Porous Gallium Research Articles

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.

Electroless deposition of gold and platinum for metallization of the intrapore space in porous gallium nitride

The metallization of porous gallium nitride (PGaN) via electroless deposition of Au and Pt is presented. PGaN is generated via the Pt-assisted electroless etching of hydride vapor phase epitaxy GaN. Ultrathin Pt films are sputtered onto the GaN surface, and etching is carried out in a 1:2:2 solution of CH 3OH:HF:H 2O 2. In the case of electrodeposition of Au, the reduction of a Au plating solution by formaldehyde results in the deposition of Au particles which tend to cluster and fill the nanopores. The high surface activity of Au towards PGaN biases Au deposition on PGaN toward cluster formation. For Pt, either Pt clusters or thin film coatings can be obtained by the careful selection of the deposition bath conditions. More importantly, in all cases the electroless deposition of metals on PGaN is extremely sensitive to the conditions of the surface and the plating bath. In particular there is a fine line between conditions which produce smooth coatings and those resulting in clustering. Moreover, the narrow size of the pores, on the order of 80 nm, raises issues of mass transport of the metal plating solution inside the pores, contributing to the production of clustered deposits. Overall the electroless deposition process is more controllable for the deposition of Pt than of Au.

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 .

Morphology evolution and luminescence properties of porous GaN generated via Pt-assisted electroless etching of hydride vapor phase epitaxy GaN on sapphire

Porous gallium nitride (PGaN) is produced by Pt-assisted electroless etching of hydride vapor phase epitaxy (HVPE)–GaN. Ultrathin Pt films are sputtered onto the GaN surface, and etching is carried out in a 1:2:1 solution of CH3OH:HF:H2O2. The evolution of the morphology proceeds by first forming a network of small pores, after which a ridge-trench morphology evolves, with ridges separated by a porous network in trenches between the ridges. As the etch progresses further the ridges evolve to a maximum size and then start to disappear. The formation and evolution of the ridge-trench morphology is explained by the presence of two different etch rates, an enhanced etch rate which generates the porous network and a slower etch rate that leads to the terraces of the ridge morphology. The rate at which the morphology evolves depends on the carrier concentration, with more heavily doped samples etching faster. In all cases, the final depth of the trenches between ridges is independent on the thickness of the starting GaN film. Cathodoluminescence (CL) spectroscopy of the unintentionally doped and the Si doped HVPE materials produce PGaN which shows only band gap emission at 368 nm before and after etching with only small shifts in the wavelength of maximum emission. The intensity of CL emission decreases with etch time as the GaN is consumed. CL spectroscopy and imaging show the ridges to be optically inactive, suggesting that the ridges might arise from grain boundaries or dislocations present in the starting GaN material.

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.

Template Assisted Design of Microporous Gallium Nitride Materials.

AbstractFor Abstract see ChemInform Abstract in Full Text.

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.

Template assisted design of microporous gallium nitride materials

For the first time the synthesis of a porous gallium nitride material (GaN1.15H1.18O0.06C0.04) with narrow pore size distribution in the micropore regime using a nonylamine assisted sol-gel route is reported. The yellow solid has a very high micropore volume of 0.176 cm3 g(-1) comparable to that of zeolitic solids.

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.

Morphology and luminescence of porous GaN generated via Pt-assisted electroless etching

Porous gallium nitride (PGaN) is produced by Pt-assisted electroless etching of GaN. Ultrathin Pt films are sputtered onto the surface of GaN, and etching is carried out in a 1:2:1 or 1:2:2 solution of CH3OH:HF:H2O2. Etching proceeds by first forming a network of small pores, after which ridge structures form, with the porous network in trenches between the ridges. As the etch progresses further the sidewalls of the ridges become steeper, and then the ridges start to disappear. Cathodoluminescence (CL) spectroscopy and imaging show the ridges to be optically inactive, suggesting that the ridges might arise from grain boundaries or dislocations present ins the starting GaN material. CL emission is confined to the porous areas between the ridges. CL properties of the PGaN vary depending on the source of the original, nonporous GaN material. Undoped and unintentionally doped hydride vapor phase epitaxy materials produce PGaN which shows only band gap emission at 368 nm before and after etching, whereas PGaN produced from the Si-doped metalorganic chemical vapor deposited material exhibits two blueshifted luminescence bands at 358 and 326 nm. The origin of the 358 nm blueshifted emission can plausibly be explained by quantum confinement effects, but the 326 nm band can only be explained by other mechanisms, such as the creation of specific surface states by etching. The etched morphology is very similar among the different types of GaN substrates used, however the difference in light emission properties must be ascribed to growth conditions, or to the nature and concentration of the dopants.

Gallium nitride: Method of defect characterization by wet oxidation in an oxalic acid electrolytic cell

A method is suggested to characterize the defect density of a given gallium nitride film, through anodic etching. We report that oxalic acid etches gallium nitride, at room temperature and pressure in an electrolytic cell. Etching is theorized to proceed by attacking those points where planar defects and threading dislocations terminate on the GaN surface. The effect of varying electrolysis voltage on the etch pit density of the porous gallium nitride thus formed is investigated. Etch pit densities on the order of 4×1010 pits/cm2 were measured. The use of etch pit density to estimate defect density of a given GaN film is discussed.

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.

Poly(p-phenylene vinylene)/porous GaP composite materials

We report results obtained from physical characterization of thin composite films made by mixing poly(p-phenylene vinylene) (PPV) and porous gallium phosphide (GaP) particles with different GaP concentrations. Optical measurements including Raman, infrared and photoluminescence show that the GaP particles were progressively incorporated to the polymer film which fills the powder pores. The fabricated films possess properties of both materials with a weak contribution of GaP to the optical properties, the main interaction effect consists in a blue shift in the emission spectra of PPV. Diodes using composites as an active layer exhibit higher conductivity than that of PPV, leading to an enhancement of the light emission. The improvement in optical and electrical performance of the composite based light emitting diodes (LEDs) makes this new composite a good candidate for display applications.