Martin Steglich

The structural and optical properties of black silicon by inductively coupled plasma reactive ion etching

Black Silicon nanostructures are fabricated by Inductively Coupled Plasma Reactive Ion Etching (ICP-RIE) in a gas mixture of SF6 and O2 at non-cryogenic temperatures. The structure evolution and the dependency of final structure geometry on the main processing parameters gas composition and working pressure are investigated and explained comprehensively. The optical properties of the produced Black Silicon structures, a distinct antireflection and light trapping effect, are resolved by optical spectroscopy and conclusively illustrated by optical simulations of accurate models of the real nanostructures. By that the structure sidewall roughness is found to be critical for an elevated reflectance of Black Silicon resulting from non-optimized etching processes. By analysis of a multitude of structures fabricated under different conditions, approximate limits for the range of feasible nanostructure geometries are derived. Finally, the technological applicability of Black Silicon fabrication by ICP-RIE is discussed.

Improvement of Ge-on-Si photodiodes by black silicon light trapping

A light-trapping scheme for normal incidence Ge-on-Si photodiodes, utilizing needle-like black silicon nanostructures is presented. Simulations reveal that light absorption in thin rear Ge films can be enhanced several times due to both the antireflection and the scattering effect of the nanostructure. It is shown that especially films with thicknesses ≤100 nm benefit from the black silicon nanostructure, e.g., resulting in a 5 to 10 times higher absorptance at 1500 nm for a 100 nm thick film. Theoretical predictions are experimentally proved by reflectance-transmittance measurements of crystalline Ge films sputtered on black silicon substrates.

A normal-incidence PtSi photoemissive detector with black silicon light-trapping

A normal-incidence light-trapping scheme relying on black silicon surface nanostructures for Si-based photoemissive detectors, operating in the IR spectral range, is proposed. An absorptance enhancement by a factor of 2–3 is demonstrated for technologically most relevant, ultrathin (2 nm–3 nm) PtSi rear layers on Si. It is shown that this increase can be translated into an equivalent increase in responsivity because of the absorption limitation of detector performance. Pd2Si/p-Si detectors with black silicon are suggested as promising candidates for room temperature detection in the third optical window with an expected external quantum efficiency in the range of 9%–14%.

Heteroepitaxial Ge-on-Si by DC magnetron sputtering

The growth of Ge on Si(100) by DC Magnetron Sputtering at various temperatures is studied by Spectroscopic Ellipsometry and Transmission Electron Microscopy. Smooth heteroepitaxial Ge films are prepared at relatively low temperatures of 380°C. Typical Stransky-Krastanov growth is observed at 410°C. At lower temperatures (320°C), films are essentially amorphous with isolated nanocrystallites at the Si-Ge interface. A minor oxygen contamination at the interface, developing after ex-situ oxide removal, is not seen to hinder epitaxy. Compensation of dislocation-induced acceptors in Ge by sputtering from n-doped targets is proposed.

Ge-on-Si photodiode with black silicon boosted responsivity

Normal-incidence Ge-on-Si photodiodes with 300 nm thick intrinsic Ge absorber layer and black silicon light-trapping are fabricated and analyzed with regard to their responsivity. Compared to a standard Ge-on-Si photodiode without black silicon, the black silicon device exhibits a 3-times increased responsivity of 0.34 A/W at 1550 nm. By that, the problematic bandwidth-responsivity trade-off in ultrafast Ge-on-Si detectors can be widely overcome. The black silicon light-trapping structure can be applied to the device rear during back-end processing.

Self-organized, effective medium black silicon antireflection structures for silicon optics in the mid-infrared

Thanks to its high quality and low cost, silicon is the material of choice for optical devices operating in the mid-infrared (MIR; 2 μm to 6 μm wavelength). Unfortunately in this spectral region, the refractive index is comparably high (about 3.5) and leads to severe reflection losses of about 30% per interface. In this work, we demonstrate that self-organized, statistical Black Silicon structures, fabricated by Inductively Coupled Plasma Reactive Ion Etching (ICP-RIE), can be used to effectively suppress interface reflection. More importantly, it is shown that antireflection can be achieved in an image-preserving, non-scattering way. This enables Black Silicon antireflection structures (ARS) for imaging applications in the MIR. It is demonstrated that specular transmittances of 97% can be easily achieved on both flat and curved substrates, e.g. lenses. Moreover, by a combined optical and morphological analysis of a multitude of different Black Silicon ARS, an effective medium criterion for the examined structures is derived that can also be used as a design rule for maximizing sample transmittance in a desired wavelength range. In addition, we show that the mechanical durability of the structures can be greatly enhanced by coating with hard dielectric materials like diamond-like carbon (DLC), hence enabling practical applications. Finally, the distinct advantages of statistical Black Silicon ARS over conventional AR layer stacks are discussed: simple applicability to topological substrates, absence of thermal stress and cost-effectiveness.

AZO/Ag/AZO transparent conductive films: correlation between the structural, electrical, and optical properties and development of an optical model

Multilayer transparent electrodes consisting of a tri-layer structure of Al-doped zinc oxide and silver (AZO/Ag/AZO) prepared by inline DC magnetron sputtering at room temperature were investigated. The AZO working gas pressure during deposition was identified as a crucial parameter to influence both the transmittance and sheet resistance of the transparent electrode. By a reduction of the pressure to an optimal value of 0.15 Pa, highest Figure-of-Merit values reported so far for suchlike prepared AZO/Ag/AZO systems could be achieved. In the course of layer characterization, a clear correlation between the coating microstructure and measured electrical and optical properties could be established. Furthermore, we present a model that describes the transmittance spectra of real-structure AZO/Ag/AZO tri-layer systems in a quantitative manner while explicitly considering the specific optical response of the AZO-silver interfaces. Using a generalized Maxwell-Garnett approach with a Gaussian distribution of the Depolarization factors, the interface roughness was described as an effective interfacial layer leading to an improved agreement between measured and simulated spectra.