Porous GaN Research Articles

GaN nanostructure for hydrogen gas sensor

The growth of heterostructure of n-type GaN/AlN/Si(111) is carried out using the molecular beam epitaxy (MBE) Veeco model Gen II system. The surface morphology of the as-grown GaN sample showed pits on the GaN surface in a ratio small than those found by other research groups. Porous GaN samples were synthesized by an electrochemical etching technique combined with increasing the current density to 75 mA/cm2. The formation of pore structures are of different sizes, the etched surface became hexagonal, and pore structures are confined to a smaller size. The PL results showed greater blue shift luminescence in comparison to results found by other research groups. The reduction in crystallite size is confirmed by an increase in the broadening of XRD spectra. Raman spectra also displayed a strong band at 522 cm−1 from the Si(1 1 1) substrate, and a small band at 301 cm−1. These are due to the acoustic phonons of Si. Two Raman active optical phonons are assigned to h-GaN at 139 cm−1 and 568 cm−1, due to E2 (low) and E2 (high) respectively. The sensitivity of the gas sensor is increased as a function of the hydrogen flow rate and they became much higher compared to the as-grown sample.

Hierarchically porous GaN thin films fabricated using high fluence Ar ion implantation of epitaxial GaN on sapphire

• We show the fabrication of hierarchical pores in GaN by means of Ar ion implantation. • This approach is free of the limitations of anodic etching of porous GaN. • There is no need for uniform current spreading to obtain uniform porosity. • The submission focuses on the material properties not on the implantation process. • We demonstrate different damage behavior for single and double ionized Ar. We show a method to fabricate porous gallium nitride (GaN) films without the need for wet chemical etching nor high donor concentrations in the starting material. The pores in epitaxial GaN films were formed using 200 keV Ar + ion implantation at fluences ranging from 2.5∙10 16 Ar + ∙cm −2 to 10 17 Ar + ∙cm −2 . The properties of the created nanostructures are discussed by means of complementary structural, optical and electrical studies. Electrochemical capacitance of the samples in an aqueous electrolyte is studied to deconvolute information about the surface area available after the implantation. Single and double ionized implantation is compared in the discussion and different damage in the two cases is shown. The results show information complementary to the current models of damage buildup in gallium nitride films upon high fluence irradiation as well as present a means of fabricating on-wafer porous areas for future GaN-based devices. The method overcomes the drawbacks of the commonly applied anodic etching: it does not require a high GaN donor concentrations nor difficult setups for achieving uniform etching. Furthermore, it can be applied locally using appropriate masking of the selected areas with a thick photoresist or other strippable inorganic ion stopping film.

Hollow porous GaN nanofibers gas sensor for superior stability and sub-ppb-level NO2 gas detection

Accurate detection of NO2 gas with low concentration and high stability is of great significance to ecological protection and human health. In this work, hollow porous GaN nanofibers were fabricated by a low-cost solvothermal method and controlled nitridation process. Systematic characterizations results demonstrate large specific surface and abundant adsorpation sites of the GaN nanofibers. Excellent NO2 sensing performances were tested at 200 °C, including the high sensitivity of 22.9 (200 ppm), rapid response time within 11 s, high selectivity, and low detection limit (0.5 ppb). Furthermore, the anti-interference ability, cross-sensitivity characteristics, and long-term stability of the sensor in the actual working environment were also systematically investigated. The surface depletion layer model and the change of the potential barrier were used to explain the enhanced gas-sensing performance. The fabricated GaN sensor offered a promising candidate for real-time monitoring of sub-ppb-level NO2 gas in actual environments.

Assessment of the Enhancement for the Excitation Emission in Porous Gan using Photoluminescence

ABSTRACT: This work aims to assess the enhancement of optical properties for porous GaN nanostructures, which fabricated by Photoelectrochemical etching under different current densities. The changing of optical properties for different samples were investigated by Photoluminescence (PL) spectroscopy. A strong near band-gap-edge emission (NBE) was detected with peak energy 3.40 eV for as-grown and sample etched at 5mA/cm2, while its 3.41 and 3.42 eV for samples etched with 10 and 20mA/cm2 respectively. Also, another peak emission from the sapphire substrate at peak 1.7 eV was observed. The PL peak intensity of the porous samples have increased with increasing the porosity, while the FWHM of the near-band-edge peak was decreased in 5 and 10mA/cm2 samples compared to as-grown non-etched sample, indicated that the pore size decreased with etching current density and porosity. Finally, the change of refractive with porosity was investigated in the porous GaN nanostructure.

Study of Pore Geometry and Dislocations in Porous GaN Based Pseudo-Substrates Using TEM

Nitride based light emitting diodes (LEDs) have demonstrated very high efficiency in the blue and green wavelength ranges. For full color LED based displays, efficient and small sized devices emitting in the three primary colors - red, green and blue are required. While phosphide based red LEDs are highly efficient, high surface recombination velocities limit their scaling to small dimensions. Nitride emitters can be scaled to smaller sizes, however for longer emission wavelength such as yellow, orange, and red, the indium content in the active region of the devices needs to be >30%. Due to the large lattice mismatch between GaN and InN, the needed high indium content introduces a large strain in the structure, often resulting in poor material quality. Recently, a novel technique to mitigate this lattice mismatch induced strain was demonstrated with the use of compliant porous GaN underlayer based substrates. In our previous reports, we have discussed the surface morphology, strain state evolution and device demonstrations using these substrates, however a detailed discussion regarding the pore cross-sections and dislocation propagation for each generation of these porous GaN substrates was yet to be reported. In this work, we will discuss the cross-sectional transmission electron microscopy (TEM) results of these substrates across multiple stages of the substrate technology development.

Fully Relaxed, Crack-Free AlGaN with upto 50% Al Composition Grown on Porous GaN Pseudo-Substrate

Fully relaxed, crack free, smooth AlxGa1−xN layers with up to 50% Al composition were demonstrated on pseudo-substrates composed of dense arrays of 10 × 10 µm2 compliant porous GaN-on-porous-GaN tiles. The AlGaN layers were grown in steps for a total of 1.3 µm. The growth conditions necessary to demonstrate high quality films at higher Al compositions also suppressed any sidewall growth.

Green edge emitting lasers with porous GaN cladding.

GaN lasers with green emission wavelength at λ = 510 nm have been fabricated using novel nano-porous GaN cladding under pulsed electrical injection. The low slope efficiency of 0.13 W/A and high threshold current density of 14 kA/cm2 are related to a combination of poor injection efficiency and high loss, analyzed by the independent characterization methods of variable stripe length and segmented contacts. Continuous wave operation showed narrowed spectra and augmented spontaneous emission.

Graphene/porous GaN Schottky Betacell

Betavoltaic batteries have advantages over other batteries due to their high energy density, long life, insensitivity to the environment, and small size. This study aims to design a new structure for betavoltaic batteries using porous GaN with a wide energy bandgap. Many nanowires, coated with graphene, are formed on the surface of GaN. Graphene makes a Schottky junction with GaN and creates an internal electric field to separate electron-hole pairs in the depletion region. In our simulation, Ni63 with an activity of 1 mCi, radiant energy of 17 keV, and radiant flux of 6 Pa is used for beta radiation. The structure is simulated in a TCAD 3D simulator, and an auxiliary function written in C++ is employed to simulate the beta beam. This function provides the spatial distribution of the generated electron-hole pairs in GaN as the input data to our simulator to calculate the battery characteristics. The simulation results indicated that the open-circuit voltage, short-circuit current, fill factor (FF), output power, and efficiency were obtained as 1.22 V, 61.5 nA, 37%, 28.4 nW, and 28.4%, respectively. The effect of changes in the radioisotope source activity and temperature on the betavoltaic battery performance is also investigated. The use of porous GaN, along with the graphene as the semi-metal in Schottky junctions with GaN, has caused a good increase in the performance of our proposed betavoltaic cell in comparison with previously published works.

Nano-porous GaN cladding and scattering loss in edge emitting laser diodes.

We report continuous wave operation of electrically injected InGaN laser diodes using nano-porous GaN n-side cladding with 33% porosity. At 454 nm emission wavelength, the pulsed injection slope efficiency is 0.24 W/A with a high loss of 82 cm-1. The considerable 60 cm-1 of excess loss of the nano-porous clad lasers is attributed to scattering at pores in unintentionally 3% porosified layers, supported by numerical modeling. Simulations comparing porous GaN cladding to AlInN cladding for lasers operating at 589 nm indicate that the porous cladding provides similar internal loss and lower thermal impedance.

Demonstration of device-quality 60% relaxed In0.2Ga0.8N on porous GaN pseudo-substrates grown by PAMBE

Achieving high-quality, relaxed InGaN substrates for longer-wavelength light emitting diodes (LEDs) is of great interest for the development of micro-LED based display technology. This work demonstrates molecular beam epitaxy (MBE)-grown In0.2Ga0.8N with a strain relaxation of 60% corresponding to an equivalently fully relaxed In composition of 12%. This was done by growing on a GaN-on-porous GaN pseudo-substrate (PS). The surface morphology of this film was found to be free of V-defects on the surface and with a threading dislocation density comparable to that of the GaN layers beneath. While InGaN grown on planar GaN-on-sapphire substrates remained nearly strained to the GaN underlayer, InGaN grown under identical conditions on PS displayed elastic-like relaxation. Furthermore, an increase in indium (In) composition was observed for the InGaN grown on PS. Where past work of InGaN grown on porous GaN PS by metalorganic chemical vapor deposition also resulted in relaxed InGaN templates suitable for device application, the surfaces of these relaxed films exhibited V-defects for thicker layers. Employing MBE, thicker films with higher In composition can be achieved with smooth surface morphology, thus enabling pseudo-substrates with a wide range of lattice constants. These pseudo-substrates of varying in-plane lattice constant are attractive for III-nitride based optoelectronics, particularly for green, amber, and red micro-LEDs.

Self-assembled single-crystal bimodal porous GaN exhibiting a petal effect: application as a sensing platform and substrate for optical devices.

This paper investigates the petal effect (hydrophobicity and strong adhesion) observed on single-crystal bimodal porous GaN (porous GaN), which has almost the same electrical properties as bulk GaN. The water contact angles of porous GaN were 100°–135° despite the intrinsic hydrophilic nature of GaN. Moreover, it was demonstrated that the petal effect of porous GaN leads to the uniform attachment of water solutions, enabling highly uniform and aggregation-free attachment of chemicals and quantum dots. These results indicate that porous GaN can be applied in quantum dot light-emitting diodes and as an analytical substrate.

Narrowband near-ultraviolet photodetector fabricated from porous GaN/CuZnS heterojunction

Narrowband photodetection systems are widely used in fluorescence detection, artificial vision and other fields. In order to realize the narrow spectral detection of special band, it is traditionally necessary to integrate broadband detectors with optical filters. However, with the development of detection technology, higher requirements have also been placed on the power consumption, size, and cost of the detection system, and the applications of traditional narrowband photodetectors with complex structures and high costs are limited. Thus, a filterless, narrowband near-ultraviolet photodetector based on a porous GaN/CuZnS heterojunction is demonstrated. The porous GaN thin films with low defect density and CuZnS thin films with high hole conductivity are fabricated by photoelectrochemical etching and water bath growth methods, respectively, and the porous GaN/CuZnS heterojunction near-ultraviolet photodetectors are thus fabricated. Benefiting from the porous structure of GaN and the optical filtering effect of CuZnS, the photo-dark current ratio of the device exceeds four orders of magnitudes under –2 V bias and 370 nm light illumination; more importantly, the device has an ultra-narrowband near-ultraviolet photoresponse with a full width at half maximum of <8 nm (peak at 370 nm). In addition, the peak responsivity, external quantum efficiency and specific detectivity reach 0.41 A/W, 138.6% and 9.8×10<sup>12</sup> Jones, respectively. These excellent device performances show that the near-ultraviolet photodetectors based on porous GaN/CuZnS heterojunctions have broad application prospects in the field of narrow-spectrum ultraviolet photodetection.

The Mechanism of Charge Flow and Electric Current in Porous GaN Thin Films during Photo Electrochemical Etching

The Mechanism of Charge Flow and Electric Current in Porous GaN Thin Films during Photo Electrochemical Etching

Tailoring Wettability Properties of GaN Epitaxial Layers through Surface Porosity Induced during CVD Deposition.

Porous GaN epitaxial layers were prepared using single-step chemical vapor deposition (CVD) through the direct reaction of ammonia with gallium. The degree of porosity and pore diameters in the resulting GaN were analyzed by means of SEM and AFM and were found to depend on the GaN deposition time. Furthermore, the evolution of the contact angle of a droplet of water located on the surface of these GaN epitaxial layers with the deposition time was investigated. We observe a transition from the hydrophilic regime to the hydrophobic regime for deposition times longer than 15 min. The observed dependence of GaN hydrophobicity on its degree of porosity is discussed and explained in the framework of the Cassie–Baxter model.

Fabrication of porous gallium nitride (GaN) utilizing two-step etching method and control of its optical properties

Fabrication of porous gallium nitride (GaN) utilizing two-step etching method and control of its optical properties

Improvement of Porous GaN-Based UV Photodetector with Graphene Cladding

This work presents the role of graphene in improving the performance of a porous GaN-based UV photodetector. The porous GaN-based photodetector, with a mean pore diameter of 35 nm, possessed higher UV sensitivity, about 95% better compared to that of the as-received (non-porous) photodetector. In addition, it exhibits a lower magnitude of leakage current at dark ambient, about 70.9 μA, compared to that of the as-received photodetector with 13.7 mA. However, it is also highly resistive in nature due to the corresponding electrochemical process selectively dissolute doped regions. Herein, two types of graphene, derived from CVD and the electrochemical exfoliation (EC) process, were cladded onto the porous GaN region. The formation of a graphene/porous GaN interface, as evident from the decrease in average distance between defects as determined from Raman spectroscopy, infers better charge accumulation and conductance, which significantly improved UV sensing. While the leakage current shows little improvement, the UV sensitivity was greatly enhanced, by about 460% and 420% for CVD and EC cladded samples. The slight difference between types of graphene was attributed to the coverage area on porous GaN, where CVD-grown graphene tends to be continuous while EC-graphene relies on aggregation to form films.

Electrochemical Tuning of GaN Porosity for Advanced Photonics Applications

The nanoporous GaN from electrochemical process have mainly found roles as strain-free template for light-emitting diodes1, laser diodes2, Bragg reflectors3 and for photonic and optoelectronic applications4.Based on previous researches, a complete review exists on GaN electrochemical and photo-electrochemical etching5: it shows the impact of different parameter on pores morphology and its physical properties. However, according to our knowledge, no literature depicts a systemic pure electrochemical study enabling fine-tuning of porosification process.Our work proposes a three electrodes set up to study the porosification reactions by dynamic electrochemical methods such as cyclic voltammetry (CV) and chronoamperometric (CA) to understand and control the GaN anodic oxidation. It aims to highlight the influence of crucial parameters, relative to both electrochemical and semi-conductor contribution, on the electrochemical signal and on the physico-chemical characteristic of the obtained porous GaN. The main objective is to have a new diagnostic tool for GaN porosification mechanism to better tune the final porous structure.Therefore, the impact of dopant concentration and the influence of thickness of the doped GaN layer was studied by CV analyses and CA method. Coupling the electrochemical curves with SEM analyses enables to define four regimes in the electrochemical charge-transfer: pre-breakdown, porosification zone, a transition phase and electropolishing (figure 1).Moreover, this work show that higher is the doping rate, thinner is the space charges region (SCR): as a result, the conductivity of the n-GaN working electrode increases, and the passage of electrons through the SCR requires lower electric field. This lead to a shift of the CV curves to lower potentials. On the other hand, the range of porosification potentials reduces.The pore morphologies are directly related to the thickness of the space charge region (SCR) which depend on the doping rate and the applied potential:- The nucleation zone corresponds to low potentials region. In this range, dislocations and v-pits control the pores formation. At the tips of these impurities, the radius curvature becomes small enough that a low electric field is sufficient to move electrons through the SCR. In the non-peak areas of the defects, the thickness of SCR is larger and the electric field in the nucleation zone is not sufficient for the electrons passage. Hence the low porosity.- In porosification zone, the electric field is sufficient to accelerate the electrons passage through the SCR, which favors the propagation of nanopores in GaN. The result is the formation of pores on the entire surface, the higher the imposed potential, the higher the pore density and the porosity.The effect of the doped GaN layer thickness was also inspected with CV and CA. The CV curves show that the intensity of the current peaks increases with increasing doped layer thickness because the quantity of available charges is higher. The current drop results from charge consumption in the doped layer.The kinetics of the electrochemical charge transfer reaction between GaN and the electrolyte depends on several parameters. On the semiconductor side, it depends on the doping rate and the quantity of available charges; these two parameters control the creation of holes at the SC/electrolyte interface. On the electrolyte side, it depends on the concentration and the composition of the electrolyte, which control the diffusion of anions towards the SC/electrolyte interface.The cyclic voltammetry curves obtained on different types of electrolyte show that depending on the concentration and the nature of the electrolyte, the charge transfer regime could be diffusional which influences the rate of porosification and thus the pore morphology.The SEM images complete the electrochemical study and confirm that the porosification of GaN depends on:- The doping rate in the semiconductor, which controls the thickness of the SCR in the SC.- The concentration and nature of the electrolyte, which controls the diffusion of material from the electrolyte to the SC/electrolyte interface.- The applied potential, which controls the rate of the charge transfer reaction between the SC and the electrolyte. 1 K.J. Lee, S.-J. Kim, J.-J. Kim, K. Hwang, S.-T. Kim, and S.-J. Park, Optics Express 22, A1164 (2014). 2 S.-M. Lee, S.-H. Gong, J.-H. Kang, M. Ebaid, S.-W. Ryu, and Y.-H. Cho, Opt. Express, OE 23, 11023 (2015). 3 G.-Y. Shiu, K.-T. Chen, F.-H. Fan, K.-P. Huang, W.-J. Hsu, J.-J. Dai, C.-F. Lai, and C.-F. Lin, Scientific Reports 6, 29138 (2016). 4 S.H. Park, G. Yuan, D. Chen, K. Xiong, J. Song, B. Leung, and J. Han, Nano Lett. 14, 4293 (2014). 5 P.H. Griffin and R.A. Oliver, Journal of Physics D-Applied Physics 53, 383002 (2020). Figure 1

Suppression of wear in gallium nitride fibers reinforced fluoropolymer composites: Synergistic effects of load support and tribochemistry

A multifunctional hypothesis of filler selection for PTFE suggests ultralow wear (10−7 mm3/Nm) depends on the filler’s ability to both support the normal load by being large in size and to induce favorable tribochemistry by being weak in aggregate strength. This study is the first to test the independent effects of preferential load support and tribochemistry on PTFE wear by studying the wear reducing properties of a porous GaN fiber, a fully dense SiO2 fiber and microsized GaN particles. The results found the only filler that induce ultralow wear is the GaN fiber which was microns in size and friable under shear stresses. This study reinforces the leading hypothesis that a mechanistic synergy is required for ultralow wear PTFE composites.

Patterned III‐Nitrides on Porous GaN: Extending Elastic Relaxation from the Nano‐ to the Micrometer Scale

Investigations of the use of patterned porous GaN underlayers to achieve elastic relaxation of lattice‐mismatched top layers such as InGaN and AlGaN are reviewed. Thereby, the degree of relaxation of the top layers increases with the porosity and associated decrease in mechanical hardness of the porous GaN underlayers as well as with decreasing pattern size and increase in thickness of the lattice‐mismatched top layers, following the trends observed for nanostructures. Micrometer‐sized, high‐fill‐factor GaN‐on‐porous‐GaN tile arrays are used as universal substrate for the deposition of InGaN and AlGaN. Due to the elastic nature of the relaxation process, the threading dislocation density in the epitaxial material grown on top of the GaN‐on‐porous‐GaN tiles is equivalent to that in the GaN base material. InGaN and AlGaN layers grown on top of relaxed or partially relaxed InGaN or AlGaN underlayers by metal–organic chemical vapor deposition display a higher indium or aluminum content compared to their counterparts deposited on coloaded planar GaN reference wafers due to the decreased lattice mismatch. First applications of the patterned porous GaN‐based technology for optoelectronic and electronic devices are presented and serve as a pathway toward future implementations.

Activated hybrid g-C3N4/porous GaN heterojunction for tunable self-powered and broadband photodetection

Top-down contamination-free uniform etching techniques for preparing porous micro-structure of GaN to enhance its optical absorption has long been explored to arrive at a convenient and efficient strategy. Besides, appropriate heterointerfacing to enhance the photoexcited charge transport with minimal recombination and quenching for consistent and efficient photodetection has been a challenging issue. Herein, we demonstrate one-step solvothermal etching for preparing macroporous GaN (m-GaN), followed by its activation with 2D g-C3N4 to fabricate type II heterojunction for broadband and self-powered (360–635 nm) photodetector (PD). Additionally, tunability in photoresponse and self-powered operation is achieved by impregnating g-C3N4 in thermally reduced graphene oxide (rGO) and chemically modified graphene oxide (CMGO) in separate experiments. With the optimized thickness and composition of CMGO:g-C3N4 on m-GaN, the developed PD displays remarkable photoresponse with broadened photosensitivity and estimated responsivity, detectivity, and external quantum efficiency of 3 A/W, ~1014 Jones, and 103%, respectively. CMGO treated devices exhibit better self-powered characteristics compared to rGO due to improved carrier transport from g-C3N4 and higher built-in potential developed with m-GaN. Under laboratory testing environment, the devices' stability has been recorded for 40 days without noticing any decay in dark current and photoresponse. This work suggests the application of hybrid g-C3N4/m-GaN heterojunction for stable, high-performance, and self-powered photodetection. Further, the modification in band-structure and optical properties of g-C3N4 with graphene oxides of different reduction levels can be leveraged in functional optoelectronics.