6.4 SELF-ORDERED QUANTUM WIRES

The self-limiting V-groove profiles discussed in the previous sections can serve as seeds for the formation of QWRs and related heterostructures, owing to the characteristic evolution of growth on a nonplanar substrate described in Section 6.2. Starting from a self-limiting V-groove profile (e.g., Al _xGa _{1- x}As), and then switching to growth of a lower bandgap layer (e.g., Al _yGa _{1- y} As, y < x), which is also characterized by a larger "effective" diffusion length, results in expansion of the nanofacets at the bottom of the groove. This is accompanied by a higher growth rate at the bottom facets. The result is the formation of a crescent-shaped, lower-bandgap structure, which can confine charge carriers in the lateral direction (see Figure 6.27). Capping the crescent with a higher bandgap material yields a buried QWR structure whose interfaces are all formed during growth, and are thus potentially defect-free. Notice that, in addition to the 3D barrier material surrounding the QWR crescent, 2D barriers are formed, due to the growth of the lower-bandgap layer on the sidewalls of the groove. In the case of an alloy barrier (e.g. AlGaAs), a 2D VQW barrier can also form above and below the wire. A complex 1D heterostructure confined by barriers of different dimensionalities is thus realized.

Figure 6.27: Schematic cross section of a single-QWR heterostructure formed between two faceted V-groove profiles grown by OMCVD in a [01 1] channel etched in a (100)-GaAs substrate.