Growth modes in homoepitaxy on vicinal GaAs(110) surfaces

Surface Science 424 (1999) L309–L313

Surface Science Letters

Growth modes in homoepitaxy on vicinal GaAs(110) surfaces P. Tejedor a,b, *, P. Sˇmilauer a,c, B.A. Joyce a a Semiconductor Materials IRC, Imperial College, London SW7 2AB, UK b Instituto de Ciencia de Materiales de Madrid (C.S.I.C.), Cantoblanco, 28049Madrid, Spain c Institute of Physics of the ASCR, Cukrovarnicka´ 10, 162 53Praha 6, Czech Republic Received 21 December 1998; accepted for publication 5 January 1999

Abstract The growth of GaAs by molecular beam epitaxy on (110) substrates vicinal to (111)A has been systematically studied by atomic force microscopy at different As/Ga flux ratios and growth rates. The striking variety of surface morphologies observed after deposition of 250 ML at 450°C has been correlated with four distinct growth regimes and a diagram of growth regimes as a function of growth conditions has been constructed. We discuss the microscopic origin of the various surface morphologies in terms of growth modes and relative adatom populations. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Atomic force microscopy; Gallium arsenide; Growth modes; Molecular beam epitaxy; Surface structure, morphology, roughness, and topography; Vicinal single crystal surfaces

Epitaxial growth of GaAs by molecular beam epitaxy (MBE ) on (110) vicinal substrates offers a new and attractive way to fabricate electronic devices based on laterally ordered nanostructures [1,2]. A prerequisite to obtaining such structures is to have a deep understanding of the growth mechanisms that would allow an accurate control of the morphological instabilities that originate on the vicinal surface during growth. In a previous paper [3], we investigated the temperature-dependent formation of morphological instabilities during growth of GaAs on (110) vicinal substrates misoriented by 1.5° towards (111)A and identified two distinct growth regimes: At substrate temperatures between 450 and 500°C, macrosteps are * Corresponding author. Fax: +34 91 3720623; e-mail: [email protected].

created by step bunching, whereas above 550°C the surface becomes unstable against transverse meandering [4]. A ripple pattern morphology [5] with ridges running in the direction of the tilt is formed. In a subsequent study that dealt with the temporal evolution of the surface morphology [6 ] we found that, at long deposition times, this ripple pattern morphology gives rise to large three-dimensional pyramidal features similar to those observed on a singular GaAs(110) surface [7]. In addition, a transition from step bunching to the ripplepattern structure was observed to take place during growth under suitable growth conditions. In this Letter, we use atomic force microscopy (AFM ) to study the effect of the V:III flux ratio and growth rate on the surface morphology of GaAs grown by MBE on (110) vicinal substrates. Our aim is to better understand the origin of the

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growth modes on this surface at low temperatures. In the following, we show that the Ga-supply limited region, where the step-bunching instability is typically observed, comprises three distinct growth regimes: step propagation is the only growth mode at the lowest values of the Ga flux, whereas nucleation of two-dimensional (monolayer or bilayer) islands on the terraces is observed at later stages of growth for higher Ga fluxes. A third regime is found for high As/Ga flux ratios and high growth rates which is characterized by the formation of ripples in the direction of the pre-existing step edges, , possibly as a result of growth in the stepdecoration mode. Finally, growth in the As-limited region leads to rapid roughening of the vicinal surface at low As/Ga flux ratios and high growth rates. All the experiments reported here were carried out in a standard VG Semicon V80H MBE system. Arsenic was supplied using a valved cracker cell ( EPI V-500 As) working at 430°C, which provided mechanical control over the resulting As flux. Ga 4 and As fluxes were calibrated using RHEED 4 intensity oscillations on a GaAs(001) substrate. The epi-ready semi-insulating GaAs(110) substrates misoriented towards (111)A by 1.5° (American Xtal Technology) were bonded with In to Mo disks and outgassed in vacuum for 45 min at 350°C. Oxide removal was then carried out in the growth chamber at ca 620°C under an As flux 4 of 5×1015 molecules cm−2 s−1, with the initial amorphous RHEED pattern being replaced rapidly by a clear (1×1) pattern. Subsequently, ˚ -thick layers of GaAs were grown at 500 A 450°C using Ga fluxes that varied between 6×1013 and 6×1014 atoms cm−2 s−1 while the As flux was adjusted between 2×1014 and 4 1×1016 molecules cm−2 s−1 to provide As/Ga flux ratios from 3:1 to 20:1. Under these conditions, the GaAs growth rate varied between 0.1 and 1 mm h−1. During and after growth, the RHEED pattern displayed very sharp streaks in both [001] and [11: 0] azimuths. All samples were quenched to room temperature immediately after deposition with the arsenic cell shutter open down to 400°C to avoid any modification of the as-grown surface morphology. The surface morphology of the

samples was examined by AFM (Burleigh Instruments, Inc.) using the constant force contact mode. Etched single-crystalline silicon tips were ˚ , and a sidewall used with an end radius of 100 A angle of 35.3°. Scan rates of 0.35 lines per second were used and data were taken at 256 points per scanline. The AFM images shown in Fig. 1 illustrate the variation in morphology of the GaAs(110) vicinal ˚) surface upon deposition of ca 250 ML (~500 A of GaAs at 450°C at different As/Ga flux ratios (3:1–20:1) and growth rates (0.1–1 mm h−1). The images are 2×2 mm scans taken in the [001] direction. Examination of these images and their crosssections along the [001] direction revealed the striking variety of growth scenarios that arise on this surface when the relative populations of Ga and As adatoms are varied at a fixed temperature. Analysis of these data produced the map of the growth regimes with growth conditions shown in Fig. 2. The cross-section scans shown in Fig. 3 encompass the four growth regimes (regions I–IV in Fig. 2) identified. According to the kinetic analysis reported in Ref. [12], the GaAs growth rate on the [110] surface under the conditions studied here is limited by the supply of Ga with the exception of the experiments carried out at the lowest As/Ga flux ratios and the highest growth rates, where GaAs growth is As-limited. The results indicate that at low temperatures, the GaAs(110) vicinal surface is unstable against step bunching in a wide range of growth conditions. The average height of the macrosteps formed by step bunching varies ˚ and the average terrace width between 16 and 24 A ˚ , with the widest terraces between 1150 and 2000 A being found in the samples grown at the lowest As/Ga flux ratios and growth rates. The formation of step bunches, similar to the effect of a negative Ehrlich–Schwoebel barrier [9,10] is caused by preferential incorporation of Ga adatoms into step edges from the upper terrace [3]. In region I, which comprises samples grown at the lowest growth rates (6×1013–1.5× 1014 Ga atoms cm−2 s−1), growth proceeds by step propagation. A common characteristic of the samples in this growth region is that, although the terraces run parallel to the [11: 0] direction, a clear tendency to form [11: 2]-type step edges is observed

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Fig. 1. AFM images (2×2 mm) showing the variation in surface morphology of the GaAs(110) vicinal surface after the deposition ˚ (~250 ML) of GaAs at a constant temperature of 450°C and different As/Ga ratios and growth rates. of 500 A

at any As/Ga flux ratio. However, two different scenarios are observed in this growth region depending on the As/Ga flux ratio. At low and intermediate As/Ga ratios (≤10:1) growth proceeds by ‘‘conventional’’ step flow in the [001] direction, as illustrated in the cross-section shown ˚) in Fig. 3a, where trains of monolayer-high (~2 A steps can be clearly seen on the terraces. In contrast, the growth direction is reversed at high As/Ga ratios (20:1) and step flow of terraces in the [001: ] direction is observed, as depicted in Fig. 3b. Interestingly, a similar tendency for growth to progress in the [001: ] direction on the GaAs(110) singular surface at high As/Ga ratios ˚ s−1¬ (6:1) and low growth rates (0.14 A 0.05 mm h−1) in the so-called step decoration mode has been recently reported by Holmes et al.[7]. At higher growth rates (1.5×1014–3×

1014 Ga atoms cm−2 s−1) but still in the Ga-supply limited growth regime (region II ), the growth mode changes from pure step flow to step flow accompanied by nucleation of islands on the terraces, cf Fig. 3c. Initial step bunching causes the average terrace size to increase. Since only adatoms deposited up to a certain distance from the step edges can reach them, an increasingly higher number of adatoms accumulates in the middle of the terrace during growth, until the critical density for island nucleation is reached. The islands thus formed are incorporated into advancing steps before they can coalesce with each other. Growth still takes place by step propagation, but step advancement is disrupted by the presence of the islands, causing the step bunching to cease. Both monolayer- and bilayer-high islands coexist on the terraces, although the former dominate at low

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Fig. 2. Growth diagram determined from AFM studies for the homoepitaxial growth of GaAs on GaAs(110) misoriented towards (111)A at 450°C.

As/Ga flux ratios and the latter at intermediate As/Ga flux ratios (~10:1). At the highest As/Ga flux ratios, the island density is very low, but increases significantly as the As/Ga flux ratio is decreased or the growth rate is increased at a fixed low As/Ga ratio. In addition, [112: ]-type step edges are formed preferentially at low As/Ga flux ratios, while [110: ]-type step edges predominate at intermediate and high As/Ga flux ratios (Fig. 1). At the lowest As/Ga flux ratios and highest growth rates (6×1014 Ga atoms cm−2 s−1), (region III in Fig. 2), the vicinal surface roughens due to extensive nucleation of islands on the terraces ( Fig. 1). The cross-section of the sample grown at 1 mm h−1 and As/Ga flux ratio of 3:1 shown in Fig. 3d demonstrates that growth initially proceeds via step flow (despite the low As availability) and gives way to nucleation of three-dimensional islands when the incorporation of Ga adatoms into islands dominates over the incorporation into step edges. It should be noted that in this regime, the effective As population on the surface is rather low due to its low incorporation rate coefficient in comparison with that on the (001) surface. The overall GaAs growth rate in this region is thus limited by the rate of As incorporation via dissociation of the As* interme2

Fig. 3. AFM cross-section scans along the [001] direction corresponding to GaAs samples grown at 450°C on a GaAs(110) vicinal substrate at various As/Ga ratios and growth rates: (a) As/Ga ratio of 3:1 and 0.1 mm h−1 (region I ); (b) As/Ga ratio of 20:1 and 0.1 mm h−1 (region I ); (c) As/Ga ratio of 3:1 and 0.5 mm h−1 (region II ); (d) As/Ga ratio of 3:1 and 1 mm h−1 (region III ); and (e) As/Ga ratio of 10:1 and 1 mm h−1 (region IV ).

diate species [8,11]. Preferential alignment of the step edges along [11: 2] is observed. A very interesting change in growth morphology is observed in region IV, which includes the samples grown at the highest growth rates (3x1014–6×1014 Ga atoms cm−2 s−1) and As/Ga flux ratios (≥10:1). All samples in this growth region exhibit the formation of ridges running along the direction of the pre-existing step edges, [11: 0] as shown in Fig. 1. The cross-section scan shown in Fig. 3e corresponds to the sample grown at 1 mm h−1 with an As/Ga flux ratio of 10:1. The typical peak-to-valley height of the ridges has been

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˚ . In contrast with [11: 2]-type estimated to be 12 A steps, which are known to consist of consecutive (001) ‘‘nanofacets’’ [7], the [11: 0]-type steps formed in this growth region appear to be formed by both (001) and (111)A ‘‘nanofacets’’, cf Fig. 3e. This tendency to form (111)A ‘‘nanofacets’’ becomes more pronounced with increasing growth rate. We suggest that growth proceeds by step decoration, in a way similar to that reported by Zhang et al. [12] for InAs growth at 480°C on GaAs(110) substrates misoriented towards (111)A. TEM studies carried out by these authors showed that InAs decorates preferentially those step whose edges run along [11: 0] rather than those along [11: 2], leading to the formation of (111)A facets, in good agreement with the presence of (111) streaks in the RHEED pattern. Comparison of our results with the growth model proposed by Holmes et al. [7] suggests that the ultimate development of a surface growing in the step decoration mode is a sawtooth waveform similar to the ridge structure shown in Fig. 3e. Growth in the step decoration mode is analogous to step flow growth, with the exception that nucleation occurs on the upper terrace, above the step edge. Although step decoration seems the most likely mechanism responsible for the ridge morphology found in region IV, a detailed analysis of the temporal evolution of the vicinal GaAs(110) surface during growth would be useful to ascertain the microscopic origin of the observed morphology. Finally, it should be noted that the type of step edge formed on the vicinal GaAs(110) surface depends strongly on the relative As and Ga populations and, ultimately, on the reaction kinetics. When the As or/and the Ga populations on the surface are low and far from the 1:1 stoichiometric ratio, the As–Ga interaction kinetics at the step edges are very slow and the step-edge atoms are allowed to rehybridize their dangling bond charge density and adopt a stable configuration [13], for

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example, [11: 2]. In contrast, when both As and Ga are supplied in excess, the 1:1 ratio is readily achieved and the kinetics of GaAs formation at step edges are much faster, leading to the formation of highly reactive [11: 0]-type step edges [13].

Acknowledgement This work was supported by the Engineering and Physical Sciences Research Council ( EPSRC ) UK under Grant No. GR/97540. P.T. acknowledges financial support from the Ministerio de Educacio´n y Ciencia of Spain (Ref. No. PR95-143). P.Sˇ. acknowledges financial support from the Grant Agency of the Czech Republic (Grant No. 202/96/1736), the Volkswagen Stiftung, and the Royal Society ( UK ).

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