Radiative Tunneling Research Articles

Self-organized quantum wires on semiconductor surfaces: the new frontier provided by reduced dimensionality

Abstract Microscopic growth and reaction on the solid surfaces often results in the self-organization of surface atoms into various nanostructures. A very interesting type of such nanostructures is the 1D atomic chains, or the surface quantum wires (SQW). The angle-resolved photoemission using synchrotron radiation and scanning tunneling microscopy (STM) studies are performed to investigate the possible 1D electronic anomaly of the well-ordered surface phases composed of self-organized SQW on Si and SiC surfaces. Three different types of SQW systems are studied. The metastable SQW of group III (Al and In) and group IV (Pb) adsorbates on the Si(001)2×1 surface are shown to have common surface band structures, which suggest no obvious 1D electronic coupling but strong local covalent bond formation. The highly-stable Si SQW on the Si-rich SiC(001) surface exhibit surface state bands with lack of dispersions. In contrast, the indium SQW formed on Si(111) (the 4×1-In surface) has been known to have 1D metallic surface states. We have predicted and find a reversible transition from the 4×1 phase into a semiconducting 4×2 phase. The 1D charge density wave (CDW) along the SQW is observed directly by STM at low temperature. The Fermi contours of the metallic phase exhibit a perfect nesting matching precisely with the CDW periodicity. It is thus clearly concluded that this phase transition is of a Peierls type, demonstrating the possibility of investigating a variety of interesting 1D physical properties for the 1D SQW systems formed on solid surfaces.

Radiation and electrical stress-induced hole trap-assisted tunneling currents in ultrathin gate oxides

Based on carrier-separation measurement on pMOSFET, we show the existence of hole trap-assisted tunneling (HTAT) current after 10 keV X-ray irradiation on ultrathin gate oxide. The characteristics of this current have been studied in detail and compared with the corresponding current due to electrical stress. No essential difference is found between the HTAT currents due to ionizing radiation and electrical stress. The results indicate that these two currents have similar origin.

Time-resolved photoluminescence study of hydrogenated amorphous silicon nitride

Time-resolved measurements of photoluminescence were carried out on a hydrogenated amorphous silicon nitride film prepared by low-pressure chemical vapor deposition. When excited with 5.0-eV photons, photoluminescence occurs over a broad spectrum ranging from 1.8 to 3.6 eV. The peak energy of this photoluminescence varies with time from several nanoseconds to nearly 1 ms. These results are explained by a combination of an excitonlike recombination process and a radiative tunneling recombination process of photogenerated carriers within the band-tail states, which are affected by the contributions of thermalization, the Coulombic interaction, and the extent of localization.

Structure and magnetism of self-organized Co islands

This investigation turns its attention to magnetic effects of cobalt islands on a non-magnetic substrate. The preparation technique of MBE enables the creation of fiat and well-ordered surfaces of hcp-Co(0001) if the evaporation onto a W(110) crystal is carried out at ambient temperatures. Subsequent heating to about. 1000 K causes a phase transition from thin filmq to islands. The investigations are carried out in VUV photoemission with polarized synchrotron radiation, LEED and Scanning Tunneling Microscopy (STM). We employ the technique of magnetic linear dichroism in the angular distribution of photoemission (MLDAD) from valence bands. The MLDAD effect shows significant differences between a flat cobalt layer and cobalt islands. Variations due to the excitation energies are discussed with respect to a fully-relativistic bulk-band-structure calculation.

Radiative and nonradiative recombination in polymerlikea−C:Hfilms

Localized electronic states corresponding to \ensuremath{\pi} electrons in hydrogenated amorphous carbon have been investigated using photoluminescence (PL) and optical-absorption spectroscopies. Carbon films contain \ensuremath{\pi}-bonded ``grains'' with different configurations which spatially confine photogenerated electron-hole pairs. A model of PL in $a\ensuremath{-}\mathrm{C}:\mathrm{H},$ including excitonlike radiative recombination and electron tunneling to nonradiative acceptor centers at constant energy, is proposed to explain the following results. The broad PL emission spectra are composed of three bands (at 2.20--2.40 eV, 2.65 eV, and 2.95 eV, independent of the excitation energy) arising from different structural units containing $\mathrm{C}\mathrm{=}\mathrm{C}$ double bonds. Resonance features have been evidenced in the PL excitation spectra and attributed to excitation of confined electron-hole pairs similar to confined exciton optical transitions. Excitonic behavior is consistent with the redshift of peak (b) excitation resonance observed as a function of increasing gap (decreasing \ensuremath{\varepsilon}). Relative values of the radiative rates deduced from PL efficiency indicate that the exciton radius has the same order of magnitude for all three emissive centers. Nonradiative recombination cannot be explained by multiphonon emission processes. The dependence of PL efficiency on the density of acceptor sites (deduced from $\ensuremath{\pi}\ensuremath{-}{\ensuremath{\pi}}^{*}$ optical transitions) can rather be described by electron tunneling from a confined exciton toward nonradiative localized ${\ensuremath{\pi}}^{*}$ states arising from distorted \ensuremath{\pi}-bonded sites. This mechanism accounts for the anticorrelation of the PL efficiency with the optical gap, the decrease of the decay time as a function of emission energy, and the relative enhancement of the blue emission (2.95-eV peak) as a function of decreasing optical gap. In contrast, the band-tail model proposed earlier leads to unrealistically low values of the electron Bohr radius.

Radiative tunneling recombination and luminescence of trapezoidal δ-doped superlattices

Radiative recombination is investigated in the authors’ previously proposed δ-doped superlattice, which can be grown from one of several well-known single-crystal semiconductors of the type InSb, InAs, or GaAs. The energy diagram of the superlattice consists of alternating trapezoidal n-type and p-type potential wells for electrons and holes. An equation for the radiative recombination rate is derived for such a trapezoidal superlattice, and it is shown that, owing to the spatial separation of electrons and holes, the radiative lifetime can attain values of the order of 1 ms, and that it depends weakly on the temperature. This result is attributable to the fact that radiative recombination in a trapezoidal superlattice is governed by optical tunneling transitions of electrons from states near the bottom of the n-type wells to states near the bottom of the p-type wells. An expression is obtained for the luminescence spectrum of the superlattice, where the spectral maximum corresponds to a photon energy much smaller than the width of the semiconductor band gap and can be situated in the far-infrared range. It is noted that such a trapezoidal superlattice can be an efficient converter of thermal radiation into very long-wavelength radiation.

Non-radiative distant pair recombination in amorphous silicon

A brief review of the quadrature frequency-resolved spectroscopy (QFRS) data shows that the radiative tunnelling (RT) model, commonly used to interpret the recombination kinetics in amorphous silicon, must be rejected. The dependence of the QFRS spectra on the excitation rate reveals that the shortening of the carrier lifetimes, which is observed for high excitation rates, is not a matter of decreasing electron-hole separations. Instead, it is induced by a directly competing non-radiative recombination channel. In this paper, the distant pair transition is identified as the origin of this channel, ruling out the possibility that it may be related to Auger recombination, as was previously suggested. This is done by calculating the QFRS signal as a function of the excitation rate, subsequently comparing the results with the respective experimental data. It is argued that the data are clearly in favour of an excitonic recombination model.

Structure determination of the indium induced Si(001)-(4 × 3) reconstruction by surface X-ray diffraction and scanning tunneling microscopy

The indium-induced Si(001)-(4 × 3) reconstruction has been investigated by surface X-ray diffraction (SXRD) measurements with synchrotron radiation and scanning tunneling microscopy (STM). The Patterson function analysis enables us to exclude In dimers as a structural element in this reconstruction. We present a new structural model which includes 6 In atoms threefold coordinated to Si atoms and 5 displaced Si atoms per unit cell. Relaxations down to the sixth layer were determined. ‘Trimers’ made up of InSiIn atoms are a key structural element.

Radiative and nonradiative tunneling processes in silicon

An excitation transfer among carbon-oxygen and vacancy-oxygen defects in electron irradiated silicon was studied by means of the photo-Electron Spin Echo (photo-ESE) technique. Analysis of the appropriate rate equations enabled us to estimate quantum yield of nonradiative tunneling among (C-O) and (V-O) centers. These processes were found to be several orders of magnitude more efficient than donor-acceptor pair radiative recombination and bound exciton radiative transitions.

Frequency up-conversion and time-dependent tunneling of electromagnetic radiation in step-ionized plasmas*)

The frequency spectra is evaluated and the energy balance investigated for the transmitted and reflected transient electromagnetic waves that are generated when a monochromatic source drives a finite width plasma in which a step increase in density occurs. The source wave undergoes up-conversion to a frequency which depends on the initial and final plasma frequencies. When the final density is many times the plasma critical density transmission resonances result in multiple peaks in the frequency spectra. A tunneling effect is described in which a burst of energy is transmitted from the plasma immediately following the density transition.

Influence of high-order dispersion on self-focusing. I. Qualitative investigation

The self-focusing, described by the nonlinear Schrödinger equation with a higher-order derivative term, appearing from dispersive corrections, is considered. A qualitative investigation shows that this term, even if it is small, may play an important role in the final state of the self-focusing. Depending on the sign of a coefficient before this term, it may lead either to a tunneling of the self-trapped radiation, which finally results in defocusing, or to a steady homogeneous wave beam.

On the Tunneling Among Shallow and Deep Centers in ZnS

Radiative tunneling among randomly dstributed donors (D) and acceptors (A) is one of the most efficient processes responsible for the visible luminescence (VL) in many semiconductors (SCs). However, the quantum yield of these transitions may be strongly affected by presence of the other, usually deep, centers. The best known deactivators (killers) of VL in ZnS are transition metal (TM) impurities. In the previous papers [1, 2] two deactivation mechanisms were identified. Recently O,Donnel et al. [3] proposed a new one which can limit the donor—acceptor pair (DAP) luminescence in ZnSe:Fe. During this process an electron tunnels from a donor (D) to iron instead of acceptor (Α) (Fig. 1). In this paper we present the results of photoESR studies of recharging process in ZnS crystals. By selecting the experimental temperature and conditions we could obtain the metastably occupied D, A, Fe and Cr centers with their depopulation monitored via the studies of Fe 3+ and Cr+ ESR signal intensities. At low temperature this depopulation occurs only due to tunneling transitions of e.g. A Cr type. Hence, the decay kinetics of the Fe3+ and Cr+ ESR signals enables us to estimate the tunneling efficiencies among activator and killer centers. The effIciency of the DAP luminescence is usually characterized by two parameters: W0 and α. This comes from the fact that the tunneling rate W(r) between given donor and an acceptor located at a distance r is taken in the form:

Observation of radiative and nonradiative tunneling in GaAs/AlxGa1−xAs heterojunction bipolar transistors with compositionally graded base-emitter heterojunctions

A comparative experimental study of 77-K current gain and electroluminescent (EL) spectra in Npn GaAs/AlxGa1−xAs heterojunction bipolar transistors (HBTs) with either compositionally graded or abrupt base-emitter heterojunctions is reported. Shifting-peak spectra associated with radiative tunneling of carriers into the base-emitter heterojunction space-charge region are observed to dominate the 77-K EL spectra of HBTs with linearly graded emitters. The 77-K EL spectra of the abrupt HBTs are characterized by a peak whose energy position is invariant with respect to base-emitter bias and corresponds to recombination of electrons diffusing across the HBT base. Comparison of EL peak intensity and transistor base current indicates that the graded HBT current gain at 77 K is determined by nonradiative tunneling of electrons and holes to deep levels at the base-emitter junction. The loss of minority carriers to radiative and nonradiative tunneling current mechanisms in the graded HBT causes the 77-K current gain of the graded HBT to be significantly lower than that of the abrupt HBT.

Tunneling of radiation from a single-mode fiber waveguide with a partial cladding

A theoretical study is made of the tunneling of radiation from a single-mode fiber waveguide with a partial cladding. A simple method is proposed for the calculation of the attenuation of the power of the tunneling mode. The results obtained are of fundamental importance in the development and fabrication of fiber-optic directional couplers. The calculated dependences agree with the experimental results obtained by other authors.

A novel superlattice infrared source

A novel superlattice infrared source is proposed. The device utilizes radiative intersubband transitions and resonant tunneling phenomena in a finite superlattice. Theoretical estimates of the radiative transition lifetime and resonant tunneling time are given, which show the possibility of obtaining an infrared light-emitting diode, or even a laser. The device has a narrow emission spectrum, and the wavelength can be adjusted by varying device parameters. The device design has no long wavelength limitation. A specific design of an AlGaAs-GaAs 10-μm superlattice infrared source is discussed.

Time-resolved photoluminescence in amorphous phosphorus

Time-resolved photoluminescence spectroscopy has been used to study recombination in amorphous phosphorus. Excitation with 30 ps pulses at 532 nm confirms our earlier observation that the luminescence band which dominates in the first few hundred nanoseconds following excitation shifts down in energy 180 meV by 200 ns, and that the band is not a superposition of different bands. The decay of luminescence excited by pulses of ~5 ps duration is shown to be consistent with a radiative tunnelling model in the time range 100–700 ps.

Upper hybrid emission from a magnetized gas discharge plasma

Measurements are presented of electromagnetic radiation emitted perpendicular to a strong magnetic field in a gas discharge. The discharge plasma is characterized by electron densities in the range 1010-3*1012 cm-3, an electron temperature of approximately 2 eV, and a dilute energetic electron tail extending out to the discharge voltage (<or approximately=200 V). Emission measurements are made in the frequency range 12-18 GHz. It is shown that the emission is predominantly in the extraordinary mode, originates at the upper hybrid layer in the radially inhomogeneous plasma, and is generated by the energetic electron tail. Combined with earlier detailed measurements of the energetic electron properties it is shown that the emitted power varies linearly with the hot electron density in the vicinity of the upper hybrid radius and varies as the 3/2 power of the tail temperature. Absolute power estimates are provided to link the authors measurements to the fundamental emission parameters. There are indications that tunneling of radiation is not playing a role in these experiments, but this conclusion requires a lot of further study.

Resonant tunneling of electromagnetic radiation in superlattice structures

The propagation of electromagnetic radiation in superlattice structures is considered. We present calculations of tunneling transmissivities for photons through GaAs–AlGaAs superlattice structures. Resonant tunneling (i.e., 100% transmissivity) occurs at incidence with wave numbers near the quasi-guided modes of the structure.

Hopping-Type Electronic Processes in Pre-Breakdown Electrical Fields in Insulators with Defect Centers

Field-induced electronic jumps over the system of impurity centers in ionic insulators in pre-breakdown electric fields are discussed. An alkali halide crystal with electrically neutral F-color centers and negatively charged F'-centers is used as a model system. The Radiative Tunnel Transitions (RTT) of an outer electrons of F'-centers over the network of F-sites under the action of an applied electric field, could provide a specific contribution to the total electronic conductivity. This mechanism has similarities with activationless hopping conductivity in strong electric fields in compensated semiconductors. The processes of inter-electronic energy transfer could provide yet another contributing factor towards the enhancement of the pre-breakdown electronic conductivity.

Energy transfer processes in non-ohmic hopping conductivity in a strong electric field

Energy transfer processes between jumping current carriers in non-ohmic activationless F − 1 4 -hopping conductivity in a strong electric field are considered. An energy gained during the downward spontaneous hopping of one electron can be radiationlessly transferred to another electron enabling it to make an upward energy hopping transition. It is shown that the contribution from these two-electron four-site energy transfer processes into the total hopping conductivity may be comparable or even exceed the contribution from one-electron radiative tunnel transitions. Numerical estimates are based on an example of beta-rhombohedral crystalline boron which exhibits F − 1 4 -field law.