Universal behavior of magnetoresistance in quantum dot arrays with different degree of disorder

Magnetoresistance in two-dimensional array of Ge/Si quantum dots was studied in a wide range of zero-magnetic field conductances, where the transport regime changes from hopping to diffusive one. The behavior of magnetoresistance is found to be similar for all samples - it is negative in weak fields and becomes positive with increase of magnetic field. The result apparently contradicts to existing theories. To explain experimental data we suggest that clusters of overlapping quantum dots are formed. These clusters are assumed to have metal-like conductance, the charge transfer taking place via hopping between the clusters. Relatively strong magnetic field shrinks electron wave functions decreasing inter-cluster hopping and, therefore, leading to a positive magnetoresistance. Weak magnetic field acts on "metallic" clusters destroying interference of electron wave function corresponding to different paths (weak localization) inside clusters. The interference may be restricted either by inelastic processes, or by the cluster size. Taking into account WL inside clusters and hopping between them within the effective medium approximation we extract effective parameters characterizing charge (magneto) transport.