Inelastic electron tunneling spectroscopy on MOS structures with very thin oxide films

Inelastic electron tunneling spectroscopy at 4.2 K was used to investigate the defect structure of MOS capacitors with very thin SiO₂ films. Samples were degeneratelyP-andB-doped Si substrates, oxidized in O₂ at 600°C and provided with evaporated Pb, Au, In, Al or Mg electrodes. The observed peaks in the second derivative of theI-U characteristic were assigned to the excitation of phonons and of vibrational modes of the dopants and impurities. The results were found to correlate with infrared data. In addition, a distinct effect of Si/SiO₂ interface states on the characteristic was found.     

Evidence for multiphonon emission from interface states in MOS structures

Transient capacitance measurements on MOS structures give strong evidence for a temperature-dependent capture cross-section for electrons at the continuously distributed interface states. A theoretical model that explains this behavior is lattice-relaxation multiphonon emission.     

Photoelectric spectroscopy of oxide states with MOS structures atT=79 K

AtT=79 K illumination effects with visible and UV light on the drain current were studied forn-channel enhancement-type MOS transistors. The results show that the response of photoelectric measurements is due to electron excitation from oxide states into the silicon surface layer (positive changes of drain current). The oxide states lying near the bottom of the silicon dioxide conduction band are distributed in energy. Oxide states having captured a hole can be discharged by electrons excited from the silicon conduction or valence band (negative changes of drain current) in combination with a tunneling process.     

Spin-dependent resonant tunneling through quantum-well states in magnetic metallic thin films

Quantum-well (QW) states in nonmagnetic metal films between magnetic layers are known to be important in spin-dependent transport, but QW states in magnetic films remains elusive. Here we identify the conditions for resonant tunneling through QW states in magnetic films and report first principles calculations of Fe/MgO/FeO/Fe/Cr and Co/MgO/Fe/Cr. We show that, at resonance, the current increases by 1 to 2 orders of magnitude. The tunneling magnetoresistance ratio is much larger than in simple spin tunnel junctions and is positive (negative) for majority- (minority-) spin resonances, with a large asymmetry between positive and negative biases. The results can serve as a basis for novel spintronic devices.     

Resonant tunneling through electronic trapping states in thin MgO magnetic junctions

We report an inelastic electron tunneling spectroscopy study on MgO magnetic junctions with thin barriers (0.85-1.35 nm). Inelastic electron tunneling spectroscopy reveals resonant electronic trapping within the barrier for voltages V>0.15 V. These trapping features are associated with defects in the barrier crystalline structure, as confirmed by high-resolution transmission electron microscopy. Such defects are responsible for resonant tunneling due to energy levels that are formed in the barrier. A model was applied to determine the average location and energy level of the traps, indicating that they are mostly located in the middle of the MgO barrier, in accordance with the high-resolution transmission electron microscopy data and trap-assisted tunneling conductance theory. Evidence of the influence of trapping on the voltage dependence of tunnel magnetoresistance is shown.     

Size quantization of electronic states in very thin platinum films

The Quantum Size Effect (QSE) is expected to exist in metal films. It has been discussed in a number of theoretical papers. The experimental verification is difficult because of the very short Fermi wave length of the charge carriers in metals. Surface roughness of the films and diffuse scattering of the charge carriers from the film surface usually prevent the observation of QSE. We succeeded in preparing Pt-films with surface roughness below 0.3 nm, which also exhibited partly specular reflection of the electrons from the film surface. Oscillations of the conductivity with the film thickness, due to QSE, were observed in the thickness range from 0.5 nm to 2.0 nm. Band splitting was observed in the thickness range from 5 nm to 20 nm by means of tunneling spectroscopy. From the results we suggest that electrons as well as holes in closed Fermi surfaces participate in the QSE. Holes in open Fermi surfaces, despite their larger density, could not be observed. The measurements allow the determination of the Fermi energy and the effective masses of both electrons and holes in quasi amorphous films.     

Direct resonant interband tunneling into exciton states in multiquantum well structures

Resonant Zener tunneling is investigated in a multiquantum-well p-i-n diode at cryogenic temperatures. Current-voltage characterization in presence of high magnetic fields demonstrates the dominant role of the electron–hole Coulomb interaction in driving this process. After a theoretical modelization in a variational scheme we conclude that tunneling electrons are directly injected into exciton states. We also suggest that similar transport experiments can be an alternative exciton spectroscopic technique with respect to the more conventional optical ones.     

Phonon-assisted tunneling in Schottky barriers

Experimental current-voltage characteristics and their temperature dependencies for AlGaAs Schottky barrier structures are shown to be in agreement with the recent theory of phonon-assisted tunneling.     

Detecting electronic states at stacking faults in magnetic thin films by tunneling spectroscopy

Co islands grown on Cu(111) with a stacking fault at the interface present a conductance in the empty electronic states larger than the Co islands that follow the stacking sequence of the Cu substrate. Electrons can be more easily injected into these faulted interfaces, providing a way to enhance transmission in future spintronic devices. The electronic states associated with the stacking fault are visualized by tunneling spectroscopy, and its origin is identified by band structure calculations.     

Tail states in clean superconductors with magnetic impurities

We analyze the behavior of the density of states in a singlet s-wave superconductor with weak magnetic impurities in the clean limit. By using the method of optimal fluctuation and treating the order parameter self-consistently we show that the density of states is finite everywhere in the superconducting gap, and that it varies as ln(N(E) proportional to -/E-Delta(0)/((7-d)/4) near the mean field gap edge Delta(0) in a d-dimensional superconductor. In contrast to most studied cases the optimal fluctuation is strongly anisotropic.     

Local polariton states in polar crystals with impurities

We show that an impurity embedded in an ionic crystal can give rise to a novel kind of local states. These states exist within a polariton gap of a material and are a mix of excitations of the crystal, such as phonons or excitons, and the transverse electromagnetic field. Electromagnetic components of the states along with the corresponding excitations of the material are localized in the vicinity of an impurity.     

Tail states in a superconductor with magnetic impurities

A field theoretic approach is developed to investigate the profile and spectrum of subgap states in a superconductor subject to a weak magnetic impurity potential. Such states are found to be associated with spatially inhomogeneous instanton configurations of the action.     

The Au-SiO x -a-Si:H structures with very thin anodic oxide layers

Au-SiO _x -a-Si:H MIS structures with anodic oxide layers of thickness from 1.4 to 7.2 nm are investigated. The formation of the superthin SiO _x layers enables us to obtain photosensitive diode structures. These structures have a high photocurrent-to-dark current ratioi _ph/i _d >10⁴ and the dark resistanceR10¹⁰ for the reverse bias. The analysis of IR reflectance absorption spectra in the region of longitudinal Si-O stretching modes allows us to make a conclusion on the dependence of structure-chemical properties of superthin anodic oxids on their thickness.We thank Ing. J. Stuchlík from Czechoslovak Academy od Sciences for kindly manufacturing the a-Si:H films for us.     

Analysis of A-DLTS spectra of MOS structures with thin NAOS SiO2 layers

A set of MOS structures with thin SiO₂ layers prepared by nitric acid oxidation (NAOS) method was investigated using acoustic deep level transient spectroscopy (A-DLTS) to explain the role of annealing treatment (post-oxidation annealing (POA) and post-metallization annealing (PMA)) at different conditions on the distribution of interface states. The activation energies of interface states and the corresponding capture cross-section were calculated both from Arrhenius plots constructed for individual peaks of the A-DLTS spectra and applying the method of modeling of measured acoustic spectra. The energy distribution of the interface states was determined also from the dependence of acoustoelectric response signal (ARS) on the external bias voltage (U _ac - V _G curves). By comparing the A-DLTS spectra, U _ac - V _G characteristics and some electrical measurements (G-V, I-V curves) of investigated MOS structures with no treatment with those treated with POA and/or PMA, the role of individual treatments was observed. The definite decrease of the interface states in the structures with the PMA treatment in comparison with the POA treatment was confirmed too.     

Silicon MOS structures with nonstoichiometric metal-oxide semiconductors

MOS structures are formed by oxidative annealing of tin, tungsten, palladium, nickel, and zinc thin films on silicon, and their high-frequency C-V characteristics are measured. The energy spectra of the density of surface states taken of SnO_{2 ? x} , WO_{3 ? x} and PdO_x nonstoichiometric oxides are found to have common features.