Logical operation based on polarization encoding of light is important for future data transmission and information processing. However, in the terahertz (THz) region, chiral materials with large optical activity are not available in nature, and the effective manipulation of polarization states remains challenging. Here, the authors demonstrate a double-layer bi-anisotropic metamaterial that consists of planar spiral and cut-wire layers separated by a polyimide film. Strong asymmetric polarization rotation of two orthogonal linear polarizations can be observed around 0.53 THz. By investigating the correlation between two linear polarization states before and after the spiral-wire metamaterial at this frequency, a controlled-NOT (CNOT) gate operating on two linear-polarization-based qubits is further exploited. The processing mechanism of the asymmetric rotation and CNOT gate is attributed to the scattering of dipole momentum based on classical multipole theory. This polarization processor's architecture is promising for robust and energy-efficient THz polarization control, and also provides an effective path for the development of future optical supercomputing technology.

•Temperature dependent mobility and low-frequency noise of a-IGZO TFTs are studied.•The dominant scattering mechanism of channel carriers is found Coulomb scattering.•The border trap density and the distribution of filled interface traps are deduced.•Annealing at higher temperature is found effective to reduce border trap density.In this work, the interface properties of amorphous indium–gallium–zinc oxide thin film transistors annealed at different temperatures ranging from 150 to 250 °C are studied by temperature dependent mobility and low-frequency noise (LFN) characterizations. The dominant scattering mechanism for carrier transport is found to be Coulomb scattering based on gate bias and temperature dependent mobility measurement. Meanwhile, as the annealing temperature increases, the dominant mechanism of LFN within the device channel varies from carrier number fluctuation to carrier mobility fluctuation. The border trap density as well as the distribution properties of charged border traps is deduced. The present results suggest that annealing at higher temperature has a more remarkable effect on removing deeper border traps than traps closer to the channel/dielectric interface.

•Breakdown properties of GaN HEMTs are studied by drain current injection method.•Buffer-leakage induced breakdown is revealed at high drain current injection level.•The buffer-leakage induced breakdown should be defect-related in present devices.Off-state breakdown characteristics of AlGaN/GaN high-electron-mobility transistors have been studied based on drain current injection method. It is found that at low drain current injection level, the observed premature breakdown is caused by excess gate-to-drain leakage current. Nevertheless, at high drain injection current level, buffer-leakage-dominated breakdown proceeds gate-leakage-dominated breakdown as the gate bias increases from pinch-off voltage to deep-depletion voltage. In both breakdown regions, the breakdown voltages show negative temperature coefficients. The buffer-leakage-induced breakdown should be defect-related, which is confirmed by temperature-dependent buffer leakage measurements.

•MSM photodetectors are directly fabricated on Fe-doped semi-insulating HVPE GaN template.•The photodetector exhibits low dark current and acceptable UV-to-visible rejection ratio.•The high-density defect states within the GaN:Fe layer have a strong influence on the performance of the photodetectors.In this work, metal-semiconductor-metal ultraviolet (UV) photodetectors (PDs) are directly fabricated on Fe-doped semi-insulating GaN template grown by hydride vapor phase epitaxy. Under 20 V bias, the PD exhibits a room temperature low dark current of <2.5 pA, a UV-to-visible rejection ratio of up to 1 × 102 and a corresponding maximum quantum efficiency of ∼10%. At higher temperature, both the photo-current and dark current of the PD increase, and a reduced UV-to-visible rejection ratio is observed. A temporal response and recovery time of less than 2 ms is obtained for the PD, nevertheless its photocurrent is found to decrease continuously under stable UV illumination. There is evidence that the high-density trap states and recombination centers within the GaN:Fe layer have a strong influence on the photo-response characteristics of the PDs. This study provides a possible route to fabricate low cost GaN-based UV PDs with reasonable performance.

In this work, Schottky barrier diodes with vertical geometry were fabricated on low-defect-density homoepitaxial GaN for studying the reverse leakage mechanism of GaN-based Schottky contact. A leakage current model based on electron transmission primarily through linear defects like dislocations was suggested to explain the reverse current–voltage characteristics measured between 300 and 410 K, in which electrons from contact metal overcome the locally height-reduced Schottky barrier through thermionic-field emission.Highlights► Vertical Schottky diodes are fabricated on low-defect-density homoepitaxial GaN. ► The diode shows low leakage current on the order of ∼10−11 under −10 V bias. ► A leakage model based on electron transmission along linear defects are suggested. ► Electrons overcome the height-reduced Schottky barrier through TFE emission.

•The electrical properties of a-IGZO TFTs are measured as a function of temperature.•Larger subthreshold swing and reduced drain current are observed at low temperature.•Carrier transport mechanisms at various temperature and biasing regimes are discussed.The electrical properties of amorphous indium–gallium–zinc oxide thin film transistors are measured in the temperature range from 70 to 300 K. The device shows normal enhancement mode operation with significantly reduced drain current at low temperature. Its turn-on voltage and subthreshold swing decrease as temperature increases. The transport mechanisms of channel electrons are analyzed based on the evolution of field-effect mobility and channel conductance as a function of temperature and gate bias. It is suggested that in low temperature range, the dominant carrier transport mechanism is hopping between localized band-tail states. As temperature increases, multiple trapping and release plays a role in the whole carrier transport process. Meanwhile, in high gate bias range when the Fermi level moves above the mobility edge, band transport starts to dominate.

The distribution of deep-level traps at atomic-layer-deposited Al2O3/n-GaN interface is studied by photo-assisted high-frequency capacitance–voltage (C–V) method. The dark C–V curve of the fabricated metal–oxide-semiconductor (MOS) capacitors shows positive-shifted ideal MOS characteristics with deep-depletion behavior. The electrons trapped at deep states can be neutralized by holes generated by ultra-violet (UV) light illumination. An interface-trap-related voltage stretch-out of the C–V curve is then obtained by sweeping the capacitor from depletion back to accumulation after UV illumination, at which electrons gradually inject back into the emptied donor-like trap states. By comparing the voltage shift of the post-UV curve with an “ideal” trap-free C–V curve obtained by a parallel-shift of the dark C–V curve, a decayed interface trap profile towards the mid-bandgap is derived, yielding a peak value of ∼(1–2) × 1012 eV−1 cm−2 at ∼0.3 eV below the conduction band edge of GaN.Highlights► The distribution of deep-level traps at the ALD-Al2O3/n-GaN interface is studied by a photo-assisted C–V method. ► The influence of donor-like traps within the Al2O3 layer is considered. ► A decayed interface trap profile towards the mid-bandgap of GaN is derived.

The efficiency droop behavior of GaN-based light emitting diodes (LEDs) is studied when the LEDs are under reverse-current and high-temperature stress tests respectively. It is found that reverse-current stress mainly induces additional non-radiative recombination centers within the active region of InGaN/GaN multiple quantum wells, which degrade the overall efficiency of the GaN LED under test but push the peak-efficiency-current towards higher magnitude. The up-shift of peak-efficiency-current can be explained by a rate-equation model in which the newly-created defects by reverse-current stress enlarge the dominant low-current region of non-radiative recombinations. Comparatively, high-temperature stress mainly increases the series resistance of the LED under test. Although the overall efficiency of the GaN LED also drops, there is no shift of peak-efficiency-current induced by the high-temperature stress.Research highlights► Efficiency droop of GaN LEDs was studied under reverse-current and high-temperature stress. ► Reverse-current stress could cause an up-shift of peak-efficiency-current of the LEDs. ► High-temperature stress has no impact on the efficiency roll-off point. ► The observations can be explained by a rate-equation model.

Metal-semiconductor–metal ultraviolet photodetectors are fabricated on low-defect-density homoepitaxial GaN layer on bulk GaN substrate. The dislocation density of the homoepitaxial layer characterized by cathodoluminescence mapping technique is ∼5 × 106 cm−2. The photodetector with a high UV-to-visible rejection ratio of up to 1 × 105 exhibits a low dark current of <2 pA at room temperature under 10 V bias. The photo-responsivity of the photodetector gradually increases as a function of applied bias, resulting in a photodetector quantum efficiency exceeding 100% at above medium bias. The photo-responsivity also shows a dependence on the incident optical power density and illumination conditions. The internal gain mechanism of the photodetector is attributed to photo-generated holes trapped at the semiconductor/metal interface as well as high-field-induced image-force lowering effect.Research highlights► MSM UV photodetectors are fabricated on low-defect-density homoepitaxial GaN layer. ► The photodetector exhibits ultra-low dark current and high UV/visible rejection ratio. ► The internal gain is attributed to hole trapping at interface and image-force lowering effect.

Cathodoluminescence (CL) spectroscopy and mapping techniques were used to study defect and impurity distributions in free-standing bulk GaN substrates prepared by hydride vapor-phase epitaxy. It was found that, in the bulk GaN substrates investigated, dislocation clusters appearing as dark cores in the CL map were surrounded by bright disk-like regions with higher luminescence efficiency than that of the outside areas. This large-area luminescence nonuniformity disappeared in homoepitaxial GaN grown on top of the GaN substrate. Schottky barrier diodes fabricated on the homo-epilayer exhibited low average reverse leakage current, while dislocation clusters duplicated from the original bulk GaN substrate still limited device yield.

We report the fabrication of specially-designed Schottky rectifiers with thin Schottky metal on bulk GaN substrate through a homoepitaxial process. The Schottky rectifiers show consistent easy turn-on behaviors but with a wide distribution of reverse leakage current levels. A turn-on voltage as low as 0.9 V with a low on-resistance of 2.56 mΩ cm2 was obtained. The easy turn-on properties can be attributed to the low series resistance achieved by fabricating vertical geometry devices on highly conductive GaN substrate. Electron-beam induced current analysis indicates that although the dislocation density in the homo-epilayer is generally low and on the order of ∼1 × 107 cm−2, mesoscopic defects such as grain boundaries and “dislocation walls” are present in the epilayer and serve as strong recombination centers. By comparing the reverse leakage current levels and the EBIC images of selected devices, we found that the “dislocation walls” observed are most likely to be the major source of high reverse leakage current. This study not only shows the promise of making high-performance GaN power devices based on homoepitaxial growth approach, but also indicates the importance of successful substrate polishing to the development of bulk-GaN-based devices.