Direct Tunneling Process Research Articles

Osmium Related Deep Levels in Indium Phosphide

Deep energy levels arising from doping of n- and p-type InP with the 5d transition metal Os are studied, using electrical and optical deep level transient spectroscopy. Electrically active Os-related deep level concentrations up to 8.2 × 1015 cm—3 are observed representing up to 20% of the chemical concentration. Three prominent levels with low-electric-field activation energies of EV +0.29, EC — 0.32 and EV +0.53 eV, labelled OsA, Os1 and OsB, respectively, are identified. Double-correlation deep level transient spectroscopy measurements show strong dependence of carrier emission rates on the electric field for all the three levels. All three levels exhibit a stronger field effect than that reported for any other deep level in InP. The field effect can be interpreted in terms of a phonon-assisted tunneling mechanism, merging in a direct tunneling process at high field strengths for both Os1 and OsA. Detailed data are obtained on the carrier capture cross-sections and their temperature dependence for all three levels. In the case of OsA the data can be interpreted in terms of a multiphonon capture model.

Experimental study of the quasi-breakdown failure mechanism in 4.5 nm-thick SiO 2 oxides

In this study, we have investigated the electrical properties of the failure mode referred as quasi-breakdown or soft-breakdown in MOS capacitors on p-type substrate with an oxide thickness of 4.5 nm. Quasi-breakdown appears during high field stresses as a sudden increase between two and four orders of magnitude in the gate current over the whole gate voltage range, but remains undetected in C( V) characteristics between 20 Hz and 100 kHz. Quasi-breakdown was systematically triggered during negative gate voltage stresses after a threshold between 10 and 15 C/cm 2 was reached in the injected charge, this threshold being independent of the stressing oxide field. A very weak temperature dependence and a low frequency noise in the gate current were also observed. The I( V) characteristics are found to follow a first-order exponential law versus the gate voltage, indicative of a direct tunneling process, which could result from a local lowering of the oxide thickness resulting from a sudden metal/isolant transition in a localized region of the oxide near the anode due to oxide defects.

Ａｌ／Ａｌ‐ｏｘｉｄｅ／Ｍ／Ａｌ（Ｍ＝Ｆｅ，Ｎｉ）接合の非弾性電子トンネル分光

Inelastic-electron-tunneling spectroscopy (IETS) was used to investigate the electron states of the interfaces of Al/Al-oxide/Fe(dFe)/Al and Al/Al-oxide/Ni(dNi)/Al tunnel junctions. The conductance curves for all junctions showed a minimum around the zero-bias voltage. The IET spectra showed a strong positive peak around 4 mV, corresponding to the minimum of conductance curves, while another broad peaks was observed for junctions with dNi, dFe≥10A, The peak position was different from that assigned to the Al-O LO phonon mode observed for Al/Al-oxide/Al junctions. It was confirmed by magnetization measurements that a ferromagnetic layer was formed for films with dNi≥30A and dFe≥10A. These results were investigated in relation to the direct and inelastic tunneling process due to magnons.

Microstructured gold/Langmuir–Blodgett film/gold tunneling junctions

We prepared symmetric sandwich structures of the sequence gold/Langmuir–Blodgett film/gold by employing a novel evaporation technique. As molecular systems, we used an octasubstituted palladiumphthalocyanine and a perylene-3,4,9,10-tetracarboxyldiimide derivative. Electronic transport measurements show a tunneling characteristic which arises from a direct tunneling process through molecular states. The current/voltage curves are symmetric for positive and negative bias proving the identical electronic behavior of the two organic/inorganic interfaces at the top and bottom electrode.

Rings in above-threshold ionization: A quasiclassical analysis.

A generalized strong-field approximation is formulated to describe atoms interacting with intense laser fields. We apply it to determine angular distributions of electrons in above-threshold ionization (ATI). The theory treats the effects of an electron rescattering from its parent ion core in a systematic perturbation series. Probability amplitudes for ionization are interpreted in terms of quasiclassical electron trajectories. We demonstrate that contributions from the direct tunneling processes in the absence of rescattering are not sufficient to describe the observed ATI spectra. We show that the high-energy portion of the spectrum, including recently discovered rings (i.e., complex features in the angular distributions of outgoing electrons) are due to rescattering processes. We compare our quasiclassical results with exact numerical solutions.

Theoretical aspects of electronically stimulated desorption of physisorbed atoms

We present a unified theoretical approach to various aspects of the dynamics of desorption of neutrals from physisorbed layers on metals triggered by electron or photon irradiation. The basic mechanism, as described by Antoniewicz, is an ionization to a more strongly bound state, acceleration towards the surface, reneutralization, and desorption. The quantum-mechanical framework is presented first and is followed by its classical approximation. A fully quantum-mechanical treatment and its quasiclassical approximation (in the classical trajectory approximation) are then used. Desorption yields, and the kinetic energy distributions of desorbing neutrals obtained in the classical and in the fully quantum-mechanical approach are numerically compared and a comparison with the existing experimental data is made. The classical trajectory approximation is applied to study the position dependence of the neutralization rate influenced by the dynamics of the motion of the ion for the direct and Auger electron tunneling processes from the metal onto the adsorbed ion. The obtained neutralization rate is compared with the one following from the conventional perturbation theory. Deviations of the motion of the ion from that caused by the ideal image potential and desorption via the chemisorbed excited state, as proposed by Jennison et al., are briefly discussed.

Photoinduced electron transfer involving localized electronic states

It is assumed that large numbers of localized states which exist within the band gap of thin or highly disordered surface films take part in the photoexcitation process. Model calculations have been performed which describe the photoexcitation of an electron into one such localized state and its subsequent escape from this state to yield a photocurrent. The Poole—Frenkel effect, which describes the lowering of the escape barrier due to a superimposed field, direct tunneling and phonon-assisted tunneling processes are considered. The effects of various parameters, such as the effective mass of the electron and the dielectric constant of the material, on the escape probability are studied. These results are compared to photo-electrochemical studies of ion implanted HfO 2 films.

Effect of Elastic Strain on Interband Tunneling in Sb-Doped Germanium

The effects of uniaxial compression and of hydrostatic pressure on the direct and indirect tunneling processes in germanium tunnel diodes have been studied experimentally under forward and reverse bias at 4.2\ifmmode^\circ\else\textdegree\fi{}K and compared with Kane's theory. The diodes were formed by alloying indium doped with $\frac{3}{8}%$ gallium on (100) and (110) faces of germanium bars containing an antimony concentration of 5.5\ifmmode\times\else\texttimes\fi{}${10}^{18}$/${\mathrm{cm}}^{3}$. The first order change of the tunneling current with stress was measured at fixed bias voltages. For biases smaller than 8 mV the current is direct and not affected by the relative shifts of the (111) conduction band valleys. In the bias range of indirect tunneling the anisotropic tunneling from the (111) valleys was observed in agreement with theory. In the range of direct tunneling to the (000) conduction band the current change is correlated with the stress induced change of the direct band gap and of the energy separation between the (111) and (000) conduction bands. This separation was found to be 0.160\ifmmode\pm\else\textpm\fi{}0.005 eV at zero stress in agreement with optical measurements on degenerate germanium. Some details of the bias dependence of the pressure effect including some fine structure at small biases remain unexplained.

Indium arsenide tunnel diodes

The tunnel-current density through a very narrow p-n junction depends exponentially on the reduced effective mass of the charge carriers and the bandgap. Therefore, indium arsenide appears to be a promising material for fast tunnel diodes at room temperature ( m r⋍0·05 , E g⋍0·33 eV), giving higher frequency limits for a given doping level or lower doping (lower capacity) for a given gain-bandwidth product. Tunnel diodes have been made by alloying InCd and InZn dots on InAs crystals which had been doped with 2 × 10 16 to 6 × 10 18 Te atoms/cm 3. The resulting I–V characteristics vary with doping level and temperature from classical p-n curves to Esaki curves of 10:1 peak to valley current ratios. The best ratios at room temperature are about 2:1. Good units with high current density do not show any structure indicative of phonon participation. However, lower-current units do show two conductance minima in the rising part of the forward characteristic at 4°K. The high-current units also show a different temperature dependence of the peak current and of the peak voltage than those of lower current density. This leads to the tentative conclusion that the characteristics of the low-current units are determined by phonon-assisted tunneling, while in the high-current units the direct tunneling process is dominant. In the I–V characteristic of units made from low-doped material an interesting cusp structure is found. This resembles the “phonon” structure above, but is dependent on the surface condition of the diode. A measurement of the rise time of a switching pulse across the diode proved to be limited by the circuit to 10 −9 sec. This and other indirect measurements allow an estimate of a gain-bandwidth product, for the faster diodes, of 3 kMc/s.