Hans Lüth

Porous silicon multilayer optical waveguides

Optical waveguiding is demonstrated in porous silicon multilayers. Depth variations in porosity, and therefore refractive index, are achieved by switching between high and low current densities during the anodic etch process. Planar waveguiding has been demonstrated at λ = 1.28 μm. The wavelength range has been extended to the visible (λ = 0.6328 μm) by oxidising the samples to produce layered porous oxide structures. Two-dimensional strip-loaded waveguides have been produced, for both the visible and infrared, by etching into each top layer through a pre-deposited photolithographically-defined mask.

Interface and Wetting Layer Effect on the Catalyst-Free Nucleation and Growth of GaN Nanowires†

To avoid catalyst-induced contaminations that might alter the electronic properties of the material, catalyst-free growth is preferable. However, the nucleation and growth mechanisms of GaN wires in the catalyst-free procedure are still under debate. Two mechanisms are usually invoked for the nucleation and the growth of NWs. One is based on the Ga-droplet formation followed by the well-established vapor–liquid–solid mechanism, [13] and the other is based on small GaN clusters as nucleation seeds and a vapor–solid growth process. [12] In the present work the formation of crystalline GaN nanoclusters as possible NW precursors in catalyst-free plasma-assisted MBE (PAMBE) growth is studied by high-resolution transmission electron microscopy (HRTEM) imaging. Details of the interface between the GaN layer and the substrate are investigated and discussed in connection with the mechanism of catalyst-free NW growth. GaN NWs were grown at 7808C by PAMBE without the use of catalysts, on clean Si(111) and oxidized Si(100) substrates, according to the procedure described elsewhere. [6] The GaN NWs were investigated by HRTEM using a JEOL 3000F

Origin of improved RF performance of AlGaN/GaN MOSHFETs compared to HFETs

In this paper, the influence of a 10-nm-thick silicon-dioxide layer, as a passivation or as a gate insulation, on the performance of heterojunction field-effect transistors (HFETs) and metal-oxide-semiconductor HFETs (MOSHFETs), based on an undoped AlGaN/GaN heterostructure on a SiC substrate, was investigated. Channel-conductivity results yield a nearly 50% increase of mobility in the MOSHFET samples compared to the unpassivated HFETs. This increase of the transport properties of the MOSHFET channel is confirmed by a similar 45% increase of the cutoff frequency, from 16.5 to 24 GHz. Hall measurements, however, show a 10% decrease of the mobility in the heterostructure with a SiO2 top layer. In this paper, the superior performance of the MOSHFET transistor, in contradiction to the Hall results, is attributed to the screening of the Coulomb scattering of the charged surface defects by the gate-metallization layer

A vertical resonant tunneling transistor for application in digital logic circuits

A vertical resonant tunneling transistor (VRTT) has been developed, its properties and its application in digital logic circuits based on the monostable-bistable transition logic element (MOBILE) principle are described. The device consists of a small mesa resonant tunneling diode (RTD) in the GaAs/AlAs material system surrounded by a Schottky gate. We obtain low peak voltages using InGaAs in the quantum well and the devices show an excellent peak current control by means of an applied gate voltage. A self latching inverter circuit has been fabricated using two VRTTs and the switching functionality was demonstrated at low frequencies.

Constant-Current-Mode LAPS (CLAPS) for the Detectionof Penicillin

A software feedback control system for the constant-current-mode operation of the light-addressable potentiometric sensor (LAPS) was developed. The constant-current-mode LAPS (CLAPS) is suitable for online monitoring and recording of changes in the pH value or the ion concentration. An enzyme LAPS was fabricated by adsorptive immobilization of penicillinase on the pH-sensitive layer of Ta2O5. This sensor was operated in the constant-current-mode and the detection limit for penicillin G was found to be at least as low as 100 µM.

Electronic and vibrational properties of semiconducting crystalline FeSi2 layers grown on Si(111)

Semiconducting FeSi2 thin layers have been grown on Si(111) by solid state reaction under ultrahigh vacuum (UHV) conditions. The different reaction steps in the temperature range 380–750 °C were followed in situ by Auger electron spectroscopy (AES) and electron energy‐loss spectroscopy (EELS). FeSi2 is formed between 550 and 680 °C. At higher temperatures the silicide thin film disrupts forming islands as shown by EELS and ex situ medium‐energy ion scattering (MEIS). The low energy excitation spectra of the thin FeSi2 layers were measured by high‐resolution electron energy‐loss spectroscopy (HREELS). Surface phonon excitations at 50 meV energy are observed and explained in the framework of dielectric theory of surface scattering. This is the first time that Fuchs–Kliewer surface phonons, typical for heteropolar crystals such as GaAs, ZnO, etc. are so clearly observed on a silicide surface by HREELS. Their presence gives evidence of the semiconducting character of the FeSi2 overlayer. A broad loss structur...

Novel liquid sensor based on porous silicon optical waveguides

The introduction of solvents into the pores of optical waveguides formed using porous silicon is shown to dramatically reduce the interfacial scattering loss of the waveguides (by as much as 34-dB cm/sup -1/ in one example), in a reversible manner. The degree of loss reduction is dependent on the type of solvent introduced. These observations, combined with the fact that a substantial portion of the guided-mode field interacts with the solvent introduced into the pores, indicate that an enhanced sensitivity for sensor applications may be achievable across a broad range of operational wavelengths.

Flux Quantization Effects in InN Nanowires

InN nanowires, grown by plasma-enhanced molecular beam epitaxy, were investigated by means of magnetotransport. By performing temperature-dependent transport measurements and current measurements on a large number of nanowires of different dimensions, it is proven that the carrier transport mainly takes place in a tube-like surface electron gas. Measurements on three representative nanowires under an axially oriented magnetic field revealed pronounced magnetoconductance oscillations with a periodicity corresponding to a single magnetic flux quantum. The periodicity is explained by the effect of the magnetic flux penetrating the coherent circular quantum states in the InN nanowires, rather than by Aharonov-Bohm type interferences. The occurrence of the single magnetic flux quantum periodicity is attributed to the magnetic flux dependence of phase-coherent circular states with different angular momentum quantum numbers forming the one-dimensional transport channels. These phase coherent states can exist because of the almost ideal crystalline properties of the InN nanowires prepared by self-assembled growth.

The application of porous silicon to optical waveguiding technology

The porosification of silicon can be achieved by the partial electrochemical dissolution (anodization) of the surface of a silicon wafer. The degree of porosity is dependent on the anodization parameters and can generally be controlled within the constraints imposed by substrate dopant type and concentration. Control of porosity leads to control of refractive index, and therein lies the concept of using porous silicon as an optical waveguide. We discuss porous silicon wavegides, for the visible to the infrared, produced by a number of approaches: 1) epitaxial growth onto porous silicon (where the porous layer acts as a substrate for a higher refractive index waveguide epilayer); 2) ion implantation (where either selective areas of high electrical resistivity can be produced, which act as a barrier against porosification, or where the surface of a porosified layer is amorphised to form a waveguide; 3) porous silicon multilayers (where the anodization parameters are periodically varied to produce alternate layers of different porosity and thus refractive index); and 4) oxidation of porous silicon (where a porosified layer is oxidized to form a graded-index, dense or porous, oxide waveguide).

AlN and GaN epitaxial heterojunctions on 6H–SiC(0001): Valence band offsets and polarization fields

From a series of in situ photoemission experiments macroscopic electric fields are clearly demonstrated in SiC/AlN, SiC/GaN, and GaN/AlN heterostructures grown by molecular beam epitaxy on 6H–SiC(0001). A significant contribution is due to the spontaneous polarization; the piezoelectric term alone would not explain the sign of the field measured in SiC/AlN. The experimental field has lower intensity as compared to theory: the role of electronic gap states at the surface is pointed out. A self-consistent tight-binding approach which is able to describe polarization fields, dielectric screening, and free carrier screening is applied for a more consistent theoretical discussion of the experimental data. The valence band offset (VBO) has been determined for all heterojunctions under study and the apparent dependence on the overlayer thickness, due to the presence of the strong polarization fields, has been pointed out in view of a correct determination of the VBO. The VBOs at the heterojunctions obtained by e...