The effect of illumination on transport properties of the two-dimensional hole gas (2DHG) in Si–SiGe heterostructures is found to irreversibly alter its 2D transport properties, analogous to the persistent photoconductivity effect in GaAs-based devices. The relatively small change of the 2D hole concentration from 3.75×1011cm−2 before illumination to 4.23×1011cm−2 after illumination is accompanied by a significant increase in the in-plane effective mass from (0.23–0.25)me to (0.32–0.33)me, and an even larger increase in the quantum lifetime. To evaluate the g-factor of this highly spin degenerate 2DHG we use highly sensitive magneto photoconductivity measurements to obtain g*=9.1±0.1 after illumination, compared to an estimate for dark state g*=8.5±0.5.

Magneto-transport measurements of the 2D hole system (2DHS) in p-type Si - heterostructures identify the integer quantum Hall effect (IQHE) at dominantly odd-integer filling factors and two low-temperature insulating phases (IPs) at and , with re-entrance to the quantum Hall effect at . The temperature dependence, current - voltage characteristics, and tilted field and illumination responses of the IP at indicate that the important physics is associated with an energy degeneracy of adjacent Landau levels of opposite spin, which provides a basis for consideration of an intrinsic, many-body origin.

Low-temperature magnetotransport measurements of 2D electron and hole systems (2DES, 2DHS) in high quality n- and p-type modulation-doped SiSi1 − xGex heterostructures (respectively) have been extended to high magnetic fields (50 T) and low temperatures (30 mK). For the high-mobility 2DES in n-Si, a two-valley system, signatures of the fractional quantum Hall effect (FQHE) in the region v < 1 (one valley occupied, lowest spin state) usually observed in GaAsAlGaAs are replicated out to v = 2/5 at B ≈ 48T. For 1 < v < 2, however, (both valleys occupied, lowest spin state), prominent FQHE states such as v = 5/3 are absent, indicative of the importance of valley occupation. For the 2DHS in p-Si1 − xGex, in addition to the QHE, two low-temperature insulating phases (IP) are identified at v = 1.5 and v ≲ 0.5 (B ≈ 30T), with re-entrance to the QHE regime at v = 1 (B ≈ 15T). The IP centred at v = 1.5 has the characteristics of a Hall insulator but is unanticipated by the global phase diagram for 2D systems. The important physics associated with the IP is related to an energy degeneracy of adjacent Landau levels of opposite spin.

A highly sensitive photoconductivity (PC) technique has been applied to undoped GaAs-AlGaAs micro-structures with either a single interface or a single quantum well and a gate-controlled variable density of electrons or holes. In zero magnetic field measurements we have found a wide absorption band below the fundamental gap of bulk GaAs, which correlates with the population of 2D states. The PC technique has potential for spectroscopic studies of semiconductor nano-structures, such as quantum wire devices, which present difficulties for other conventional spectroscopy methods.

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 nu < 1 (one valley occupied) usually observed in GaAs are replicated out to nu = 2/5(B similar to 40 T). For 1 < nu < 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 nu = 1.5 and nu < 1/2(B similar to 30 T). The IP at nu = 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 nu = 1.5. Magneto-photoconductivity measurements of these samples closely resemble d rho(xx)/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.