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GaAs quantum dots
Surface Science 492 (2001) 345±353
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Shape and surface morphology changes during the initial stages of encapsulation of InAs/GaAs quantum dots P.B. Joyce, T.J. Krzyzewski, P.H. Steans, G.R. Bell, J.H. Neave, T.S. Jones * Department of Chemistry, Centre for Electronic Materials and Devices, Department of Chemistry, Imperial College of Science, Technology and Medicine, South Kensington, London SW7 2AY, UK Received 9 February 2001; accepted for publication 26 July 2001
Abstract The change in shape and surface morphology of InAs/GaAs(0 0 1) quantum dots (QDs) during their initial encapsulation by GaAs has been studied using re¯ection high energy electron diraction (RHEED) and scanning tunnelling microscopy (STM). The shape of the QDs changes signi®cantly during the earliest stages of overgrowth. In situ RHEED measurements show a signi®cant chevron angle change after deposition of only 2 ML of GaAs, before losing all crystallographic structure as more GaAs is deposited. STM results indicate signi®cant surface mass transport, the height of the QDs decreases faster than the rate at which the GaAs capping layer is deposited and the QDs eectively ``collapse'' during the earliest stages of encapsulation. The area of the partially capped QDs increases signi®cantly with increasing GaAs deposition and there is an anistropic increase in shape with signi®cant elongation along the [ 1 1 0] azimuth. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Single crystal epitaxy; Indium arsenide; Gallium arsenide; Quantum eects
1. Introduction The growth of self-assembled InAs quantum dots (QDs) on GaAs(0 0 1) surfaces, particularly via molecular beam epitaxy (MBE), continues to stimulate very high levels of research activity. The fundamental aspects of epitaxial growth in this complex strained system are of great interest, as are the potential applications of `capped' InAs/ GaAs QD structures in optoelectronics. These structures are based on QD arrays buried by sub-
* Corresponding author. Tel.: +44-20-7594-5794; fax: +4420-7594-5801. E-mail address: [email protected] (T.S. Jones).
sequent GaAs or Inx Ga(Al)1 x As overgrowth. While much eort has been devoted to understanding the processes involved during QD nucleation and growth, far less attention has been paid to the overgrowth and capping processes, despite the fact that capping is an essential step in the production of QD-based devices. Similarly, QD properties such as the shape, strain state and composition pro®le have been rather more closely studied for uncapped QDs than for capped QDs. It is principally these properties which must be de®ned in order to assist realistic calculations of the electronic and optical properties of capped QD systems. There is still considerable controversy over the shape of uncapped InAs QDs, with a variety of
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structures proposed over the last few years [1±8]. These have been based on a number of experimental techniques including scanning tunnelling microscopy (STM) [3,5±7], atomic force microscopy [4] and re¯ection high energy electron diraction (RHEED) [1,4,5]. Several high- and low-index crystallographic planes have been suggested as candidates for stable QD surfaces. However a recent study of refraction eects in electron scattering from InAs/GaAs QDs has called into question the simple interpretation of commonly observed ``chevrons'' in RHEED as relating to well-de®ned QD side facets [9]. As well as QD shape, the composition [5,8] and strain state [8] of uncapped QDs have also been investigated experimentally. The properties of capped QDs have also been studied, using experimental techniques such as transmission electron microscopy (TEM) [10±13], cross-sectional STM (XSTM) [14±17], scanning TEM (STEM) with local energy dispersive X-ray analysis (EDX) [18,19] and incident ion beam techniques [20,21]. Several studies have highlighted signi®cant dierences of shape [10,13,15,20±23], strain state [20,21] and composition [18,19] between capped and uncapped structures. However, most theoretical calculations of the electronic and optical properties of InAs/GaAs QDs have employed model QD structures reminiscent of uncapped QDs (e.g. Refs. [24±26]). Experimental evidence for comparison with such calculations, primarily obtained using photoluminescence, is based on studies of capped QDs. Signi®cant differences from highly symmetric, ideal uncapped QD structures are expected [27]. In this paper, we concentrate on the changes that a single layer of InAs/GaAs QDs undergoes during the initial stages of encapsulation with GaAs. We demonstrate using RHEED and STM, that from the ®rst monolayer (ML) of GaAs overgrowth, there is a rapid change in shape and size, which leaves the fully encapsulated layer showing little morphological resemblance to the original uncapped QDs. The results con®rm: that it is a rather dubious procedure to relate measured optical and electronic properties, or theoretical calculations of electronic structure, to the shape, size and composition of the initial uncapped QDs.
2. Experimental details The growth of the samples was performed in two dierent MBE systems; one purpose built with speci®c modi®cations for RHEED analysis, and a second combined MBE-STM system (DCA Instruments/Omicron Gmbh), speci®cally designed for detailed STM analysis, but also equipped with RHEED. For both systems, the Ga and In sources were calibrated using RHEED intensity oscillation measurements of homoepitaxial growth on GaAs(0 0 1) and InAs(0 0 1) substrates. Epi-ready GaAs(0 0 1) substrates (n Si doped) were used for the growth of the QDs and the sample preparation in both systems involved outgassing at 300 °C, before removal of the native oxide layer under an As2 ¯ux at 620 °C. A 0.6 lm thick GaAs buer layer was grown at 590 °C, the substrate temperature was then reduced to 510 °C and the sample annealed for 5 min at that temperature. This led to a c 4 4 RHEED pattern. The substrate temperature was calibrated by monitoring the oxide desorption temperature, the 2 4 to c 4 4 change in reconstruction, and optical pyrometer measurements. The QD samples examined in the RHEED system were grown by depositing 2 ML of InAs at an In ¯ux of 1:08 1014 atoms cm 2 s 1 , which corresponds to a deposition rate of 0.2 ML s 1 . The samples were subsequently capped with GaAs using a Ga ¯ux of 6:26 1014 atoms cm 2 s 1 (1.0 ML s 1 ), and throughout QD and capping layer growth the substrate temperature was 510 °C. The QD samples used for STM analysis were grown using an In ¯ux of 7:02 1013 atoms cm 2 s 1 (0.13 ML s 1 ) and encapsulated with a Ga ¯ux of 3:13 1014 atoms cm 2 s 1 (0.5 ML s 1 ). The amount of InAs deposited was ®xed at 2.3 ML and the GaAs capping thickness varied up to 30 ML. The samples were subsequently transferred via a rapid quench into the STM chamber. The process of quenching was completed within approximately ®ve seconds with the samples cooling very quickly to room temperature. This provides a distinct advantage over conventional MBE systems, as it eectively `freezes' the growth and removes additional eects caused by sample annealing, for example alloying and segregation. Deliberate
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annealing of partly capped QDs at the growth temperature for <30 s under an As2 ¯ux prior to quenching causes only minor changes in the surface reconstruction and no observable changes in the overall surface morphology. Constant current STM images were obtained with a sample bias of 3.5 V and tunnelling currents of 0.1±0.3 nA. 3. Results and discussion It is well established that the RHEED pattern is a sensitive monitor of the 2D ! 3D growth mode transition in self-assembled QD formation. The streaky pattern that characterises the formation of the 2D wetting layer during the initial stages of InAs deposition changes dramatically at the onset of 3D island formation. The amount of InAs required for the transition depends on growth temperature [5] (typically between 1.4 and 2.0 ML), but is independent of the In ¯ux. A typical diffraction pattern recorded along the [ 1 1 0] direction after 2 ML InAs deposition is shown in Fig. 1(a). Characteristic chevrons are observed along the [ 1 1 0] direction with a half angle of 25° (1:5°). The presence of chevrons is generally believed to relate to crystallographically well-de®ned facet planes which form the QD sidewalls [1,4,5]. However, recent calculations of refraction eects [9] suggest that QD sidewalls with a well-de®ned polar angle relative to the (0 0 1) surface (close to 25°) combined with a variable azimuthal angle result in the observed chevron patterns. It should be noted that chevrons were not observed in the diraction pattern recorded along the orthogonal [1 1 0] direction, although chevrons were seen along other azimuths. Fig. 1 shows how the RHEED patterns evolve along [ 1 1 0] with increasing GaAs overgrowth; (a) (1 ML), (c) 14 A (5 ML) uncapped QDs, (b) 2.8 A (18 ML). The chevrons that are and (d) 50.4 A characteristic of 3D island formation are still present after 1 ML GaAs deposition, but the half angle decreases to 20° (1.5°). This is consistent with a reduction in the polar angle of the QD sidewalls to 20° [9] indicating a `¯attening' of the QDs. As more GaAs is deposited the chevrons continue to collapse further into the rods and it
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becomes increasingly dicult to extract any characteristic chevron angles above 2 ML GaAs thickness; the chevrons require 5 ML of GaAs to be deposited before they completely collapse onto the underlying diraction rods. The RHEED pattern becomes rather weak and the diraction features poorly de®ned. The ``dim-diuse'' nature of the pattern continues during deposition of the next 5 ML of GaAs, before the pattern starts to recover and resemble the RHEED pattern obtained from the 2 4 reconstructed GaAs(0 0 1) buer layer. It should be noted that these three stages of evolution; disappearance of the chevrons, dim-diuse patterns and recovery, were also observed for the overgrowth of QDs formed at lower temperatures, although the amounts of GaAs required for each stage dier. For example, at 450 °C, the chevrons collapse completely after deposition of 3 ML GaAs, the dim-diuse pattern is observed for the next 3±5 ML of GaAs, and complete recovery requires 7 ML of GaAs. The initial disappearance of the chevrons in the RHEED patterns is consistent with major morphological changes arising from the complex dynamic processes occurring during the earliest stages of encapsulation, i.e. Ga incorporation, In segregation and resulting diusion from the top to the base of the 3D islands. The 3D features in the RHEED pattern persist for longer at the highest temperatures, consistent with the initial 3D islands being larger [5] and requiring more GaAs for subsequent encapsulation. The appearance of the weak and diuse RHEED patterns before recovery suggests the presence of a disordered or ``liquidlike'' phase on the surface, consistent with a high degree of mass transport during encapsulation. 2 ) The sequence of STM images (2000 2000 A in Fig. 2 illustrate how the surface morphology evolves during deposition of the GaAs capping layer. Fig. 2(a) shows a representative image for the uncapped QDs; for these growth conditions the number density is 6 1010 cm 2 , and the ave respecrage height and width are 41 and 220 A tively. The STM image in (b) corresponds to of GaAs. At this stage in overgrowth with 4.2 A the capping process the morphology of the surface appears very similar to that of the uncapped QDs, however there are signi®cant changes in their size.
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Fig. 1. RHEED patterns recorded along the [1 1 0] azimuth for (a) uncapped InAs/GaAs(0 0 1) QDs, and after overgrowth with (b) 1 ML, (c) 5 ML and (d) 18 ML of GaAs. (e) and (f) are close-ups of the chevrons from (a) and (b) respectively, and highlight the reduction in the chevron angle with GaAs deposition. The chevron angle is measured by using an enlarged RHEED image to draw lines through the midpoints of several cross-sections taken at dierent points along the length of the chevron. The angle between the centroids gives a value for the chevron angle with an accuracy of 3°.
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2 ) at various stages of GaAs overgrowth: (a) uncapped InAs/GaAs QDs; (b)±(e) Fig. 2. Filled states STM topographs (2000 2000 A 2 ) recorded after 5 ML GaAs GaAs cap thickness of 1.5, 2.5, 5 and 30 ML respectively. (f) A higher resolution image (200 200 A overgrowth showing the coexistence of the locally ordered 2 4 and disordered 1 3 reconstructed domains.
and the The average height is reduced to 35 A Fig. 2(c) length increases along [ 1 1 0] by 50 A. GaAs deposition. shows the morphology after 7 A Major changes are now apparent with the average despite only height of the QDs reduced by 15 A,
of GaAs being deposited during the capping 7A procedure. The QD width, de®ned along [1 1 0], and the corresponding has increased by 50 A The surface length, de®ned along [1 1 0], by 100 A. morphology around the QDs also changes at this
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stage of encapsulation with the QDs surrounded by 2D islands, which have coalesced with adjacent 2D islands. Close inspection of the 2D islands indicate a 1 3 surface reconstruction, similar to the reconstruction observed in the wetting layer of the uncapped QDs and generally indicative of an Inx Ga1 x As surface alloy [28]. It is likely therefore that these 2D islands are also an Inx Ga1 x As alloy. Further changes occur when more GaAs is de the posited. For a GaAs capping thickness of 14 A, surface consists of large 3D features of ill-de®ned shape and the majority of islands have undergone coalescence with neighbouring islands. The surface structure is highly disordered, but it is possible to distinguish local patches of 2 4 reconstruction both on top of the 3D features and in the surrounding areas. This is highlighted in the high 2 ) shown in Fig. 2(f). resolution image (200 200 A It is important to note that the RHEED patterns observed at this cap thickness are no longer characterised by transmission diraction spots (Fig. 1(c)). STM height cross-sections taken across the mounds reveal discrete steps which are 3 A high and suggest the presence of monolayer GaAs GaAs, the surface steps. After deposition of 84 A is very rough and the surface reconstruction completely 2 4, although 3D features can still be observed where the GaAs has grown on top of the original QDs (Fig. 2(e)). The changes in QD size as a function of capping thickness provide important information about the processes that occur during the overgrowth of the QDs, and these are quanti®ed in Fig. 3. The schematic in Fig. 3(a) shows how the height and lateral dimensions of the partially capped QDs are de®ned with respect to the amount of GaAs overgrowth. Fig. 3(b) shows the change in QD height (hQD ) as a function of the GaAs capping layer thickness. The solid line indicates the height of the QDs expected (ho hcap ) if the GaAs simply grew up and around the QDs. The measured QD height (hQD ) decreases at a signi®cantly faster rate than the height increase of the capping layer and it appears that the QDs collapse rapidly during the initial stages of overgrowth. Fig. 3(c) shows how the lateral dimensions (d) of the QDs taken along the [1 1 0] and [ 1 1 0] direction are aected by increasing GaAs cap thickness. There is a rapid in-
crease in size along both directions, but the change is highly anisotropic, the width along [1 1 0] increasing by a factor of 1.75, and the length measured along [1 1 0] increasing by a factor of GaAs. The error 2.8 after deposition of 45 A bars in Fig. 3 represent a combination of statistical variation among the mounds or QDs and the experimental error. In the case of the height data the experimental error is approximately 1 ML thickness, while for the diameters the error relates to the diculty in de®ning mound edges, particularly at high capping thicknesses. It is straightforward to measure the total volume per unit area of the QDs prior to capping and during the initial stages of overgrowth (<4 ML) when the QDs are still well de®ned. At later stages of capping, however, the mounds become too ill de®ned to calculate their volumes. Note that no assumption about the QD shape need be made since the volumes are directly integrated from STM topographs. Prior to capping the QD volume 3 cm 2 and this rises to 3:5 is 2 0:2 1016 A 16 3 0:2 10 A cm 2 after deposition of 1.5 ML of GaAs. This signi®cant increase is due to In±Ga intermixing in and around the QDs and there is no evidence for any reduction of the QD volume caused by In evaporation [10]. The measured changes in height, length and width are consistent with the RHEED results, which show that the facet angle of the 3D islands changes during the earliest stages of encapsulation. It is clear that the structure and composition of the QDs undergo rapid changes, with the incident Ga atoms being incorporated into the structure and In atoms diusing from the top to the base of the 3D islands. In eect the 3D islands appear to ``collapse'' down into the surface during the initial encapsulation process. The apparent collapse of the QDs raises important questions regarding the stability of the QDs. Uncapped QDs grown under a range of growth rates are highly stable to post-growth annealing, whereas the addition of small amounts of GaAs leads to dramatic changes in surface structure and morphology. During capping the surface area is reduced rather quickly. If the surface energy of the InAs island sides is increased signi®cantly by the addition of Ga adatoms, as may
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Fig. 3. (a) Schematic showing how the size of the QDs was obtained from STM images after GaAs overgrowth; d is the lateral dimensions of the QDs, hQD the height of the partially capped QDs, ho the initial QD height and hcap the height of the GaAs capping of the partially capped QDs as a function of the GaAs cap thickness. The solid layer. (b) The measured change in height (hQD =A) horizontal line is the height of the partially capped QDs (ho hcap ) expected if the GaAs simply grows up and around the QDs. (c) The eect of overgrowth on the lateral dimensions (d) of the QDs. The solid circles show the increase in width along [1 1 0] and the triangles represent the increase in length along [1 1 0].
be the case for a stable, strained and reconstructed side facet, this would provide a strong driving force for the reduction in surface area as the islands collapse during encapsulation. Clearly, the entropy of mixing is also increased during this process, although this may not be signi®cant compared with the change of surface and strain energy. Furthermore, strain relief by relaxation at the top of the QDs becomes less ecient. The observed changes in size and shape, and the implication for composition, have important consequences for any modelling of the electronic structure of capped InAs/GaAs QDs. For exam-
ple, calculations for idealised QDs based on square based pyramids and comprising 100% InAs are clearly inappropriate and any theoretical model must take into account the intermixing of Ga and In atoms, both prior to and during the overgrowth process. For example, recent STEMEDX measurements [19] have shown that the degree of intermixing after overgrowth can be as large as 40%, even though the uncapped QDs (grown at very low deposition rates) are virtually pure InAs [29]. The dramatic shape changes which occur during the initial stages of capping clearly provide a mechanism for intermixing based on
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surface diusion rather than bulk In±Ga interdiffusion, which is presumably a very slow process at temperatures around 510 °C. Finally, our results may also provide an explanation for the often reported vertical self-alignment that occurs when multilayer QD structures are grown and separated by only a thin GaAs GaAs) [16±18,30, spacer layer (typically <150 A 31]. The self-alignment is generally attributed to strain ®elds arising from the underlying QD layer. Fig. 2(e) shows a typical starting surface for the growth of the second layer. The presence of mounds and the highly disordered surface morphology after GaAs deposition on top of the ®rst layer of QDs is likely to have a strong in¯uence on the nucleation of the second layer of QDs. We suggest that second layer QD nucleation on top of these mounds may provide an additional mechanism for strain relief and lead to the observed selfalignment. 4. Conclusions In summary, we have shown that the shape of InAs/GaAs QDs changes dramatically during GaAs encapsulation. In situ RHEED measurements show that the sidewalls of the QDs become shallower and eventually lose all their crystallographic information. STM images reveal that the QDs collapse rapidly with the average height decreasing faster than the rate at which the GaAs is deposited. The area of the QDs increases considerably, the processes being highly anisotropic with signi®cant lengthening occurring along 1 1 0. The capping of InAs/GaAs QDs with GaAs clearly has important implications for the size, shape and composition of the QDs, which are key parameters in determining the electronic properties of these important structures. Our results suggest that the surface species are highly mobile during encapsulation and a signi®cant amount of mass transport and alloying occurs. In view of these results, it is essential that the changes in the shape and composition of the QDs during capping have to be fully considered when proposing models to account for the novel electronic properties of these con®ned structures.
Acknowledgements This work was supported by the EPSRC, UK, who also provided studentships for PBJ and TJK. GRB is grateful to the Ramsay Memorial Trust for the provision of a Research Fellowship, funded in part by VG Semicon Ltd. (UK).
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[26] W. Yang, H. Lee, T.J. Johnson, P.C. Sercel, A.G. Norman, Phys. Rev. B 61 (2000) 2784. [27] R. Heitz, O. Stier, I. Mukhametzhanov, A. Madhukar, D. Bimberg, Phys. Rev. B 62 (2000) 11017. [28] J.G. Belk, C.F. McConville, J.L. Sudijono, T.S. Jones, B.A. Joyce, Surf. Sci. 387 (1997) 213. [29] P.B. Joyce, T.J. Krzyzewski, G.R. Bell, T.S. Jones, S. Malik, D. Childs, R. Murray, Phys. Rev. B 62 (2000) 10891. [30] I. Mukhametzhanov, R. Heitz, J. Zeng, P. Chen, A. Madhukar, Appl. Phys. Lett. 73 (1998) 1841. [31] G.S. Solomon, J.A. Trezza, A.F. Marshall, J.S. Harris Jr., Phys. Rev. Lett. 76 (1996) 952.
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Shape and surface morphology changes during the initial stages of encapsulation of InAs/GaAs quantum dots P.B. Joyce, T.J. Krzyzewski, P.H. Steans, G.R. Bell, J.H. Neave, T.S. Jones * Department of Chemistry, Centre for Electronic Materials and Devices, Department of Chemistry, Imperial College of Science, Technology and Medicine, South Kensington, London SW7 2AY, UK Received 9 February 2001; accepted for publication 26 July 2001
Abstract The change in shape and surface morphology of InAs/GaAs(0 0 1) quantum dots (QDs) during their initial encapsulation by GaAs has been studied using re¯ection high energy electron diraction (RHEED) and scanning tunnelling microscopy (STM). The shape of the QDs changes signi®cantly during the earliest stages of overgrowth. In situ RHEED measurements show a signi®cant chevron angle change after deposition of only 2 ML of GaAs, before losing all crystallographic structure as more GaAs is deposited. STM results indicate signi®cant surface mass transport, the height of the QDs decreases faster than the rate at which the GaAs capping layer is deposited and the QDs eectively ``collapse'' during the earliest stages of encapsulation. The area of the partially capped QDs increases signi®cantly with increasing GaAs deposition and there is an anistropic increase in shape with signi®cant elongation along the [ 1 1 0] azimuth. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Single crystal epitaxy; Indium arsenide; Gallium arsenide; Quantum eects
1. Introduction The growth of self-assembled InAs quantum dots (QDs) on GaAs(0 0 1) surfaces, particularly via molecular beam epitaxy (MBE), continues to stimulate very high levels of research activity. The fundamental aspects of epitaxial growth in this complex strained system are of great interest, as are the potential applications of `capped' InAs/ GaAs QD structures in optoelectronics. These structures are based on QD arrays buried by sub-
* Corresponding author. Tel.: +44-20-7594-5794; fax: +4420-7594-5801. E-mail address: [email protected] (T.S. Jones).
sequent GaAs or Inx Ga(Al)1 x As overgrowth. While much eort has been devoted to understanding the processes involved during QD nucleation and growth, far less attention has been paid to the overgrowth and capping processes, despite the fact that capping is an essential step in the production of QD-based devices. Similarly, QD properties such as the shape, strain state and composition pro®le have been rather more closely studied for uncapped QDs than for capped QDs. It is principally these properties which must be de®ned in order to assist realistic calculations of the electronic and optical properties of capped QD systems. There is still considerable controversy over the shape of uncapped InAs QDs, with a variety of
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structures proposed over the last few years [1±8]. These have been based on a number of experimental techniques including scanning tunnelling microscopy (STM) [3,5±7], atomic force microscopy [4] and re¯ection high energy electron diraction (RHEED) [1,4,5]. Several high- and low-index crystallographic planes have been suggested as candidates for stable QD surfaces. However a recent study of refraction eects in electron scattering from InAs/GaAs QDs has called into question the simple interpretation of commonly observed ``chevrons'' in RHEED as relating to well-de®ned QD side facets [9]. As well as QD shape, the composition [5,8] and strain state [8] of uncapped QDs have also been investigated experimentally. The properties of capped QDs have also been studied, using experimental techniques such as transmission electron microscopy (TEM) [10±13], cross-sectional STM (XSTM) [14±17], scanning TEM (STEM) with local energy dispersive X-ray analysis (EDX) [18,19] and incident ion beam techniques [20,21]. Several studies have highlighted signi®cant dierences of shape [10,13,15,20±23], strain state [20,21] and composition [18,19] between capped and uncapped structures. However, most theoretical calculations of the electronic and optical properties of InAs/GaAs QDs have employed model QD structures reminiscent of uncapped QDs (e.g. Refs. [24±26]). Experimental evidence for comparison with such calculations, primarily obtained using photoluminescence, is based on studies of capped QDs. Signi®cant differences from highly symmetric, ideal uncapped QD structures are expected [27]. In this paper, we concentrate on the changes that a single layer of InAs/GaAs QDs undergoes during the initial stages of encapsulation with GaAs. We demonstrate using RHEED and STM, that from the ®rst monolayer (ML) of GaAs overgrowth, there is a rapid change in shape and size, which leaves the fully encapsulated layer showing little morphological resemblance to the original uncapped QDs. The results con®rm: that it is a rather dubious procedure to relate measured optical and electronic properties, or theoretical calculations of electronic structure, to the shape, size and composition of the initial uncapped QDs.
2. Experimental details The growth of the samples was performed in two dierent MBE systems; one purpose built with speci®c modi®cations for RHEED analysis, and a second combined MBE-STM system (DCA Instruments/Omicron Gmbh), speci®cally designed for detailed STM analysis, but also equipped with RHEED. For both systems, the Ga and In sources were calibrated using RHEED intensity oscillation measurements of homoepitaxial growth on GaAs(0 0 1) and InAs(0 0 1) substrates. Epi-ready GaAs(0 0 1) substrates (n Si doped) were used for the growth of the QDs and the sample preparation in both systems involved outgassing at 300 °C, before removal of the native oxide layer under an As2 ¯ux at 620 °C. A 0.6 lm thick GaAs buer layer was grown at 590 °C, the substrate temperature was then reduced to 510 °C and the sample annealed for 5 min at that temperature. This led to a c 4 4 RHEED pattern. The substrate temperature was calibrated by monitoring the oxide desorption temperature, the 2 4 to c 4 4 change in reconstruction, and optical pyrometer measurements. The QD samples examined in the RHEED system were grown by depositing 2 ML of InAs at an In ¯ux of 1:08 1014 atoms cm 2 s 1 , which corresponds to a deposition rate of 0.2 ML s 1 . The samples were subsequently capped with GaAs using a Ga ¯ux of 6:26 1014 atoms cm 2 s 1 (1.0 ML s 1 ), and throughout QD and capping layer growth the substrate temperature was 510 °C. The QD samples used for STM analysis were grown using an In ¯ux of 7:02 1013 atoms cm 2 s 1 (0.13 ML s 1 ) and encapsulated with a Ga ¯ux of 3:13 1014 atoms cm 2 s 1 (0.5 ML s 1 ). The amount of InAs deposited was ®xed at 2.3 ML and the GaAs capping thickness varied up to 30 ML. The samples were subsequently transferred via a rapid quench into the STM chamber. The process of quenching was completed within approximately ®ve seconds with the samples cooling very quickly to room temperature. This provides a distinct advantage over conventional MBE systems, as it eectively `freezes' the growth and removes additional eects caused by sample annealing, for example alloying and segregation. Deliberate
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annealing of partly capped QDs at the growth temperature for <30 s under an As2 ¯ux prior to quenching causes only minor changes in the surface reconstruction and no observable changes in the overall surface morphology. Constant current STM images were obtained with a sample bias of 3.5 V and tunnelling currents of 0.1±0.3 nA. 3. Results and discussion It is well established that the RHEED pattern is a sensitive monitor of the 2D ! 3D growth mode transition in self-assembled QD formation. The streaky pattern that characterises the formation of the 2D wetting layer during the initial stages of InAs deposition changes dramatically at the onset of 3D island formation. The amount of InAs required for the transition depends on growth temperature [5] (typically between 1.4 and 2.0 ML), but is independent of the In ¯ux. A typical diffraction pattern recorded along the [ 1 1 0] direction after 2 ML InAs deposition is shown in Fig. 1(a). Characteristic chevrons are observed along the [ 1 1 0] direction with a half angle of 25° (1:5°). The presence of chevrons is generally believed to relate to crystallographically well-de®ned facet planes which form the QD sidewalls [1,4,5]. However, recent calculations of refraction eects [9] suggest that QD sidewalls with a well-de®ned polar angle relative to the (0 0 1) surface (close to 25°) combined with a variable azimuthal angle result in the observed chevron patterns. It should be noted that chevrons were not observed in the diraction pattern recorded along the orthogonal [1 1 0] direction, although chevrons were seen along other azimuths. Fig. 1 shows how the RHEED patterns evolve along [ 1 1 0] with increasing GaAs overgrowth; (a) (1 ML), (c) 14 A (5 ML) uncapped QDs, (b) 2.8 A (18 ML). The chevrons that are and (d) 50.4 A characteristic of 3D island formation are still present after 1 ML GaAs deposition, but the half angle decreases to 20° (1.5°). This is consistent with a reduction in the polar angle of the QD sidewalls to 20° [9] indicating a `¯attening' of the QDs. As more GaAs is deposited the chevrons continue to collapse further into the rods and it
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becomes increasingly dicult to extract any characteristic chevron angles above 2 ML GaAs thickness; the chevrons require 5 ML of GaAs to be deposited before they completely collapse onto the underlying diraction rods. The RHEED pattern becomes rather weak and the diraction features poorly de®ned. The ``dim-diuse'' nature of the pattern continues during deposition of the next 5 ML of GaAs, before the pattern starts to recover and resemble the RHEED pattern obtained from the 2 4 reconstructed GaAs(0 0 1) buer layer. It should be noted that these three stages of evolution; disappearance of the chevrons, dim-diuse patterns and recovery, were also observed for the overgrowth of QDs formed at lower temperatures, although the amounts of GaAs required for each stage dier. For example, at 450 °C, the chevrons collapse completely after deposition of 3 ML GaAs, the dim-diuse pattern is observed for the next 3±5 ML of GaAs, and complete recovery requires 7 ML of GaAs. The initial disappearance of the chevrons in the RHEED patterns is consistent with major morphological changes arising from the complex dynamic processes occurring during the earliest stages of encapsulation, i.e. Ga incorporation, In segregation and resulting diusion from the top to the base of the 3D islands. The 3D features in the RHEED pattern persist for longer at the highest temperatures, consistent with the initial 3D islands being larger [5] and requiring more GaAs for subsequent encapsulation. The appearance of the weak and diuse RHEED patterns before recovery suggests the presence of a disordered or ``liquidlike'' phase on the surface, consistent with a high degree of mass transport during encapsulation. 2 ) The sequence of STM images (2000 2000 A in Fig. 2 illustrate how the surface morphology evolves during deposition of the GaAs capping layer. Fig. 2(a) shows a representative image for the uncapped QDs; for these growth conditions the number density is 6 1010 cm 2 , and the ave respecrage height and width are 41 and 220 A tively. The STM image in (b) corresponds to of GaAs. At this stage in overgrowth with 4.2 A the capping process the morphology of the surface appears very similar to that of the uncapped QDs, however there are signi®cant changes in their size.
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Fig. 1. RHEED patterns recorded along the [1 1 0] azimuth for (a) uncapped InAs/GaAs(0 0 1) QDs, and after overgrowth with (b) 1 ML, (c) 5 ML and (d) 18 ML of GaAs. (e) and (f) are close-ups of the chevrons from (a) and (b) respectively, and highlight the reduction in the chevron angle with GaAs deposition. The chevron angle is measured by using an enlarged RHEED image to draw lines through the midpoints of several cross-sections taken at dierent points along the length of the chevron. The angle between the centroids gives a value for the chevron angle with an accuracy of 3°.
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2 ) at various stages of GaAs overgrowth: (a) uncapped InAs/GaAs QDs; (b)±(e) Fig. 2. Filled states STM topographs (2000 2000 A 2 ) recorded after 5 ML GaAs GaAs cap thickness of 1.5, 2.5, 5 and 30 ML respectively. (f) A higher resolution image (200 200 A overgrowth showing the coexistence of the locally ordered 2 4 and disordered 1 3 reconstructed domains.
and the The average height is reduced to 35 A Fig. 2(c) length increases along [ 1 1 0] by 50 A. GaAs deposition. shows the morphology after 7 A Major changes are now apparent with the average despite only height of the QDs reduced by 15 A,
of GaAs being deposited during the capping 7A procedure. The QD width, de®ned along [1 1 0], and the corresponding has increased by 50 A The surface length, de®ned along [1 1 0], by 100 A. morphology around the QDs also changes at this
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stage of encapsulation with the QDs surrounded by 2D islands, which have coalesced with adjacent 2D islands. Close inspection of the 2D islands indicate a 1 3 surface reconstruction, similar to the reconstruction observed in the wetting layer of the uncapped QDs and generally indicative of an Inx Ga1 x As surface alloy [28]. It is likely therefore that these 2D islands are also an Inx Ga1 x As alloy. Further changes occur when more GaAs is de the posited. For a GaAs capping thickness of 14 A, surface consists of large 3D features of ill-de®ned shape and the majority of islands have undergone coalescence with neighbouring islands. The surface structure is highly disordered, but it is possible to distinguish local patches of 2 4 reconstruction both on top of the 3D features and in the surrounding areas. This is highlighted in the high 2 ) shown in Fig. 2(f). resolution image (200 200 A It is important to note that the RHEED patterns observed at this cap thickness are no longer characterised by transmission diraction spots (Fig. 1(c)). STM height cross-sections taken across the mounds reveal discrete steps which are 3 A high and suggest the presence of monolayer GaAs GaAs, the surface steps. After deposition of 84 A is very rough and the surface reconstruction completely 2 4, although 3D features can still be observed where the GaAs has grown on top of the original QDs (Fig. 2(e)). The changes in QD size as a function of capping thickness provide important information about the processes that occur during the overgrowth of the QDs, and these are quanti®ed in Fig. 3. The schematic in Fig. 3(a) shows how the height and lateral dimensions of the partially capped QDs are de®ned with respect to the amount of GaAs overgrowth. Fig. 3(b) shows the change in QD height (hQD ) as a function of the GaAs capping layer thickness. The solid line indicates the height of the QDs expected (ho hcap ) if the GaAs simply grew up and around the QDs. The measured QD height (hQD ) decreases at a signi®cantly faster rate than the height increase of the capping layer and it appears that the QDs collapse rapidly during the initial stages of overgrowth. Fig. 3(c) shows how the lateral dimensions (d) of the QDs taken along the [1 1 0] and [ 1 1 0] direction are aected by increasing GaAs cap thickness. There is a rapid in-
crease in size along both directions, but the change is highly anisotropic, the width along [1 1 0] increasing by a factor of 1.75, and the length measured along [1 1 0] increasing by a factor of GaAs. The error 2.8 after deposition of 45 A bars in Fig. 3 represent a combination of statistical variation among the mounds or QDs and the experimental error. In the case of the height data the experimental error is approximately 1 ML thickness, while for the diameters the error relates to the diculty in de®ning mound edges, particularly at high capping thicknesses. It is straightforward to measure the total volume per unit area of the QDs prior to capping and during the initial stages of overgrowth (<4 ML) when the QDs are still well de®ned. At later stages of capping, however, the mounds become too ill de®ned to calculate their volumes. Note that no assumption about the QD shape need be made since the volumes are directly integrated from STM topographs. Prior to capping the QD volume 3 cm 2 and this rises to 3:5 is 2 0:2 1016 A 16 3 0:2 10 A cm 2 after deposition of 1.5 ML of GaAs. This signi®cant increase is due to In±Ga intermixing in and around the QDs and there is no evidence for any reduction of the QD volume caused by In evaporation [10]. The measured changes in height, length and width are consistent with the RHEED results, which show that the facet angle of the 3D islands changes during the earliest stages of encapsulation. It is clear that the structure and composition of the QDs undergo rapid changes, with the incident Ga atoms being incorporated into the structure and In atoms diusing from the top to the base of the 3D islands. In eect the 3D islands appear to ``collapse'' down into the surface during the initial encapsulation process. The apparent collapse of the QDs raises important questions regarding the stability of the QDs. Uncapped QDs grown under a range of growth rates are highly stable to post-growth annealing, whereas the addition of small amounts of GaAs leads to dramatic changes in surface structure and morphology. During capping the surface area is reduced rather quickly. If the surface energy of the InAs island sides is increased signi®cantly by the addition of Ga adatoms, as may
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Fig. 3. (a) Schematic showing how the size of the QDs was obtained from STM images after GaAs overgrowth; d is the lateral dimensions of the QDs, hQD the height of the partially capped QDs, ho the initial QD height and hcap the height of the GaAs capping of the partially capped QDs as a function of the GaAs cap thickness. The solid layer. (b) The measured change in height (hQD =A) horizontal line is the height of the partially capped QDs (ho hcap ) expected if the GaAs simply grows up and around the QDs. (c) The eect of overgrowth on the lateral dimensions (d) of the QDs. The solid circles show the increase in width along [1 1 0] and the triangles represent the increase in length along [1 1 0].
be the case for a stable, strained and reconstructed side facet, this would provide a strong driving force for the reduction in surface area as the islands collapse during encapsulation. Clearly, the entropy of mixing is also increased during this process, although this may not be signi®cant compared with the change of surface and strain energy. Furthermore, strain relief by relaxation at the top of the QDs becomes less ecient. The observed changes in size and shape, and the implication for composition, have important consequences for any modelling of the electronic structure of capped InAs/GaAs QDs. For exam-
ple, calculations for idealised QDs based on square based pyramids and comprising 100% InAs are clearly inappropriate and any theoretical model must take into account the intermixing of Ga and In atoms, both prior to and during the overgrowth process. For example, recent STEMEDX measurements [19] have shown that the degree of intermixing after overgrowth can be as large as 40%, even though the uncapped QDs (grown at very low deposition rates) are virtually pure InAs [29]. The dramatic shape changes which occur during the initial stages of capping clearly provide a mechanism for intermixing based on
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surface diusion rather than bulk In±Ga interdiffusion, which is presumably a very slow process at temperatures around 510 °C. Finally, our results may also provide an explanation for the often reported vertical self-alignment that occurs when multilayer QD structures are grown and separated by only a thin GaAs GaAs) [16±18,30, spacer layer (typically <150 A 31]. The self-alignment is generally attributed to strain ®elds arising from the underlying QD layer. Fig. 2(e) shows a typical starting surface for the growth of the second layer. The presence of mounds and the highly disordered surface morphology after GaAs deposition on top of the ®rst layer of QDs is likely to have a strong in¯uence on the nucleation of the second layer of QDs. We suggest that second layer QD nucleation on top of these mounds may provide an additional mechanism for strain relief and lead to the observed selfalignment. 4. Conclusions In summary, we have shown that the shape of InAs/GaAs QDs changes dramatically during GaAs encapsulation. In situ RHEED measurements show that the sidewalls of the QDs become shallower and eventually lose all their crystallographic information. STM images reveal that the QDs collapse rapidly with the average height decreasing faster than the rate at which the GaAs is deposited. The area of the QDs increases considerably, the processes being highly anisotropic with signi®cant lengthening occurring along 1 1 0. The capping of InAs/GaAs QDs with GaAs clearly has important implications for the size, shape and composition of the QDs, which are key parameters in determining the electronic properties of these important structures. Our results suggest that the surface species are highly mobile during encapsulation and a signi®cant amount of mass transport and alloying occurs. In view of these results, it is essential that the changes in the shape and composition of the QDs during capping have to be fully considered when proposing models to account for the novel electronic properties of these con®ned structures.
Acknowledgements This work was supported by the EPSRC, UK, who also provided studentships for PBJ and TJK. GRB is grateful to the Ramsay Memorial Trust for the provision of a Research Fellowship, funded in part by VG Semicon Ltd. (UK).
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