Porous GaN Research Articles

Defected Photonic Crystal Array Using Porous GaN as Malaria Sensor

A defective one-dimensional photonic crystal is investigated as a biosensor to detect malaria disease. The proposed photonic structure is air/(GaN/Porous GaN)N/Sample/(GaN/Porous GaN)N/Substrate. The red blood cells sample of the human being is used as a sample defect in the proposed optical device. The pioneer transfer matrix method is used to analyze the transmittance spectra. A change in sample refractive index highly affects the transmittance resonant peak and this shift in the peak plays a key role in the operation of the device. The relatively high figure of merit of 1022 RIU−1 with a sensitivity of 1472 nm/RIU and quality factor of 1076 is detected. The proposed sensor is relatively better than others available to detect malaria disease.

Effect of ultraviolet and room lights on porous GaN films using photo-assisted electrochemical etching

We fabricated high reflectivity GaN/nanoporous (NP) GaN distributable Bragg reflector (DBR) structures by electrochemical etching of HF solution in ultraviolet (UV)- and room-light. Compared with in room-light, the DBR fabricated in UV-light has higher reflectivity. To elucidate the difference of these two DBRs, the effect of lights on the etching breakdown voltage, pore morphology, crystal quality and photoluminescence (PL) properties was studied in detail. By comparison, the NP-GaN film obtained under the UV-light has lower breakdown voltage, higher porosity, higher crystal quality and bigger red-shift of PL position, meaning that the DBR obtained in the UV-light has higher quality and reflectivity.

Investigation on the Effect of Current Density on Porous GaN Fabricated by UV-assisted Electrochemical Etching

Investigation on the Effect of Current Density on Porous GaN Fabricated by UV-assisted Electrochemical Etching

Fabrication of layer-ordered porous GaN for photocatalytic water splitting

Ordered nanoporous structures are promising for multifunctional devices and systems. Here, we demonstrate the controllable layer-ordered porous GaN structures using selective electrochemical etching of the n-GaN layers from n-GaN/u-GaN multilayers. The shape of layer-ordered porous structures can be modulated to dome-shaped or triangular by changing the electrochemical etching solution. Compared with the corresponding planar GaN, the photocurrent of the porous GaN increases by ~2.7 times in photocatalytic water splitting process. The enhanced photocatalytic performance is mainly attributed to the increased specific surface area and the modulation of the electric fields of incident light in the porous structures as simulated using finite-difference time-domain method. This approach can help to design and diversify highly ordered nanostructures for the III-nitride semiconductor, which lays a foundation for photocatalytic or other optoelectronic applications.

Fabrication of a 2 inch free standing porous GaN crystal film and application in the growth of relaxed crack-free thick GaN

This paper describes the fabrication of a 2 inch free standing porous GaN crystal film and the application in the growth of relaxed crack-free thick GaN.

Improved H2 detection performance of GaN sensor with Pt/Sulfide treatment of porous active layer prepared by metal electroless etching

High-performance chemiresistor gas sensor made of sulfide porous GaN decorated with Pt nanoparticles, which shows tunable sensor response and enhanced sensitivity. The fabricated gas sensors show detection of H2 down to 30 ppm at 23 °C after sulfide treatment and Pt decorated porous GaN. The response time and recovery time were equal to 47 s and 113 s, respectively. Density functional theory simulations were used to support the detection mechanism based on sulfide treatment. Adsorption energy calculations showed that H adsorption energy is lowered by the simultaneous presence of S and Pt on the GaN (0001) surface. The density of states (DOS) calculations revealed possibility of bond strengthening when Pt and S is adsorbed on GaN surface along with H, arising from the hybridization of d and p orbitals of Pt and S with that of H 1s orbitals.

Large-area surface-enhanced Raman spectroscopy substrate by hybrid porous GaN with Au/Ag for breast cancer miRNA detection

Surface Enhanced Raman Spectroscopy (SERS) method is widely used as a powerful undamaged tool to analysis low concentration of molecules, which gives great potential for bio-detection. In this work, large area SERS substrates were fabricated by hybrid large area porous GaN nanostructures with Au/Ag. By controlling porous GaN morphology, the Au/Ag layer thickness and morphology, the detection limit of 1.13 × 10-13 M and enhancement factor of 2.5 × 108 of t he large area SERS substrate were obtained with good uniformity and stability by using R6G as SERS probe. The high performance of the SERS substrate is attribute to the electromagnetic enhancement after hybrid GaN nanostructure with Au /Ag, which conformed by finite difference time domain method (FDTD) simulation. The potential application for the detection of miRNA (miR-K12-5-5p as breast cancer related biomarker) was evaluated with fluorescent molecular (FAM) as Raman target molecular and a sensitive miR-K12-5-5p detection with a limit down to 8.84 × 10-10 M was achieved, which confirming the potentialities of the Au/Ag/ porous GaN hybrid SERS substrate for early-cancer diagnosis, and giving great potential to develop ultrasensitive SERS biosensors for various biological and chemical analyses.

Enhanced piezoelectricity and electromechanical efficiency in semiconducting GaN due to nanoscale porosity

Electrical polarization phenomena in GaN are important as they have significant impact on the operation of modern day energy efficient lighting and are fundamental to GaN-based high power and high frequency electronics. Controlling polarization is beneficial for the optimization of these applications. GaN is also piezoelectric, and therefore mechanical stress and strain are possible handles to control its polarization. Nonetheless, polar semiconductors in general, and GaN in particular, are weak piezoelectric materials when compared to ceramics, and are therefore not considered for characteristic electromechanical applications such as sensing, actuation and mechanical energy harvesting. Here, we examine the effect of nanoscale porosity on the piezoelectricity of initially conductive GaN. We find that for 40% porosity, the previously conductive GaN layer becomes depleted, and exhibits enhanced piezoelectricity as measured using piezoresponse force microscopy, as well as by using a mechanical energy harvesting setup. The effective piezoelectric charge coefficient of the porous GaN, d33,eff, is found to be about 8 pm/V which is 2-3 times larger than bulk GaN. A macroscale device comprising a porous GaN layer delivered 100 nW/cm2 across a resistive load under a 150 kPa mechanical excitation. We performed finite element simulations to analyze the evolution of the piezoelectric properties with porosity. The simulations suggest that increased mechanical compliance due to porosity gives rise to the observed enhanced piezoelectricity in GaN. Furthermore, the simulations show that for stress-based excitations, the porous GaN electromechanical figure of merit is increased by an order of magnitude and becomes comparable to that of barium titanate piezoceramics. In addition, considering the central role played by GaN in modern electronics and optoelectronics, our study validates a very promising research direction when considering stress-based electromechanical applications which combine GaN's semiconducting and piezoelectric properties.

Method of growing elastically relaxed crack-free AlGaN on GaN as substrates for ultra-wide bandgap devices using porous GaN

Crack-free 1.3 μm thick elastically relaxed AlxGa1−xN layers were demonstrated on compliant high fill-factor 10 × 10 μm2 tile patterned GaN-on-porous-GaN pseudo-substrates (PSs). Porous GaN was utilized as a semi-flexible underlayer. The AlxGa1−xN layer was grown in steps of 200, 300, 400, and 400 nm. While the AlxGa1−xN layer regrown on a co-loaded GaN-on-sapphire template cracked after deposition of only 200 nm AlGaN, on the GaN-on-porous GaN patterned substrates, no cracks were observed, and smooth films were achieved. In addition, an enhanced aluminum uptake was observed as the AlGaN growth progressed on the GaN-on-porous-GaN PS, owing to the composition pulling effect, until the point when the tiles started to coalesce. Upon further regrowth on these coalesced tiles, the aluminum uptake saturated, while the surface remained crack-free and smooth.

Color-tunable <10 μ m square InGaN micro-LEDs on compliant GaN-on-porous-GaN pseudo-substrates

In this study, two methods to tune the emission wavelength of micro-LEDs fabricated on tile patterned compliant GaN-on-porous-GaN pseudo-substrates (PSs) are presented. The mechanical flexibility of porous GaN was utilized to relax the strain induced during the growth of light-emitting diode (LED) structures with n- and p-InGaN layers and enhance the indium incorporation via the composition pulling effect. The first approach involved only varying the size of the PS square tiles used for LED structure regrowth, from 20 × 20 μm2 to 8 × 8 μm2. Higher n-InGaN base layer relaxation with decreasing tile size resulted in a red shift of emission from 525 nm to 561 nm with no change in the growth conditions. The second method involved changing the mole fraction, x, of the n-InxGa1−xN base layer of the LED structure from 0.04 to 0.09 by reducing growth temperature, while maintaining the high temperature growth conditions of the multi-quantum well (MQW) and p-InGaN targeting 530 nm emission. The resulting wavelength shift was a remarkable 536–616 nm due to the stronger composition pulling effect, providing a pathway to enable high indium content MQW active regions to be grown at high temperature.

First experimental demonstration and analysis of electrical transport characteristics of a GaN-based HEMT with a relaxed InGaN channel

In this paper, we demonstrate the first GaN/InGaN high electron mobility transistor with a partially relaxed InGaN channel, realized by utilizing a porous GaN buffer obtained by the selective and controlled electrochemical etch of GaN. According to reciprocal space map analysis, ∼70% of InGaN relaxation relative to GaN was achieved by this process. With the transfer length method and extracted charge profile, the relaxed InGaN channel demonstrates ∼9.6% two-dimensional electron gas mobility enhancement with respect to the strained InGaN channel which is consistent with a reduced effective mass predicted by theoretical analysis based on Vegard’s law. On comparing the output characteristics of devices with relaxed and strained InGaN channel, the relaxed channel devices show a 10% improvement of on-resistance while maintaining similar saturation current. A non-uniform charge distribution was found in the relaxed InGaN channel, which can be attributed to the strain relaxation fabrication process.

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.

Enhancing Performance of Porous Si-Doped GaN Based MSM Photodetector Using 50 Hz ACPEC

In this work, we report the formation of porous Si-doped GaN films under a novel alternating current (sine-wave a.c. (50 Hz)) photo-assisted electrochemical etching (ACPEC) conditions. The formation of porous Si-doped GaN by the novel ACPEC is performed in the same electrolyte concentration (4% KOH) used in common dc constant current electrochemical etching process. Ultra-violet (UV) illumination is used to assist in the generation of electron-hole pairs, where etching proceeds through the oxidation and consequently, dissolution of the semiconductor surface. The ac formed porous Si-doped GaN with excellent structural and optical properties. According to the FESEM micrographs, the GaN thin films exhibit a homogeneous nanoporous structures with spatial nano-flakes arrangement. The porous layer exhibited a substantial photoluminescence (PL) intensity enhancement with red-shifted band-edge PL peaks associated with the relaxation of compressive stress. The shift of E2 (high) to the lower frequency in Raman spectra of the porous GaN films further confirms such a stress relaxation. Electrical characterizations of the MSM photodiodes were carried out by using current-voltage (I-V) measurements indicated that the devices were highly sensitive to ambient light.

Comparative Studies between Porous Silicon and Porous P-Type Gallium Nitride Prepared Using Alternating Current Photo-Assisted Electrochemical Etching Technique

Porous n-type Si and porous p-type GaN nanostructures were fabricated using alternating current photo-assisted electrochemical (ACPEC) etching in 1:4 volume ratio of hydrofluoric acid (HF) and ethanol (C2H5OH) for a duration of 30 min. The proposed approach to this work was to study pore formation on the Si and p-GaN substrates in the aspects of morphological and structural changes. The morphological and structural properties of porous Si and porous p-type GaN samples have been studied using field emission scanning electron microscopy (FESEM) measurement, energy-dispersive X-ray spectroscopy (EDX), atomic force microscopy (AFM), and high-resolution X-ray diffraction (HR-XRD) in comparison to the respective as-grown sample. FESEM analysis revealed that uniform pore size with triangular-like shape was formed in porous Si sample while circular-like shape pores were formed in the porous p-type GaN sample. AFM measurement revealed that the root-mean-square surface roughness of porous Si and porous p-type GaN was 6.15 nm and 5.90 nm, respectively. Detailed investigation will be presented in this work to show that ACPEC etching technique is a viable technique to produce porous nanostructures in different substrates.

Growth of strain-relaxed InGaN on micrometer-sized patterned compliant GaN pseudo-substrates

The compliant behavior of high fill-factor 10 × 10 μm2 square patterned 60–140 nm thick GaN-on-porous-GaN tiles was demonstrated by utilizing porous GaN as a semi-flexible underlayer. High resolution x-ray diffraction measurements showed a larger a-lattice constant of InGaN layers deposited on these patterned GaN-on-porous GaN pseudo-substrates in comparison to those deposited on co-loaded planar GaN-on-sapphire templates. Additionally, InGaN based light emitting diode (LED) structures deposited on these GaN pseudo-substrates exhibited room temperature electroluminescence at 547 nm compared to 506 nm for the LED structures grown on co-loaded planar GaN on sapphire templates, corresponding to a redshift of around 40 nm. The longer emission wavelength was associated with the higher indium incorporation into the InGaN quantum wells deposited on the compliant GaN pseudo-substrates, owing to a reduced lattice mismatch between the quantum well and the n-InGaN base layers grown on the compliant pseudo-substrates, due to the composition pulling effect.

Morphological, Structural and Optical Characteristics of Porous GaN Fabricated by UV-Assisted Electrochemical Etching

The fabrication of porous GaN (PGaN) by UV-assisted electrochemical etching with a variations of current densities (40, 60, and 80 mA/cm2) for 60 min in electrolytes consisting of 4% KOH are reported. Field Emission Scanning Electron Microscopy (FESEM), Energy Dispersive X-Ray (EDX), Atomic Force Microscopy (AFM) and X-ray Diffraction (XRD) were used to characterize the morphological and structural characteristics of the PGaN. All PGaN sample prepared by electrochemical etching technique produced a hexagonal-like pore shape. FESEM images demonstrated that the pore uniformity and porosity are affected significantly by the current density. The PGaN sample fabricated with 80 mA/cm2 produces a uniform and high porosity structure compared to other PGaN sample. This shows that the morphology and structural characteristic of PGaN are increase with the increase of current density. The EDX result revealed significant Ga and N atom presence in all samples. However, the O atom only presence in sample etched with 80 mA/cm2 implying that the etching process is occur vigorously in this sample. The AFM verified that the surface roughness and the pore depth are increased as current density increased. There were relatively large variations of the peak intensities for 2Theta-scan patterns as exposed by XRD. The peak shift for PGaN sample relative to as-grown was inconsistent and the changed was relatively small. Raman intensity found to be enhanced with the increase in current density and among the PGaN sample, the E2(high) peak for sample prepared with 60mA/cm2 and 80mA/cm2 was observed to be slightly shifted to lower frequency. The PL spectra displayed that the porosity has high impact on the PL peak intensity. . Overall, this proved that with the usage of low power UV light, the pore structure still can be produced as good as pore structure fabricated with high power UV light.

Dislocations as channels for the fabrication of sub-surface porous GaN by electrochemical etching

Porosification of nitride semiconductors provides a new paradigm for advanced engineering of the properties of optoelectronic materials. Electrochemical etching creates porosity in doped layers while leaving undoped layers undamaged, allowing the realization of complex three-dimensional porous nanostructures, potentially offering a wide range of functionalities, such as in-distributed Bragg reflectors. Porous/non-porous multilayers can be formed by etching the whole, as-grown wafers uniformly in one simple process, without any additional processing steps. The etch penetrates from the top down through the undoped layers, leaving them almost untouched. Here, atomic-resolution electron microscopy is used to show that the etchant accesses the doped layers via nanometer-scale channels that form at dislocation cores and transport the etchant and etch products to and from the doped layer, respectively. Results on AlGaN and non-polar GaN multilayers indicate that the same mechanism is operating, suggesting that this approach may be applicable in a range of materials.

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.

Effect of KOH Concentration on the Properties of Undoped Porous GaN on Sapphire Substrate Prepared by UV assisted Electrochemical Etching

Effect of KOH Concentration on the Properties of Undoped Porous GaN on Sapphire Substrate Prepared by UV assisted Electrochemical Etching

Comparison study of GaN films grown on porous and planar GaN templates**Project supported by the National Key R&D Program of China (Grant No. 2017YFB0404201), the State Key R&D Program of Jiangsu Province, China (Grant No. BE2019103), the Six-Talent Peaks Project of Jiangsu Province, China (Grant No. XCL-107), the Fund from the Solid-state Lighting and Energy-saving Electronics Collaborative Innovation Center, PAPD,

The GaN thick films have been grown on porous GaN template and planar metal-organic chemical vapor deposition (MOCVD)-GaN template by halide vapor phase epitaxy (HVPE). The analysis results indicated that the GaN films grown on porous and planar GaN templates under the same growth conditions have similar structural, optical, and electrical properties. But the porous GaN templates could significantly reduce the stress in the HVPE-GaN epilayer and enhance the photoluminescence (PL) intensity. The voids in the porous template were critical for the strain relaxation in the GaN films and the increase of the PL intensity. Thus, the porous GaN converted from β-Ga2O3 film as a novel promising template is suitable for the growth of stress-free GaN films.