Low-temperature transport and photoconductivity response of high mobility Si-SiGe heterostructures in strong magnetic fields

Abstract

Low-temperature (mK) magneto-transport and photoconductivity measurements of 2D electron and hole systems (2DES, 2DHS) in high quality n-and p-type modulation-doped Si-SiGe heterostructures (respectively) have been extended to high magnetic fields. For the high mobility 2DES in n-Si, a two valley system, signatures of the fractional quantum Hall effect (FQHE) in the region ν < 1 (one valley occupied) usually observed in GaAs are replicated out to ν = 2/5(B ∼ 40 T). For 1 < ν < 2 however (two valleys occupied) prominent FQHE states are absent. For the 2DHS in p-SiGe, in addition to the QHE two low temperature insulating phases (IP) are identified at ν = 1.5 and ν < 1/2(B ∼ 30 T). The IP at ν = 1.5 has the characteristics of a Hall insulator and our measurements indicate that the important physics is related to an unusual degeneracy of adjacent Landau levels (LL) of opposite spin. Illuminating the p-SiGe samples results in a lifting of the LL degeneracy together with a quenching of the IP at ν = 1.5. Magneto-photoconductivity measurements of these samples closely resemble dρxx/dB and provide an additional probe of this indirect bandgap system. The incorporation of a dilution refrigerator in pulsed magnets with CuAg conductor for extended use at 60 T is described.