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We report an experimental study indicating ultrafast creation and annihilation of space-charge domains in a semiconductor superlattice under the action of a THz field. Our experiment was performed for an InGaAs/InAlAs superlattice with the conduction electrons undergoing miniband transport. We applied to a superlattice a dc bias that was slightly smaller than a critical bias necessary for the formation of space-charge domains caused by a static negative differential conductivity. Additionally subjecting the superlattice to a strong THz field, resulted in a dc transport governed by the formation of domains if the frequency of the field was smaller than an upper frequency limit (~3 THz). From this frequency limit for the creation and annihilation of domains we determined the characteristic time of the domain buildup. Our analysis shows that the buildup time of domains in a wide miniband and heavily doped superlattice is limited by the relaxation time due to scattering of the miniband electrons at polar optic phonons. Our results are of importance for both an understanding of ultrafast dynamics of pattern formation in nanostructures and the development of THz electronic devices.

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 study electron miniband magneto-transport in GaAs/AlAs superlattices with AlAs barriers of two monolayers. Here, the two-dimensional AlAs barrier comprises an in-plane distribution of AlAs islands. We use magneto-transport to probe both the rate and the anisotropy of the electron scattering associated with the islands.

We report the use of an InGaAs/InAlAs superlattice for detection of THz radiation pulses generated by a free-electron-laser (FELIX). The detector showed a response corresponding to a reduction of the direct current through the superlattice. The current reduction is attributed to the THz-field induced modulation of Bloch oscillations performed by miniband electrons. The detector response was measured in a frequency range between 4 and 12 THz and showed strong minima at the frequencies of infrared active transverse optic phonons.

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.

We discuss the feasibility of a semiconductor superlattice oscillator which exploits the quenching of propagating dipole domains for the generation of submillimeter waves. We studied the dynamics of electrons in a semiconductor superlattice by performing a simulation based on a drift-diffusion model, taking into account feedback from a resonant circuit. The simulation delivers propagating dipole domains which are quenched before they reach the anode. The periodic formation and quenching of domains creates a self-sustained oscillation of the current through the superlattice. The frequency of the oscillation can be more than three times higher than without feedback. We suggest that with already existing superlattices an oscillator working in the quenched domain mode can be realized up to almost 500 GHz.

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 observed a self-sustained current oscillation in an InGaAs/InAlAs superlattice at room temperature. The oscillation gave rise to microwave generation at a fundamental frequency around 2 GHz and higher harmonics. The power at the first harmonic was 0.1 mW, corresponding to an efficiency (i.e. ratio of microwave power to dc power) of 3.5%. We attribute the current oscillation to travelling dipole domains.

In order to investigate the modification of excitation and decay of core holes by condensation and adsorption in the simplest possible cases, as well as the coupling to atomic motion, we have studied the Ar2p and Kr3d excitation regions in Ar and Kr mono- and multilayers on Ru(001). Using synchrotron radiation from BESSY (Berlin), total and Auger electron yields and yields of desorbing ions and neutrals, as function of photon energy, as well as decay electron spectra for specific primary excitations have been measured. The main results are: Multilayers: Energies for resonant core excitations are shifted to higher values by 0.6 to 1 eV compared to free atoms; for Kr, surface (smaller shifts) and bulk excitations can be distinguished. Autoionization and normal Auger spectra are clearly different, as in the gas phase. Besides desorption of neutral atoms as in the valence region, singly and doubly charged ions as well as ionic clusters are desorbed. Monolayers: The shift of resonant excitation energies relative to the gas phase is smaller than for condensed layers. Decay spectra for resonant and non-resonant excitations are identical, as for strongly coupled chemisorbates, proving that charge exchange with the metal is fast compared to core decay. As in the valence region, only neutral atoms desorb. The results shed light on the screening and charge transfer behaviour and on the mechanisms of stimulated desorption which are operative in them.

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.

We have measured the tunneling conductance of YBa2Cu3O7/Pb planar tunnel junctions fabricated on superconducting (110)‐oriented YBa2Cu3O7 thin films exhibiting zero‐resistance superconducting transition temperatures (Tc’s) between 88 and 89.5 K. The conductance is the same as we and other groups have measured on (100)‐ and (103)‐oriented YBa2Cu3O7/Pb junctions and quantitatively different from c‐axis tunneling. The ab‐plane transport is comparable to that measured in (001)‐oriented YBa2Cu3O7 thin films, and the transport anisotropy varies between 65 and 80 from 300 K to Tc, respectively. This anisotropy is higher than that previously measured in both (110)‐oriented and c‐axis aligned (100)‐oriented films, but it is comparable to that measured in single crystals.

We theoretically studied the possibility of frequency multiplication using propagating dipole domains which are induced in a semiconductor superlattice by microwave radiation. We have investigated the dynamics of electrons in a superlattice submitted to both a static voltage and a microwave field by performing a simulation based on a drift-diffusion model and incorporating current-limiting boundary conditions. The motion of electrons in the superlattice was governed by an Esaki–Tsu drift velocity field characteristic with a negative differential mobility above a critical electrical field. The simulation delivered, for a static voltage larger than a critical voltage, the periodic formation and annihilation of propagating dipole domains and, as a consequence, a reduction of the direct current through the superlattice. Our simulation showed that an additional microwave field can periodically induce and subsequently quench domains giving rise to a strongly anharmonic current. The anharmonicity of the current is the origin for the generation of higher harmonics of the microwave field. Both the formation and annihilation of a domain can take place within a time of about 1 ps suggesting that the mechanism of domain induction and quenching can be used for generation of radiation up to almost 1 THz .

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.

We observed that the propagation of a dipole domain in a GaAs/AlAs superlattice can be controlled by an external high-frequency field. The doped GaAs/AlAs superlattice showed a negative differential conductance giving rise to propagating dipole domains. Depending on the frequency of the high-frequency field, either frequency-locked or quasiperiodic propagating domain states occurred. The behavior can be described by a ``devil's staircase,'' where in the locked state the ratio of the external frequency and the domain transit frequency is a rational number. A theoretical analysis indicates that the high-frequency field controls the domain propagation by a modification of both the domain velocity and the shedding of the domains from the cathode.

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)

Photon stimulated desorption of neutrals and ions from condensed films of H2O, NH3 and CH4 has been investigated in the C 1s, N 1s and O 1s core excitation regimes, respectively, and compared to absorption data obtained via electron-NEXAFS. Desorption of neutral molecules and of H+ as well as fragment and cluster ions has been observed. The PSD yields of neutrals, molecular ions and ionic clusters, as well as the corresponding total-yield and/or Auger-NEXAFS spectra show identical structures. This suggests that X-ray induced ESD, probably via double and multiple valence ionization, is the dominant mechanism for desorption of these species. The PSD yield of H+ ions, however, exhibits significant discrepancies from these XESD derived NEXAFS spectra. On H2O and NH3, where it is by far the most abundant ion, it is dominated by a narrow peak right at the onset of desorption. The intensity of this threshold peak shows a remarkable polarization dependence. On CH4, the H+ signal is small, and no enhancement is observed at any photon energy; there is an overall shift of 0.2 eV of the H+-NEXAFS structure to lower energy as compared to absorption. The mechanisms of these effects are discussed.

Because of their fundamental interest, electronic sputtering processes of Van-der-Waals bound solids have been investigated by many researchers /1/. Most of these studies dealt with pure solids of only one component, though for a variety of applications (e.g. space science or cryogenic devices) the investigation of electronically stimulated reactions and erosion of alloys might be even more important. Moreover, studies of such layers could shed light on the mechanisms by which the electronic excitation is transferred to nuclear motion, especially if resonant excitation by monochromatic photons from synchrotron sources is investigated. Tuning the wavelength to a suitable absorption line makes the localisation of the primary excitation on selected constituents of the “mixed ice” possible. We have therefore investigated total electron yield (TEY), and neutral and ionic photon stimulated desorption yields (n-PSD, i-PSD) of pure and mixed ice layers of Ar, O2, CO, and H2. Erosion of Ar has been studied before by desorption and luminescence spectroscopy and the prevailing sputtering mechanisms for valence excitation are well understood (see, e.g., ref.2 and references therein). O2 and CO are interesting candidates because the sputtering mechanisms seem to be different in the two cases. For O2, a high relaxation energy of up to 7 eV due to dissociative recombination of the molecular ion is expected to be the main “power source” for desorption /3/.

Frequency multiplication of microwave radiation by use of space charge domains in a GaAs/AlAs superlattice is reported. A microwave field triggered the formation and annihilation of domains giving rise to the generation of harmonics. The new method of frequency multiplication, which is demonstrated for 20 GHz radiation, should be applicable to the generation of radiation up to almost 1 THz.

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%.

We investigate the electronic properties of wide miniband (\ensuremath{\sim}100 meV) GaAs/AlAs superlattice structures in which the tunnel barriers are formed from planar arrays of AlAs islands. By studying the effect of an in-plane magnetic field on the miniband conduction, we determine the strength and anisotropy of the rate of electron scattering by the island arrays.