Photo-assisted Electrochemical Etching Research Articles

Morphological and structural characteristics of Gallium Nitride (GaN) porosity using image processing

Attaining a generalized porosity measurement based on the conventional measuring methods is unfeasible due to thin and complex layers on Gallium Nitride (GaN) film. Therefore, there is an impetus for developing a method of estimating general fabric porosity via image processing techniques. This study is aimed to investigate and validate a new technique for extracting the GaN porosity in terms of morphological and structural characteristics using an image processing technique. The anodization on porous GaN films is prepared by using direct current photo-assisted electrochemical etching (DC-PECE) etching technique. The quantitative structural characteristics based on mathematical morphology is analyzed using Field Emission Scanning Electron Microscopy (FESEM) and Atomic Force Microscopy (AFM). To validate the method, the evaluation of porous GaN quality is performed through a non-destructive investigation of its nanostructures using adapting image analysis techniques. Porosity of the structures obtained by calculating the areas occupied by the pores. The quantitative results were obtained and showing good agreement between two modes of measurement and calculation (percentage porosity and pore depth) based on the image-processing data with 91 % and 78 % correlation coefficient.

Influence of different etching methods on the structural properties of porous silicon

PurposePorous silicon (Si) was fabricated by using three different wet etching methods, namely, direct current photo-assisted electrochemical (DCPEC), alternating CPEC (ACPEC) and two-step ACPEC etching. This study aims to investigate the structural properties of porous structures formed by using these etching methods and to identify which etching method works best.Design/methodology/approachSi n(100) was used to fabricate porous Si using three different etching methods (DCPEC, ACPEC and two-step ACPEC). All the samples were etched with the same current density and etching duration. The samples were etched by using hydrofluoric acid-based electrolytes under the illumination of an incandescent lamp.FindingsField emission scanning electron microscopy (FESEM) images showed that porous Si etched using the two-step ACPEC method has a higher porosity and density than porous Si etched using DCPEC and ACPEC. The atomic force microscopy results supported the FESEM results showing that porous Si etched using the two-step ACPEC method has the highest surface roughness relative to the samples produced using the other two methods. High resolution X-ray diffraction revealed that porous Si produced through two-step ACPEC has the highest peak intensity out of the three porous Si samples suggesting an improvement in pore uniformity with a better crystalline quality.Originality/valueTwo-step ACPEC method is a fairly new etching method and many of its fundamental properties are yet to be established. This work presents a comparison of the effect of these three different etching methods on the structural properties of Si. The results obtained indicated that the two-step ACPEC method produced an etched sample with a higher porosity, pore density, surface roughness, improvement in uniformity of pores and better crystalline quality than the other etching methods.

Fabrication of GaN nanowires containing n+-doped top layer by wet processes using electrodeless photo-assisted electrochemical etching and alkaline solution treatment

We attempted to fabricate GaN nanowires by electrodeless photo-assisted electrochemical (PEC) etching and successive alkaline solution treatment. The sample consisting of n+-doped and unintentionally doped GaN grown on a GaN substrate was selectively etched using a Ti mask in a mixed solution of K2S2O8 and KOH under UV light radiation. This specific layer structure required a relatively concentrated KOH solution for etching. The PEC etching resulted in the formation of a tapered cone structure of GaN with its top diameter determined by the mask size. Successive KOH treatment after PEC etching yielded GaN nanowires with diameters of about 220 nm.

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.

Wet etching for isolation of N-polar GaN HEMT structure by electrodeless photo-assisted electrochemical reaction

Electrodeless photo-assisted electrochemical etching was performed in an N-polar GaN high-electron-mobility transistor to obtain device isolation with a flat wet etching surface. The root mean square roughness of the surface after 30 nm etching was 3.4 nm, and a relatively flat etched surface was confirmed. The resistance between the electrodes changed from around 102 Ω to approximately 108 Ω by 30 nm etching.

An integrated micro electro mechanical system–based silicon wet etching process and nano carbon materials used for improving micro direct methanol fuel cells performance

This study integrates “TMAH wet etching” “photo-assisted electrochemical etching’, “PtRu binary metal chemical plating”, and “methanol modification” techniques to develop a micro direct methanol fuel cell. The battery electrode plate is made into microchannel + porous silicon (PS) and microchannel + through-silicon via (TSV) plate electrodes using a silicon wafer as the substrate. When the anode and cathode use a PS electrode and TSV electrode, respectively, the maximum open-circuit voltage is 0.4 V, which is 1.5 times the cell using the PS electrode and 6.7 times the cell using the TSV electrode. The PtRu binary metal is uniformly coated on the surface of a graphene and carbon nanotube to form a compound catalyst layer (PtRu/G-CNT), where the Pt and Ru contents are 34.1 wt% and 2.6 wt%, respectively. In the half-cell test, the oxidation current peak of PtRu/G-CNT is 5 mA/cm2, which is 2.02 times and 2.4 times that of Pt/G-CNT and PtRu/G, respectively. When the fuel is mixed with 0.1% surfactant of sodium dihexyl sulfosuccinate, the maximum power density is 0.336 MW/cm2, and the maximum open-circuit voltage is 0.48 V, which are 1.4 times and 1.2 times the case without a surfactant, respectively.

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.

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.

Ultra-sensitive UV sensors based on porous silicon carbide thin films on silicon substrate

In this report, high-performance ultraviolet (UV) detectors were designed based on porous silicon carbide (SiC) thin films on silicon (Si) substrate. The results can broaden the applications of porous SiC structures in sensing applications. Here, n-type Si (100) was used as a substrate for epitaxial growth of SiC thin films. In order to fabricate porous SiC thin films, a two-electrode, photo-assisted electrochemical etching process was carried out using an integrated current source. The improvement in optical characteristics of porous SiC/Si was reported by tuning the anodization current density. It was illustrated that current density is an effective parameter for controlling the morphology of porous samples. The optical properties of samples were studied using photoluminescence (PL) and optical reflectometry. The results showed that by applying an optimized value of etching current density, the optical reflectivity is decreased which is due to the elevated specific surface area of porous samples that captures the incident light and reduces the reflection. Enhancement in porosity of optimized porous SiC/Si sample was illustrated by its elevated PL intensity. The UV sensing capability of fabricated metal-semiconductor-metal (MSM) detectors based on porous SiC/Si samples with different etching current densities was investigated. The device based on the optimized porous sample showed enhanced sensitivity (54.51) to UV illumination due to the elevated photogenerated current. Moreover, the ultrafast sensing behavior of this device indicates its improved optoelectrical performance in UV detection.

Crystal orientation dependence of alternating current photo-assisted (ACPEC) porous silicon for potential optoelectronic application

Purpose Porous silicon (PS) was successfully fabricated using an alternating current photo-assisted electrochemical etching (ACPEC) technique. This study aims to compare the effect of different crystal orientation of Si n(100) and n(111) on the structural and optical characteristics of the PS. Design/methodology/approach PS was fabricated using ACPEC etching with a current density of J = 10 mA/cm2 and etching time of 30 min. The PS samples denoted by PS100 and PS111 were etched using HF-based solution under the illumination of an incandescent white light. Findings FESEM images showed that the porous structure of PS100 was a uniform circular shape with higher density and porosity than PS111. In addition, the AFM indicated that the surface roughness of porous n(100) was less than porous n(111). Raman spectra of the PS samples showed a stronger peak with FWHM of 4.211 cm−1 and redshift of 1.093 cm−1. High resolution X-ray diffraction revealed cubic Si phases in the PS samples with tensile strain for porous n(100) and compressive strain for porous n(111). Photoluminescence observation of porous n(100) and porous n(111) displayed significant visible emissions at 651.97 nm (Eg = 190eV) and 640.89 nm (Eg = 1.93 eV) which was because of the nano-structure size of silicon through the quantum confinement effect. The size of Si nanostructures was approximately 8 nm from a quantized state effective mass theory. Originality/value The work presented crystal orientation dependence of Si n(100) and n(111) for the formation of uniform and denser PS using new ACPEC technique for potential visible optoelectronic application. The ACPEC technique has effectively formed good structural and optical characteristics of PS.

Photoassisted Electrochemical Micropatterning of Gold Film.

Electrochemical etching is a powerful and popular method for fabricating micropatterns on metal substrates for use in electronic devices, electrochemical sensors, and plasmonic substrates. In order to achieve micropatterning, either a prepatterned insulating layer (mask) or a scanning microelectrode is often required to selectively trigger electrochemical etching at the desired locations. In the present work, we employed a well-focused light beam to enable the photoassisted electrochemical etching of gold film with a spatial resolution close to the optical diffraction limit (∼300 nm). It was found that the simultaneous application of light irradiation and appropriate potential were critical for the oxidative dissolution (i.e., etching) of gold to occur. Superior controllability of light beam allowed for the direct-write micropatterning without the need of mask or probe. Etching kinetics and mechanism were also studied by monitoring the dynamic evolution of optical transparency with a conventional transmission bright-field microscope, together with characterizations on the as-obtained patterns with atomic force microscopy and electron microscopy. This study is anticipated to contribute a feasible method for the micropatterning of gold film with implications for nanoelectronics and electrochemical sensors.

Electrodeless photo-assisted electrochemical etching of GaN using a H3PO4-based solution containing S2O82– ions

Electrodeless photo-assisted electrochemical etching was successfully demonstrated using a H3PO4-based solution containing S2O82– ions. The pH value of the solution changed under UVC illumination, clearly showing that SO4·– radicals were produced from S2O82– ions by absorbing UVC light. The production rate of SO4·– radicals maintained a constant value over the wide pH range of the solution, leading to etching rates and surface roughness comparable to those obtained in KOH-based solutions. The positive-type photoresist was applicable as the etching mask for the H3PO4-based solution. This finding will contribute to the development of a simple wet etching process suitable for the manufacturing of GaN-based devices.

Simple wet-etching technology for GaN using an electrodeless photo-assisted electrochemical reaction with a luminous array film as the UV source

A simple setup for electrodeless photo-assisted electrochemical (PEC) etching was discussed from the viewpoint of the experimental geometry, in which the sample was dipped into the electrolyte under ultraviolet (UV) irradiation. Sulfate radicals () were produced from K2S2O8 with UV light as the oxidizing agent; this consumed the extra UV photogenerated electrons, making it electrodeless. The transmittances were measured for various concentrations of K2S2O8 (aq.) to adjust the electrolyte depth. The effect of tetramethylammonium hydroxide post-treatment was also examined. The results indicate that damage-free PEC etching is feasible for everyone, even those who are not familiar with electrochemistry.

Modification of Surface Properties of Silicon Wafers by Laser-Assisted Electrochemical Etching

In this paper, the structural properties of porous silicon layer PSL were reported. Photo-assisted (laser) electrochemical etching PECE technique used to fabrication PSL from n-type wafer silicon as a function of etching time. Optical microscopy OM image is confirmed that the surface topography of porous silicon layer formation was a mud-like structure. The porosity and thickness have been determined gravimetrically are varied from 61% to 82% and 7.2 µm to 9.4µm respectively. The XRD patterns show that one diffraction peak for all PSL through anodization duration and it is assigned to the (400) plane and data confirmed the porous silicon PS was nanocrystalline.

Modification of Surface Properties of Silicon Wafers by Laser-Assisted Electrochemical Etching

In this paper, the structural properties of porous silicon layer PSL were reported. Photo-assisted (laser) electrochemical etching PECE technique used to fabrication PSL from n-type wafer silicon as a function of etching time. Optical microscopy OM image is confirmed that the surface topography of porous silicon layer formation was a mud-like structure. The porosity and thickness have been determined gravimetrically are varied from 61% to 82% and 7.2 µm to 9.4µm respectively. The XRD patterns show that one diffraction peak for all PSL through anodization duration and it is assigned to the (400) plane and data confirmed the porous silicon PS was nanocrystalline.

Effects of a photo-assisted electrochemical etching process removing dry-etching damage in GaN

We investigated the ability of a photo-assisted electrochemical (PEC) etching process to remove the damage that dry etching causes in the near-surface region of GaN samples. The process consists of anodic oxidation of the GaN surface and subsequent dissolution of the oxide with a chemical treatment, and the extent of the PEC reactions depends on the total charge density transferred in them. The PEC process was conducted for samples prepared with various dry-etching conditions followed by fabrication of Schottky barrier diodes (SBDs) and metal–insulator–semiconductor (MIS) capacitors. The PEC process greatly improved the barrier height, ideality factor, and reverse leakage current of SBDs. Capacitance–voltage measurements of MIS capacitors revealed that the densities of interface states and discrete traps were both reduced by the PEC process. The results obtained here show that the PEC process can remove dry-etching damage from the GaN surface.

Effect of Different UV Light Intensity on Porous Silicon Fabricated by Using Alternating Current Photo-Assisted Electrochemical Etching (ACPEC) Technique

Alternating current (sine-wave a.c (50Hz)) photo-assisted electrochemical etching (ACPEC) process was used to produce the formation of porous silicon with different ultraviolet (UV) light intensity. The study aims to investigate the effect of different UV light illumination on the properties of porous silicon. The surface of n-type silicon (111) was selectively etched in the HF and ethanol solution with a ratio (5:20) for 30 minutes under different UV lamp intensity; 10%, 30% and 60%. The samples were characterized by using atomic force microscopy (AFM) and X-ray diffraction (XRD). The AFM measurements revealed porous silicon with 30% of UV light intensity has the highest surface roughness value with a pyramidal shape. XRD analysis showed FWHM value of etched samples increased as the crystallite size of the etched sample decreased.

Precise thickness control in recess etching of AlGaN/GaN hetero-structure using photocarrier-regulated electrochemical process

The photocarrier-regulated electrochemical (PREC) process was developed for fabricating recessed-gate AlGaN/GaN high-electron-mobility transistors (HEMTs) for normally off operation. The PREC process is based on photo-assisted electrochemical etching using low-energy chemical reactions. The fundamental photo-electrochemical measurements on AlGaN/GaN heterostructures revealed that the photo-carriers generated in the top AlGaN layer caused homogeneous etching of AlGaN with a smooth surface, but those generated in the GaN layer underneath caused inhomogeneous etching that roughens the surface. The concept of the PREC process is to supply the photo-carriers generated only in the AlGaN layer by selecting proper conditions on light wavelength and voltage. The phenomenon of self-termination etching has been observed during the PREC process, where the etching depth was controlled by light intensity. The recessed-gate AlGaN/GaN HEMT fabricated with the PREC process showed positive threshold voltage and improvement in transconductance compared to planar-gate AlGaN/GaN HEMTs.

Errata: Fabrication of through-silicon via arrays by photo-assisted electrochemical etching and supercritical electroplating

Errata: Fabrication of through-silicon via arrays by photo-assisted electrochemical etching and supercritical electroplating

Fabrication of through-silicon via arrays by photo-assisted electrochemical etching and supercritical electroplating

This paper aims to fabricate high aspect ratio through silicon via (TSV) by photo-assisted electrochemical etching (PAECE) and supercritical CO2 copper electroplating. A blind-holed silicon array was first fabricated by PAECE. By studying the etching parameters, including hydrofluoric acid concentration, etchant temperature, stirring speed, tetrabutylammonium perchlorate (TBAP) content, and Ohmic contact thickness, an array of pores with a 1∶45 aspect ratio (height=250 μm and diameter=5.5 μm) was obtained successfully. Moreover, TBAP and Kodak Photo-Flo (PF) solution were added into the etchant to acquire smooth sidewalls for the first time. TBAP was added for the first time to serve as an antistatic agent in deionized water-based etchant to prevent side-branch etching, and PF was used to degasify hydrogen bubbles in the etchant. The effect of gold thickness over Ohmic contact was investigated. Randomized etching was observed with an Au thickness of 200 Å, but it can be improved by increasing the etching voltage. The silicon mold of through-holes was filled with metal using supercritical CO2 copper electroplating, which features high diffusivity, permeability, and density. The TSV structure (aspect ratio=1∶35) was obtained at a supercritical pressure of 2000 psi, temperature of 50°C, and current density of 30 mA/cm2 in 2.5 h.