Yevgeniy Temko

Change of InAs/GaAs quantum dot shape and composition during capping

Using plan-view and cross-sectional scanning tunneling microscopy, the shape and composition of InAs/GaAs quantum dots are investigated before and after capping by GaAs. During capping, the original pyramidally shaped quantum dots become truncated, resulting in a flat (001) top facet and steeper side facets. The InAs quantum dots are found to be intermixed at their top with GaAs due to material rearrangement. Since the bottom interface of quantum dots and wetting layer is always sharp, this intermixing occurs during capping and not during quantum dot growth. Considering strain energies, a model for the capping is presented.

Shape and growth of InAs quantum dots on GaAs(113)A

The shape of InAs quantum dots (QDs), grown by molecular-beam epitaxy on the GaAs(113)A surface, is studied by in situ scanning tunneling microscopy. At an early growth stage, the QDs adopt a shape given by {110}, (111)A, and {2 5 11}A bounding facets and an unresolved rounded {001} region. At a later growth stage, the QDs become elongated along [332], with a reduction of the (111)A facet size and a flattening of the rounded region. This is explained by facet growth kinetics. The broad size distribution indicates growth limitation. The symmetry of the substrate is retained in the QDs, proving epitaxial growth.

Shape transition of InAs quantum dots on GaAs(001)

InAs quantum dots (QD’s) grown by molecular-beam epitaxy on GaAs(001) substrates were studied by atomically resolved in situ (without breaking a ultrahigh vacuum) scanning tunneling microscopy. A detailed sequence of shapes, appearing during the growth process, is evaluated: The smallest QD is terminated by {137}A bounding facets with a rhombic base. With increasing size, {1¯3¯5¯}B and {1¯1¯2¯}B facets appear in succession besides {137}A, making the QD more rounded. At a critical diameter of the base, the QD undergoes a shape transition from flat to steep by the formation of steeper {110} and {111} facets at the sides and gradually achieves a squared base. For all QD’s the {137}A facets remain on top. Almost all shapes that have been observed for InAs QD’s on GaAs(001) in earlier publications are covered.

Shape of InAs quantum dots grown on the GaAs (1̄ 1̄ 3̄) B surface

Self-organized InAs quantum dots were grown by molecular-beam epitaxy on the GaAs (113) B surface. Atomically-resolved scanning tunneling microscopy images were acquired in situ. Each quantum dot grows with the same orientation relative to the substrate, with mirror symmetry to the (110) plane perpendicular to the surface, and with its central part sitting on a flat base. The shape of the central part is given by {110} and (111) B bounding facets and a not-further-resolved round region. High-index surfaces such as (135) B, (315) B, and probably (112) B are derived for the base.

Mature InAs quantum dots on the GaAs(114)A surface

InAs quantum dots (QDs), grown by molecular-beam epitaxy on GaAs(114)A surfaces, were studied in situ by atomically resolved scanning tunneling microscopy. At their mature stage, the QDs present a complicated but regular shape being bound by flat {110}, (111)A, and {2 5 11}A facets, and a steep part composed of rather variable combinations of {110}, (111)A, {111}B, and {2 5 11} surfaces. The QD shape can be derived from mature InAs QDs on GaAs(001).

InAs quantum dots on the GaAs(5 2 11)B surface

We report on InAs quantum dots grown by molecular-beam epitaxy on the stable GaAs(5 2 11¯)B surface discovered recently. Atomically resolved scanning tunneling microscopy images acquired in situ reveal compact quantum dots terminated by (101), (011), and (111)B facets and an unresolved vicinal (001) region. A flat base of (315)B orientation extends in front of the (101) and (111)B facets. The quantum dots exhibit a very narrow size distribution attributed to the well ordered substrate and to high nucleation efficiency.

Step structure on the GaAs(2 5 11) surface

Abstract: GaAs samples off-oriented from (2 5 11) by 1degrees and 5.2degrees were prepared by molecular-beam epitaxy and characterised in situ by scanning tunneling microscopy. Terraces of the GaAs(2 5 11) surface are separated by steps along three characteristic directions [3 1 (1) over bar], [4 5 (3) over bar], and [2 (3) over bar 1] which are equivalent to the intersections of the (0 1 1) (I I 3)A. and (I I I)A planes with the (2 5 11) plane, respectively. On the basis of atomically resolved step images, step models were developed and analyzed with help of the electron counting rule (ECR). Step bunches along [3 1 (1) over bar] form (0 1 1) facets. According to the derived models the steps along [3 1 (1) over bar] fulfill the ECR. whereas the steps along [4 5 (3) over bar] violate it. Steps along [2 (3) over bar 1] violate the ECR as well. but may fulfill it over mesoscopic areas.

Growth nuclei and surface defects on GaAs((1)over-bar(1)over-bar(3)over-bar)B

Abstract Growth nuclei and surface defects are studied with scanning tunneling microscopy on the GaAs( 1 1 3 )B(8×1) surface prepared by molecular beam epitaxy. Besides steps, three types of distortions of the (8×1) reconstructed surface are found. First, there are growth nuclei formed by small cluster of Ga and As atoms. Second, there are zig–zigzag or zigzag–zag irregularities in the zigzag chains of the (8×1) reconstruction. Third, complete zigzag chains are added or removed. Based on the analysis of these surface defects, the epitaxial growth on the GaAs( 1 1 3 )B(8×1) surface is discussed.

InAs quantum dots on GaAs((1)over-bar(1)over-bar(2)over-bar)B

InAs quantum dots (QDs) were prepared by molecular beam epitaxy on GaAs(1¯1¯2¯)B substrates. Shape and size distribution of the QDs were investigated using in situ scanning tunneling microscopy as function of preparation temperature between 435 and 550°C. The wetting layer is not flat but undulated in submicrometer scale in a similar way as the bare substrate. The atomic structure of the wetting layer is the same as found for the flat base of InAs QDs grown on GaAs(1¯1¯3¯)B substrates. The shape of the QDs is given by {110}, (1¯1¯1¯)B, and {1¯4¯3¯}B bounding facets and a round vicinal (001¯) region. Unexpectedly, the number density increases and the size distribution sharpens, when the growth temperature is increased from 435to470°C, which is attributed to lattice defects incorporated into the QDs during growth at 435°C.