S. Zangooie

Ellipsometric characterization of anisotropic porous silicon Fabry–Pérot filters and investigation of temperature effects on capillary condensation efficiency

Porous silicon Fabry-Perot optical filters with reflectivity peaks in the visible and near infrared spectral range have been manufactured electrochemically and characterized with variable angle spectroscopic ellipsometry. Generalized ellipsometry and the Bruggeman effective medium approximation were employed to study the anisotropy of the samples including determination of the tilt of the optic axis relative to the sample normal. At a wavelength of 700 nm, the difference between the real parts of the ordinary and extraordinary indexes of refraction of the low and high index sublayers were 0.05 and 0.07, respectively. In addition, the effect of temperature on the spectral redshifts caused by capillary condensation of water and toluene vapors in the porous material are investigated. It is demonstrated that a temperature decrease causes the spectral shifts to occur at lower partial pressures, indicating a more effective vapor capture at lower temperatures. Finally, it is shown that this phenomenon can be use...

Protein Adsorption in Thin Porous Silicon Layers

Porous silicon layers with thicknesses in the range 100-400 nm and average porosities in the range 38-71% were prepared by electrochemical anodization. Variable angle spectroscopic ellipsometry was ...

Infrared ellipsometry characterization of porous silicon Bragg reflectors

We investigate porous silicon Bragg reflectors in a nondestructive manner using variable angle-of-incidence infrared spectroscopic ellipsometry. In addition to the thickness, volume porosity, inhomogeneity, and optical anisotropy, properties of the solid content of the porous material are investigated in terms of dielectric function and surface chemistry. The material was found to have positive birefringence. The high sensitivity of the technique is employed to detect and identify infrared resonant absorptions related to different Si-H as well as Si-O-Si vibrational modes. The average electrical resistivity of the solid content of the porous material is determined to be 0.03 Omega cm, which is larger than the corresponding bulk value of 0.019 Omega cm. Furthermore the average carrier concentration in the porous material shows a decrease from 6.2 x 10(18) cm(-3) to 4 x 10(18) cm(-3).

Self-organization in porous 6H-SiC

Pores in porous 6H‐SiC were found to propagate first nearly parallel with the basal plane and gradually change direction and align with the c axis. As a consequence, well-defined columnar pores were formed. It was shown that the rate of change of propagation directions was influenced by the etching parameters, such as hydrofluoric acid concentration and current density. Larger currents resulted in formation of larger pores. Pore sizes were found to increase with depth due to a decrease of the acid concentration. In addition, due to chemical etching effects, larger pore sizes were obtained close to the sample surface. SiC is a promising candidate material for high-power, high-temperature, high-frequency, and optical device applications. These are mainly due to the high-saturation electron drift velocity, thermal conductivity, and breakdown field of the material. 1 A spin off from SiC materials research results from the high surface area material obtained by electrochemical dissolution of the crystalline bulk material in hydrofluoric acid (HF) solutions. 2‐5 The resulting structures may find applications in disciplines such as sensor technologies, considering the superior mechanical, thermal, and electronic properties, as well as long time stability of the materials compared with other porous structures such as porous silicon (PS). 6 In addition, creating quantum wires through the etching process resulting in an increase of the band gap compared with the bulk material may provide future optoelectronics application possibilities. Strong photo- and electroluminescence above the band gap and in the blue and ultraviolet (UV) part of the spectrum has also been reported. 7,8 Therefore, an accurate understanding of the influence of the etching conditions on the morphological properties of the material is required. In a recent report we emphasized the anisotropy in pore propagation in nitrogen-doped 4H‐SiC. 9 Pores were found to initially propagate nearly parallel to the sample surface and gradually change

Microstructural and infrared optical properties of electrochemically etched highly doped 4H–SiC

Pores in porous 4H–SiC are found to propagate first nearly parallel with the basal plane and then gradually change plane of propagation towards the direction of the c axis. A similar anisotropy in pore propagation is found in porous 6H–SiC. A disordered phase is encountered at the interface between crystalline SiC and the pores. Formation of this phase was attributed to the etching conditions. Characterization of the material with nondestructive infrared spectroscopic ellipsometry in the photon energy range 0.062–0.62 eV provides average thickness and porosity in good agreement with electron microscopy observations. Anodization of SiC introduces remarkable changes to the reststrahlen band. A shallow minimum at 0.113 eV is attributed to the Berreman effect. In addition, a sharp peak at 0.126 eV is discussed to be related to the in-depth inhomogeneity and particle shape effects in the material.

Surface, Pore Morphology, and Optical Properties of Porous 4H-SiC

Electron microscopy and spectroscopic ellipsometry are used to investigate the microstructural and optical properties of porous SiC. It is discussed that lc dislocations are likely to play a role in the initial phases of the anodization process in terms of formation of nanometer sized holes. Pores are found to initially propagate nearly parallel with the sample surface and gradually change direction toward the c axis. Pore sizes are found to increase with depth toward the interface between the porous SiC and the substrate. A different pore morphology is found along the internal surfaces of micropipes, where structures are relatively large in size and appear spherical in shape. The anisotropy in pore propagation influences the etch rate, which varies in a nonlinear manner with the anodization time. The etching rate is also influenced by the larger absorptivity of the porous layers caused by formation of a disordered phase at the interface between the crystalline SiC and the pores. Ellipsometric analysis of porous SiC layers yields thicknesses and porosities in good agreement with the electron microscopy observations. Optical properties of the solid content of the porous layers are significantly different from those of hulk crystalline SiC, and depend on the etching time and sample thickness.

Infrared response of multiple-component free- carrier plasma in heavily doped p-type GaAs

Spectroscopic ellipsometry is used to measure the dielectric function of heavily doped p-type GaAs for wave numbers from 100 to 2000 cm−1. Due to partial filling of the heavy- and light-hole valence bands, heavy holes as well as light holes form a multiple-component plasma coupled with longitudinal optical phonons. Line-shape analysis of the infrared response allows differentiating between light- and heavy-hole contributions to the carrier plasma, and the results observed suggest nonparabolicity effects of the heavy- and light-hole valence bands in GaAs.Spectroscopic ellipsometry is used to measure the dielectric function of heavily doped p-type GaAs for wave numbers from 100 to 2000 cm−1. Due to partial filling of the heavy- and light-hole valence bands, heavy holes as well as light holes form a multiple-component plasma coupled with longitudinal optical phonons. Line-shape analysis of the infrared response allows differentiating between light- and heavy-hole contributions to the carrier plasma, and the results observed suggest nonparabolicity effects of the heavy- and light-hole valence bands in GaAs.

Infrared optical properties of aged porous GaAs

Aging properties of porous GaAs were investigated nondestructively using variable angle of incidence infrared spectroscopic ellipsometry. In addition to the thickness and volume porosity, properties of the solid part of the porous material are investigated in terms of the long-wavelength dielectric function and chemical composition. The high sensitivity is employed to detect and identify infrared resonant absorptions related to different vibration modes of cubic and amorphous As 2 O 3 . Resonances centered at 333.3, 480, 785.8, 838, and 1045.5 cm −1 are from cubic As 2 O 3 , whereas resonances centered at 350, 490, and 808.5 cm −1 are from amorphous As 2 O 3 .

Porous Anodic 4H-SiC: Thickness Dependent Anisotropy in Pore Propagation and Ellipsometric Characterization

Pores in porous 4H-SiC propagate first nearly parallel with the sample surface and gradually change direction tc,wards the direction of the c-axis. A disordered region at the interface between crystalline SiC and the pores is encountered ed, which significantly influences the optical response of the material. Thickness and porosity of porous SIC as obtained using variable angle of incidence spectroscopic ellipsometry show good agreement with electron microscopy observations.