GaN-based Laser Research Articles

On the formation of cleaved mirror facets of GaN-based laser diodes—A comparative study of diamond-tip edge-scribing and laser scribing

The formation of cleaved mirror facets of GaN-based laser diodes on c-plane GaN substrates is investigated using either diamond-tip edge-scribing or laser scribing with a skip-and-scribe method. The yield of properly cleaved facets was significantly improved to around 80% when using the laser skip-and-scribe method instead of diamond scribing. In addition, the cleavage planes of laser scribed samples showed fewer terraces than those of diamond scribed samples. The performance of broad area laser diodes with proper facet quality is shown to be independent of the scribing methods studied.

Comparative study of the differential resistance of GaAs- and GaN-based laser diodes

The differential resistance curves of GaAs- and GaN-based laser diodes (LDs) are experimentally and numerically investigated. It is found that the dependence of the differential resistance upon the injection current differs in the GaAs- and GaN-based LDs mainly in two aspects. The first is the kink polarity of the differential resistance in the vicinity of the threshold current, and the second is the behavior of the differential resistance curve beyond the threshold current. Self-consistent calculation results suggest that the LD kink and its polarity are determined by the superposition effects of the n-side, active and p-side regions of LDs. It is found that this kink mainly differs in the differential resistance curves of the active region, while the difference in the behavior of the differential resistance curve after the lasing threshold is ascribed to a resistance change in the p-side region caused by a reduced ideality factor.

Investigation of rapid degradation in GaN-based blue laser diodes

Investigation of the degradation modes in GaN-based blue laser diodes grown by metal organic chemical vapor deposition (MOCVD) is carried out. Early failure of the LDs happened at the initial stage of the aging. After analysis of the electrical and thermal characteristics, local degradation of the active region is observed. Further investigation on the microstructures of the local regions shows that the early failure of the LDs is due to the local structure damage with the formation of gallium metal.

Room-temperature CW operation of a nitride-based vertical-cavity surface-emitting laser using thick GaInN quantum wells

We demonstrated a room-temperature (RT) continuous-wave (CW) operation of a GaN-based vertical-cavity surface-emitting laser (VCSEL) using a thick GaInN quantum well (QW) active region and an AlInN/GaN distributed Bragg reflector. We first investigated the following two characteristics of a 6 nm GaInN 5 QWs active region in light-emitting diode (LED) structures. The light output power at a high current density (∼10 kA/cm2) from the 6 nm GaInN 5 QWs was the same or even higher than that from standard 3 nm 5 QWs. In addition, we found that hole injection into the farthest QW from a p-layer was sufficient. We then demonstrated a GaN-based VCSEL with the 6 nm 5 QWs, resulting in the optical confinement factor of 3.5%. The threshold current density under CW operation at RT was 7.5 kA/cm2 with a narrow (0.4 nm) emission spectrum of 413.5 nm peak wavelength.

Precision UV laser scribing for cleaving mirror facets of GaN-based laser diodes

Laser scribing with a nanosecond-pulsed UV laser operating at 355 nm was used to create precise perforation for die separation of GaN-based laser diodes. Machining depth of single- and multiple-pass scribing was investigated. For pulse energies between 1 and 45 µJ at a pulse repetition frequency of 20 kHz and single scan at 100 mm/min, scribe depths from 15 to 180 µm were obtained. Processing parameters were adjusted to minimize the formation of microcracks due to laser-induced local heating. By using the laser skip-and-scribe technique, the propagation of the cleavage plane could be controlled, irregular breaking could be minimized, and die yield could be improved. Smooth mirror facets with low density of terraces were formed by cleaving. In the vicinity of the laser-treated zone, no detrimental effects on the crystal quality of the multi-quantum wells could be detected by cathodoluminescence. The electro-optical characteristics of broad-area laser diodes fabricated by the laser-assisted process were similar to the ones fabricated using the conventional diamond-tip edge-scribing technique that suffers from low die yield. Our results demonstrate that nanosecond-pulsed UV laser scribing followed by cleaving is a powerful technique for the formation of mirror facets of GaN-based laser diodes.

Differential resistance of GaN-based laser diodes with and without polarization effect.

In this paper, we used numerical calculation and simulation to investigate the differential resistance of GaN-based laser diodes (LDs) with and without polarization effect. We confirmed the existence of a kink at the vicinity of threshold current in the differential resistance curve of GaN-based LDs and found that the kink polarity can be reversed dependent on the polarization effect. The serial parasitic diodes should be included in the theoretical analysis of the equivalent circuit of the LD devices. We determined that the superposition effects of the n-side, active, and p-side regions of the LDs caused the kink and its polarity. We also found that the differential resistance before and after the threshold was dominated by the p-side region and its gradual reduction is related to an electron overflow into p-side. Finally, we studied the effects of cavity facet reflectivity on the kink.

Index-Antiguiding in Narrow-Ridge GaN-Based Laser Diodes Investigated by Measurements of the Current-Dependent Gain and Index Spectra and by Self-Consistent Simulation

The threshold current density of narrow (1.5 {\mu}m) ridge-waveguide InGaN multi-quantum-well laser diodes, as well as the shape of their lateral far-field patterns, strongly depend on the etch depth of the ridge waveguide. Both effects can be attributed to strong index-antiguiding. A value of the antiguiding factor R = 10 is experimentally determined near threshold by measurements of the current-dependent gain and refractive index spectra. The device performances are simulated self-consistently solving the Schr\"odinger-Poisson equations and the equations for charge transport and waveguiding. Assuming a carrier-induced index change which matches the experimentally determined antiguiding factor, both the measured high threshold current and the shape of the far-field pattern of lasers with shallow ridges can be reproduced theoretically.

Generation of a 2.2 nJ picosecond optical pulse with blue-violet wavelength using a GaInN master oscillator power amplifier

We report the generation of a picosecond optical pulse with 2.2 nJ pulse energy at blue-violet wavelengths using a GaN-based mode-locked laser diode (MLLD) and a semiconductor optical amplifier (SOA). The picosecond optical pulse generated by MLLD at a frequency of 812 MHz was amplified effectively by SOA. We optimized SOA with a widely flared waveguide structure to generate a high optical pulse energy.

Kinetics of catastrophic optical damage in GaN-based diode lasers

Catastrophic optical damage (COD) in 450 nm emitting InGaN/GaN diode lasers is artificially provoked by applying single sub-microsecond current pulses of increasing amplitude. Studying a batch of devices in which COD does not represent the main degradation mode, we find that COD is a ‘hot’ process. It becomes re-ignited in subsequent pulses. During the process, the spatial filamentation changes abruptly and the outer appearance of the damage pattern is predominantly created within the initial pulse. The process can cause material ejection out of the front facet as shown by thermography.

Room-temperature continuous-wave operation of GaN-based vertical-cavity surface-emitting lasers fabricated using epitaxial lateral overgrowth

We have successfully demonstrated the room-temperature continuous-wave operation of GaN-based vertical-cavity surface-emitting lasers (VCSELs) with all-dielectric reflectors, which were fabricated using epitaxial lateral overgrowth. The VCSELs exhibited a threshold current of 8 mA and a threshold voltage of 4.5 V at a lasing wavelength of 446 nm. The maximum output power was 0.9 mW for an 8-µm-diameter current aperture, which was made possible because of the high thermal conductivity of the GaN substrate.

On the importance of cavity-length and heat dissipation in GaN-based vertical-cavity surface-emitting lasers.

Cavity-length dependence of the property of optically pumped GaN-based vertical-cavity surface-emitting lasers (VCSELs) with two dielectric distributed Bragg reflectors was investigated. The cavity lengths were well controlled by employing etching with inductively coupled plasma and chemical mechanical polishing. It was found that the lasing characteristics including threshold, slope efficiency and spontaneous emission coupling factor were substantially improved with reducing the cavity length. In comparison with the device pumped by a 400 nm pulsed laser, the lasing spectrum was featured by a red shift and simultaneous broadening with increasing the pumping energy of a 355 nm pulsed laser. Moreover, the lasing threshold was much higher when pumped by a 355 nm pulsed laser. These were explained by taking into account of the significant heating effect under 355 nm pumping. Our results demonstrate that a short cavity length and good heat-dissipation are essential to GaN-based VCSELs.

Demonstration of GaN‐based near UV Laser Diodes

Laser Technik JournalVolume 12, Issue 2 p. 16-16 CommunicationsFree Access Demonstration of GaN-based near UV Laser Diodes Realization of cw-operating LDs with emission wavelengths near 386 nm for spectroscopy, metrology, and lithography First published: 01 April 2015 https://doi.org/10.1002/latj.201590018AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume12, Issue2April 2015Pages 16-16 RelatedInformation

The research on temperature distribution of GaN-based blue laser diode

Blue GaN-based laser diodes (LDs) have been characterized by thermal infrared imaging and transient thermal technique to obtain temperature distributions along the optical resonant cavity under continuous-wave (CW) operation. The highest temperature occurred at the emitting facet and the second higher temperature occurred in the other reflecting facet under injection current I=100mA with platform temperature of 308.15K. The results are attributed to the nonradiative recombination in the active region and the reabsorption of laser light inside the cavity. And due to the different of optical flow density, the highest temperature appears in emitting facet. Based on diode forward voltage with temperature, transient thermal technique is used to obtain the temperature distribution along the top-down direction of LDs by the structure function method. The thermal resistance from chip to case is 47.3K/W. The chip’s thermal resistance is the main contribution.

Longitudinal modes evolution of a GaN-based blue laser diode

Abstract Spectral emission of a single-transverse Fabry–Perot GaN-based blue laser diode was experimentally investigated to study the evolution of its longitudinal modes. A 0.003-nm resolution spectrometer was employed to detect and record emission spectra of the laser diode in the wavelength range between 440 and 450 nm as a function of the operating current and temperature. The stability of the laser was investigated over continuous 14 h by monitoring variations in emitted power and central wavelength and found to exhibit excellent stability. The longitudinal mode spacing of 0.0548 nm was found to agree with corresponding calculated mode spacing. The longitudinal modes were observed to shift at rates of 0.0045 nm/mA and 0.0154 nm/°C toward longer wavelengths. Similarly, the gain was observed to shift towards longer wavelengths but with a rate of 0.0432 nm/°C.

Effects of Lattice Types on GaN-Based Photonic Crystal Surface-Emitting Lasers

GaN-based photonic crystal (PC) surface emitting lasers with different lattice types including hexagonal, square, and honeycomb lattice were fabricated and characterized. Laser characteristics including threshold conditions, lasing spectra, polarization, and divergence angle of devices with different lattice have been studied and discussed. The threshold energy density of the device with honeycomb, hexagonal, and square lattice was measured to be approximately 1.6, 2.3, and 3.8 mJ/cm2 at room temperature, showing superior characteristics of applying honeycomb as PC lattice patterns. Moreover, the experimental results were matched well to the theoretical prediction using the multiple scattering method. Finally, the laser characteristics of devices with three lattice types such as divergence angle and degree of polarization (DOP) were analyzed and discussed. The device with honeycomb lattice shows low threshold, high DOP of 86%, and low divergence angle of 1.3° among three lattice types, demonstrating its potential in the future surface emitting laser sources.

Effect of quantum well non-uniformities on lasing threshold, linewidth, and lateral near field filamentation in violet (Al,In)GaN laser diodes

The lateral near field patterns and filamentation effects of gain guided broad area (Al,In)GaN-based laser diodes emitting around 415 nm have been investigated. Diodes from the same laser bar, which are close to each other, show nearly the same number and widths of filaments. Comparison of different bars, which are from the same wafer but further apart from each other, reveals that a higher number of filaments correlates with a higher laser threshold and broader spectral linewidth. Cathodoluminescence mappings at 80 K show strong variations of the quantum well band gap and hence of the emission wavelength for the bars with strong filamentation. These observations confirm previous theoretical predictions stating that large band gap fluctuations increase the threshold current and spectral linewidth. Furthermore, filamentation is enhanced as well because of a reduction of the carrier diffusion length.

Nano-optical analysis of GaN-based diode lasers

Near-field scanning optical microscopy (NSOM) is applied for analyzing GaN-based diode lasers. The measurements are carried out at the front facets of standard devices without any additional preparation. Four different schemes for luminescence and photocurrent detection are applied. The results allow for a detailed analysis of the epitaxial layer sequence, the waveguide mode, the impact of defect absorption, and efficiencies of carrier transfer into the quantum well. Moreover, the effective potential profile as formed by both layer structure and doping profile is imaged. Features being spatially separated by only 30 nm are safely resolved. Our results pave the way towards non-destructive nanoscopic analysis of wide-bandgap optoelectronic devices.

GaN-based vertical-cavity laser performance improvements using tunnel-junction-cascaded active regions

This Letter investigates the output power enhancement achieved by tunnel junction insertion into the InGaN multi-quantum well (MQW) active region of a 410 nm vertical-cavity surface-emitting laser which enables the repeated use of carriers for light generation (carrier recycling). While the number of quantum wells remains unchanged, the tunnel junction eliminates absorption caused by the non-uniform MQW carrier distribution. The thermal resistance drops and the excess bias lead to a surprisingly small rise in self-heating.

Fabrication and characteristics of a GaN-based microcavity laser with shallow etched mesa

In this work, we have developed a simple GaN-based microcavity (MC) with an intracavity shallow etched mesa. The textured GaN-based MC incorporated two high-reflectivity dielectric Bragg mirrors and an InGaN/GaN multiple quantum well with a shallow etched mesa as an optical confined structure. Lasing and transverse optical confinement characteristics have been verified by measuring devices with different mesa diameters. A quality factor (Q) of 2600 and a threshold energy of 30 nJ have been observed in a 10-µm-diameter device. Such a cavity structure could be implanted into electrically pumped GaN vertical-cavity surface-emitting lasers for supporting efficient transverse confinement.

Stimulated emission in GaN-based laser diodes far below the threshold region

We identify that the stimulated emission of GaN laser diodes (LDs) emerges far below the traditionally recognized threshold from both optical and electrical experiments. Below the threshold, the linear-polarized stimulated emission has been the dominating part of overall emission and closely related to resonant cavity. Its intensity increases super linearly with current while that of spontaneous emission increases almost linearly. Moreover, the separation of quasi-Fermi levels of electrons and holes across the active region has already exceeded the photon emission energy, namely, realized the population-inversion.