ARTICLE IN PRESS Microelectronics Journal 40 (2009) 476– 478
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Surfactant-mediated growth of InAs– GaAs superlattices and quantum dot structures grown at different temperatures M. Alduraibi, C. Mitchell, S. Chakraborty, M. Missous Microelectronics and Nanostructures Group, School of Electrical and Electronic Engineering, The University of Manchester, Sackville Street, Manchester, PO Box 88, M60 1QD, UK
a r t i c l e in f o
a b s t r a c t
Available online 22 July 2008
The structural and optical qualities of superlattice InAs–GaAs structures and quantum dots (QDs), grown by molecular beam epitaxy (MBE) at low (250 1C) and normal (450 1C) growth temperatures, have been investigated. The InAs layers (3 monolayers) were grown under conditions where only the indium beam impinged upon the growth surface (surfactant growth mode). This growth mode still resulted in the formation of QDs at normal growth temperatures, but with dot sizes that were much smaller than those for ‘‘normal’’ growth of 3 ML InAs–GaAs QD structures. In addition, at low temperature under such ‘‘arsenic-free’’ conditions a very high quality InAs–GaAs superlattice structure with 3 ML of InAs was formed, as demonstrated by transmission electron microscopy (TEM). This is a direct conﬁrmation that the critical thickness of InAs can be extended well beyond the 1.7 ML limit seen at higher growth temperatures. & 2008 Elsevier Ltd. All rights reserved.
Keywords: Molecular beam epitaxy Surfactant growth Quantum dots Low-temperature growth
1. Introduction Semiconductor quantum dots (QDs) have been extensively investigated in recent years due to their unique optical properties arising from their discrete energy levels, which make them useful for optoelectronic applications. InAs QDs have attracted much attention due to the effective bandgap energy, which allows emission and absorption at telecommunication wavelengths (1.3 and 1.55 mm) where silica ﬁbres have minimum losses. In addition, low-temperature (LT)-grown InAs–GaAs superlattices are useful materials for ultrafast optical switches due to the short carrier lifetime caused by the incorporation of excess arsenic into the crystal as point defects [1,2]. Molecular beam epitaxy (MBE) is one of the key techniques used to fabricate self-organized QD structures, formed by the Stranski–Krastanow growth mode. It has been observed that the growth conditions have an important effect on the properties of self-organized QDs, such as their density and size distribution . Several studies have been made on single-plane InAs QDs, grown on GaAs at different growth temperatures [4,5]. However, relatively little has been reported on stacked binary InAs QDs grown at normal growth temperature (450 1C) and InAs–GaAs superlattices grown at LT (250 1C). It has been found that, under normal growth conditions (NGC), a structure of 3 monolayers (MLs) of stacked binary InAs–GaAs QDs contained volcano-like defects . The effect of such defects
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(M. Missous). 0026-2692/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.mejo.2008.06.055
on the quality of the material structure was clearly observed in double crystal X-ray diffraction (DCXRD) measurements, which showed broad satellite peaks. The work presented here demonstrates that using only the indium beam (surfactant growth) during the growth of thick InAs layers (3 MLs) improves the structural quality of InAs–GaAs QDs and superlattices enormously. Detailed DCXRD, transmission electron microscopy (TEM), photoluminescence (PL) and atomic force microscopy (AFM) measurements are reported for samples grown using this technique at both low and high temperatures for the ﬁrst time.
2. Experiments Two novel surfactant-grown structures were investigated in this study (identiﬁed as sample no. 1949 and no. 1753) alongside similar samples grown under NGC. All samples were grown using an Oxford Instruments VG V90H MBE reactor on semi-insulating (1 0 0) GaAs substrates. The growth of sample no. 1949 started with a 170 nm buffer layer, grown at 580 1C. Then, the substrate temperature was decreased to 450 1C for the growth of ten periods of 25 nm GaAs layer (spacer) followed by an InAs layer with a nominal thickness of 2.9 MLs. The sample was left uncapped for the purpose of AFM imaging. The growth of the InAs layers was performed with just the indium beam impinging on the surface, surfactant growth; under these conditions the group V uptake was provided by the residual arsenic in the growth chamber. The growth rate of GaAs and InAs was 1 and 0.033 ML s 1, respectively. The second sample (sample no. 1753) has an identical structure
ARTICLE IN PRESS M. Alduraibi et al. / Microelectronics Journal 40 (2009) 476–478
and was grown under the same growth conditions, i.e. surfactant growth, except that the substrate temperature was reduced to 250 1C to grow the InAs–GaAs superlattice. Samples having the same structure were also grown at 250 1C, but under NGC. A Bede QC200 was used for DCXRD studies with a 1.54056 A˚ wavelength source and randomly polarised beam. A Ge crystal was used as the reference crystal for the GaAs substrate with (0 0 4) reﬂection plane geometry. For TEM studies, cross-sectional samples parallel to the (11 0) plan were prepared using the Ar-ion milling technique. The cross-sectional TEM imaging was taken using a Philips CM200 microscope operating at 200 kV with the sample aligned with /11 0S zone axes of GaAs parallel to the electron beam. AFM images of QDs were all made with contact mode scans. An Accent RPM2000 system was used for room temperature PL measurements with an 11 mW laser (532 nm wavelength) and either a silicon CCD detector (550–900 nm) or an InGaAs detector (900–1800 nm). Fig. 2. Cross-sectional TEM images of (a) sample no. 1949, grown at normal temperature and (b) sample no. 1753, grown at low temperature.
The DCXRD rocking curve of sample no. 1949 (InAs–GaAs QDs grown at normal temperature) shows an improvement in the quality of the structure in comparison with the sample grown under NGC. This improvement is apparent from the sharpness of the satellite peaks, Fig. 1, in contrast to the broader peaks observed by Ng et al. . In that study the broadening and reduction in intensity of the satellite peaks around the main Bragg peak was shown to be due to the formation of extended, volcano-like defects, which degrade the quality of the structure. The DCXRD rocking curves therefore indicate that surfactant growth of InAs–GaAs QDs results in high-quality crystal structures free of volcano-like defects. This assertion is supported by the TEM images, Fig. 2a, where the strained regions around the InAs QDs appear in every InAs layer, including at the surface of the sample. TEM images of this sample show QD correlation between the InAs layers; however, there was no strain coupling between any InAs adjacent layers that leads to the formation of extended crystal defects. In comparison, the LT surfactant sample (no. 1753) shows a superlattice structure with no QDs (Fig. 2b). This is probably due to the low-substrate temperature during the growth that reduces the adatom migration length . Moreover, the DCXRD spectrum was very well deﬁned (Fig. 1) and could hardly be distinguished from the surfactant-grown structure at high temperature. The periodic and well-deﬁned satellite peaks in addition to the TEM images show that an InAs–GaAs superlattice with 3 MLs of InAs was successfully grown at LT under arsenic-free conditions. Such
-3000 -1000 ω-2θ (arcsec)
Fig. 1. DCXRD rocking curves of InAs–GaAs QDs, sample no. 1949, and LT InAs–GaAs superlattice sample no. 1753.
-1000 ω-2θ (arcsec)
Fig. 3. DCXRD spectrum and TEM image (inset) of InAs–GaAs superlattice with 3 MLs InAs layers grown by the normal growth technique at low substrate temperature.
structure, LT growth of 3 ML InAs layers, could not be obtained with an impinging As ﬂux (NGC) as the InAs turned into a halolike structure within 1.5 ML. In this case, both GaAs and InAs ﬁlms were 3D and as a consequence both DCXRD and TEM showed no presence of a superlattice, see Fig. 3, where only the substrate peak is seen in the DCXRD rocking curve and no superlattice is visible in the TEM. This is due to the increase in the sticking coefﬁcient of the As adatoms at low substrate temperature, which in turns affects the movement of the indium adatoms and effectively acts as a diffusion barrier to their movement. A similar study by Missous  showed that the provision of stoichiometric beams at LT leads to 2D growth in the case of GaAs. It can be argued here that supplying an indium beam only and relying on the surrounding As in the chamber is akin to stoichiometric growth. In addition, at LT the diffusion coefﬁcient of group-III adatoms (migration length) is proportional to 1/T, where T is the substrate temperature [8,9]. Thus, growing InAs layers using only the In beam leads to a larger diffusion length for the In atoms and, consequently, the formation of two-dimensional layers with a thickness as high as 3 MLs, instead of the formation of a halo-like structure. At room temperature, sample no. 1949 shows a strong PL emission at about 1.246 mm with full-width at half-maximum (FWHM) of around 50.5 nm (40.5 meV), indicating a high uniformity in the size of the QDs. In addition, the PL spectrum of this sample shows another peak at around 1.18 mm with
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M. Alduraibi et al. / Microelectronics Journal 40 (2009) 476–478
Gaussian fit Peak 1
1.2 PL intensity (a.u.)
A detailed AFM study of the QDs in sample no. 1949 is shown in Fig. 5. QDs are well deﬁned with a dot density of around 1 109 cm 2. The average height of the QDs was about 4.7 nm and the width was about 97 nm. Unlike previous reports on 3 ML InAs QD grown under NGC , there was no presence of volcano-like defects.
Gaussian fit Peak 2
0.6 0.4 0.2 0 1000
1200 Wavelength (nm)
Fig. 4. Room temperature PL spectrum of sample no. 1949 showing Gaussian ﬁt for each peak. Inset shows the PL peak without ﬁtting.
InAs–GaAs layers were grown at normal and low growth temperatures by MBE under arsenic-free conditions. At normal growth temperature this growth mode still resulted in the formation of QDs, where a ten-period structure has been shown to be of high crystalline quality and free from the formation of any extended defects. In contrast to normal growth condition, at LT surfactant growth resulted in the formation of a high-quality InAs–GaAs superlattice with 3 ML-thick InAs layers. DCXRD studies showed this structure to be very well deﬁned, retaining a high degree of lattice integrity, and could hardly be distinguished from the surfactant-grown structure at normal temperature. These results indicate that the presence of excess arsenic at LT was directly responsible for inhibiting the growth of InAs thin ﬁlms. Therefore, surfactant-mediated growth, which occurs under near stoichiometric growth conditions, must be responsible for the integrity of the thick 3 ML InAs ﬁlms and the subsequent epitaxy of the LT-InAs–GaAs superlattice. References
Fig. 5. 3D AFM of sample no. 1949.
intensity almost ten times less than the intensity of the main peak and having a FWHM of around 90 nm (80.5 meV). Fig. 4 shows this spectrum with a Gaussian ﬁt for the peaks. The presence of these two peaks suggests that there are two different interband transitions in the QDs. One of these is the recombination of ground-state electrons with ground-state holes, whereas the smaller peak could come from the recombination of ground-state electrons with holes in the ﬁrst excited state [10,11]. Sample no. 1753 showed no noticeable PL emission. It is probable that this was due to the incorporation of the point defects at LTG [12,13], which act as nonradiative centres .
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