InAs quantum dots on GaAs()BT. Suzuki, Y. Temko, M. C. Xu, and K. Jacobi Citation: J. Appl. Phys. 96, 6398 (2004); doi: 10.1063/1.1811387 View online: http://dx.doi.org/10.1063/1.1811387 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v96/i11 Published by the American Institute of Physics. Related ArticlesPlanar arrays of magnetic nanocrystals embedded in GaN Appl. Phys. Lett. 101, 081911 (2012) Structural, morphological, and magnetic characterization of In1−xMnxAs quantum dots grown by molecular beamepitaxy J. Appl. Phys. 112, 034317 (2012) Subbandgap current collection through the implementation of a doping superlattice solar cell Appl. Phys. Lett. 101, 073901 (2012) Carrier localization in InN/InGaN multiple-quantum wells with high In-content Appl. Phys. Lett. 101, 062109 (2012) Eliminating the fine structure splitting of excitons in self-assembled InAs/GaAs quantum dots via combinedstresses Appl. Phys. Lett. 101, 063114 (2012) Additional information on J. Appl. Phys.Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors

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InAs quantum dots on GaAs „1̄1̄2̄…BT. Suzuki, Y. Temko, M. C. Xu, and K. Jacobia)

Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany

(Received 4 August 2004; accepted 7 September 2004)

InAs quantum dots(QDs) were prepared by molecular beam epitaxy on GaAss1̄1̄2̄dB substrates.Shape and size distribution of the QDs were investigated usingin situ scanning tunnelingmicroscopy as function of preparation temperature between 435 and 550 °C. The wetting layer isnot flat but undulated in submicrometer scale in a similar way as the bare substrate. The atomicstructure of the wetting layer is the same as found for the flat base of InAs QDs grown on

GaAss1̄1̄3̄dB substrates. The shape of the QDs is given by{110}, s1̄1̄1̄dB, and h1̄4̄3̄jB bounding

facets and a round vicinals001̄d region. Unexpectedly, the number density increases and the sizedistribution sharpens, when the growth temperature is increased from 435 to 470 °C, which isattributed to lattice defects incorporated into the QDs during growth at 435 °C. ©2004 AmericanInstitute of Physics. [DOI: 10.1063/1.1811387]

I. INTRODUCTION

High-index GaAs and InAs surfaces have attracted muchinterest in connection with quantum dot(QD) preparation. Arecently reported high-index GaAss2 5 1 1dA surface1–4 andits s137dA subunit form bounding facets on InAs QDs grownon the GaAs(001),5,6 GaAss113dA,6–9 GaAss114dA,10 and

GaAss1̄3̄5̄dB.11 Another high-index GaAss1̄3̄5̄dB surface hasbeen discovered recently, which is facetted into vicinal

s2̄5̄11̄dB surfaces for the bare surface, but becomes flat bydeposition of a small amount of InAs, accompanied by a

cs232d reconstruction.12 The s1̄3̄5̄dB facet forms a flat base

of InAs QDs grown on GaAss1̄1̄3̄dB13,14 and

GaAss2̄5̄11̄dB.15

The GaAss1̄1̄2̄dB surface, which we use as substratehere, has also interesting properties. It contributes to the flat

base of the InAs QDs on GaAss1̄1̄3̄dB, although its surfacestructure is rather disordered.13 InAs or InGaAs depositionon this surface has been reported to induce QDs or quantumdashes formation.16–19 The bare surface structures of the

GaAss1̄1̄2̄dB have been already investigated in our previousstudies.20–24 There are two surface structures, depending onthe preparation temperature. A Ga-rich structure forms above520 °C with an interesting self-organized surface corruga-tion: 12 nm wide stripes form with two{110} side facets. AnAs-rich structure forms below 520 °C that exhibits locally ahighly disordered 231 reconstruction and some undulationon a submicrometer scale.24 Moreover, a complex facetedstructure has been reported near the transitiontemperature.20–23

In the present study, we investigated shape and size dis-

tribution of the InAs QDs grown on GaAss1̄1̄2̄dB. We pre-pared our samples by molecular beam epitaxy(MBE) and

usedin situ scanning tunneling microscopy(STM) for analy-sis.

II. EXPERIMENTAL METHODS

The experiments were carried out in a multichamber ul-trahigh vacuum system consisting of a surface-analysis, aMBE- and an STM-chamber(Park Scientific Instruments,VP2).25 STM tips were clipped from a tungsten wire andcleaned by electron bombardment in the STM chamber.Samples with a typical size of about 5310 mm2 were cut

from a GaAss1̄1̄2̄dB wafer (n type, Si doped, carrier concen-tration 6.33 –34.031017 cm−3, wafer technology). Thesamples were cleaned by several ion bombardment and an-nealing cycles. Afterwards a GaAs buffer layer about 50 nmthick was deposited using MBE at 500–550 °C. After thegrowth, the samples were kept under As2 flux for about10 min. Then the samples were cooled down to435–500 °C, and kept at these temperatures for about10 min under As2 flux. Then InAs was deposited. Thesample heater and the In- and As-Knudsen cells were shutoff, as soon as the reflection high-energy electron diffraction(RHEED) pattern changed from streaky to spotty indicatingQD formation. Then the samples were transferred to theSTM chamber within one minute without breaking vacuum.All STM images were taken at room temperature. The nomi-nal amount of InAs deposited onto the substrate was0.71±0.02 nm, 0.64±0.02 nm, and 0.56±0.12 nm for435 °C, 470 °C, and 500 °C, respectively, at a growth rateof about 0.004–0.007 nm/s. Beam equivalent pressure ratioof As2 to In was 40–50 at an As2 pressure of<4.5310−7 mbar.

III. RESULTS AND DISCUSSION

A. Structure determination during growth

RHEED patterns from the GaAss1̄1̄2̄dB substrate surfaceare shown in Figs. 1(a) and 1(b) before and in Figs. 1(a8) and1(b8) after InAs deposition at 470 °C. The electron beam

a)Author to whom the correspondence should be addressed; electronic mail:Jacobi@fhiberlin.mpg.de

JOURNAL OF APPLIED PHYSICS VOLUME 96, NUMBER 11 1 DECEMBER 2004

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was directed alongf1̄1̄1g for Figs. 1(a) and 1(a8) and along

f11̄0g for Figs. 1(b) and 1(b8). Periodic reflections are seen inFig. 1(a) as indicated by arrowheads. The distance betweenthem corresponds to 0.41 nm in real space, which agreeswith the unit-cell vector length s0.40 nmd of the

GaAss1̄1̄2̄dB surface perpendicular tof1̄1̄1g. In addition,weak half-order reflections are seen—as noted bys1/2,0d inFig. 1(a)—which indicate that the surface has a twofold pe-riodicity in this direction. The reflections are more diffuse in

the f11̄0g direction, as shown in Fig. 1(b), which indicatesthat the surface structure(in real space) is disordered perpen-

dicular tof11̄0g. These RHEED patterns agree with our pre-vious, atomically resolved STM images that exhibited a dis-ordered 231 reconstruction under As-rich preparationconditions.24 At about 520 °C a phase transition occurs be-tween the As rich, disordered 231 reconstruction below anda Ga rich, corrugated structure above 520 °C.24 Thus, allInAs QDs discussed in this contribution were grown on theAs rich, disordered, and 231 reconstructed surface at tem-peratures between 430 and 500 °C.

After InAs deposition, the RHEED pattern changed fromstreaky to spotty as shown in Figs. 1(a8) and 1(b8), whichindicates the formation of QDs in the Stranski-Krastanovgrowth mode. The InAs deposition was stopped as soon asthis change was recognized by eye on the RHEED screen.Fundamental reflections remain, but half-order reflectionsdisappear after this transition, as shown in Fig. 1(a8). A so-called chevron structure is not seen both in Figs. 1(a8) and1(b8). A chevron structure is due to electron reflections fromfacets tilted around an axis, that is an intersection line be-tween the substrate plane and the electron scattering plane,and has been observed for the(137) facets for InAs QDs onGaAs(001).26

B. Overview STM images and size distribution

Figure 2 shows overview STM images after the InAsgrowth at sample temperatures of(a) 435, (b) 470, and(c)500 °C. QDs form at all growth temperatures. The numberdensities are 43109, 331010, and 23109 cm−2 for 435,470, and 500 °C, respectively. The QD formation variesquite unexpectedly with growth temperature. There is a well-known empirical rule that the size of the QDs increases withgrowth temperature and the number density of the QDsdecreases.14,27The results presented here do not comply withthis rule: By raising the temperature from435 °C to 470 °C the number density increases.

Another interesting observation can be made from over-view images such as those presented in Fig. 2: The wettinglayer is not flat but undulated on a submicrometer scale. Themaximum corrugation is about 4 nm. The undulation seemsto reproduce the initial bare surface that is also undulated ona submicrometer scale.24 The atomic structure of the wettinglayer is described later in detail when presenting Fig. 7.

Although there are some reports that InAs deposition on

the s1̄1̄2̄dB surface induces InAs quantum-dash(QDH)formation,16,19 we could not find any evidence for QDHs.Even for growth temperatures up to 550 °C, that is, abovethe transition temperature between the As-rich and the Ga-rich structures of the bare surface, no QDHs but large InAsclusters with ill-defined shapes are formed(not shown here).

Figure 3 shows the size distributions of QDs grown atvarious temperatures. The size is measured at the foot of the

QDs alongf111̄g. The QDs show an unexpected behaviorwith the growth temperature as already mentioned duringpresentation of the overview STM images in Fig. 2. The sizeof the QDs in Fig. 3(a) ranges from 10 to 50 nm with apeak at 20 nm. The sizes are not as small as expected for thelow preparation temperature of 435 °C. Also, the distribution

FIG. 1. RHEED patterns of the GaAss1̄1̄2̄dB surface, before(a, b) and aftersa8 ,b8d InAs deposition at 470 °C.

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is broad with many relatively large QDs, one of which isindicated by an arrowhead in Fig. 2(a). Moreover, the num-ber density is not large. As the growth temperature increasesfrom 435 °C in Fig. 3(a) to 470 °C in Fig. 3(b), the mean

size of the QDs does not increase. The size distribution be-comes much sharper—loosing the tail to larger sizes—andthe number density increases by more than one order of mag-nitude. The remaining very weak tail between 35 and 90 nmis attributed to the relatively small number of the large dots,of which one is marked by an arrowhead in Fig. 2(b). Asmall number of such large QDs have been observed for allQD assembles, which have been studied in our laboratory onGaAs substrates of different orientation so far, and are attrib-uted to dislocated QDs which grow larger due to reducedstrain. The dislocations are thought to be created to reducethe strain.11 As the growth temperature is increased from470 °C in Fig. 3(b) to 500 °C in Fig. 3(c), the changes insize distribution complies with the general rule: The QD size

FIG. 2. Overview STM images after InAs growth at(a) 435 °C,(b) 470 °C,and (c) 500 °C. The sample bias isV=−3 V and the tunneling current isI=0.1 nA. The size of the images is 5003500 nm2.

FIG. 3. Size distribution of InAs QDs grown on GaAss1̄1̄2̄dB at 435 °C(a),470 °C (b), and 500 °C(c).

6400 J. Appl. Phys., Vol. 96, No. 11, 1 December 2004 Suzuki et al.

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increases with the size distribution ranging from30 to 55 nm and a peak at 35 nm. The number density de-creases by nearly one order of magnitude.

C. Atomically resolved shape of InAs quantum dotson GaAs „1̄1̄2̄…B

In Fig. 2 it can be seen already that the QDs are ofdifferent size but of a typical shape at least with respect totheir lateral confinement at the wetting layer. It is very inter-esting to zoom in and to determine the three dimensional(3D) shape of a typical QD. 3D STM images of two QDsof slightly different shape, both grown at 435 °C, are shownin Figs. 4(a) and 4(b). The azimuthal orientation of the QDscould be exactly determined from the atomically resolvedstructure of the wetting layer that is shown below in Fig. 8.

The QDs are mirror symmetric with respect to thes11̄0dplane normal to the surface, which reflects the symmetry ofthe substrate. This proofs epitaxial growth. Interestingly,

some QDs[see Fig. 4(b)] are elongated alongf111̄g. In thecase of thes113dA substrate we could correlate a similarelongation with a faceting of the round part 4 which obvi-ously slowed down the growth speed of the round part alongthis direction.7

The QDs in Fig. 4 are terminated by four facets num-bered 1–4 in Fig. 4(a) The QDs grown at 470 °C exhibitquite a similar shape and are not shown separately. In Fig. 5three QDs of different size are shown which were grown at500 °C. These QDs differ from those shown in Fig. 4 by theadditional facets 5 and 6. Also in this case elongation intodifferent directions is observed.

Typical bounding facets of the QDs of Figs. 4 and 5 aredepicted in Fig. 6. The individual facets were atomically re-solved by STM, which allowed determining the orientationand surface structure of each facet. A high-resolution STMimage of the facet 1 in Fig. 4 is shown in Fig. 6(a). Whitelines mark a rectangular unit cell on the facet. The facet isinclined to the substrate bys29±2d°; the lengths of the unit-cell vectors and the angle between them areu1

=0.54±0.04 nm,u2=0.41±0.02 nm, ands98±3d°, respec-tively. The geometric values are 30°,u1=0.55s0.52d nm, u2

=0.41s0.38d nm, and 98°. Thus, the facet 1 is identified as

s01̄1̄d and facet 2 ass1̄01̄d. This assignment is also valid forthe 500 °C QDs in Fig. 5.

The facet 3 exhibits two different structures as shown inFigs. 6(b) and 6(c). The first structure shown in 6(b) is iden-

tified as s1̄1̄1̄dB-232 reconstruction from the followingdata: The facet is inclined to the substrate bys19±2d° andexhibits a rhombic unit cell as marked in Fig. 6(b). Thelengths of the unit-cell vectors and the angle between themarev1=s0.87±0.07d nm, v2=s0.78±0.04d nm, ands57±3d°,respectively.[Geometric values: 19°,v1=0.86s0.80d nm, v2

=0.83s0.77d nm, 59°] The second structure is shown in Fig.

6(c) and is identified ass1̄1̄1̄dB-sÎ193Î19d reconstructionfrom the ringlike structures on the facet that are0.93±0.07 nm in diameter. The same ringlike structures have

been observed on thes1̄1̄1̄dB facets of InAs QDs grown on

the GaAss1̄1̄3̄dB13 and s1̄3̄5̄dB11 substrates. It is known that

the 232 reconstruction is stabilized at lower temperatureand the less As-richsÎ193Î19d reconstruction at highertemperature.28,29 This agrees with our observation here that

the s1̄1̄1̄dB facet is 232 reconstructed for QDs prepared at435 °C[Fig. 4 and Fig. 6(b)] andsÎ193Î19d reconstructedfor QDs prepared at 500 °C[Fig. 5 and Fig. 6(c)]. Remark-ably, only the smallest QD in Fig. 5(b) is completelysÎ193Î19d reconstructed whereas for the larger QDs this recon-struction exists only near to the wetting layer. For the as-sumption of a temperature gradient across the QD its size iscertainly too small. However, it may also be connected witha reduced flux of In arriving from the wetting layer by dif-fusion at the top.

A high-resolution STM image of the facet 6 of the500 °C QD in Fig. 5 is shown in Fig. 6(d). The facet isinclined to the substrate bys27±2d°. The lengths of the unit-cell vectors and the angle between them arew1

=s0.87±0.03d nm, w2=s0.93±0.04d nm, ands119±8d°, re-

FIG. 4. 3D STM images of InAs QDs grown at 435 °C(V=−3 V, I

=0.1 nA). The size at the base alongf111̄g is noted.

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spectively. (Geometric values: 28°,w1=0.93s0.86d nm, w2

=0.95s0.89d nm, 110°) Thus, the facet 6 is identified as

s4̄1̄3̄dB and correspondingly the facet 5 ass1̄4̄3̄dB. This is

the first indication thath1̄3̄4̄jB may be a stable surface on anInAs QD.

The areas 4 in Figs. 4 and 5 do not exhibit any orderedsurface structure(not shown here in detail) but are roundedas obvious from both figures. From pure geometrical consid-

erations, the rounded area should consist ofs001̄d and its

vicinals which are inclined tos1̄1̄2̄dB by 35°. The area

4—lying opposite tos1̄1̄1̄dB—is disordered for the QDs onnearly all high-index surfaces we have studied. The reasonfor this is not clear yet and is hoped to be given by futuresimulations of the QD growth. Finally, a schematic drawingof the QD shape is depicted in Fig. 6(e). The QD exhibits the

s11̄0d plane normal to the substrate as the only plane of(mirror) symmetry. This demonstrates perfect epitaxialgrowth as it sustains the symmetry of the substrate surface. Alarge part of the QD is terminated by the facets tilted to thesubstrate by up to 30°, i.e., the QD is a rather flat entity inline with our earlier results on different substrates.6

D. Observation of lattice defects in InAs quantum dotson GaAs „1̄1̄2̄…B

Interestingly, about 50%, 20%, and 0% of the observedQDs, grown at 435 °C, 470 °C, and 500 °C, respectively,exhibit lattice defects observable on the bounding facets bySTM. A high-resolution STM image of such a lattice defect

appearing on as01̄1̄d facet is shown in Fig. 7(a). Rows ofbright humps that correspond to As dangling bonds of the

typical Ga-As zigzag rows run alongf011̄g. In the upper andthe right-hand side of Fig. 7(a) areas are marked by arrow-heads, in which the rows are shifted aside along[100] byabout 2/3 row spacings. Quite obviously, the observed de-fects are stacking faults in{111} planes. The stacking-faults

penetrate the QD perfectly alongs11̄1d and s111̄d planes,respectively, i.e., establish area defects. In addition, anotherlattice defect is seen in the lower side of the figure marked bya large arrowhead: An additional single step emerges near thecrossing point of the two stacking faults. Thus, the defect isa screw dislocation. The screw dislocation penetrates the

QDs alongf01̄1̄g establishing a line defect. The same latticedefects have been observed on the InAs QDs grown on

GaAss1̄3̄5̄dB recently, and have been discussed in detail pre-senting atomic structure models.11 A similar lattice defect is

observed on thes1̄1̄1̄dB-232 facet, as shown in Fig. 7(b)where an additional single step emerges near the middle ofthe facet as marked by an arrowhead. Thus, the defect is ascrew dislocation again, but the screw dislocation penetrates

the QD alongf1̄1̄1̄g in this case, instead of alongf01̄1̄g. It isnoted that the squarelike unit cell, marked by dotted lines inthe right-hand side of Fig. 7(b), is related to an out-of-phase

boundary of thes1̄1̄1̄dB-232 reconstruction(see also Fig.1(b) in Ref. 28), and is not related to any lattice defect pen-etrating the bulk crystal.

FIG. 5. 3D STM images of InAs QDs grown at 500 °C(V=−3 V, I =0.1 nA). The size at the base alongf111̄g is noted.

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The unexpected variation of the QD ensembles when thetemperature changes from 435 °C to 470 °C as reported inFigs. 2 and 3 can be explained by the lattice defects of theQDs. As the temperature increases, many QDs change from adisordered to the well ordered coherent state, i.e., duringgrowth at the higher temperature most defects can heal outalready during growth. The coherent QDs are strained andhence the size distribution becomes sharper. Moreover, whenthe number of incoherent QDs decreases, many coherentQDs do not have incoherent QDs in their neighborhood, andtherefore are not incorporated into the incoherent QD duringthe anticipated ripening process. Hence, the number densitystays high, instead of being reduced. Broad size distributionand occurrence of lattice defects are experimentally corre-lated here again.

E. InAs wetting layer on GaAs „1̄1̄2̄…B

The wetting layer is very important for the QD growthsince it is the template for the diffusion of the incoming Inatoms and As2 molecules to the growing QDs. The STM

images in Fig. 8 show the morphology and structure of the

wetting layer on the GaAss1̄1̄2̄dB substrate. Many flat ter-races can be seen in Fig. 8(a). The terraces are not very wideand are frequently interrupted by steps. This gives rise to theundulated morphology, as shown in Fig. 2. An enlarged STMimage in Fig. 8(b) shows the surface structure on a terrace.Although it is difficult to recognize the exact unit cell, rowsof bright humps are seen which can be connected by lines ata given separationx=s0.98±0.06d nm, in accordance with

the periodicity length alongf111̄g of 1.05s0.98d nm at a InAs

sGaAsds1̄1̄2̄dB surface. Moreover, at the side wall of step

bunches alongf312̄g, thes3̄1̄5̄dB facets form. Thus, the struc-ture of the wetting layer is exactly the same to that of the flat

base on the InAs QDs grown on the GaAss1̄1̄3̄dB.17 The

GaAss1̄3̄5̄dB surface has been discovered recently; it is fac-

etted into vicinals2̄5̄1̄1̄dB surfaces for the bare surface, butbecomes flat by deposition of a small amount of InAs, ac-companied by acs232d reconstruction.12

FIG. 6. High-resolution STM images of(a) facet 1,(b),(c) facet 3 and(d)facet 6(V=−3 V, I =0.1 nA). (e) A schematic drawing of the 3D shape ofthe QD.

FIG. 7. High-resolution STM images of lattice defects on(a) s01̄1̄d and(b)

s1̄1̄1̄dB facets (V=−3 V, I =0.1 nA). The sizes of the images are(a) 30330 nm2 and (b) 13313 nm2.

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IV. CONCLUSIONS

Shape and size distribution of the InAs QDs grown on

the GaAss1̄1̄2̄dB by MBE were investigated usingin situ

STM. The shape of the QDs is given by{110}, s1̄1̄1̄dB, and

h1̄4̄3̄jB bounding facets and a rounded vicinals001̄d region.

The QDs exhibit thes11̄0d plane normal to the substrate asthe only plane of(mirror) symmetry. This demonstrates per-fect epitaxial growth as it mirrors the symmetry of the sub-strate surface. A large part of the QD is terminated by facetstilted to the substrate by up to 30°: The QDs are rather flat

entities. This is the first indication thats1̄3̄4̄dB may be astable surface on the InAs QDs.

The QDs show an unexpected behavior for growth tem-peratures changing between 435 °C and 470 °C. As growthtemperature increases, the size of the QDs does not simply

increase, but the size distribution drastically becomes sharp,and the number density increases. This phenomenon is attrib-uted to lattice defects incorporated in the QDs at the lowergrowth temperature. A wide size distribution is correlatedhere again with incorporation of lattice defects.

The wetting layer is not flat but undulated on a sub-micrometer scale and reflects the structure of the bare sub-strate surface. The atomic structure of the wetting layer isexactly the same to that of the flat base of InAs QDs grown

on the GaAss1̄1̄3̄dB substrate.

ACKNOWLEDGMENTS

The authors thank G. Ertl for support and P. Geng andM. Richard for technical assistance.

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FIG. 8. STM images of the wetting layer(V=−3 V, I =0.1 nA). The sizes ofthe images are(a) 1003100 nm2 and (b) 27327 nm2.

6404 J. Appl. Phys., Vol. 96, No. 11, 1 December 2004 Suzuki et al.

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