Kuang Yao Chen

Probing Landau quantization with the presence of insulator?quantum Hall transition in a GaAs two-dimensional electron system

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, magneto-resistivity oscillations due to Landau quantization 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 localization.

A delta-doped quantum well system with additional modulation doping

A delta-doped quantum well with additional modulation doping may have potential applications. Utilizing such a hybrid system, it is possible to experimentally realize an extremely high two-dimensional electron gas (2DEG) density without suffering inter-electronic-subband scattering. In this article, the authors report on transport measurements on a delta-doped quantum well system with extra modulation doping. We have observed a 0-10 direct insulator-quantum Hall (I-QH) transition where the numbers 0 and 10 correspond to the insulator and Landau level filling factor ν = 10 QH state, respectively. In situ titled-magnetic field measurements reveal that the observed direct I-QH transition depends on the magnetic component perpendicular to the quantum well, and the electron system within this structure is 2D in nature. Furthermore, transport measurements on the 2DEG of this study show that carrier density, resistance and mobility are approximately temperature (T)-independent over a wide range of T. Such results could be an advantage for applications in T-insensitive devices.

Probing the onset of strong localization and electron–electron interactions with the presence of a direct insulator–quantum Hall transition

Abstract 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 on 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 for the observed transition in our study.

Electron-electron interactions in Al0.15Ga0.85N/GaN high electron mobility transistor structures grown on Si substrates

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.

Al0.15Ga0.85N∕GaN high electron mobility transistor structures grown on p-type Si substrates

We report on magnetotransport studies of Al0.15Ga0.85N/GaN high electron mobility transistor (HEMT) structures grown on p-type Si(111) substrates. Both weak localization (WL) and electron-electron interaction (EEI) correction terms to the conductivity of the SiN treated HEMT are smaller than those of the untreated HEMT. Since both WL and EEL corrections tend to decrease the conductivity of an AlGaN/GaN HEMT structure, our SiN treatment is useful for enhancing the performance of GaN-based HEMT structures grown on Si, which is compatible with the mature Silicon CMOS technology.

Controllable Disorder in a Hybrid Nanoelectronic System: Realization of a Superconducting Diode

We have studied a hybrid nanoelectronic system which consists of an AlGaAs/GaAs two-dimensional electron gas (2DEG) in close proximity (~70 nm) to an Al superconducting nanofilm. By tuning the current through the Al film, we can change the conductance of the 2DEG and furthermore vary the effective disorder in the Al superconducting film in a controllable way. When a high current is injected into the film, screening which couples the Al film and the 2DEG results in a collapse of anti-symmetric behavior in the current-voltage characteristics, V(I) ~ -V(-I), which holds true in a conventional superconductor. Our results may open a new avenue of experimentally realizing a superconducting diode.

Electron heating and huge positive magnetoresistance in an AlGaAs∕GaAs high electron mobility transistor structure at high temperatures

We have performed magnetoresistivity measurements ρxx(B) on an AlGaAs∕GaAs high electron mobility transistor (HEMT) structure at high temperatures T. The observed positive magnetoresistance (MR) in the HEMT structure can be greatly enhanced simply by increasing the driving current. At T=80K and B=6T, the MR value can be increased from ∼150% to 4000%, almost a 30-fold increase when a large current of 40μA is applied. Such results are due to electron heating effects and our data lay the foundation for practical magnetic device applications which can be readily combined with high-speed electronics, high-frequency amplifiers, and radar using the mature GaAs-based HEMT technology.

Huge positive magnetoresistance in a gated AlGaAs∕GaAs high electron mobility transistor structure at high temperatures

Magnetoresistivity measurements on a gated AlGaAs∕GaAs high electron mobility transistor (HEMT) structure were performed at high temperatures T. By changing the applied gate voltage Vg, we can investigate the observed huge positive magnetoresistance (PMR) at different effective disorder and density inhomogeneity within the same HEMT structure. The observed PMR value increases with increasing disorder in the depletion mode (Vg⩽0). Moreover, the PMR value is not limited by the quality of the HEMT structure at T=80K. Such results pave the way for low-cost, high-throughput GaAs-based HEMT fabrication for future magnetic sensing and recording devices fully compatible with the mature HEMT technology.

Magnetotransport in an aluminum thin film on a GaAs substrate grown by molecular beam epitaxy

Magnetotransport measurements are performed on an aluminum thin film grown on a GaAs substrate. A crossover from electron- to hole-dominant transport can be inferred from both longitudinal resistivity and Hall resistivity with increasing the perpendicular magnetic field B. Also, phenomena of localization effects can be seen at low B. By analyzing the zero-field resistivity as a function of temperature T, we show the importance of surface scattering in such a nanoscale film.

Probing mobility gaps at resistivity minima in the integer quantum hall effect

Magneto-transport measurements are performed on the AlGaAs/GaAs quantum Hall (QH) devices fabricated recently by our group. A series of Hall plateaus are observed with increasing the perpendicular magnetic field, and the mobility gaps resulting from localization effects are investigated at the minima in the longitudinal resistivity. Only the gap corresponding to the filling factor i = 2 is close to the expected cyclotron energy, and our study supports that the low-field QH conductors may suffer problems due to insufficient localization. The anomalous change on the Hall slope is observed when the i = 3 plateau is destroyed by the large current.