| We present the fabrication and transport properties of high quality quasi two-dimensional (2D) systems with an additional degree of freedom along the direction perpendicular to the 2D plane. The electron or hole systems were realized in remotely doped GaAs/Al{dollar}sb{lcub}rm x{rcub}{dollar}Ga{dollar}sb{lcub}rm 1-x{rcub}{dollar}As heterostructures grown by molecular beam epitaxy. The additional degree of freedom is achieved in two ways: (i) by increasing the thickness of the carrier system, or (ii) by placing two or more 2D systems in close proximity. In both cases the electric subband energy levels are lowered significantly compared to the case of single-layer 2D systems, and typically more than one subband is occupied. We characterized these systems by comparing the results of a self-consistent calculation with experimental data from various techniques, such as capacitance-voltage measurements, analysis of Shubnikov-de Haas oscillations, or magnetoresistance measurements with in-plane magnetic field. Magnetotransport measurements in these systems at high magnetic field reveal novel phenomena not observed in thin or single-layer 2D electron systems. Examples include: (1) a collapse of the energy gap of the fractional quantum Hall states as the width of the electron system increases, (2) a reentrant insulating phase at high filling factor in a wide hole system, and (3) anomalously strong fractional quantum Hall states in a three-layer electron system. |
