Eva Vazsonyi

Anisotropic etching of silicon in a two-component alkaline solution

The application of anisotropic alkaline etching for the fabrication of micromechanical parts is mostly based on the strong angular dependence of the etch rates in single-crystal silicon. In some applications, the smooth, defect-free etched surface is of high importance and from this point of view the strong anisotropy is disadvantageous. With this consideration, the composition of the alkaline etching solution has been changed by the addition of strong oxidizing agent. In such an etching solution, a mirror-like and defect-free silicon surface has been obtained, even after long etching times. In parallel, the reduction of the anisotropy, i.e. the decrease of the ratio (R001/R111) of the etch rates has occurred. In (001) oriented wafers, when the pattern alignment follows the 100 directions, this type of anisotropic etching produces vertical walls. The absolute values of the etch rates in all crystallographic directions are relatively low and they change with the doping concentration. The strong oxidizing component is supposed to ensure homogeneous oxidation (i.e. passivation layer formation) on the whole surface even though there are differences in the activation energy of the surface states. In this process the dissolution of the passivation layer is the rate-limiting chemical reaction.

The role of hydrogen in luminescence of electrochemically oxidized porous Si layer

Abstract Remarkable improvements of photoluminescence (PL) efficiency together with large blueshift of PL peak wavelength have been achieved by electrochemical oxidation. It is known from Fourier transform infrared (FT-IR) measurements that the SiO 2 covering on pore walls has discontinuities where the surface states are terminated by hydrogen. Dramatic degradation of luminescence transitions in anodically oxidized PSL, due to thermal hydrogen desorption, indicates the importance of hydrogen in electrochemical surface passivation.

Characterization of an Integrable Single-Crystalline 3-D Tactile Sensor

Porous-Si-micromachining technique was used for the formation of single-crystalline force-sensor elements, capable of resolving the three vector components of the loading force. Similar structures presented so far are created from deposited polycrystalline Si resistors embedded in multilayered SiO2/Si3N4 membranes, using surface micromachining technique for a cavity formation. In this paper, the authors implanted four piezoresistors in an n-type-perforated membrane, having their reference pairs on the substrate in order to form four half bridges for the transduction of the mechanical stress. They successfully combined the HF-based porous-Si process with conventional doping and Al metallization, thereby offering the possibility of integration with readout and amplifying electronics. The 300times300 mum2 membrane size allows for the formation of large tactile arrays using single-crystalline-sensing elements of superior mechanical properties. They used the finite-element method for modeling the stress distribution in the sensor, and verified the results with real measurements. Finally, they covered the sensors with different elastic silicon-rubber layers, and measured the sensors altered properties. They used continuum mechanics to describe the behavior of the rubber layer

Nanocrystal characterization by ellipsometry in porous silicon using model dielectric function

Porous silicon layers were prepared by electrochemical etching of p-type single-crystal Si (c-Si) of varying dopant concentration resulting in gradually changing morphology and nanocrystal (wall) sizes in the range of 2–25nm. We used the model dielectric function (MDF) of Adachi to characterize these porous silicon thin films of systematically changing nanocrystal size. In the optical model both the surface and interface roughnesses have to be taken into account, and the E0, E1, and E2 critical point (CP) features are all described by a combination of several lineshapes (two-dimensional CP, excitonic, damped harmonic oscillator). This results in using numerous parameters, so the number of fitted parameters were reduced by parameter coupling and neglecting insensitive parameters. Because of the large number of fitted parameters, cross correlations have to be investigated thoroughly. The broadening parameters of the interband transitions in the measured photon energy range correlate with the long-range orde...

Morphological investigation of porous samples by resonant backscattering spectrometry

Abstract When performing backscattering spectroscopy measurements slightly above the energy where a sharp resonance exists in the elastic scattering cross section a characteristic resonance peak appears in the energy spectra of the backscattered particles. Unlike homogeneous samples where the position and width of this peak are mainly determined by the experimental set-up (incident energy, resonance width, etc.) for porous materials the peak width depends on the structure of the sample. This effect is caused by fluctuations in the stopping power along the trajectories of incident and scattered ions. Using the 3045 keV resonance in the 16 O(α,α) 16 O reaction for analysing oxidised porous silicon samples, it was demonstrated both experimentally and theoretically that: (i) the widening of the resonance peak is easily observable, (ii) it is closely related to the actual morphology of the samples and (iii) it can be applied to determine morphological details, as porosity, average pore diameter and anisotropy of the pore directions.

Characterization of ITO/porous silicon LED structures

Abstract The electrical behavior and the electroluminescence (EL) obtained from n- and p-type ITO/porous silicon LEDs have been characterized simultaneously at different temperatures. Stability and aging in air were investigated, and means for avoiding their detrimental effects in the experiments are suggested. The dominating current carrying mechanism responsible for visible light emission in both substrate types has been identified to be Fowler–Nordheim tunneling. This emphasizes the contribution of embedded nanoparticles (quantum dots) rather than the role of nanowires in efficient EL.

Densification and noble gas retention of ion-implanted porous silicon

Abstract To investigate the gas retention behaviour of porous materials, 500 keV He and 100 keV Ar ion implantations were carried out into porous silicon samples of columnar structure. Samples of different porosities were irradiated using a current density of ∼ 6 × 10 12 ions/cm 2 /s up to a fluence of 10 × 10 17 and 10.3 × 10 16 ions/cm 2 , for He + and Ar + , respectively. The quantity of the retained gas was determined by Rutherford backscattering spectrometry (RBS). It was found that in contrast to the case of compact materials, the gas retention is negligible for the lowest fluence implantations (∼ 5 × 10 16 and ∼ 1 × 10 15 ions/cm 2 , for He and Ar respectively). At higher fluences the retention rate gradually increases, approaching the rate of full retention. For He implantation the retention behaviour shows a strong porosity dependence. The above phenomenon is accompanied by the densification of the material as it was observed by scanning electron microscope on the fractured samples and also indicated by the gradual depression of the whole implanted area according to surface profiler measurements. The densification grows until the whole implanted layer (from the surface to the implanted range) almost completely transforms into compact material. Possible mechanisms responsible for the above phenomena are also discussed.

Isotropic texturing of multicrystalline silicon wafers with acidic texturing solutions [solar cell manufacture]

The limitations associated with the most widely used anisotropic etching techniques in industrial multicrystalline solar cell manufacturing processes can be reduced by using acidic etching solutions. Optimised etching conditions were found which lower the reflectance from multi-c wafers below levels obtained by anisotropic texturing with NaOH solutions. The surfaces of acidic textured wafers show a very homogeneous distribution of the reflected light. Solar cells produced on acidic textured wafers show higher performance than in the case of alkaline texturing.

Experimental proof for nanoparticle origin of photoluminescence in porous silicon layers

A series of nonlinear phenomena in the excitation intensity dependence of photoluminescence (PL) was observed in porous silicon (PS) at room temperature. From a low level of excitation, the blue shift of the PL spectra was detected followed by complete saturation of the integrated PL intensity, without detectable change in the spectral position. This behavior may well be experimental proof of the nonparticle origin of PS light emission.

Characterization of the anisotropic etching of silicon in two-component alkaline solution

The characteristics of wet chemical etching processes were compared for crystalline Si in aqueous solutions of pure NaOH and NaOH with added oxidizing agent NaOCl (sodium-hypochlorite). Due to the addition of NaOCl to the reference NaOH solution, the etch rate decreased for the {1?0?0} and {1?1?0} planes and increased for the {1?1?1} plane. The difference between the activation energy of these etching processes for the three different crystallographic directions became rather small due to the combined effect of the oxidation and the subsequent etching sub-processes. This means that the temperature dependence of the number of kinks as chemical oxidation sites is very similar for the different planes. For the {1?1?1} plane the oxidation sub-process has a higher efficiency resulting in an increased etch rate whereas the etch rate decays for the {1?0?0} and {1?1?0} planes due to enhanced passivation, and these two phenomena contribute to the decrease of the anisotropy. Due to the addition of NaOCl, the oxidation sub-process is more isotropic but still site dependent. In the practice, the etched surface remains of good quality without defect development. In the presence of the oxidizing agent, the difference of the etch rates in the 1?0?0 and 1?1?0 directions is reversed to R100 < R110. This fact can be exploited in the formation of novel 3D structures on the (1?0?0) wafers by forming a square-masking pattern aligned precisely to the 1?0?0 directions.