Disorder-induced features of the transverse resistance in a Si-MOSFET in the quantum Hall effect regime

We investigate two features of the transverse resistance R_xy in a Si-MOSFET in the quantum Hall effect regime. The first, the "overshoot" phenomenon, is observed at filling factor v = 3. In this case, when the magnetic field increases and the filling factor v approaches v = 3, R_xy overshoots its normal plateau value of 1/3(h/e²). However, if magnetic field increases further, R_xy decreases to its normal value. This effect has been previously observed in other two-dimensional systems, particularly, in a more perfect Si/SiGe heterostructure. Comparing the two Si systems, Si-MOSFET and Si/SiGe, with similar electron density and different mobility clearly shows that the increase in disorder leads to significant enhancement of the overshoot. The second effect is reported here for the first time and is characteristic only of Si-MOSFET. It was observed that when the filling factor approaches v = 5, R_xy, which is already at the plateau of 1/4(h/e²), abruptly decreases to 1/5(h/e²) and then sharply returns to 1/4(h/e²). This "deep" effect is more pronounced at higher electron concentration for which v = 5 is achieved at stronger magnetic fields. The explanation is based on the assumption of the equality between the disorder-induced broadening of adjacent valley-split Landau bands and the energy gap between them. In the narrow interval of magnetic fields near v = 5, the gap increases sharply due to exchange enhancement of the valley splitting, which results in a corresponding drop in R_xy.