On Surfaces Other Than 001

The surface structures and surface energies of substrate materials are important parameters in determining the mode of epitaxial growth. Higher index substrates often generate QDs different from those grown on the (001) surface. For example, Lobo and Leon [73] investigated InGaAs QDs on GaAs (11«) (n = 0, 1, and 3) surfaces using AFM. They found trapezoidal islands on (110) with the parallel edges of each trapezoid lying along [110], equilateral triangular islands on (111)B (they speculated that the island edges might parallel to (110)), lens-shaped islands on (311)B, and lens-shaped islands elongated along [233] on (311)A. Because of the observation of small lens-shaped islands on (110) and (111)B, they argued that large polygonal islands possibly begin as coherent lens-shaped islands and change shape beyond the coherent-incoherent transition (dislocations were seen in these large polygonal islands). Alternatively, the triangular and trapezoidal shapes may be the most energetically favorable for coherent islands and dislocation introduction may occur later.

Among all the investigated substrates with higher index surfaces, substrates with (311) surfaces have produced the highest quality QDs [147], while GaAs substrates with (111)A surface [148, 149] produced no QD. Notzel et al. [150] grew InGaAs/AlGaAs heterostructure on GaAs(311)B using metalorganic vapor-phase epitaxy. They found that during growth interruption after subsequently growing an AlGaAs buffer layer and an InGaAs layer, the strained InGaAs layer self-organizes into orderly arranged nano disks. The nano disks were found to be covered with AlGaAs, forming clearly faceted islands (see Fig. 7). However, the facet planes were not identified. The fact that no AlGaAs was deposited after the InGaAs was deposited implies lateral mass transport from the AlGaAs buffer layer to the InGaAs QDs. Notzel et al. later [147] compared the morphologies of InGaAs grown on GaAs(«11)A/B (n = 2, 3, and 5) with a thick Al0 5Ga0 5As film as the buffer layer. They found that among all of these high-index surfaces (311)B provides the best alignment and uniformity, while (311)A presents wirelike structures with the wire axis approximately along [233].

Lacombe et al. [151] investigated Ga02In08As/InP(311)B QDs using TEM and AFM and found that the QDs are generally bounded by low-index facets {001}, {111}B, and {110} which make angles to the substrate surface of 25°, 29°, and 31°, respectively. A very similar shape but with more facets ({315} and (112)B) on the skirts of QDs has been seen in the MBE grown InAs/GaAs(113)B system by Suzuki et al. [152]. They presented in-situ shape determination of the QDs using atomically resolved STM. The shape and facet plane indices determined by Suzuki et al. [152] are shown in Figure 8. The fact that QDs grown on (113)B

Figure 7. A three-dimensional AFM image of In0 2Ga0 8As/Al0 5 Ga0 5 As on GaAs(311)B. Reprinted with permission from [147], R. Notzel et al., Appl. Phys. Lett. 65, 2854 (1994). © 1994, American Institute of Physics and Prof. T. Fukui.

have low-index facets is important for QD growth because low-index facets normally have lower surface energies and therefore reduce the barrier for QD formation. This is supported by the observation that In08Ga02As QDs were easier to form on InP(113)B than on InP(001) [153].

Omi and Ogino [154, 155] grew coherent Ge islands on Si(113) using MBE at 400 °C to 550 °C and_found the islands to be wire-shaped, elongated along [332], and bounded by {519} facets. The width of the islands is between 16 and 32 nm, while the length is between 10 and 600 nm. They found the wire-shaped islands were transformed from equiaxial islands and they explained the transformation in terms of the theory of Tersoff and Tromp [96]. Zhu et al. [156] also reported Ge/Si(113) islands elongated

Figure 8. (a) and (b) three-dimensional STM images of an InAs/GaAs(113)B QD with the image size of 42 x 42 nm2 and (c) a schematic model of the QD shape. Reprinted with permission from [152], T. Suzuki et al., Appl. Phys. Lett. 80, 4744 (2002). © 2002, American Institute of Physics and Prof. K. Jacobi.

Figure 8. (a) and (b) three-dimensional STM images of an InAs/GaAs(113)B QD with the image size of 42 x 42 nm2 and (c) a schematic model of the QD shape. Reprinted with permission from [152], T. Suzuki et al., Appl. Phys. Lett. 80, 4744 (2002). © 2002, American Institute of Physics and Prof. K. Jacobi.

along [332] but bounded by facets including (111), {516}, {517}, and {518} with (113) as the top facets.

Atomic force microscopy and TEM investigation [157] revealed that coherent Ge/Si(211) islands grown by MBE are of irregular hexagonal shapes elongated along (111) and bounded by {111} and {113} facets, whereas large dislocated Ge/Si(211) islands are mainly bounded by {111} and {001} facets. Truncated tetrahedral-shaped islands have been observed in Ge/Si(111) [158-160], and they have a triangular base bounded by three (110) equivalent edges and with (111) as the top facet. The Ge/Si(111) island shape appears to reflect the symmetry of the (111) substrate surface.

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