Laser Lift-off Technique Research Articles

Laser lift-off technique for freestanding GaN substrate using an In droplet formed by thermal decomposition of GaInN and its application to light-emitting diodes

We developed a laser lift-off technique for a freestanding GaN substrate using an In droplet formed by thermal decomposition of GaInN. A combination of an In droplet formed by thermal decomposition of GaInN during growth and a pulsed second-harmonic neodymium-doped yttrium aluminum garnet laser (λ = 532 nm) realized the lift-off GaN substrate. After laser lift-off of the GaN substrate, it was used to achieve 380 nm ultraviolet light-emitting diodes with light output enhanced 1.7-fold. In this way, the light extraction can be improved by removing the GaN substrate.

Nanogenerators: Highly‐Efficient, Flexible Piezoelectric PZT Thin Film Nanogenerator on Plastic Substrates (Adv. Mater. 16/2014)

A highly-efficient, lightweight, flexible, piezoelectric lead zirconate titanate (PZT) thin-film nanogenerator is demonstrated by employing the laser lift-off transfer technique and a lateral electrode structure. As reported by K. J. Lee and co-workers on page 2514, this large-area PZT thin-film nanogenerator can convert to the highest output performance from a slight mechanical deformation and provides the feasibility of fully flexible self-powered electronics.

(Invited) High Brightness, Large Scale GaN Based Light-Emitting Diode Grown on 8-Inch Si Substrate

GaN based light-emitting diodes(LEDs) grown on a Si substrate is one of promising future technologies in LED industry due to its scalability and costeffectiveness compare to the conventional sapphire based LEDs.One of major technical issues in GaN/Si LED is the severemismatch oflattice and thermal expansion coefficientsbetween GaNand Si, which induces significant tensile strain in the GaNepilayer that often leads to wafer cracking. Optical loss by the absorbing Si substrate is another barrier to overcome for realizing high-brightness LEDs competitive to those on sapphire or SiC substrates.In this study, we demonstrated blue LEDs grown on 8-inch diameter Si substrate and fabricated high brightness devicesutilizing vertical chip process Thecrack-free InGaN/GaN LED waferswith strain-engineered buffer structure were grown on 8-inch diameter Si(111) substrates by metalorganic chemical vapor deposition (MOCVD) system (Veeco TurboDisk k465i).The optimized growth conditions of AlN buffer layer improved the crystalline quality of GaNepilayers. The FWHM of (002) and (102) ω-scan is 280 and 420 arcsec, respectively. These values are comparable to the best reported so far for GaN on silicon. The surface morphology of GaN layer investigated using atomic force microscopy (AFM) exhibits very smooth surface and clear atomic step flow patterns. The root-mean-square (RMS) roughness is 0.15 nm in a 5x5 μm2 scan area.For LED on 8-inch Si, MQW PL wavelength uniformity of ~0.5% (2.4 nm) has been achieved using the proper strain/curvature engineering with optimized wafer carrier designs. These are among the lowest values reported to date.After the epitaxial growth with silicon substrate, a 1mm × 1mm sized vertical type LED(VLED) was fabricated using both wafer bonding process and Si removing process. While the GaN on sapphire employs a laser lift-off technique to remove the substrate, we developed the wet etching process in order to remove the silicon substrate without any damageon the surface of LED structure. The electro-optical characteristics of the fabricated VLED in this work shows good efficiency(almost 1W light power at 1A inject current) with operating voltage of 4.0V.

GaN-based substrates and optoelectronic materials and devices

In order to solve the problems of GaN heteroepitaxy on sapphire substrate, some techniques were explored. Freestanding GaN substrates have been made by hydride vapor phase epitaxy (HVPE), laser lift-off (LLO), and chemical mechanical polishing techniques. Wafer bending and cracking in the HVPE growth were partly settled by pulsed flow modulation method. High-crystal quality was established for 1.2 mm thick GaN substrate by X-ray diffraction measurement, in which the full width of half maximum values were 72, 110 arcsec for (102), (002) peaks. A novel micro-size patterned sapphire substrate (PSS) and a nano PSS were also fabricated. High-power vertical structure light emitting diodes (VSLEDs) have been developed by Au–Sn eutectic wafer bonding, homemade micro-area LLO, and light extraction structure preparation. The high-injection-level active region with low temperature GaN sandwiched layers was used for low-efficiency droop. The light output power of VSLED was achieved as 400 mW driven at 350 mA, and the dominant wavelength is about 460 nm. The structures and properties of strain modulated superlattices (SLs) and quantum wells as well as advanced simulation of carriers transport across the electron blocking layer were investigated in laser diodes. The hole concentration was achieved as high as 1.6 × 1018 cm−3 in AlGaN/GaN SLs:Mg by inserting an AlN layer. High-quality AlGaN epilayers and structures were grown by MOCVD. Some device structures of UV LEDs and detectors were demonstrated. The emission wavelength of 262 nm UV LED has been successfully fabricated. At last, high-quality InN and InGaN materials for solar cell were grown by boundary-temperature-controlled epitaxy and growth-temperature-controlled epitaxy. Hall-effect measurement showed a recorded electron mobility of 3,280 cm2/(V s) and a residual electron concentration of 1.47 × 1017 cm−3 at 300 K.

InGaN-based resonant-cavity light-emitting diodes with dielectric-distributed Bragg reflectors

An InGaN-based resonant-cavity light-emitting diode (RC-LED) chip with a large chip area of 1 × 1 mm2 was fabricated using ZrO2/SiO2 dielectric-distributed Bragg reflectors (DBRs) on both sides of the cavity. After the growth of InGaN-based n- and p-epitaxial layers on a sapphire substrate, ZrO2/SiO2 layers for one DBR were deposited on it; then, holes with ∼10-µm diameters were patterned through the DBR layers. The DBR layer’s side of the whole sample was thermally bonded to a silicon substrate by using Au bonding metal; then, the sapphire substrate of the sample was removed using a laser lift-off technique. ZrO2/SiO2 layers for the other DBR were deposited on n-GaN for complete formation of the cavity. Through the electrical and the optical characterizations, we showed that our RC-LED has the improved optical output power and forward directionality in comparison with a conventional LED.

Enhancement of light extraction efficiency in sub-300 nm nitride thin-film flip-chip light-emitting diodes

We report on the improved light extraction efficiency of 287nm small periphery AlGaN/AlGaN thin-film flip-chip (TFFC) light-emitting diodes (LEDs) fabricated using the laser lift-off technique. After sapphire substrate removal, the exposed AlN N-face was etched in a KOH solution. The result was a 1.5-fold increase in the TFFC device output power at 20mA dc current injection, which was attributed to the AlN surface texturing. TFFC die encapsulation process produced an additional 1.34-fold enhancement of the emitted light power. More importantly, the encapsulated thin-film LEDs exhibited a similar reliability performance compared to encapsulated conventional FC LEDs processed from the same epiwafer, and with no noticeable degradation of the encapsulating material optical properties.

Auto-Split Laser Lift-Off Technique for Vertical-Injection GaN-Based Green Light-Emitting Diodes

An auto-split laser lift-off (LLO) method for fabrication of vertical-injection GaN-based green light-emitting diodes (ASV-LEDs) is demonstrated. The ASV-LEDs exhibited a significant improvement in the light output and thermal dissipation, as compared with that of conventional LEDs on sapphire. The intrinsic physical mechanism of the auto-split LLO technique is studied by a Frank-Read dislocation clustering model. The laser energy density and mesa spacing are shown to be key factors in the auto-split LLO method. It is believed that this method offers an alternative way to fabricate high-performance GaN-based thin-film LEDs.

Shock-induced brittle cracking in HVPE-GaN processed by laser lift-off techniques

A study on brittle cracking in GaN films processed by laser lift-off is presented. Two kinds of cracks were found in the N-polar face of GaN after the laser lift-off process, namely perpendicular cracks along the {1 −1 0 0} planes and lateral cracks along the (0 0 0 −1) plane, respectively. Single-shot laser damage is studied to understand the cracking mechanism. The damage morphology indicates that the GaN material on the edge of the laser ablation area experiences three loading modes: shear stress PS, longitudinal compressive stress PL and transverse tensile stress PT. Under shock PL, lateral cracks likely appear and extend from the illuminated region along the interface in mode I. Furthermore, two different kinds of perpendicular cracks were found, namely shear cracks (PC I) and deflection cracks (PC II). A strong PS gives rise to PC I while a cooperative action of PL and PT results in PC II. In addition, there exist a critical effective spot size dPth and a critical ratio of the laser spot size dL to the effective spot size dP, when cracks occur over them.

Local stress distribution in GaN vertical light-emitting diodes fabricated using CLO and LLO methods

We report on the local stress distribution in a GaN-based vertical light-emitting diode (V-LED) fabricated using two types of separation methods: a chemical lift-off (CLO) procedure and a laser lift-off (LLO) technique. The CLO LED exhibits a stronger donor-bound exciton (D0X) emission than the LLO LED, owing to its textured surface morphology and lower amount of damage to the structure. On the basis of the photoluminescence and Raman spectroscopy results, we determine that the CLO GaN LED has an 82 MPa lower residual stress than the LLO GaN LED. Therefore, the CLO technique can be considered as a more effective method to fabricate stress-relieved high-brightness LEDs.

Vertical InGaN light-emitting diode with a retained patterned sapphire layer

We present an efficient vertical InGaN light-emitting diode (LED) in which the proposed vertical LEDs were fabricated with patterned sapphire substrates (PSS) using thinning techniques. After the thinning of sapphire substrate, selective dry etching process was performed on the remainder sapphire layer to expose the n-GaN contact layer instead of removing the sapphire substrate using the laser lift-off technique. These processes feature the LEDs with a sapphire-face-up structure and vertical conduction property. The PSS was adopted as a growth substrate to mitigate the light-guided effect, and thereby increase the light-extraction efficiency. Compared with conventional lateral GaN LEDs grown on PSS, the proposed vertical LEDs exhibit a higher light output power and less power degradation at a high driving current. This could be attributed to the fact that the vertical LEDs behave in a manner similar to flip-chip GaN/sapphire LEDs with excellent heat conduction.

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.

Fabrication of Vertical-Structured GaN-Based Light-Emitting Diodes Using Auto-Split Laser Lift-Off Technique

Vertical-structured GaN-based light-emitting diodes (V-LEDs) were successfully fabricated using auto-split laser lift-off (LLO) technique. Compared to regular sapphire-substrate LED, the forward voltage of the V-LED at 20 mA is about 5% lower, while the light output power is about 43% higher. For V-LED, the saturation behavior of the light output power (Lop) is not observed when the injection current is increased to 480 mA, while the Lop of regular LED starts to decrease at around 110 mA. These improved results can be attributed to the total effect of less current crowding, surface roughening on n-GaN layer, highly reflective Ag mirror and good thermal conductivity of the electroplated Ni. Finally, mechanisms of the auto-split LLO technique are discussed based on one-dimensional heat equation. It is shown that the auto-split LLO process is determined by the vapor pressure of N2 gas, which is strongly dependent on the density of the laser energy.

Vertical InGaN light-emitting diodes with a sapphire-face-up structure

Vertical GaN-based light-emitting diodes (LEDs) were fabricated with a Si substrate using the wafer-bonding technique. Lapping and dry-etching processes were performed for thinning the sapphire substrate instead of removing this substrate using the laser lift-off technique and the thinning process associated with the wafer-bonding technique to feature LEDs with a sapphire-face-up structure and vertical conduction property. Compared with conventional lateral GaN/sapphire-based LEDs, GaN/Si-based vertical LEDs exhibit higher light output power and less power degradation at a high driving current, which could be attributed to the fact that vertical LEDs behave in a manner similar to flip-chip GaN/sapphire LEDs with excellent heat conduction. In addition, with an injection current of 350 mA, the output power (or forward voltage) of fabricated vertical LEDs can be enhanced (or reduced) by a magnitude of 60% (or 5%) compared with conventional GaN/sapphire-based LEDs.

Performance of GaN Vertical Light Emitting Diodes Using Wafer Bonding Process with Al-Alloyed Graphite Substrate

We report on the vertical-structure light emitting diodes (VLEDs) fabricated with wafer bonding method using Al-alloyed graphite and Si supporter. VLEDs with Al-alloyed graphite produced no crack during/after laser lift-off (LLO) techniques while the wafer crack took place using Si supporter because of the difference of thermal expansion coefficients between Si and sapphire. The performance of VLEDs with wafer bonding method using Al-alloyed graphite supporter was compared to those fabricated by Cu plating methods. The output power of the chips with wafer bonding method was nearly same as the one with Cu-plating method. However, the forward voltage of VLEDs with wafer bonding method was higher than those with Cu-plating method. In the terms of reliabilities the wafer bonding process is more preferable to Cu-plating and our report proposes that Al-alloyed graphite could be one of promising candidates for the supporters in wafer bonding process.

GaN-based LEDs with a high light extraction composite surface structure fabricated by a modified YAG laser lift-off technology and the patterned sapphire substrates

Vertical structure LEDs have been fabricated with a novel light extraction composite surface structure composed of a micron grating and nano-structure. The composite surface structure was generated by using a modified YAG laser lift-off technique, separating the wafers from cone-shaped patterned sapphire substrates. LEDs thus fabricated showed the light output power increase about 1.7–2.5 times when compared with conventional vertical structure LEDs grown on plane sapphire substrates. A three-dimensional finite difference time domain method was used to simulate this new kind of LED device. It was determined that nano-structures in composite surface patterns play a key role in the improvement of light extraction efficiency of LEDs.

Transferring Thin Film GaN LED Epi-Structure to the Cu Substrate by Chemical Lift-Off Technology

In this paper, a patterned sacrificial layer structure comprised of 3 μm-wide SiO2 narrow strips and 3 μm spacing between strips fabricated on sapphire substrate was proposed. The structure was used not only to improve GaN epilayer quality, but also as a sacrificial layer structure. The GaN light emitting diodes (LEDs) covered by the Cu substrate was fabricated after the GaN epitaxy layer was grown on the sapphire by lateral epitaxial overgrowth. Chemical etching using hydrofluoric acid was conducted to remove the aforementioned SiO2 strips, and hence transform them into narrow tunnels. The GaN LEDs with the Cu substrate detached from the sapphire substrate after interface strain was increased, which demonstrates that n-side-up vertical GaN LEDs could be more easily obtained using chemical lift-off technology without laser damage compared with the laser lift-off technique.

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

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.

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

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

Substrate-free large gap InGaN solar cells with bottom reflector

In this study, we report on the realization of the In 0.085Ga 0.915N p– i– n solar cell by low-pressure metalorganic vapor phase epitaxy (MOVPE). The w–2θ scans of (0 0 0 2) reflection observation indicate good suppression of phase separation for the p– i– n solar cells with a 150-nm-thick i-In 0.085Ga 0.915N epilayer. The sharp and narrow signals in the photoluminescence spectra of In 0.085Ga 0.915N provided evidence for high material quality, even through the epilayer thickness exceeded its critical value. Furthermore, the laser lift-off (LLO) technique is used to fabricate the substrate-free thin-film solar cells (TF-SCs) with a bottom reflector. Although the samples have suffered from thermal-gradient-induced damage during LLO process, the fabricated TF-SCs still exhibit a low forward voltage of 3.3 V at 20 mA along with an ideality factor of 3.1. Finally, since unabsorbed photons can be reflected by the bottom reflector, the TF-SCs show a 13.6% increase in short-circuit current density as compared to their counterparts without a bottom reflector.

GaN/metal/Si heterostructure fabricated by metal bonding and laser lift-off

A process methodology has been adopted to transfer GaN thin films grown on sapphire substrates to Si substrates using metal bonding and laser lift-off techniques. After bonding, a single KrF (248 nm) excimer laser pulse was directed through the transparent sapphire substrates followed by low-temperature heat treatment to remove the substrates. The influence of bonding temperature and energy density of the excimer laser on the structure and optical properties of GaN films were investigated systemically. Atomic force microscopy, X-ray diffraction and photoluminescence measurements showed that (1) the quality of the GaN film was higher at a lower bonding temperature and lower energy density; (2) the threshold of the energy density of the excimer laser lift-off GaN was 300 mJ/cm2. The root-mean-square roughness of the transferred GaN surface was about 50 nm at a bonding temperature of 400 °C.