The high brightness GaN-based Light-Emitting Diodes (LEDs) have made possible applications in traffic signals, backlight in liquid crystal displays, and solid state lighting. However, LED devices still require further improvement of the optical output power. The limitation of optical power is mainly due to the low internal quantum efficiency and extraction efficiency. The low internal quantum efficiency results from the high Threading Dislocation (TD) density of GaN films grown on foreign substrates. The low light extraction efficiency is related to the large difference in the refractive indices between GaN and air.
The Patterned Sapphire Substrates (PSS) technique, as a mask-less means, has attracted much attention due to the uninterrupted single growth process. Simulations of the ray tracing of light indicate that the PSS can also redirect photons that were originally emitted out of the escape cone back into the escape cone on the top of device surfaces. And epitaxial lateral overgrowth (ELOG) significantly reduces the TD density in GaN films.
Until now, most patterned sapphire substrates are fabricated by traditional lithography and etching. In this system, patterns on mold are transfered to SiO2 or SiN by lithography, which can be used as a mask for sapphire in next etching. Instead of traditional lithography, we use hybrid mold nano-imprint lithography to realize the first pattern transfer from mold to SiO2 or SiN. In this way, lower cost and mass production will be achieved. In addition, nano-scale patterns will be easier to obtain, which lead to higher output power shown in others reports.

Figure 3.2.1 Schematic of transport in super-lattice structures

Research of bulk homogeneous materials for thermoelectric applications began in the 1950s, but the development was limited in the next several decades. By the introduction of materials and concepts based on nano-structuring, especially on super-lattice structures, the field has dramatically gone forward over the past 15 years. Nano-fabrication of semiconductors in the form of quantum well and super-lattice now plays an important role in commercial products such as high electron mobility transistors, compact disc laser diodes and solid-state-coolers. Quantum confinement effects and intensive scatterings of phonons created by nano-structures are used to significantly enhance thermoelectric performance (Figure 3.2.1). We are now widely targeting ZT ≥ 3~4 to make these solid-state systems competitive with traditional energy conversion systems.

(a) The schematic of Seebeck coefficient measurements in nanostructures and the circuit diagram of the home-made setup in our lab. (b) The mocroscopy pictures of devices for the Seebeck coefficient measurements of nanowires. (c) The experimental results of the Seebeck coefficient measurements in Bi2Te3 thin films.

(a) The schematic of three omega (3ω) method and the theoretical model of it. (b) The circuit diagram of three omega method in our lab. (c) The schematic of differential 3ω method and the experimental results. (d) The experimental results of slope 3ω method.

(a) The structures of Si-based Nano-Phononic Crystals and the distribution of mean free path in corresponding structures. (b) The dispersion of phonons in Nano-Phononic Crystals. (c) The theoretical calculation results of designed structures compared with the bulk Silicon.

　　Model:built by laboratory
　　Application:
　　1.Thermal conductivity measurement of bulk and film materials.
　　2.Thermal conductivity measurement of heterostructure interfaces.
　　3.Thermal conductivity measurement fo liquid, nanoparticles and polymer.
　　4.Thickness, sound speed and elasticity modulus of nano films.
　　Performance index: