Ignaz Eisele

Vertical Si-Metal-Oxide-Semiconductor Field Effect Transistors with Channel Lengths of 50 nm by Molecular Beam Epitaxy

The growth conditions in molecular beam epitaxy (MBE) were studied for the fabrication of vertical Si-metal-oxide-semiconductor field effect transistors (MOSFET) with channel lengths down to 50 nm. The short channel length imposes severe constraints on the doping profile. MBE growth provided a steepness of 10 nm/dec for boron and 2 nm/dec for antimony. The sharpness of the doping profile was sustained throughout the process by keeping all process temperatures below 700° C. The high crystal quality and the well-defined doping profile was verified by the good performance of a triangular barrier diode. A vertical n-MOSFET with an estimated channel length of 50 nm was grown. The drain and gate characteristics were discussed for a source drain voltage regime from U sd=0 V to 1 V.

Reliable hybrid GasFETs for work-function measurements with arbitrary materials

Abstract We have developed a novel hybrid technique for manufacturing gas-sensing field-effect structures (GasFETs) with an air gap. To prove the efficiency of the device, gas measurements with the well-known Pt (sensitive layer) and H2 (gas) system have been carried out at room temperature. Samples produced identically show reproducible signals of the same magnitude. Simultaneously, we have recorded the electrical response of a platinum-coated Kelvin probe. Results obtained by the hybrid GasFETs are transferrable to those obtained by the Kelvin probe. With the new technology for device fabrication, our transducer meets the requirements for measuring work-function changes caused by gases on a wide variety of sensitive layers.

Ozone detection in the ppb range with work function sensors operating at room temperature

Thin evaporated films of potassium iodide (KI) are highly sensitive to ozone in work function measurements. Among various other materials KI is well suited for limit detection in the ppb range in normal humid air with work function sensors like SGFET. The cross-sensitivities to other oxidants are comparatively small which is due to a redox system of highly oxidated states on the layer surface as is found by XPS analysis.

Velocity saturation in short channel field effect transistors

Abstract The high electric field transport of short channel MOS transistors has been investigated. Saturation of the electron velocity leads to strong inhomogeneous channel conditions which have to be accounted for by a self consistent analysis of the experimental results. It follows that the saturation value of the drift velocity in the inversion channel is close to the bulk value of 107 cm sec−1.

Evidence for mobility domains in (100) silicon inversion layers

Abstract In proof of the existence of domains with different mobilities measurements of Shubnikov-de Haas oscillations, piezoresistance, and transverse “Hall” field due to mobility anisotropy are provided. The energy splitting between the ground state subbands of different valleys has been determined.

Effective masses in (100) silicon inversion layers

Abstract Shubnikov-de Haas oscillations at low electron concentrations have been studied under mechanical stress. The effective cyclotron mass of the anisotropic valleys has been determined to be m c m 0 = 0.43 . Additional piezo-resistance and anisotropic conductivity measurements proved a complete transfer of electrons from isotropic into anisotropic valleys. The existence of domains is necessary for the explanation of the experimental results.

Surface quantum oscillations in (100) inversion layers under uniaxial stress

Abstract Surface quantum oscillations have been measured with uniaxially stressed (100) n-type inversion layers. A relation between mechanical stress and cyclotron mass mc has been observed. In the quantum limit the two-fold valley degeneracy is lifted by about 1 meV with compression.

Valley degeneracy and mobility anisotropy under mechanical stress on (111) silicon inversion layers

Abstract Shubnikov-de Haas oscillations, piezoresistance, Hall mobility, and transverse “Hall” field due to mobility anisotropy have been studied on n-channel (111) Si inversion layers. The valley degeneracy was found to be 2 between 1.7 and 300 K. Under uniaxial mechanical stress the initially isotropic conductivity became strongly anisotropic. All results can be described by the existence of domains in the inversion layer.

Evaluation of gas mixtures with different sensitive layers incorporated in hybrid FET structures

Abstract We have developed a new hybrid technique for gas sensors based on field effect structures (GASFET). A wide range of materials can be used as sensitive layer (e.g., inorganic or organic, metal or oxide) for a selective detection of different gases. GASFET with an air gap using as sensitive layer different materials (Pt, SnO2, Ga2, O3, V2O5) were built. The interactions of these different sensitive layers with various gases were investigated. Gas measurement were carried out at room temperature (25 °C) and at 70 °C. Work function changes of these senstitive layers upon exposure to hydrogen, ammonia, nitrogen dioxide and carbon monoxide as well as all mixtures of two gases were recorded. The carrier gas was synthetic air. Matrices representing the signals as a function of two gas concentrations were obtained. By means of pattern recognition and analytical techniques we are able to identify the gaseous species and to evaluate their concentration if only one gas is present in the gas mixture. When two gases are mixed, they can be identified and their concentrations can be approximated.

Stress and intersubband correlation in the silicon inversion layer

Abstract The influence of uniaxial mechanical stress on transport phenomena of (100) and (111) silicon inversion layers has been studied. From magneto-quantum oscillations it is evident that at low temperatures the valley degeneracy remains 2, independent of surface orientation and stress. In order to understand the experimental results, coupling effects between different conduction band valleys are assumed. This can lead to a domain like structure which causes an additional scattering. Piezoresistance, anisotropic conductivity, and Hall as well as field effect mobility are analyzed with respect to these coupling effects.