Laser Lift-off Process Research Articles

Preparation on transparent flexible piezoelectric energy harvester based on PZT films by laser lift-off process

The piezoelectric energy generation properties of transparent flexible devices (TFD) based on PbZr0.52Ti0.48O3 (PZT) films, which were fabricated by laser lift-off (LLO) process, were studied for a piezoelectric energy harvester. Through the introduction of indium-tin-oxide (ITO) and polyethylene terephthalate (PET) substrates, TFDs were implemented, respectively. The TFDs based on PZT films generated an AC-type output signal and output power of 8.4nW/cm2, at periodically bending and releasing motion. In addition, inverted output signals were observed when the manufactured TFDs were connected to the measuring equipment in reverse and were bended to the reverse direction, demonstrating that the generating signals originated from the piezoelectric effect of TFDs. The experimental results clearly showed that the TFDs based PZT film have potential for use in next generation of electronic devices applications such as flexible devices, transparent electronics, and energy harvester.

193 nm excimer laser lift-off for AlGaN/GaN high electron mobility transistors

AlGaN/GaN HEMTs grown on both-side-polished sapphire substrates were successfully lifted-off with a 193-nm UV excimer laser system. The photon energy of the 193 nm laser is larger than the band gap of AlN and thus it can be used to lift-off AlGaN HEMT structures with AlN or AlGaN interfacial layers grown on sapphire substrates prior to growth of the GaN buffer layers. The lifted-off HEMT chip was warped and showed 25–42% reduction of the saturation drain current. There was no degradation observed either in the forward or reverse gate current-voltage (I-V) characteristics or on the drain punch-through voltage. Based on comparisons of cross-sectional electron micrographs, no additional dislocations were created in the HEMT structures after the laser lift-off process. Reduction in saturation drain current was attributed to relaxation of the lifted-off HEMT structures. Newton's rings and Raman spectrum E2 peak shifts were used to estimate the strain relaxation of the laser lifted-off samples.

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.

Fabrication of flexible device based on PAN-PZT thin films by laser lift-off process

The ferroelectric properties of flexible devices based on 0.05Pb(Al0.5Nb0.5)O3–0.95Pb(Zr0.52Ti0.48)O3+0.7wt.%Nb2O5+0.5wt.%MnO2 (PAN-PZT) thin films, which were fabricated using a laser lift-off (LLO) process, were investigated. The flexible devices based on PAN-PZT thin films were coated with a sacrificial layer, which prevented or minimized damage during LLO process. The structural and electrical properties of the PAN-PZT thin films before and after LLO process demonstrated that the crystallographic and ferroelectric properties of the device were retained after LLO process. Flexible devices based on PAN-PZT thin films coated with a sacrificial layer may be fabricated using the LLO process for the production of flexible electronic devices.

GaN-based light-emitting diodes on origami substrates

GaN-based light-emitting diodes (LEDs) were transferred to paper substrates after a laser lift-off (LLO) process with an ArF excimer laser system (λ = 193 nm) to remove the sapphire substrate and produce freestanding blue LED templates. The threshold voltage (∼2.7 V), current-voltage characteristics, and peak emission wavelength (442 nm) were not changed after the paper substrate was subsequently wrinkled. We were able to demonstrate transfers to both planar and folded (origami) paper structures, showing the promise of the LLO process for transferring LEDs to arbitrary surfaces.

Influence of laser lift-off on optical and structural properties of InGaN/GaN vertical blue light emitting diodes

The influences of the laser lift-off (LLO) process on the InGaN/GaN blue light emitting diode (LED) structures, grown on sapphire substrates by low-pressure metalorganic chemical vapor deposition, have been comprehensively investigated. The vertical LED structures on Cu carriers are fabricated using electroplating, LLO, and inductively coupled plasma etching processes sequentially. A detailed study is performed on the variation of defect concentration and optical properties, before and after the LLO process, employing high-resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM) observations, cathodoluminescence (CL), photoluminescence (PL), and high-resolution X-ray diffraction (HRXRD) measurements. The SEM observations on the distribution of dislocations after the LLO show well that even the GaN layer near to the multiple quantum wells (MQWs) is damaged. The CL measurements reveal that the peak energy of the InGaN/GaN MQW emission exhibits a blue-shift after the LLO process in addition to a reduced intensity. These behaviors are attributed to a diffusion of indium through the defects created by the LLO and creation of non-radiative recombination centers. The observed phenomena thus suggest that the MQWs, the active region of the InGaN/GaN light emitting diodes, may be damaged by the LLO process when thickness of the GaN layer below the MQW is made to be 5 μm, a conventional thickness. The CL images on the boundary between the KrF irradiated and non-irradiated regions suggest that the propagation of the KrF laser beam and an accompanied recombination enhanced defect reaction, rather than the propagation of a thermal shock wave, are the main origin of the damage effects of the LLO process on the InGaN/GaN MQWs and the n-GaN layer as well.

Chemical Lift-Off of Blue Light-Emitting Diodes Grown on Sapphire Substrate with an Oxide-Patterned Sacrificial Layer

A high quality GaN epilayer was successfully grown on a sapphire substrate with patterned SiO2 as a sacrificial structure utilizing the direct heteroepitaxial lateral overgrowth method. The light-emitting diode (LED) structure was subsequently grown on the GaN epilayer and fabricated using a chemical lift-off process. After SiO2 was removed with a HF solution, the LEDs with Cu substrate could be separated from the sapphire substrate due to an increase in the GaN/sapphire interfacial strain. Similar forward voltage and output power characteristics were obtained for vertical LEDs fabricated using chemical lift-off and laser lift-off processes.

Ultrafast pump-probe microscopy with high temporal dynamic range

Ultrafast pump-probe microscopy is a common method for time and space resolved imaging of short and ultra-short pulse laser ablation. The temporal delay between the ablating pump pulse and the illuminating probe pulse is tuned either by an optical delay, resulting in several hundred femtoseconds temporal resolution for delay times up to a few ns, or by an electronic delay, resulting in several nanoseconds resolution for longer delay times. In this work we combine both delay types for temporally high resolved observations of complete ablation processes ranging from femtoseconds to microseconds, while ablation is initiated by an ultrafast 660 fs laser pump pulse. For this purpose, we also demonstrate the calibration of the delay time zero point, the synchronization of both probe sources, as well as a method for image quality enhancing. In addition, we present for the first time to our knowledge pump-probe microscopy investigations of the complete substrate side selective ablation process of molybdenum films on glass. The initiation of mechanical film deformation is observed at about 400 ps, continues until approximately 15 ns, whereupon a Mo disk is sheared off free from thermal effects due to a directly induced laser lift-off ablation process.

Thermo-mechanical solution of film/substrate systems under local thermal load and application to laser lift-off of GaN/sapphire structures

Film/substrate structures may undergo a localized thermal load, which can induce stresses, deformation and defects. In this paper, we present the solutions of temperature and stresses in a film/substrate structure under a local thermal load on the film surface. Then, the generalized Stoney formula, which connects the curvature of deformation and the stress field is obtained. The present solution takes into account the non-uniformity of the temperature field both in the width and thickness directions of the film. The thermo-mechanical solution is applied to the analysis of the temperature distribution, stresses, and damage of a GaN/sapphire system during the laser lift-off (LLO) process. It is shown that the laser with the Gaussian distribution of energy density causes much smaller tensile stresses at the edge of the heated area in the film than the laser with the uniform distribution of energy density, and thus can avoid damage to the GaN films separated from the substrate.

Simulation and experimental study of ArF 193 nm laser lift-off AlGaN/GaN high electron mobility transistors

A finite element simulation was used to estimate the temperature distributions within AlGaN/GaN high electron mobility transistors (HEMTs) during the laser lift-off process. The time-dependent simulation showed that a thin layer of GaN at the GaN/sapphire interface was heated up to around 1600 K in less than 25 ns by a pulsed laser exposure with a duration of 25 ns and a fluence of 800 mJ/cm2 to decompose this GaN layer into Ga and nitrogen. Experimentally, there was a threshold fluence around 550 mJ/cm2, corresponding to 1300 K at the GaN/sapphire interface, for partially lifting off the HEMT structure from the sapphire. The simulated temperature at the GaN/sapphire interface with a fluence of 420 mJ/cm2 never reached above 1000 K, however, the HEMT structure was lifted-off by multiple laser exposures at this fluence. Therefore, instead of thermally induced decomposition, the lift-off mechanism could also be through the Ga–N bond breaking during the multiple lower-fluence high-energy 193 nm laser exposures.

Control of the free carrier concentrations in a Si-doped freestanding GaN grown by hydride vapor phase epitaxy

Freestanding GaN was grown on sapphire substrate in a horizontal HVPE system, followed by laser lift-off process. The thickness of the freestanding GaN samples was about 350μm. Si was used as an n-type dopant and the carrier concentrations were non-destructively measured by micro-Raman spectroscopy. The carrier concentrations of GaN samples were changed from 0.62 to 3.7×1018/cm3 by varying the Si/Ga ratios from 0.01 to 0.04. Crystalline quality of the freestanding GaN was characterized by X-ray diffraction (XRD). The full-width at half-maximum (FWHM) values of the rocking curves of the (102) plane was changed from 96.9 to 317.5arcsec depending on Si/Ga ratios. This work provides Si/Ga ratios to obtain various carrier concentrations of the freestanding GaN by HVPE, and proposes micro-Raman spectroscopy as a sensitive technique for carrier concentration measurement.

Formation of Cu Substrates with W Addition for Vertical Structure Light Emitting Diodes using Electroplating Method

Pure Cu substrate has been previously electroplated on p-GaN side of vertical structure GaN-based light emitting diode (LED) to mechanically hold thin GaN-based multilayer after removal of a sapphire substrate by laser lift-off process. The ductile Cu substrate needs to be mechanically strengthened to prevent the GaN-based multilayer from being damaged during vertical LED chip fabrication. In this study, the formation of Cu substrate with W addition on Au-seeded LED wafer by electroplating is introduced. W presents as WO3 in the Cu substrate. W content of W-added Cu deposits was dependent on constant reduction potential in an acidic citrate bath. The reduction potential also influenced directly the electrical resistivity, hardness, and residual stress of W-added Cu substrates. Applying moderate reduction potentials like −1.1 V resulted in 1.5 times larger hardness than that of pure Cu substrate by adding around 1.0 at % W to Cu. Nevertheless, the electrical resistivity of W-added Cu substrate was maintained as low as that of pure Cu substrate. At more positive reduction potentials than −1.1 V, very low residual stress between 4 and 42 MPa was measured. Wafer bowing induced by large residual stress of W-added Cu substrate can cause serious mechanical damage to LED structure.

Resonant-cavity blue light-emitting diodes fabricated by two-step substrate transfer technique

GaN-based blue resonant-cavity light-emitting diodes (RCLEDs) have been fabricated by the two-step substrate transfer technique. The prefabricated structure was separated from its sapphire substrate by laser lift-off (LLO) process and was then transferred to a Si permanent substrate. By optimising the LLO process and adopting an indium tin oxide thin film and two high-reflectivity dielectric distributed Bragg reflectors, a narrow linewidth of 0.3 nm at 461.2 nm, corresponding to a quality factor of approximately 1530, has been observed from a blue RCLED under a current density of 2 kA/cm2. It indicates that a high quality optical resonator has been realised by this approach.

Characteristics of a Transparent GaON Sacrificial Layer for Transfer of Devices from Sapphire to a Flexible Substrate

We investigated the characteristics of GaON films grown by reactive magnetron sputtering on sapphire substrate for use as a sacrificial layer in a laser lift-off (LLO) process. Due to the desirable bandgap (4.58 eV) of the amorphous GaON film, which is suitable for absorbing 248 nm KrF excimer laser (5.0 eV) energy, the amorphous GaON layer was easily separated from the original sapphire substrate through irradiation of the KrF excimer laser. It was found that irradiation from a KrF excimer laser (550 mJ/cm2) at the a-GaON/sapphire interface led to rapid thermal decomposition of the a-GaON, producing metallic Ga and O2 and N2 gases and inducing separation from the sapphire substrate. This indicates that amorphous GaON film is a promising sacrificial layer for use in the LLO process for transfer of devices from a rigid sapphire substrate to flexible substrates.

Pump-Probe Investigations on the Laser Ablation of CIS Thin Film Solar Cells

A picosecond laser lift-off process of an about 0.5μm thick Mo layer on a glass substrate can be induced by irradiating the layer from the glass side. A pump-probe setup for time- and space resolved microscopy is used to investigate the ablation process in a time domain from a few picoseconds up to 3.6ns. The results display a bulging of Mo within 1ns followed by a lift-off at times past 3.6ns. Furthermore, the setup is used for in situ investigations of a multi pulse ablation through the whole layer stack (ZnO/CIS/Mo/Glass) of a CIS thin film solar cell.

GaN-Based Thin Film Vertical Structure Light Emitting Diodes Fabricated by a Modified Laser Lift-off Process and Transferred to Cu

GaN-based thin film vertical structure light-emitting diodes (VS-LEDs) were fabricated by a modified YAG laser lift-off (LLO) process and transferred to Cu substrates. With a comparison of the electrical and optical properties of conventional LEDs on sapphire substrates and of lateral structure thin film LEDs by a KrF LLO process, the vertical structure of LLO LEDs shows obvious superiority. LLO VSLEDs made by modified YAG LLO process show less increase of leakage current than the devices made by conventional KrF LLO process. Furthermore, owing to the well current spreading and less current path, the ideality factors and series resistance of vertical structure LEDs reduce greatly and the efficiency increases more obviously than the lateral structure LEDs, which is also reflected on the relative L-I curves. The output power of vertical structure LEDs is over 3 times greater than that of the lateral structure LLO LEDs within 300mA.

InGaN-Based Resonant-Cavity Light-Emitting Diodes with a ZrO2/SiO2 Distributed Bragg Reflector and Metal Reflector

We fabricated InGaN-based resonant-cavity light-emitting diodes (RC-LEDs) in which one of the reflectors forming the cavity was made of Ag metal and the other was a ZrO2/SiO2 dielectric distributed Bragg reflector (DBR). Ag metal was deposited onto an InGaN epitaxial layer grown on a sapphire substrate. A ZrO2/SiO2 DBR was deposited onto n-GaN after a Ag/InGaN/sapphire sample was bonded to a silicon substrate using Au bonding metal and the sapphire substrate was removed by a laser lift-off process. Multiple emission modes with a narrowed spectrum were observed with corresponding dips in the reflectance of the resonant cavity, which was examined by microreflectance measurements. The output intensity of the RC-LED was higher than that of a conventional LED and the turn-on voltage increased with a slight decrease in series resistance. The output power of the RC-LED in a 1 ×1 mm2 chip increased linearly with the injection of current up to a 500 mA.

Hybrid metamaterial design and fabrication for terahertz resonance response enhancement

Planar hybrid metamaterial with different split ring resonators (SRR) structure dimensions are fabricated on silicon substrates by femtosecond (fs) laser micro-lens array (MLA) lithography and lift-off process. The fabricated metamaterial structures consist of: (a) uniform metamaterial with 4 SRRs at same design and dimension as a unit cell and (b) hybrid metamaterial with 4 SRRs at same design but different dimensions as a unit cell. The electromagnetic field responses of these hybrid and single dimension metamaterial structures are characterized using a terahertz (THz) time-domain spectroscopy. Transmission spectra of these metamaterial show that a broader resonance peak is formed when 2 SRRs are close to each other. FDTD simulation proves that there is a strong mutual coupling between 2 SRRs besides a strong localized electric field at the split gap, which can enhance the electric field up to 364 times for tunable, broad band and high sensitivity THz sensing. Meanwhile, the strong coupling effect could lead to the formation of an additional resonance peak at approximately 0.2 THz in the THz spectra regime.

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.

Two inch large area patterning on a vertical light-emitting diode by nano-imprinting technology

A vertical light-emitting diode (LED) with a chip size of 500 × 500 µm2 was fabricated by the laser lift-off (LLO) process of an InGaN-based blue LED wafer. After the LLO process, photonic crystal patterns by UV nano-imprint lithography were formed on the n-GaN top layer of the vertical LED over the entire area with a diameter of 2 inches. As the result of n-GaN patterning, light output power of the vertical LED with photonic crystals was increased by up to 44% compared to that of the vertical LED without a photonic crystal at a driving current of 1000 mA.