Xiaotie Wang

GaN-based light-emitting diodes on origami substrates

GaN-based light-emitting diodes (LEDs) were transferred to paper substrates after a laser lift-off (LLO) process with an ArF excimer laser system (λ = 193 nm) to remove the sapphire substrate and produce freestanding blue LED templates. The threshold voltage (∼2.7 V), current-voltage characteristics, and peak emission wavelength (442 nm) were not changed after the paper substrate was subsequently wrinkled. We were able to demonstrate transfers to both planar and folded (origami) paper structures, showing the promise of the LLO process for transferring LEDs to arbitrary surfaces.

Low-temperature, site selective graphitization of SiC via ion implantation and pulsed laser annealing

A technique is presented to selectively graphitize regions of SiC by ion implantation and pulsed laser annealing (PLA). Nanoscale features are patterned over large areas by multi-ion beam lithography and subsequently converted to few-layer graphene via PLA in air. Graphitization occurs only where ions have been implanted and without elevating the temperature of the surrounding substrate. Samples were characterized using Raman spectroscopy, ion scattering/channeling, SEM, and AFM, from which the degree of graphitization was determined to vary with implantation species, damage and dose, laser fluence, and pulsing. Contrasting growth regimes and graphitization mechanisms during PLA are discussed.

Thermal simulation of laser lift-off AlGaN/GaN high electron mobility transistors mounted on AlN substrates

A finite element simulation was used to estimate the temperature rise of laser lift-off AlGaN/GaN high electron mobility transistors (HEMTs) mounted on AlN substrates via silicone elastomer under various conditions of power density on the transistor and for a range of elastomer thicknesses. The thermal response was compared to HEMTs fabricated on SiC and sapphire substrates. The maximum temperature of the transistor occurred in the gate area at a power level of 15 W, and was only 25 K higher than HEMTs fabricated on SiC. The transit time required for steady-state temperature of the flip-chip bonding device is in the range of a millisecond, which is faster than that of most power switch applications.

GaN-based light-emitting diodes by laser lift-off with micro- and nano-sized reflectors

GaN-based light-emitting diodes (LEDs) were grown on a patterned sapphire substrate (PSS) containing hemispheres on the growth surface. Free-standing LED structures were obtained by removing the PSS using laser lift-off technique. The N-face GaN surface with micron-sized concave hemisphere structures, which had been located between the sapphire and the GaN film, was then exposed and photo-electrochemically etched using a 2M KOH solution to create nano-sized pyramids. Aluminum was deposited on the roughened N-face GaN as a reflective layer. The roughened aluminum reflectors, consisting of micron-sized hemisphere structures and nano-sized pyramids, enhanced the light extraction efficiency through multiple scattering events of photons and randomized the directions of the photons. The subsequent enhancement in electroluminescence was 13% compared with an untextured control LED.

Synthesis of graphene and graphene nanostructures by ion implantation and pulsed laser annealing

In this paper, we report a systematic study that shows how the numerous processing parameters associated with ion implantation (II) and pulsed laser annealing (PLA) can be manipulated to control the quantity and quality of graphene (G), few-layer graphene (FLG), and other carbon nanostructures selectively synthesized in crystalline SiC (c-SiC). Controlled implantations of Si− plus C− and Au+ ions in c-SiC showed that both the thickness of the amorphous layer formed by ion damage and the doping effect of the implanted Au enhance the formation of G and FLG during PLA. The relative contributions of the amorphous and doping effects were studied separately, and thermal simulation calculations were used to estimate surface temperatures and to help understand the phase changes occurring during PLA. In addition to the amorphous layer thickness and catalytic doping effects, other enhancement effects were found to depend on other ion species, the annealing environment, PLA fluence and number of pulses, and even las...

193 nm excimer laser lift-off for AlGaN/GaN high electron mobility transistors

AlGaN/GaN HEMTs grown on both-side-polished sapphire substrates were successfully lifted-off with a 193-nm UV excimer laser system. The photon energy of the 193 nm laser is larger than the band gap of AlN and thus it can be used to lift-off AlGaN HEMT structures with AlN or AlGaN interfacial layers grown on sapphire substrates prior to growth of the GaN buffer layers. The lifted-off HEMT chip was warped and showed 25–42% reduction of the saturation drain current. There was no degradation observed either in the forward or reverse gate current-voltage (I-V) characteristics or on the drain punch-through voltage. Based on comparisons of cross-sectional electron micrographs, no additional dislocations were created in the HEMT structures after the laser lift-off process. Reduction in saturation drain current was attributed to relaxation of the lifted-off HEMT structures. Newtons rings and Raman spectrum E2 peak shifts were used to estimate the strain relaxation of the laser lifted-off samples.

Simulation and experimental study of ArF 193 nm laser lift-off AlGaN/GaN high electron mobility transistors

A finite element simulation was used to estimate the temperature distributions within AlGaN/GaN high electron mobility transistors (HEMTs) during the laser lift-off process. The time-dependent simulation showed that a thin layer of GaN at the GaN/sapphire interface was heated up to around 1600 K in less than 25 ns by a pulsed laser exposure with a duration of 25 ns and a fluence of 800 mJ/cm2 to decompose this GaN layer into Ga and nitrogen. Experimentally, there was a threshold fluence around 550 mJ/cm2, corresponding to 1300 K at the GaN/sapphire interface, for partially lifting off the HEMT structure from the sapphire. The simulated temperature at the GaN/sapphire interface with a fluence of 420 mJ/cm2 never reached above 1000 K, however, the HEMT structure was lifted-off by multiple laser exposures at this fluence. Therefore, instead of thermally induced decomposition, the lift-off mechanism could also be through the Ga–N bond breaking during the multiple lower-fluence high-energy 193 nm laser exposures.