Research Center for Advanced Functional Materials and Laser Communication Systems

Studies of the dislocation structure in heterolayers based on materials with hexagonal symmetry

Theoretical studies are being carried out at the International Research Center of Functional Materials and Devices of Optoelectronics to describe and predict defects in various semiconductor heterostructures. In particular, a theoretical model is proposed that describes the critical conditions for the formation of misfit dislocations (MDs) in III-nitrides. Figure 1 illustrates a semipolar heterostructure, which consists of substrate and layer with MDs formed in accordance to wurtzite crystallography. The MDs in Fig. 1 are mixed dislocations, which have both edge and screw components. These MDs can be originated as a result of basic (Fig. 1, a) or prismatic (Fig. 1, b). Fig. 1, c shows sessile MD.

Figure 1 – Schematics of misfit dislocations (MDs) in a semipolar III-nitride heterostructures.

(a) Misfit dislocation originated from basal slip (BSMD), (b) misfit dislocation originated from prismatic slip (PSMD), and (c) sessile misfit dislocation (SMD). b is dislocation Burgers vector.

Figure 2 compares the theoretical calculations with experimental data based on the dependencies of the critical film thickness on the amount of Al in AlGaN/GaN heterostructures (Fig. 2, a). The symbols correspond to layer thicknesses h, at which the presence of MDs and stress relaxation were detected in the experiments. (see also the table in Fig. 2; the presence of MDs is marked with a tick, the absence of MDs is marked with a cross). The nucleation of MDs is possible in the region above the corresponding curves. This approach is used to determine the lower boundary of the possibility of MDs formation. Figure 2, b-d shows the panchromatic cathodoluminescent images of Al_0.13Ga_0.87N/GaN heterostructures with different film thicknesses. In Fig. 2, b, only threading dislocations are visible; in Fig. 2, c, MDs originated from basal slip; in Fig. 2, d, MDs originated from basal and prismatic slip.

Figure 2 – Comparison of the results of theoretical calculations with experimental data. (a) dependencies of the critical film thickness h_c on composition x of Al in the Al_xGa_1-_xN/GaN heterostructure. Symbols indicate layer thicknesses h at which the presence of MDs and stress relaxation were detected in the experiments; open symbols correspond to PSMDs and filled symbols correspond to BSMDs. The table presents the experimental data: a tick corresponds to the observation of MDs, and a cross indicates MDs absence in the heterostructure. (b-c) panchromatic cathodoluminescent images taken at 5 kV excitation voltage for Al_0.13Ga_0.87N/GaN heterostructures with different film thicknesses: (b) 23 nm, (c) 180 nm, (d) 360 nm.