Laser Lift-off Technique Research Articles

Roughening surface morphology on free-standing GaN membrane with laser lift-off technique

An ultraviolet (UV) laser lift-off (LLO) technique was presented to form a roughened surface morphology on GaN membrane grown by metalorganic chemical vapor deposition (MOCVD). The etched surface showed cone-like structures on a free-standing GaN membrane. Based on the scanning electron microscopy (SEM) and atom force microscopy (AFM) measurements, the etching mechanism was proposed, which was related to the different decomposition depth caused by the dislocations in the GaN membrane. The etching efficiency and morphology of GaN by the LLO technique and the photoelectrochemical (PEC) wet etching technique was compared and analyzed. This roughed cone-like surface morphology by LLO can enhance the external efficiency of vertical structure n-side-up GaN-based light-emitting diodes (LEDs) simultaneously while being released of the performance constrains impeded by sapphire.

Fabrication of Dicing-Free Vertical-Structured High-Power GaN-Based Light-Emitting Diodes With Selective Nickel Electroplating and Patterned Laser Liftoff Techniques

Through the use of selective nickel (Ni) electroplating, patterned laser liftoff technique, and surface roughing of the top n-GaN epilayer, a novel process for the fabrication of vertical-structured metal-substrate GaN-based light-emitting diodes (VM-LEDs) to avoid difficulties in Ni substrate dicing and improve device yield was proposed and demonstrated. In conjunction with a sidewall passivation with SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and keeping the size of epilayer smaller than that of Ni island, a considerable improvement in yield and device performance were shown. As compared to conventional lateral-structured GaN-based LEDs, VM-LEDs show an increase in light output power about 174% at 350 mA with a significant decrease in forward voltage from 3.5 to 3.17 V

Vertical AlGaN deep ultraviolet light emitting diode emitting at 322nm fabricated by the laser lift-off technique

A vertical AlGaN deep ultraviolet (DUV) light emitting diode (LED) emitting at 322nm was fabfricated by the laser lift-off technique. The emission area extended to the entire electrode uniformly, and the current crowding was suppressed effectively in the devices. As a result, the differential conductance of the vertical LED was improved by a factor of 5 and the operation voltage was reduced to half, compared to that of the lateral LED. The self-heating effect was effectively suppressed even at high-current-density operation. The vertical structure in the high resistive AlGaN LED has potential application in high-power AlGaN DUV devices.

Experimental and numerical investigation on GaN/Al 2O 3 laser lift-off technique

Transient temperature field of GaN films has been analyzed by finite element method for laser lift-off (LLO) technique. Temperature distribution in GaN films irradiated by pulse laser with different energy density as a function of time and depth has been simulated. The results show that the high temperature region in GaN films localizes within about 70 nm below the GaN/Al 2O 3 interface. It is also found that the energy density of the pulse laser should be confined to a certain range decided by the simulation to realize damage-free GaN films LLO from sapphire substrates. LLO experiments have also been carried out using KrF excimer laser with 248 nm wavelength and 30 ns pulse width. GaN films are separated from sapphire substrates using the parameters obtained from the simulation. Atomic force microscopy, scanning electron microscopy and photoluminescence measurements show that the surface morphology and optical characteristic of GaN films degrade a little after LLO.

Blue flip-chip AlGaInN LEDs with removed sapphire substrate

Characteristics of AlGaInN LEDs with removed sapphire substrate are studied. To remove the substrate from a finished LED crystal mounted by the flip-chip method onto a silicon wafer, the laser lift-off technique was used. To raise the light output efficiency, a scattering profile was formed on the n-GaN surface by ion etching in a Cl2: Ar gas mixture. This resulted in the 25–30% increase in the external quantum efficiency of LEDs. The LEDs fabricated in this way demonstrate stable operation at drive currents of up to 300 mA with an optical power as high as 110 mW.

The use of transparent conducting indium-zinc oxide film as a current spreading layer for vertical-structured high-power GaN-based light-emitting diodes

In this study, the performance of vertical-structured high-power GaN-based light-emitting diodes (VM-LEDs) with a transparent and low-resistant indium-zinc oxide (IZO) film as a current spreading layer (CSL) was investigated. Nickel electroplating and patterned laser liftoff techniques were employed for the transfer of sapphire substrate to nickel substrate. The novel IZO CSL atop n-side-up VM-LEDs offering benefits of superior current spreading ability, larger extraction efficiency, and lower forward voltage drop was demonstrated. As compared to the regular LED without IZO CSL, the use of an IZO CSL with an optimum thickness of around 300 nm leads to an increase in light output power by 97.1 (67.8)% and a decrease in forward voltage drop by 4.9 (15.5)% under an injection current of 350 (800) mA

Improvements of N-Side-up GaN Light-Emitting Diodes Performance by Indium–Tin-Oxide/Al Mirror

An n-side-up vertical conducting GaN/mirror/Si light-emitting diode (LED) has been fabricated using a combination of wafer-bonding, laser lift-off, and surface texturing techniques. The indium–tin-oxide (ITO)/Al and Pd mirror were chosen to improve optical reflectivity and contact resistance with p-GaN. It was found that the LEDs with higher reflectivity mirrors present better optical properties. A GaN/ITO/Al/Si LED with surface texturing presents maximum luminance intensity with five times the magnitude of the original planar GaN/sapphire LED at 20 mA. The output power characteristic of a GaN/ITO/Al/Si LED is 8.9 mW, and it is higher than that of the original GaN/sapphire (4.5 mW) LED at 20 mA. With high current injection, the surface textured GaN/ITO/Al/Si LED also shows more stable luminance intensity and output power. The junction temperature is measured by the diode forward method. As the dc forward current increases to 200 mA, the junction temperatures of the GaN/mirror (ITO/Al)/Si and GaN/sapphire LED are 87 and 158 °C, respectively. Obviously, the Si substrate provides good thermal dissipation.

Effect of Surface Treatment on the Performance of Vertical-Structure GaN-Based High-Power Light-Emitting Diodes with Electroplated Metallic Substrates

Large-area (0.6 ×0.6 and 1 ×1 mm2) highly-efficient GaN-based light-emitting diodes (LEDs) with a vertical-conducting structure (VM-LEDs), using a patterned laser lift-off technique and a Ni electroplating process as well as a surface treatment of the top n-GaN epilayer by plasma and chemical etching, were successfully fabricated and investigated. Compared to regular LEDs of the same size, both the forward voltage drop and the light output power (Lop) of the VM-LED were substantially improved. With inductively coupled plasma (ICP) etching followed by an additional KOH etching and an HF/HCl treatment on the n-GaN layer, an increase in Lop by 227% (195%) at 350 (800) mA has been achieved for the (1 ×1 mm2)-sized VM-LEDs.

Fabrication of Thin-GaN LED Structures by Au–Si Wafer Bonding

Using Au-Si wafer bonding and laser lift-off (LLO) techniques, an light emitting diode (LED) GaN epi layer was successfully transferred onto a Si substrate. After the wafer bonding, a KrF excimer laser was used to separate the GaN layer from the grown sapphire substrate. The Raman spectra results show that the quality of the transferred GaN epi layer did not change; the initial compressive stress level of the GaN epi layer was relieved by 202 MPa after transferring. The transferred GaN epi layer was further processed for use in a thin-GaN LED device. The luminance-intensity-current voltage curve results indicate a forward voltage of 3.4 V, and a luminance intensity of 204 mcd at 20 mA.

Use of patterned laser liftoff process and electroplating nickel layer for the fabrication of vertical-structured GaN-based light-emitting diodes

The fabrication process and performance characteristics of a vertical-structured GaN-based light-emitting diode (VM-LED) employing nickel electroplating and patterned laser liftoff techniques are presented. As compared to regular LED, the forward voltage drop of the VM-LED at 20–80 mA is about 10%–21% lower, while the light output power (Lop) is more than twice in magnitude. Especially, the Lop exhibits no saturation or degradation at an injection current up to 520 mA which is about 4.3 times higher than that of the regular one. Substantial improvements in the VM-LEDs performances are mainly attributed to the use of metallic substrate which results in less current crowding, larger effective area, and higher thermal conductivity.

Surface Texturing for Wafer-Bonded Vertical-Type GaN/Mirror/Si Light-Emitting Diodes

An n-side-up GaN/mirror(Pd/Au)/Si light-emitting diode (LED) with surface texturing has been fabricated by a combination of wafer-bonding, laser lift-off, and surface texturing techniques. Two concentrations of KOH solution were used to roughen the n-GaN surface. In order to obtain a uniformly roughened surface, the solution was heated instead of being subjected to photoirradiation. The GaN/Pd/Au/Si LEDs with surface texturing exhibited a maximum luminance intensity of 130 mcd (at 20 mA) with a forward voltage of 3.2 V. The luminance intensity is over two times larger than that of the original planar GaN/sapphire LEDs (at 20 mA). Under high current injection, the surface textured GaN/Pd/Au/Si LEDs also showed a more stable luminance intensity. This feature is attributed to the Si substrate providing a good heat sink and surface roughening enhancing the external quantum efficiency. Furthermore, the n-side-up GaN/mirror(Pd/Au)/Si LEDs with surface texturing have been demonstrated to have high reliability.

Growth of thick GaN layers with hydride vapour phase epitaxy

In this paper we describe recent experimental efforts to produce high quality thick (⩾300 μm) GaN layers on sapphire, the removal of such a layer from the sapphire substrate, and the properties of the so obtained free-standing GaN material. The growth process is described in some detail in the vertical reactor geometry used in this work. Defects like dislocations, micro-cracks and pits produced during growth are discussed, along with procedures to minimize their concentration on the growing surface. The laser lift-off technique is shown to be a feasible technology, in particular if a powerful laser with a large spot size can be used. A major problem with the free-standing material is the typically large bowing of such a wafer, due to the built in defect concentrations near the former GaN-sapphire interface. This bowing typically causes a rather large width of the XRD rocking curve of the free-standing material, while optical data confirm virtually strain free material of excellent quality at the top surface.

Fabrication of Large-Area GaN-Based Light-Emitting Diodes on Cu Substrate

A large-area GaN-based light-emitting diode (LED) 1000×1000 µm2 in size with a p-side down configuration was fabricated using wafer bonding and laser lift-off (LLO) techniques. The thin GaN LED was transferred onto a copper substrate without peeling or cracks. The large-area LEDs showed a uniform light-emission pattern over entire defined mesa area without a transparent contact layer on the p-type GaN. The operating current of the large-area LEDs can be driven up to 1000 mA with continuously increasing light output-power. The light output-power is 240 mW with a driving current of 1000 mA. Large-area emission and high current operation make the LLO-LEDs applicable to high-power LED applications.

Preparation of PNN-PZT Thick Film on Pt/Ti/SiO2/Si Substrate by Laser Lift-Off Process

Thick PNN-PZT films, about 25 μm, were successfully prepared by combining the screen printing and laser lift-off techniques. PNN-PZT films screen printed on sapphire substrate, after sintering at 1200°C (3 h), are secondary free perovskite, possessing good ferroelectric properties (Pr = 16.4 μC/cm2, Ec = 12.6 kV/cm). Silver-bonding and laser lift-off processes can successfully transfer the PNN-PZT films from the sapphire to silicon substrates, maintaining ferroelectric properties. The resulted PNN-PZT/Si thick films possess larger electrical polarization: (Pr = 16.5 μ C/cm2) with slightly larger coercive field (Ec = 24.6 kV/cm).

Vertical-conducting p-side-up GaN/mirror/Si light-emitting diodes by laser lift-off and wafer-transfer techniques

A p-side-up GaN/Ag/Au–Sn/Si light-emitting diode (LED) has been fabricated by a combination of laser lift-off and wafer-transfer techniques. The Ag/Au-Sn mirror structure was chosen to favor high optical reflectivity, low contact resistance with n-GaN, vertical conducting, and low-temperature (∼300 °C) wafer bonding. The GaN/Ag/Au–Sn/Si LED presented a maximum luminance intensity of 129 mcd (@ 20 mA) with a forward voltage of 3.3 V. The luminance intensity is over two times in magnitude as compared with that of the original planar GaN/sapphire LED. Under high current injection, the GaN/Ag/Au-Sn/Si LED also showed a more stable emission wavelength. This feature is attributed to the Si substrate providing a good heat sink. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

GaN/Mirror/Si Light-Emitting Diodes for Vertical Current Injection by Laser Lift-Off and Wafer Bonding Techniques

A p-side-up GaN/mirror/Si light-emitting diode (LED) for vertical current injection has been fabricated by laser lift-off and wafer bonding techniques. A variety of metallic mirrors (Au, Al, and Ag) were chosen to improve the optical reflectivity and contact resistance with n-GaN. The GaN/mirror/Si LED with a silver mirror achieved a maximum luminance intensity of 45 mcd (20 mA) with a low forward voltage of 3.5 V. This luminance intensity is over two times that of the original planar GaN/sapphire LED. Under high current injection, the GaN/mirror(Ag)/Si LED also showed a more stable emission wavelength than the planar GaN/sapphire LED. This can be explained by the fact that the Si substrate provides a good heat sink and alleviates the joule heating problem. On the basis of these results, the p-side-up structure confirms the possibility of the simultaneous realization of a lower contact resistance and higher reflectivity for GaN/mirror/Si LEDs.

Performance enhancement of InGaN light-emitting diodes by laser lift-off and transfer from sapphire to copper substrate

Laser lift-off technique was employed to carry out transfer of prefabricated InGaN multiple-quantum-well light-emitting diodes (LEDs) from sapphire onto Cu substrate. Silver epoxy was used as the bonding material. Characterization results showed tremendous device improvements in terms of maximum allowable current, light output power, and reliability from the use of conductive Cu substrate. LEDs on Cu could withstand a maximum current of 530 mA before breakdown while those on sapphire could only withstand 350 mA. At 40 mA, light output power of LEDs on sapphire and Cu was 0.74 and 0.95 mW, respectively. In addition, reliability test at constant current of 300 mA showed improvement in light output power for LEDs on Cu whereas LEDs on sapphire suffered deterioration with time.

Study of GaN light-emitting diodes fabricated by laser lift-off technique

The fabrication process and performance characteristics of the laser lift-off (LLO) GaN light-emitting diodes (LEDs) were investigated. The LLO-GaN LEDs were fabricated by lifting off the GaN LED wafer structure grown on the original sapphire substrate by a KrF excimer laser at 248 nm wavelength with the laser fluence of 0.6 J/cm2 and transferring it onto a Cu substrate. The LLO-GaN LEDs on Cu show a nearly four-fold increase in the light output power over the regular LLO-LEDs on the sapphire substrate. High operation current up to 400 mA for the LLO-LEDs on Cu was also demonstrated. Based on the emission wavelength shift with the operating current data, the LLO-LEDs on Cu show an estimated improvement of heat dissipation capacities by nearly four times over the light-emitting devices on sapphire substrate. The LLO process should be applicable to other GaN-based LEDs in particular for those high light output power and high operation current devices.

Vertical InGaN‐based blue light emitting diode with plated metal base fabricated using laser lift‐off technique

A vertical InGaN-based blue light emitting diode (LED) with a reflective metal electrode and thick gold plated base free from insulating sapphire is demonstrated. A 50 μm-thick gold plated layer on a highly reflective platinum p-side ohmic contact is formed on the InGaN-based LED epi-structure, followed by irradiation with high power ultra-violet pulsed laser from the backside of the original sapphire substrate. The laser processing, so called laser lift-off technique, enables the separation of sapphire by decomposition of interfacial GaN layer. No transparent electrode is necessary for the LED because the current can spread well through the highly conductive n-GaN layer so that uniform blue light is emitted through the n-GaN layer. The electroluminescence (EL) spectra show small multi peaks due to the resonance within the formed vertical cavity. In addition, the measured thermal resistance of the vertical LED is smaller than that of the conventional LED on poor heat-conductive sapphire. The vertical LED would increase the brightness together with smaller chip size and lower series resistance. This would be advantageous espacially for high power and highly efficient LEDs applicable to future solid state lighting. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Preparation of ferroelectric Pb(Zr 1− xTi x)O 3/Si films by laser lift-off technique

Materials characteristics and ferroelectric properties of thick xPb(Ni 1/3Nb 2/3)O 3-(1- x)Pb(Zr 0.52Ti 0.48)O 3 ( x=0.3), PNN-PZT films, which were prepared by doctor-blade tape-casting and laser lift-off processes, were investigated. The underlying buffer materials were observed to influence significantly the densification behavior for the PNN-PZT sapphire layers. For PNN-PZT/sapphire films sintered without buffer layer, the substrates impose pronounced constraint on materials shrinkage, inducing the formation of porosity for the PNN-PZT layers. The proportion of pores is pronouncedly reduced when using a Pb(Zr 0.52Ti 0.48)O 3 thin layer (<0.3 μm) prepared by metal-organic decomposition process as buffer layer. The PNN-PZT films transferred to silicon substrates by laser lift-off processes exhibit large electrical polarization characteristics (Pr=41.9 μC/cm 2), with small coercive field (Ec=35.6 kV/cm).