O. Baumgartner

Diagonal-transition quantum cascade detector

We demonstrate the concept of diagonal transitions for quantum cascade detectors (QCD). Different to standard, vertical QCDs, here the active transition takes place between two energy levels in adjacent wells. Such a scheme has versatile advantages. Diagonal transitions generally yield a higher extraction efficiency and a higher resistance than vertical transitions. This leads to an improved overall performance, although the absorption strength of the active transition is smaller. Since the extraction is not based on resonant tunneling, the design is more robust, with respect to deviations from the nominal structure. In a first approach, a peak responsivity of 16.9 mA/W could be achieved, which is an improvement to the highest shown responsivity of a QCD for a wavelength of 8 μm at room-temperature by almost an order of magnitude.

A bi-functional quantum cascade device for same-frequency lasing and detection

We demonstrate a bi-functional quantum cascade device that detects at the same wavelength as it coherently emits. Our fabricated device operates at room-temperature with a pulsed peak power emission of 45 mW and a detector responsivity of 3.6 mA/W. We show how to compensate the intrinsic wavelength mismatch between the laser and the detector, based on a bound-to-continuum design. An overlap between the laser and the detector spectra was observed from 6.4 μm to 6.8 μm. The electro-luminescence spectrum almost perfectly matches the detector spectrum, overlapping from 6.2 μm to 7.1 μm.

Understanding correlated drain and gate current fluctuations

Recently, some experimental groups have observed the occurrence of correlated drain and gate current fluctuations, which indicate that both currents are influenced by the charge state of the same defect. Since the physical reason behind this phenomenon is unclear at the moment, we evaluated two different explanations: The first model assumes that direct tunneling of carriers is affected by the electrostatic field of the charged defect. Interestingly, this model inherently predicts the gate bias and temperature dependences observed in the experiments and is therefore quite promising at a first glance. In the second model, our multi-state defect model is employed to describe trap-assisted tunneling as a combination of two consecutive nonradiative multi-phonon transitions - namely hole capture from the substrate followed by hole emission into the poly-gate. The latter transition is found to be in the weak electron-phonon coupling regime, which requires the consideration of all band states instead of only the band edges. Our investigation shows that the electrostatic model must be discarded since it predicts only small changes in the gate current while the extended variant of the multi-state defect model delivers quite promising results.

Modeling of high-k-Metal-Gate-stacks using the non-equilibrium Green’s function formalism

A high-k-metal-gate stack has been investigated using an open boundary model based on the non-equilibrium Greenpsilas function formalism. The numerical energy integration, which is crucial because of the very narrow resonant states, is pointed out in detail. The model has been benchmarked against the established classical and closed boundary Schrodinger-Poisson model. In contrast to the established models, the solution covers distinct resonant states with a realistic broadening and results in a major difference in the current density spectrum.

On the validity of momentum relaxation time in low-dimensional carrier gases

The momentum relaxation time (MRT) is widely used to simplify low-field mobility calculations including anisotropic scattering processes. Although not always fully justified, it has been very practical in simulating transport in bulk and in low-dimensional carrier gases alike. We review the assumptions behind the MRT, quantify the error introduced by its usage for low-dimensional carrier gases, and point out its weakness in accounting for inter-subband interaction, occurring specifically at low inversion densities.

A detailed evaluation of model defects as candidates for the bias temperature instability

Despite its long research history, the bias temperature instability (BTI) is still not fully understood. Recent advances on both the experimental and theoretical side have deepened our understanding of the phenomenon, but the microscopic origin is still unknown. We report on a detailed evaluation of atomistic models of the oxygen vacancy and the hydrogen bridge defects in SiO2 as candidates for the defect responsible for the BTI. For this purpose, time constants are calculated using a combination of atomistic and semiconductor device modeling. These time constants are then compared to electrical measurement data obtained from BTI experiments on individual defects in small-area MOS transistors. The inherent uncertainty in the energetic position of the energy levels in the density functional calculation with respect to the device simulation is accounted for using an empirical energy shift. Very good agreement with the experimental data is found for the hydrogen bridge defect, while for the oxygen vacancy severe discrepancies between the predicted behavior and the experimental observation arise.

Efficient simulation of quantum cascade lasers using the Pauli master equation

A transport model for quantum cascade lasers based on the Pauli master equation is presented. An efficient Monte Carlo solver has been developed. The numerical methods to reduce the computational cost are discussed in detail. Finally, the simulator is used to obtain current-voltage characteristics as well as microscopic quantities of a mid infrared QCL structure.

Electronic band structure modeling in strained Si-nanowires: Two band k · p versus tight binding

The subband structure of silicon nanowires has gained much interest recently. Nanowires with diameters below 10 nm are predicted to have a significantly altered subband structure compared with bulk silicon. The effective mass approximation fails to describe these alterings correctly, and so far the semiempirical tight binding method and first principles calculations were used to investigate them. In this paper we present an approach based on a two band k · p description of the conduction band minima. The method excels in simplicity of modeling and versatility including the ability to model strain effects on the subband structure.

Coupling of non-equilibrium Green’s function and Wigner function approaches

We propose a coupling scheme, where the advantages of the coherent Greenpsilas function formalism are combined with the ability of the Wigner formalism to account for phase-breaking processes of interaction with phonons and other lattice imperfections. The Greenpsilas function formalism is used to calculate the coherent Wigner function which provides the initial condition in an equation, from which corrections due to phonon interactions can be calculated. A variety of possible approaches to the obtained equation are considered, and the case where the initial condition is small is investigated numerically.

Direct tunneling and gate current fluctuations

A comprehensive study of correlated gate leakage and drain current fluctuations in nMOS devices using non-equilibrium Greens function calculations has been carried out. A simulation model combining 3D self-consistent electrostatic potentials accounting for random discrete dopants and charged oxide traps with a 1D and 2D transport description of direct-tunneling gate leakage has been developed. The influence of the charge state of the trap on the direct-tunneling current has been investigated. A considerable local change in current density around the trap has been observed. By varying the position of the trap it has been found that oxide defects close to the drain and source regions have a higher impact on the gate leakage. A statistical analysis of nMOSFETs by varying the configuration of the random discrete dopants has been performed. The trap has been positioned close to the drain to achieve a worst-case scenario. The reduction in direct-tunneling current due to charging of a single trap has been calculated for each device. Gate current reductions below one percent have been found. The experimentally measured large gate leakage fluctuations can thus not be accounted for with direct tunneling.