Shapes of QDs in Other Material Systems with 001 Surfaces

Using TEM to investigate the shape of MOCVD grown InP/GaInP(001) islands, Georgsson et al. [134] found that both buried and unburied islands have truncated pyramidal shapes with a hexagonal base bounded by (001), {110} and {111} facets. Figure 6 shows the cross-section images taken from [110] and [1l0] of InP/GaInP(001) islands together

Figure 5. A comparison of an experimental plan-view image of a buried InIGa1_IAs/GaAs QD (a) and a simulated image based on a lens-shaped model buried QD (b).

with the schematic shape model deduced from the information provided by plan-view and cross-section TEM images. Theoretical investigations [135] using a hybrid approach that combines density-functional theory calculations of the surface energies and continuum elasticity theory calculations of bulk deformation energies, suggest that the InP/GaInP(001) island shape shown in Figure 6 is close to thermodynamic equilibration.

Arlery et al. [136] characterized GaN/AlN(0001) QDs in the wurtzite GaN crystal structure using cross-section highresolution TEM images [136, 137] together with RHEED data [138, 139] and deduced that the QDs are truncated pyramids with a hexagonal base, an aspect ratio of 1/5, and a contact angle between the QD sidewalls and the substrate plane of 30°. Wurtzite GaN QDs with the aspect ratio of 1/5 and zinc-blende GaN QDs with an aspect ratio of 1/10 were also reported by Daudin et al. [140].

Although AFM [141, 142] and optical characterizations [143, 144] have suggested the existence of QD structures in II-VI systems, detailed structural information of II-VI QDs has been rare. The first clear image of II-VI QDs was produced by Arita et al. [145]. They used AFM to investigate CdSe/ZnSe islands grown on GaAs(001) by MOCVD and showed a structure with {311}A facets along the [011] direction.

Figure 6. Cross-section TEM images of an unburied IP/GaIP(001) island taken from [110] (a) and [110] (b). A schematic model of the island shape deduced from the combination of plan-view and cross-section TEM images is shown in (c). Reprinted with permission from [134], K. Georgsson et al., Appl. Phys. Lett. 67 2981 (1995). © 1995, American Institute of Physics and Prof. W. Seifert.

0 0

Post a comment