Naturally formed In x Al1−x As/In y Al1−y As vertical superlatticesSung Won Jun, TaeYeon Seong, J. H. Lee, and Bun Lee Citation: Applied Physics Letters 68, 3443 (1996); doi: 10.1063/1.115787 View online: http://dx.doi.org/10.1063/1.115787 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/68/24?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Study of In x Ga1−x As/InAs y P1−y structures lattice mismatched to InP substrates J. Appl. Phys. 80, 6229 (1996); 10.1063/1.363699 Fabrication and microscopic photoluminescence imaging of ridgetype InGaAs quantum wires grown on a (110)cleaved plane of AlGaAs/GaAs superlattice Appl. Phys. Lett. 69, 1294 (1996); 10.1063/1.117396 Wannier–Stark localization in strained barrier GaAs/In X Al1−X As superlattices J. Appl. Phys. 80, 2285 (1996); 10.1063/1.363057 Evidence of Γ–X sequential resonant tunneling in GaAs/AlAs superlattices Appl. Phys. Lett. 69, 520 (1996); 10.1063/1.117773 Measurement of index of refraction of In x Al1−x As epitaxial layer using in situ laser reflectometry Appl. Phys. Lett. 68, 2355 (1996); 10.1063/1.115856

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Naturally formed In xAl12xAs/In yAl12yAs vertical superlatticesSung Won Jun and Tae-Yeon Seonga)Kwangju Institute of Science and Technology (K-JIST), Kwangju 506-303, Korea

J. H. Lee and Bun LeeElectronics and Telecommunications Research Institute, Taejon 303-505, Korea

~Received 25 March 1996; accepted for publication 11 April 1996!

InxAl12xAs/InyAl12yAs vertical superlattices~VS! were naturally formed by phase separationduring the growth of InAlAs/InP layers at temperatures in the range 565–615 °C by metalorganicchemical vapor deposition. The VS lies perpendicular to the~001! growth plane. As the growthtemperature increased from 565 to 615 °C, the VS decreased in thickness from;15 to;6 nm, andbecame less planar and uniform. Transmission electron diffraction results showed the occurrence ofCuPt-type ordering in some of the layers. Band-gap reduction of;300 meV was observed in theInAlAs layers grown at temperatures ranging from 565 to 615 °C. Such a large reduction inband-gap energy was attributed to combined effects of the VS and CuPt-type ordering. ©1996American Institute of Physics.@S0003-6951~96!q1324-1#

There is increasing evidence that natural superlatticescan be spontaneously formed, either by phase separation oratomic ordering, during the growth of nominally homoge-neous epitaxial layers composed of a wide range of ternaryIII–V alloys.1 Seonget al.2,3 investigating molecular beamepitaxy ~MBE! InAsySb12y layers showed that for growthtemperaturesT<400 °C, phase separation occurred sponta-neously during growth, the individual layers consisting ofthin platelets oriented approximately parallel to the surfacewhich alternated from Sb rich to As rich, and these weretermed natural strained layer superlattices~NSLS!. CuPt-type atomic ordering, resulting in monolayer$111% naturalsuperlattices, is observed in a wide range of MBE and met-alorganic chemical vapor deposition~MOCVD!-grownIII–V alloy ~001! layers.4 It is well believed that the occur-rence of CuPt-type ordering is directly related to surfaceprocesses. Models1,4,5 based on surface reconstruction havebeen proposed to explain the ordering mechanism. Energycalculations6 predicted that the ordering produces significantband-gap narrowing and experimental results showed this tobe the case for InGaAs7 and InAlAs8 layers.

InAlAs ternary alloy has attracted a great deal of atten-tion because of its potential applications in high-speed elec-tronic and optoelectronic devices. Normanet al.9 investigat-ing MOCVD InAlAs layers grown at 600 and 650 °C andBaxter et al.10 investigating a MBE InAlAs layer grown at530 °C reported the presence of CuPt-type ordering in theirsystems. Gomyoet al.8 investigating MBE InxAl12xAs lay-ers grown in the range 415–570 °C showed that triple-periodordering and CuPt-type ordering occurred on$111%A and$111%B planes, respectively. Photoluminescence~PL! mea-surements showed that the triple-period and CuPt-type order-ing produced band-gap reductions of;80 and;30 meV,respectively. In this letter we report~a! the first observationof naturally formed InxAl12xAs/InyAl12yAs vertical super-lattices ~VS! in the layers of nominal compositionIn0.52Al0.48As grown on exactly~001!-oriented InP sub-strates,~b! the occurrence of CuPt-type ordering in some of

the layers, and~c! a large band-gap reduction of;300 meVPL peak energy in the InxAl12xAs/InyAl12yAs VS grown inthe range 565–615 °C.

Nominally undoped In0.52Al0.48As layers were grown ina low-pressure MOCVD reactor using trimethylindium, tri-methylaluminum, and arsine as sources. The substrates wereFe-doped semi-insulating~001! InP on which In0.52Al0.48Aslayers;500 nm thick were grown at a rate of 0.5 nm/s withtemperatures in the range 565–800 °C. For each specimen,two orthogonal@110# and@1̄10# cross-section and@001# plan-view thin films were prepared by mechanical polishing andAr1 ion milling using a N2 cold stage, and examined bytransmission electron microscope~TEM! and transmissionelectron diffraction ~TED!. PL measurements were per-formed using 1 m double monochromator and liquid N2cooled Ge detector.

@1̄10# cross-section~002! TEM dark-field ~DF! imagesof the layers of nominal composition In0.52Al0.48As grown at565, 590, and 615 °C are shown in Figs. 1~a!, 1~b!, and 1~c!,respectively. The contrast in these images is sensitive to avariation in alloy composition which was qualitatively deter-mined from energy dispersive x-ray~EDX! analysis. The im-ages show quasiperiodic vertical band-like contrast with thebands lying perpendicular to the~001! growth plane. Thebands alternate in contrast from bright to dark, and the bandswithin each layer correspond to material with two differentalloy compositions, indicating that phase separation has oc-curred. The abrupt change in contrast between the bands in-dicates that there are relatively sharp boundaries~with 0.5nm resolution! between the two phases and these are termedvertical superlattices~VS!. For the In0.52Al0.48As layersgrown in the range 680–800 °C, the analogous@1̄10# cross-section~002! DF images~not shown! showed a single uni-form contrast because the layers were of uniform composi-tion. In Fig. 1~d! is shown@110# cross-section~002! TEMDF image of the In0.52Al0.48As layers grown at 615 °C. Theimage shows that the layer is relatively uniform, indicatingthat there are no vertical bands in this cross section. As thegrowth temperature increased from 565 to 615 °C, the VSa!Electronic mail: tyseong@matla.kjist.ac.kr

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became less planar and uniform, and decreased in thicknessfrom ;15 to;6 nm.

In Fig. 2 is shown@001# plan-view TEM DF image ofIn0.52Al0.48As layer grown at 565 °C. The image shows qua-siperiodic band-like contrast with the bands lying parallel tothe@1̄10# direction. This is in good agreement with the cross-section DF results@Fig. 1~a!#. Plan-view~002! images fromthe layers grown at 590 and 610 °C showed similar bands,although the band thickness and uniformity depended on thegrowth temperature.

Figure 3~a! shows @110# TED pattern taken from anIn0.52Al0.48As layer grown at 615 °C, which shows the mainspots and 1/2$111% superlattices spots, indicating the pres-ence of CuPt-type ordering on the~1̄11! and ~11̄1! planes.The superlattice spots are elongated and tilted;16° from the@001# direction. The superlattice spots are joined together by@001# lines of weak diffuse diffracted intensity. The exist-ence of weak diffuse intensity could be attributed to eithersharp planar defects in the ordered domains11 or more likely

the elongation and inclination of ordered microdomains.12,13

@110# TED examinations showed that as the growth tempera-ture increased from 565 to 800 °C, the ordering increased,reached a maximum at;650 °C and then decreased. Thelayers grown at temperaturesT>700 °C showed no evidencefor CuPt-type ordering. @1̄10# TED patterns fromIn0.52Al0.48As layers grown in the range 565–615 °C re-vealed diffuse streaks passing through the main spots, whichlie along the@110# direction. The length of streaks remainsunchanged with increasing order of the Bragg reflections.The length of streaks slightly increased with increasing thegrowth temperature. For example, for the 615 °C layer@Fig.3~b!#, the diffuse streak length of;1/25g~220! correspondsto quasi-periodic vertical band structures with a wavelengthof 6–8 nm.

Chenget al.14 showed that vertical quantum wells lyingalong the @001# growth direction formed in MBE(GaAs)n /(InAs)n short-period superlattices~SPS! grown on~001! InP and suggested that the strain induced from the de-viation of superlattice periodicity in the SPS could be re-sponsible for the formation of the vertical quantum well. Thepresent InAlAs VS is different from the growth behaviorsobserved by Chenget al.,14 since in this work uniform singlethick In0.52Al0.48As layers were grown. The present VS isalso different from phase-separated NSLS observed bySeonget al.2,3 Although the precise mechanisms are notclear yet, phase separation as a result of spinodal decompo-sition could be responsible for the VS.

Surprisingly, PL measurements~not shown! showed alarge reduction in band-gap of;300 meV in the 615 °Clayer ~as compared to unstrained bulk specimen!, whereas asfor the 800 °C layer, band-gap energy was fairly close to that

FIG. 1. @1̄10# cross-section~002! TEM dark-field ~DF! images fromMOCVD In0.52Al0.48As layers grown at~a! 565 °C, ~b! 590 °C, and~c!615 °C showing quasiperiodic vertical band-like contrast with the bandslying perpendicular to the~001! growth plane.~d! @110# cross-section~002!TEM DF image from the In0.52Al0.48As layer grown at 565 °C showing noband-like contrasts.

FIG. 2. @001# plan-view DF image from the 565 °C layer@Fig. 1~a!# show-ing quasiperiodic band-like contrast with the bands lying along the@1̄10#direction.

FIG. 3. ~a! @110# and ~b! @1̄10# cross-section TED patterns fromIn0.52Al0.48As layers grown at 615 °C.@110# pattern shows main lattice spotsand 1/2$111% superlattice spots, and@1̄10# pattern shows diffuse streakspassing through the main spots along the@110# direction.

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of unstrained bulk specimen. A similar large band-gap reduc-tion was also observed in the 565 and 590 °C layers. De-tailed studies of the optical properties of the InAlAs layerswill be published in the near future.15 Gomyoet al.8 showedthat CuPt-type ordered InAlAs MBE layers grown in therange 520–570 °C experienced a band-gap reduction of;30meV. The band-gap reduction in the present InAlAs layers isfar greater than that observed by Gomyoet al.8 To ourknowledge, this is the biggest redshift associated with naturalsuperlattices reported so far. It is noted that the 565 °C layershowed no CuPt-type ordering. Thus these results indicatethat a combination of the VS and CuPt-type ordering wouldproduce such a large redshift.

The authors wish to thank KOSEF and Korea Telecomfor financial support.

1A. G. Norman, T-Y. Seong, I. T. Ferguson, G. R. Booker, and B. A.Joyce, Semicond. Sci. Technol.8, S9 ~1993!, and references therein.

2T-Y. Seong, A. G. Norman, I. T. Ferguson, J. L. Hutchinson, G. R.Booker, R. A. Stradling, and B. A. Joyce, Inst. Phys. Conf. Ser.117, 485~1991!.

3T-Y. Seong, A. G. Norman, I. T. Ferguson, and G. R. Booker, J. Appl.Phys.73, 8227~1993!.

4G. B. Stringfellow and G. S. Chen, J. Vac. Sci. Technol. B9, 2182~1991!;A. G. Norman, T-Y. Seong, B. A. Philips, G. R. Booker, and S. Mahajan,Inst. Phys. Conf. Ser.134, 279 ~1993!, and references therein.

5B. A. Philips, A. G. Norman, T-Y. Seong, S. Mahajan, G. R. Booker, M.Skowronski, J. P. Harbison, and V. G. Keramidas, J. Cryst. Growth140,249 ~1994!.

6S-H. Wei and A. Zunger, Appl. Phys. Lett.58, 2684~1991!.7D. J. Arent, M. Bode, K. A. Bertness, S. R. Kurtz, and J. M. Olson, Appl.Phys. Lett.62, 1806~1993!.

8A. Gomyo, K. Makita, I. Hino, and T. Suzuki, Phys. Rev. Lett.72, 673~1994!.

9A. G. Norman, R. E. Mallard, I. J. Murgatroyd, G. R. Booker, A. H.Moore, and M. D. Scott, Inst. Phys. Conf. Ser.87, 77 ~1987!.

10C. S. Baxter, W. M. Stobbs, R. F. Broom, and J. P. Reithmaier, J. Cryst.Growth131, 419 ~1993!.

11I. J. Murgatroyd, A. G. Norman, and G. R. Booker, J. Appl. Phys.67,2310 ~1990!.

12T-Y. Seong, G. R. Booker, A. G. Norman, and I. T. Ferguson, Appl. Phys.Lett. 64, 3593~1994!.

13TEM DF images using the 1/2~3̄31! superlattice reflection of the InAlAslayers grown at 615 °C revealed microdomains which were elongated andinclinded;16° anticlockwise from the@1̄10# direction.

14K. Y. Cheng, K. C. Hsieh, and J. N. Baillargeon, Appl. Phys. Lett.60,2892 ~1992!.

15J. H. Lee, B. Lee, and T-Y. Seong~unpublished!.

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