Virtual-surfactant mediated epitaxy of InAs on GaAs(001) studied by scanning tunneling microscopy

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Journal of Crystal Growth 167 (1996) 440-445

Virtual-surfactant mediated epitaxy of InAs on GaAs( 001) studied by scanning tunneling microscopy J. Behrend *, M. Wassermeier, K.H. Ploog Paul-Drude-lnstitut fiir Festki~rperelektronik, Hausvogteiplatz 5 - 7, D- 10117 Berlin, Germany Received 20 February 1996

Abstract

The mechanism of virtual-surfactant mediated molecular beam epitaxy (MBE) of InAs on GaAs(001) was investigated by in situ scanning tunneling microscopy (STM) and reflection high-energy electron diffraction (RHEED). InAs layers with thicknesses ranging from 1 to 20 monolayers (ML) do not exhibit any strain driven morphological phase transition when grown under In-rich conditions. RHEED and STM confirm a well-ordered (4 × 2) reconstruction of these In-terminated surfaces. Three different atomic structure models that agree with the STM images are discussed. Large STM scans reveal a characteristic morphology of rectangularly shaped islands and step edges with a large degree of anisotropy. We attribute these new findings to a special strain reducing growth mode that is related to the In-rich (4 × 2) surface reconstruction.

1. I n t r o d u c t i o n

Recently, heterostructures c o m p o s e d of InAs/GaAs(001) have attracted a lot of interest for technological applications as well as for fundamental studies. Due to the considerable lattice mismatch of about 7%, a critical thickness exists where a strain driven morphological phase transition from two-dimensional (2D) layer-by-layer growth to three-dimensional (3D) nucleation occurs. For the InAs/GaAs(001) system this transition starts between 1.7 and 2.5 monolayers (ML) depending on the misorientation of the GaAs substrate and the growth conditions [1]. This island formation has recently been rediscovered and described as a self-

* Corresponding author. Fax: + 4 9 30 203 77 201; E-mail: behrend@pdil .iaas-berlin.de.

organized phenomenon and proposed to synthesize during molecular beam epitaxy (MBE) low-dimensional quantum structures [2-4]. Ultra-thin InAs quantum wells (QW) grown below the critical thickness exhibit a very good structural perfection, as shown by high-resolution transmission electron microscopy (HRTEM), and a remarkably high photoluminescence (PL) intensity [5,6]. The virtual-surfactant growth method [7] allows the growth of InAs on GaAs(001) with a layer thickness much larger than the critical value and with superior structural and optical quality avoiding the 3D nucleation. This growth mechanism was related to kinetic limitations to both In and As adatom mobility to be present under In-rich conditions in absence of a direct A s 4 flux. This behavior is expected to favor the 2D growth mode for the InAs inhibiting the islanding and relaxation to the equilibrium state of the sub-

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J. Behrend et al. / Journal of Crystal Growth 167 (1996) 440-445

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Fig. 1. STM image and RHEED pattern of a 20 ML thick InAs(001) layer with (4 X 2) reconstruction grown under In-rich virtual-surfactant conditions. The lower part shows a contour plot of the same image explaining the phase boundaries. The labels A, B, and C are explained in the text.

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strate-film system. HRTEM images of such samples [8] show the high structural perfection of the strained InAs layer without any 60 ° misfit dislocations. This results in drastically enhanced PL intensities and narrower PL line widths compared to relaxed InAs quantum wells of the same thickness grown under normal As-rich conditions [7,9]. In this paper we report the first STM investigations of InAs layers grown on GaAs(001) using In-rich virtual-surfactant growth conditions to provide a more detailed understanding of this special growth mode.

2. Experiment The InAs films were grown by solid-source MBE on 0.5 /xm thick GaAs buffer layers deposited at 580°C on (001)-oriented GaAs substrates with 0.5 ° miscut towards [lll]A. The substrate temperature during InAs deposition was kept at 450°C, and a nominal growth rate of 0.1 to 0.3 M L / s , depending on the layer thickness ranging from 1 to 20 ML in different samples, was used. To achieve the virtualsurfactant growth conditions the substrate temperature was lowered from 580 to 450°C and the shutter of the As cell was closed so that only the residual AS 4 atmosphere was used for the subsequent InAs deposition. The well-defined (2 X 4) RHEED pattern of the GaAs(001) surface, established during the buffer layer growth, changes immediately to a clear (4 × 2) symmetry when the shutter of the In cell is opened. Under these arsenic-deficient conditions RHEED shows a clear (4 X 2) pattern with a bright Laue circle, as shown in Fig. 1. The pattern remains for the entire thickness of the grown InAs layer, evidencing that a 2D growth mode is in operation. For comparison, a 1 /zm thick InAs layer was grown in the normal anion-stable way using an A s 4 flux of about 2 × 10 - 6 Torr and a growth rate of 0.5 M L / s at 430°C. In this case the morphological phase transition was observed between 1.5 and 2.5 ML followed by the relaxation of the layer due to the creation of misfit dislocations. After 1 /zm of InAs growth the RHEED pattern displayed a (2 X 4) symmetry with bright spots on the Laue circle evidencing a wellordered surface reconstruction. For the in situ STM imaging directly after growth, the resulting surfaces were quenched to room tern-

perature and subsequently transferred into the STM chamber having a base pressure below 1 × 10 -11 Torr, which is connected to the MBE growth chamber via a gate valve. The filled states STM images were obtained with etched tungsten tips at tunneling currents of 100-300 pA at a bias voltage of 2.5 V.

3. Results and discussion The STM image of Fig. 1 shows the surface of a 20 ML thick InAs layer grown under virtual-surfactant conditions. During the InAs growth a well-defined (4 × 2) RHEED pattern with bright spots on a Laue circle appears indicating a smooth and ordered surface [7,9]. The STM image confirms this notion and reveals a well-ordered and homogeneous

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J. Behrend et a l . / Journal of Crystal Growth 167 (1996) 440-445

ladder-type reconstruction. Bright rows that run along the [110] direction produce the four-fold periodicity of 1.6 nm. The two-fold symmetry (0.8 nm) is formed by the chain of protrusions between the bright rows. The chains are sometimes interrupted by missing protrusions (see label A in the image). Neighboring chains of protrusions may be shifted by 0.4 nm with respect to each other forming a phase boundary along the [110] direction (see label B in the image). Sometimes a phase shift also occurs within a row resulting in a kink of a phase boundary (see label C in the image). A similar STM image was recently reported for In coverages of 1 ML on GaAs(001) [10]. In that case the In was deposited on the substrate at room temperature, followed by annealing of the sample to 400°C. This procedure also resulted in (4 × 2) reconstructed In-terminated surfaces. Our STM images of

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the In-rich (4 X 2) surface look different to previously published data of the (4 X 2) reconstruction on homoepitaxially grown or relaxed InAs layers [1 l]. This might either be due to the different preparation conditions (the samples were grown As-rich and no or only residual strain was involved) or due to the different resolution of the STMs. The vertical corrugation of the ladder-type reconstruction measured by STM is smaller than that of the As-terminated InAs surface but larger than that of the Ga-terminated (4 X 2) reconstructed GaAs surface. The appearance of two different structural elements (rows and protrusions) suggest the assignment of each to one species (In and As). The complex convolution of topological and electronical contrast in the STM images allows three possible configurations of the atomic structure, as shown in Fig. 2. All three models contain In and As dimers in the

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Fig. 3. Large scan STM image of a 20 ML thick InAs(001) layer grown under In-rich conditions showing the unusual morphology of the surface as a consequence of kinetic limitations of In and As incorporation.

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Fig. 4. STM image of a 20 ML thick InAs(001) layer grown under In-rich conditions proving that almost all the step edges and islands follow exactly the two principal axis of the cubic crystal symmetry.

first and second layer of the crystal, respectively, but in different arrangements with respect to the rows and protrusions of the STM image. The different assignment of In and As dimers to rows and protrusions leads to different In coverages and a change of the surface stoichiometry. Based on the Biegelsen structure [12] the first model of Fig. 2 contains three In dimers in the uppermost layer forming the protrusions and the rows correspond to the As dangling bonds of the second layer. The second model shown in Fig. 2 contains only one In dimer in the uppermost layer forming the rows and perpendicularly oriented to that one As dimer every other lattice constant in the second layer corresponding to the protrusions between the rows. These two models are both consistent with the electron counting argument. The third model, where four electrons are missing per unit mesh, consists of a single In dimer in the uppermost layer forming the protrusions and a continuously

filled sequence of perpendicularly oriented As dimers in the second layer for the rows in the STM image. Common to all models is that In in the uppermost layer and the As in the second layer results in the same topographic contrast, i.e. brightness, in the STM image. This is in agreement with the experimental finding on different GaAs surfaces, that the group III element typically shows less contrast than the group V element [13,14]. The In coverages are 75% for model 1 and 25% for models 2 and 3. From the preparation procedure we expect to end up with a high In coverage and favor model 1. Fig. 3 shows a large scan STM image of the same surface. Compared to InAs and GaAs(001) surfaces grown under As-rich conditions, where the morphology is determined by growth instabilities, like meandering, [15] the situation for the (4 × 2) InAs(001) surface prepared under In-rich conditions is very different. As shown in the STM image of Fig. 4,

J. Behrend et al. / Journal of Crystal Growth 167 (1996) 440-445

almost all the step edges are straight and follow the two principal axis o f the cubic crystal symmetry. The islands are of exact rectangular shape and elongated along the [110] direction, thus exhibiting a large degree o f anisotropy. Also a strong step bunching and multilayer-high steps are observed. The defect density on the terraces due to holes and missing unit meshes is obviously also reduced as compared to the As-rich grown InAs(001) surface. A similar behavior was found on relaxed InAs(001) surfaces during the transition of the reconstruction from (2 X 4) to (4 X 2) where domain structures with very straight step edges were formed [16]. In our case, however, the observed morphology covers the entire surface and may rather be related to the short migration length of both As and In adatoms during growth reported to be induced by the special growth conditions [7].

4. Summary W e have investigated the structure and morphology of the In-rich (4 X 2) reconstructed InAs(001) surface resulting from the virtual-surfactant mediated growth on GaAs(001). Our S T M images reveal a ladder-type reconstruction with (4 x 2) symmetry different from that previously observed on homoepitaxially grown or relaxed InAs layers. Due to the virtual-surfactant growth conditions the investigated surface exhibits a very unique morphology observed in large scan STM images.

Acknowledgements The authors would like to thank H.P. Sch6nherr for his expert help with the M B E setup and J.L.

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Lazzari and A. Trampert for valuable discussions. Part of this work was sponsored by the Bundesministerium fiir Bildung, Wissenschaft, F o r s c h u n g und Technologie and the Deutsche Forschungsgemeinschaft (SFB 296).

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