Formation process for and strain effect in InAs quantum dots grown on GaAs substrates by using molecular beam epitaxy

Solid State Communications 130 (2004) 473–476 www.elsevier.com/locate/ssc

Formation process for and strain effect in InAs quantum dots grown on GaAs substrates by using molecular beam epitaxy M.D. Kima, D.H. Leea, T.W. Kimb,*, S.G. Kimc a

Department of Physics, Chungnam National University, 220 Gung-dong, Yuseong-gu, Daejeon 305-764, South Korea Advanced Semiconductor Research Center, Division of Electrical and Computer Engineering, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, South Korea c Department of Mobile Communications, Joongbu University, Gumsan-gun, Chungnam 132-940, South Korea

b

Received 29 September 2003; accepted 23 February 2004 by T.T.M. Palstra

Abstract Reflection high-energy electron diffraction (RHEED) and atomic force microscopy (AFM) measurements were used to investigate the dependences of the formation process and the strain on the As/In ratio and the substrate temperature of InAs quantum dots (QDs) grown on GaAs substrates by using molecular beam epitaxy. The thickness of the InAs wetting layer and the shape and the size of the InAs QDs were significantly affected by the As/In ratio and the substrate temperature. The strains in the InAs layer and the GaAs substrate were studied by using RHEED patterns. The magnitude in strain of the InAs QDs formed at a low substrate temperature was larger than that in InAs QDs grown at high substrate temperature. The present results can help to improve the understanding of the formation process and the strain effect in InAs QDs. q 2004 Elsevier Ltd. All rights reserved. PACS: 68. 55. 2a; 68. 55. Jk; 68.65. Hb Keywords: A. Nanostructure; B. Crystal growth; C. Surface electron diffraction

Recently, rapid developments and improvements in monolayer growth techniques such as molecular beam epitaxy (MBE) have made it possible to produce defect-free quantum dots (QDs) on semiconductors [1,2]. The formations of self-assembled InAs QDs on GaAs substrates are particularly interesting due to their potential applications in nanoscale optoelectronic devices, such as QD lasers [3,4], optical memories [5], and infrared photodetectors [6,7]. The initial growth of a lattice-mismatched epilayer on a substrate due to the Stranski– Krastanow mode progresses in the two dimensions; then, three-dimensional (3D) QDs are formed on a residual two-dimensional (2D) wetting layer (WL). An appropriate regulation of the growth parameters, such as the As/In ratio and the substrate temperature, may provide the * Corresponding author. Tel.: þ82-2-2290-0354; fax: þ 82-22292-4135. E-mail addresses: [email protected] (T.W. Kim), mdkim@ cnu.ac.kr (M.D. Kim). 0038-1098/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2004.02.043

possibility for precisely controlling the sizes and the shapes of the QDs. Since studies concerning 3D growth modes are required for strain relaxation and for the formation of dislocation-free QDs, systematic studies of the formation processes for and strain effect in InAs QDs grown on GaAs substrates are very important if the device efficiencies are to be enhanced. This letter reports data on a formation process for obtaining InAs QDs of uniform size by changing the As/In ratio and the growth temperature and on the strain effect in InAs QDs grown on GaAs substrates by using MBE. Reflection high-energy electron diffraction (RHEED) and atomic force microscopy (AFM) measurements were performed to investigate in situ the formation and the surface morphology, respectively, of the InAs QDs. The dependence of the growth mode on the As/In ratio and the substrate temperature was investigated, and the strain in the InAs layer was determined on the basis of the RHEED patterns.

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The samples used in this study were grown on semiinsulating, (100)-oriented GaAs substrates by using a Riber 32 MBE system. The substrate native oxides were thermally removed at 540 8C under an As4 pressure of approximately 1025 Torr. We investigated the dependence of the formation processes for the InAs WL and the QDs by varying the In/As flux ratio and the substrate temperature. The In/As flux ratios during the growth were 20, 40, 55, and 85, and the entire growth process was monitored in situ by using RHEED patterns [8 – 11]. The RHEED patterns were controlled in real time, and the incident electron beam was parallel to either the [110] or the ½110 azimuth. The intensity variation of the specular beam and the primary streak of the RHEED pattern were recorded. At the same  diffraction time, the spacing between the (10) and the ð10Þ streaks of the [110] azimuth was monitored by using a charge coupled device camera; then, the spacing was analyzed with an image processing system. AFM measurements were carried out in order to analyze the surface morphology of the sample and the shapes and the sizes of the QDs. The RHEED pattern is a sensitive monitor for investigating of the growth mode transition from the 2D to the 3D mode in the formation of self-assembled QDs [12]. A streaky RHEED pattern indicates that the WL layer is a 2D layer. As soon as 3D islands are formed, the streaky pattern abruptly changes to a spotty one. The thickness and the quality of the InAs WL depend significantly on the growth conditions. The transition time from the streaky to the spotty pattern is related to the size of the InAs QDs. Fig. 1 shows the primary (10) streak intensities taken from the RHEED patterns of the InAs surfaces grown at 430 8C with As/In ratios of 85, 55, 40, and 20. Regions I, II, and III are attributed to the GaAs buffer layer, the InAs WL, and the

Fig. 1. Intensities of the RHEED patterns along the primary (10) rod along the [110] azimuths for the InAs layer as a function of the growth time for As/In ratios of (a) 85, (b) 55, (c) 40, and (d) 20 at a substrate temperature of 430 8C. The insert represents an atomic force microscopy image of InAs quantum dots grown at an As/In flux ratio of 40.

InAs QDs, respectively. The arrows indicate the transition time from the 2D to the 3D stage of the InAs growth mode. The intensity of the RHEED pattern dramatically increases just after the formation of the InAs QDs. In addition, as soon as the InAs WL starts to grow, the intensity of the RHEED pattern increases slightly and then decreases, forming a hump, as shown in Fig. 1. Even though the initial stage time of the lump is independent of the As/In ratio, the full width at half maximum (FWHM) and the intensity of the lump slightly increase with decreasing As/In ratio. AFM images show that the size and the density of the QDs increase with decreasing As/In ratio. This increase of the size and the density of the QDs with decreasing As/In ratio can be explained by taking into account the strain effect between the InAs layer and the GaAs substrate. The height, diameter, and density of the InAs QDs grown with an As/In ratio of 40, determined from the AFM image, are 6 nm, 35 nm, and 5 £ 1010 cm22, respectively. The InAs layer during the initial 0.2-monolayer (ML) growth follows a 2D growth process to maintain thermodynamically stability, and the InAs WL receives a compressive stress during a metastable growth period. Thus, the RHEED intensity of the InAs layer increases slightly due to the surface stress. The thickness of the InAs WL increased from 1 to 1.5 ML with decreasing As/In flux ratio from 85 to 40, as shown in Fig. 1(a) – (c). However, when the As/In ratio was decreased far to 20, the thickness of the InAs WL decreased, as shown in Fig. 1(d). This behavior is related to a reduction in the In diffusion length at an As/In ratio of 20. The intensity of the RHEED pattern qualitatively corresponds to the magnitude of the As coverage. When the InAs WLs are completely formed, the RHEED intensity rapidly increases. This result indicates that the surface roughness decreases due to strain relaxation. However, after the QDs are formed, the intensities are almost constant because of the strain relaxation. Since the InAs QDs are formed by increasing the thickness of the InAs layer, the intensity variation during the formation of the QDs is important for obtaining defectfree QDs. Fig. 2 shows the specular beam (00) and primary streak (10) intensities of the RHEED patterns for the InAs layer as function of the growth time for As/In ratio of 85 at a substrate temperature 480 8C. The transition point from 2D to 3D growth cannot be clearly observed in the specular beam. However, when the substrate temperature was increased from 430 to 480 8C, the thickness of the InAs WL increased from 1 to 1.4 ML, as shown in Figs. 1(a) and 2. The increase in the InAs WL with increasing substrate temperature is attributed to an enhancement of the In diffusion length at higher growth temperatures. The intensity of the specular beam indicates that the InAs grows initially as a 2D layer, and followed by 3D islands. The intensity of the specular beam in region II decreases dramatically at the initial growth stage of the InAs layer. On the other hand, the intensity of the primary streak dramatically increases just

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due to the strain effect. The shape of the QDs is affected by the As/In ratio during the growth of the InAs layer. Domed islands are formed by adjusting the As/In ratio, as shown in the inset of Fig. 3. The strain in the InAs layer was investigated as a function of the growth time. The spacing ðdÞ between the  diffraction streaks can be determined from (10) and the ð10Þ the RHEED patterns, and d represents the reciprocal distance of the in-plane lattice parameter. The surface lattice parameter indicates the characteristics of the strain relaxation mechanisms. Fig. 4(a) and (b) shows the dependence of the strain ( f ) on the growth time for the InAs layers grown at 480 and 430 8C, respectively. The insets represent the RHEED patterns obtained at the initial and the final stages of the InAs QD formation. The value of the f can be calculated by using the equation f ¼ ðaInAs 2 aGaAs Þ=aInAs ; Fig. 2. Specular beam (00) and primary streak (10) intensities of the RHEED patterns along the [110] azimuths for the InAs layer as functions of the growth time for an As/In ratio of 85 and a substrate temperature of 480 8C. The insert is an atomic force microscopy image of InAs quantum dots grown at an As/In flux ratio of 85 and a substrate temperature of 480 8C.

after InAs QD formation. After the growth was interrupted, the intensity increased due to thermal annealing. The dependence of the width/height ratio along the [110]  directions of the InAs QDs grown at 430 8C and the ½110 for different As/In ratios, as determined from the profile of the AFM images, is shown in Fig. 3. The ratio between the width and the height of the QDs for the [110] direction is almost constant regardless of the variation in the As/In ratio. However, the average width of the QDs grown at low As/In  direction become smaller, and that ratio along the ½110 decrease indicates that the In molecules are conglomerated

Fig. 3. Width/height ratio of the InAs QDs along the [110] and the  azimuths determined from the atomic force microscopy ½110 images as functions of the As/In ratio at a substrate temperature of 430 8C. The insert is an atomic force microscopy image of InAs domed islands grown at an As/In ratio of 40 and a substrate temperature of 430 8C.

Fig. 4. Strain for the InAs layer, determined from the spacing d  diffraction streaks of the RHEED between the (10) and the ð10Þ pattern, as a function of the growth time grown at substrate temperatures of (a) 480 8C and (b) 430 8C. The inserts are RHEED patterns of (i) the initial stage and (ii) the final stage of the InAs QDs.

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where aInAs is the lattice constant of the strained InAs layer, and aInAs is that of the strain-free InAs layer. As Fig. 4(a) shows, as soon as the InAs WL is formed, the value of the strain increases slightly; then, decreases smoothly. This result is in reasonable agreement with the variation of the RHEED intensity and is indicative of the existence of a hump resulting from the strain. The strain dramatically increases and decreases at the initial and final stages of QD formation, and the strain increases slightly after growth interruption of the InAs layer. The strain behavior in Fig. 4(b) for As/In ¼ 20 is similar to that in Fig. 4(a), except that the magnitude of the strain increases more significantly just before and after the formation of the QDs. This result indicates that, just before and after the QD formation, the strain in the InAs QDs grown at low temperatures is larger than in the InAs QDs grown that at high temperatures. A remarkable sustained tensile strain is observed in the InAs layers grown on GaAs substrates. Even though this behavior is not yet understood, the variation of the strain effect can help improve understanding of ways to control the shapes and sizes of QDs. The strains in the InAs WL and in the InAs QDs depend significantly on the As/In flux ratio and the growth temperature. This observation suggests that the strain in the InAs layer is initially relaxed due to the formation of surface islands rather than to the formation of dislocations. Therefore, the dislocations generated in QDs can be removed by controlling the As/In ratio and the growth time. In summary, the dependences of the formation process and the strain effect on the As/In ratio and the substrate temperature of InAs QDs grown on GaAs substrates were investigated by using RHEED patterns and AFM images. The thickness of the InAs wetting layer and the shapes and the sizes of the InAs QDs were significantly affected by the As/In ratio and the substrate temperature. These observations can help to improve understanding of the formation

process for and the strain effects in InAs QDs grown on GaAs substrates.

Acknowledgements This work was supported by the Nano Research and Development Program (Grant No. M1-0212-04-0002).

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