Atomic-scale electronics in semiconductors

A dopant atom in a semiconductor, the solid state analogue of a hydrogen atom, has a Bohr radius of several nanometers. Because this length scale is close to being accessible by modern nanolithography, detection and control of charge and spin in a semiconductor down to the level of individual dopant atoms is within reach and provides the unique opportunity to study, manipulate, and utilize a single atom's wave function. We have performed electrical transport measurements across epitaxial defect-free nanometer-sized Schottky diodes. These were formed by self-assembled CoSi2-islands on Si(111) and contacted with the tip of a scanning tunneling microscope (STM). Greatly enhanced conductance was observed in diodes which were small compared to the Debye length in the semiconductor. The observed behavior can be understood qualitatively from a decreased barrier width for smaller diodes. On highly doped substrates, we find that individual dopant atoms even dominate the transport characteristics of our nanometer sized devices, due to their random distribution in the space charge region. The ability to observe the energy levels of single dopant atoms is essential for experimental studies of individual wave functions in a semiconductor. Preliminary results in a fabrication method for nano-devices approaching the size regime necessary for the observation of single dopants demonstrate the feasibility of our STM-based measurement method for this purpose. The most straightforward means to address an individual impurity is manipulation of its wave function with a gate. As a first approach to this problem, we theoretically studied the effect of a homogeneous electric or magnetic field on the energy levels of shallow impurities in silicon, taking the bandstructure into account. Furthermore, we used a description as hydrogen-like impurities for accurate computation of energy levels and lifetimes up to large electric fields. A similar description was used in a realistic device geometry, in which a small nearby gate influences a single dopant atom. This knowledge is particularly important for the development of a dopant-atom based quantum computer.Applied Science

Multi-Subband Monte Carlo simulations of ION degradation due to fin thickness fluctuations in FinFETs

The impact of fin thickness nonuniformities on carrier transport in n-type FinFETs is analyzed with a Multi-Subband Monte Carlo technique, which allows for an accurate description of the quasi-ballistic transport taking place in short channel devices and which comprises the dominant scattering mechanisms as well as a semi-empirical technique to handle quantization effects in the transport direction. We found that the impact of channel thickness discontinuity on the on-current is larger when the nonuniformities are located close to the Virtual Source of the device. Furthermore, the sensitivity of the on-current to thickness nonuniformity is essentially the same when considering devices with different crystal orientations. Comparison with drift-diffusion simulations reveals substantial differences in the predicted trends of the sensitivity of the drain current to thickness fluctuations in these nanoscale devices

A PSP-based small-signal MOSFET model for both quasi-static and nonquasi-static operations

In this paper, a small-signal MOSFET model is described, which takes the local effects of both velocity saturation and transverse mobility reduction into account. The model is based on the PSP model and is valid for both quasi-static and nonquasi-static (NQS) operations. Recently, it has been found that, in the presence of velocity saturation, the low-frequency capacitances cannot be determined from the Ward-Dutton charge-partitioning scheme. By use of the small-signal model developed in this paper, it is demonstrated that, in the presence of velocity saturation, no terminal drain and source charges exist, from which the capacitances can be derived. The small-signal model enables the determination of the correct capacitive behavior in the presence of velocity saturation. Furthermore, it is demonstrated how the small-signal model can be used to determine the number of collocation points needed in the large-signal NQS PSP model. Finally, inclusion of the local variation of mobility reduction due to the vertical electrical fields provides insight into the approach commonly applied in compact modeling, where these fields are replaced by global ones depending on the terminal voltages only

Stark effect in shallow impurities in Si

Applied Science

Conductance distribution in nanometer-sized semiconductor devices due to dopant statistics

Applied Science

Group-theoretical analysis of double acceptors in a magnetic field: Identification of the Si:B+ ground state

Applied Science

Photonic integrated brillouin optical time domain reflection readout unit

Brillouin scattering can be used for determining strain distributions along fibers. This is a convenient way to monitor the structural integrity of large constructions. However, the high cost of the optical circuitry has prevented wide use of this technique. We report on a design study on an integrated optical circuit for a Brillouin optical time domain reflection readout unit with low-cost potential. The circuit contains narrow linewidth tuneable distributed Bragg reflector (DBR) lasers, photodiodes with an optical mixer for coherent detection, a 10-bit digitally switched delay line for frequency tuning, and a switching fabric that allows three modes of operation. Such a circuit can be realized through one of the upcoming foundries in the field of photonic integratio

Band Offset Measurements on Ultra-Thin (100) SOI MOSFETs

This work shows experimental evidence of structural quantum confinement showing up in the electrical device characteristics through a widening of the band gap. In this work, subthreshold currents in long channel ultra-thin SOI MOSFETs with (100) crystal orientation have been analyzed for various temperatures and different silicon body thicknesses in order to extract shifts in the band edges. Although the offsets in both the valence band and conduction band contribute to the total band gap, this work concentrates on the valence band offset, as the investigated devices are p-MOSFETs. Likewise, changes in the conduction band can be measured on n-type devices. The valence band edge was found to move downward for decreasing silicon body thickness, corresponding to a widening of the band gap. This implies that devices with an extremely thin semiconductor body exhibit a stronger temperature dependence. Good agreement with theory was observed