Martin Schade

High-resolution investigations of ripple structures formed by femtosecond laser irradiation of silicon

We report on the structural investigation of self-organized periodic microstructures (ripples) generated in Si(100) targets after multishot irradiation by approximately 100-fs to 800-nm laser pulses at intensities near the single shot ablation threshold. Inspection by surface sensitive microscopy, e.g., atomic force microscopy (AFM) or scanning electron microscopy (SEM), and conventional and high-resolution transmission electron microscopy reveal complex structural modifications upon interaction with the laser: even well outside the ablated area, the target surface exhibits fine ripple-like undulations, consisting of alternating crystalline and amorphous silicon. Inside the heavily modified area, amorphous silicon is found only in the valleys but not on the crests which, instead, consist of highly distorted crystalline phases, rich in defects.

Proof of damage-free selective removal of thin dielectric coatings on silicon wafers by irradiation with femtosecond laser pulses

The microstructural impact of selective femtosecond laser ablation of thin dielectric layers from monocrystalline silicon wafers was investigated. Various spots opened by 280 fs laser pulses at λ = 1.03 μm wavelength and 50 fs pulses at 800 nm, respectively, were analyzed in detail using Raman and transmission electron microscopy. The results show clearly that the thin dielectric films can be removed without any detectable modification of the Si crystal structure in the opened area. In contrast, in adjacent regions corresponding to laser fluence slightly below the breaking threshold, a thin layer of amorphous silicon with a maximum thickness of about 50 nm is found at the Si/SiO2 interface after laser irradiation. More than one pulse on the same position, however, causes structural modification of the silicon after thin film ablation in any case.

Microscopic Origins of Catastrophic Optical Damage in Diode Lasers

Extremely early phases of the catastrophic optical damage (COD) process in 808-nm emitting GaAs/Al0.35Ga0.65 As high-power diode lasers are prepared by the application of short single current pulses. Typical energy entries during these pulses are on the order of 100 nJ within several 100 ns. The resulting defect pattern is investigated by high-resolution microscopy. The root of the COD is found to be located at the waveguide of the laser structure. Analysis of material composition modifications as a result of early COD phase points to melting being involved in the process. During recrystallization, an Al-rich pattern is formed that encloses a volume of a few cube micron of severely damaged material.

Equilibrium shapes of polycrystalline silicon nanodots

This study is concerned with the topography of nanostructures consisting of arrays of polycrystalline nanodots. Guided by transmission electron microscopy (TEM) measurements of crystalline Si (c-Si) nanodots that evolved from a “dewetting” process of an amorphous Si (a-Si) layer from a SiO2 coated substrate, we investigate appropriate formulations for the surface energy density and transitions of energy density states at grain boundaries. We introduce a new numerical minimization formulation that allows to account for adhesion energy from an underlying substrate. We demonstrate our approach first for the free standing case, where the solutions can be compared to well-known Wulff constructions, before we treat the general case for interfacial energy settings that support “partial wetting” and grain boundaries for the polycrystalline case. We then use our method to predict the morphologies of silicon nanodots.

Regular arrays of Al nanoparticles for plasmonic applications

Optical properties of aluminium nanoparticles deposited on glass substrates are investigated. Laser interference lithography allows a quick deposition of regular, highly periodic arrays of nanostructures with different sizes and distances in order to investigate the shift of the surface plasmon resonance for, e.g., photovoltaic, plasmonic or photonic applications. The variation of the diameter of cylindrical Al nanoparticles exhibits a nearly linear shift of the surface plasmon resonance between 400 nm and 950 nm that is independent from the polarization vector of the incident light. Furthermore, particles with quadratic or elliptic base areas are presented exhibiting more complex and polarization vector dependent transmission spectra.

Investigation of the chemical composition profile of SiGe∕Si(001) islands by analytical transmission electron microscopy

The authors have determined the composition profile within individual Si1−xGex nanoscale islands on Si(001). Samples have been grown by means of liquid phase epitaxy in the Stranski-Krastanov mode. By applying electron energy loss spectroscopy, the intensities of Si K and Ge L edges have been measured to determine the relative atomic concentration of germanium. The quantification of the composition suggests a profile comprising of two regions with different linear concentration gradients.

Long-time feedback in self-organized nanostructures formation upon multipulse femtosecond laser ablation

Self-organized nanostructures (ripples) on the target surface after multi-pulse femtosecond laser ablation exhibit, obviously, a positive multi-pulse feedback in the self-organization process. Experiments on different targets (CaF2, Si) investigate this feedback in more detail, in particular its dynamics. The influence of pulse number and time separation between successive pulses on both the size and the complexity of the nanostructures as well as the size of the modified surface area is studied. In addition to a dependence on the coupled dose, confirming incubation effects previously observed on ablation efficiency, both modified area as well as pattern feature size and complexity decrease with increasing pulse-to- pulse delay between 1 ms and 1 s, indicating an unexpectedly long lifetime of the feedback. Further, for silicon, a persisting modification of the crystalline structure is found well beyond the ablation spot, though no apparent change in surface morphology can be seen. Mapping the band-to-band photoluminescence displays a spatially modulated dramatic increase of non-radiative recombination compared to unaffected material.

Spectroscopic Investigation of Silicon Polymorphs Formed by Indentation

Silicon polymorphs have been prepared by means of scratching or indentation of Si(100) surfaces. Different indenter types have been used in order to validate the independence of silicon polymorph formation from indenter geometry. The formation of silicon polymorphs could be verified by registering the loading-displacement curves. Related to the maximum loads applied, only the formation of the meta-stable silicon phases SI-III, Si-IV and Si-XII has been observed, what has been verified by Raman spectroscopy. Four different ways of the preparation of electron transparent samples are presented and compared. Finally, a first electron energy loss spectrum of certain silicon polymorphs is shown.

Mechanism of selective removal of transparent layers on semiconductors using ultrashort laser pulses

The process of ultrashort laser-assisted selective removal of thin dielectric layers from silicon substrates has a large potential for technological applications, the most straightforward one being an energy-efficient and environmentally compatible method to produce contact openings on crystalline silicon solar cells. Using photon energies above the band gap energy, ablation of such thin transparent layers is possible without noticeable damage of the silicon substrate. To understand in detail the physics behind this damage-free delamination, experiments with a variety of laser parameters were done, utilizing in particular wavelengths from UV to mid-infrared and pulse durations between 50 and 2000 fs. Experiments were also conducted using different transparent materials on silicon, e.g. SiO2 and SixNy. The ablated regions were carefully analyzed by light microscopy (LM), atomic force microscopy (AFM), Raman spectroscopy (RS), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). The results give evidence that the mechanism of damage-free ablation is initiated by ultrafast creation of electron-hole plasma by the ultrashort laser pulse itself followed by non-thermal decomposition of an ultrathin Si layer of a few nm thickness only. This process works best in the region of moderate substrate absorption, i.e. using laser photon energies only slightly above the band gap, and for the shortest pulses. In contrast, laser energy input into the dielectric layer by addressing either the UV absorption or a vibrational resonance (e.g. at λ = 9.26 μm for SiO2) allowed ablation only in connection with partial damage of the substrate.

Investigation of the change in the microstructure of thin p-type Bi-Sb-Te thermoelectric films after heat treatment

Thermoelectric devices utilizing the Peltier and the Seebeck effect have been widely used for cooling and power generation applications in the last decades. Typically they are built from bulk materials. The energy conversion efficiency of such devices depends on the thermoelectric figure-of-merit (z·T) of the thermoelectric materials. In spite of numerous works to improve the z·T values of thermoelectric materials - which are dependent on the Seebeck coefficient S, the electrical conductivity σ, and the thermal conductivity κ (z = S2·σ / κ) - only a value around unity could be reached for bulk thermoelectric materials at room temperature. However, for low dimensional nanostructures, such as nanowires or superlattices, it has been predicted that the z·T values can be enhanced significantly compared to bulk materials due to quantum-confinement effects [1]. Beside this effect the progressive miniaturization requires thin thermoelectric films with Seebeck coefficients and electrical conductivities comparable to the bulk materials.