Laser lift-off technique and the re-utilization of GaN-based LED films grown on sapphire substrate

Abstract

A thin GaN LED film, grown on 2-inch-diameter sapphire substrates, is separated by laser lift-off. Atom force microscopy (AFM) and the double-crystal X-ray diffraction (XRD) have been employed to characterize the performance of GaN before and after the lift-off process. It is demonstrated that the separation and transfer processes do not alter the crystal quality of the GaN films obviously. InGaN/ GaN multi-quantum-wells (MQW's) structure is grown on the separated sapphire substrate later and is compared with that grown on the conventional substrate under the same condition by using PL and XRD spectrum.GaN-based electronic and optoelectronic devices are used widely [1] . However, it is difficult to get the large-size single GaN crystal, and GaN films are usually grown on dissimilar substrates such as sapphire. Sapphire as the most commonly used substrate still imposes constrains on the GaN film quality due to the mismatch of lattice and the thermal-expansion coefficient between the sapphire and GaN [2] . The sapphire substrate also inhibits the performance of LEDs, LDs and transistor devices due to its poor thermal and electrical conductivity.The poor thermal conductivity of sapphire prevents efficient dissipation of heat generated by GaN-based high-current devices, and consequently inhibits the performance of the devices. In addition, for devices processed on sapphire substrates, all contacts must be made from the topside, which will result in a spreading-resistance penalty and will increase operating voltages [3] . To solve these problems, GaN epilayers can be transferred to another substrate with good thermal conductivity, such as Si or metallic materials, by using wafer bonding and laser lift-off (LLO) techniques.The lift-off process (LLO) is performed using an ultraviolet (UV) laser. The laser light is irradiated from the back surface of the sapphire substrate, and will be absorbed by the interface of GaN and sapphire, yielding metallic Ga and gas N 2 .Then the GaN and the sapphire will be separated [4][5][6][7][8] .The steps of grinding and polishing process must be taken on the rear surface of the sapphire wafer after the electrode preparation in commercial LEDs. However, due to the high rigidity and fragility of sapphire with a hardness level of 9 in the classical Mohs hardness scale, the mechanical process of sapphire substrate is hard, which increases the difficulty of the mechanical process and the re-utilization of sapphire. In fact, the above problems can be solved and the sapphire substrates will be re-utilizated by LLO technique to transfer the GaN film from the Si substrates. In this paper, GaN LED films, grown on 2-inch-diameter sapphire substrates, are bonded and separated by LLO. Then a InGaN/ GaN multiquantum-wells (MQW's) structure is grown on the separated sapphire substrate again and is compared with that grown on the conventional substrate under the same condition.The InGaN/GaN multiple-quantum-wells (MQW's) LED structures used in this paper are grown on (0001)-oriented sapphire substrates...

Al composition dependency of interface phonon in the wurtzite quantum cascade laser

“…Sun [4] and Huang [5] proposed to use a GaN-based system with large longitudinal-optical (LO)-phonon energy (~90 meV) for THz QCLs to achieve the high temperature operation. The GaN-based material usually crystallizes in the hexagonal wurtzite structure [6,7] . Every primitive cell in wurtzite contains four atoms, and nine optical and three acoustical branches are presented.…”