Embedded interdigital transducers for high frequency surface acoustic waves on GaAs

We investigate high-performance, high-frequency interdigital transducers (IDTs) for the generation of surface acoustic waves(SAWs) on GaAs substrates, where the metal fingers are embedded in the substrate. We demonstrate that the acoustic reflections and the scattering of the surface modes into the substrate become considerably reduced in these transducers, leading to an increased output power. The finger embedding process is particularly relevant for the generation of powerful beams of high-frequency SAWs on weak piezoelectric substrates (such as most of the semiconducting materials) using long IDTs. We also show that the reflection reduction is important for the design of focusing single-finger IDTs, since it minimizes the effects of the finger grating on the angular dependence of the phase velocit

Latest Developments from the S-DALINAC*

The S-DALINAC is a 130 MeV superconducting recirculating electron accelerator serving several nuclear and radiation physics experiments as well as driving an infrared free-electron laser. A system of normal conducting rf resonators for noninvasive beam position and current measurement was established. For the measurement of gamma-radiation inside the accelerator cave a system of Compton diodes has been developed and tested. Detailed investigations of the transverse phasespace were carried out with a tomographical reconstruction method of optical transition radiation spots. The method can be applied also to non-Gaussian phasespace distributions. The results are in good accordance with simulations. To improve the quality factor of the superconducting 3 GHz cavities, an external 2K testcryostat was commissioned. The influence of electro-chemical polishing and magnetic shielding is currently under investigation. A digital rf-feedback-system for the accelerator cavities is being developed in order to improve the energy spread of the beam of the S-DALINAC. * Supported by the BMBF under contract no. 06 DA 820, the DFG under contract no. Ri 242/12-1 and -2 and the DFG Graduiertenkolleg 'Physik und Technik von Beschleunigern

Quantum Hall effect in a high-mobility two-dimensional electron gas on the surface of a cylinder

The quantum Hall effect is investigated in a high-mobility two-dimensional electron gas on the surface of a cylinder. The novel topology leads to a spatially varying filling factor along the current path. The resulting inhomogeneous current-density distribution gives rise to additional features in the magneto-transport, such as resistance asymmetry and modified longitudinal resistances. We experimentally demonstrate that the asymmetry relations satisfied in the integer filling factor regime are valid also in the transition regime to non-integer filling factors, thereby suggesting a more general form of these asymmetry relations. A model is developed based on the screening theory of the integer quantum Hall effect that allows the self-consistent calculation of the local electron density and thereby the local current density including the current along incompressible stripes. The model, which also includes the so-called `static skin effect' to account for the current density distribution in the compressible regions, is capable of explaining the main experimental observations. Due to the existence of an incompressible-compressible transition in the bulk, the system behaves always metal-like in contrast to the conventional Landauer-Buettiker description, in which the bulk remains completely insulating throughout the quantized Hall plateau regime

THE MULTIVALUED DISTRIBUTION OF ELECTRONS BETWEEN EQUIVALENT VALLEYS

Dans le Si-n la distribution multiforme d'électrons chauds entre des vallées équivalentes conduit à la formation de domaines de haut champ en vertu de la conductivité négative différentielle du type N et de la formation de structure des couches avec des champs opposés de Sasaki pour des températures au-dessous de 50 K.In n-Si the multivalued distribution of hot electrons among the valleys (mde) leads to static high field domains on account of N-type ndc and to layers of opposite Sasaki fields for temperatures below 50 K

Near-field phase shift photolithography for high-frequency SAW transducers

Optical lithography has been widely used in mass production of various electronic devices, mainly because of its high throughput capability. However, the resolution in conventional lithography is diffraction limited. Cost issues, on the other hand, make slower but higher resolution methods, like electron beam lithography, unattractive for industrial applications. In order to be able to continue the use of optical lithography, new schemes were developed that enhance the resolution. Phase-shifting masks, for example, alter both the amplitude and the phase of the exposing light and lead to higher resolution. Using the related phase edge method it has been shown that 100 nm features can be produced using 248 nm light. Furthermore, employing an elastomeric phase mask, commercially available photo resist, and incoherent light, down to 90 nm features were demonstrated. Here, we report on the application of a near-field phase shift technique on the fabrication of SAW transducers. This simple and low cost technique is best suited for the fabrication of SAW structures, where the metallization ratio is different from 1:1, like Narrow Gap Floating Electrode Unidirectional Transducers (NG-FEUDTS)

Spontaneous Hall effect in MBE grown Fe layers on GaAs(3 1 1) and GaAs(3 3 1) substrates

The spontaneous Hall effect in Fe layers epitaxially grown on GaAs(3 1 1)A and GaAs(3 3 1)A substrates is utilised to characterise the electronic and magnetic properties of the layers. We observe a very unusual hysteresis behaviour, which arises from the in-plane magnetisation with the easy axes along the [233] and [110] direction for the GaAs(3 1 1)A and GaAs(3 3 1)A substrates, respectively. A non-vanishing transverse magnetoresistance appears even for in-plane magnetic fields. We identify the origin of the non-vanishing transverse magnetoresistance as the spin–orbit interaction between conducting electrons and impurities. Different models, including out-of-plane magnetisation of the Fe-layers grown on high index GaAs substrates due to the stepped heterointerface and spin-orbit interaction governed by the magneto-crystalline anisotropy of bulk Fe or of the heterointerface, are discussed to explain the unusual ‘in-plane Hall effect’

Electron-beam-induced modifications of electronic properties in GaN-based quantum well structures

The electronic properties of (In,Ga)N/GaN quantum wells fabricated by MOCVDvary significantly during investigations using low-energy electron beam irradiation (LEEBI) suchas cathodoluminescence (CL) if a certain exposure dose is exceeded. For unintentionally dopedstructures, we observe a simultaneous LEEBI-induced activation of donors and acceptors. Thus,the resistivity of the layers is not varied, while the quantum efficiency and optical transition energyincreases significantly by LEEBI. A p-n structure is turned towards flat band conditions duringLEEBI indicating an electron beam induced passivation of acceptors in the p-type layer