Wet Chemical Polishing Research Articles

On-wafer fabrication of cavity mirrors for InGaN-based laser diode grown on Si

Direct bandgap III-V semiconductor lasers grown on silicon (Si) are highly desired for monolithic integration with Si photonics. Fabrication of semiconductor lasers with a Fabry–Pérot cavity usually includes facet cleavage, however, that is not compatible with on-chip photonic integration. Etching as an alternative approach holds a great advantage in preparing cavity mirrors with no need of breaking wafer into bars. However, gallium nitride (GaN) sidewalls prepared by dry etching often have a large roughness and etching damages, which would cause mirror loss due to optical scattering and carrier injection loss because of surface non-radiative recombination. A wet chemical polishing process of GaN sidewall facets formed by dry etching was studied in detail to remove the etching damages and smooth the vertical sidewalls. The wet chemical polishing technique combined with dry etching was successfully applied to the on-wafer fabrication of cavity mirrors, which enabled the realization of room temperature electrically injected InGaN-based laser diodes grown on Si.

Optimization of a Michigan-type silicon microprobe for infrared neural stimulation

This paper presents a Michigan-type deep brain silicon optrode capable of delivering infrared light into the neural tissue. Silicon optrodes were fabricated by deep reactive ion etching (DRIE) and subsequent wet chemical polishing. The proposed method is feasible to reduce the sidewall roughness significantly and is able to turn the substrate material into an infrared waveguide of sufficient efficiency for infrared neural stimulation. The advantage of our approach is that the fabrication process is fully compatible with that of functional neural microelectrodes. Moreover, there is no need to add further waveguide layers on top of such devices aiming IR stimulation, which may induce less tissue trauma. Our design also facilitates the spatially controlled illumination of the tissue through integrated micromirrors and microlenses and the precise alignment of an optical fiber through an integrated guide slot. An average system efficiency of 22.1% with a Gaussian beam profile (NA=0.13) was achieved by reducing the RMS roughness of the sidewall down to 8.7nm using 1310nm wavelength illumination and coupled core diameter of 9μm. To our knowledge, this is the first investigation of the optical properties of a Michigan-type silicon microprobe aiming infrared neural stimulation at the substrate level.

Passivation of Inline Wet Chemically Polished Surfaces for Industrial PERC Devices

We study the impact of the residual surface morphology resulting from rear side wet chemical polishing of initially random pyramid textured surfaces on large area industrial rear surface thermal oxide passivated Cz Si solar cells. Cell parameters for three different surfaces (low, medium and high removal of Si) and a fully textured rear surface as reference are presented for solar cells with rear sides metallized either by physical vapour deposition or by screen printing technique. For both metallization schemes a flattened surface clearly results in higher values for open circuit voltage and short circuit current density and thus also cell efficiency. Median efficiencies up to 19.4% (239cm2, as processed) and stabilized peak efficiency of 19.3% (confirmed by Fraunhofer ISE CalLab) are reached. Carrier lifetime and quantum efficiency measurements confirm that the rear surface recombination and injection dependence strongly reduces for flattened surfaces, which is attributed to changes in crystal orientation and reduced surface area.