Controllable step bunching induced by Si deposition on the vicinal GaAs(001) surface

Surface Science 459 (2000) L482–L486 www.elsevier.nl/locate/susc

Surface Science Letters

Controllable step bunching induced by Si deposition on the vicinal GaAs(001) surface Z.M. Wang, L. Da¨weritz *, K.H. Ploog Paul-Drude-Institut fu¨r Festko¨rperelektronik, Hausvogteiplatz 5-7, D-10117 Berlin, Germany Received 6 December 1999; accepted for publication 13 April 2000

Abstract The evolution of the morphology of the vicinal GaAs (001) surface induced by Si deposition was studied by scanning tunneling microscopy. The observed step bunching is accompanied by a spatial separation of surface phases with different Si coverages on terraces and in step regions. The spacing and height of the bunches depend on the substrate temperature. A model is proposed to account for these effects by considering a kinetic pathway of the surface to an accessible lowest energy state. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Adatoms; Gallium arsenide; Models of surface kinetics; Molecular beam epitaxy; Scanning tunneling microscopy; Silicon; Step formation and bunching; Vicinal single crystal surfaces

Step bunching is a phenomenon in which the surface breaks up into regions with high step densities and regions with little or no steps. The study of this phenomenon is important because of the increasing use of its resulting surfaces as templates for growth of quantum wires and dots [1– 4]. In principle, the height and spacing of the bunches depending on the number of steps in the bunches can be controlled and hence provide an excellent possibility for tailoring of template structures. The surface reconstruction and the introduction of impurities or adsorbates are known to play an important role in determining the step bunching behavior. However, the physical origin of the step bunching is not clear in most cases, and several * Corresponding author. Fax: +49-30-20377-201. E-mail address: [email protected] (L. Da¨weritz)

mechanisms [5–8], either kinetic or thermodynamic, have been proposed for an explanation. In this letter, we discuss the step bunching observed on the vicinal GaAs (001) surface induced by Si deposition, which can be controlled by the substrate temperature. A simple analysis, treating each bunch period as a local lowest energy configuration limited by kinetic processes, shows that this behavior can be expected if two different reconstructions coexist on the surface. The samples were grown on vicinal GaAs (001) substrates misoriented 2° towards the [111]A direction in a molecular beam epitaxy (MBE ) growth chamber, equipped with high-energy electron diffraction (RHEED) and reflectance difference spectroscopy ( RDS). After oxide desorption and the growth of a 300 nm GaAs buffer layer, the substrate temperature and As 4 beam-equivalent pressure were adjusted in order

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to achieve the (2×4)a reconstructed surface, on which the surface atom mobility is known to be high, as confirmed previously by RHEED and RDS studies [9,10]. Subsequently, 0.1 ML (monolayer) Si was deposited with a flux of about 5.0×1011 atoms cm−2 s−1 in 60 s pulses and 180 s interruption. Thereby, the (2×4)a reconstruction was converted to (3×2), as indicated by the RHEED pattern [9]. The resulting surface was quenched by decreasing both the substrate temperature and the As beam-equivalent pressure in a 4 controlled manner by keeping the RHEED pattern and RDS signal unchanged. After the As source 4 was closed completely at a substrate temperature of 500°C, the sample was rapidly transferred under ultra-high vacuum to the scanning tunneling microscopy (STM ) chamber for analysis. In this study, we concentrate on 0.1 ML Si-on-GaAs (001); investigations with different Si coverages are published elsewhere [11]. All STM scans were taken at room temperature using tungsten tips. The tunneling current was set constant between 0.1–0.4 nA, and the negative sample bias was 2–4 V. The STM images shown in Fig. 1 demonstrate the structural transformation from a regular step train to step bunches explicitly. On the clean surface with the (2×4)a reconstruction, as shown in Fig. 1a, regular step-terrace arrays with relatively straight step edges parallel to [1: 10] were observed. The mean terrace width was about 8 nm, as expected from the miscut. The step with a height of 0.28 nm comprises one As-atomic and one Ga-atomic layer. The (2×4)a reconstruction is characterized by two As dimers on the top layer with a complete second Ga layer and Ga dimerization[12]. The As dimer-pair rows of the (2×4)a reconstruction running along [1: 10] are clearly visible in Fig. 1a but with a high kink density, which is expected for this surface phase [13]. Fig. 1b shows the surface structure after 0.1 ML Si deposition at 590°C. The Si deposition induces the (3×2) reconstruction within the original (2×4)a structure. The (3×2) structure is terminated by Si dimers occupying Ga sites with the dimer bond along [110], as assessed by previous RDS measurements [9,10]. The height of the steps between the two different reconstructions (As and Si termi-

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Fig. 1. STM images of GaAs(001) surface miscut by 2° towards [111]A: (a) bare surface, (b) surface after Si deposition of 0.1 ML at 590°C.

nated, respectively) is expected to be 0.14 nm, i.e. one atomic layer. The two different coexisting reconstructions are well separated and are accompanied by step bunching of (2×4)a reconstructed bare GaAs regions with a high step density and (3×2) reconstructed Si-covered wide terraces

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without steps. The spatial separation of surface phases with different Si coverages results in the formation of ordered ribbon-like Si structures on GaAs (001). The average spacing of the bunches is about 50 nm, as demonstrated more clearly by the larger-area STM image of Fig. 2a. Most of the wide terraces resulting from the bunching have a length of more than 1 mm. A similar step bunching was observed for Si deposition performed at a substrate temperature of 550°C but with a narrower period, as shown in Fig. 2b. The average spacing and length of the bunches in this case are about 15 and 400 nm, respectively. It is generally accepted that there are three possible modes by which to describe the heteroepitaxial growth: the island growth ( Volmer–Weber) mode, the layer-by-layer (Frank–van der Merwe) mode, and the layer-plus-island (Stranski– Krastanov) mode. All of these modes only consider the morphology evolution of the epitaxial layer with the simplifying assumption that the substrate is immobile during growth. However, the surface evolution induced by 0.1 ML Si deposition, as shown in Fig. 1, indicates that mass transport between several planes takes place, leading to bunches containing about six steps of monolayer height. The phenomenon is similar to the transformation of vicinal Si(111) from a surface composed of (1×1) terraces separated by singleatomic-height steps at high temperatures to facets separated by large (7×7) terraces at low temperatures, which has been interpreted in terms of the equilibrium crystal shapes [14,15]. In an STM study [16 ], it was observed that kinetic limitations lead to uniformly spaced bunches. In the present case, it is not the change in temperature but the Si coverage of the GaAs(001) surface that disturbs the original equilibrium state. Recent studies have shown that GaAs MBE growth may proceed under conditions much closer to thermodynamic equilibrium than has been believed before and that a high density of mobile Ga adatoms exists on the GaAs(001) surface in the presence of an As flux, 4 even without depositing any material [17–19]. In the present experiment, the initial surface with monolayer steps correponds to the lowest energy state that is kinetically accessible under the given preparation conditions. The Si deposition induces

Fig. 2. Large-area STM images of GaAs(001) surface miscut by 2° towards [111]A after Si deposition of 0.1 ML at (a) 590°C and (b) 550°C.

a new reconstruction, (3×2), disturbing the original state. The surface atoms must re-arrange to search for the possible lowest energy states. The variation in surface free energy ( f ) with sur step density (tan h), where h is the misorientation,

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Fig. 3. Schematic illustration of step bunching mechanism induced by the coexistence of two different reconstructions indicated by the gray scale. (a) Configuration of ideal step array on the initial surface. (b) Configuration corresponding to the case of f ( f )>f +f . (c) Configuration corresponding to aa bb ab ba the case of f +f >f and f >f . For explanation, see text. ab ba aa bb aa

is governed by[20] f =f +f tan h+g tan3 h (1) sur terr step s–s where the surface tension of the terrace ( f ), the terr step formation energy ( f ) and the free energy step of step–step interaction ( g ) depend on the surs–s face reconstructions. If the whole surface is homogeneously reconstructed, the movement of steps is independent of the step formation energy, and the step–step interaction is the driving force for the surface evolution [20]. When two different reconstructions [(2×4)a and (3×2) in this experiment, denoted as a and b, respectively] coexist on the surface, there are four types of steps to be considered (cf. Fig. 3): steps with both upper and lower a- (b-) reconstructed terraces labeled as S (S ) aa bb and steps with upper a- (b-) reconstructed terraces and lower b- (a-) reconstructed terraces labeled as S (S ). The free energy for creating a correab ba sponding step is denoted by f (x, y=a, b). The xy step formation energy is expected to play a crucial role in determining the resulting step configuration from the analysis below. The starting surface shown in Fig. 3a is characterized as regular stepterrace arrays with all terraces a-reconstructed, as we observed for the original (2×4)a reconstructed vicinal GaAs (001) surface. For clarity, perfectly

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periodic arrangements, as shown in Fig. 3, are assumed. When the b-reconstruction nucleates on the initial surface, and f ( f )>f +f , the step aa bb ab ba structure with the lowest energy is that shown in Fig. 3b, which resembles the surface resulting from a normal step flow mode. If f +f >f and ab ba aa f >f , the thermodynamically lowest energy state bb aa corresponds to two completely separated phases with the steps concentrated in the a-reconstructed regions. Considering kinetic factors[21], however, only local lowest energy states with b-domains of size, L, as shown in Fig. 3c will be reached in practice. The Si-induced bunching observed on the GaAs (001) surface indicates that the free energy to create a step within the (3×2) reconstructed regions is higher than that to create a step within the (2×4)a reconstructed regions, which is implied also from the different appearances of the two reconstructions. The (3×2) structure is more ordered than the (2×4)a structure, as shown in Fig. 1. In general, a stronger lateral interaction between surface species is expected for an ordered reconstruction[22], and hence a higher step formation energy. The domain size will depend on the substrate temperature through an activation process. When increasing the substrate temperature, the mobility of surface atoms is enhanced, and the steps can rearrange themselves on a larger length scale, leading to a larger bunch spacing. The results of Fig. 2 support the conclusion that at a fixed Si coverage, the configuration with a larger domain size is of a lower surface energy than that with a smaller domain size. The temporal behavior of the domains, i.e. the growth power law, is not covered by this study. Such measurements would be very useful to distinguish between different possible mechanisms that can be involved in the observed phase separation. Si deposition introduces (3×2) reconstructed domains nucleated randomly as the initial distribution. Subsequently, the domain distribution can continue to evolve by long-range diffusion of impinging Si atoms, ripening in which large domains grow at the expense of smaller domains, and coalescence of domains. Although the dominating mechanism is still unclear, the misorientation of the initial surface is expected to play an impor-

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tant role in determining the evolution of the domains according to the above analysis, which has been demonstrated by the RDS transients and the RHEED intensity recordings [23]. The present experiment concerning the vicinal surface misoriented 2° towards the [111]A direction reveals a remarkably uniform distribution of step bunches, as shown in Fig. 2. Such a surface provides an excellent template for further growth of lowdimensional structures. The uniform distribution of the bunches is coupled with the step type, as revealed by comparative studies for the singular surface and the vicinal surface 2° misoriented towards the [111]B direction[24]. In summary, the controllable step bunching induced by Si deposition on the vicinal (001) GaAs surface is understood in terms of a local lowest energy state defined by kinetic factors in a system of two coexisting surface reconstructions. It is expected that this approach to understand the incorporation processes of impurities at an atomic level is not limited to the case considered in this study but is also applicable to pseudobinary III– V semiconductor alloys, such as Al Ga As, x 1−x which show a considerable richness of growth phenomena.

Acknowledgement The authors greatly acknowledge technical assistance by P. Schu¨tzendu¨be.

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