GaN-based Vertical-cavity Surface-emitting Lasers Research Articles

Enhanced performance of GaN based VCSELs through graded electron-blocking layer design

Electron leakage from the active region to the p-type region restricts the performance of GaN-based Vertical-Cavity Surface-Emitting Lasers (VCSELs). AlGaN EBL can decrease the leakage current, but also raises the hole injection barrier and reduce the hole injection efficiency. Then, it is important to design EBL structures that can enhance both electron blocking and hole injection. In this study, VCSEL devices with three different EBL, basic structure Al0.6Ga0.4N EBL labeled as Device A, newly proposed 16 nm thick Al0.75–0.80Ga0.25–0.20N EBL labeled as Device B, and 18 nm thick Al0.75–0.80Ga0.25–0.20N EBL labeled as Device C, respectively, are designed and analyzed using PICS 3D simulation. Device A represents a basic VCSEL structure, while Device C incorporates a graded electron-blocking layer (EBL) with adjusted thickness. Through simulation results, it is observed that the introduction of a graded EBL in Device C leads to significant performance enhancements compared to the basic structure of Device A. Specifically, the graded EBL effectively reduces band bending and increases the electron barrier height, thereby improving carrier confinement and reducing electron leakage. Additionally, the utilize of a Graded structure in Device C aids in strain relief at the layout between the QB and EBL, resulting in improved electron-blocking capability and potentially enhanced hole transport characteristics. These findings underscore the importance of EBL grading in optimizing the performance of VCSELs, highlighting its potential for advancing the efficiency and functionality of these semiconductor devices. Power of the device C is being improved upto 4.16%, similarly the conduction band barrier height is improved upto 26.6% which is beneficial for better VCSEL performance as it enhances electron confinement in the active region, leading to increased efficiency and reduced carrier leakage and valance band barrier height decreases upto 6.52% and threshold current is decreased upto 4.8% so if valence band barrier height decrease the hole injection efficiency increases and if threshold current decreases the emitting power of the device increase.

Over 20% wall plug efficiency of on-wafer GaN-based vertical-cavity surface-emitting laser

We demonstrated an over 20% wall plug efficiency of an on-wafer GaN-based vertical-cavity surface-emitting laser (VCSEL) with a 5 μm aperture. The cavity length of the VCSEL was 4λ optical length, containing a 3.7λ part of GaN-based layers controlled with an in situ reflectivity spectra measurement and a 0.3λ part of an ITO electrode and a Nb2O5 spacer layer adjusted with an ex situ measurement. In order to calibrate the thickness of ITO and N2O5, we directly evaluated resonance wavelength shifts of a 4λ GaN cavity test structure with additional ITO or N2O5 depositions on the cavity. We then fabricated GaN-based VCSELs with various aperture sizes from 5 to 20 μm by implementing the tuned ITO electrode and the tuned N2O5 spacer layer into the 4λ cavity of the VCSELs. The GaN-based VCSEL with an 8 μm aperture showed a light output power of 13.1 mW and an emission wavelength of 417.7 nm, which was only a 0.3 nm away from a designed wavelength.

Demonstration of III-nitride vertical-cavity surface-emitting lasers with a topside dielectric curved mirror

We report long cavity (65λ) GaN-based vertical-cavity surface-emitting lasers (VCSELs) with a topside dielectric concave mirror, an ion implanted current aperture, and a bottomside nanoporous GaN distributed Bragg reflector. Under pulsed operation, a VCSEL with a 10 μm aperture and a curved mirror with a radius of curvature of 120 μm had a threshold current density of 14 kA cm−2, and a maximum output power of 370 μW for a lasing mode at 404.5 nm. The longitudinal performance has a side-mode suppression ratio of 30 dB up to a current density of approximately 40 kA cm−2. Multiple transverse mode profiles are observed across several devices.

Impacts of SiO2-Buried Structure on Performances of GaN-Based Vertical-Cavity Surface-Emitting Lasers

Impacts of SiO₂-Buried Structure on Performances of GaN-Based Vertical-Cavity Surface-Emitting Lasers

Simulation of performance enhancement of GaN-based VCSELs by composition gradient InGaN last-quantum barrier

Electron leakage in the active region decreases the internal quantum efficiency and impedes the performance of gallium nitride (GaN)-based vertical-cavity surface-emitting lasers (VCSELs). In this study, we propose a novel InGaN last-quantum barrier (LQB) structure with gradient indium (In) composition, and the device performance was simulated by the commercial software PICS3D. Compared with the device with conventional GaN LQB, the electron leakage is greatly reduced and the hole injection efficiency is also improved by the graded LQB structure. Consequently, the threshold current is reduced by 44%, and output power is increased by 392% in GaN-based VCSEL based on composition gradient InGaN LQB. The composition gradient InGaN can also allow us to increase the thickness of the LQB in epitaxy without degrading the carrier injection efficiency due to the reduced polarization in the LQB. The results of this study suggest that the composition gradient InGaN LQB is promising for the realization of high-performance GaN-based VCSELs.

Current spreading structure of GaN-based vertical-cavity surface-emitting lasers.

Indium tin oxide (ITO) is often used as a current spreading layer in the GaN-based vertical-cavity surface-emitting lasers (VCSELs). However, the absorption coefficient of ITO is significant, which reduces the laser output power, raises the threshold, and makes VCSELs hardly lase in the ultraviolet range. To find a transparent conductive structure that can replace ITO, we propose a periodic p-AlGaN/u-GaN/p-GaN structure. In the simulation of light-emitting diodes, the optimized parameter is obtained with multi-period 10 nm p-Al0.1Ga0.9N/2 nm u-GaN/8 nm p-GaN combined with n-GaN/n-Al0.2Ga0.8N in the n region. Applying the structure to 435 nm VCSELs and comparing it to a common VCSEL with the ITO current spreading layer, it can be found that the new structure reduces the threshold from 9.17 to 3.06 kA/cm2. The laser power increases from 1.33 to 15.4 mW. The optimized structure has a high laser power and a lower threshold, which can be used in future investigations.

Effects of different current confinement layers in GaN-based VCSELs

For GaN-based vertical-cavity surface-emitting lasers (VCSELs), a suitable current confinement layer is essential for high-performance devices. The effect of different current confinement layers, including SiO2, AlN, and diamond, on the performance of GaN-based VCSELs was compared through simulation. The devices’ heat dissipation and current confinement characteristics were analyzed based on the electro-opto-thermal model. Considering thermal management, the diamond was a better candidate under high injected current. Benefiting from the excellent heat dissipation, the device with diamond shows a significant improvement in output power and the thermal roll-over current. This work gives a superior option for the current confinement layer and can be helpful for future design and fabrication of high-power GaN-based VCSELs.

Long-Cavity M-Plane GaN-Based Vertical-Cavity Surface-Emitting Lasers with a Topside Monolithic Curved Mirror

We report long-cavity (60.5 λ) GaN-based vertical-cavity surface-emitting lasers with a topside monolithic GaN concave mirror, a buried tunnel junction current aperture, and a bottomside nanoporous GaN distributed Bragg reflector. Under pulsed operation, a VCSEL with a 9 µm aperture had a threshold current density of 6.6 kA/cm2, a differential efficiency of 0.7%, and a maximum output power of 290 µW for a lasing mode at 411 nm and a divergence angle of 8.4°. Under CW operation, the threshold current density increased to 7.3 kA/cm2, the differential efficiency decreased to 0.4%, and a peak output power of 130 µW was reached at a current density of 23 kA/cm2.

In situ cavity length control of GaN-based vertical-cavity surface-emitting lasers with in situ reflectivity spectra measurements

We developed an in situ cavity length control of GaN-based vertical-cavity surface-emitting lasers (VCSELs) with an in situ reflectivity spectra measurement. Firstly, a temperature dependence of a center wavelength of a 40-pair AlInN/GaN DBR was investigated, resulting in a 23 nm redshift from RT to GaN growth temperature of 1100 °C. Secondly, a periodic oscillation of a reflectivity intensity during the GaN cavity growth on the DBR was clearly observed, providing precise cavity length information. Thirdly, based on both the results, we performed the in situ GaN cavity length control, showing an accuracy within a 0.5% error. We finally demonstrated the in situ cavity length control of an actual GaN-based VCSEL structure containing n-layers, a GaInN active region, and p-layers by selecting different resonance wavelengths along with the corresponding growth temperatures for the layers.

Influence of anti-phase surface relief structure on optical mode and laser output power for 450 nm GaN-based VCSELs

For GaN-based vertical-cavity surface-emitting lasers (VCSELs), single-mode output and high power are difficult to be achieved simultaneously. In this report, we propose increasing the output power while taking the optical field into account by engineering the anti-phase surface relief structure and current aperture size for GaN-based VCSELs. We find that the proposed anti-phase surface relief structure helps to get Gaussian-shaped near-field and far-field patterns for VCSELs. However, such structure suffers from mirror loss and causes decreased laser power. Hence, the current injection aperture size has to be reduced so that the overlap level between the carrier profile and the anti-phase surface relief region can be decreased. Therefore, the VCSEL structure with enhanced laser power and dominating fundamental laser mode can be finally designed.

Highly efficient operation and uniform characteristics of curved mirror vertical-cavity surface-emitting lasers

This study obtained highly uniform and efficient GaN-based vertical-cavity surface-emitting lasers with curved mirrors from a single wafer. The average threshold current (I th) and the optical output power (P max) of 14 chips measured up to 7.0 mA were 0.64 mA and 4.5 mW, respectively. The standard deviations of I th and P max were 6.7% and 5.1%, respectively. Additionally, the best chip showed maximum values of wall plug efficiency and output power of 13.4% and 7.6 mW, respectively, at 5.2 mA and 12.8 mA operating currents.

Narrow Emission of Blue GaN-Based Vertical-Cavity Surface-Emitting Lasers With a Curved Mirror

We report a narrow divergence angle in a blue gallium-nitride-based visible vertical-cavity surface-emitting laser (GaN-VCSEL) with a curved mirror. This device exhibited continuous-wave operation at a wavelength of 447.9 nm. The full-width at half-maximum divergence angle of the 8 μm-aperture VCSEL was 3.9° when the device was operated at a current of 1.2 × Ith, where Ith is the threshold current. In this type of cavity, the radius of curvature (ROC) of the curved mirror determines the near-field pattern (NPF) and far-field pattern (FFP). The authors fabricated devices having ROC up to 988 μm, which is more than 10 times greater than the ROC of the mirror used in the previously reported structures, allowing this narrow emission. The I–L characteristics showed that the Ith value increased with increasing ROC. The present study investigates the mechanism behind this tendency in detail.

Milliwatt-class Green and Blue GaN-based Vertical-Cavity Surface-Emitting Lasers

Milliwatt-class Green and Blue GaN-based Vertical-Cavity Surface-Emitting Lasers

Quantum barriers with a polarization self-screening effect for GaN-based VCSELs to increase the electron-hole stimulated recombination and output performance

Hole injection is one of the fundamental limitations that affect the lasing power for GaN-based vertical-cavity surface-emitting lasers (VCSELs). In this report, a GaN-based VCSEL with a composition gradient quantum barrier (CGQB) structure is proposed and investigated. The designed InxGa1-xN quantum barrier has a linear gradient level of InN composition along the [0001] orientation, which is effective in reducing the energy band barrier height for holes. Furthermore, the polarization-induced bulk charges that are generated in the proposed quantum barriers can reduce the electric field magnitude in quantum wells, which is known as the polarization self-screening effect. Therefore, the hole injection and the electron-hole stimulated recombination rate can be both enhanced. We also find that although the hole injection can be enhanced and the polarization induced electric field in the quantum wells can be reduced, an increased gradient level of the InN composition for the polarization self-screened quantum barriers is not always favored. The reduced quantum barrier height will redistribute the energy subbands and make peak gain not coupled with the cavity resonance wavelength, which will decrease the lasing power. Hence, to avoid the substantial variation of the subbands, we suggest that the polarization self-screened active region shall possess properly thick quantum wells for maximizing the lasing power. Moreover, the optimized active region design can increase the 3dB frequency.

Monolithic high-index contrast grating mirror for a GaN-based vertical-cavity surface-emitting laser

In this paper, a pulsed electrically pumped GaN-based vertical-cavity surface-emitting laser (VCSEL) with one dielectric distributed Bragg reflector and one n-GaN monolithic high-index contrast grating (MHCG) mirror was demonstrated at room temperature. The reflectance of the n-GaN MHCG and cavity mode behaviors of the VCSEL with MHCG for varying n-GaN thickness, MHCG pattern diameter, and current aperture size were numerically investigated. Measured characteristics of the fabricated device showed that the lasing action started at an injection current of 10.2 mA, corresponding to a current density of about 15.1 kA/ cm 2 . Above threshold, the measured slope efficiency was 6.2 × 10 − 3 W / A , and the output power was 0.13 mW at 30 mA. Moreover, the measured lasing peak occurring at 403.4 nm and the longitudinal mode spacing of 5.6 nm were in good agreement with simulations. The incorporation of an n-GaN MHCG mirror not only greatly simplified the fabrication but also substantially improved the lasing characteristics in comparison to the previous work applying TiO 2 HCG mirrors.

Dynamic characteristics and device degradation of GaN-based vertical-cavity surface-emitting laser with an AlInN/GaN distributed Bragg reflector

The dynamic characteristics of GaN-based vertical-cavity surface-emitting laser (VCSEL) with an AlInN/GaN semiconductor distributed Bragg reflector has been reported for the first time. The max dynamic lasing frequency of 1.1 GHz with a wavelength of 415 nm and a threshold current of 7 mA has been demonstrated. Moreover, at the pulse lasing operation, both the steep lasing waveforms and the tailing spontaneous emission waveforms were observed in driven with the square pulse waveforms. The dynamic lasing frequency driven by the sine waveform seems to be related to this steep lasing waveform at the square pulse operation. During these dynamic measurements, the lasing characteristics of the VCSEL were gradually degraded, and the lasing power is decreased by −7 dB at the same bias condition after several hours.

Aperture diameter dependences in GaN-based vertical-cavity surface-emitting lasers with nano-height cylindrical waveguide formed by BCl3 dry etching

We demonstrated GaN-based vertical-cavity surface emitting lasers (VCSELs) with 5–30 μm wide nano-height cylindrical waveguide formed by BCl3 etching. A 5 nm-depth etching with BCl3 showed the most efficient current blocking at the interface of the etched p++-GaN and an ITO electrode among the cases with BCl3, Ar, or O2, which could be due to not only etching damages but also diffused B atoms into the etched surface. While room-temperature continuous-wave operations of the VCSELs with the large apertures were demonstrated, maximum light output power values of the large aperture VCSELs seemed limited by nonuniform current injection and device thermal resistances.

Room-temperature continuous-wave operations of GaN-based vertical-cavity surface-emitting lasers with buried GaInN tunnel junctions

We report GaN-based vertical-cavity surface-emitting lasers (VCSELs) with buried GaInN tunnel junctions (TJs). Under room-temperature continuous-wave operation, the VCSEL with an 8 μm aperture exhibited a low operating voltage of 5.3 V at 10 kA cm−2 and a differential resistance of 110 Ω. In addition, the VCSEL with a 10 μm aperture showed a threshold current of 14.4 mA and a peak output power of 2.0 mW. An influence of an absorption loss at the GaInN TJ on laser performance was estimated using an analysis of a one-dimensional optical intensity profile based on a cross-sectional STEM image.

Cavity designs for nitride VCSELs with dielectric DBRs operating efficiently at different temperatures

This study set out to analyze the performance of modern nitride GaN-based tunnel-junction vertical-cavity surface-emitting lasers (VCSELs) at various ambient and elevated temperatures, with the aid of an advanced self-consistent optical-electrical-recombination-thermal model. Particular attention was given to the impact that both the length of the laser cavities and the mirror reflectivities have on laser operation across the temperature range. The optimal number of pairs in the top Distributed Bragg Reflector (DBR) was determined for stable operation at high temperatures and relatively high output powers. The effect on laser performance of expanding the metallization around the bottom DBR was also investigated.

Improved carrier confinement and stimulated recombination rate in GaN-based vertical-cavity surface-emitting lasers with buried p-AlGaN inversion layer

Abstract A GaN-based vertical cavity surface emitting laser (VCSEL) featuring a buried ring-shape p-Al0.10Ga0.90N inside n-GaN contact layer for lateral electron confinement is proposed. The p-AlGaN layer inserted in n-GaN forms an n-p-n structure, acting as a potential barrier to prevent vertical electron migration outside the aperture of the VCSEL, where optical gain is accumulated. By adjusting the thickness and position of the p-AlGaN layer, electron concentration and stimulated recombination rate in the aperture of the VCSEL increased significantly. Consequently, the output power of VCSEL with buried p-AlGaN layer increases by 57% compared to the conventional VCSEL at an injection current of 10 mA. The detailed mechanism responsible for this enhancement is further explored. This work suggests that the introduction of the buried p-AlGaN layer in VCSEL can provide new line of thought in achieving effective current confinement in the development of high-efficient, low-threshold solid-state lasers.