Midgap Energy Research Articles

Concept of Double Peak electric field distribution in the development of radiation hard silicon detectors

The concept of Double Peak (DP) electric field distribution is considered for the analysis of operational characteristics of irradiated silicon detectors. The key point of the model is trapping of equilibrium carriers to the midgap energy levels of radiation-induced defects, which leads to a non-uniform distribution of space charge concentration with positively and negatively charged regions adjacent to the p + and n + contacts, respectively. In our new development of the DP model we consider a non-depleted base region in between the space charge regions as a high resistivity bulk, which operates as a drift region with a non-negligible electric field. Electric field characteristics of detectors processed from n-type MCZ Si wafers using various technological procedures, and irradiated by 1 MeV neutrons and 24 GeV/ c protons, have been compared. Electric field profiles have been reconstructed from DP pulse response of heavily irradiated detectors and calculated by the simulation of DP electric field distribution caused by carrier trapping. It is shown that detectors from n-type MCZ Si irradiated by 24 GeV/ c protons do not show typical space charge sign inversion up to the irradiation fluence of about 2.2×10 15 p/cm 2 and the region with a positive charge dominates over a negatively charged region.

Photon energy up-conversion in colloidal TiO2 nanorods

Nonaqueous reaction between titanium(IV) chloride and benzyl alcohol was used for the synthesis of highly crystalline TiO2 nanorods. The X-ray diffraction measurements proved the exclusive presence of the anatase phase. The photoluminescence of anatase TiO2 nanorods with several well-resolved emission bands was observed, and quantum yield at room temperature was found to be 2.5×10−3. Photon energy up-conversion from colloidal TiO2 nanorods was observed at low excitation intensities. The energy of the up-converted photoluminescence spans the range of energies displayed by the normal photoluminescence emission of the TiO2 nanorods. The explanation for the photon energy up-conversion involves mid-gap energy levels.

Time-Resolved Electric Force Microscopy of Charge Trapping in Polycrystalline Pentacene

Here we introduce time-resolved electric force microscopy measurements to directly and locally probe the kinetics of charge trap formation in a polycrystalline pentacene thin-film transistor. We find that the trapping rate depends strongly on the initial concentration of free holes and that trapped charge is highly localized. The observed dependence of trapping rate on the hole chemical potential suggests that the trapping process should not be viewed as a filling of midgap energy levels, but instead as a process in which the very creation of trapped states requires the presence of free holes.

Photoluminescence of Anatase and Rutile TiO2 Particles

Nonaqueous reactions between titanium(IV) chloride and alcohols (benzyl alcohol or n-butanol) were used for the synthesis of anatase TiO2 particles, while rutile TiO2 particles were synthesized in aqueous media by acidic hydrolysis of titanium(IV) chloride. The X-ray diffraction measurements proved the exclusive presence of either the anatase or the rutile phase in prepared samples. The photoluminescence of both kinds of particles (anatase and rutile) with several well-resolved peaks extending in the visible spectral region was observed, and the quantum yield at room temperature was found to be 0.25%. Photon energy up-conversion from colloidal anatase and rutile TiO2 particles was observed at low excitation intensities. The energy of up-converted photoluminescence spans the range of emission of normal photoluminescence. The explanation of photon energy up-conversion involves mid-gap energy levels originating from oxygen vacancies.

Third Harmonic Generation Susceptibility of Polyacetylene in Terms of the Chain Temperature

The tight binding Hamiltonian of Su, Schrieffer, and Heeger has been used to describe the physical properties of the polyacetylene chain. The chain temperature is calculated from the potential energy which is directly related to the CH group displacement. Two different forms of the displacements have been considered: random and Gaussian. The third harmonic generation susceptibility spectra have been plotted in terms of the temperature using the four-wave mixing formula. The results show that a second peak at the mid gap energy has occurred for room temperature which is in good agreement with the experimental data.

Doping of Cadmium Telluride with Germanium, Tin, and Lead

The transition of Ge-, Sn-, and Pb-doped CdTe crystals to semi-insulating behavior is analyzed in terms of a charge compensation model which includes shallow donor and acceptor states and defects with midgap energy levels. Model predictions are compared with experimental resistivity data for different doping levels. The deep levels are assumed to be due to dopant atoms. The results indicate that deep stoichiometric defects (V Cd and TeCd) have an insignificant effect on the electrical properties of the material.

Improvement of the current-voltage characteristics of a tunneling dielectric by adopting a Si3N4∕SiO2∕Si3N4 multilayer for flash memory application

Superior characteristics of an atomic-layer-deposited (ALD) Si3N4 layer and Si3N4∕SiO2∕Si3N4 stacked layers as a tunneling gate dielectric for nonvolatile flash memory application are reported. Compared to a single layer of SiO2 electric field-sensitive characteristics were obtained by barrier profile engineering with a stacked layer; a lower leakage current at a low field and a higher leakage current at a high field. The stacked dielectric layer showed Fowler–Nordheim tunneling. However, the interfacial potential barrier profile was somewhat smoothed by chemical interaction between the individual layers. The interfacial trap density of this dielectric with an ALD Si3N4 bottom layer was as low as 4×10−10∕cm2eV near the mid-gap energy state, but the reoxidation process degraded the interface quality. The degradation mechanism was studied.

Characterization of transport and charge collection in high resistivity CdTe by photoelectric measurements

A systematic study of photoelectric properties of high resistivity CdTe (undoped and Cl, In doped) has been undertaken in the configuration, where the sample surface is illuminated in a direction parallel to the applied electric field to create experimental conditions similar to those used by measurements of detection properties of X-ray and gamma-ray detectors. A detailed investigation of photoconductivity was performed at above bandgap energy 1.55 eV in dependence of the illumination intensity and the applied voltage. It was observed, that the transport in the material is generally nonlinear in respect to both the illumination intensity and the applied voltage. Drift-diffusion equations for photogenerated electron and hole transport and the Poisson equation were solved numerically using a model including one near midgap energy level. It was found, that this level in detector-grade samples acts as a weak hole trap and has a moderate concentration /spl sim/10/sup 14/ cm/sup -3/. Simulations of electric field in dependence of illumination intensity show the formation of a dead layer extending up to 100 /spl mu/m for a flux of 10/sup 14/ cm/sup -2/ s/sup -1/.

Midgap States in Antiferromagnetic Heisenberg Chains with a Staggered Field

We study low-energy excitations in antiferromagnetic Heisenberg chains with a staggered field which splits the spectrum into a longitudinal and a transverse branch. Bound states are found to exist inside the field induced gap in both branches. They originate from the edge effects and are inherent to spin-chain materials. The sine-Gordon scaling h(2/3)(s)[log(h(s)](1/6) (h(s), the staggered field) provides an accurate description for the gap and midgap energies in the transverse branch for S=1/2 and the midgap energies in both branches for S=3/2 over a wide range of magnetic field; however, it can fit other low-energy excitations only at much lower field. Moreover, the integer-spin S=1 chain displays scaling behavior that does not fit this scaling law. These results reveal intriguing features of magnetic excitations in spin-chain materials that deserve further investigation.

Surface passivation of GaAs using a Ge interface control layer

AbstractThe effective surface passivation is a key to achieve useful metal–insulator–semiconductor devices on GaAs. In this work, the composition and structure of the pyrolytic GeOxNy film deposited on GaAs in the hydrazine vapour has been studied by Auger electron spectroscopy and transmission electron microscopy. The capacitance‐voltage characteristics was used to investigate the GeOxNy–GaAs interface in the frequency range of 1 kHz–1 MHz. The deposited insulator film has a layered structure and consists of a thin (∼2–4 nm) amorphous Ge interface control layer adjacent to the GaAs, followed by the outer GeOxNy film. It is found that a high quality interface is formed with the minimum interface state density of 3 × 1011 cm–2 eV–1 at midgap energies in GaAs. It is suggested that a good bond matching at the GaAs–Ge interface and coherent termination of bonds at both sides of the Ge interface control layer are the reason for perfect passivating properties of the GeOxNy film. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Magneto-Optical and Transport Properties of GaAs/AlAs:Yb Superlattices

We have grown Yb-doped GaAs/AlAs superlattice samples with molecular beam epitaxy method and measured magnetic field dependence of photoluminescence and resistance up to 15 T. Two additional PL peaks in the mid-gap energy region were found only in the case of selective excitation of the well layer. These peaks show anomalous oscillatory behavior, which can be understood as high density accumulation of two-dimensional electrons. These results indicate that photoexcited electron-hole system in the well layers rapidly transferred to Yb-related traps in the barrier layers.

Electrical and materials characterization of GSMBE grown Si1−x−yGexCy layers for heterojunction bipolar transistor applications

This paper reports on detailed materials and electrical characterization of strain-compensated Si1−x−yGexCy (x ⩽ 11%, y ⩽ 1%) layers synthesized by gas source molecular beam epitaxy (GSMBE). Materials assessment included the application of spectroscopic ellipsometry (SE), x-ray diffraction (XRD) and high-resolution cross-section transmission electron microscopy (HRXTEM). An almost fully strain-compensated Si0.88Ge0.11C0.01 alloy with high substitutional carbon incorporation (XRD) and well-defined structure with coherent interfaces (HRXTEM) was produced. An increase of C content (up to 1.6%) resulted in a tensile strained SiGeC alloy and degraded electrical characteristics. The minority-carrier lifetime is a parameter strongly dependent on mid-gap energy band levels, which determine the leakage currents in heterojunction bipolar transistors, and it was assessed here using the capacitance-transient technique. Metal–oxide–semiconductor (MOS) test capacitors were fabricated on Si/SiGe/Si and Si/SiGeC/Si structures using low-temperature plasma oxidation for dielectric growth. The strain-free Si0.88Ge0.11C0.01 layers exhibit a minority-carrier lifetime, τg, of ∼0.1–0.2 µs which constitutes degradation of the carrier lifetime by two orders of magnitude compared to control SiGe layers. The obtained range of τg is nevertheless sufficient for the successful application of epitaxial strain-compensated SiGeC layers in high performance heterojunction bipolar transistors (HBTs).

Double peak electric field distortion in heavily irradiated silicon strip detectors

Non-uniform distribution of the electric field outlined as double peak distortion (DPD) is considered for heavily irradiated silicon strip detectors, which were developed for the CERN-ATLAS semiconductor tracker. DPD originates from the non-uniform accumulation of electrons and holes from the bulk generated current that are captured by radiation induced defects: deep acceptors and donors with mid-gap energy levels. This corresponds to the formation of the low electric field region in the detector central part that consequently will delay charge collection. The electric field distributions at different reverse biases, fluences and detector operational temperatures are calculated using a one-dimensional Poisson equation as it was done earlier for pad detectors. It has been shown that due to the electric field focusing at the strips the DPD effect is more pronounced for strip detectors as compared to that in pad detectors. The double peak electric field distribution is evinced experimentally in current pulse response shape when the charge is collected along the strip axis.

Evidence for interstitial hydrogen as the dominant electronic defect in nanometer alumina films

We provide experimental and first principles theoretical evidence that electron transport through alumina films on Al(111) occurs through electron tunneling mediated by interstitial (i.e., nonbonded) H defects within the 6-nm amorphous surface oxide. Experiments indicate field-assisted tunneling into a defect level near midgap in the oxide and conductance that increases with increasing H content. Using density functional theory, we find interstitial neutral H indeed produces a midgap trap. Moreover, with a negatively charged H, the full ${1s}^{2}$ orbital is almost at the same mid-gap energy. This shows ${\mathrm{H}}^{\ensuremath{-}}$ electron-electron interactions are well screened in this wide gap insulator and can mediate conduction.

Intrinsic electron accumulation at clean InN surfaces.

The electronic structure of clean InN(0001) surfaces has been investigated by high-resolution electron-energy-loss spectroscopy of the conduction band electron plasmon excitations. An intrinsic surface electron accumulation layer is found to exist and is explained in terms of a particularly low Gamma-point conduction band minimum in wurtzite InN. As a result, surface Fermi level pinning high in the conduction band in the vicinity of the Gamma point, but near the average midgap energy, produces charged donor-type surface states with associated downward band bending. Semiclassical dielectric theory simulations of the energy-loss spectra and charge-profile calculations indicate a surface state density of 2.5 (+/-0.2)x10(13) cm(-2) and a surface Fermi level of 1.64+/-0.10 eV above the valence band maximum.

Improvements in Reliability and Leakage Current Properties of HfO[sub 2] Gate Dielectric Films by In Situ O[sub 3] Oxidation of Si Substrate

thin films were atomic layer deposited on Si wafers at 300°C using and A thin layer at the interface was grown by the in situ oxidation immediate before film deposition. This thin layer resulted in no flatband shift with the thermal treatment and a much smaller flatband voltage shift under the voltage stress, in addition to a more than ten times smaller leakage current density than a film without the oxidation. A very small interfacial trap density near the midgap energy was obtained. © 2004 The Electrochemical Society. All rights reserved.

High-k properties of atomic-layer-deposited HfO2 films using a nitrogen-containing Hf[N(CH3)2]4 precursor and H2O oxidant

HfO 2 thin films were deposited on HF-dipped Si wafers at 300 °C using an atomic-layer-deposition technique with N-containing Hf[N(CH3)2]4 and H2O as the precursor and oxidant, respectively. A thin interfacial SiNx layer was spontaneously formed at the HfO2/Si interface during film growth. This interfacial SiNx layer prevented substrate Si diffusion into the HfO2 film. Therefore, the reduction in the capacitance density as a result of post-annealing at 800 °C was minimized. The leakage current density was also reduced due to the more amorphous-like structure of the film. Furthermore, the interfacial trap density (Dit) of <5×1010 cm−2 eV−1 near the midgap energy states was obtained from an as-deposited film that has a capacitance equivalent thickness of 1.8 nm. This Dit value was comparable to that of the well-grown SiO2/Si interface. However, the Dit slightly increased after post-annealing as a result of the increased N concentration at the interface, but it was still <1×1011 cm−2 eV−1.

High-k ZrO2 Gate Dielectric on Strained-Si

ABSTRACTThe growth and structural characterization of UHV-compatible LPCVD grown strained-Si layer on linearly graded relaxed SiGe layer and the electrical properties of the high-k ultrathin ZrO2 films deposited on strained-Si layer using microwave-plasma CVD at low temperature (150°C) are reported. The strained-Si layer has been characterized using AFM, TEM and Raman spectroscopy. The C-V and G-V characteristics of ZrO2 films have been used to calculate the interface trap density, Dit, near the midgap energy, and the fixed oxide charge density, Qf/q. These are found to be 2.24 × 1012 cm−2 eV−1 and 1.45 × 1011 cm−2, respectively. Poole-Frenkel (PF) conduction mechanism is found to dominate the current conduction at room temperature.

Average-bond-energy method in Schottky barrier height calculation

Ten barrier heights of metal-semiconductor contacts are calculated by taking the average-bond-energy as the reference level- The coincidence degree of the calculational values and the experimental values is as good as that of Tersoff's charge-neutrality point method in theoretical calculation of metal-semiconductor contacts- The calculational results are much better than that of Harrison's tight-binding method and Cardona's dielectric midgap energy method-

Impurity band in clean superconducting weak links

Weak impurity scattering produces a narrow band with a finite density of states near the phase difference $\phi =\pi$ in the mid-gap energy spectrum of a macroscopic superconducting weak link. The equivalent distribution of transmission coefficients of various cunducting quantum channels is found.