Porous Gallium Phosphide Research Articles

Porous GaP/g‐C3N4 Photoanode for Enhanced Hydrogen Production

AbstractThe g‐C3N4 decorated porous gallium phosphide have been fabricated by a facile electrophoretic deposition (EPD) process. The morphology, element composition and light absorption of the GaP/g‐C3N4 photoanode were observed using field‐emission scanning electron microscopy, X‐ray photoelectron spectroscopy, ultraviolet and visible spectrophotometer, respectively. When acting as photoanode, porous GaP/g‐C3N4 serves as a direct Z‐scheme system, where photogenerated holes in GaP are expended by electrons generated from the g‐C3N4, inhibiting the corrosion of GaP. Therefore, the porous GaP/g‐C3N4 showed a larger photocurrent density, which is 2.1 times as large as that of the porous GaP without g‐C3N4, and a more stable photocurrent density for over 10000 s (at 0 V vs RHE). Thus, this work delivers a practical way to improve the photoelectrochemical stability and property of III–V semiconductor materials, which could be used in solar energy conversion fields.

Investigation of the porous GaP layers' chemical composition and the quality of the tests carried out

Purpose: The purpose of this study is to establish the quality of tests for determining the chemical composition of the porous surface obtained by the method of electrochemical etching, based on the indicators of convergence and reproducibility of the results. Design/methodology/approach: The method of electrochemical etching was used to obtain layers of porous gallium phosphide, which can be used as buffer layers for nitrides formation on their basis. Por-GaP was formed in a solution of hydrofluoric acid at a current density of j =100 mA/cm2, etching was carried out for 20 minutes. The resulting structures sulfide passivation was carried out, thus avoiding the formation of an oxide film on the samples surfaces. For this purpose, porous gallium phosphide samples were kept in a sulfide solution for 10 minutes and dried in a stream of nitrogen. The chemical composition of the porous GaP surface layers has been investigated. To do this, the method of electronprobe INCA Energy microanalysis was used. The research was carried out on the entire surface of the sample in order to calculate the total spectrum of the elemental composition of the sample under study. Findings: It was established that during anodizing, the stoichiometry of the crystals investigated did not shift significantly towards the excess of gallium atoms. Oxygen is present at an insignificant concentration of 0.3%. This indicates the effectiveness of conducting the sulfide passivation of the sample surface following the electrochemical treatment. The presence of fluorine atoms that appeared on the surface as a result of the reaction with the electrolyte during etching, is observed in extremely low concentration. Experimental studies have shown that the sample chosen can be used as a standard enterprise sample when analyzing the chemical composition of the surface of porous gallium phosphide due to its convergence, reproducibility, homogeneity. In addition, the given method for determining the standard sample can be applied to other porous semiconductors. Conducting such studies is an important technological task that will allow us to create a series of standard samples of porous semiconductors of A3B5 group. Research limitations/implications: The research was carried out for porous gallium phosphide samples synthesized in the solution of hydrofluoric acid, though, carrying out of similar experiments for por-GaP obtained in other conditions, is necessary. Practical implications: The studies of the reproducibility and convergence of the experiment have an important practical significance, since it is the reproducibility of the experiment results that is the main problem to modern material science, all the more to nanoengineering. Therefore, the technique proposed, will allow the synthesis of por-GaP layers with adjustable properties, will facilitate their widespread implementation in the real sectors of industry. The obtained porous layers can be used as standard samples. Originality/value: The main problem in the nanostructures synthesis is the adjustability of properties. The technique presented allows to form stable layers of the porous gallium phosphide with the surface chemical composition known in advance. In addition, carrying out the sulfide passivation allows to stabilize por-GaP properties, which is an important process task. For the first time, a study of the presented technique as to convergence, reproducibility and homogeneity, was conducted.

Synthesis and microstructure of p-type porous gallium phosphide layers

Gallium phosphide is a semiconductor material of great interest for new modern LED technologies. HereGallium phosphide is a semiconductor material of great interest for new modern LED technologies. Herewe report the results of research on obtaining of nanostructured porous gallium phosphide layers byelectrochemical etching of the monocrystalline (100) p-type GaP substrate surface. The structure andsurface morphology of obtained samples were studied by scanning electron microscopy, atomic forcemicroscopy and Raman spectroscopy. The chemical composition of nanostructured GaP surfaces wasstudied by energy dispersive X-ray spectroscopy, which showed that the pore formation duringelectrochemical etching are mostly due to dissolution of phosphorus atoms rather than gallium .It wasfound that shape and size of structures are strongly depending on electrochemical etching conditions. Atconstant applied voltage and varied current we observe that the pore size is increasing from 10 μm to 50μm, and at fixed current, but varying the voltage the pore diameters can be achieved in the range between100 to 200 nanometers.

Electrochemical Formation and Microstructure of Porous Gallium Phosphide

Electrochemical formation and microstructure of porous GaP have been investigated. Nanostructured porous GaP layers of thickness up to … 20 „m were fabricated on n-type (111)-oriented crystalline c-GaP substrates. Studies of microstructure of porous GaP in dependence on electrolyte type and regimes of technological procedure have been carried out by scanning electron microscopy. The samples were characterized by spectroscopic ellipsometry in visible and near UV spectral range. The investigations have shown that the structure and optical response of porous GaP can be eciently controlled by technological procedure of electrochemical formation. The shape and dimension of pores can be varied from nanometer-scaled cylindrical pores to GaP nanorods.

Dye Sensitization of Porous Gallium Phosphide

not Available.

Experimental observation of second-harmonic generation and diffusion inside random media

We have experimentally measured the distribution of the second-harmonic intensity that is generated inside a highly scattering slab of porous gallium phosphide. Two complementary techniques for determining the distribution are used. First, the spatial distribution of second-harmonic light intensity at the side of a cleaved slab has been recorded. Second, the total second-harmonic radiation at each side of the slab has been measured for several samples at various wavelengths. By combining these measurements with a diffusion model for second-harmonic generation that incorporates extrapolated boundary conditions, we present a consistent picture of the distribution of the second-harmonic intensity inside the slab. We find that the ratio l2ω/Lc of the mean free path at the second-harmonic frequency to the coherence length, which was suggested by some earlier calculations, cannot describe the second-harmonic yield in our samples. For describing the total second-harmonic yield, our experiments show that the scattering parameter at the fundamental frequency k1ωl1ω is the most relevant parameter in our type of samples.

Spatial Extent of Random Laser Modes

We have experimentally studied the distribution of the spatial extent of modes and the crossover from essentially single-mode to distinctly multimode behavior inside a porous gallium phosphide random laser. This system serves as a paragon for random lasers due to its exemplary high index contrast. In the multimode regime, we observed mode competition. We have measured the distribution of spectral mode spacings in our emission spectra and found level repulsion that is well described by the Gaussian orthogonal ensemble of random-matrix theory.

The role of phase-matching and nanocrystal-size effects in three-wave mixing and CARS processes in porous gallium phosphide

Nonlinear-optical interactions, such as second-harmonic and sum-frequency generation and coherent anti-Stokes Raman spectroscopy (CARS), are investigated in porous GaP for the first time by means of a novel laser source based on mode-locked picosecond Nd3+:YVO4 laser and subsequent continuum generation in an optical fiber. The efficiency of the former two nonlinear optical processes is shown to be strongly dependent on the wavelengths of the interacting waves and tends to increase with the decrease of the excitation wavelength. The power of the generated second-harmonic and sum-frequency increases by a factor of 2 and 30, respectively, compared to the crystalline GaP. In contrast, the CARS signal in porous GaP is found to be less efficient than one in crystalline GaP. The observed results are explained in terms of competition of the phase-matching effects in GaP nanocrystals and the enhanced photon lifetime in scattering porous GaP layers.

Electrochemical Formation of Porous GaP in Aqueous HNO[sub 3

Porous gallium phosphide was produced by anodic etching of n-type GaP in an aqueous solution of 1 M HNO3. At potentials lower than 15.5 V, yellow porous samples were obtained, of which the pore size was around 45% larger than the pores obtained in 0.5 M H2SO4. At potentials around 15.5 V electropolishing occurred. No pores formed when GaP was etched at potentials in this range. At potentials higher than approximately 15.5 V the current density increased strongly with increasing potential, and very large pores formed. It was found that NO 3 - and SO 4 2- only play a role in the chemical dissolution of the oxide layer that forms during etching.

Disorder-correlated enhancement of second-harmonic generation in strongly photonic porous gallium phosphide

We report an order of magnitude enhancement of second-harmonic generation (SHG) from porous gallium phosphide relative to SHG in crystalline gallium phosphide. Optical heterodyning measurements of photon free-path length reveal a correlation between SHG enhancement and disorder of the porous material.

Porous gallium phosphide: challenging material for nonlinear‐optical applications

Electrochemically produced porous GaP layers demonstrate strong non-Rayleigh light scattering in visible range. Moreover, (110) porous GaP layers exhibit in-plane birefringence. Both properties offer much promises for enhanced nonlinear-optical processes. We report experimental studies of spectral and orientation dependences of the second-harmonic generation in (110) and (111) porous GaP layers. An order of magnitude increase of the second-harmonic intensity was found in the strongly scattering porous GaP layers in comparison with monocrystalline GaP. The spectral dependence of the second-harmonic intensity was discussed in terms of the phase matching and light localization phenomena. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Photo‐electrochemical formation of porous GaP

Electrochemical etching formation mechanisms of porous gallium phosphide are investigated under different conditions. Anodic etching of single crystalline n-type GaP was performed in H2SO4 0.5 M aqueous solution both under dark condition and by shining the samples with the 351 nm line of an Argon laser. Different reaction mechanisms are found as evidenced by the analysis of the current-voltage characteristics. Moreover, a dependence of the porosity on the laser power density is detected. A quantitative estimate of the porosity is given by studying the Frohlich modes through micro Raman Spectroscopy. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A model for pore growth in anodically etched gallium phosphide

The electrochemical etching process of porous gallium phosphide was studied by means of the characteristic current–potential (I–V) curves. Measurements were performed in H2SO4 0.5-M aqueous solution both in the dark and by illuminating the samples with the 351-nm line of an argon laser. Raman spectroscopy was applied to investigate the surface morphology of the samples prepared under different anodizing conditions within the potentiostatic regime. Based on a few reasonable assumptions, a simple model of pore growth is proposed. The enhancing effect in current intensity due to the branching of pores and the opposite effect due to a concomitant decrease in the effective cross area available for carrier transport are accounted for to explain the main features of the recorded I–V curves.

Methods for Increasing the Efficiency of Nonlinear Optical Interactions in Nanostructured Semiconductors

Methods for increasing the efficiency of the optical second-and third-harmonic generation in gallium phosphide and silicon nanostructures formed by electrochemical etching of crystalline semiconductors are discussed. The efficiency of nonlinear optical interactions can be increased by using phase matching in anisotropic nanostructured semiconductors that exhibit form birefringence or by increasing the local field, as in scattering in macroporous semiconductors. The efficiencies of third-harmonic generation in porous silicon and of second-harmonic generation in porous gallium phosphide are found to increase by more than an order of magnitude.

Second-harmonic generation in strongly scattering porous gallium phosphide

Second-harmonic generation in a strongly scattering macroporous GaP layer with typical sizes of GaP nanocrystals ranging from 200 to 500 nm is studied using nanosecond pulses of an optical parametric oscillator. As the pump wavelength decreases from 1.6 to 1.0 μm the second-harmonic yield from porous GaP increases more than one order of magnitude relative to the second-harmonic yield from crystalline GaP. This enhancement of the second-harmonic generation for shorter wavelengths correlates with the increase in the efficiency of light scattering, indicating the importance of scattering effects in nonlinear-optical processes in strongly scattering random media .

Efficient second-harmonic generation by scattering from porous gallium phosphide

We experimentally study second-harmonic generation by femtosecond Cr: forsterite-laser radiation scattered on the surface of porous gallium phosphide with characteristic pore sizes and distances between the pores comparable with the second-harmonic wavelength. The intensity of the second-harmonic signal from samples with initial crystallographic surface orientations (110) and (111) is more than an order of magnitude higher than the intensity of the second harmonic generated in reflection from single-crystal gallium phosphide. The efficiency of second-harmonic generation by macroporous gallium phosphide substantially increases as the pump wave-length becomes shorter. The influence of light localization and scattering effects on the enhancement of second-harmonic generation and polarization properties of the second-harmonic is discussed.

Experimental determination of the effective refractive index in strongly scattering media

Measurements of the angular-resolved-optical transmission through strongly scattering samples of porous gallium phosphide are described. Currently porous GaP is the strongest-scattering material for visible light. From these measurements the effective refractive index and the average reflectivity at the sample interface can be obtained. These parameters are of great importance for an accurate interpretation of optical experiments, and are for the first time determined in strongly scattering samples.

Raman-active modes of porous gallium phosphide at high pressures and lowtemperatures

Porous gallium phosphide (GaP) with a honeycomb-likemorphology and a skeleton relative volume concentration c = 0.7was investigated by Raman spectroscopy under pressure up to 10 GPa at T = 5 K.The porous samples were prepared by electrochemical etching. The transverseoptical (TO) and longitudinal optical (LO) mode frequencies were found to shiftwith pressure similarly to those of bulk GaP. As in bulk GaP, the TO feature ofthe porous GaP exhibits a pressure-induced narrowing which is interpreted interms of a Fermi resonance. The scattering intensity observed on thelow-frequency side of the LO mode is attributed to surface-related Fröhlich modescattering. The latter results are interpreted on the basis of an effective mediumexpression for the dielectric function. The Raman spectra indicate that both themorphology and degree of porosity are unaffected by pressure in the rangeinvestigated.

Tunable photonic strength in porous GaP

The light-scattering properties of porous gallium phosphide, prepared by electrochemical etching, are investigated. We show that the photonic strength of the porous semiconductor can be tuned from weak to extremely strong. This tunability is related to the density and size of the pores, which are controlled by the dopant density of the GaP crystals, and the etching potential. Moreover, electrochemical etching does not introduce any significant optical absorption, which makes porous GaP suitable for many photonic applications.

Optical phonons of circular wires in porous GaP

The reflection spectra of porous gallium phosphide samples are investigated in the far infrared region of wave numbers (10–700 cm−1). In addition of the longitudinal and transverse optical phonon modes corresponding to the bulk material, additional vibrational modes are detected. Their number and spectral position are correctly described by a model of a dispersive dielectric medium under the assumption that porous gallium phosphide is formed by crystallites whose shape is close to cylindrical. It is concluded that such vibrational modes are optical phonons confined by the volume of the quantum wire. The experimental optical reflection spectra are used to obtain estimates of the average diameter of nanocrystallites forming the porous GaP layer.