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We attempt to improve the light extraction of AlGaN-based deep-ultraviolet light-emitting diodes (DUV-LEDs) by introducing a highly transparent p-type layer together with reflective Rh/Al p-type electrodes. The p-GaN contact layer is thinned to balance the Ohmic contact and DUV light transmittance, which helps the Rh/Al p-type electrodes realize high reflection as well as good electrical performance. After optimization, the Rh/Al reflective p-type electrodes present reflectance of greater than 70% and specific contact resistivity of 3.75 × 10−4 Ω·cm2. Due to the improvement in the light extraction efficiency, the highest wall-plug efficiency of 278 nm DUV-LEDs is improved by 57% compared to the conventional configuration with Ni/Au as the p-type electrodes.

It is empirically well acknowledged that C doping makes GaN high-resistive. However, the detailed doping type and high-resistivity mechanisms of C doped GaN, which are extremely essential for GaN power electronics, still remain unclear. In this work, we clarify the mutative (from downward to upward) shift of the Fermi level and the n-type conductivity in heavily C doped GaN grown by MOCVD for the C concentration increases over a critical value, by combining photo-assisted KPFM and Seebeck coefficient measurements. We also discuss the reverse transition of Fermi level and ultimately n-type conductivity should be attributed to the self-compensation and other donor-type compensation centers introduced along with C doping.

We demonstrate 1.5 μm thick buffer-free GaN layers directly on physical vapor deposited (PVD) AlN/Si templates via delayed coalescence growth. The full width of half maximum of the X-ray diffraction rocking curves for GaN (002)/(102) planes are 525/527 arcsec. The PVD-AlN with low density and large size AlN nuclei, combined with 3D growth mode of the GaN, contribute to the delayed coalescence growth and thus crack-free GaN layers. The PVD-AlN can also effectively suppress the Ga/Al diffusion and lead to a low radio frequency loss of 0.20 dB mm−1 at 10 GHz for the GaN layers.

AlGaN with a high Al fraction and low-dimensional structure is an important material for deep ultraviolet (DUV) optoelectronic devices. However, due to a lack of high-quality and low-cost homogeneous AlN substrates, AlN is mainly prepared by heteroepitaxy on sapphire with a large lattice mismatch. The resulting defect density and residual stress in AlN films become a bottleneck for enhancing the performance of AlGaN-based DUV light-emitting devices. In this paper, the research advances in epitaxial growth and threading dislocation (TD) control of AlN on sapphire substrates at Peking University are described, including decreasing the tilt of AlN grains by nitridation pretreatment of sapphire, forming TD loops by growth mode alternations, bending and terminating TDs at the sidewall of voids by an image force on nano-patterned sapphire substrates, promoting the climbing and meeting of TDs by extrinsic supersaturated vacancies, etc. Based on high-quality AlN with a TD density of ∼108 cm−2, 276 nm DUV light-emitting diodes are fabricated with a light output power of 17.3 mW at an injection current of 100 mA.

The conductive channel induced by aluminum (Al) diffusion into high-resistivity Si substrates is one of the main contributors to RF loss of GaN-on-Si RF devices. However, it has been unclear how and when the Al diffuses into the substrates. Here, we study the origins of the Al impurity in the substrates. It’s revealed that trimethylaluminum (TMAl) flow rate during the pretreatment is a considerable contributor to the Al diffusion. By restricting the TMAl preflow rate, high-quality GaN layers with RF loss of 0.3 dB mm−1 at 10 GHz and dislocation density of 1.6 × 109 cm−2 have been achieved on Si substrates.

High-quality AlN with uniform in-plane strain has been attempted with preset strain modulation on nano-patterned AlN templates (NPATs). It is found that this strain preset frame can effectively improve both the tilt and twist features of AlN on NPATs, further greatly decreasing threading dislocation density. More importantly, the AlN epilayer after completing coalescence can maintain the in-plane uniform compressive strain. Adopting AlN templates achieved in this scheme, the chip-on-wafer light output power (LOP) of AlGaN light-emitting diode (LED) reaches 10.2 mW at 100 mA with single emission peak at 280 nm, which increases by 22.3% than the LOP of LED device without adopting this strain preset frame.

Research on lightweight advanced materials that combine heat insulation and sound isolation becomes more attractive day by day. In this study, a 3D spacer structural Kevlar/polyimide composite (3DSSK/PIC) with light weight, heat insulation and sound absorbing properties was prepared by a combination of structural design, hand paste resin method and hot-pressing process. The fabricated 3DSSK/PIC possesses more than 80% of void as well as the ultra-light density (0.2 g/cm3). Consequently, the thermal insulation performance of 3DSSK/PIC (34%) is superior to that of polystyrene foam (adiabatic ratio ΔT ~20%) and similar to that of wood block (ΔT ~35%). Furthermore, the sound absorption coefficient of the 3DSSK/PIC is higher than 0.2 in the range of 600 ~ 2500 Hz, demonstrating its super sound absorption performance. In addition, compressive tests and SEM images were conducted to characterize the mechanical and structural performance. Therefore, the ultra-light 3D fabric reinforced composite with ultra-high thermal insulation and sound-absorbing properties justify a great potential in a wide variety of applications in the industry engineering, automobile, or aerospace.

The stress evolution of AlN grown on nano-patterned substrates has been investigated. Two sources of tensile stress exist: one originates from the coalescence of grain islands in AlN nucleation layer, and then the voids provide a channel for the gradual release of tensile stress to nearly stress-free state accompanied by the lateral growth process of AlN columns on the mesas. The other originates from the contacting of adjacent columns to complete coalescence process, which is responsible for residual tensile stress in the top AlN epilayer after completing coalescence. This understanding is certainly of great significance in AlN-based material and devices.

A comprehensive strategy of crystal quality control for AlN grown on a nano-patterned sapphire substrate has been explored based on the period size effect. It is found that the crystalline perfection of AlN can be greatly improved by enlarging the period size from 1.0 to 1.4 μm, and the X-ray diffraction ω-scan FWHM values for (0002) and (10-12) planes reach 162 and 181 arcsec, respectively, owning to the significantly reduced area ratio of the coalescence zone. Our results indicate the pattern design requires a critical balance between reducing the area ratio of the coalescence zone and decreasing the coalescence thickness.

We demonstrate that different doping approaches can significantly influence the lattice locations of carbon (C) in GaN grown by MOCVD. For intrinsically doped GaN with TMGa as the C source, an annealing induced switching process of C atoms from Ga sites to N sites is observed, revealing the existence of substitutional C atoms on both Ga and N sites. While for extrinsically doped GaN with propane as the C precursor, C atoms incorporate almost on N sites. From the viewpoint of growth dynamics, we propose a mechanism for C incorporation into different lattice locations.

Monoclinic gallium oxide (Ga2O3) has been grown on (0001) sapphire (Al2O3) substrate by plasma-assisted molecular beam epitaxy (PA-MBE). The epitaxial relationship has been confirmed to be [010]( \begin{document}$\bar{2}01$\end{document} ) β-Ga2O3||[ \begin{document}$01\bar{1}0$\end{document} ](0001)Al2O3 via in-situ reflection high energy electron diffraction (RHEED) monitoring and ex-situ X-ray diffraction (XRD) measurement. Crystalline quality is improved and surface becomes flatter with increasing growth temperature, with a best full width at half maximum (FWHM) of XRD ω-rocking curve of ( \begin{document}$\bar{2}01$\end{document} ) plane and root mean square (RMS) roughness of 0.68° and 2.04 nm for the sample grown at 730 °C, respectively. Room temperature cathodoluminescence measurement shows an emission at ~417 nm, which is most likely originated from recombination of donor–acceptor pair (DAP).

Fundamental principles of science are now increasingly employed for the manufacturing of innovative textile products which protect the consumer from dangers and risks; protection from extreme environment is one of the properties of textile products. This research is based on study of concentrations of phase change material (PCM) in textiles which can produce thermo-regulating characteristics to control body temperature useful for various daily wear and technical textiles. The latent heat which is absorbed during the process of phase change is studied and discussed in this research. Polyester cotton bland fabrics were treated with phase change material (microencapsulated paraffin wax) by a pad dry cure. These microcapsules are bounded onto the fabric surface by using a binder. The microcapsules and their effectiveness are tested by using DSC (Differential Scanning Calorie Meter). The testing is further extended to the physical property in term of air permeability, stiffness and tensile strength to ensure the performance properties of the textile fabric. All the properties of treated fabric were evaluated with respect to the add-on of binder

AlGaN-based deep ultraviolet light-emitting diodes adopting modulated-taper design for p-AlGaN layer have been investigated. It is found that remarkable enhancements both in light output power (LOP) and internal quantum efficiency (IQE) can be realized using the modulated-taper design compared to the conventional single-Al-component AlGaN ones. It is verified that the upward energy band of the last quantum barrier, especially symmetric ones, can increase effective potential barrier height for electrons and at the same time, can decrease the barrier height for holes, which is beneficial for high hole concentration distribution within the quantum wells, therefore accounting for the improved IQE as well as LOP.

Performances of Ga- and N-polarity solar cells (SCs) adopting gradient-In-composition intrinsic layer (IL) are compared. It is found the gradient ILs can greatly weaken the negative influence from the polarization effects for the Ga- polarity case, and the highest conversion efficiency (η) of 2.18% can be obtained in the structure with a linear increase of In composition in the IL from bottom to top. This is mainly attributed to the adsorptions of more photons caused by the higher In composition in the IL closer to the p-GaN window layer. In contrast, for the N-polarity case, the SC structure with an InGaN IL adopting fixed In composition prevails over the ones adopting the gradient-In-composition IL, where the highest η of 9.28% can be obtained at x of 0.62. N-polarity SC structures are proven to have greater potential preparations in high-efficient InGaN SCs.

The transition from bound exciton to free exciton and exciton–phonon interaction in an AlN epilayer have been investigated by time resolved deep ultraviolet photoluminescence spectroscopy. Based on the analysis of the energy position (S-shaped dependence with temperature), integrated intensity as well as decay time, the main X peak located at 6.06 eV at 7.7 K is assigned to originate from radiative recombination of excitons bound to some unintentionally doped Si or O impurities. While the other two peaks on the lower energy side should be from the bound exciton's phonon replicas. The corresponding small Huang–Rhys factor indicates weak interaction between phonon and bound excitons, in comparison to the case of free exciton, for which our experimental results are in good agreement with the theoretical calculation of the Huang–Rhys factors.

Two series of AlGaN samples with different stresses were designed to investigate the effect of stress on the Al composition. X-ray diffraction reciprocal space mapping (XRD RSM) demonstrated that the AlGaN epilayers with different stresses have large Al composition differences despite the same growth conditions. The largest Al composition difference reached up to 21.3%, which was also confirmed using secondary ion mass spectroscopy (SIMS). This result is attributed to a large stress discrepancy in the AlGaN epilayers. Finally, the dependences of the solid-phase Al composition on the gas-phase Al composition under different stresses were systematically analyzed.

InAlN has been studied by means of temperature-dependent time-integrated photoluminescence and time-resolved photoluminescence. The variation of PL peak energy did not follow the behavior predicted by Varshni formula, and a faster redshift with increasing temperature was observed. We used a model that took account of the thermal activation and thermal transfer of localized excitons to describe and explain the observed behavior. A good fitting to the experiment result is obtained. We believe the anomalous temperature dependence of PL peak energy shift can be attributed to the temperature-dependent redistribution of localized excitons induced by thermal activation and thermal transfer in the strongly localized states. V-shaped defects are thought to be a major factor causing the strong localized states in our In0.153Al0.847N sample.

Transport properties of InN layers with both lattice polarities are reported. It is proposed that grain boundaries form potential barriers for electrons and thus lead to a reduction of electron mobility. However, these grain boundaries are important scattering centers only in the N-polarity InN films, but not in the In-polarity ones, which is consistent with the grain feature of the N-polarity InN surfaces. The carrier mobility in grain boundaries is estimated to be about 75 cm2 V-1 s-1 at room temperature for our sample, far less than the carrier mobility in the grains.

The spatial distribution features of indium composition in In0.17Al0.83N epilayers have been investigated by means of cathodoluminescence and photoluminescence. It is demonstrated that there is excellent compositional homogeneity in In0.17Al0.83N, implying there is almost no phase separation that would cause compositional inhomogeneity. This result is quite different from the case in InxGa1-xN alloys. Based on the analysis of the temperature–mole fraction (T–x) phase diagram, we believe that the In0.17Al0.83N alloy is metastable against local decomposition. The high kinetic energy barrier caused by the considerable covalent bond mismatch between Al–N and In–N is supposed to account for the realization of such a metastable state.

Thin tungsten nitride (WNx) films were produced by reactive DC magnetron sputtering of tungsten in an Ar—N2 gas mixture. The films were used as Schottky contacts on AlGaN/GaN heterostructures. The Schottky behaviours of WNx contact was investigated under various annealing conditions by current-voltage (I—V) measurements. The results show that the gate leakage current was reduced to 10−6 A/cm2 when the N2 flow is 400 mL/min. The results also show that the WNx contact improved the thermal stability of Schottky contacts. Finally, the current transport mechanism in WNx/AlGaN/GaN Schottky diodes has been investigated by means of I—V characterisation technique at various temperatures between 300 K and 523 K. A TE model with a Gaussian distribution of Schottky barrier heights (SBHs) is thought to be responsible for the electrical behaviour at temperatures lower than 523 K.

We systematically investigated the effect of the thermal oxidation treatment on the performance of AlGaN/GaN high electron mobility transistors (HEMTs). The HEMTs with thermal oxidation treatment exhibit four orders of magnitude reduction in gate leakage current, 80% reduction of trap density, and more than two times improvement of off-state drain breakdown voltage, compared with those shown by HEMTs without thermal oxidation treatment. The simplicity in the thermal oxidation treatment process, coupled with the drastic improvement in device performance render the thermal oxidation treatment highly promising for GaN-based microwave power amplifier applications in communication and radar systems.

In this study, an optimal vibration-based energy harvesting system using magnetostrictive material (MsM) was designed and tested to enable the powering of a wireless sensor. In particular, the conversion efficiency, converting from magnetic to electric energy, is approximately modeled from the magnetic field induced by the beam vibration. A number of factors that affect the output power such as the number of MsM layers, coil design and load matching are analyzed and explored in the design optimization. From the measurements, the open-circuit voltage can reach 1.5 V when the MsM cantilever beam operates at the second natural frequency 324 Hz. The AC output power is 970 µW, giving a power density of 279 µW cm − 3. The attempt to use electrical reactive components (either inductors or capacitors) to resonate the system at any frequency has also been analyzed and tested experimentally. The results showed that this approach is not feasible to optimize the power. Since the MsM device has low output voltage characteristics, a full-wave quadrupler has been designed to boost the rectified output voltage. To deliver the maximum output power to the load, a complex conjugate impedance matching between the load and the MsM device is implemented using a discontinuous conduction mode (DCM) buck-boost converter. The DC output power after the voltage quadrupler reaches 705 µW and the corresponding power density is 202 µW cm − 3. The output power delivered to a lithium rechargeable battery is around 630 µW, independent of the load resistance.

A conformal load-bearing antenna structure (CLAS) combines the antenna into a composite structure such that it can carry the designed load while functioning as an antenna. In this paper, two types of new 3D integrated microstrip antennas (3DIMAs) with different feeding methods are designed to work at the radar L-band. Different from the conventional CLAS, the radiating patch and the ground plane of the 3DIMA are both composed of woven conductive wires and are bonded into the 3D composite physically by Z-yarns, greatly improving the damage tolerance of the antenna. The return loss of the coaxial-fed antenna is ?13.15?dB with a resonant frequency of 1.872?GHz, while that of the microstrip-fed antenna is ?31.50?dB with a resonant frequency of 1.33?GHz. Both of the 3DIMAs have similar radiation patterns to that of the traditionally designed microstrip antenna. In addition, an experimental investigation of the impact response of the coaxial-fed 3DIMA was carried out and the results showed the radiation pattern had almost no change even when the antenna received an impact energy of 15?J, exhibiting superior impact resistance to that of a conventional microstrip antenna.