Charge Neutrality Level Research Articles

Band alignment between GeTe and SiO2/metals for characterization of junctions in nonvolatile resistance change elements

GeTe materials were characterized using x-ray photoelectron spectroscopy in both the amorphous and crystalline states. Valence and conduction band alignments relative to a SiO2 reference were measured to allow the GeTe band diagram, work function, and electron affinity to be inferred. Hole barrier heights was also studied for several metal/GeTe systems (metal=Al,Ni,W) to extract the charge neutrality level of these interfaces for GeTe in both the crystalline and amorphous states. Near perfect Fermi-level pinning was observed for crystalline GeTe in contact with all of the metals with much less pinning observed for amorphous GeTe.

Ambiguity in the magnitude and direction of the derived interface dipole in lanthanum aluminate heterostructures: Implications and proposed solution

The electric field strength of interface dipoles cannot be measured directly but can be derived from measurable electronic properties such as the valence band offset (VBO) and electron affinity using photoemission techniques. In this study, we found that the measurements of these two values are affected by differential charging and surface contaminants, respectively. This can affect both the polarity and the strength of the derived interface dipole and therefore might have implications regarding the understanding of oxide-semiconductor band alignment. Our overall band lineup and derived interface dipole in lanthanum aluminate (LAO) heterostructures agree excellently with a popular charge-neutrality level model. This would not be possible without the accurate measurement of VBO and electron affinity in LAO heterostructures.

Modification of the structural lattice parameters and electron spectra of n-GaAs films on sapphire on irradiation with reactor neutrons

Changes in the structural parameters of epitaxial GaN films on sapphire (n-GaN/Al2O3(0001)) induced by irradiation with reactor neutrons with integrated fluences up to 7.25 × 1019 fn cm−2 (φfn/φtn ≈ 1) and subsequent isochronal annealing at temperatures up to 1000°C are studied. Measurements of the lattice parameters a and c of the irradiated n-GaN films show that the parameter c increases by 0.38% and the parameter a remains almost unchanged. From theoretical estimations, it follows that, in the irradiated n-GaN film, the elastic tensile stress along the c axis is as high as ∼1.5 GPa, whereas the compression stress in the basal plane of the unit cell is about −0.5 GPa. The tension of the irradiated GaN film along the hexagonal axis induces a decrease in the band gap Eg by 37 meV and a lowering of the charge neutrality level by 22 meV with respect to the corresponding parameters in the initial GaN film on sapphire. The parameter c changed by irradiation with reactor neutrons by Δc can be recovered by annealing in the temperature range 100–1000°C, with the basic stage of annealing at about 400°C.

Effect of pressure and mechanical stress on the electronic properties of AlN and GaN

The fundamental properties of the AlN and GaN compounds with a wurtzite structure under external hydrostatic pressure, uniaxial mechanical stress σ⊥ along the hexagonal axis, and biaxial mechanical stress σ⊥ in the basal plane of the unit cell have been considered in terms of first-principles calculations in the frame-work of the density functional theory. The pressures of the phase transitions from the structures of wurtzite and zinc blende to the structure of rock salt have been obtained. The behavior of the structural parameters, interband transitions, and positions of the charge neutrality level has been investigated. The calculated pressure coefficients of the band gap are as follows: ∂E g /∂p = 40.9 meV/GPa, −∂E g /∂σ | = −4.2 meV/GPa, and −∂E g /∂σ ⊥ = 45.2 meV/GPa for AlN and ∂E g /∂p = 33.0 meV/GPa, −∂E g /∂σ | = 23.6 meV/GPa, and −∂E g /∂σ ⊥ = 9.6 meV/GPa for GaN. The pressure coefficients of the charge neutrality level in almost all cases are substantially smaller than the corresponding values obtained for the band gap E g .

The Effect of Donor/Acceptor Nature of Interface Traps on Ge MOSFET Characteristics

In this paper, the acceptor and donor nature of interface traps are investigated using conductance and interface trap time constant measurements on Ge n- and p-type metal-oxide-semiconductor field-effect transistors (N-and PMOSFETs). The presence of acceptor-type interface trap states in the valence-band side of Ge band gap is confirmed by these measurements. Electron trapping by the acceptor-type interface states and their effect on Ge N- and PMOS performance are discussed. The high density of the acceptor-type interface traps found to be degrading Ge NMOSFET performance, while it is not a concern for Ge PMOSFETs because of the position of charge neutrality level in Ge. Trapped charge calculations show that reducing the interface trap density by the ozone oxidation mitigates the electron trapping by the acceptor-type traps, which otherwise degrade Ge NMOSFET performance. By engineering the gate dielectric interface of Ge NMOSFETs, 40% improvement in inversion electron mobility is reported. Improvement of 2.5× over universal hole mobility is achieved for Ge PMOSFETs.

Integration of Self-Assembled Redox Molecules in Flash Memory Devices

Self-assembled monolayers (SAMs) of either ferrocenecarboxylic acid or 5-(4-Carboxyphenyl)-10,15,20-triphenyl-porphyrin-Co(II) (CoP) with a high- dielectric were integrated into the Flash memory gate stack. The molecular reduction-oxidation (redox) states are used as charge storage nodes to reduce charging energy and memory window variations. Through the program/erase operations over tunneling barriers, the device structure also provides a unique capability to measure the redox energy without strong orbital hybridization of metal electrodes in direct contact. Asymmetric charge injection behavior was observed, which can be attributed to the Fermi-level pinning between the molecules and the high- dielectric. With increasing redox molecule density in the SAM, the memory window exhibits a saturation trend. Three programmable molecular orbital states, i.e., CoP <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sup> , CoP <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-</sup> , and CoP <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2-</sup> , can be experimentally observed through a charge-based nonvolatile memory structure at room temperature. The electrostatics is determined by the alignment between the highest occupied or the lowest unoccupied molecular orbital (HOMO or LUMO, respectively) energy levels and the charge neutrality level of the surrounding dielectric. Engineering the HOMO-LUMO gap with different redox molecules can potentially realize a multibit memory cell with less variation.

Limits to doping in oxides

The chemical trends of limits to doping of many semiconducting metal oxides is analyzed in terms of the formation energies needed to form the compensating defects. The $n$-type oxides are found to have high electron affinities and charge neutrality levels that lie in midgap or the upper part of their gap, whereas $p$-type oxides have small photoionization potentials and charge neutrality levels lying in the lower gap. The doping-limit energy range is found to vary with the bulk free energy of the compound.

Role of a universal branch-point energy at ZnO interfaces

Contemporary interface state models were tested using the barrier heights of laterally homogeneous ZnO Schottky contacts. A strong correlation was found between barrier height and metal Miedema electronegativity, indicating that interfacial chemical bonding may play an important role in the charge transfer at metal-ZnO interfaces. The charge neutrality level of metal-ZnO interface states was found to lie in the band gap, $0.90\ifmmode\pm\else\textpm\fi{}0.05\text{ }\text{eV}$ below the conduction-band minimum. This is in contrast to the electrical nature of ZnO free surfaces and heterojunctions, and with quasiparticle band-structure calculations which have previously placed the branch-point energy of ZnO in the first conduction band. This large discrepancy indicates that we still lack a comprehensive predictive model for these interfaces.

Fermi-level pinning and charge neutrality level in nitrogen-doped Ge2Sb2Te5: Characterization and application in phase change memory devices

We study the dependence of the hole barrier height at the metal/α-Ge2Sb2Te5 interface as a function of nitrogen doping in Ge2Sb2Te5 as well as the vacuum work function of the metal. Materials parameters such as the band gap, dielectric constant, and electron affinity values of these nitrogen-doped films were also determined. All Ge2Sb2Te5 films studied in this work are amorphous. Following further physical analysis, the effective work functions of metals on nitrogen-doped Ge2Sb2Te5 films were obtained and found to differ from that of their values in vacuum. This led to the extraction of the slope parameter Sx and charge neutrality level ΦCNL which characterizes Ge2Sb2Te5. Appreciable metal Fermi-level pinning to the charge neutrality level of Ge2Sb2Te5, which is located near the valence band edge, was observed. We then demonstrate application of the extracted parameters to obtain the band alignment of α-Ge2Sb2Te5 with various other materials such as SiO2, giving good agreement with experimental results.

Charge neutrality level and electronic properties of GaSe under pressure

Calculations of lattice parameters and electron band spectra of GaSe have been carried out from the first principles. The dependence of these parameters on hydrostatic compression as high as 5 GPa and on homogeneous biaxial tensile and compressive stresses (from −3 to 3 GPa) in the basal plane of the unit cell is considered. The calculations adequately reproduce the experimental features of the major interband transitions in GaSe under a hydrostatic pressure and, in the absence of experimental data, predict the dependence of structural and electronic properties of GaSe under the effect of a biaxial stress. On the basis of calculated band spectra, the energy position of the local charge neutrality level CNL, E v + 0.8 eV, has been determined and electronic properties of the as-grown material and energy diagrams for interphase boundaries in GaSe have been analyzed.

Single crystalline CoFe/MgO tunnel contact on nondegenerate Ge with a proper resistance-area product for efficient spin injection and detection

We report the proper resistance-area products in the single crystalline bcc CoFe/MgO tunnel contact on nondegenerate n-Ge desirable for efficient spin injection and detection at room temperature. The electric properties of the single crystalline CoFe(5.0 nm)/MgO/n-Ge(001) tunnel contacts with an ultrathin MgO thickness of 1.5, 2.0, and 2.5 nm have been investigated by the I-V-T and C-V measurements. Interestingly, the crystalline tunnel contact with the 2.0-nm MgO exhibits the Ohmic behavior with the RA products of 5.20×10−6/1.04×10−5 Ω m2 at ±0.25 V, satisfying the theoretical conditions required for significant spin injection and detection. We believe that the results are ascribed to the presence of MgO layer between CoFe and n-Ge, enhancing the Schottky pinning parameter as well as shifting the charge neutrality level.

Direct evidence of metallicity atZnO (0001¯)−(1×1)surfaces from angle-resolved photoemission spectroscopy

We have investigated the stability of O-polar $\text{ZnO}\text{ }(000\overline{1})\text{\ensuremath{-}}(1\ifmmode\times\else\texttimes\fi{}1)$ surfaces with photoemission spectroscopy and low-energy electron diffraction. We present direct evidence of quasi-two-dimensional electron gas (2DEG) formation $({N}_{2D}\ensuremath{\le}2\ifmmode\times\else\texttimes\fi{}{10}^{13}\text{ }{\text{cm}}^{\ensuremath{-}2})$ at the O-face from our angle-resolved photoemission spectroscopy studies. These findings are discussed in terms of hydrogenation of the O-face by chemisorptions process and the energetic location of the charge-neutrality level of ZnO. Our results reveal insight into the role of hydrogen in forming the quasi-2DEG and stabilizing the O-polar nonreconstructed surface.

On the Choice of High-$\kappa$ Dielectrics for Metal Nanocrystal Memory to Improve Data Retention

An investigation on the optimized choice of high-κ gate dielectrics and metal nanocrystals (NCs) for improving retention capability of a flash memory is presented. The influence of charge neutrality level (CNL) of gate dielectrics on retention time is quantified. Results show that retention efficacy of metal NCs with high work function (WF) is adversely affected when embedded in certain oxides with high CNLs. This unravels that both the CNL and WF must be tuned together to maximize the retention period.

Doping-induced modulation of electrical and optical properties of silicon nitride

This work presents first-principle calculations of electronic structure and optical properties of doped α-Si 3N 4. It is found that B and P impurities form shallow acceptor and deep donor bands, respectively, in the band gap of α-Si 3N 4. Analysis of the charge neutrality level indicates that bipolar doping of α-SiN x is possible and that both n- and p-type electrical conductivity can be expected. This result can be helpful to extend the list of device applications of SiN x . Furthermore, it is shown that upon heavy doping with these impurities, the optical properties of the material are modified by doping. Both the refractive index and extinction coefficients are increased over the photon energy range 0–4 eV as a result of the doping.

Proposal of a new physical model for Ohmic contacts

Ohmic contacts are crucial for both device applications and the study of fundamental physics. In this study, we propose a new physical model for Ohmic contacts based on the detailed considerations of a metal/semiconductor interface, such as charge neutrality level concept, with which it is possible to describe the real situation precisely. Our proposed model contains many defect energy levels that originate from vacancies and impurities located in the vicinity of the metal/semiconductor interface, within the energy range of the Schottky barrier height. Moreover, we calculate the current–voltage characteristics based on our model. Our calculated results show that our model reveals linear Ohmic I–V characteristics and dense defect energy level distribution in the energy range of the Schottky barrier height is crucial for obtaining Ohmic I–V characteristics.

Characteristics of surface states and charge neutrality level in Ge

The characteristics of surface states on Ge are investigated using metal/insulator/semiconductor structures and the conductance technique. Evidence of acceptor-like and donor-like surface states on the valence band side of the Ge bandgap is shown by trap time constant analysis. The dependency of trap time constants on trap energy separation from band edge, capture cross section, and temperature are studied through conductance measurements and simulations. The effect of surface states on the location of charge neutrality level at the Ge surface and the consequences such as surface conductivity and negative charge buildup at the interface are discussed.

Charging energy, self-interaction correction and transport energy gap for a nanogap organic molecular junction

A C 60 molecule sandwiched between two Au(1 1 1) based tips is used as a model system to analyze the formation of nanogap molecular junctions including charging effects and self-interaction corrections. In our approach, we combine a DFT-LDA calculation of the structural and electronic properties of the system with an analysis of the interface barrier formation based on the charge transfer and the organic Charge Neutrality Level, as described by the Induced Density of Interface States model. This allows us to determine, as a function of the tip/C 60 distance, the self-interaction correction to the frontier molecular orbitals levels, the transport energy gap and the metal/organic barrier height. Our approach offers a fully consistent technique for analyzing a metal/organic interface at the molecular level.

The level of local charge-neutrality and pinning of the Fermi level in irradiated nitrides wz-III-N (BN, AlN, GaN, InN)

On the basis of the density functional theory and general gradient approximation and the method of special points, the electronic spectra and the energy position of the local charge-neutrality level are calculated for the wurtzite BN, AlN, GaN, and InN compounds with the use of different heuristic models. It is shown that, with an increasing atomic mass of the cation in the wz-III-N compounds, the charge-neutrality level shifts from the position near the midgap in the BN and AlN, to the upper part of the band gap in GaN, and to the allowed energy region of the conduction band in InN. Such shifts define the semi-insulator properties of BN and AlN, the n-type conductivity of GaN, and the n+-type conductivity of InN on saturation of the wz-III-N compounds with intrinsic defects induced by high-energy irradiation.

InSb–TiOPc interfaces: Band alignment, ordering and structure dependent HOMO splitting

Thin films of titanyl phthalocyanine (TiOPc) have been adsorbed on InSb(1 1 1) (3 × 3) and InSb(1 0 0) c(8 × 2) surfaces and studied with respect to their electronic structure using photoemission (PES), density functional theory (DFT) and scanning tunneling microscopy (STM). The interface chemical interaction is weak in both cases; no adsorbate induced surface band bending is observed and the energy level alignment across the interface is determined by the original position of the substrate Fermi level and the charge neutrality level of the molecule. Room temperature adsorption results in disordered films on both surfaces. The behaviors after annealing are different; on InSb(1 0 0) well-ordered molecular chains form along and on top of the In-rows, whereas on (1 1 1) no long range order is observed. The disorder leads to intermolecular interactions between the titanyl group and neighboring benzene rings leading to a split of TiOPc HOMO (highest occupied molecular orbital) by as much as 0.8 eV.

Electronic properties and pinning of the Fermi level in irradiated II–IV–V2 semiconductors

Experimental data on the electronic properties of II–IV–V2 semiconductors irradiated with high-energy particles (electrons, protons, and neutrons) are reported. The limiting electrical characteristics of irradiated ternary compounds are evaluated and compared with the corresponding data for the binary III–V analogues of the compounds. Particular emphasis is placed on the determination of the limiting position of the Fermi level, Flim, in irradiated II–IV–V2 compounds. The results of calculations of the energy position of the intrinsic charge-neutrality level are presented.