Photon assisted tunneling in superconductor-normal metal point contacts at far-infrared frequencies

The response of normal metal-superconductor point contacts to radiation at frequencies up to 2.5 THz is studied experimentally. The results can be analyzed in terms of the so called Photon Assisted Tunneling (PAT) effect and are in excellent agreement with recent theoretical predictions.     

Microwave assisted tunneling

Results obtained from experiments on microwave assisted tunneling are found to be in excellent agreement with the existence of geometrical resonances in the superconducting junction structure.     

Photon emission with the scanning tunneling microscope

By placing a photon detector near the tip-sample region of a scanning tunneling microscope, we have measured isochromat photon-emission spectra of polycrystalline tantalum and Si(111)7×7 at photon energies of 9.5 eV. Such spectra contain electronic-structure information comparable to inverse photoemission spectroscopy, but with high lateral/spatial resolution. The implications of this new observation are discussed.     

Theory for impurity assisted tunneling

A tunneling formalism is presented which goes beyond the usual Transfer-Hamiltonian model. In order to study many body interactions an expression for the conductance is derived in terms of the one particle scatteringt-matrix.     

Theory of photon assisted tunneling

The widespread use of the transfer Hamiltonian model in interpreting electron tunneling should be rejected, and replaced by the current density operator formalism. Starting from first principles of quantum field theory, we present a more comprehensive theory of photon assisted tunneling. The possible application to mixer theory up to the Exahertz frequency region is outlined.     

Photon tunneling scanning image separation method and instrument

A patent for the new photon scanning tunneling microscopy (PSTM) is described in this paper, called the Tunneling Scanning Image Separation Method and Instrument. We call the new instrument described here the Image Separated-PSTM (IS-PSTM). There may be some false image information with the first generation PSTM in the mixed image of the topography and refractive index of a general optical sample. With this new method and instrument we can eliminate this false image information and separate the surface topography image and the distribution image of varying refractive indices of a general optical sample from a mixed image of PSTM.     

X-ray diffractometry diagnosis of laser diffusion of aluminum into silicon

High-resolution X-ray diffractometry was used to study alterations of the structure of single-crystal silicon taking place in the process of nonequilibrium solid-state diffusion of aluminum atoms occurring under heating of the near-surface layer by radiation of a CO₂ laser with pulse durations 1, 2, and 3 s. Crystal lattice deformation profiles, diffusion lengths, and densities of dislocation loops have been determined.     

Single-electron tunneling in silicon nanostructures

We present a brief overview on different realizations of single-electron devices fabricated in silicon-on-insulator films. Lateral structuring of highly doped silicon films allows us to observe quasi-metallic Coulomb blockade oscillations in shrunken wires where no quantum dot structure is geometrically defined. Embedding quantum dot structures into the inversion channel of a silicon-on-insulator field-effect transistor Coulomb blockade up to 300 K is observed. In contrast to the quasi-metallic structures, in these devices the influence of the quantum mechanical level spacing inside the dot becomes visible. Suspending highly doped silicon nanostructures leads to a novel kind of Coulomb blockade devices allowing both high-power application as well as the study of electron–phonon interaction. Received: 14 April 2000 / Accepted: 17 April 2000 / Published online: 6 September 2000     

Transient tunneling spectroscopy (TTS) study of electron tunneling from phosphorus atoms in silicon

Recently developed method of transient tunneling spectroscopy (TTS) is applied to investigate the tunneling dynamics of electrons from phosphorus atoms to the silicon conduction band. In contrast to the conventional constant-current spectroscopic tunneling techniques, in TTS one monitors the evolution of the tunneling process in time. Various difficulties, which may be encountered in the measurements of the tunneling time by TTS, are discussed and illustrated. The temperature dependence of the tunneling time for an isolated phosphorus atom is presented, and possible mechanisms responsible for the decrease of the tunneling time with the lattice temperature T, at T15 K, are discussed.     

Inelastic tunneling assisted by molecular impurities

A recently published many-body theory of tunneling is used to calculate the effect of a molecular impurity localized in the barrier. The variation of the conductance with the position of the impurity is worked out for finite temperatures in a renormalized perturbation treatment.     

Photon emission during intraband tunneling through quantum well structures

Radiative transitions associated with intraband electron tunneling through DC biased quantum well structures are analyzed theoretically. Spontaneous emission and stimulated emission of photons within the quantum well structure are calculated and estimates are made of the radiative transition rate in comparison with the damping loss. The absence of an inherent long wavelength emission cutoff is in contrast with interband transition devices and suggests applications of intraband transition devices as far infrared or microwave sources.     

Photon scanning tunneling microscope with a nonresonance atomic-force regime

A design of a photon scanning tunneling microscope is presented. The shear-force regime and the advantages of nonresonance excitation of the probe are discussed in detail. It is suggested that the replica method be used to estimate the size of the active part of the probe. Zh. Tekh. Fiz. 68, 51–58 (September 1998)     

Photon tunneling contributions to extinction for laboratory grown hexagonal columns

A new treatment predicting the extinction and absorption properties of ice particles is evaluated in this study using laboratory measurements of the extinction efficiency, Q_ext. In this treatment, the degree of ‘photon tunneling’ for ice crystals is unspecified, and laboratory measurements of Q_ext were used in conjunction with this scheme to quantify the significance of this process by determining a tunneling factor, denoted t_f. The term tunneling here refers to the interaction of a particle with radiation outside its area cross-section. A t_f of 1.0 corresponds to tunneling exhibited by ice spheres as predicted by Mie theory, while a t_f of 0 indicates no tunneling.

The laboratory work entailed Fourier transform infrared spectroscopy (FTIR) for optical depth measurements in an ice cloud grown in a chamber, over a wavelength range of 2–18 μm. From these measurements, the extinction efficiency Q_ext as a function of wavelength was determined. Ice particle size spectra were measured in the cloud chamber, and were used to predict Q_ext using the radiation scheme noted above and also using a new implementation of T-matrix, which is based on the exact geometry of a ‘pristine’ hexagonal ice crystal, without approximating the crystal as a spheroid.

Results show that t_f values determined from the laboratory measurements and the new radiation scheme are qualitatively in agreement with t_f values based on fundamental theory. Mean Q_ext errors (relative to measured Q_ext) over all wavelengths sampled were 3.0% when using a constant optimized t_f in the radiation scheme, and 2.3% when using a t_f scheme based on complex angular momentum theory. Moreover, Q_ext as predicted from T-matrix over the wavelength interval 8–12 μm is also in excellent agreement with the measured Q_ext. A single wavelength calculation at 14 μm was performed using the finite difference time domain (FDTD) and T-matrix methods, both of which agreed precisely with the measured Q_ext value. This validates the integrity of T-matrix, FDTD, the new radiation scheme, and the laboratory measurements for the corresponding range of wavelengths and size parameters. Collectively, these results indicate the tunneling contributions predicted for solid hexagonal columns are realistic.     

Penetration of electronic states from silicon substrate into silicon oxide

The depth profiling of O 1s energy loss in silicon oxide near the SiO₂/Si interface was performed using extremely small probing depth. As a result, the energy loss of O 1s photoelectrons with threshold energy of 3.5 eV was found. This value of 3.5 eV is much smaller than the SiO₂ bandgap of 9.0 eV, but quite close to direct interband transition at Γ point in energy band structure of silicon. This can be explained by considering the penetration of electronic states from silicon substrate into silicon oxide up to 0.6 nm from the interface. In addition, the penetrating depth is larger than the thickness of the compositional transition layer.     

Paramagnetic defects in silicon/silicon dioxide systems

Paramagnetic defect centers in Si/SiO₂ systems have been observed by direct ESR, optically-induced ESR, and NMR relaxation of liquids at the outer oxide surface. In general, all the defects reported elsewhere were confirmed, but with some significant discrepancies in character. The P_B center was observable even at room temperature. The P_C center was found to exist much deeper in the silicon than previously determined, and it is tentatively identified to be neutral iron. Surface liquid relaxation is very strong on oxidized crushed silicon, is not dependent on liquid composition, and suggests a strong wide-line spin center in the outer oxide surface. The optically activated spin center created by HF/HNO₃ etches was found not to involve H₂O or OH functionalities, and appears to be a nitrogenous radical. The optical defect center lies within the silicon, and its presence warrants caution in use of HNO₃-based etches in wafer processing. Oxides prepared at elevated pressures show fewer P_A and P_C defects than those produced by conventional processing, which indicates potential merit in pressure oxidation methods.     

Excimer-laser-induced micropatterning of silicon dioxide on silicon substrates

Highly resolved micropatterns induced on SiO₂-coated Si sample surfaces have been investigated using a KrF excimer laser (λ: 248 nm and τ: 23 ns). Uniform micropatterns were observed to form in the oxide layer after laser-induced melting of interfaces. The pattern size can be controlled either by the laser parameters or even by the oxide layer thickness. SEM analysis identified that the micropatterns were virtually initiated at the molten interface and the oxide layer followed the interface patterning to change its profile. Simulation of laser interaction with double-layered structures indicated that the oxide layer could melt or be ablated due to interface superheating when it was deposited on a highly absorbing Si substrate. IR analysis has demonstrated that the structural properties of the SiO₂ layer undergo no appreciable changes after laser radiation. This process provides a possible basis for its application in micropatterning of transparent materials using excimer lasers. Received: 4 September 2000 / Accepted: 13 September 2000 / Published online: 30 November 2000     

Model of photoluminescence from ion-synthesized silicon nanocrystal arrays embedded in a silicon dioxide matrix

A four-level model of photoluminescence from Si nanocrystal arrays embedded in a SiO₂ matrix is suggested. The model allows for thermally activated transitions between singlet and triplet levels in the exchange-split energy state of an exciton in an excited silicon nanocrystal. An expression is derived for the temperature dependence of the intensity of photoluminescence monochromatic components. A correlation is found between the amount of splitting and the emitted photon energy by comparing model data with our experimental data for ion-synthesized Si nanocrystals in a SiO₂ matrix. The model explains the finiteness of the photoluminescence intensity at temperatures close to 0 K and the nonmonotonicity of the temperature run of the intensity.