| We realized very high quality degenerate electron systems in selectively-doped GaAs/{dollar}rm Alsb{lcub}x{rcub}Gasb{lcub}1-x{rcub}As{dollar} heterostructures fabricated by molecular beam epitaxy. The use of specific growth techniques, like Si {dollar}delta{dollar}-doping and the substitution of {dollar}rm Alsb{lcub}x{rcub}Gasb{lcub}1-x{rcub}{dollar}As layers with thin superlattices, helped optimize conditions for minimum disorder. Several types of low-disorder systems were subsequently realized: (1) two-dimensional electron systems at (100) interfaces, (2) quasi-three-dimensional systems in wide parabolic wells, (3) double layer electron systems in wide square wells, and (4) two-dimensional hole systems at (311)A interfaces. Low-temperature magnetotransport experiments revealed the dramatic effects of increased electron layer thickness, interlayer correlations, and larger effective mass on fractional quantum Hall states and reentrant insulating phases at low Landau level fillings. |
