GaN Cavity Research Articles

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.

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.

Lasing mode manipulation in a Benz-shaped GaN cavity via the Joule effect of individual Ni wires

Silicon-based gallium nitride lasers are considered potential laser sources for on-chip integration. However, the capability of on-demand lasing output with its reversible and wavelength tunability remains important. Herein, a Benz-shaped GaN cavity is designed and fabricated on a Si substrate and coupled to a Ni metal wire. Under optical pumping, excitation position-related lasing and exciton combination properties of pure GaN cavity are studied systematically. Under electrically driven, joule thermal of Ni metal wire makes it easy to change the temperature of the cavity. And then, we demonstrate a joule heat-induced contactless lasing mode manipulation in the coupled GaN cavity. The driven current, coupling distance, and excitation position influence the wavelength tunable effect. Compared with other positions, the outer ring position has the highest lasing properties and lasing mode tuning abilities. The optimized structures demonstrate clear wavelength tuning and an even mode switch. The thermal reduction of the band gap is identified to account for the modification of the lasing profile, but the thermo-optic effect is non-negligible under a high-driven current.

Low threshold lasing in Al-decorated GaN microdisk on silicon substrate

In spite of the unique advantages of nitride microcavities laser, the optical loss and threshed values are still high in most GaN microdisk laser integrated on silicon substrate. Herein, we fabricated a GaN microdisk cavity pivoted on Si substrate using standard semiconductor process. Al nanoparticles (NPs) with diameter below 100 nm were then decorated on the GaN cavity as surface plasmons (SPs) gain to enhance the lasing performance. SPs coupling properties and photoluminescence (PL) enhancement of Al decorated cavities were studied via excitation power-dependent PL and time-resolved PL measurements. Low optical loss caused by device suspension and SPs coupling induced Purcell Factor enhancement significantly improve the lasing properties. A spontaneous enhancement in PL (by 1.75 folds) along with altered lasing characteristics, including accelerated exciton recombination, reduced lasing threshold value (by 5 folds approximately), slight lasing intensity improvement and redshift of the resonant mode, were observed.

Side-mode suppression in ultraviolet quasi-semicircle microlaser cavity

We report a quasi-semicircle GaN ultraviolet microcavity laser. Poincaré map of the quasi-semicircle cavity shows periodic, quasi-periodic and chaotic optical trajectories. Finite-element-method optical near-field distribution patterns of the periodic optical modes agree with the optical loops in the quasi-semicircle cavity. Based on the numerical results, optically pumped lasing experiment shows that ultraviolet multimode lasing action from different optical resonance modes can be effectively stimulated at low pumping intensity in the quasi-semicircle GaN cavity, and the side-mode suppression phenomenon can be obtained at high pumping intensity. Because the possible period optical loops are quasi-periodic and unstable, the quasi-whispering-gallery mode lasing with higher quality factor prefer to dominate the lasing action under high pumping. The experimental and simulated lasing modes suggest that side-mode suppression phenomenon can be caused by the mode competition between quasi-whispering-gallery modes and other quasi-periodic modes.

Demonstration of polarization control GaN-based micro-cavity lasers using a rigid high-contrast grating reflector

We reported on GaN microcavity (MC) lasers combined with one rigid TiO2 high-contrast grating (HCG) structure as the output mirror. The HCG structure was directly fabricated on the GaN structure without an airgap. The entire MC structure comprised a bottom dielectric distributed Bragg reflector; a GaN cavity; and a top HCG reflector, which was designed to yield high reflectance for transverse magnetic (TM)- or transverse electric (TE)-polarized light. The MC device revealed an operation threshold of approximately 0.79 MW/cm2 when pulsed optical pumping was conducted using the HCG structure at room temperature. The laser emission was TM polarized with a degree of polarization of 99.2% and had a small divergence angle of 14° (full width at half maximum). This laser operation demonstration for the GaN-based MC structure employing an HCG exhibited the advantages of HCGs in semiconductor lasers at wavelengths from green to ultraviolet.

Impact of Silver Surface Morphology on the Wall Plug Efficiency of Blue Vertical Light-Emitting Diodes

Due to merits of good thermal conductivity and vertical current distribution, vertical light-emitting diodes (VLEDs) have been widely studied for many years. However, the wall plug efficiency (WPE) remains limited by the light trapping effect in GaN cavity and difficulties to achieve low-resistance ohmic contacts to p-GaN. As one of the solutions, Ni-/Ag-based mirror layers as ohmic contacts have attracted much attention due to the high work function of Ni and high reflectivity of Ag for blue light. In this paper, a Ni/Ag/Ni/Ag/Ti metal stack was deposited separately on the surface of p-GaN. A high WPE of 45.7% for GaN-based blue VLEDs on Si substrates has been achieved at the mean diameter (69 nm) of Ag grains and root-mean-square (rms) surface roughness (2.06 nm) of Ag mirror layers. There are three impacts on VF: formation of NiO, growth of Ag grain, and increases of Ag oxide. The impacts of surface topography on optical properties are investigated by the Mie scattering theory for Ag grains and surface scattering theory for roughness surface of Ag mirror layer. The conclusion shows that: 1) with growth of Ag grain, the absorption of Ag grains is heightened obviously and the intensity distribution of scattering light with scattering angle is changed from uniformity to irregularity and 2) since the magnitudes of specular reflectance are dramatically larger than the scattering about 1011 times, the specular reflectance of Ag roughness surface totally covers the attribution of scattering and is only decided by rms values and wavelength of incident light.

Lasing Characteristics of a Metal-Coated GaN Shallow Grating Structure at Room Temperature

This study demonstrated a metal-coated GaN laser from a defect cavity with shallow grating structure. With a structure optimization for surface plasmon waves, the shallow grating can support a high quality factor (Q) resonant mode, which leads to a lasing action in the metal-coated GaN cavity. The defect lasing mode was observed at a wavelength of approximately 365 nm at room temperature. The defect grating structure was 840 nm in period, 220 nm in width, and 200 nm in height, and the cavity length of the defect region was 300 nm. The quality factor was 480 and the threshold power density was 4.6 W/cm 2 . Lasing modes were also characterized with FEM simulation. Furthermore, the optical image and far-field angle of the metal-coated defect mode GaN grating structure were analyzed, and the lasing behavior of the defect grating structure was attributed to the defect mode.

Electroluminescence from InGaN quantum dots in a fully monolithic GaN/AlInN cavity

We present for the first time electroluminescence from InGaN quantum dots inside a monolithic nitride based cavity. The structure consists of a 40-fold bottom GaN/Al 0.82In 0.18N distributed Bragg reflector (DBR), a single InGaN quantum dot layer inside a 5 λ n-type (bottom) and p-type (top) doped GaN cavity and a 10-fold GaN/Al 0.82In 0.18N top DBR. Structural properties have been investigated by scanning transmission electron microscopy. Optical reflectivity measurements are in good agreement with calculations which predict a peak reflectivity of 92% and a quality factor of 220. Electroluminescence shows a pronounced emission at the spectral position of the cavity mode near 500 nm.

Structural Effects on Highly Directional Far-Field Emission Patterns of GaN-Based Micro-Cavity Light-Emitting Diodes With Photonic Crystals

This study theoretically and experimentally investigates the highly directional far-field emission patterns of GaN photonic crystal (PhC) micro-cavity light-emitting diodes (MCLEDs) depending on varying structural parameters. Angular-spectra-resolved electroluminescence measurements reveals the behavior of guided-mode extraction which is significantly affected by the structural parameters of GaN PhC MCLEDs, where the GaN cavity thickness decides the extracted guided mode numbers, PhC lattice constant influences the distribution of far-field emission, and PhC hole depth affects the interaction with guided modes. The proposed GaN ultrathin MCLED (uMCLED) with PhC lattice constant of 420 nm and deep hole depth of 250 nm exhibited a maximum output light extraction efficiency of 248% under one-watt input power compared to GaN non-PhC uMCLED and produced a directional far-field emission pattern at half intensity near 17^ . The present results indicate that highly directional light extraction enhancement could contribute to developments of many applications, especially for etendue-limited applications such as pico- projectors.

Strong light-matter coupling in bulk GaN-microcavities with double dielectric mirrors fabricated by two different methods

Two routes for the fabrication of bulk GaN microcavities embedded between two dielectric mirrors are described, and the optical properties of the microcavities thus obtained are compared. In both cases, the GaN active layer is grown by molecular beam epitaxy on (111) Si, allowing use of selective etching to remove the substrate. In the first case, a three period Al0.2Ga0.8N/AlN Bragg mirror followed by a λ/2 GaN cavity are grown directly on the Si. In the second case, a crack-free 2 μm thick GaN layer is grown, and progressively thinned to a final thickness of λ. Both devices work in the strong coupling regime at low temperature, as evidenced by angle-dependent reflectivity or transmission experiments. However, strong light-matter coupling in emission at room temperature is observed only for the second one. This is related to the poor optoelectronic quality of the active layer of the first device, due to its growth only 250 nm above the Si substrate and its related high defect density. The reflectivity spectra of the microcavities are well accounted for by using transfer matrix calculations.

Optical investigation of a hybrid GaN based microcavity with AlInN/GaN bottom and dielectric top distributed Bragg mirror

A hybrid GaN based microcavity was fabricated to investigate light matter coupling. The sample consists of a lattice matched AnInN/GaN bottom and a dielectric upper Bragg mirror. To increase the coupling strength an InGaN/GaN multi quantum well was embedded inside a 3 λ / 2 ( λ = 435 nm ) GaN cavity. The emission characteristics of the structure without top Bragg mirror were analyzed using photoluminescence spectroscopy. Two emission peaks at nearly fixed energy but changing intensity ratio with respect to the position on the sample arise. A detuning of the cavity mode and the excitonic emission into resonant condition is demonstrated by scanning across the wafer and temperature dependent measurements at about 50 K. Angle resolved photoluminescence investigations at the full hybrid structure exhibit two emission peaks. Predominantly excitonic behavior was found for the high and photonic-like behavior for the low energy peak. No clear anticrossing behavior was observed.

Microstructural characterisation of a prototype layer structure for a GaN-based photonic crystal cavity

A semiconductor multilayer consisting of an n-type GaN/sapphire pseudo-substrate with a double sacrificial layer (20 nm of InGaN and 20 nm of InAlN) and a GaN cavity structure on top incorporating an InGaN quantum dot active layer was grown. Atomic force microscopy (AFM) was used to measure the morphology of the upper surface of each layer of similar test structures and to assess the morphological evolution of the full structure during its growth. Transmission electron microscopy (TEM) was also used to assess defect incorporation into the structure. AFM showed that the incorporation of a double sacrificial layer was observed to have no detrimental affect on either the formation of the quantum dots, or the morphology of the top GaN capping layer. TEM highlighted the occurrence of threading dislocations propagating through the sacrificial layers into the capping GaN layer, but did not resolve the occurrence of defect generation in the sacrificial layers.

Theoretical analysis of effect of temperature on threshold parameters and field intensity in GaN material based heterostructure

Temperature dependent analysis to achieve better performance by reducing threshold current requirements and field intensity has been carried out for GaN/AlGaN heterostructure lasers. The mirror loss in the GaN cavity has been obtained as a function of temperature and cavity length. The quantum efficiency has been deduced for different values of cavity length. Dependence of recombination rate on band gap and temperature has been investigated. Threshold current density has been deduced for GaN lasers and effect of temperature on it has been investigated. The near field intensity analysis has been carried out at different temperatures for 10% aluminum mole fraction in GaN/AlGaN heterostructure lasers. Furthermore, the effective index and FWHM of near field has been investigated as a function of temperature. It has been deduced from our analysis that temperature has a dominant effect on the threshold conditions and near field intensity in the wide band gap GaN based lasers.

Crack-free fully epitaxial nitride microcavity with AlGaN∕GaN distributed Bragg reflectors and InGaN∕GaN quantum wells

Using metal-organic chemical vapor deposition, we have fabricated fully epitaxial nitride microcavties with AlGaN∕GaN distributed Bragg reflectors and InGaN quantum wells as the light emitter. To solve the problem of cracking, a thin AlN anticracking layer was used. The samples were characterized using transmission electron microscope, reflectivity spectroscopy, and photoluminescence spectroscopy. A cavity quality factor of 200 was obtained and the spontaneous emission of cavity mode was measured from a 1λ GaN microcavity, with 40-pair Al0.24Ga0.76N∕GaN distributed Bragg reflectors as the bottom and top reflectors and three period In0.10Ga0.90N∕GaN quantum wells in the GaN cavity layer.

Room-temperature polariton luminescence from a bulk GaN microcavity

We report strong exciton-photon coupling at room temperature in a hybrid high quality bulk $3\ensuremath{\lambda}∕2$ GaN cavity with a bottom lattice-matched $\mathrm{Al}\mathrm{In}\mathrm{N}∕\mathrm{Al}\mathrm{Ga}\mathrm{N}$ distributed Bragg reflector through angle-resolved polarized photoluminescence (PL). Coupling of the optically active free excitons (${X}_{A}$, ${X}_{B}$, and ${X}_{C}$) to the cavity mode is demonstrated, with their contribution to the PL spectra varying with polarization. Under TE polarization, exciton oscillator strengths for ${X}_{A}$ and ${X}_{B}$ are about one order of magnitude larger than in bulk GaAs. Photoluminescence exhibits a strong bottleneck effect despite its thermal lineshape.

Crack-free fully epitaxial nitride microcavity using highly reflective AlInN∕GaN Bragg mirrors

We report the growth over 2 in. sapphire substrates of crack-free fully epitaxial nitride-based microcavities using two highly reflective lattice-matched AlInN∕GaN distributed Bragg reflectors (DBRs). The optical cavity is formed by an empty 3λ∕2 GaN cavity surrounded by AlInN∕GaN DBRs with reflectivities close to 99%. Reflectivity and transmission measurements were carried out on these structures, which exhibit a stopband of 28 nm. The cavity mode is clearly resolved with a linewidth of 2.3 nm. These results demonstrate that the AlInN∕GaN system is very promising for the achievement of strong light–matter interaction and the fabrication of nitride-based vertical cavity surface emitting lasers.

Determination of AlxGa1xN refractive indexes at low and room temperature, in the 300–600 nm range, for the optimisation of GaN-based microcavities

AlxGa1–xN alloys are deposited on (111)Si substrate by molecular beam epitaxy. The Al composition is deduced from energy dispersive X-ray spectroscopy and photoluminescence experiments. The refractive index n and the extinction coefficient k, at room temperature, in the 300–600 nm range, are extracted from a two-stage procedure based on spectroscopic ellipsometry and reflectivity measurements. In the transparent region, the refractive index is determined as a function of wavelength, using a Sellmeier law. The accurate knowledge of the later is essential for the fabrication of optimised GaN-based microcavities. For this purpose, two simulations of microcavities with nitride-based distributed Bragg reflectors are presented from the refractive indexes determined in this work at 5 K. The first one is constituted by a (λ/2) GaN cavity embedded between two Al0.19Ga0.81N/Al0.675Ga0.325N distributed Bragg reflectors. In the second one, a lower aluminium composition (0.47) is used in place of (0.675) in order to reduce the strain effects due to the difference between the lattice parameters

Modelling and Spectroscopy of GaN Microcavities

Two GaN microcavities have been grown on Si(111) substrates by molecular beam epitaxy. The first one is constituted of a (λ/2) GaN cavity with 12 (λ/4) AlN/Al0.20Ga0.80N bilayers as the top mirror, the Si substrate acting as the bottom mirror. The second one is a (2λ) GaN cavity sandwiched between 13 and 15 AlN/Al0.20Ga0.80N periods, respectively, for the top and bottom mirrors. Reflectivity and photoluminescence experiments were carried out under normal incidence at low temperature (T = 5 K). The reflectivity spectra reveal a cavity mode whose linewidth is about 30 meV. Our experiments demonstrate that the cavity mode controls the GaN emission. The modelling of the luminescence spectra as a function of the cavity resonance energy allows to estimate the spectral broadening of the GaN emission line to about 50 meV; this value seems to be responsible for the non-observation of the strong coupling.