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An AlGaN/GaN metal-heterostructure-metal (MHM) ultraviolet (UV) photodetector employing lateral Schottky contacts was fabricated and characterized at different temperatures. As the temperature increased from 25 to 250 °C, the photoresponsivity of the MHM photodetector increased 3.5 times. This was attributed to the spontaneous-polarization-induced spatial separation of the photogenerated electrons and holes and the increased optical absorption at higher temperatures. Meanwhile, the decay time constant of the photocurrent became approximately three orders of magnitude smaller. With the enhanced photoresponsivity and the decreased response time constant, kilohertz optical switching of the MHM photodetector was recorded at 250 °C. The AlGaN/GaN MHM photodetector, sharing the same GaN-on-Si electronics platform, provides an applicable candidate for an all-GaN integrated UV sensing and amplifying system for high-temperature applications.

In this work, the breakdown characteristics and the electroluminescence (EL) spectra of a Schottky-metal/p-GaN/AlGaN/GaN device under forward bias were investigated at different temperatures. The failure of the metal/p-GaN junction, which was caused by electron transport in the p-type Schottky junction, was identified as the first step in the device breakdown process. The breakdown voltage increased with higher temperatures. Under a forward bias of 8 V, the intensity of the EL emission increased more than two orders of magnitude, while the current increased by a factor of 4 as the temperature increased from 0 °C to 200 °C. This unambiguously demonstrated thermally enhanced hole injection at the Schottky-metal/p-GaN interface. We proposed that more electrons were annihilated by the thermally enhanced hole injection, resulting in the positive temperature dependence of the device breakdown.

Defect mediated nonradiative recombination limits the photoluminescence (PL) quantum yield of transition metal dichalcogenide monolayers (MLs). In this work, the enhancement of the PL intensity of ML WS2 is reported in a van der Waals heterostructure of WS2 ML and InGaN quantum dots (QDs) under excitation with photon energy larger than the bandgaps of both WS2 and QDs. The mechanism of this PL enhancement is due to the aggregation of excitons in WS2 ML toward the QD sites to form an interfacial bound state, which effectively mitigates the influence of defects. This exciton aggregation induced enhancement of PL intensity is more pronounced at low temperatures and under low power excitations. The lifetime and diffusion coefficient of the excitons in WS2 ML are also measured to validate the aggregation scenario.Defect mediated nonradiative recombination limits the photoluminescence (PL) quantum yield of transition metal dichalcogenide monolayers (MLs). In this work, the enhancement of the PL intensity of ML WS2 is reported in a van der Waals heterostructure of WS2 ML and InGaN quantum dots (QDs) under excitation with photon energy larger than the bandgaps of both WS2 and QDs. The mechanism of this PL enhancement is due to the aggregation of excitons in WS2 ML toward the QD sites to form an interfacial bound state, which effectively mitigates the influence of defects. This exciton aggregation induced enhancement of PL intensity is more pronounced at low temperatures and under low power excitations. The lifetime and diffusion coefficient of the excitons in WS2 ML are also measured to validate the aggregation scenario.

In this work, by using an on-chip integrated Schottky-on-heterojunction light-emitting diode (SoH-LED) which is seamlessly integrated with the AlGaN/GaN high electron mobility transistor (HEMT), we studied the effect of on-chip light illumination on the de-trapping processes of electrons from both surface and bulk traps. Surface trapping was generated by applying OFF-state drain bias stress, while bulk trapping was generated by applying positive substrate bias stress. The de-trapping processes of surface and/or bulk traps were monitored by measuring the recovery of dynamic on-resistance Ron and/or threshold voltage Vth of the HEMT. The results show that the recovery processes of both dynamic Ron and threshold voltage Vth of the HEMT can be accelerated by the on-chip SoH-LED light illumination, demonstrating the potentiality of on-chip hybrid opto-HEMTs to minimize the influences of traps during dynamic operation of AlGaN/GaN power HEMTs.

We report that a simple metal-AlGaN/GaN Schottky diode is capable of producing GaN band-edge ultraviolet emission at 3.4 eV at a small forward bias larger than ∼2 V at room temperature. Based on the surface states distribution of AlGaN, a mature impact-ionization-induced Fermi-level de-pinning model is proposed to explain the underlying mechanism of the electroluminescence (EL) process. By experimenting with different Schottky metals, Ni/Au and Pt/Au, we demonstrated that this EL phenomenon is a “universal” property of metal-AlGaN/GaN Schottky diodes. Since this light-emitting Schottky diode shares the same active structure and fabrication processes as the AlGaN/GaN high electron mobility transistors, straight-forward and seamless integration of photonic and electronic functional devices has been demonstrated on doping-free III-nitride heterostructures. Using a semitransparent Schottky drain electrode, an AlGaN/GaN high electron mobility light-emitting transistor is demonstrated.

An array of Nb strips was fabricated on a Bi2Te3 thin film. Transport measurements show that an upturn of the resistance occurs below the Nb superconducting transition temperature. Corresponding to this resistance upturn, a differential resistance peak around zero bias and a prominent negative magnetoresistance at low magnetic fields are observed. The resistance upturn, differential resistance peak, and negative magnetoresistance can be suppressed by increasing measurement current, temperature, and applied magnetic field. We explain these phenomena in terms of an energy barrier effect induced by the Nb superconducting gap and the suppression of Andreev reflection due to the low transparency at the Nb-Bi2Te3 interface.

A linear magnetoresistance (LMR) with strong temperature dependence and peculiar non-symmetry with respect to the applied magnetic field is observed in high-index (221) Bi2Se3 films. Different from the LMR observed in the previous studies which emphasize the role of gapless linear energy dispersion, this LMR is of disorder origin and possibly arises from the electron surface accumulation layer of the film. Besides, an abnormal negative magneto-resistance that shows a non-monotonic temperature dependence and persists even at high temperatures and in strong magnetic fields is also observed.

Linear magneto-resistance is observed in high magnetic field in topological insulator Bi2Se3 films. As revealed by tilted magnetic field measurement, this linear magneto-resistance is associated with the gapless topological surface states and of quantum origin. In the ultra-thin limit, the inter-surface tunneling induced surface state gap opening quenches the linear magneto-resistance. Instead, weak negative magneto-resistance is observed in high magnetic fields in ultra-thin films.

The effect of fluorine-plasma (F-plasma) implantation on the current-voltage (I-V) and electroluminescence (EL) characteristics of Ni/Au-AlGaN/GaN Schottky diodes have been investigated. The observed EL spectrum is dominated by the GaN near band edge emissions. The threshold current of the EL emission for F-plasma implanted diodes is significantly lower than that for the previously reported diodes without the F-plasma implantation. This reduction of threshold current results from the presence of negatively charged F-centers in AlGaN layer which leads to upward band bending of AlGaN layer and enhanced hole injection by multi-step tunneling process through AlGaN layer. The magnitude of the upward banding in AlGaN layer is estimated to be 0.36 eV by analyzing the forward-biased I-V characteristics.

Electroluminescence (EL) from a forward biased Ni/Au-AlGaN/GaN Schottky diode was observed and studied. According to the EL spectra, which were dominated by the GaN near band edge emissions, holes were injected into the GaN layer. It was found that the turn-on voltage of the EL emission coincided with the second turn-on voltage that indicates the appearance of a second conduction channel in the diode’s forward current-voltage characteristics. This coincidence implied that the second conduction channel originated from the hole injection. A model based on the Fermi-level depinning caused by the current induced interface states ionization is proposed to explain the hole injection process and the observed EL characteristics.

Fluorine plasma treatment technique can effectively incorporate fluorine atoms into the AlGaN barrier, depleting the two-dimensional electron gases (2DEGs) in the channel of AlGaN∕GaN high electron mobility transistors and converting the device from depletion mode to enhancement mode. To reveal the underlying physical mechanisms, temperature dependent persistent photoconductivity (PPC) and Hall measurements are conducted in AlGaN∕GaN heterostructures treated by CF4 plasma. Weakly temperature dependent 2DEG mobility and much more pronounced PPC effect are observed in the F-treated sample. An energy barrier of 624meV for electrons recaptured by the F-related centers is extracted from the PPC decay behaviors.

Anomalous photocurrent was observed in an epitaxial Fe∕Zn0. 96Fe0. 04S Schottky diode grown by molecular beam epitaxy. The temperature dependent decay behavior of the anomalous photocurrent has been studied. A model based on the photoionization of the acceptorlike interface states is proposed to explain this anomalous phenomenon. By fitting the decay curves of the anomalous photocurrent at different temperatures, ionization energy of the interface states is obtained from an Arrhenius plot of the decay time constants. We believe that these interface states are associated with some complex (Fe, S) defects formed at the Fe∕ZnFeS interface.