Florentina Niebelschütz

Strain- and pressure-dependent RF response of microelectromechanical resonators for sensing applications*

MEMS resonators bear great potential for applications as RF sensors, filters and oscillators, e.g., in life sciences or information technology. A semiconductor fabrication process has been applied to prepare resonant AlN and SiC beams operating at frequencies between 0.1 and 2.1 MHz. The metallized beams were actuated in a permanent magnetic field of about 0.5 T by the Lorentz force. For systematic studies of the resonant frequencies and quality factors, the induced voltage was measured using time domain and frequency domain techniques. Resonator geometry, material and ambient pressure were varied to attain a generalized understanding of the RF performance. The dependence of the resonant frequency on tensile axial strain has been derived analytically and extended to include highly strained beams. Based on these formulas, accurate detection of the residual layer strain after fabrication is presented. To describe the quality factor a chain of beads model has been applied successfully. The influences of the beam width and the pressure-dependent viscosity on the model parameters are analyzed.

Piezoelectric actuation of (GaN/)AlGaN/GaN heterostructures

A detailed analysis of the piezoelectric response of (GaN/)AlGaN/GaN heterostructures is reported. The electromechanical properties of two types of heterostructures with an Al content of 31% are compared. Only a single two-dimensional electron gas (2DEG) is formed for samples with thin GaN cap layers, while both a 2DEG and a two-dimensional hole gas coexist in the case of thick GaN caps. The lower GaN layer represents the mechanically supporting layer, while the AlGaN film, and in some cases an additional GaN cap layer, serves as the piezoelectrically active layers for actuation. The 2DEG (at the lower AlGaN/GaN interface) provides the conducting channel which was used as back electrode for the applied external voltage. Electroreflectance spectroscopy is applied in order to determine the electric field distribution across the whole structure as a function of the applied voltage. It is found that only a part of the modulation voltage drops across the active region. Piezoelectric force microscopy yields the...

Coalescence aspects of III-nitride epitaxy

In this work, coalescence aspects of wurtzite-III-nitride epitaxy are addressed. The coalescence phenomena have been studied in thin epilayers by means of electron and atomic force microscopies, and electron and x-ray diffractions. This study generalizes the growth parameters responsible for the rapid coalescence of III-nitride films, and describes the coalescence qualitatively and, partly, analytically for the case of heteroepitaxy in nonequilibrium conditions. Coalescence time and the corresponding diffusion coefficients at elevated temperatures were estimated for GaN and InN depositions. The rate of coalescence has been found to impact on the structure and morphology of III-nitride epilayers. A simple growth model was suggested to explain the formation of domain boundaries and (0001) stacking faults formed during the coalescence. In particular, it is shown that two adjacent and tilted, hexagonal-shaped 2H domains may form a noncoherent boundary explicitly along a {11¯00} plane. We also suggest that the...

Tuning Residual Stress in 3C-SiC(100) on Si(100)

Germanium modified silicon surfaces in combination with two step epitaxial growth technique consisting in conversion of the Si(100) substrate near surface region into 3C-SiC(100) followed by an epitaxial growth step allows the manipulation of the residual strain. The morphology and the residual strain in dependence on the Ge coverage are only affected by the Ge quantity and not by the growth technique. The positive effect of the Ge coverage is attributed to changes in the morphology during the conversion process, as well as to a reduced lattice and thermal mismatch between SiC and Si in consequence of alloying the near surface region of the Si substrate with Ge.

Response of Nerve Cell to Inhibitor Recorded by Aluminium-Gallium-Nitride FET

In this work we report on the recording of extracellular potential of NG108-15 nerve cells as response to diisopropylfluorophosphate (DFP) using an open gate aluminium gallium nitride/gallium nitride (AlGaN/GaN) field effect transistor. The biocompatibility study of our GaN materials with NG108-15 nerve cells shows a proliferation rate of about 95%. The DFP was added to the medium with and without adherent cells and we record the source-drain current (IDS) of the AlGaN/GaN field effect transistor versus time. The cells react very differently to the inhibitor in the case of repeated titrations of the DFP inhibitor. A saturation concentration was determined, above which no further cell reaction was detectable. Sensor reaction without cells exhibits a clearly distinguishable behavior.

Composition and Interface Chemistry Dependence in Ohmic Contacts to GaN HEMT Structures on the Ti/Al Ratio and Annealing Conditions

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.

Gas Pressure Sensing Based on MEMS Resonators

MEMS resonators bear great potential for applications as RF sensors, filters and oscillators, e.g., in life sciences or information technology. Resonant AlN and SiC beams with operation frequencies between 0.01 and 3.4 MHz have been prepared using a semiconductor fabrication process. The metallized beams were actuated in a permanent magnetic field of about 0.5 T by the Lorentz force. The resonant response was detected in the frequency domain. Resonator geometry and material were varied to attain a generalized understanding of the RF performance in dependence of the ambient pressure. In particular the quality factor shows a high sensitivity on pressure, allowing potential application as absolute pressure sensor. Theoretical models have been applied that match well to the measurement.

Performance Modification of SiC MEMS

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.

Isotropic Etching of SiC

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.

Temperature Facilitated ECR-Etching for Isotropic SiC Structuring

In order to realize complex three dimensional or free standing structures on SiC substrates, an undercut, i.e. a selective isotropic etching process of SiC, is required. This was realized using an electron cyclotron resonance etching set up with pure SF6 and a SF6/Ar gas composition at elevated substrate temperatures. Above 350°C a significant lateral etch component was observed, which rose to a value of 50-70 nm/min increasing the substrate temperature up to 570°C during the etching process. Depending on substrate temperature the etching profiles and surface roughness were studied. Based on an analysis of the influence of microwave power, working pressure, bias voltage, gas flow and gas mixture on the etching behavior a novel isotropic, high selective, residue free etch process for SiC was developed, which allows for example the fabrication of piezoelectric actuated AlGaN/GaN resonators grown on SiC substrates.