GaN Distributed Bragg Reflectors Research Articles

Electrochemical etching of n-type GaN in different electrolytes

Nanosized pores were introduced into GaN epitaxial films via electrochemical (EC) etching in strong electrolytes including NaOH, HNO3, NaNO3, and NaCl. The growth behaviors of GaN nanopores show a strong correlation with the electrolyte types under the same applied bias and electrolyte concentration. Specifically, the resulting morphologies (e.g. pore growth direction and pore size) were mainly pertinent to the anions instead of cations. Pore sizes as small as a few nanometers and pore directions between the horizontal and vertical directions can be adjusted by selecting the electrolytes and etching voltages. The depletion model was employed to interpret the carrier flow pathways, and the pore propagation direction can be regarded as the sum of etch rate vectors of intended etching direction and preferential oxidation direction. Finally, the near-infrared nanoporous (NP) GaN distributed Bragg reflectors (DBRs) were manufactured by the EC etching in different electrolytes. Our study points out that the type of anions in electrolyte greatly impacts pore morphologies of EC etched GaN:Si, thereby affecting the future applications.

Design and Simulation of InGaN-Based Red Vertical-Cavity Surface-Emitting Lasers.

We propose a highly polarized vertical-cavity surface-emitting laser (VCSEL) consisting of staggered InGaN multiple quantum wells (MQWs), with the resonance cavity and polarization enabled by a bottom nanoporous (NP) n-GaN distributed Bragg reflectors (DBRs), and top TiO2 high-index contrast gratings (HCGs). Optoelectronic simulations of the 612 nm VCSEL were systematically and numerically investigated. First, we investigated the influences of the NP DBR and HCG geometries on the optical reflectivity. Our results indicate that when there are more than 17 pairs of NP GaN DBRs with 60% air voids, the reflectance can be higher than 99.7%. Furthermore, the zeroth-order reflectivity decreases rapidly when the HCG's period exceeds 518 nm. The optimal ratios of width-to-period (52.86 ± 1.5%) and height-to-period (35.35 ± 0.14%) were identified. The staggered MQW design also resulted in a relatively small blue shift of 5.44 nm in the emission wavelength under a high driving current. Lastly, we investigated the cavity mode wavelength and optical threshold gain of the VCSEL with a finite size of HCG. A large threshold gain difference of approximately 67.4-74% between the 0th and 1st order transverse modes can be obtained. The simulation results in this work provide a guideline for designing red VCSELs with high brightness and efficiency.

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.

Fabrication of InGaN/GaN multiple quantum well embedded on nanoporous GaN mirror under different acid electrolytes

Large-size (2 inch) InGaN/GaN multiple quantum wells (MQW) embedded on nanoporous (NP)-GaN distributed Bragg reflectors (DBR) were fabricated by electrochemical (EC) etching at different acid electrolytes (e.g. HF, oxalic, and HNO3 acids). The sample obtained in HNO3 electrolyte presents the highest reflectance and the strongest photoluminescence (PL) intensity, whereas the sample fabricated by the HF etching has uneven surface, leading to very low reflectance. Compared with the as-deposited sample, the HNO3 etching leads to a 5-fold enhancement of PL intensity of etched sample, probably resulting from two factors which reflection of NP-GaN DBR mirror enhanced the light output, and amplify spontaneous emission (ASE) via a resonant cavity increased PL performance.

Pixelated vertical-cavity surface-emitting laser arrays from colloidal quantum dot films

Pixelated arrays of vertical-cavity surface-emitting lasers from colloidal quantum dot thin films have been demonstrated. CdSe/ZnCdS core/shell structured colloidal quantum dots have been used as laser gain medium. 10 pairs of alternating layers between GaN and nanoporous GaN consisted highly reflective distributed Bragg reflectors, exhibiting more than 99% of reflectivity in the spectral range of 600 nm–650 nm. Close-packed colloidal quantum dot solid thin films were sandwiched between the two GaN/nanoporous GaN distributed Bragg reflectors to achieve well-defined lasing outputs under pulsed optical excitation with the lasing threshold as low as 11.0 μJ/cm2. The resulting CQD-VCSEL devices exhibited a lasing output power of 1.5 μW and a conversion efficiency of 11.4%. Finally, a two-axis scanning micro-mirror was used to complete the 25 (5 by 5) pixelated laser arrays system.

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.

Investigation of InGaN-Based Green Micro-Photonic-Crystal- Light-Emitting-Diodes with Bottom, Nanoporous, Distributed Bragg Reflectors

In this work, an InGaN-based, green micro-photonic crystal-light-emitting-diode (µ-PCLED), which incorporates a nanoporous, GaN-distributed Bragg reflector (DBR) to form a Fabry–Perot (FP) cavity, was fabricated and characterized. Simulations for the µ-PCLED’s optical features were systematically performed and analyzed. Numerical results revealed that the p-GaN photonic crystal (PC) with a filling factor of 0.3 is beneficial for improving the coupling constants of the first- and second-order Bragg diffractions. In addition, based on the product of quantum well (QW) and PC confinement factors, four to six pairs of InGaN QWs should be the preferable design. In order to achieve single-wavelength emission and small full-width at half-maximum (FWHM), the thickness of the n-GaN layer was controlled to be thinner than 920 nm, leading to more than 20 nm wavelength separation between two adjacent FP modes. Experimentally, the fabricated InGaN-based µ-PCLED with a mesa diameter of 30 µm can emit 545 nm green light with FWHM of about 10 nm and negligible blue-shift of about 3 nm in spontaneous emission under the injection current of 1 to 10 mA. Our simulation and experimental results demonstrate that the p-GaN PC design can effectively resolve the wavelength instability issue.

Effect of distributed Bragg reflectors on photoluminescence properties of CH3NH3PbI3 film

The nanoporous (NP) GaN distributed Bragg reflector (DBR) was prepared by using electrochemical etching. Then the NP-GaN DBR was pretreated by using ozone treatment. Atomic force microscopy and X-ray diffraction (XRD) were used to investigate the influence of ozone treatment on the structure of the substrates. The hybrid organic–inorganic CH3NH3PbI3 perovskite films were grown on the NP-GaN DBR and reference substrates by using a one-step solution method. The XRD and field emission scanning electron microscopy test results indicate the high quality of the prepared CH3NH3PbI3 perovskite films. The photoluminescence intensity of the prepared CH3NH3PbI3 perovskite film on the NP-GaN DBR substrate is ~ 3.5 times higher than that of the film on the reference substrate, with a 3.6 nm spectral blue-shift. The enhancement should be contributable to amplify spontaneous emission by resonant cavity, while the blue-shift could be contributable to stoichiometric difference of the films on different substrates.

Nearly Lattice-Matched GaN Distributed Bragg Reflectors with Enhanced Performance.

Heavy silicon-doping in GaN generally causes a rough surface and saturated conductivity, while heavily silicon-doped n++-AlGaN with ≤5% aluminum can maintain an atomically flat surface and exhibit enhanced conductivity. Given this major advantage, we propose using multiple pairs of heavily silicon-doped n++-Al0.01Ga0.99N and undoped GaN instead of widely used multiple pairs of heavily silicon-doped n++-GaN and undoped GaN for the fabrication of a lattice-matched distributed Bragg reflector (DBR) by using an electrochemical (EC) etching technique, where the lattice mismatch between Al0.01Ga0.99N and GaN can be safely ignored. By means of using the EC etching technique, the n++-layers can be converted into nanoporous (NP) layers whilst the undoped GaN remains intact, leading to a significantly high contrast in refractive index between NP-layer and undoped GaN and thus forming a DBR. Our work demonstrates that the NP-Al0.01Ga0.99N/undoped GaN-based DBR exhibits a much smoother surface, enhanced reflectivity and a wider stopband than the NP-GaN/undoped GaN-based DBR. Furthermore, the NP-Al0.01Ga0.99N/undoped GaN-based DBR sample with a large size (up to 1 mm in width) can be obtained, while a standard NP-GaN/undoped GaN-based DBR sample obtained is typically on a scale of a few 100 μm in width. Finally, a series of DBR structures with high performance, ranging from blue to dark yellow, was demonstrated by using multiple pairs of n++-Al0.01Ga0.99N and undoped GaN.

Preparation of large-area, high quality, free-standing GaN distributed Bragg reflectors

In this paper, we propose a strategy for the preparation of large-area, high quality, free-standing nanoporous (NP) GaN distributed Bragg reflector (DBR) membranes via electrochemical (EC) etching followed by annealing in NH3 ambient. In a centimeter-level macroscopic area, the free-standing DBR membranes show smooth and crack-free surface, which are attributed to curvature-driven mass-transport during the annealing. Due to high peak reflectivity (>80%) and low root-mean-square roughness (∼0.934 nm) of the DBR membrane, the separation strategy introduces a paradigm to prepare high quality, free-standing semiconductor membranes.

Eu-doped Lu2O3 epitaxial films with an embedded nanoporous GaN distributed Bragg reflectors

The GaN/nanoporous GaN (NP-GaN) distributed Bragg reflectors (DBRs) with high reflectivity (>95%) at specific wavelengths were designed and fabricated by an electrochemical (EC) etching technique, and then as-grown GaN and DBR were used to grow Eu-doped Lu2O3 high-quality films by Pulsed Laser Deposition (PLD) method. Structural analyses confirmed the thin film grown at 480 °C has highest crystalline quality. The epitaxial relationship between the film and substrate was determined as Lu2O3 (222)∥GaN (0002) with Lu2O3[1‾10]∥GaN[1‾21‾0]. The photoluminescence (PL) intensity of the Eu-doped Lu2O3 film with an embedded DBR substrate was 3.4 times higher than for reference film on as-grown GaN, which should be attributed to in-coupling and out-coupling of the excitation and emission lights. The enhancement effects of light-coupling were explained by multiple reflection model. Hence, this strategy can be used to enhance the optical signal intensity of scintillators by embedding DBR substrates with proper Bragg wavelength.

Enhancement of two-photon-excited fluorescence by distributed Bragg reflectors

Blend films of 4,4,4-tris[3methylphenyl(phenyl)amino]triphenylamine and tris-[3-(3-pyridyl)mesityl] borane were successfully prepared on gallium nitride wafer. Like other two-photon-absorbing (TPA) materials, the blend film has relatively low luminous efficiency. To improve luminous efficiency, the blend film was embedded on nanoporous(NP)-GaN distributed Bragg reflector (DBR) fabricated by electrochemical etching. By comparison, we found that the photoluminescence intensity of the blend film on the DBR is 2.5 times bigger than that of the film on as-grown GaN, which should be contributed to light interference enhancement in the blend film. The results indicate that NP-GaN DBRs are promising candidates for improving luminous efficiency of TPA materials.

Preparation and improved photoelectrochemical properties of InGaN/GaN photoanode with mesoporous GaN Distributed Bragg reflectors

We fabricate the large-area and highly reflective mesoporous GaN (MP-GaN) Distributed Bragg reflectors (DBRs) via an electrochemical etching technology in HNO3 solution for the first time and then use the DBRs as the template in growth of InGaN/GaN structure by the metal organic chemical vapor deposition (MOCVD) technology. The InGaN/GaN structure with MP-GaN DBRs acts as the photoanode during photoelectrochemical (PEC) water splitting experiments in 1 M HBr solution for the first time. Compared to the sample without MP-GaN DBRs, the InGaN/GaN structure with the MP-GaN DBRs presents significantly increased photoluminescence (PL) intensity, which is attributed to light reflection effect of the bottom MP-GaN DBRs and improved internal quantum efficiency (IQE) of the multiple quantum wells (MQWs) layer. More interestingly, the InGaN/GaN structure with the MP-GaN DBRs presents markedly improved PEC properties such as a low onset voltage (−0.61 V vs. RHE), a 4.5-fold enhancement in photoconversion efficiency and good stability.

Fabrication and applications of wafer-scale nanoporous GaN near-infrared distributed Bragg reflectors

The wafer-scale, near-infrared, and nanoporous (NP)-GaN distributed Bragg reflectors (DBRs) were fabricated by using an electrochemical anodization method for the first time. The peak reflectivity of the DBR mirrors on the sapphire substrates is ~95% in the range of 550 nm–1750 nm with different stop-band widths. To explore its potential applications, a lead-free all-inorganic perovskite film with a PL emission in the near-infrared region was grown on a DBR mirror. Compared to the reference perovskite film, the photoluminescence intensity of the perovskite film on the DBR substrate presents more than 4-fold enhancement, which is in agreement with the calculated value. The performance enhancement should be contributable to light-coupling enhancement of emission light. The near-infrared NP-GaN DBRs pave the way for developing a range of perovskite devices for broadband and large-area applications.

Sequential plan-view imaging of sub-surface structures in the transmission electron microscope

Transmission electron microscopy (TEM) is a central technique for the characterisation of materials at the atomic scale. However, it requires the sample to be thin enough to be electron transparent, imposing strict limitations when studying thick structures in plan-view. Here we present a method for sequential plan-view TEM that allows one to image complex structures at various depths. The approach consists of performing an iterative series of front-side ion milling followed by TEM imaging. We show it is possible to image how the sample properties vary with depth up to several microns below the surface, with no degradation of the sample and imaging conditions throughout the experiment. We apply this approach to 3D cavities in mesoporous GaN distributed Bragg reflectors, demonstrating the ability to characterise the morphology of the pores, local crystal features and chemical composition through the multilayer structure. The same workflow can be applied to a variety of complex micron-scale systems which are by nature too thick for standard TEM analysis, and can also be adapted for profiling samples in cross-section.

Tamm plasmons in metal/nanoporous GaN distributed Bragg reflector cavities for active and passive optoelectronics.

We theoretically and experimentally investigate Tamm plasmon (TP) modes in a metal/semiconductor distributed Bragg reflector (DBR) interface. A thin Ag (silver) layer with a thickness (55 nm from simulation) that is optimized to guarantee a low reflectivity at the resonance was deposited on nanoporous GaN DBRs fabricated using electrochemical (EC) etching on freestanding semipolar (2021¯) GaN substrates. The reflectivity spectra of the DBRs are compared before and after the Ag deposition and with that of a blanket Ag layer deposited on GaN. The experimental results indicate the presence of a TP mode at ∼ 454 nm on the structure after the Ag deposition, which is also supported by theoretical calculations using a transfer-matrix algorithm. The results from mode dispersion with energy-momentum reflectance spectroscopy measurements also support the presence of a TP mode at the metal-nanoporous GaN DBR interface. An active medium can also be accommodated within the mode for optoelectronics and photonics. Moreover, the simulation results predict a sensitivity of the TP mode wavelength to the ambient (∼ 4-7 nm shift when changing the ambient within the pores from air with n = 1 to isopropanol n = 1.3), suggesting an application of the nanoporous GaN-based TP structure for optical sensing.

The relationship between the three-dimensional structure of porous GaN distributed Bragg reflectors and their birefringence

Porous GaN distributed Bragg reflectors offer an opportunity to provide the high reflectance, lattice-matched components required for efficient GaN vertical cavity surface emitting lasers. The birefringence of these structures is, therefore, of key interest as it could be used to control the polarization of the emitted light. Here, we present a detailed analysis of the optical birefringence for both laterally etched, patterned structures and self-assembled radial porous structures. We correlate this with the 3D structure of the pores, which we measure through the use of FIB milling and serial block-face SEM imaging. This is a powerful method for imaging the internal nanostructure of the sample and allows the internal pore morphology to be viewed in a reconstruction of any 3D plane. We measure the birefringence of our porous GaN layers as Δn=0.14 with a lower refractive index parallel to the pores (∥) than perpendicular to them (⟂). Using finite element modeling, we accurately reproduce the experimentally observed birefringence trends and find that this can be done by modeling GaN as a perfect dielectric. This indicates that the birefringence arises from the limited width across the pores. This also shows that standard modeling approaches can be used to design porous GaN birefringent devices effectively.

Fabrication and optoelectronic properties of Ga2O3/Eu epitaxial films on nanoporous GaN distributed Bragg reflectors

Nanoporous GaN distributed Bragg reflectors (DBRs) with a high reflectivity (~ 92%), which was fabricated via a doping selective electrochemical etching process, were used to deposit Eu-doped β-Ga2O3 films by using pulsed layer deposition. Structural and chemical composition analyses indicated that the 900 °C-annealed film in air has the best crystalline quality and highest photoluminescence (PL) efficiency. The epitaxial relationship between the β-Ga2O3: Eu film and DBR mirror was Ga2O3 ( $$\overline{2}$$ 01)║GaN (0001) with Ga2O3 [010]║GaN [ $$\overline{1}$$ 2 $$\overline{1}$$ 0]. Compared to the Eu:Ga2O3 film on reference template, the 900 °C-annealed film on the DBR mirror presented a ~ 20-fold enhancement in the PL emission. The performance enhancement was attributed to light-coupling enhancement of the buried DBR mirror. Because of the good electrical properties of the annealed films, the fabricated DBR substrates pave the way for developing a range of rare-earth-doped Ga2O3 optoelectronic devices.

High Responsivity and Wavelength Selectivity of GaN‐Based Resonant Cavity Photodiodes

AbstractThe implementation of blue‐light photodiodes based on InGaN in emerging technologies, such as free‐space visible light communication (VLC), requires transformative approaches toward enhanced performance, miniaturization, and integration beyond current Si‐based technologies. This work reports on the design and realization of high‐performance InGaN‐based resonant cavity photodiodes with high‐reflectivity lateral porous GaN distributed Bragg reflectors. The well‐controlled porosification of GaN on the 2‐inch wafers enables design and fabrication of optical components, unlocking the potential of nitride semiconductors for several applications. These resonant‐cavity‐enhanced photodiodes, which have a 12 nm‐thick optically active region, exhibit a high responsivity (≈0.1 A W−1) to blue‐light even without any externally applied voltage. Furthermore, the device can operate as both an emitter and a detector of visible light at well‐defined wavelengths with spectral overlap between the electroluminescence emission and photocurrent responsivity, meeting the requirement of wavelength selectivity, thermal stability, and low‐power consumption for VLC, with potential for integration of different functionalities, that is, light emission and detection, on a single chip without additional light filters.

Structural characterization of porous GaN distributed Bragg reflectors using x-ray diffraction

Porous GaN distributed Bragg reflectors (DBRs) provide strain-free, high-reflectivity structures with a wide range of applications across nitride optoelectronics. Structural characterization of porous DBRs is currently predominantly achieved by cross-sectional scanning electron microscopy (SEM), which is a destructive process that produces local data and has accuracy limited to around 3% by instrument calibration uncertainty. Here, we show that high-resolution x-ray diffraction (XRD) offers an alternative, nondestructive method for characterizing porous nitride structures. XRD scans of porous GaN DBRs show that despite the constant lattice parameter across the DBR layers, characteristic satellite peaks still arise, which are due to the interference between x-rays reflected from the porous and nonporous layers. By comparing the intensities and positions of the satellite peaks through diffraction patterns simulated from a kinematic model, the structural properties of the porous GaN DBRs can be analyzed. Using our method, we have measured a series of DBRs with stop bands from the blue wavelength region to the IR and compared their structural values with those from SEM data. Our results show that the XRD method offers improvements in the accuracy of determining layer thickness, although uncertainty for the value of porosity remains high. To verify the results gained from the XRD and SEM analysis, we modeled the optical reflectivity using the structural values of both methods. We found that the XRD method offered a better fit to the optical data. XRD, therefore, offers accurate, nondestructive characterization of porous DBR structures based on macroscale measurements and is suitable for full wafer analysis.