Self-organized CdSe quantum dots onto cleaved GaAs (110) originating fromStranski–Krastanow growth modeHyun-Chul Ko, Doo-Cheol Park, Yoichi Kawakami, Shizuo Fujita, and Shigeo Fujita Citation: Applied Physics Letters 70, 3278 (1997); doi: 10.1063/1.118427 View online: http://dx.doi.org/10.1063/1.118427 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/70/24?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Optimization of growth conditions of type-II Zn(Cd)Te/ZnCdSe submonolayer quantum dot superlattices forintermediate band solar cells J. Vac. Sci. Technol. B 31, 03C119 (2013); 10.1116/1.4797486 Photoluminescence of CdSe quantum dots and rods from buffer-layer-assisted growth Appl. Phys. Lett. 88, 121906 (2006); 10.1063/1.2187411 CdSe self-assembled quantum dots with ZnCdMgSe barriers emitting throughout the visible spectrum Appl. Phys. Lett. 85, 6395 (2004); 10.1063/1.1834993 Quantum dot formation by segregation enhanced CdSe reorganization J. Appl. Phys. 92, 6546 (2002); 10.1063/1.1516248 Investigations on the Stranski–Krastanow growth of CdSe quantum dots Appl. Phys. Lett. 76, 418 (2000); 10.1063/1.125773

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Self-organized CdSe quantum dots onto cleaved GaAs (110) originatingfrom Stranski–Krastanow growth mode

Hyun-Chul Ko,a) Doo-Cheol Park, Yoichi Kawakami, Shizuo Fujita, and Shigeo FujitaDepartment of Electronic Science and Engineering, Kyoto University, Kyoto 606-01, Japan

~Received 24 February 1997; accepted for publication 15 April 1997!

Self-organized CdSe/ZnSe quantum dots~QDs! have been fabricated on GaAs~110! crystalsurfaces, which were obtained by cleaving GaAs~100! wafers in ultrahigh vacuum. CdSe showeda conventional Stranski–Krastanow growth mode on the ZnSe~110! lower cladding layer, whosesurfaces are atomically flat. The wetting layers, which are compose of quantum wells with wellwidths of 1, 2, and 3 monomolecular layers, showed sharp photoluminescence~PL!. The fabricatedCdSe QDs showed intense green PL spectra, whose peak is located at 2.192 eV, with a linewidth of0.24 eV. The state filling effect in CdSe QDs was also observed by employing excitation powerdependence of the PL intensity. ©1997 American Institute of Physics.@S0003-6951~97!03324-X#

In recent years, a great deal of research has been devotedto the self-organization phenomena in order to fabricate highquality quantum dot~QD! structures.1–3 The Stranski–Krastanow~SK! mode is one promising method, where is-land structures are self-formed on a two-dimensional~2D!wetting layer, as a result of the transition of the growthmode, namely, from the 2D to three-dimensional~3D! one ata certain layer thickness. The main merit of such a techniqueis both the easiness of fabrication without lithographic pro-cess and high optical quality due to low interface defects. Ina III–V material system like InAs/GaAs, highly uniformQDs structures have been fabricated with well-establishedtechniques.1,3 In the II–VI material system, most of the QDstructures have been synthesized as colloids embedded in aglass matrix. In these materials, interesting optical propertiessuch as zero-dimensional behavior of excitons and excitonspin dynamics have been reported.4,5 However, from an ap-plication view point, it is very important to fabricate QDstructures with an epitaxial growth technique. Very recently,Xin et al.have reported that CdSe QDs grown on the~001!-oriented ZnSe buffer layers, in which no clear evidence wasshown for the achievement of the SK mode.6 This is prob-ably because of the difficulty in the preparation of the atomi-cally flat cladding layer on GaAs~001!.

CdSe has a large exciton binding energy~15 meV!.7

Moreover, the exciton binding energy can be increased re-markably in low-dimensional structures. Therefore, if highquality quantum dot structures are fabricated, a new exci-tonic devices, which acts in the green spectrum region, canbe expected even at room temperature. However, althoughthe CdSe/ZnSe strained system is very similar to InAs/GaAs,which has a 7% lattice mismatch, it is very difficult to obtainthe quantum effect in the nanostructures of CdSe because ofthe small exciton Bohr radius of CdSe (;3 nm).

In this letter, we investigated the fabrication and opticalproperties of self-organized CdSe/ZnSe QDs originatingfrom the SK mode, which were grown on GaAs~110! sur-faces cleaved in ultrahigh vacuum~UHV! by molecularbeam epitaxy~MBE!. The CdSe island structure showedzero-dimensional optical properties, while wetting layersshowed 2D ones.

Epitaxial growth was carried out with the MBE systemconsisting of two chambers; one for the cleaving of the sub-strate and the other for the epitaxial growth. The detailedprocess was described elsewhere.8

The sample structures are very similar to the singlequantum well ~SQW! with ultrathin active layers. Aftercleavage of the substrate in UHV, a lower cladding layer~100 nm in thickness! of ZnSe, CdSe well layers@1–10monolayers~ML !# and upper barrier layers~50 nm in thick-ness! of ZnSe were successively grown on the cleaved sur-face. Samples A, B, C, and D have 1, 2, 4, and 10 ML CdSein active layers, respectively. Growth interruption at the het-erointerfaces was not adopted. As for the fabrication of~110!-oriented quantum structures by MBE, it has beenfound that the growth interruption at heterointerfaces causedthe 3D growth mode in our previous study.10 The growthmode was monitored from the observation ofin situ reflec-tion high-energy electron diffraction~RHEED! patterns withan electron acceleration voltage of 20 kV. Typical growthconditions were as follows: growth temperature of 250 °C,beam equivalent pressure ratiop(VI)/ p(II) of 1.0, andgrowth rate of 0.1mm/h. The photoluminescence~PL! spec-tra of the samples were measured with a cooled chargecoupled device in conjunction with 50 cm monochrometer.An He–Cd laser~325 nm line! with a power density of1.5 W/cm2 was used as an excitation source.

The SK mode during the growth of the CdSe active layeron the ZnSe lower cladding layer was confirmed by the ob-servation of the RHEED patterns. The ZnSe epitaxial layerwas coherently grown on the GaAs~110! cleaved surface upto more than 120 nm with 2D layer-by-layer growth showingthe oscillations of RHEED specular beam intensity.9 Duringthe growth of ZnSe lower cladding layers, the RHEED pat-terns maintained very streaky (131) structures. However,the streaky lines of the RHEED patterns were slightly dif-fused if the growth of CdSe was initiated, and they changedto spotty features suddenly after the deposition of about 3ML CdSe. The patterns recovered gradually were streakywith the growth of the upper cladding layer of ZnSe. Theseresults suggest that the growth mode of CdSe on ZnSe~110!changes from the 2D to the 3D mode, i.e., the conventionalSK growth mode. The critical thickness of the growth modechange is 3 ML in these growth conditions.a!Electronic mail: ko@kuee.kyoto-u.ac.jp

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The atomic force microscope~AFM! image after depo-sition of CdSe with a 10 ML thickness on the ZnSe lowercladding layer without the upper cladding layer is shown inFig. 1. The AFM observations were performed immediatelyafter growth. Island structures with average base diametersof 47 nm and height of 10 nm were observed. The islanddensity was 1.73109 cm22, which is almost two orders ofmagnitude lower than that of typical InAs quantum dotsgrown on GaAs~001!.3 The estimation of the exact size ofthe islands is also quite difficult since the AFM images of theCdSe islands without the upper cladding layer changes withthe time.6 Nevertheless, the actual size of the island struc-tures with the upper cladding layer are expected to be smallerthan that mentioned above. This is because the AFM tends tooverestimate the size if the height of islands changesabruptly.

Figure 2 shows the PL spectra of the samples with dif-ferent active layer thicknesses measured at 22 K. Sample Awith 1 ML CdSe showed a sharp main peak at 2.719 eV witha PL peak from the ZnSe cladding layers at 2.804 eV locatedat the high-energy shoulder of the main PL peak. Sample B

with 2 ML CdSe also revealed the main peak at 2.577 eV.Sample C, which has a 4 ML deposition quantity of CdSe,has a peak at 2.519 eV with unresolved shoulders on thehigh-energy side and weak and broad PL at 2.215 eV.Sample D, which has a 10 MLs of CdSe active layer showedsix peaks of PL, which are composed of broad but intenseluminescence at 2.192 eV and several relatively weak butsharp peaks at higher energy than 2.4 eV. In sample D, thebroad PL peak located at 2.192 eV is considered to be origi-

FIG. 3. The calculated optical transition energy of CdSe/ZnSe SQW as afunction of well width. The exciton binding energy of CdSe is calculated byvariational method.

FIG. 1. Atomic force microscope image of CdSe/ZnSe quantum dots with aCdSe deposition thickness of 10 MLs. No upper cladding layer is depositedto the surface. The scanned size is 0.8mm30.8mm. ~a! Top view and~b!side view images.

FIG. 2. Photoluminescence spectra of the samples with CdSe depositionthickness. Samples A, B, C, and D have 1, 2, 4, and 10 MLs of CdSe inactive layers, respectively. The excitation power was 1.5 W/cm2.

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nating from the CdSe islands, while PL peaks above 2.4 eVare attributed to the CdSe wetting layers with various thick-nesses respectively. It is expected that the local variation ofthickness in the wetting layer occurs due to the step bunch-ing or bilayer growth, as has been frequently observed in theMBE growth on ~110!-oriented surfaces.11 Consequently,well-defined PL peaks originating from the CdSe wettinglayer with the thickness of ML units indicate the formationof high optical properties of the single QWs.

The optical transition energy ofn51 heavy-hole exci-tons was calculated for the CdSe/ZnSe QWs coherentlygrown on the GaAs~110!-oriented surface, where the wellwidth dependence of the exciton binding energy was takeninto account by means of the variational method.12 As shownin Fig. 3, the calculated transition energy of the CdSe/ZnSeQWs with the 1, 2, and 3 MLs of active layer thickness at 22K is estimated to be 2.67, 2.61, and 2.50 eV, respectively.These results fairly agree with the peaks observed in the PLmeasurements. The small redshift of the 1 and 2 MLs QWpeaks in sample D compared to sample A is considered to bedue to the partial relaxation of strains. Xinet al. also re-

ported similar results for the CdSe QDs grown on GaAs~001!. They observed a very weak peak at 2.61 eV as awetting QW, which exactly corresponds to the 2 MLs wet-ting QW of this work. Unfortunately, there was no evidenceof the SK growth mode in their work, which is probablyowing to the 3D growth as a result of the rough surfacemorphology of the lower cladding layer.

To confirm the origin of the emission, PL spectra weremeasured as a function of the excitation power intensity.Figure 4 shows the excitation power dependence of the PLspectra of sample D. The PL intensity (IPL) can be expressedas a function of the excitation laser power (I ext

a ) as IPL}I ext

a , wherea is the nonlinear component. As it can be seenin the Fig. 4~b!, the intensity of QD transitions increasesapproximately linearly with the excitation laser intensity. Onthe contrary, the intensity of QW transitions increased superlinearly with the increase of excitation intensities. In otherwords, the PL intensity of the QD peak is gradually saturatedas the excitation power intensity increased due to the over-flowing carriers after the filling of the state in QDs, resultingin the enhanced recombination of carriers in the QW of thewetting layer regions and consequent suppressing lumines-cence from QDs. The nonlinear components of the QWpeaks depend on the wetting QW thickness. Thea value isbigger for thinner QWs.

In conclusion, we have fabricated CdSe/ZnSe QDs onthe GaAs~110! surfaces obtained by cleaving a GaAs~100!wafer in UHV. The growth mode of CdSe on the ZnSe~110!lower cladding layer showed the conventional SK mode. Thesharp PL spectra from the CdSe wetting layers ascribed tothe emission from the QWs with 1, 2, and 3 MLs thickness.The fabricated CdSe QDs revealed the intense green PLspectrum, whose peaks were located at 2.2 eV.

The authors thank Dr. T. K. Yoo of the LG ElectronicsResearch Center for his encouragement. This work was sup-ported in part by the Kyoto University–Venture BusinessLaboratory~KU-VBL ! project and also by the LG Electron-ics Research Center in Korea.

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13Details of the calculations are out of the scope of this study and will bereported elsewhere.

FIG. 4. Photoluminescence spectra of the CdSe/ZnSe QDs with 10 MLs ofthe CdSe layer at various excitation intensity. The broken line represents thepeaks originating from emissions from QDs and QWs.~b! Intensity evolu-tion of each peak as a function of excitation power.

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