Separation of Thin GaN from Sapphire by Laser Lift-Off Technique

Abstract

Laser lift-off of GaN from sapphire substrates has become a viable technique to increase the brightness of GaN-based light-emitting diodes (LEDs). The LEDs free from sapphire exhibit high luminous efficiency by placing highly reflective electrode on the back side. The devices serve low series resistance together with low thermal resistance taking advantages of the vertical structure. Thinner epitaxial structure is desired to serve better device performance, however, cracks in the film after the lift-off limits the minimum thickness. In this paper, successful laser lift-off of very thin GaN with the thickness down to 4 µm is described. The established laser lift-off system utilizes homogenized beam-profile of the employed neodymium-doped yttrium aluminium garnet (Nd:YAG) third harmonic laser in which optimization of the laser fluence minimizes the thickness of the decomposed GaN. It is also revealed by calculation that the compressive stress in the thin GaN is increased by reducing the thickness. It is demonstrated that the lattice of the GaN is relaxed after the laser lift-off, which is confirmed by photoluminescence (PL) and X-ray diffraction (XRD) measurements. In addition, reduction of the wafer bowing of GaN on sapphire is experimentally confirmed after the laser irradiation with the formation of metal Ga in between the interface.

Flexible Crossbar‐Structured Resistive Memory Arrays on Plastic Substrates via Inorganic‐Based Laser Lift‐Off

“…A XeCl excimer laser with a wavelength of 308 nm, which is well‐established in the display industry for low‐temperature poly‐silicon (LTPS), was irradiated through the back side of the glass substrate to reduce the adhesion between the exfoliation layer and the substrate, thus finally detaching just the upper inorganic layer from the substrate, as shown in Figure a(ii). This ILLO process is explained by localized melting, vaporizing, or dissociating of the laser‐reactive exfoliation layer as a result of the laser–materials interaction. In previous studies, we had demonstrated the easy transfer of a large‐area lead zirconate titanate (PZT) thin film and gallium nitride (GaN) thin film onto flexible substrates via a laser lift‐off (LLO) process, demonstrating that PZT, GaN, and other inorganic materials can be employed as the exfoliation layer for flexible electronics.…”

Structural Modifications in Free-Standing InGaN/GaN LEDs after Femtosecond Laser Lift-Off

“…The laser provides a maximum output power of 8 W at 520 nm emission wavelength with a repetition rate of 200 kHz and a pulse length of <400 fs. Unlike with many other lift-off processes presented in literature, the beam profile in our setup is not widened using a beam shaper [2,3]. Instead, the beam is guided through a focusing objective and scanned across the sample by a galvanometer scanner.…”

“…Although the required energy densities are an order of magnitude higher than for conventional liftoff [2], the mostly non-thermal nature of the material decomposition at the interface may lead to a gentle lift-off. Moreover, extension of the process to UV LED structures is conceivable, as the lift-off is not directly dependent on the band gap of the semiconductor material.…”

“…The light is transmitted through the sapphire, then absorbed in the first GaN layers at the interface, leading to local decomposition [1]. In conventional lift-off processes, high-power laser sources with photon energies above the GaN bandgap of 3.4 eV (corresponding to 365 nm) are utilized, usually excimer lasers (emission wavelength of 248 for KrF and 308 nm for XeCl) or frequency-tripled Nd:YAG lasers (355 nm) with pulse widths in the nanosesond regime and energy densities of several 100 mJ/cm 2 [2].…”

Effects of polarity and surface treatment on Ga- and N-polar bulk GaN

“…Undoped, bulk GaN samples were grown by halide vapor phase epitaxy (HVPE) at Kyma Technologies, Inc. After deposition, the GaN templates were removed from their sapphire substrates using a laser lift-off process 19 and were subsequently polished down to a thickness of $400-450 lm. The surfaces were then finished with either a mechanical polish (MP) or chemical mechanical polish (CMP).…”