Direct formation of self-assembled quantum dots under tensile strain by heteroepitaxyof PbSe on PbTe (111)M. Pinczolits, G. Springholz, and G. Bauer Citation: Applied Physics Letters 73, 250 (1998); doi: 10.1063/1.121770 View online: http://dx.doi.org/10.1063/1.121770 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/73/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Gigantic uphill diffusion during self-assembled growth of Ge quantum dots on strained SiGe sublayers Appl. Phys. Lett. 96, 141909 (2010); 10.1063/1.3383241 Near-equilibrium strain relaxation and misfit dislocation interactions in PbTe on PbSe (001) heteroepitaxy Appl. Phys. Lett. 83, 5160 (2003); 10.1063/1.1633675 Controlling the size and density of self-assembled PbSe quantum dots by adjusting the substrate temperatureand layer thickness Appl. Phys. Lett. 81, 2457 (2002); 10.1063/1.1509116 Self-organized lateral ordering for vertically aligned PbSe/PbEuTe quantum-dot superlattices Appl. Phys. Lett. 80, 1273 (2002); 10.1063/1.1447319 Oswald ripening and shape transitions of self-assembled PbSe quantum dots on PbTe (111) during annealing Appl. Phys. Lett. 77, 2991 (2000); 10.1063/1.1323733

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Direct formation of self-assembled quantum dots under tensile strainby heteroepitaxy of PbSe on PbTe „111…

M. Pinczolits, G. Springholz,a) and G. BauerInstitut fur Halbleiterphysik, Johannes Kepler Universita¨t Linz, 4040 Linz, Austria

~Received 12 March 1998; accepted for publication 12 May 1998!

Direct growth of tensile-strained PbSe quantum dots by molecular beam epitaxy on 5.5% latticemismatched PbTe~111! is investigated by atomic force microscopy andin situ reflection highenergy electron diffraction. After wetting layer formation, two types of PbSe islands are formeddistinguishable in size and ordering behavior. All islands exhibit a well defined pyramidal shapewith triangular base and steep~100! side facets. In addition, the dots exhibit a remarkably narrowsize distribution with a relative variation of height and width as low as67%. © 1998 AmericanInstitute of Physics.@S0003-6951~98!04728-7#

Strained-layer heteroepitaxy has been recently devel-oped as a novel technique for direct fabrication of self-assembled semiconductor nanostructures with quantum con-finement in lateral as well as vertical direction.1–3 It is basedon the general fact that for deposition of highly strained het-eroepitaxial layers, nanometer sized three dimensional is-lands~3D! are spontaneously formed on the epitaxial surfaceonce a critical thickness is exceeded~Stranski–Krastanowgrowth mode!. This fundamental growth mode instability isdriven by the substantial reduction of strain energy allowedby the lateral deformation of the freestanding 3D islands,which for highly strained layers by far outweighs the in-crease in free surface energy. Since the fully coherent self-assembled 3D islands are defect free, they can be utilized forfabrication of novel optoelectronic devices such as quantumdot lasers4,5 with unique properties due to the deltalike elec-tronic density of states. Up to now the growth of self-assembled quantum dots has been restricted to material sys-tems under compressive strain. This is because mostheterostructures are grown on Si or GaAs substrates and be-cause of the general trend that within each class of semicon-ductor compounds the energy band gap decreases with in-creasing lattice constant. In addition, recent work hassuggested quite a different growth behavior fortensile thanfor compressive-strained layers. For Ga0.75In0.25As on InP,e.g., the formation of 3Dholes instead of islands has beenreported,6 and for Si0.5Ge0.5 on relaxed Si12xGex buffer lay-ers, no 3D growth instability was found for layers undertensile strain.7 In both works, however, the misfit strain wasrestricted to values below or equal to 2%, which is not muchabove the critical strain for which coherent 3D islanding isknown to take place.

To clarify this issue we have investigated the growthproperties of a new heteroepitaxial system, namely, PbSe onPbTe~111!. These narrow gap semiconductors, used for fab-rication of midinfrared diode lasers,8,9 crystallize in the rocksalt crystal structure and, in contrast to most other heteroepi-taxial systems, a hightensile strain is present in the PbSelayer due to the large 5.5% lattice mismatch. From our sys-

tematic growth studies, we observe spontaneous formation of3D PbSe islands on the surface once a critical coverage isexceeded. At first these islands are arranged in one dimen-sional chain like structures, while homogenous island nucle-ation sets in only at a larger coverage. All islands exhibit anidentical highly faceted pyramidal shape with a remarkablynarrow size distribution. This makes the material systemvery well suited for the fabrication of homogenous quantumdots for optoelectronic applications in the midinfrared spec-tral region.

Molecular beam epitaxy~MBE! growth was carried outin a Riber 1000 MBE system. First, 2–4mm fully relaxedPbTe buffer layers were predeposited on freshly cleavedBaF2 ~111! substrates.10 For PbSe growth, a substrate tem-perature of 380 °C and a growth rate of 0.08 ML/s was usedin all experiments @for PbSe one monolayer (ML)53.54 Å]. In situ growth studies were performed by reflec-tion high energy electron diffraction~RHEED!. Aftergrowth, the samples were rapidly cooled to room tempera-ture to preserve the epitaxial surface structure. Atomic forcemicroscopy~AFM! was carried out in air soon after removalof the samples from the MBE system. AFM measurementswere done in contact mode using sharpened microlevers orultralevers of Park Scientific Instruments. Due to the finitesize of the AFM tips~nominal tip radius of 20 and 10 nm,respectively!, the apparent shape of 3D surface features inthe AFM images represents a convolution of the real surfacemorphology with the tip shape function.

For the PbTe buffer layers, the RHEED pattern exhibitssharp diffraction spots arranged on the lowest order Lauecircle, as is typical for highly perfect 2D surfaces. As shownin Fig. 1~a!, when PbSe is deposited on this surface, initially,this pattern remains unchanged, indicating a perfect 2Dgrowth mode. At a critical coverage ofuc

A51.5 ML, how-ever, 3D transmission diffraction spots start to appear@seeFig. 1~b!#, indicating the onset of island formation well be-fore the first misfit dislocations can be formed. At 3 MLcoverage@Fig. 1~c!#, the 2D diffraction spots are completelyreplaced by the transmission electron diffraction pattern.From intensity versus coverage measurements shown in Fig.1~d!, the onset of this roughening transition was found to bereproducible at 1.5 ML, but an additional kink is observed ina!Electronic mail: g.springholz@hlphys.uni-linz.ac.at

APPLIED PHYSICS LETTERS VOLUME 73, NUMBER 2 13 JULY 1998

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the traces at a coverage ofucB52.4 ML, indicating an accel-

eration of the roughening process. Similar measurementswere carried out also for PbSe12xTex on PbTe~111! withvarying alloy composition. Apart from the slight increase ofthe critical coverageuc for the 2D–3D growth transitionwhen the misfit strain decreases, we find that epitaxialgrowth remains purely two dimensional when the misfitstrain becomes less than 1.6%~Se content,30%!. From theRHEED patterns, we have also determined the in-plane PbSelattice constant as a function of layer thickness. Up to 1.5ML, it is found to be identical to that of the PbTe bufferlayer, then, however, correlated with the onset of surfaceroughening it rapidly decreases, reaching 95% of the bulkPbSe lattice constant already at 4 ML coverage. Thus, the 3Dislands are almost immediately fully relaxed which is consis-tent with the highly effective strain relaxation known for 3Dislanding.

To characterize the evolution of surface morphology inmore detail, PbSe layers with different thicknesses were pre-pared under identical growth conditions. After growth thesurfaces were imaged by AFM and representative images areshown in Fig. 2. For PbSe layers below 1.5 ML coverage, thesurface structure is almost identical to that of the PbTe bufferlayers ~see, e.g., Ref. 10!, with the terraced surface stepstructure well resolved by AFM@Fig. 2~a!#. Thus, growthproceeds in a pure 2D step flow growth mode forming acomplete wetting layer. In contrast to the rounded steps typi-cal for the PbTe surfaces, however, the steps on the highlystrained PbSe layer exhibit a strong tendency for undulationformation. This is even more pronounced in the AFM imagefor 2 ML coverage@Fig. 2~b!#, and indicates that the energygained by strain relaxation at the step edges outweighs theenergy required for creation of additional step length. In ad-dition, already at 1 ML coverage, a few isolated small is-lands of 3 to 4 ML in height appear on the PbSe surface@Fig.2~a!#. These precursors for 3D islands nucleate preferentiallyat the undulated surface step edges, but also in the middle ofwider terraces.

As shown in Fig. 2~b!, when the PbSe coverage exceedsthe critical coverage of 1.5 ML, large 3D islands of about150 Å in height appear on the surface. Surprisingly, thesedots are not distributed randomly over the epitaxial surface,but tend to order inpairs, chainsor even closedrings. At afirst glance, one would assume that this special arrangementis caused by the stepped surface structure of the wettinglayer; however, from a closer inspection we do not find aclear-cut correlation between the nucleation centers and theunderlying step structure. Comparing a large number of theseislands we find that the dots are remarkably well calibratedin size, in particular no dots less than 70 Å in height areobserved at this coverage. Except for the penetration pointsof threading dislocations present already in the buffer layer,no preferential nucleation sites can be identified for the dots.This indicates that a one dimensional cooperative nucleationmechanism is operative at this growth stage, in analogy toobservations for annealed Ge/Si~100! structures.11

When the PbSe coverage exceeds about 2.4 ML, a sec-ond type of PbSe dots appear in the AFM images@see Fig.2~c!#. These dots~named type B! are much smaller in sizethan the previous large dots~type A!, and they nucleate at

FIG. 1. ~a!–~c!: RHEED images observed for PbSe deposition on PbTe~111! at coverages of 1, 2, and 3 ML. The intensities of the specular spotand the ~222! transmission spot is shown in~d! as a function of PbSecoverage, with the onset of 3D island formation indicated by the arrows atuc

A51.5 ML anducB52.4 ML.

FIG. 2. Atomic force micrographs (434 mm2) of PbSe deposited on PbTe~111! at coverages of 1~a!, 2 ~b!, and 3 ML ~c!. As is indicated by the arrows,two types of PbSe dots are formed on the surface starting at a critical coverage of 1.5 ML for the type A dots and of 2.4 ML for the dots of type B.

251Appl. Phys. Lett., Vol. 73, No. 2, 13 July 1998 Pinczolits et al.

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random surface sites between the existing A dots. As shownin Fig. 3, while the density of the already existing large dotssaturates or even slightly decreases once nucleation of thesmall dots sets in, the density of the small dots rapidly in-creases with coverage and at 6 ML exceeds that of the largedots by almost two orders of magnitude. At larger coverages,the two types of dots can still be distinguished, and in thehistograms of the dot heights~not shown here!, two wellseparated peaks corresponding each to one of the character-istic island types are observed. At 5 ML coverage, averagePbSe dot height is 200 Å for the large dots~A! and about 150Å for the smaller dots~B!.

Initially, it was thought that the bimodal size distributioncould be due to the presence of two different distinct islandshapes, as has been reported for Ge dots on Si~100! preparedby chemical vapor deposition.12,13Because of the high aspectratio of the PbSe islands, however, when using standardAFM tips the dot shape in the images mainly reflected therounded shape of the AFM tips. Thus, true information onthe island shapes could be obtained only with selected ul-trasharp AFM tips with nominal tip radius of 100 Å and 10°opening angle. From such images we found thatall PbSeislands exhibit a highly faceted pyramidal shape, as is shownin Fig. 4 for 3.5 ML PbSe coverage as an example. Lineprofile measurements show that the extremely steep pyramidside faces are all inclined by 55° with respect to the epitaxialsurface, and in addition they are all aligned parallel to thethreefold ^110& directions within the~111! surface plane.Therefore, the pyramid side faces correspondexactly to thethree by 54.7° inclined$100% planes, which are the lowestfree energy surfaces of the lead salt compounds. It is notedthat the such defined pyramidal shape corresponds to aheight-to-base aspect ratio of 1:2.2, which is much largerthan what has been observed for other material systems.1,12

Since the same island shape is observed for both types A andB PbSe dots, we attribute the formation of the two differentislands not to differences in islands energies, but rather to adifferent nucleation mechanism. This is also supported bythe fact that the lateral arrangement is completely differentfor the two types of islands.

Perhaps the most striking feature of the PbSe quantumdots is their remarkably narrow size distribution. We findthat except at the onset of nucleation the relative heightvariation is612%–17% for the large dots~A!, and67%–

15% for the smaller dots~B!. Even more notably, the sizedispersion is found todecreasewith increasing coverage,such that the narrowest distribution is observed after comple-tion of island nucleation, i.e., at 4–8 ML coverages. This isvery much in contrast to the InAs/GaAs system, where thesize dispersion is found to increase with InAs coverage.1,2

We attribute this effect to the invariant highly faceted PbSeisland shape and the large stress concentration around theisland edges due to the steep side facets.

In conclusion, we have demonstrated the growth of self-assembled quantum dots for a material system undertensilestrain. Due to the pyramidal shape and the high aspect ratioof the 3D islands formed during PbSe/PbTe growth, we haveobtained a remarkably narrow size distribution, which offerspromising perspectives for incorporation of PbSe quantumdots in midinfrared lead salt diode lasers. This work wassupported by the O¨ NB Jubilaumsfonds~No. 6690! and theFWF ~No. 11557!, Austria.

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FIG. 4. Enlarged 3D AFM image of the PbSe islands at a coverage of 3.5ML, showing their highly faceted pyramidal shape with their base allaligned along the110& directions on the surface. The steep$100% side facetsare inclined by 54.7° out of the surface.

FIG. 3. Areal density of the two types of PbSe dots A and B vs PbSecoverage on PbTe~111!. The arrows indicate the onset of island formationat the critical coveragesuc

A51.5 ML and ucB52.4 ML for types A and B

dots, respectively. The inset shows the region at the critical coverages in anenlarged scale.

252 Appl. Phys. Lett., Vol. 73, No. 2, 13 July 1998 Pinczolits et al.

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