Wilfried Erfurth

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.

Nucleation-Induced Self-Assembly of Multiferroic BiFeO3-CoFe2O4 Nanocomposites

Large areas of perfectly ordered magnetic CoFe2O4 nanopillars embedded in a ferroelectric BiFeO3 matrix were successfully fabricated via a novel nucleation-induced self-assembly process. The nucleation centers of the magnetic pillars are induced before the growth of the composite structure using anodic aluminum oxide (AAO) and lithography-defined gold membranes as hard mask. High structural quality and good functional properties were obtained. Magneto-capacitance data revealed extremely low losses and magneto-electric coupling of about 0.9 μC/cmOe. The present fabrication process might be relevant for inducing ordering in systems based on phase separation, as the nucleation and growth is a rather general feature of these systems.

Strain relaxation in nanopatterned strained silicon round pillars

Periodic arrays of strained Si (sSi) round nanopillars were fabricated on sSi layers deposited on SiGe virtual substrates by electron-beam lithography and subsequent reactive-ion etching. The strain in the patterned sSi nanopillars was determined using high-resolution UV micro-Raman spectroscopy. The strain relaxes significantly upon nanostructuring: from 0.9% in the unpatterned sSi layer to values between 0.22% and 0.57% in the round sSi pillars with diameters from 100 up to 500nm. The strain distribution in the sSi nanopillars was analyzed by finite element (FE) modeling. The FE calculations confirm the strain relaxation after patterning, in agreement with the results obtained from Raman spectroscopy.

UV-Raman imaging of the in-plane strain in single ultrathin strained silicon-on-insulator patterned structure

Confocal UV-Raman with glycerin-immersed high numerical aperture objective lens was used to probe the local strain in individual strained Si structures. The investigated structures were fabricated from 15 nm thick strained silicon-on-insulator substrates with a tensile strain of 0.8%. Two-dimensional maps of the postpatterning strain were obtained for single structures with lateral dimension of 500 nm. We found that the strain measured at the center partially relaxes and drops to 0.67% as a result of patterning-induced free surfaces. This relaxation increases toward the edges following nearly a parabolic behavior. A different strain behavior was observed for larger structures.

Nanoscale patterning induced strain redistribution in ultrathin strained Si layers on oxide

We present a comparative study of the influence of the thickness on the strain behavior upon nanoscale patterning of ultrathin strained Si layers directly on oxide. The strained layers were grown on a SiGe virtual substrate and transferred onto a SiO(2)/Si substrate using wafer bonding and hydrogen ion induced exfoliation. The post-patterning strain was evaluated using UV micro-Raman spectroscopy for thin (20 nm) and thick (60 nm) nanostructures with lateral dimensions in the range of 80-400 nm. We found that about 40-50% of the initial strain is maintained in the 20 nm thick nanostructures, whereas this fraction drops significantly to approximately 2-20% for the 60 nm thick ones. This phenomenon of free surface induced relaxation is described using detailed three-dimensional finite element simulations. The simulated strain 3D maps confirm the limited relaxation in thin nanostructures. This result has direct implications for the fabrication and manipulation of strained Si nanodevices.

The complex evolution of strain during nanoscale patterning of 60 nm thick strained silicon layer directly on insulator

The strain behavior in nanoscale patterned biaxial tensile strained Si layer on insulator is investigated in 60-nm-thick nanostructures with dimensions in the 80–400 nm range. The in-plane strain is evaluated by using UV micro-Raman. We found that less than 30% of the biaxial strain is maintained in the 200×200 nm2 nanostructures. This relaxation, due to the formation of free surfaces, becomes more important in smaller nanostructures. The strain is completely relieved at 80 nm. This phenomenon is described based on detailed three-dimensional finite element simulations. The anisotropic relaxation in rectangular nanostructures is also discussed.

Strained Silicon on Wafer Level by Wafer Bonding: Materials Processing, Strain Measurements and Strain Relaxation

Different methods to introduce strain in thin silicon device layers are presented. Uniaxial strain is introduced in CMOS devices by process-induced stressors allowing the local generation of tensile or compressive strain in the channel region of MOSFETs. Biaxial strain is introduced by growing thin silicon layer on SiGe buffer and transferring it to an oxidized silicon substrates. The latter forms strained silicon on insulator (SSOI) wafer characterized by tensile strain only. Future CMOS device technologies require the combination of the global strain of SSOI substrates with local stressors to increase the device performance.

Flexible replication technique for high-aspect-ratio nanostructures.

A flexible, nondestructive, and cost-effective replication technique for nanostructures is presented. The advantages of the process are: 1) it allows for tailoring structural parameters of the replica (e.g., line width) nearly independent of the structural geometry of the master; 2) it allows for replication of high-aspect-ratio structures also in polymer materials from solution (especially noncurable polymers) such as polystyrene and polymethylmethacrylate; 3) it includes an easy separation process, thus preserving the master for repeated use. Linear grating replicas with line widths ranging from 88 to 300 nm are obtained using a single nanostructured master. Nanofibers and complex nanopatterned replicas are achievable. The presented technique and its flexibility show that atomic layer deposition is a unique tool for the preparation of high-efficiency polarizer diffractive optics, photonics, electronics, and catalysts.

On the electronic properties of a single dislocation

A detailed knowledge of the electronic properties of individual dislocations is necessary for next generation nanodevices. Dislocations are fundamental crystal defects controlling the growth of different nanostructures (nanowires) or appear during device processing. We present a method to record electric properties of single dislocations in thin silicon layers. Results of measurements on single screw dislocations are shown for the first time. Assuming a cross-section area of the dislocation core of about 1 nm2, the current density through a single dislocation is J = 3.8 × 1012 A/cm2 corresponding to a resistivity of ρ ≅ 1 × 10−8 Ω cm. This is about eight orders of magnitude lower than the surrounding silicon matrix. The reason of the supermetallic behavior is the high strain in the cores of the dissociated dislocations modifying the local band structure resulting in high conductive carrier channels along defect cores.

PHASE SEPARATION BY INTERNAL OXIDATION AND REDUCTION IN A Cu-5at%Ni-ALLOY

The basic idea of their experiments is to separate the constituents of a completely miscible alloy by internal oxidation and following reduction. A detailed theoretical description of the internal oxidation and reduction can be found elsewhere. The feasibility of the described phase separation procedure is demonstrated for a completely miscible Cu-5at%Ni alloy. The components of this alloy fulfills the requirements for chemical behavior and diffusivity.