Papers by Author: Volker Cimalla

Abstract: Ohmic properties, thermal stability and surface morphology of Al-based and non-aluminium metallizations are investigated in dependence on the annealing temperature and initial composition. Non-aluminium contacts show poor ohmic properties, while contact resistivity of 3.47x10-5 Ω.cm2 is achieved for Ti/Al/Ti/Au metallization with a former-Ti/Al ratio of (30 wt.% /70 wt.%). Thermal properties of the Al-based metallization are improved by application of Mo layer as a barrier under the upper Au film of the contact structure. These contacts show excellent thermal stability at operating temperatures as high as 400oC. The less Al amount in the contact composition and Mo barrier layer contribute to the smoother surface and better edge acuity.

Abstract: With the increasing requirements for microelectromechanical systems (MEMS) regarding stability, miniaturization and integration, novel materials such as wide band gap semiconductors are receiving more attention. The outstanding properties of group III-nitrides offer many more possibilities for the implementation of new functionalities and a variety of technologies are available to realize group III-nitride based MEMS. In this work we demonstrate the application of these techniques for the fabrication of full-nitride MEMS. It includes a novel actuation and sensing principle based on the piezoelectric effect and employing a two-dimensional electron gas confined in AlGaN/GaN heterostructures as integrated back electrode. Furthermore, the actuation of flexural and longitudinal vibration modes in resonator bridges are demonstrated as well as their sensing properties.

Abstract: Electrical, thermal and chemical properties of Ti/Al/Ti/Au ohmic contacts with different former Ti-Al ratio are investigated for application in GaN HEMTs. Lowest resistivity of 4.22x10-5 Ω.cm2 has been obtained to the channel of the HEMT structure. It is found out that the initial Ti/Al ratio influences the optimal annealing temperature at which the lowest resistivity is obtained and the element distribution and interface chemistry of the annealed contacts. XPS analysis revealed two compounds contributing to ohmic properties: an intermetal compound AlAu2 in the contact layer and a semimetal TiN at the interface with GaN.

Abstract: The adjustment of the properties of 3C-SiC based MEMS devices, i.e. the quality factor and resonant frequency, was achieved by changing the residual stress and the 3C-SiC material quality of the SiC-layers grown on Si(111) by manipulating the nucleation conditions and growth conditions with Ge deposition prior to the carbonization and epitaxial growth. Previous Raman analysis of the SiC-layers and measured resonant frequencies and quality factors of the processed MEMS show a dependence on the Ge amount at the interface of the Si/SiC heterostructure, which allows to adjust the MEMS properties to the requirements needed for certain applications.

Abstract: Isotropic etching of silicon carbide was achieved using a capacitive coupled parallel plate reactor in plasma etching mode and SF6 at elevated substrate temperatures. It was observed to be remarkable at substrate temperatures above 350°C. The influence of chamber pressure, masking materials, rf-power and substrate temperature were analyzed. Thereby, 8.5° off-axis oriented 4HSiC wafers exhibit a larger vertical and lateral etching rate compared to on-axis oriented SiC wafers. Additionally, the erosion of nitrogen containing masking material results in a reduction of the etching rates.

Abstract: The resonant frequencies and quality factors of MEMS and NEMS depend critically on the layer quality and the residual stress in the SiC/Si heterostructure. It is demonstrated, that FTIRellipsometry is a suitable technique for monitoring the inhomogeneous residual stress inside SiC/Si heterostructures containing thin layers and their variation with during processing.

Abstract: The influence of the growth conditions on the 3C-SiC layer quality in terms of crystallinity, morphology and residual strain was investigated. In dependence on the chosen growth conditions the stress state can be varied between inhomogeneous and homogeneous strain. For the reduction of the residual strain an alternative route for the improvement of the epitaxial growth of 3CSiC( 100) on Si(100) was developed. It consists in covering the silicon wafers with germanium prior to the carbonization step. The achieved improvement in the residual strain and crystalline quality of the grown 3C-SiC layers is comparable to SOI substrates. These beneficial effects were reached by using a Ge coverage in the range of 0.5 to 1 monolayer with respect to the silicon surface.

Abstract: In this paper the multifariousness of SiC/Si heterostructures for device and sensor applications will be demonstrated. 3C-SiC based microelectromechanical resonator beams (MEMS) with different geometries actuated by the magnetomotive effect operating under ambient conditions were fabricated. The resonant frequency reaches values up to 2 MHz. The applications of these resonators are the measurement of the viscosity of liquids or mass detection. Furthermore, photonic devices in the form of SiC/Si infrared gratings for wavelength and polarization filters in infrared spectra are processed. SiC wear protection for a dosing system with the possibility to dose nano- or picoliter droplets of water based liquids as well as SiC nanomasking for catalytic agent nanostructures are demonstrated.

Abstract: The effects of argon and nitrogen bombardment of 3C-SiC surfaces at acceleration voltages below 2 keV were studied by stylus profilometry, reflectometry, reflection high energy electron diffraction and Auger electron spectroscopy (AES). The erosion rate of the SiC surface was determined. It was found that the sputtering rate for argon was three times higher compared to nitrogen. AES measurements revealed argon and nitrogen incorporation at a depth of a few nanometers as well as stoichiometric changes at the same depth scale independent of the acceleration voltage. In the case of the interaction of nitrogen ions with the 3C-SiC surface the formation of a SiCNalloy was detected.

Abstract: In the present work an UHVCVD method was developed which allows the epitaxial growth of 3C-SiC on Si substrates at temperatures below 1000°C. The developed method enable the growth of low stress or nearly stress free single crystalline 3C-SiC layers on Si. The influence of hydrogen on the growth process are be discussed. The structural properties of the 3C-SiC(100) layers were studied with reflection high-energy diffraction, atomic force microscopy, X-ray diffraction and the layer thickness were measured by reflectometry as well as visible ellipsometry. The tensile strain reduction at optimized growth temperature, Si/C ratio in the gas phase and deposition rate are demonstrated by the observation of freestanding SiC cantilevers.