Chemical etching of (100) GaAs in a sulphuric acid-hydrogen peroxide-water system

A detailed investigation of the etching of (100) GaAs in H₂SO₄-H₂O₂-H₂O systems has been made. The influence of the concentration of particular etchant components on etching rate and on the shape of the crystal surface was examined. From these results the Gibbs' triangle of etching bath compositions was divided into parts corresponding to the various states of the crystal surfaces and various etching mechanisms. The shape of the crystal surface after etching was closely related to the profiles of the grooves etched in the [110] direction in the same solution.     

交替刻蚀制备有序锯齿形硅纳米线阵列及其光学性能研究

采用金属催化化学刻蚀法(MCCE),以金属Ag为催化剂,在HF与H2O2体系中通过交替刻蚀在P(111)硅衬底上制备出锯齿形硅纳米线阵列.利用扫描电子显微镜对硅纳米线的形貌进行了表征,研究了HF浓度与H2O2浓度对纳米线刹蚀方向的调控作用.选取不同的HF与H2O2浓度配比,分别对硅基底各向同性刻蚀与各向异性刻蚀进行调控,使得刻蚀方向对溶液浓度的变化能够快速响应.在溶液Ⅰ([HF]=2.3 mol/L,[H2O2]=0.4 mol/L)与溶液Ⅱ([HF]=9.2 mol/L,[H2O2]=0.04 mol/L)中交替刻蚀,制备出刻蚀方向高度可控的大规模锯齿形硅纳米线.利用紫外-可见分光光度计对锯齿形硅纳米线的减反射性能进行研究,结果表明,其表现出优异的减反特性,最低反射率为5.9％.纳米线形貌的高度可控性使其在微电子器件领域也具有巨大的应用前景.     

Formation of high aspect ratio GaAs nanostructures with metal-assisted chemical etching

Periodic high aspect ratio GaAs nanopillars with widths in the range of 500-1000 nm are produced by metal-assisted chemical etching (MacEtch) using n-type (100) GaAs substrates and Au catalyst films patterned with soft lithography. Depending on the etchant concentration and etching temperature, GaAs nanowires with either vertical or undulating sidewalls are formed with an etch rate of 1-2 μm/min. The realization of high aspect ratio III-V nanostructure arrays by wet etching can potentially transform the fabrication of a variety of optoelectronic device structures including distributed Bragg reflector (DBR) and distributed feedback (DFB) semiconductor lasers, where the surface grating is currently fabricated by dry etching.     

Optimisation of the epi-ready semi-insulating GaAs wafer preparation procedure

The surface quality is crucial for growth of epitaxial layers on III-V semiconductor substrates. In this work the procedures of epi-ready semi-insulating (SI) GaAs wafer preparation were developed. The atomic force microscopy (AFM), triple crystal X-ray diffraction (TCD) and X-ray photoelectron spectroscopy (XPS) were used to monitor morphology and composition of substrates with different chemical treatment history. We propose an optimised epi-ready SI GaAs wafer preparation procedure involving NH₄OH:H₂O₂:H₂O/NaOCl:H₂O₂:H₂O etching/polishing.     

XeF(2) gas-assisted focused-electron-beam-induced etching of GaAs with 30 nm resolution

We demonstrate the gas-assisted focused-electron-beam (FEB)-induced etching of GaAs with a resolution of 30 nm at room temperature. We use a scanning electron microscope (SEM) in a dual beam focused ion beam together with xenon difluoride (XeF(2)) that can be injected by a needle directly onto the sample surface. We show that the FEB-induced etching with XeF(2) as a precursor gas results in isotropic and smooth etching of GaAs, while the etch rate depends strongly on the beam current and the electron energy. The natural oxide of GaAs at the sample surface inhibits the etching process; hence, oxide removal in combination with chemical surface passivation is necessary as a strategy to enable this high-resolution etching alternative for GaAs.     

Novel optical and structural properties of porous GaAs formed by anodic etching of n+-GaAs in a HF:C2H5OH:HCl:H2O2:H2O electrolyte: effect of etching time

Porous GaAs layers have been formed by anodic etching of n ⁺-type GaAs (10.0) substrates in a HF:C₂H₅OH:HCl:H₂O₂:H₂O electrolyte. A dramatic impact of etching time on the optical and structural properties of porous GaAs layer is demonstrated. The nano/micro-features of porous GaAs layers are revealed by scanning electron microscopy (SEM) imaging. Two-peak room temperature photoluminescence (PL), “blue-green” and “green-yellow”, is obtained in all prepared porous GaAs samples. Proper adjustment of etching time is found to produce a white color layer, instead of the usual dark gray color of porous GaAs. This is found to cause vast enhancement in the intensity of the visible PL in porous GaAs layer. Chemical composition and structural characterization by means of X-ray photoelectron spectroscopic (XPS), X-ray diffraction (XRD), and micro-Raman spectroscopy, confirm that this layer is characterized with monoclinic β-Ga₂O₃ rich surface. Etching time induced—modification of structural and chemical properties of porous GaAs layer is discussed and correlated to its PL behavior. It is inferred that the “blue-green” PL in porous GaAs can be ascribed to different degrees of quantum confinement in GaAs nanocrystallites, whereas, the “green-yellow” PL is highly influenced by the As₂O₃ and Ga₂O₃, content in the porous GaAs layer. In addition, the reflectance measurements reveal an anti-refection trend of behavior of porous GaAs layers in the spectral range (500–1,100 nm).     

Hierarchical silicon etched structures for controlled hydrophobicity/superhydrophobicity

Silicon surface hydrophobicity has been varied by using silane treatments on silicon pyramid surfaces generated by KOH anisotropic etching. Results demonstrated that by altering the surface hydrophobicity, the apparent contact angle changed in accord with the Wenzel equation for surface structures with inclined side walls. Hierarchical structures were also constructed from Si pyramids where nanostructures were added by Au-assisted electroless HF/H2O2 etching. Surface hydrophobicity and superhydrophobicity were achieved by surface modification with a variety of silanes. Stability of the Cassie state of superhydrophobicity is described with respect to the Laplace pressure as indicated by the water droplet meniscus in contact with the hierarchical structures. The contact angle hysteresis observed is also discussed with respect to water/substrate adhesion.     

纳米多孔硅的电化学制备及性能研究

纳米多孔硅是一种潜在的化学和生物传感材料,本文采用电化学腐蚀法制备纳米多孔硅。采用SEM技术分析多孔硅的表面形貌,研究了腐蚀条件对多孔硅的孔隙率、厚度、I-V特性的影响。结果表明,多孔硅的孔隙率随着腐蚀电流密度和腐蚀时间的增加而呈线性增大趋势;其厚度随着腐蚀电流密度的增加而近似呈线性增大趋势,随腐蚀时间的成倍增加而显著增大;其I-V特性表现出非整流的欧姆接触。     

Synthesis of 1D regular arrays of gold nanoparticles and modeling of their optical properties

Self-organized formation of uniform coating of semiconductor substrate by metal nanoparticles offers a convenient and efficient access to large-scale arrays of uniform metal-semiconductor nanostructures. We used a cheap and facile method of photoinduced chemical gold deposition from an aqueous or alcohol gold salt solution onto semiconductor surface (GaAs, InP). By controlling of both the solution composition and the deposition conditions, gold particles of 10-50 nm in diameter were obtained and the gold covering degree of the semiconductor surface was varied in a wide range. Morphology of the nano/micro structures formed was characterized by atomic force microscopy and scanned electron microscopy with local element analysis. The investigations show that the semiconductor surface patterning can be used for the selective deposition of gold nanoparticles, because they are located predominantly at the tops of the microrelief. We have used specially textured by the anisotropic chemical etching microrelief surfaces of semiconductor single crystal as templates and have obtained nanoparticle arrays in the shape of 1D systems of near parallel quasiperiodical wires. For the periodic 1D array of metal nanowires built into the air-semiconductor interface the spectral and angular dependencies of the transmittance/reflectance of the polarized light have been obtained theoretically using differential formalism. These dependencies demonstrate non-monotonic behaviour at surface plasmon polariton excitation conditions and show possibility of designing functional subwavelength devices.     

Micropolarizer fabricated from CaCO3 by anisotropic wet etching

A micropolarizer is fabricated on a birefringent crystal, calcite, by anisotropic wet etching. This device consists of a v-groove grating on calcite, covered by an index-matching material. The grating is fabricated by acid wet etching. When its acid concentration and stirring speed are altered, the etching mechanism can be controlled within the surface-reaction regime. This results in anisotropic etching, which produces a v-groove grating on a calcite surface. This v-groove grating can be fabricated to have a period as small as 2 microm. To the best of our knowledge, this type of v-groove calcite grating is reported for the first time. Although the transmission efficiency of this device is wavelength dependent, a broadband micropolarizer can be made by gluing together two devices with periods of 12 and 16 microm.     

Evolution of two-dimensional crystal morphologies by surface diffusion with anisotropic surface free energies

The Wulff shape of a crystal surface in equilibrium under anisotropic surface free energy has been widely reported and a typical differentiable anisotropic free energy functional form has been used in illustrations. Here we study the evolution of an arbitrary initial two-dimensional crystal shape to equilibrium and we classify the anisotropy in three cases. We find that when the typical surface free energy is critically anisotropic, multiple equilibrium states exist and the evolved equilibrium shape depends on the initial crystal configuration. Numerical simulations show that corners and planar facets develop on the surface in evolution. In cases of severe anisotropy, small surface facets and coarsening can occur. In contrast, when the typical surface free energy is mildly anisotropic, the evolving surface is smooth and the equilibrium shape is unique for a variety of the initial crystal shapes.     

Investigations of surface layers produced by chemical treatment of GaP

The composition and structure of etched (100) GaP surfaces were studied just after etching and during aging. The two etchants, aqua regia and bromine in methanol, act through diffusion-limited and kinetically controlled processes respectively. The surface layer was found to have a substoichiometric oxide composition. The structural state of the surface etched with aqua regia could be explained by the chemical behaviour of the gallium ions in the aqueous medium, i.e. amorphous and porous surface layers containing water were produced at room temperature, whilst a dense crystalline water free layer was formed at boiling temperature. The bromine etchant also produced a water free surface. Just after etching (1.5–2) × 10¹⁵ O atoms cm^-2 and about 2 × 10¹⁵ disordered Ga atoms cm^-2 were produced by the acid etchant on the surface (determined by backscattering analysis). These values were not dependent on the etching temperature, in agreement with the diffusion-limited character of the aqua regia action.     

Wet etching of GaN,AlN, and SiC: a review

The wet etching of GaN, AlN, and SiC is reviewed including conventional etching in aqueous solutions, electrochemical etching in electrolytes and defect-selective chemical etching in molten salts. The mechanism of each etching process is discussed. Etching parameters leading to highly anisotropic etching, dopant-type/bandgap selective etching, defect-selective etching, as well as isotropic etching are discussed. The etch pit shapes and their origins are discussed. The applications of wet etching techniques to characterize crystal polarity and defect density/distribution are reviewed. Additional applications of wet etching for device fabrication, such as producing crystallographic etch profiles, are also reviewed.     

Novel anti-reflection technology for GaAs single-junction solar cells using surface patterning and Au nanoparticles

Single-junction GaAs solar cell structures were grown by low-pressure MOCVD on GaAs (100) substrates. Micro-rod arrays with diameters of 2 microm, 5 microm, and 10 microm were fabricated on the surfaces of the GaAs solar cells via photolithography and wet chemical etching. The patterned surfaces were coated with Au nanoparticles using an Au colloidal solution. Characteristics of the GaAs solar cells with and without the micro-rod arrays and Au nanoparticles were investigated. The short-circuit current density of the GaAs solar cell with 2 microm rod arrays and Au nanoparticles increased up to 34.9% compared to that of the reference cell without micro-rod arrays and Au nanoparticles. The conversion efficiency of the GaAs solar cell that was coated with Au nanoparticles on the patterned surface with micro-rod arrays can be improved from 14.1% to 19.9% under 1 sun AM 1.5G illumination. These results show that micro-rod arrays and Au nanoparticle coating can be applied together in surface patterning to achieve a novel cost-effective anti-reflection technology.     

Bottom-up photonic crystal lasers

The directed growth of III-V nanopillars is used to demonstrate bottom-up photonic crystal lasers. Simultaneous formation of both the photonic band gap and active gain region is achieved via catalyst-free selective-area metal-organic chemical vapor deposition on masked GaAs substrates. The nanopillars implement a GaAs/InGaAs/GaAs axial double heterostructure for accurate, arbitrary placement of gain within the cavity and lateral InGaP shells to reduce surface recombination. The lasers operate single-mode at room temperature with low threshold peak power density of ～625 W/cm2. Cavity resonance and lasing wavelength is lithographically defined by controlling pillar pitch and diameter to vary from 960 to 989 nm. We envision this bottom-up approach to pillar-based devices as a new platform for photonic systems integration.     

Control of bio-MEMS surface chemical properties in micro fluidic devices for biological applications

Surface chemistry of silicon/glass based bio-MEMS was controlled by depositing plasma polymerized acrylic acid (ppAc) films at two different electrode positions in a two-stage plasma reactor. AFM and XPS were used to characterize the surface roughness and surface chemistry of the films, respectively. The surface of bio-MEMS was highly functionalized with carboxylic/ester functionalities with a very good surface uniformity. The proportion of carbon atoms as C-OX, C(==O)OX functionalities was decreased and an increase in C==O functionalities was observed when the electrode position was increased from the mesh. These functionalized bio-MEMS devices have advantages in fabrication of reusable micro fluidic devices and the variation of fluid velocity by changing the surface properties may be used to develop a micro-mixing system to control the mixing ratio of different fluids for different biological and chemical applications.     

Fabrication of high aspect ratio silicon micro-tips for field emission devices

The evolution of higher order {221} and {331} crystal planes during corner undercutting in the anisotropic etching of (100) silicon is discussed, and the occurrence of highly vertical (72.5°) {311} planes unique to KOH etches are demonstrated. Using a combined etching technique, very high aspect ratio micro-tips are formed and their distinct advantages for vacuum microelectronics and field-emission devices (FED) are described. This revised version was published online in November 2006 with corrections to the Cover Date.     

硅量子线的研究

利用常规硅工艺的反应离子刻蚀、各向异性化学腐蚀、热氧化和超低压ＣＶＤ生长技术，成功地硅单晶衬底上制作了硅／二氧化硅异质界面结构超精细硅量子线。本项研究结果对开展低维量子结构物理及硅量子器件的研究具有十分重要的意义。     

Laser etching and dry processing

Controlled, anisotropic etching of different materials commonly used in microelectronics is an important processing step in microfabrications. During recent years it has been demonstrated that lasers can be used for initiating and enhancing the etching process in many gas-solid (dry processing) and liquid-solid (wet processing) systems. The laser-induced reaction could be either photochemical or thermochemical. Using laser etching technique a variety of materials such as Al, Ta, Ni/Fe, GaAs, InP, Si, SiO₂ mylar, different polymers and superconducting materials have been processed. In this paper we briefly review these laser etching experiments.     

Nanoembossed polymer substrates for biomedical surface interaction studies

Biomedical devices are moving towards the incorporation of nanostructures to investigate the interactions of biological species with such topological surfaces found in nature. Good optical transparency and sealing properties, low fabrication cost, fast design realization times, and biocompatibility make polymers excellent candidates for the production of surfaces containing such nanometric structures. In this work, a method for the production of nanostructures in free-standing sheets of different thermoplastic polymers is presented, with a view to using these substrates in biomedical cell-surface applications where optical microscopy techniques are required. The process conditions for the production of these structures in poly(methyl methacrylate), poly(ethylene naphthalate), poly(lactic acid), poly(styrene), and poly(ethyl ether ketone) are given. The fabrication method used is based on a modified nanoimprint lithography (NIL) technique using silicon based moulds, fabricated via reactive ion etching or focused ion beam lithography, to emboss nanostructures into the surface of the biologically compatible thermoplastic polymers. The method presented here is designed to faithfully replicate the nanostructures in the mould while maximising the mould lifetime. Examples of polymer replicas with nanostructures of different topographies are presented in poly(methyl methacrylate), including nanostructures for use in cell-surface interactions and nanostructure-containing microfluidic devices.