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We studied the effects caused by Joule heating in GaAs/AlAs superlattices. We measured the current–voltage characteristics of superlattice mesas of different cross sections. Each current–voltage characteristic showed a corresponding peak current followed by a region of negative differential conductivity, according to miniband transport in the superlattice. We found that Joule heating in the mesa reduced the peak-current density. Current jumps in the region of negative differential conductivity disappeared for large cross-section mesas indicating a suppression of electron domains. The results are of significance for the development of high-power superlattice oscillators for submillimeter radiation.

We have analysed the core-to-bound excitations in condensed NH 3 and CH 4 in the N1s and C1s regions, respectively, utilizing highly resolved synchrotron radiation. Photoabsorption (from partial electron yields) and photodesorption of H + ions as functions of photon energy were measured from condensed layers of varied thicknesses (100 ML, 10 ML, 1 ML on a 1 ML Xe spacer layer). Clear evidence for repulsive (or at most extremely weakly bound) core excitations for NH 3 , and for bound core excitations with vibrational resolution for CH 4 , has been obtained. Surface and bulk excitations can be well distinguished using the graded surface sensitivity of the two signals from the various layers. For the surface excitations, surface symmetry breaking both in terms of electronic and vibronic states has been deduced from polarization-dependent behaviour. Possible information on Rydberg and valence contents of the final states involved, and on contributions of ultrafast dissociation, are discussed.

We report on frequency multiplication of microwave radiation by propagating space-charge domains in a semiconductor superlattice; the domains were due to a negative differential mobility of miniband electrons. We irradiated an arrangement of two GaAs/AlAs superlattices, mounted in a rectangular waveguide system, with microwave radiation (frequency near 70 GHz) and observed the generation of harmonics; the conversion of radiation power to the third harmonic showed a remarkable efficiency (5%). A theoretical study, based on a drift-diffusion model for the miniband electrons, suggests that the microwave radiation has driven the formation and annihilation of space-charge domains and that the resulting anharmonic current was the source of the frequency-multiplied radiation. Our results indicate that frequency multiplication by space-charge domains in a semiconductor superlattice can be exploited for efficient generation of submillimeter-wave radiation.

Photon stimulated desorption (PSD) spectra of ions (N+, N+2, and N2+) by N 1s → 1πg and N 1s → Rydberg level excitations have been recorded from solid nitrogen for different polarization and detection conditions, and compared with electron yield data. Vibrational fine structure is clearly resolved also in photon stimulated desorption (PSD) spectra; its details depend on layer preparation, but also on the type of signal. Detailed evaluation should allow conclusions on the nature and the dynamics of the process of bond breaking and on the influence of the surroundings on it. Exploiting alignment effects by properly setting polarization and detection angles, symmetries of electronic excited states can be well analysed even for samples consisting of randomly oriented molecules.

Excitation of exciton pairs in the bulk and on the surface of solid Ar by single photon absorption are investigated via photon stimulated desorption of ions. For films of various thicknesses, line shapes and branching ratios into Ar+ and Ar2+ emission are studied. For the photon energy range from 23 to 29 eV, four bands can be discriminated which exhibit different contributions from bulk and surface excitations, respectively.

We report millimeter wave generation by a self-sustained current oscillation in a doped InGaAs/InAlAS wide miniband superlattice. The superlattice (miniband width 160 meV) showed, at room temperature, a current voltage characteristic with negative differential conductance. Coupled to a high-frequency circuit, the superlattice generated millimeter waves, at a frequency (55 GHz) which was tunable by half a percent by changing the bias voltage. The power (0.3 mW) corresponded to an efficiency (i.e., ratio of microwave power to dc power input) of 0.3%. We attribute the microwave generation to a current oscillation caused by traveling dipole domains.

Presents experimental evidence for ultra-fast domain formation in a semiconductor superlattice. An InGaAs/InAlAs superlattice device was mounted in a corner cube. The superlattice device showed a region of negative differential conductance in the current-voltage characteristic for voltages larger than an onset voltage. When the superlattice device was irradiated with radiation smaller than 1.2 THz, the onset voltage shifted, with increasing THz radiation power, to lower voltages. Taking an analysis based on a drift-diffusion model into account, we suggest that dipole domains form within about 400 fs.

By photoabsorption in the Ar 2p range of Ar multilayer condensates, distinct contributions of surface and bulk excitations can be distinguished in the characteristic core resonances detected by partial-electron-yield near-edge x-ray-absorption fine-structure spectra. Compared to the corresponding bulk/gas-phase features, the surface excitations are shifted by ∼0.5 eV towards lower/higher photon energy. Deexcitation of core-to-bound 4s resonances in these condensates leads to distinct spectator-shifted Auger features (2h le final states)

The authors report the generation of radiation at 147 GHz with an InGaAs/InAlAs superlattice oscillator. The generation is attributed to current oscillation due to propagating dipole domains. These are a consequence of the negative differential mobility caused by the miniband transport. The power of the emission was /spl sim/80 /spl mu/W with a DC to RF conversion efficiency of 0.5%.