Structural and optical characterization of self-assembled InAs-GaAs quantum dots grown on high index surfaces

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MicroelectronicsJournal28 (I997) 933-938

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Structural and optical characterization of self-assembled InAs-GaAs quantum dots grown on high index surfaces M. Henini ~*, S. Sanguinetti 2, L. Brusaferri 2, E. Grilli 2, M. Guzzi 2, M.D. Upward ~, P. Moriarty ~ and P.H. Beton t Department of Physics, University of Nottingham, University Park, Nottingham NG7 2RD, UK 2I.N.F.M and Dipartimento di Fisica, Universita' degli Studi, Via Celoria 16, 1-20133 Milano, Italy

The structural and the optical propertics of lnAs layers grown on high index GaAs surfaces by molecular beam epitaxy are investigated in order to understand the formation and the self-organization of quantum dots (Ql)s) on novel index surfaces. Four different GaAs substrate orientations have been examined, namely, (111)B, (311)A, (311)B and (100). The (100) surface was used as a reference sample. STM pictures exhibit a uniform QI) coverage for all the samples with the exception of (111)B, which displays a surface characterized by very large islands and where STM pictures give no evidence of QD formation. The photoluminescence (PL) *Corresponding author.

spectra of GaAs (100) and {311 } samples show typical QD features with PL peaks in the energy range 1.15-1.35eV with comparable efficiency. No significant quenching of PL up to temperatures as high as 70 K was observed. These results suggest that the high index substrates are promising candidates for production of high quality selfassembled QD materials for application to photonics. ,~i 1997 Elsevier Science Ltd.

1. Introduction

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t p r e s e n t there is a great interest in the laterally c o n f i n e d OD n a n o m e t e r s t r u c -

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M. Henini et al./Self-organization of quantum dots

tures, or quantum dots (QDs). The main driving force which supports the study of QDs is the possibility of using their unique properties, such as the atomic-like electronic density of states and the predicted enhanced linear and nonlinear optical properties [1-3], to fabricate novel and strongly improved photonic and electronic devices. The possibility of direct formation of QDs by an cpitaxial technique is of substantial importance. This goal can be achieved by the reorganization of strained epilayers, which, over a certain coverage, undergo a 2D-3D phase transition in the growth and self-organize in nanoscale islands [4-7]. This phenomenon permits one to achieve a high level of quantum confinement without any patterning of the surface before deposition. It is of fundamental importance to understand the process of nucleation and growth of this self-assembled QD and its dependence on the growth conditions. An interesting field of investigation is the use of different substrate orientations in order to change the reaction kinetics and the microscopic patterning of the surface. This approach for achieving selforganization and ordering of quantum disks of strained InGaAs on GaAs was used by N6tzel et al. [8]. Consequently, we have studied the behaviour of InAs strained epilayers on high index GaAs substrates. The results of the present work demonstrate the importance of surface orientation on the final shape, size and arrangement of the self-assebled InAs/GaAs QD.

consisted of the fbllowing layers, ill order of growth from the substrate: a 0.3/2nl thick undoped GaAs buffer followed by a 15 x (3.8 nm A10.33Ga0.67As+3.4nm GaAs) superlattice, a 0.2/lm undoped GaAs, 1.8 monolayers of InAs, and a 30nm undoped GaAs capping layer. The growth temperature was 630°C as monitored by a pyrometer, except during the growth of InAs and capping layer when the temperature was lowered to 500°C. The growth rates are 1 monolayer (ML) per second for GaAs, 0.5ML/sec for AlAs and 0.066ML/sec for lnAs. The formation of lnAs islands was monitored by R.HEEI), and the average thickness of InAs deposited is 1.8 ML.

The epitaxial layers were deposited by molecular beam epitaxy (MBE) using a Varian Gen-II system on liquid encapsulated Czochralski semi-insulating GaAs substrates with the following orientations: (100)-t-0.5 °, (111)B 2 ° offtowards (100), (311)A and (311)B-4-0.5°.

At the growth temperature of 630°C, a minimum As/Ga ratio was chosen to achieve a (2x4) R.HEED pattern during the growth on (100). RHEED patterns were not checked for the other planes. The samples were rotated during growth to improve uniformity. After growth, the epitaxial surfaces were examined using a Nomarski phase-contrast optical microscope and were found to be mirror smooth and nearly defect-free. The size and density of InAs quantum dots were estimated by STM from samples of the same desigm but with the growth terminated after depositing the haas layers. For these samples, following growth of the InAs/ GaAs structures, a protective amorphous As layer was deposited. The samples were then transferred, through air, to the UHV STM system. After degassing at 200°C overnight, the protective As capping layer was thermally desorbed at 300°C. (It should be noted that, for all samples, there was no change in surface morphology following subsequent anneals up to a temperature of 350°C.) The STM data were acquired using a commercially available system with electrochemically etched W tips that were cleaned by electron bombardment heating in UHV.

The structures which were grown simultaneously on the four different GaAs substrates

PL spectral characterization was performed on the l~)ur surfaces at different temperatures, in

2. Experimental details

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Microelectronics Journal, Vol. 28, Nos 8-10

the 2-70 K range. Tile PL was excited with an Ar' laser in multiline mode, with power density as low as 2.5W/cm 2. The spot size was about 300/tm FWHM. In order to exclude band filling phenomena, which may affect the QD's PL spectral shape, the linearity of the PL intensity with the excitation power density was tested over three order of magnitude, from 0.05 to 50W/cm 2. The luminescence spectra were mcasurcd with a Fourier transform spectrometer operating with a InGaAs photodetector. 3. Results and discussion

Figure 1 shows representative STM images of GaAs (100), (111)B, (311)A and (311)B samples with equivalent InAs coverages. In each case a 1.8ML coverage, with respect to the GaAs (100) surface, was deposited. Owing to the differences in surface atom density between the substrates, the actual coverages for GaAs (111)B and GaAs{311} are 1.4 and 2.8 ML, respectively. Under the particular growth conditions used in this study we observed quantum dot formation only on the GaAs(100) and {311} surfaces. The PL spectra of these surfaces (Fig. 2) show typical QD features [7], with asymmetric or structured bands of about 50 meV FWHM and different peak energies in the 1.15-1.35eV range. These spectral features suggest the presence of different QD size distribution on the three surfaces. Moreover, owing to the fact that the shape of the QD's PL spectra originates from inhomogeneous broadcning, the asymmetry in the PL peaks may stem from multimodal distribution in the QD sizes. In fact, it is clear from Fig. 1 that there are distinct differenccs in the size, shape and distribution of the InAs islands on the GaAs (100) and {311} substrates. Figure 3 shows, for each surface, histograms of the lateral sizcs and vertical heights of the islands obtained from a number of 200x2{)0nm STM images. A bimodal distribution is observcd for both

quantum dot size and height on GaAs(100). Peaks in the lateral size (diameter) distribution are observed at 14 and 20nm (For STM imaging of large surface protrusions such as the InAs islands investigated in this report, the measured lateral size represents an upper limit as tunnelling may occur between the side of the tip and the feature.) The direct observation of island formation with two predominant sizes is consistent with the asymmetric line shape of thc PL emission from the InAs/GaAs (100) sample, as reported in Fig. 2. For GaAs(311)B a single peak is observed in the size distribution of the dots corresponding to a mean diameter of 21.6nm (standard deviation: 4.1nm). The corresponding value for GaAs(100) was 18.7 nm (standard deviation: 4.2 nm). In direct contrast to the GaAs(311)B case, we notice a very broad InAs island size distribution on GaAs(311)A (Fig. 3) with a mean value of 35.8nm and a standard deviation of 9.7nm. Multimodal distribution of sizes is observed and is reflected in the structured PL spectrum of this sample. Furthermore, it is quite clcar that the islands have a considerably more anisotropic shape than that observed for GaAs (100) or GaAs (311)B. From STM images of the InAs wetting layer between the islands we can determine that the islands are preferentially aligned with the [-233] direction of the (311)A substrate. It is interesting to note that we observe the formation of faceted islands on the GaAs(311)A but not the GaAs(311)B substrate. It has previously bcen suggested that quantum dots grown on non-(100) oriented substrates will have well-developed microfacct configurations [9]. A 100x 100nm filled state STM image of the decapped InAs/GaAs(l I1)B sample is shown in Fig. lb. Due to the 2 ° miscut of the sample from thc (111)B orientation, a large number of GaAs bilayer steps, 0.33 nm high, are visible in the image. A (2x2) reconstruction [10] was clearly resolved on the terraces. STM images

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M. Henini et al./Self-organization of quantum dots

InAs/GaAs(001) 200 nm x 200 nm

InAs/GaAs(111)B 100 nm x 100 nm

InAs/GaAs(311)A 2 0 0 nm x 2 0 0 nm

InAs/GaAs(311)B 2 0 0 nm x 200 nm

Fig. 1. STM pictures (lOOx l()Onm) oflnAs/GaAs (~II)S grown on (100) (la), (111)B (lb), (311)A (lc) and (311)B (ld).

similar to that shown in Fig. lb were acquired at various positions (separated by distances of millimetres) across the sample surface. However, while Fig. lb is representative o f the majority o f the InAs/GaAs(lll)B surface, large faceted

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islands up to 200 nm in base length and 10 nm in height were also observed in isolated areas. Similar structures have previously been found to nucleate during homoepitaxial growth on 2 ° miscut GaAs(111)B surfaces [11]. We therefore

Microelectronics Journal Vol. 28, Nos 8-10

orders of magnitude lower than in the other samples and spread over the whole energy range covered by the PL emission fore1 the other surfaces. Taking into account that the PL spot size is about five orders of magnitude larger than the STM scanned area, we may argue that the PL observed in this sample originated from small and dispersed areas where defects acted as seed for a Q D formation.

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Fig. 2. Photoluminescencc spectra measured at 2K of InAs/GaAs Ql)s grown on (100) (continuous), (311)B (dash-dotted), (311)A (c.ashed) and (II1)B (dotted). The (I 11)B spectral intensity' is multipied by a factor 50.

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It is worth noticing that the PL efficiency of the {311} surfaces is comparable to that of the (10()) and that the PL integrated intensity is not appreciably quenched up to 70K. The increasing engineering opportunities (size and shape control) in addition to the high PL efficiency of the samples suggest that the high index substratcs arc promising candidates for production of high quality and customized self-assembled Q1) materials for application to photonics.

Acknowledgements 6

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Fig. 3. Histograms of the lateral sizes (left panels) and vertical heights (right panels) of the islands obtained from a number of 2()(1x 2(10nm STM images. believe that the large, faceted islands we observed arise from the growth of the GaAs epilayer and arc not related to the &posited lnAs overlayer. Unexpectedly, we observed Q I ) PL emission even from the sample grown on the (111)B surface, where S T M pictures do not show the formation o f dots. The signal was about two

This work has been supported by the Engineering and Physical Sciences Research Council (EPSRC) in the UK. We acknowledge N A T O for funds enabling the collaboration. T h e U H V STM system was supplied by Oxford Instruments SPM group, formerly WA Technology, Cambridge, UK.

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M. Henini et al./Self-organization of quantum dots

[2] Schmitt-Rink, S., Miller, D.A.B. and Chemla, D.S., Theory of the linear and nonlinear optical properties of semiconductor microcrystallites, Phys. Rev., B35 (1987) 8113. [3] Takagahara, T., Excitonic optical nonlinearity and exciton dynamics on semiconductor quantum dots, Phys. Rev., B36 (19871 9293. [4] Moison, J.M., Houzay, F., Barthe, F., Leprince, L., Andr6, E. and Vatel, O., Self-organized growth of regular nanometer-scale InAs dots on GaAs, AppI. Phys. Lett., 64 (19941 196. [5] Marzin, J.Y., Gerard, J.M., Izrael, A., Barrier, 1). and Bastard, G., Photoluminescence of single lnAs quantum dots obtained by self-organized growth of GaAs, Phys. Rev. I.x'tt., 73 (1994) 716. [6] Leonard, 11., Pond, K., and Pctroff, P.M., Critical layer thickness for self--assembled InAs islands on GaAs, Phys. Rev., B50 (1994) 11687. [7] P,uvimov, S., Werncr, P., Schecrschmidt, K., Goesele, U., Heydenreich, H., Richter, U., Lcdentsov, N.N., Grundmann, M., Bimberg, D., Ustinov,

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