Hommel, D.

Kalt, H.

Lüerssen, D.

Mikolaiczyk, M.

Okhawa, K.

Rabenstein, C.

Richter, H.

Schimmel, T.

von Freymann, G.

Wegener, M.

Freymann, G. von

KITopen

APPLIED PHYSICS LETTERS VOLUME 76, NUMBER 2 10 JANUARY 2000 First published in: Near-field photoluminescence imaging of single defects in a ZnSequantum-well structure at low temperaturesG. von Freymann, D. Lu¨erßen, C. Rabenstein, M. Mikolaiczyk, H. Richter, H. Kalt,Th. Schimmel, and M. Wegenera)Institut fu¨r Angewandte Physik, Universita¨t Karlsruhe (TH), D-76128 Karlsruhe, GermanyK. Okhawa and D. HommelInstitut fu¨r Festko¨rperphysik, Universita¨t Bremen, D-28334 Bremen, Germany~Received 10 August 1999; accepted for publication 12 November 1999!Single bright-spot, bow-tie-shaped defects in a ZnSe heterostructure are spectroscopically imagedwith 200 nm spatial resolution at a sample temperature of 20 K. The local thickness variations in thesingle ZnSe quantum-well lead to a lateral double-well potential resulting in a shift andenhancement of the quantum-well photoluminescence. The luminescence images of the ZnMgSSebarrier material are governed by nonradiative recombination at the four associated Frank partialdislocations. © 2000 American Institute of Physics. @S0003-6951~00!01702-2#Defects in semiconductor heterostructures are usuallyundesired and often thought of solely as centers of nonradi-ative carrier recombination in the quantum wells. In this con-text, it did come as a surprise to us when Ref. 1 recentlyobserved that pairs of stacking faults2,3 bounded by Frankpartial dislocations in ZnSe quantum-well structures showedup as pairs of bright spots in microphotoluminescenceimages.1 Moreover, it was shown by electron microscopy1that these bow-tie-shaped defects are very closely identical inshape, size, and orientation with respect to the crystallo-graphic axes. The aim of the present work is to clarify thenature of the resulting potential landscape in the quantumFIG. 1. ~a! Scheme of the sample-layer sequence and the bow-tie structuresrelated to pairs of stacking faults bounded by Frank partial dislocations. ~b!A topographic image taken with an atomic force microscope ~AFM! indi-cates a surface topography with a height of about 30 nm.a!Electronic mail: martin.wegener@physik.uni-karlsruhe.de2030003-6951/2000/76(2)/203/3/$17.00Downloaded 06 Mar 2009 to 129.13.72.198. Redistribution subject to EVA-STAhttp://digwell and the role of nonradiative carrier recombination in thewells and the barriers by scanning near-field optical spectros-copy at low temperatures.The sample investigated here was grown by molecular-beam epitaxy on a 300-nm-thick GaAs buffer on a GaAs~001! substrate ~Fig. 1!. This sample contains a single 4-nm-FIG. 2. Photoluminescence spectra taken at four different spatial positions~A, B, C, and D! as defined in Fig. 1. The sample temperature is T520 K, excitation with 300 nW power of 400-nm-wavelength light. Theupper part defines spectral intervals ~a!–~f!, which correspond to the imagesshown in Figs. 3~a!–3~f!. The inset shows an approach curve for the inter-vals ~a! and ~b!.© 2000 American Institute of Physics AIP license or copyright; see http://apl.aip.org/apl/copyright.jspR (Elektronisches Volltextarchiv – Scientific Articles Repository) bib.ubka.uni-karlsruhe.de/volltexte/5962000204 Appl. Phys. Lett., Vol. 76, No. 2, 10 January 2000 von Freymann et al.FIG. 3. ~Color! Photoluminescence false color images corresponding to the photon energy intervals ~a!–~f! defined at the top of Fig. 2. The sampletemperature is T520 K, excitation with 300 nW power of 400-nm-wavelength light. For clarity, the bow-tie defined in Fig. 1 is shown in each image by thegreen lines. The dotted lines are guides to the eye. The 1000 nm bar on the bottom defines the length scale. The individual photoluminescence intensity scaleson the right-hand side refer to the integrated number of photon counts in the respective spectral interval.thin ZnSe quantum well with quaternary Zn12xMgxSySe12y~ZnMgSSe! barriers with x’0.1 and y’0.1 on either side.Note that the ZnMgSSe cap layer is 17 nm thin. Ref. 1showed that pairs of stacking faults bounded by four Frankpartial dislocations originate at the GaAs/ZnSe interface andpropagate through the 1000-nm-thick ZnMgSSe barrier layertowards the surface. A bow-tie-shaped distortion of thesample surface is apparent from the topographic image@taken with an atomic-force microscope ~AFM!#, see Fig.1~b!.For the optical experiments, we frequency double about1 W of power at an 800 nm wavelength derived from a cwTi:sapphire laser in a 2-mm-thick BBO crystal to obtainabout 3 mW of power at a 400 nm wavelength. 300 nW areeffectively coupled into an optical fiber. The light propagatestowards a nanometer-scale-apex, which is formed by a two-step selective etching process. Electron micrographs of iden-tical fiber tips have been shown in Ref. 4. We scan thisDownloaded 06 Mar 2009 to 129.13.72.198. Redistribution subject touncoated tip in a constant height of 70 nm above the highestelevation of the defect ~see Fig. 1!, i.e., the feedback loop isinactive. The constant height mode is employed because theconstant distance mode is expected to be influenced by to-pography artifacts. The photoluminescence from the sample,which is held at a constant temperature of T520 K, is col-lected by the same uncoated fiber tip ~the same technique hasbeen applied to different semiconductor systems at roomtemperature, see Refs. 5, and 6! and is sent into a gratingspectrometer which is connected to a liquid-nitrogen-cooled,back-illuminated charge-coupled-device ~CCD! camera. Thespectral resolution is better than 1 meV ~full width at halfmaximum! and is indicated in Fig. 2. Typically, a total of1600 spectra are taken on a 4 mm34 mm area with 100 nmseparation between points and with an integration time of 0.5s per point. We have investigated about 20 different bow-tie-shaped defects of this sample in detail. The results shown AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp205Appl. Phys. Lett., Vol. 76, No. 2, 10 January 2000 von Freymann et al.here for one particular defect are representative for all ofthem.Selected spectra for four positions A, B, C, and D withinthe xy plane ~defined in Fig. 1! are depicted in Fig. 2. Whenpositioning the fiber tip above the homogeneous regions ofthe quantum well ~position B!, we observe photolumines-cence from the quantum well around a photon energy of\v52.831 eV, from the top ZnMgSSe cap layer around\v52.949 eV, and a much weaker contribution from thelower ZnMgSSe barrier around \v52.975 eV. The twonominally identical ZnMgSSe epilayers emit at differentwavelengths. This is very likely due to slightly different ac-tual compositions. When moving to spots near the bow-tie~see positions A, C, and D!, the relative weights of thesespectral contributions change. Furthermore, a series of strongadditional photoluminescence lines below 2.82 eV photonenergy occurs. These reproducible lines are most prominentin A and C. These lines have previously been seen in micro-photoluminescence experiments.1 Reference 1 obtained goodfits to these lines by a simple model which accounts for alocal thickening of the quantum well by an integer number ofbilayers. Thus, each photoluminescence peak corresponds toa different local quantum-well thickness. The lowest-energypeaks correspond to a thickening by as much as 12 bilayers.This shows that the thickening of 30 nm seen in Fig. 1~b! ofthe 1031-nm-thick epitaxial film is not proportionally distrib-uted over the different layers. Unfortunately, the detailedmechanism of the formation of such defects is presently notunderstood. Strain effects, on the other hand, are not likely toproduce a series of sharp photoluminescence lines. The ZnSequantum well is homogeneously strained, however, stackingfaults are not known to relax strain. A study on the formationof such defects has shown7 that the compressive strain of theZnSe layers is relaxed around the dislocation lines. Thus, theother parts of the sample remain essentially homogeneouslystrained.In order to get an overview of the behavior, we integratethe measured spectra over various intervals in photon energyand plot the resulting real-space images. The images ~a!–~f!in Fig. 3 correspond to the intervals ~a!–~f! defined at the topof Fig. 2. Figure 3~a! images the thin cap-layer lumines-cence. We interpret the measured four dark spots as beingdue to nonradiative recombination. This interpretation isconsistent with Fig. 3~c!, which shows the correspondingimage of the thick lower ZnMgSSe layer. The projection ofthe four Frank partial dislocations shown in Fig. 1 onto thexy plane results in a cross structure with reduced photolumi-nescence. The quantum-well luminescence at the spectral po-sition of the homogeneous quantum-well peak has yet a dif-ferent structure @Fig. 3~b!#. It exhibits oval-shaped minima atthe edges of the bow-tie while the total quantum-well lumi-nescence @Fig. 3~d!# shows bright spots there. This indicatesa shift of the photoluminescence towards lower photon ener-Downloaded 06 Mar 2009 to 129.13.72.198. Redistribution subject togies. This is consistent with the low-photon-energy cut of thequantum-well luminescence, Fig. 3~e! ~corresponding to athickening of the quantum well of one bilayer!, which showsoval-shaped maxima where Fig. 3~b! shows oval-shapedminima. On the very low-photon-energy end, Fig. 3~f! ~cor-responding to a thickening of the quantum well of about 12bilayers!, only the lowest-energy optical transitions contrib-ute. The measured spatial full width at half maximum of theresulting localized exciton emission is 200 nm.Finally, the approach curves to point B ~shown as aninset in Fig. 2! reveal a rise of photoluminescence intensityas the vertical distance between the fiber tip and the sample,z , approaches z50. This is indicative of an optical near-fieldresponse. ~a! and ~b! have been measured simultaneously.Note also that this rise is very much steeper when detectingthe cap-layer photoluminescence, curve ~a! in the inset ofFig. 2, as compared to the quantum-well luminescence, seecurve ~b! in the inset of Fig. 2.In conclusion, our low-temperature near-field photolumi-nescence images of a ZnSe single-quantum-well structure arean example of how much additional information one canobtain from optical spectroscopy. The optical images revealthat the local quantum-well thickness defines potentialminima for electrons and holes at the edges of the bow-tiedefect, which leads to a spectral shift and an enhancement ofthe quantum-well photoluminescence intensity. This localthickening of the quantum well is related to the surface to-pography seen by AFM, the detailed shapes, however, aredifferent. Only in the ZnMgSSe barriers, is the nonradiativerecombination associated with the four Frank partial disloca-tions the dominant effect. The size of the bow-tie is expectedto scale with the thickness of the lower ZnMgSSe barrier.Reducing this layer thickness by an order of magnitudemight lead to lateral double-well potentials on a 100 nmscale.This research has been supported by the DFG-Sonderforschungsbereich 195 and the DFG-Graduiertenkolleg 284, and is performed within the Institutfu¨r Nanotechnologie der Universita¨t Karlsruhe ~TH!.1 D. Lu¨erßen, R. Bleher, H. Richter, Th. Schimmel, and H. Kalt, Appl.Phys. Lett. 75, 3944 ~1999!.2 G. C. Hua, N. Otsuka, D. C. Grillo, Y. Fan, J. Han, M. D. Rigle, R. L.Gunshor, M. Hovinen, and A. V. Nurmikko, Appl. Phys. Lett. 65, 1331~1994!.3 L. H. Kuo, K. Kimura, A. Ohtake, S. Miwa, T. Yasuda, and T. Yao, J.Vac. Sci. Technol. B 15, 1241 ~1997!.4 Ch. Adelmann, J. Hetzler, G. Scheiber, Th. Schimmel, M. Wegener, H. B.Weber, and H. v. Lo¨hneysen, Appl. Phys. Lett. 74, 179 ~1999!.5 K.-B. Song, J.-E. Bae, K. Cho, S.-Y. Yim, and S.-H. Park, Appl. Phys.Lett. 73, 2260 ~1998!.6 D. Pahlke, F. Poser, E. Steimetz, M. Pristovsek, N. Esser, and W. Richter,Phys. Status Solidi A 170, 401 ~1998!.7 J. Tanimura, O. Wada, T. Ogama, Y. Endoh, and M. Imaizumi, J. Appl.Phys. 77, 6223 ~1995!. AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp

Near-field photoluminescence imaging of single defects in a ZnSe quantum-well structure at low temperatures

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Near-field photoluminescence imaging of single defects in a ZnSe quantum-well structure at low temperatures

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