The Au downward movement indi-cates that some of the In atoms were incorporated into thecatalyst during the InAs deposition.20 Similar to the pure InGaAsNWs, the lower part of the NW also contains continuous bumps.A EDS line scan shows a much higher In content at the edge,indicating that some of InAs are deposited onto the originalsaw-tooth structures. Few isolated islands are observed on therelatively smooth area near the NW top, as shown in Fig. 2(d)and (e). The QD is considered to be formed only by the subse-quent InAs deposition as it is not associated with twins. Theshape of the QD is irregular. No specic facets and wetting layerare observed. Thus the QD is not considered to be formed bystrain-driven S–Kmode. The formation is probably caused by anunclear adatom nucleation priority as has been reported in InAsQDs on InP NWs.8 The difficulty of S–K QDs on InGaAs NWs ismainly attributed to two factors. One is the phase separationduring the growth of Au-catalyzed InGaAs NWs, resulting in an

NW. (c) TEM image of a rough part of the NW. (d) EDS line scan results. Scale bars in

J. Mater. Chem. C, 2013, 1, 7914–7919 | 7915

Fig. 2 (a) TEM image of a single InGaAs NW after depositing InAs. (b) and (c)TEM and EDS of the lower part of the NW, respectively. (d) TEM image of the toppart of the NW. (e) HRTEM of a single QD at the NW top. The scale bar in (a), (b)and (d) is 100 nm. The scale bar in (e) is 10 nm.

Journal of Materials Chemistry C Paper

Publ

ishe

d on

10

Oct

ober

201

3. D

ownl

oade

d by

Uni

vers

ity o

f Il

linoi

s at

Chi

cago

on

22/1

0/20

14 0

1:11

:08.

View Article Online

In-rich shell which hinders the formation of QDs. The other isthe droplet-induced SFs, which are unfavorable for the QDformation as the WZ/ZB facet typically has a lower surfaceenergy than pure ZB.21 In addition, the rough surface of thelower part may hinder the diffusion of In adatoms towards thetop, which reduces the In supply for the QDs.

S–K InAs QDs on GaAs NW-based InGaAsnanoshells

Now we turn to study the formation of S–K InAs QDs on GaAsNW-based InGaAs nanoshells. Fig. 3(a) schematically shows thefabrication process. Fig. 3(b) shows the cross-section SEMimage of a single NW with a diameter of about 200 nm. We canclearly nd a lot of QDs on the NW surface. A thin bright layer isobserved between the QDs and the NW, which is attributed to

Fig. 3 (a) Schematic of the fabrication process of InAs QDs on GaAs NW-based InGshell encapsulating the GaAs NW core. (3) Growth of InAs QDs. (b) Cross-section SEMThe inset shows the top-view SEM image of pure GaAs NWs for comparison. Scale

7916 | J. Mater. Chem. C, 2013, 1, 7914–7919

the InGaAs shell. Fig. 3(c) shows the top-view SEM image of aNW root. The NW cross-section exhibits asymmetric polygonmorphology, which is different from that of a pure GaAs NWthat commonly has a regular hexagonal shape, suggesting anonuniform growth of InGaAs on the GaAs NW. As reportedpreviously, InAs preferentially grows on the {112}A sidewalls ofGaAs NWs, resulting in a truncated triangular cross-section.22

Thus in Fig. 3(c), the three longer {112} type facets indicated as(1-21), (11-2) and (-211) belong to {112}B facets on which InGaAsgrows slowly, while other shorter {112} facets belong to {112}Afacets on which InGaAs grows rapidly. Similarly, the InAs QDswere also found to preferentially grow on {112}A sidewalls of theInGaAs shell (e.g. (-1-12) facet) while they rarely grew on {112}B(e.g. (1-21) facet) due to different surface characteristics between{112}A and {112}B. Besides, the NW also consists of some {110}type facets, which are attributed to a high temperature growthof the InGaAs shell. The transformation from {112} to {110} athigh temperature is also observed in GaAs–AlInP and GaP–GaAscore–shell NWs, which is related to different surface energiesbetween the two types of facets.23,24 These {110} facets areunfavorable for the QD formation due to the low surface energyand may be eliminated in future by improving the growthconditions.25

The composition characteristics of a single NW are shown inFig. 4. From Fig. 4(a), a 20 nm thick InGaAs shell can bedistinguished easily due to the different brightness intensityfrom that of the GaAs core. Fig. 4(b) shows the EDS line scantaken along the NW width. The GaAs core, InGaAs shell andInAs QD can be clearly distinguished from the intensity changesof In and Ga elements. Fig. 4(c)–(e) show the EDS point analysisof positions 1, 2 and 3 in (a), corresponding to the QD, InGaAsshell and GaAs core, respectively. The high Ga content in QDs isprobably due to an In–Ga intermixing during the growth.26 TheInGaAs shell has an In composition of around 15%, which ismuch lower than that in the unexpectedly formed In-richInGaAs shell during the growth of InGaAs NWs and is favorablefor the QD formation due to a large lattice mismatch with InAs.

To obtain the characteristics of the structure, TEM investi-gations were conducted on single NWs. Fig. 5(a) shows a lowmagnication TEM image of a short NW section with a QD. The

aAs shells: (1) Au-catalyzed growth of the GaAs NW core. (2) Growth of an InGaAsimage of a single NW. (c) Top-view SEM image of a single NW root cross-section.

bars in (b) and (c) are 100 nm.

This journal is ª The Royal Society of Chemistry 2013

Fig. 4 (a) Scanning TEM (STEM) image of a single QD. The scale bar is 50 nm. (b) EDS line scan results of Ga and In elements. (c)–(e) EDS from detection positions 1, 2and 3 in (a), corresponding to the InAs QD, InGaAs shell and GaAs NW, respectively.

Fig. 5 (a) Low magnification TEM image of a NW section with a QD. The whitearrows indicate the interface between the QD and NW. The scale bar is 50 nm.Inset (1) shows the enlarged image of the wetting layer, while inset (2) shows thewetting layer of InAs QDs directly grown on a GaAs NW for comparison. The scalebar in the insets is 5 nm. (b) HRTEM of the InGaAs shell and the wetting layer. Theinset shows the corresponding FFT. The scale bar is 5 nm. (c) TEM of a part of theQD in (a). The arrows indicate the (1-1-1) and (-1-1-1) type SFs. The scale bar is5 nm. The inset shows the corresponding FFT. (d) TEM image of a larger QD. Thescale bar is 100 nm. The inset shows the HRTEM of a part of the QD. The arrowsindicate the (1-1-1) and (-1-1-1) type SFs. The scale bar in the inset is 5 nm.

Paper Journal of Materials Chemistry C

Publ

ishe

d on

10

Oct

ober

201

3. D

ownl

oade

d by

Uni

vers

ity o

f Il

linoi

s at

Chi

cago

on

22/1

0/20

14 0

1:11

:08.

View Article Online

distinct interface between the QD and NW indicates an epitaxialrelationship between the two structures. A thin wetting layerwith a thickness of about 3 nm is observed between the InGaAsshell and InAs QDs, indicating an S–K growth mode of QDs. Thewetting layer is a little thicker than that of InAs QDs directlygrown on a GaAs NW, which is due to a reduced strain stored inthe epitaxial layer. Fig. 5(b) shows the corresponding highresolution TEM (HRTEM) of the InGaAs shell and the wettinglayer. The InGaAs shell exhibits a perfect ZB structure, without

This journal is ª The Royal Society of Chemistry 2013

SFs or mist dislocations. As has been reported, InGaAs NWsgrown by a vapor–liquid–solid (VLS) growth process typicallyhave a lot of SFs,9,17 which is attributed to an enhanced uctu-ation in the supersaturation when two group-III elements areincorporated into the droplets.18 As the InGaAs shell in thisexperiment is formed by vapor-phase deposition, droplet-induced SFs are avoided. The high crystal quality of the InGaAsshell is also related to a small lattice mismatch between GaAsand InGaAs, as well as a suitable thickness. Based on the theoryproposed by J. W. Matthews and A. E. Blakeslee, the criticalthickness of In0.15Ga0.85As on GaAs is beyond 20 nm, which isthicker than the InGaAs shell in this experiment.26 In addition,the small dimension of NW may further increase the crystalthickness since the NW can share more strain energy.27,28

Two QDs with different size and morphology were investi-gated by TEM. Fig. 5(a) shows a smaller QD, with a length of130 nm and a height of 35 nm. This is much larger than thatdirectly grown on GaAs NWs, indicating a sufficient strainreduction by the InGaAs shell.29 The QD edges are faceted alongspecic surfaces, which are determined as (1-1-1), (0-1-1) and(-1-1-2). The morphology is a little different from that on GaAsNWs,7 probably due to different surface properties betweenInGaAs and GaAs. Fig. 5(c) shows the HRTEM image of a part ofthe QD in (a). The QD exhibits mainly ZB structure with someSFs. Fig. 5(d) shows another larger QD, with a very high lengthof 190 nm and a smaller height of 25 nm. Besides, the QD has along at (11-2) top facet other than (0-1-1). One probable reasonfor the shape transition is that the QD prefers to adopt anoptimal shape to minimize the energy. As studied theoreticallyby J. Tersoff and R. M. Tromp, the long thin shape allows betterelastic relaxation of the stress, which lowers the energy of thelarger QD.30

NW-based DWELL structure and its opticalproperties

The DWELL structure fabricated by placing QDs in a strainedQW has been widely studied in planar QD lasers as it canincrease the emission wavelength as well as improve thethreshold current density performance.31 As the NW has much

J. Mater. Chem. C, 2013, 1, 7914–7919 | 7917

Fig. 6 (a) Schematic of the NW-based “DWELL” structure for optical characterization. (b) PL of the NW-based “DWELL” structure at 77 K. The spectra are fitted by threeLorentzian line shapes. The inset shows the schematic of the conduction band structures for the NW-based DWELL structure and InAs QDs directly grown on GaAs NWs.

Journal of Materials Chemistry C Paper

Publ

ishe

d on

10

Oct

ober

201

3. D

ownl

oade

d by

Uni

vers

ity o

f Il

linoi

s at

Chi

cago

on

22/1

0/20

14 0

1:11

:08.

View Article Online

higher strain tolerance, it is possible to realize DWELL struc-tures on NWs with a wider QW composition range. To date,there have been no reports on NW-based DWELLs. Here, a NW-based DWELL was fabricated by capping the as-grown QDs withan InGaAs layer followed by a GaAs layer. The InGaAs cappinglayer was grown at 475 �C for 100 s, with the same In compo-sition as the InGaAs shell grown before. The schematic of thestructure is presented in Fig. 6(a).

Fig. 6(b) shows the corresponding PL spectra at 77 K. Threepeaks can be observed from the spectra, corresponding to theGaAs NW, InGaAs QW and InAs QD, respectively. The InGaAsQW has a peak wavelength of 912 nm, corresponding to an Incomposition of around 10%.32 The InGaAs peak has a broadlinewidth of 70 meV, which can be attributed to an inhomoge-neous In composition distribution along the NW.17 The QDshave a peak wavelength of 954 nm and a linewidth of 46 meV.Compared with the emission of InGaAs QDs directly grown onGaAs NWs,10,12,33 the wavelength is distinctly redshied (nearly70 nm), indicating that the InGaAs shell and capping layersufficiently reduce the strain. The linewidth is a little larger thanthat of InAs QDs on planar GaAs/InGaAs, as well as InAs QDs onGaAs NWs.33–35 This may be attributed to enhanced size andmorphology uctuations of QDs as a result of inhomogeneouscomposition and thickness of an InGaAs QW along the NW dueto adatom diffusion.17 In addition, the adatom diffusionmechanism of InAs QDs on NWs may also increase the sizeuctuation compared with that of QDs on a planar substrate.7

However, the linewidth here is much narrower than that of non-strain-driven InAs QDs on InP NWs, suggesting a higher qualityand better optical properties of S–K QDs.8

Summary

In conclusion, we have studied the formation of InAs QDs onternary InGaAs nanosubstrates. We nd that S–K InAs QDs aredifficult to be formed on Au-catalyzed InGaAs NWs, which isattributed to the phase separation as well as high-density of SFsinduced by the droplet. By growing a high-quality, pure ZBInGaAs shell on the GaAs NW, S–K InAs QDs are obtained, witha large size, regular shape and ZB structure. For optical char-acterization, we fabricate a DWELL structure by capping theQDs with InGaAs/GaAs double layers. The QDs have an emis-sion wavelength of 954 nm at 77 K, which is distinctly redshiedrelative to that of InGaAs QDs directly grown on GaAs NWs,

7918 | J. Mater. Chem. C, 2013, 1, 7914–7919

suggesting a sufficient strain reduction caused by the InGaAsQW. The linewidth is much narrower than that of non-strain-driven InAs QDs on InP NWs, suggesting a higher quality andbetter optical properties of S–K QDs. These results pave the wayfor the realization of S–K QDs on more types of NWs as well asobtaining longer wavelength for wider applications in theinfrared eld.

Acknowledgements

This work was supported by the National Basic ResearchProgram of China (2010CB327600), the National NaturalScience Foundation of China (61020106007, 61211120195,61077049 and 61376019), the International Science & Tech-nology Cooperation Program of China (2011DFR11010), theSpecialized Research Fund for the Doctoral Program of HigherEducation (20120005110011), the 111 Program of China(B07005), BUPT Excellent PhD Students Foundation (CX201213)and Fund of State Key Laboratory of Information Photonics andOptical Communications (Beijing University of Posts and Tele-communications), P. R. China.

References

1 R. Chen, T. D. Tran, K. W. Ng, W. S. Ko, L. C. Chuang,F. G. Sedgwick and C. Chang-Hasnain, Nat. Photonics,2011, 5, 170.

2 A. C. Scoeld, A. Lin, J. N. Shapiro, P. N. Senanayake,G. Mariani, M. Haddad, B. L. Liang and D. L. Huffaker,Appl. Phys. Lett., 2012, 101, 053111.

3 J. Wallentin, N. Anttu, D. Asoli, M. Huffman, I. Aberg,M. H. Magnusson, G. Siefer, P. Fuss-Kailuweit, F. Dimroth,B. Witzigmann, H. Q. Xu, L. Samuelson, K. Deppert andM. T. Borgstrom, Science, 2013, 339, 1057.

4 M. de la Mata, X. Zhou, F. Furtmayr, J. Teubert, S. Gradecak,M. Eickhoff, A. Fontcuberta i Morral and J. Arbiol, J. Mater.Chem. C, 2013, 1, 4300.

5 L. Pan, K. Lew, J. M. Redwing and E. C. Dickey, Nano Lett.,2005, 5, 1081.

6 D. G. Ramlan, S. J. May, J. Zheng, J. E. Allen, B. W. Wesselsand L. J. Lauhon, Nano Lett., 2006, 6, 50.

7 X. Yan, X. Zhang, X. Ren, H. Huang, J. Guo, X. Guo, M. Liu,Q. Wang, S. Cai and Y. Huang, Nano Lett., 2011, 11, 3941.

This journal is ª The Royal Society of Chemistry 2013

Paper Journal of Materials Chemistry C

Publ

ishe

d on

10

Oct

ober

201

3. D

ownl

oade

d by

Uni

vers

ity o

f Il

linoi

s at

Chi

cago

on

22/1

0/20

14 0

1:11

:08.

View Article Online

8 K. Kawaguchi, M. Heurlin, D. Lindgren, M. T. Borgstrom,M. Ek and L. Samuelson, Appl. Phys. Lett., 2011, 99, 131915.

9 M. Heiss, B. Ketterer, E. Uccelli, J. R. Morante, J. Arbiol andA. Fontcuberta i Morral, Nanotechnology, 2011, 22, 195601.

10 X. Yan, X. Zhang, X. Ren, J. Li, X. Lv, Q. Wang and Y. Huang,Appl. Phys. Lett., 2012, 101, 023106.

11 M. Heiss, Y. Fontana, A. Gustafsson, G. Wust, C. Magen,D. D. O'Regan, J. W. Luo, B. Ketterer, S. Conesa-Boj,A. V. Kuhlmann, J. Houel, E. Russo-Averchi, J. R. Morante,M. Cantoni, N. Marzari, J. Arbiol, A. Zunger,R. J. Warburton and A. Fontcuberta i Morral, Nat. Mater.,2013, 12, 439.

12 E. Uccelli, J. Arbiol, J. R. Morante and A. Fontcuberta iMorral, ACS Nano, 2010, 10, 5985.

13 W. Seifert, N. Carlsson, M. Miller, M. Pistol, L. Samuelsonand L. R. Wallenberg, Prog. Cryst. Growth Charact., 1996,33, 423.

14 G. Park, Z. Zou, O. B. Shcheki and D. G. Deppe, Appl. Phys.Lett., 1998, 73, 2564.

15 D. Guimard, R. Morihara, D. Bordel, K. Tanabe,Y. Wakayama, M. Nishioka and Y. Arakawa, Appl. Phys.Lett., 2010, 96, 203507.

16 Y. Kim, H. J. Joyce, Q. Gao, H. H. Tan, C. Jagadish,M. Paladugu, J. Zou and A. A. Suvorova, Nano Lett., 2006, 6,599.

17 D. L. Dheeraj, A. M. Munshi, M. Scheffler, A. T. J. Helvoort,H. Weman and B. O. Fimland, Nanotechnology, 2013, 24,015601.

18 Y. Guo, H. Xu, G. J. Auchterlonie, T. Burgess, H. J. Joyce,Q. Gao, H. H. Tan, C. Jagadish, H. Shu, X. Chen, W. Lu,Y. Kim and J. Zou, Nano Lett., 2013, 13, 643.

19 J. Zou, M. Paladugu, H. Wang, G. J. Auchterlonie, Y. Guo,Y. Kim, Q. Gao, H. J. Joyce, H. H. Tan and C. Jagadish,Small, 2007, 3, 389.

This journal is ª The Royal Society of Chemistry 2013

20 M. Paladugu, J. Zou, Y. Guo, G. J. Auchterlonie, H. J. Joyce,Q. Gao, H. H. Tan, C. Jagadish and Y. Kim, Small, 2007, 3,1873.

21 T. Yamashita, T. Akiyama, K. Nakamura and T. Ito, Jpn. J.Appl. Phys., 2010, 49, 055003.

22 M. Paladugu, J. Zou, Y. Guo, X. Zhang, H. J. Joyce, Q. Gao,H. H. Tan, C. Jagadish and Y. Kim, Appl. Phys. Lett., 2008,93, 201908.

23 N. Skold, J. B. Wagner, G. Karlsson, T. Hernan, W. Seifert,M. Pistol and L. Samuelson, Nano Lett., 2006, 6, 2743.

24 G. Zhang, K. Tateno, T. Sogawa and H. Nakano, Appl. Phys.Express, 2008, 1, 064003.

25 D. J. Chadi, J. Vac. Sci. Technol., B, 1985, 3, 1167.26 P. B. Joyce, T. J. Krzyzewski, G. R. Bell, B. A. Joyce and

T. S. Jones, Phys. Rev. B: Condens. Matter Mater. Phys.,1998, 58, R15982.

27 J. W. Matthews and A. E. Blakeslee, J. Cryst. Growth, 1974, 27,118.

28 Y. H. Lo, Appl. Phys. Lett., 1991, 59, 2311.29 V. M. Ustinov, N. A. Maleev, A. E. Zhukov, A. R. Kovsh,

A. Yu. Egorov, A. V. Lunev, B. V. Volovik, I. L. Krestnikov,Yu. G. Musikhin, P. S. Kopev, Zh. I. Alferov, N. N. Ledentsovand D. Bimberg, Appl. Phys. Lett., 1999, 74, 2815.

30 J. Tersoff and R. M. Tromp, Phys. Rev. Lett., 1993, 70, 2782.31 A. Stintz, G. T. Liu, H. Li, L. F. Lester and K. J. Malloy, IEEE

Photonics Technol. Lett., 2000, 12, 591.32 R. E. Nahory, M. A. Pollack, W. D. Johnston and R. L. Barns,

Appl. Phys. Lett., 1978, 33, 659.33 X. Yan, X. Zhang, X. Ren, X. Lv, J. Li, Q. Wang, S. Wei and

Y. Huang, Nano Lett., 2012, 12, 1851.34 G. T. Liu, A. Stintz, H. Li, K. J. Malloy and L. F. Lester,

Electron. Lett., 1999, 35, 14.35 J. Tatebayashi, M. Nishioka and Y. Arakawa, Appl. Phys. Lett.,

2001, 78, 3469.

J. Mater. Chem. C, 2013, 1, 7914–7919 | 7919