Magneto-transport measurements are performed on the two-dimensional electron system (2DES) in an AlGaAs/GaAs heterostructure. By increasing the magnetic field perpendicular to the 2DES, magnetoresistivity oscillations due to Landau quantisation can be identified just near the direct insulator-quantum Hall (I-QH) transition. However, different mobilities are obtained from the oscillations and transition point. Our study shows that the direct I-QH transition does not always correspond to the onset of strong localisation. * ctliang@phys.ntu.edu.tw and ochiai@faculty.chiba-u.jp PACS numbers: 72.15.Rn, 71.70.Di, The insulator to quantum Hall (I-QH) transition in a two-dimensional electron system (2DES) at low perpendicular magnetic fields B has attracted much attention [1, 2,3,4,5,6,7,8,9,10,11]. Theoretically, the direct I-QH transition from the insulator to an integer QH state of ν = 1 is forbidden in an infinite, non-interacting 2DES with arbitrary amount of disorder, where ν is the Landau level filling factor [1, 2,3]. In such a system, the only allowed state at B = 0 is the insulating one, and the 2DES undergoes the I-QH transition to enter the ν = 1 QH state [12,13]. Realistically, however, only systems of finite sizes are available, and the effects of the electron-electron (e-e) interaction are significant in some 2DESs [4,5,14,15,16,17,18]. As a result, the 2DESs may experience the direct I-QH transition from the low-field insulator to QH states of higher filling factors [2,3,4,8,16,17,18]. Such a transition can be related to the zero-field metal-insulator transition, to which e-e interaction cannot be ignored [4]. Given that most 2DESs show metallic behavior at B=0, the investigation of the direct I-QH transition at low B should be conducted in low-mobility 2DESs [1,12].

We have performed low-temperature transport measurements on a disordered two-dimensional electron system (2DES). Features of the strong localization leading to the quantum Hall effect are observed after the 2DES undergoes a direct insulator-quantum Hall transition with increasing the perpendicular magnetic field. However, such a transition does not correspond to the onset of strong localization. The temperature dependences of the Hall resistivity and Hall conductivity reveal the importance of the electron-electron interaction effects to the observed transition in our study. *

We report on experimental studies of Al0.15Ga0.85N∕GaN high electron mobility transistor (HEMT) structures grown on p-type Si (111) substrates. By introducing an ultrathin SiN layer during the crystal growth, the Hall mobility of the HEMT structure can be greatly enhanced (greater than three times). This SiN treatment technique also allows the observation of Shubnikov–de Haas oscillations which is not possible in the untreated HEMT structure. Our experimental results pave the way for the integration of AlxGa1−xN∕GaN HEMT structures with the mature Si technology in industry.

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We report on magnetotransport studies of Al0.15Ga0.85N∕GaN high electron mobility transistor (HEMT) structures grown on p-type Si (111) substrates. A small but significant decrease of the Hall slope with increasing temperature is observed. Moreover, the converted conductivities reveal that the mobility of the HEMT shows a linear dependence on temperature. All these experimental results can be ascribed to electron-electron interaction (EEI) effects in AlxGa1−xN∕GaN HEMT structures grown on Si. The existence of EEI effects can be utilized to design and optimize GaN-based quantum devices on Si such as single-electron transistors and quantum point contacts since EEI effects can strongly modify the transport in semiconductor devices.