Syed Minhaz Hossain

Role of parasitics in humidity sensing by porous silicon

Abstract Humidity sensing by porous silicon (PS) layer is commonly reported either by capacitive sensing or by conductive sensing. A critical analysis of both capacitive and conductive sensing by microporous PS layer is presented here. The influences of parasitic capacitances and resistances unavoidably associated with the active porous layer on the measured changes in capacitance and resistance of the humidity sensor with variation of humidity are analysed. The role of contact geometry, signal frequency and porosity of PS layer are also discussed. It is shown that capacitive sensing is more sensitive in low frequency range owing to the relative contributions of parasitic components.

A hygrometer comprising a porous silicon humidity sensor with phase-detection electronics

A novel hygrometer is presented, comprising a capacitive humidity sensor with a porous silicon (PS) dielectric and electronics. The adsorption of water vapor by the PS layer leading to change of its effective dielectric constant is modeled with an effective medium approximation (EMA). A simple, but precise, phase-sensitive electronic circuit has been developed. This detects any change of phase of a sinusoidal signal transmitted through the PS dielectric and correlates to ambient humidity. It is outlined how the nonlinear response of the sensor is compensated through piecewise linearization. The sensor is tested in combination with the phase detection circuitry. Excellent linearity over the entire range of relative humidity is achieved. Experimental results show a resolution better than 0.1% and an accuracy of 2% (near the transition region) and better than 0.1% (otherwise). The response time is less than 10 s with good stability.

Free standing luminescent silicon quantum dots: evidence of quantum confinement and defect related transitions.

We report the synthesis of luminescent, free standing silicon quantum dots by dry and wet etching of silicon and silicon oxide core/shell nanostructures, which are synthesized by controlled oxidation of mechanically milled silicon. Dry and wet etching performed with CF(4) plasma and aqueous HF, respectively, result in the removal of the thick oxide shell of the core/shell nanostructures and affect an additional step of size reduction. HF etch is capable of producing isolated, spherical quantum dots of silicon with dimensions ∼ 2 nm. However, the etching processes introduce unsaturated bonds at the surface of the nanocrystals which are subsequently passivated by oxygen on exposure to ambient atmosphere. The photoluminescence spectra of the colloidal suspensions of these nanocrystals are characterized by double peaks and excitation dependent shift of emission energy. Comparison of the structural, absorption and luminescence characteristics of the samples provides evidence for two competing transition processes--quantum confinement induced widened band gap related transitions and oxide associated interface state mediated transitions. The results enable us to experimentally distinguish between the contributions of the two different transition mechanisms, which has hitherto been a challenging problem.

Stability in photoluminescence of porous silicon

Abstract The effect of aging on photoluminescence (PL) of porous silicon has been studied for different storage media with a view to find suitable conditions for stabilizing the PL spectra. IR studies have been performed on both initially oxidized and oxide-free porous silicon samples to get an insight of the possible chemical changes in the porous layer after treating it in a number of environments. The changes in the PL spectra, both blue shift and red shift in appropriate environments, are ascribed to growth in Si=O bonds leading to trapped electron states at the Si/SiO2 interface in conformity with the recently proposed model [M.V. Wolkin et al., Phys. Rev. Lett. 82 (1999) 197]. In addition, a mechanism and a theoretical model for oxidation of silicon nanocrystallites in porous silicon have been proposed to explain the observed experimental results. An excellent agreement between the observed rate of shift of PL peak wavelength with storage time and that calculated from the proposed model has been found. Possibility of stabilizing the PL intensity and peak wavelength by keeping it in a non-oxidizing environment and to tune the PL peak wavelength with different storage media have also been discussed.

Silicon and silicon oxide core-shell nanoparticles: Structural and photoluminescence characteristics

We report the synthesis of spherical core-shell structures of silicon and silicon oxide by a novel route of forced external oxidation of ball milled silicon. Structural investigations reveal the formation of a crystalline silicon core surrounded by an amorphous oxide shell, with core and shell dimensions varying approximately between 4–10 and 55–170 nm, respectively. The observations suggest partial amorphization of crystalline silicon, invasive oxygen induced passivation of dangling bonds, and formation of different types of defects in the nanocrystalline silicon/silicon oxide core-shell structure, particularly at the interfaces. No detectable photoluminescence (PL) is obtained from the as-milled silicon, but the oxidized core-shell structures exhibit strong room temperature PL, detectable with unaided eye. The peak energy of the PL spectra blueshifts with an increase in excitation energy, with the peak positions varying from 2.24 to 2.48 eV under external excitation ranging from 2.41 to 3.5 eV. The obse...

Luminescent core-shell nanostructures of silicon and silicon oxide: Nanodots and nanorods

We report synthesis and luminescent characteristics of core-shell nanostructures of silicon and silicon oxide having two different morphologies—spherical (nanodot) and rodlike (nanorod), prepared by controlled oxidation of mechanically milled crystalline silicon and by exfoliation of the affected layer of porous silicon. Colloidal suspensions of these nanostructures exhibit intense room temperature photoluminescence (PL), detectable with the unaided eye. PL band peak energies of the colloidal suspensions formed from porous silicon are blue shifted by ∼1 eV compared to the as-prepared films on silicon substrate. In addition, PL spectra of all the colloidal suspensions blueshift with increase in excitation energy but the PL peaks of as-prepared porous silicon are independent of excitation. However, shape of the nanocrystals (spherical or rodlike) is found to have little effect on the emission spectra. These observations are explained in terms discretization of phonon density of states and electronic transit...

Electrode design and planer uniformity of anodically etched large area porous silicon

The structure of electrodes in the electrochemical formation cell of porous silicon greatly influences the planar uniformity of the porous silicon layer. In this paper a systematic study of the planar uniformity of large area porous silicon layer formed by different electrode structure is reported based on photoluminescence and reflectance scanning of the surface. A new design of the electrode structure is developed for achieving satisfactory planar uniformity of large area porous silicon layer.

Superlinear photovoltaic effect in Si nanocrystals based metal-insulator-semiconductor devices

Superlinear-variation in short circuit photocurrent with increasing incident optical power has been observed in metal-insulator-semiconductor structures having a silicon rich oxinitride active layer containing silicon nanocrystals. A model has been elaborated where an internal gain mechanism explains the superlinear photovoltaic effect. The internal gain mechanism is due to secondary carrier generation (SCG) from sub-bandgap levels in the nanocrystal. SCG is caused by impact excitation from the photogenerated conduction band electrons. The sub-bandgap levels are associated to traps formed at the dielectric/Si-nanocrystals interface.

Mechanism and control of formation of porous silicon onp-type Si

A simple extension of Beale’s and Lehmann’s models for the formation of porous silicon layer onp-type silicon is proposed with a view to explain the experimental conditions necessary for obtaining either uniform vertical pores or non-uniform pore branching, as desired. A uniformity parameter is defined and correlated with the measured porosity. The dependence of the porosity and the uniformity factor with the various formation parameters of porous layer are studied experimentally and explained qualitatively.

Temperature dependent photoluminescence from porous silicon nanostructures: Quantum confinement and oxide related transitions

Temperature dependent photoluminescence (PL) spectroscopy along with structural investigations of luminescent porous Si enable us to experimentally distinguish between the relative contributions of band-to-band and oxide interface mediated electronic transitions responsible for light emission from these nanostructures. Porous Si samples formed using high current densities (J ≥ 80 mA/cm2) have large porosities (P ≥ 85%) and consequently smaller (∼1-6 nm) average crystallite sizes. The PL spectra of these high porosity samples are characterized by multiple peaks. Two dominant peaks—one in the blue regime and one in the yellow/orange regime, along with a very low intensity red/NIR peak, are observed for these samples. The high energy peak position is nearly independent of temperature, whereas the yellow/orange peak red-shifts with increasing temperature. Both the peaks blue shift with ageing and with increasing porosity. The intensity of the blue peak increases whereas the yellow/orange peak decreases with i...