Karl-Fredrik Berggren

Band-gap widening in heavily Sn-dopedIn2O3

In this paper we investigate the optical properties of evaporated films of doped semiconducting In2O3 in the 2-6-eV range, i.e., around the fundamental bandgap. The study serves two main purposes: to elucidate basic properties of a heavily n-doped semiconductor, and to improve our understanding of a technologically important material which is widely used when transmittance of visible or solar radiation needs to be combined with good electrical conduction or low thermal emittance.

Reactively sputtered ZnO: Al films for energy-efficient windows

Abstract ZnO: Al films were produced by dual-target reactive magnetron sputtering. The effects of different deposition conditions were investigated. Under optimized conditions, 0.3 μm thick films had about 1% luminous absorptance, about 85% thermal IR reflectance, and a d.c. resistivity of about 5 × 10 −4 Ω cm. Band gap widening could be quantitatively explained from an effective mass model for n-doped semiconductors, provided that the polar character of ZnO was accounted for.

Calculation of scattering cross sections for increased accuracy in diagnostic radiology. I. Energy broadening of Compton‐scattered photons

In this work, scattering cross sections differential with respect to both the scattering angle and the energy of the scattered photon are derived in the relativistic impulse approximation for the light elements H, Be, and Al, and photon energies between 30 and 200 keV. The energy broadening of the scattered photons reflects the momentum distribution of the target electrons. It increases with both increasing atomic number of the scatterer and with scattering angle. Even in light elements, the energy broadening is comparable with the intrinsic energy resolution of modern Ge spectrometers. In reconstructing primary photon energy spectra by means of a Ge spectrometer and Compton scattering techniques, i.e., by measuring the photons incoherently scattered at a given angle, the energy resolution is markedly impaired compared to direct measurements in the primary beam. This is usually explained as an effect of the nonzero acceptance angle of the detector. It is shown, however, that the fundamental energy broadening of the scattered photons is alone sufficient as an explanation. The Compton scattering technique is valuable in determining energy spectra in clinical situations. Aspects of its optimal performance are discussed. The commonly used scattering angle of 90 degrees seems adequate. At small scattering angles, the incoherent-scattering cross section is badly known due to electron-electron interactions and, for photon energies less than 100 keV, coherent scattering contributes appreciably to the total scattering even in media of low atomic number. In cases where coherent scattering dominates and where the energy degradation of the incoherently scattered photons is small compared to the energy resolution of the spectrometer, the reconstruction is simplified. The double-differential cross sections derived can be used to simplify calculations of the Compton component of the mass-energy absorption coefficient.

Si/SiGe electron resonant tunneling diodes

Resonant tunneling diodes have been fabricated using strained-Si wells and strained Si0.4Ge0.6 barriers on a relaxed Si0.8Ge0.2 n-type substrate, which demonstrate negative differential resistance ...

High performance Si/Si/sub 1-x/Ge x resonant tunneling diodes

Resonant tunneling diodes (RTDs) with strained i-Si/sub 0.4/Ge/sub 0.6/ potential barriers and a strained i-Si quantum well, all on a relaxed Si/sub 0.8/Ge/sub 0.2/ virtual substrate were successfully grown by ultra high vacuum compatible chemical vapor deposition and fabricated using standard Si processing methods. A large peak to valley current ratio of 2.9 and a peak current density of 4.3 kA/cm/sup 2/ at room temperature were recorded from pulsed and continuous dc current-voltage measurements, the highest reported values to date for Si/Si/sub 1-x/Ge/sub x/ RTDs. These dc figures of merit and material system render such structures suitable and highly compatible with present high speed and low power Si/Si/sub 1-x/Ge/sub x/ heterojunction field effect transistor based integrated circuits.

Si'SiGe electron resonant tunneling diodes with graded spacer wells

Resonant tunneling diodes have been fabricated using graded Si1−xGex (x=0.3→0.0) spacer wells and strained Si0.4Ge0.6 barriers on a relaxed Si0.7Ge0.3 n-type substrate which demonstrates negative differential resistance at up to 100 K. This design is aimed at reducing the voltage at which the peak current density is achieved. Peak current densities of 0.08 A/cm2 with peak-to-valley current ratios of 1.67 have been achieved for a low peak voltage of 40 mV at 77 K. This represents an improvement of over an order of magnitude compared to previous work.

A pseudopotential calculation of the density of states of expanded crystalline mercury

The first calculation of the density of states of expanded crystalline Hg is reported for the f.c.c., b.c.c., and s.c. structures in the density range ∼ 4–9 g/cm3. The calculations are based on Animalus local pseudopotential. It is found that a band gap opens up at 6.5 g/cm3 for f.c.c., 5.5 g/cm3 for b.c.c., and 4 g/cm3 for s.c.

Electrons in one dimension

In this article, we present a summary of the current status of the study of the transport of electrons confined to one dimension in very low disorder GaAs–AlGaAs heterostructures. By means of suitably located gates and application of a voltage to ‘electrostatically squeeze’ the electronic wave functions, it is possible to produce a controllable size quantization and a transition from two-dimensional transport. If the length of the electron channel is sufficiently short, then transport is ballistic and the quantized subbands each have a conductance equal to the fundamental quantum value 2e2/h, where the factor of 2 arises from the spin degeneracy. This mode of conduction is discussed, and it is shown that a number of many-body effects can be observed. These effects are discussed as in the spin-incoherent regime, which is entered when the separation of the electrons is increased and the exchange energy is less than kT. Finally, results are presented in the regime where the confinement potential is decreased and the electron configuration relaxes to minimize the electron–electron repulsion to move towards a two-dimensional array. It is shown that the ground state is no longer a line determined by the size quantization alone, but becomes two distinct rows arising from minimization of the electrostatic energy and is the precursor of a two-dimensional Wigner lattice.

Nature of electron states and symmetry breaking in quantum point contacts according to the local spin density approximation

Electron states and local magnetization in quantum point contacts (QPCs) with different geometries and applied gate voltages are examined for a model GaAs/AlGaAs device. Using the local spin density approximation (LSDA) we recover ferromagnetic spatially split solutions in the pinch-off regime as well as antisymmetric solutions that occur with decreasing gate voltage. These kinds of spin states, which may appear in a repeated fashion in the few-electron regime, are precursors to an extended ferromagnetic state that may be associated with the 0.7 conductance anomaly. We briefly comment on some recent experiments indicating the presence of bound states (Yoon et al 2007 Phys. Rev. Lett. 99 136805). We have not found any indication of such states but suggest that the accumulations of spin and charge at the two ends of a QPC and associated singlet and triplet states are relevant in this context.

Status and perspectives of nanoscale device modelling

During the meetings of the theory and modelling working group, within the MEL-ARI (Microelectronics Advanced Research Initiative) and NID-FET (Nanotechnology Information Devices-Future and Emerging Technologies) initiatives of the European Commission, we have been discussing the current status and the future perspectives of nanoscale device modelling. The outcome of such a discussion is summarized in the present paper, outlining the major challenges for the future, such as the integration of nonequilibrium phenomena and of molecular-scale properties. We believe that modelling has a growing importance in the development of nanoelectronic devices and must therefore make a move from physics to engineering, providing valid design tools, with quantitative predictive capabilities.