S. Lenk

Metal-germanium-metal ultrafast infrared detectors

We demonstrate silicon-based ultrafast metal–semiconductor–metal (MSM) photodetectors for near infrared optocommunication wavelengths. They show a response time of 12.5 ps full width at half maximum (FWHM) at both 1300 and 1550 nm wavelengths. The overall external quantum efficiencies are 13% at 1320 nm and 7.5% at 1550 nm. The sensitive volumes are 270 nm thick Ge films, grown on Si(111) by molecular beam epitaxy. Interdigitated Cr metal top electrodes with 1.5–5 μm spacing and identical finger width form Schottky contacts to the Ge film. A Ti-sapphire femtosecond laser with an optical parametric oscillator and an electro-optic sampling system are used to evaluate the temporal response, which is limited by the transit time of the carriers between electrodes. In addition, results on Si–Ge MSM heterostructure detectors with plate capacitor geometry are presented. At 1550 nm an ultrafast response of 9.4 ps FWHM and an overall quantum efficiency of 0.9% are measured.

The Role of Oxidative Etching in the Synthesis of Ultrathin Single‐Crystalline Au Nanowires

The fabrication of ultrathin single-crystal Au nanowires with high aspect ratio and that are stable in air is challenging. Recently, a simple wet-chemical approach using oleylamine has been reported for the synthesis of Au nanowires with micrometer length and 2 nm in diameter. Despite efforts to understand the mechanism of the reaction, an ultimate question about the role of oxygen (O(2)) during the synthesis remained unclear. Here we report that the synthesis of ultrathin Au nanowires employing oleylamine is strongly affected by the amount of O(2) absorbed in the reaction solution. Saturating the solution with O(2) leads to both a high-yield production of nanowires and an increase in their length. Nanowires with diameters of about 2 nm and lengths of 8 μm, which corresponds to an aspect ratio of approximately 4000, were produced. The role of oxygen is attributed to the enhanced oxidation of twin defects on Au nanoparticles formed in the first stage of the reaction. Understanding the role of oxidative etching is crucial to significantly increasing the yield and the length of ultrathin Au nanowires.

Fabrication of epitaxial CoSi2 nanowires

We have developed a method for fabricating epitaxial CoSi2 nanowires using only conventional optical lithography and standard silicon processing steps. This method was successfully applied to ultrathin epitaxial CoSi2 layers grown on Si(100) and silicon-on-insulator substrates. A nitride mask induces a stress field near its edges into the CoSi2/Si heterostructure and leads to the separation of the CoSi2 layer in this region during a rapid thermal oxidation step. A subsequent etching step and a second oxidation generate highly homogenous silicide wires with dimensions down to 50 nm.

Luminescence lifetime of the 1.5-µm emission of ß-FeSi 2 precipitate layers in Silicon

Semiconducting iron disilicide precipitates in silicon were fabricated by ion beam synthesis and characterised by photoluminescence and TEM. We have measured the decay time of the 1.55-µm line, which was found to be 10 µs at 10 K. No evidence for a fast, direct transition could be found from the time resolved photoluminescence measurements. Furthermore, we have measured the electroluminescence decay time of a demonstrator FeSi2-Si light emitting device. This shows a fast time response of below 30 ns.

Fast time response from Si–SiGe undulating layer superlattices

We have grown Si–Si1−xGex undulating layer superlattices with x=0.39 and 0.45 by molecular-beam epitaxy on top of epitaxial implanted CoSi2 layers and fabricated vertical metal–semiconductor–metal detectors. The detectors show a quantum efficiency of 5% for the wavelength of 1320 nm and 0.9% for 1550 nm. We performed temporal response measurements, using a Ti:sapphire laser and an optical parametric oscillator which generates ultrafast pulses at infrared wavelengths. An electrical response time of 16 ps full width at half maximum was obtained at a wavelength of 1300 nm.

Epitaxial CoSi 2 -nanostructures: an approach to Si nanoelectronics

CoSi2-nanostructures were fabricated using a self-assembly process involving local oxidation of silicides (LOCOSI). The nanostructures are generated along the edge of a mask consisting of SiO2 and Si3N4, deposited by plasma enhanced chemical vapor deposition (PECVD) and patterned with conventional optical lithography. The mask induces a stress field near its edges into the underlying CoSi2/Si-heterostructure. Rapid thermal oxidation (RTO) leads to the separation of the CoSi2 layer in this region due to the concomitant anisotropic diffusion of the cobalt atoms in the stress field. Using this method, uniform gaps and narrow wires were produced from 20-30 nm-thick single-crystalline, epitaxial CoSi2-layers grown by molecular beam allotaxy (MBA) on conventional Si(100) and silicon-on-insulator (SOI) substrates. These structures with dimensions down to 40 nm can be used as building blocks for nanoscale metal-oxide-semiconductor field effect transistor (MOSFET) devices. We produced 70 nm gate-length Schottky barrier MOSFETs (SBMOSFETs) on SOI using the silicide nanostructures. These devices can be driven as both p-channel and n-channel MOSFETs without complementary substrate doping and show good I-V characteristics.

Low frequency noise in strained silicon nanowire array MOSFETs and Tunnel-FETs

MOSFETs and Tunnel-FETs (TFETs) based on arrays of nanowires (NWs) with 10 × 10 nm2 cross-section have been fabricated with strained silicon on insulator substrates. MOSFET devices show near ideal subthreshold slope close to 60 mV/dec proving excellent channel control achieved by high-k/metal gate stack. As expected fundamental differences between MOSFETs and TFETs in current-voltage characteristics are observed and analyzed. Low frequency noise spectra are studied for both types of devices. The devices show different behavior in terms of noise spectral density as a function of the applied gate voltage. A Hooge parameter of α = 7.3 × 10-3 is derived for the NW MOSFETs.

Fast IR Si/SiGe superlattice MSM photodetectors with buried CoSi 2 contacts

We fabricated vertical metal-semiconductor-metal (MSM) detectors with Si/Si1-xGex (x = 0.39 and 0.45) superlattices as active detector regions. An epitaxial buried CoSi2 contact served as the bottom electrode and a Cr contact as the top electrode. The superlattices, grown by molecular beam epitaxy, showed vertically ordered SiGe islands. With the MSM detectors we obtained quantum efficiencies of 5% for the wavelength of 1.32 µm and 1% for 1.55 µm. Using a Ti:sapphire laser and an optical parametric oscillator which generates ultrafast pulses at IR wavelengths, the temporal response of the MSM detectors was measured. The detectors showed response times of 12 ps.

Nanometer patterning of epitaxial CoSi 2 on Silicon-on-insulator substrates

Nanometer patterning of thin single crystalline CoSi2 layers on silicon-on-insulator (SOI) by local oxidation was studied. A nitride mask on top of the silicide layer was patterned by optical lithography to create the local stress. During the subsequent oxidation or oxynitridation, the silicide layer is separated at the edges of the nitride mask, where the stress is largest. The nanopatterning of the CoSi2 layers was investigated under different oxidation ambients, and with different mask thicknesses. A feature size as small as 45 nm was obtained for 24 nm epitaxial CoSi2 layers on SOI by this nanopatterning technique.

Influence of Silicon Doping on the SA-MOVPE of InAs Nanowires

The influence of Si-doping on the growth and material characteristics of InAs nanowires deposited by metal-organic vapor phase epitaxy (MOVPE) was investigated. It was observed that above a certain partial pressure ratio, doping has an influence on the morphology. The nanowires exhibit better uniformity but lower height vs. diameter aspect ratio as the supply of the dopant increases. It was consistantly found that the specific conductance of the nanowires also increases. Moreover the electrical measurements showed a transition from semiconducting to metallic behavior in the case of highly doped nanowires.