Yuhuai Liu

Silicon/Carbon Composites as Anode Materials for Li-Ion Batteries

A silicon/carbon composite has been synthesized by two poly(vinyl chloride) (PVC) pyrolysis reactions, combined with an intervening high-energy mechanical milling (HEMM) step. As lithium storage host, the composite demonstrates high initial coulombic efficiency of 82% and a large capacity at ca. 900 mAh g - 1 over 40 cycles. Controlling Li-insertion level at 600 mAh g - 1 can greatly extend the cycles to over 100. Another composite prepared by ballmilling a mixture of graphite and silicon and followed by a PVC pyrolysis process also shows both good capacity and capacity retention.

1 mW AlInGaN-based ultraviolet light-emitting diode with an emission wavelength of 348 nm grown on sapphire substrate

By introducing the AlInGaN/AlGaN quaternary system as an active region, we fabricated an UV light-emitting diode (LED) with an emission wavelength of 348 nm. The optical power is 1 mW at an injection current of 50 mA under a bare-chip geometry, which is the highest report among UV–LEDs with an emission wavelength of around 350 nm grown on sapphire substrate. It means that the optical power of such LEDs is high enough to be used in practical application. In contrast to it, a similar UV–LED based on GaN/AlGaN system as an active region has been also grown, whose optical power is less than that of the AlInGaN/AlGaN-based UV–LED by one order of magnitude. The temperature-dependent photoluminescence study indicates that there exists a strong exciton-localization effect in the AlInGaN/AlGaN material system, while there is no distinguished exciton-localization effect in the GaN/AlGaN material system. Therefore, the high performance of the AlInGaN/AlGaN-based UV–LED can be attributed to the enhanced exciton-local...

Fabrication of high performance of AlGaN/GaN-based UV light-emitting diodes

Abstract A high-performance AlGaN/GaN-based ultraviolet (UV) light-emitting diode (LED) is successfully fabricated on sapphire substrate by metal-organic-chemical-vapor-deposition technique. Generally, a p–n junction is grown on a thick GaN layer on sapphire substrate, which results in a strong internal-absorption effect. Simultaneously, a thick AlGaN cladding layer on the GaN layer also easily produces crack. In order to avoid the internal absorption, a thick AlGaN layer is immediately introduced on a thin low-temperature GaN buffer (LT GaN buffer) instead of a thick GaN layer, which successfully avoids crack formation. However, an enhanced lattice-mismatch of AlGaN/LT GaN buffer/sapphire compared with that of GaN/sapphire might result in an enhanced dislocation density, which leads to the degraded performance of UV-LED. An AlGaN/GaN supperlattice that is applied in UV-LED instead of the thick AlGaN layer strongly decreases the dislocation density, confirmed by transmission electron microscope. Furthermore, this AlGaN/GaN supperlattice successfully avoids crack formation. Consequently, the optical power of UV-LED is greatly increased. Based on the above results, we successfully fabricate a crack-free UV-LED with a high performance.

High-Performance 348 nm AlGaN/GaN-Based Ultraviolet-Light-Emitting Diode with a SiN Buffer Layer.

A 348 nm ultraviolet-light-emitting diode (UV-LED) based on an AlGaN/GaN single quantum well (SQW) with a high optical power is reported. In this structure, a thin SiN buffer is introduced before the growth of a conventional low-temperature GaN buffer layer. Such a buffer layer can dramatically reduce the density of threading dislocation as we have previously reported. Since the optical performance of UV-LED is generally known to be sensitive to the density of threading dislocations, unlike the InGaN/GaN- based blue LED, our UV-LED has a higher optical power than that of a similar structure but without a SiN buffer layer. Since our new buffer technology is much easier than the so-called epitaxial lateral overgrowth (ELO) or pendeo-epitaxy method, it is highly recommended for use in the fabrication of GaN-based optical devices, particularly AlGaN/GaN-based UV-LED.

Growth of Thick AlN Layer by Hydride Vapor Phase Epitaxy

Thick AlN crystals were grown by conventional hydride vapor phase epitaxy (HVPE) on AlN/sapphire templates under low pressure (~15 Torr) at high temperature (1100°C–1200°C). Colorless, mirror-like AlN films were obtained at the growth rates of up to 20.6 µm/h. The best root mean square (RMS) value of atomic force microscope (AFM) observations for the AlN surface was 2.34 nm. The typical values of full width half maximum (FWHM) of X-ray rocking curves for (0002) and (1012) diffraction of AlN films were 173–314 arcsec and 1574–1905 arcsec, respectively. We also investigated the influences of carrier gas, growth temperature and growth rate on the crystal quality.

Novel Composites Based on Ultrafine Silicon, Carbonaceous Matrix, and the Introduced Co-Milling Components as Anode Host Materials for Li-Ion Batteries

We report studies of the electrochemical behavior of the Si-M-C (M could be TiN, TiB 2 , graphite) composites, which were prepared from ballmilling silicon with the relatively inactive components and further followed a pyrolyzed poly(vinyl chloride) process. As lithium hosts, these composites showed large capacities of ca. 900 mAh g - 1 and good capacity retention. Research reveals that both the high-energy mechanical milling step and pyrolysis reaction play a key role in alleviating the morphology stress arising from silicon during cycling. A possible capacity decay mechanism with respect to such anode systems has also been investigated and discussed.

Novel Composite Anodes Consisting of Lithium Transition-Metal Nitrides and Transition Metal Oxides for Rechargeable Li-Ion Batteries

Lithium transition-metal nitrides are promising anode candidates for Li-ion batteries. However, lithium must be extracted from the nitrides in an initial anodic oxidation, indicating these compounds cannot directly combine with the current cathodes to constitute cells. This deterrent can be overcome by introducing a certain amount of Co 3 O 4 , which shows large capacities and relatively high oxidation/reduction potentials, into the electrodes containing the above nitrides. A thermodynamically spontaneous reaction between these two active hosts results in a delithiated state of lithium metal nitrides. Under cycling within 1.4-0 V vs Li/Li + Co 3 O 4 is relatively inert to lithium and the nitrides become electrochemically active. The composite electrodes show high first-cycle efficiency of 100%, large capacities of 500 mAh g - 1 , and excellent cyclability. Furthermore, research revealed that the composite electrodes demonstrated high cycling stability operating with polyethylene oxide (PEO) electrolytes at the elevated temperature. The reaction heating of the composite electrode under high Li utilization with PEO electrolytes via differential scanning calorimetry measurement was found to be extremely low compared with those of the lithium metal and the Li-alloy-based systems. suggesting that the composite electrodes could be promising anode candidates for all-solid-state PEO Li-ion batteries in terms of capacity, first-cycle charge efficiency, and thermal reliance.

Fabrication of High-Output-Power AlGaN/GaN-Based UV-Light-Emitting Diode Using a Ga Droplet Layer

We report a new method of increasing the output power of an ultraviolet light-emitting diode (UV-LED) based on an AlGaN/GaN single quantum well (SQW). In this method, a thin Ga droplet layer is intentionally grown before the growth of an AlGaN/GaN SQW active layer. The Ga droplet layer causes a spatial and compositional fluctuation on the SQW active layer, which induces exciton localization in the potential minima. The LEDs fabricated with the Ga droplet layer show an emission peak of 353 nm and a higher optical output power than those of the same structure but without the Ga droplet layer.

AlGaN/GaN High Electron Mobility Transistor with Thin Buffer Layers

An AlGaN/GaN high electron mobility transistor (HEMT) on a high-temperature-grown GaN buffer layer as thin as 0.35 µm has been demonstrated for the first time. The investigation of device characteristics is carried out using fat field-effect transistor (FATFET), ring-type FET and Hall measurements. The field-effect mobility obtained from the FATFET is about 1200 cm2/Vs, whereas the mobility in the buffer layers is around 200 cm2/Vs. A leakage current is found to be due to the non-semi-insulating underlying buffer layers. A two-layer model was adopted to separate the surface channel and buffer channel from Hall measurement data. The surface mobility enhancement can be attributed to the screening effect of ionized impurity and defects by the accumulated electrons.

Interactions between inversion domains and InGaN/GaN multiple quantum wells investigated by transmission electron microscopy

The propagation properties of inversion domains (IDs) in InGaN/GaN multiple quantum well (MQW) structures grown by metalorganic chemical vapor deposition have been investigated by transmission electron microscopy (TEM). The majority of the IDs, originating from the sapphire and/or buffer layer, propagate through the MQWs with normal wurtzite structure retaining their original structural features. Some of IDs could induce V-shaped pits in the MQW structures proposing a new formation mechanism for the so-called V-shaped defects. Detailed measurements show that a few IDs are found to be stopped in abnormal MQW regions, where In droplets appear due to phase separation. We presented direct evidence of pure In-phase droplets by means of high-resolution TEM. The above results provide new information on the structural defects in InGaN/GaN-based materials.