Fabrication of vertical AlGaN-based ultraviolet-B laser diodes using a laser lift-off method

Vertical AlGaN-based UV-B laser diodes were fabricated by a laser lift-off method to exfoliate sapphire substrates. These devices were processed on 1 cm2 square wafers with a polycrystalline sintered AlN substrate as a structural support for the exfoliated device. Following electrode formation and other necessary processing steps, mirrors were formed through cleavage. Subsequently, the performance of the device was evaluated by injecting a pulsed current at room temperature. Results revealed distinct characteristics, including a sharp emission at 298.1 nm, a well-defined threshold current, strong transverse-electric polarization characteristic, and a laser-specific spot-like far-field pattern, confirming the oscillation of the vertical laser diode.

These innovative devices are expected to find applications in UV curing processes, UV laser processing, laser metal processing and other laser processing, and biotechnology. 20)any of these applications require high light output power ranging from a few hundred milliwatts to several watts.However, most of the reported light output from a single LED or LD chip is only a few hundred mW at most. 21,22)herefore, the primary challenge in future studies will revolve around increasing the light output of these devices.The light output of a single LED or LD chip depends on the efficiency of each device and the input current.Thus, to boost the current injection into the device, it is essential to increase the device size.However, many AlGaN-based UV devices reported so far have used insulating materials, such as sapphire or AlN to obtain high-quality crystals, necessitating the adoption of lateral device configurations where current flows through thin-film n-AlGaN laterally within the device.While n-AlGaN can achieve relatively low resistivity values, it remains a thin-film that faces challenges in uniformly conducting current as the size of the p-electrode increases.Therefore, a pivotal shift toward vertical device configurations with p-and n-electrodes positioned opposite each other relative to the pn junction is essential.Vertical device architectures have already demonstrated success in highpower GaInN-based visible LEDs 23) and LDs. 24)However, as mentioned earlier, AlGaN-based UV devices incorporate insulating AlN and sapphire layers, making it essential to develop techniques for their removal.
Among these methods, laser lift-off (LLO), [25][26][27][28][29][30][31][32] electrochemical etching, 33) grinding and polishing, 34) and heatedpressurized water 35,36) have been reported to exfoliate sapphire substrates.We focus on achieving LLO-based exfoliation at the 1 cm 2 square wafer level using AlGaN templates grown on periodic AlN nanopillars formed through inductively coupled plasma (ICP) etching and nanoimprinting on sapphire substrates.30) This innovative approach has already been employed to demonstrate the operation of a vertical AlGaN-based UV-B LED. 31) However, vrtical AlGaN-based UV-B LDs have not been achieved through this technique until now.Herein, we report the results of RT pulsed operation of a vertical AlGaN-based UV-B LD emitting at an oscillation wavelength of 298.1 nm.This achievement was made possible by developing a cleavagebased mirror formation technique and optimizing the overall process.
The subsequent sections discuss the fabrication of the AlGaN-based laser structure, the step-by-step process for creating vertical LDs, and the methods used to assess the performance of the fabricated devices.Figures 1(a) and 1(b) depicts schematic illustrations of the wafer structure of the device used in this study and a vertical LD, respectively.In this specific device configuration, p-and n-electrodes were situated on opposite sides of the pn junction in a vertical orientation.To enable high current density operation, which is essential for laser oscillation, a current confinement structure using an insulator, Al 2 O 3 , on the p-electrode side was applied to realize a gain-guided wave laser structure.The wafers were grown on 2 inch-diameter c-plane sapphire substrates, with AlN nanopillars arranged in a triangular lattice pattern, featuring specific dimensions (300 nm height, 1 μm pitch, and 400 nm terrace width.These nanopillars were created through nanoimprinting and ICP etching using Cl 2 gas.AlGaN was subsequently regrown on the substrate, incorporating these AlN nanopillars via metalorganic vapor phase epitaxy, and the device structures were stacked on top of these nanopillars.The AlGaN fabrication conditions and device structures were similar to those reported in Refs.37 and 31, respectively.Notably, to avoid damage caused by laser beam penetration into the active and other layers during the LLO process, a 170 nm thick u-Al 0.50 Ga 0.40 N layer was intentionally inserted into the structure as a laser beam absorbing layer.This strategic addition ensured that the wavelength of light corresponding to the bandgap energy of AlN, u-Al 0.68 Ga 0.32 N, and n-Al 0.62 Ga 0.38 N layers used in LD were shorter than the light emitted during the LLO process, thus minimizing light absorption within these layers. The wafers with the fabricated device structures were cleaved leaved into 1 cm 2 squares using a laser scriber.For structural support, polycrystalline sintered AlN, posing a thermal expansion coefficient close to AlGaN and superior heat dissipation, was employed.An Au-Sn solder (∼70 wt% Au) was used to bond the device to this support substrate.After mirror polishing the back surface of the sapphire, a pulsed laser with a wavelength of 257 nm (fourth harmonic oscillation of a solid-state laser at 1028 nm) and an optical intensity of 0.53 J cm −2 was scanned from the sapphire substrate side with a spot size of 50 μm and a scan line pitch of 10 μm at a scan speed of 750 mm s −1 .Other fabrication methods, such as electrode materials and their annealing process, closely followed those detailed in Ref. 31 for fabricating Fabry-Perot-type edge-emitting LDs.Notably, the n-electrode remained unannealed owing to the use of an Au-Sn solder with a melting point of ∼280 °C.
The following is a description of the mirror formation method applied.The following process was applied in this study.Initially, approximately 95 μm-deep scribe lines were formed on the 395 μm thick polycrystalline sintered AlN substrate side via laser scribing.Simultaneously, approximately 2 mm-long scribe lines were formed on both edges of the AlGaN surface.Using a tabletop break machine (LB521, manufactured by Mitsuboshi Diamond Industry Co., Japan), edge mirrors were then formed by pressing a blade along the scribe lines on the polycrystalline sintered AlN substrate from the AlGaN side.The alignment of the blade with the scribe lines on the AlGaN surface side and the polycrystalline sintered AlN support substrate side was monitored using a charge-coupled device camera situated on the AlN support substrate side.This method facilitated the formation of favorable edge mirrors with a high degree of parallelism.The cavity length of the LD was varied from 500 to 1200 μm, and the edge surfaces of the LDs were evaluated without coatings.
The fabricated vertical UV-B LDs were monitored through differential interference microscopy and scanning electron microscopy (SEM).To assess the characteristics of the fabricated vertical devices, a prober was placed in contact with the n-electrode (cathode) and the Au-Sn solder part (anode) of the device, and pulse driving (pulse width = 50 ns, period = 500 μs) was initiated at RT (∼26 °C).LD light output was quantified by dividing the measured value, as recorded by an optical power meter (Ophir PD300-UV) with calibrated photosensitivity set at ∼3.5 cm from the edge surface of the LD, by the duty ratio.Emission spectra were captured with a spectrometer (Shamrock MCMSM-75 and ANDOR DU 420A-OE) with a resolution of 0.02 nm.Polarization characteristics were evaluated by inserting a polarizer between the spectrometer and LD.Current density values were calculated by dividing the LD injection current by the p-electrode area.
Figures 2(a) and 2(b) present differential interference micrographs of the fabricated vertical UV-B LDs observed from the AlGaN side and cross-sectional SEM images of the devices, respectively.Note that the SEM image was taken at a different location from the LD that exhibited the  characteristics described afterward due to concerns about the negative effects of SEM observation on the device.In this experiment, the devices were processed on 1 cm 2 square wafers and cleaved into a bar shape with multiple interconnected LDs, and the devices were evaluated under these conditions.Regrettably, only ∼2/3 of the devices operated effectively owing to p-electrode detachment following LLO, etc.These challenges stem from insufficient bonding strength with the support substrate, substrate warpage during crystal growth and suboptimization combinations of dielectric films and other components.These issues warrant further refinement in the selection of materials used in the process and in the process conditions.In contrast, the edge surfaces of the cleaved LDs displayed nearly perfect parallelism, suggesting the formation of a favorable cavity mirror required for LD oscillation in terms of appearance.Subsequent analysis via cross-sectional SEM image confirmed the formation of the p-electrode, n-electrode, and dielectric components.However, it was also evident that the apertures of the p-electrode and n-electrode were not perfectly aligned, revealing an alignment accuracy concern.Furthermore, at the SEM observation level, surface irregularities were observed on the cleavage surface of the LDs.It is necessary to verify whether the flatness of the cleavage surface meets the necessary standard and whether further improvements in fabrication methods and conditions are necessary.
Herein, we focus on the performance of LDs with typical characteristics featuring cavity lengths of 1200 μm and p-electrode aperture widths of 5 μm, as typical characteristics.The emission spectra at different current values, current-light intensity (I-L), and current-voltage (I-V ) characteristics of the fabricated vertical UV-B LDs are shown in Figs.3(a) and 3(b).In the emission spectrum of the fabricated LD, spontaneous emission dominates with a full width at half maximum (FWHM) of several nm up to a current density of ∼24 kA cm −2 .However, at a current density of 25 kA cm −2 , the emission transitions to an extremely sharp emission with a peak wavelength of 298.1 nm and an FWHM of <1 nm, confirming the presence of stimulated emission characteristics for the fabricated LDs.Furthermore, as the current density increases, multiple sharp peaks appear in the emission spectrum, suggesting that this      4.The ratio of transverseelectric (TE) to transverse-magnetic (TM) polarization inferred from the device to polarizer correlation exceeds two orders of magnitude, revealing that this emission is strongly TE polarized.Furthermore, the I-L characteristics exhibit a distinct threshold near the current value at which the device switches from spontaneous emission to stimulated emission, corroborating the findings from the emission spectrum results.The threshold current and corresponding threshold current density were 1.4 A and 24 kA cm −2 , respectively.Furthermore, Fig. 5 shows the emission pattern from the device at a current density of ∼28 kA cm -2 on phosphorcoated paper, revealing the appearance of a laser-specific spot-like far-field pattern above the threshold current value.Collectively, these results concluded that the fabricated LDs function as oscillating lasers, successfully demonstrating the RT operation of vertical UV-B LDs through the LLO method.Nevertheless, several challenges remain.The threshold current of vertical UV-B LDs remains higher than that of lateral LDs, and the I-V curve of vertical UV-B LDs shows that the built-in voltage of the pn junction and the operating voltage during current injection are higher than those of the lateral counterparts.These discrepancies are mainly attributed to insufficient mirror flatness and the lack of annealing treatment for the n-electrode in vertical UV-B LDs.Further studies must address these processrelated aspects to further enhance device performance.
In summary, using the LLO method, we investigated the realization of an AlGaN-based vertical UV-B LD.We optimized the cleavage method to form a favorable optical cavity in this vertical UV-B LD.Our findings confirm a sharp emission peak characteristic of stimulated emission, strong TE polarization characteristics, and threshold current in I-L characteristics for the fabricated device.Additionally, the emergence of a laser-specific spot-like far-field pattern affirmed the identity of the fabricated device as a vertical AlGaN-based UV-B LD.

Fig. 1 .
Fig. 1.(a) Cross-sectional view of the fabricated wafer structure and (b) diagrammatic view of the fabricated device.

Fig. 2 .
Fig. 2. (a) Differential interference micrograph of the fabricated device observed from the AlGaN side and (b) cross-sectional SEM image of the fabricated device.

Fig. 3 .
Fig. 3. (a) Current density dependence of emission spectrum, with an enlarged view at low current density inset in the figure and (b) I-V and I-L characteristics, with current density indicated on the upper axis.
vertical UV-B LD operates in multiple oscillation modes.This result aligns with the expectations because of the gainguided LD structure of the device.The polarization characteristics of this vertical UV-B LD at a current density of ∼28 kA cm −2 are shown in Fig.

Fig. 4 .
Fig. 4. Polarization characteristics of the fabricated device at a current density of 28 kA cm −2 .