Large Energy Shift Research Articles

Characterization of InGaP/GaAs Heterojunction Bipolar Transistors with a Heavily Doped Base

Characteristics of InGaP/GaAs heterojunction bipolar transistors (HBTs) with a heavily doped base are examined at the base doping level NB ranging from 5×1019 to 5×1020 cm-3. At NB of less than 3×1020 cm-3, the current gain is mainly determined by Auger recombination in the intrinsic base region and is inversely proportional to the square of NB. In contrast, the current gain at NB above 3×1020 cm-3 is significantly decreased. We evaluated the effective barrier height of holes between the emitter and the base by measuring temperature dependence of current gain, and found that the effective hole barrier is reduced as NB increases. This result is explained by the large energy shift of the Fermi level inside the valence band due to heavy doping, causing the increase in the back injection of holes into the emitter, and thus reducing the current gain.

Controlled coordination and oxidation states of copper and manganese cations in complex nickel–copper–cobalt–manganese oxide thin films

Thin thermistor films of Ni0.48Co0.24CuxMn2.28−xO4 (x=0.6 and 1.2) were prepared by the deposition of metalorganic solutions followed by furnace annealing at temperatures between 600 and 800 °C. Annealing temperatures are decisive factors to control the electrical properties and electronic structure. X-ray photoelectron spectroscopy revealed that the specimens contained a mixture of Cu1+ and Cu2+ cations, and the annealing caused the change of the oxidation state from Cu1+ to Cu2+, which was accompanied by the reduction of manganese cations from Mn4+ to Mn3+. The Cu 2p core level from the Cu1+ state along with Cu 3d levels showed unusually large negative binding energy shifts (2p3/2 at 930.8 eV and 2p1/2 at 950.6 eV). Extended x-ray absorption fine structure showed that all manganese ions were located in octahedral sites of the spinel lattice, and both Cu1+ and Cu2+ cations occupy the tetrahedral sites regardless of the annealing temperature. X-ray absorption near edge structure spectra of the Mn K edge confirmed the reduction of manganese at high temperature. Cu K-edge spectra confirmed the presence of the cations with anomalous position in the 3d states, thus the negative shift of the Cu1+ core was attributed to the tetrahedral coordination of these cations in the spinel structure.

Hydrogen-Bond Induced Surface Core-Level Shift in Isonicotinic Acid

Intermolecular hydrogen-bonding in thick films of isonicotinic acid evaporated onto rutile TiO2(110) has been investigated with X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). The rate of deposition is found to be a key factor in overcoming the kinetic barriers to extensive hydrogen-bond formation, which, when present, gives rise to large energy shifts between bulk and surface in both the N1s XPS and XAS. The origin of the surface core-level shift is attributed to the presence of non-hydrogen-bonded nitrogen atoms in the surface layer.

Ion Implantation Induced Interdiffusion in Quantum Wells for Optoelectronic Device Integration

AbstractIon implantation induced intermixing of GaAs/AlGaAs and InGaAs/AlGaAs quantum wells was studied using low temperature photoluminescence. Large energy shifts were observed with proton implantation and subsequent rapid thermal annealing. Energy shifts were found to be linear as a function of dose for doses as high as ∼5×1016 cm−2. Proton implantation and subsequent rapid thermal annealing was used to tune the emission wavelength of InGaAs quantum well lasers as well as detection wavelength of GaAs/AlGaAs quantum well infrared photodetectors (QWIPs). Emission wavelength of lasers showed blue shift whereas detection wavelength of QWIPs was red shifted with intermixing.

Sulfur Adsorption and Reaction with a TiO2(110) Surface: O↔S Exchange and Sulfide Formation

Upon sulfur adsorption on TiO2(110) at 600 K, all surface oxygen is replaced by sulfur. High-resolution photoemission data show a complete loss of oxygen from the surface layer, a large binding energy shift and attenuation of Ti core levels, and the presence of three different S species. The bonding of sulfur is examined using first-principles density-functional calculations and the periodic supercell approach. At saturation the top layer of the oxide surface is converted to sulfide, with the majority of sulfur buckled above the Ti lattice plane and the remaining sulfur bonded in bridging sites. A mechanism for this self-limiting thermodynamically unlikely surface reaction is proposed.

About Coulomb energy shifts in halo nuclei

We study the Coulomb energy shifts in mirror nuclei with the emphasis on the halo nuclei, ${}^{11}\mathrm{Be}$ and ${}^{11}\mathrm{N}.$ We find that even in such loosely bound nuclei the second- and higher-order Coulomb contributions are relatively small and the large relative Coulomb energy shifts are due to first order effects in the Coulomb interaction.

X-ray photoelectron spectroscopy of low surface concentration mass-selected Ag clusters

We have developed an experimental setup for the study of small mass-selected clusters delivered by soft landing to a model oxide support. An automated deposition system to achieve accurately quantified homogeneous surfaces is described which also overcomes beam instability. Finally we present some recent photoelectron spectroscopic data from the analysis of mass-selected Agn+ clusters deposited on a Xe covered Al2O3 surface. Large core-level binding energy shifts are observed as a function of deposited cluster size and diffusion/agglomeration within the noble gas layer.

Distinguishing between interface dipoles and band bending at metal/tris-(8-hydroxyquinoline) aluminum interfaces

Using a combination of photoelectron spectroscopies and ellipsometry to study the thin-film growth of Alq3 (tris-(8-hydroxyquinoline) aluminum) on Ag, we have been able to distinguish between molecular orbital energy shifts due to “band bending” and interface dipole formation. We have observed large binding energy shifts (≈0.5 eV) of the organic molecular levels at low coverages. We conclude that these shifts are consistent with the formation of a polarized molecular layer, or interface dipole, and are inconsistent with an electrostatic band-bending model.

Ultraviolet emission properties in In xGa 1− xN epitaxial layer revealed by magnetoluminescence and time-resolved luminescence studies

Two recombination channels (higher- and lower-energy states) have been found in the efficient ultraviolet (UV) emission of an In 0.08Ga 0.92N epitaxial layer. The time-resolved luminescence spectra and the temperature dependence of decay time have shown that an energy-transfer process of carriers between two states has significantly taken place. The magnetoluminescence studies have revealed from a large Landau energy shift that the higher-energy state is not related to the localized excitons, but is due to an electronic transition.

Enhanced Hole Generation in Mg-Doped AlGaN/GaN Superlattices Due to Piezoelectric Field

We investigated the electrical properties of uniformly Mg-doped AlGaN/GaN superlattices (SLs) with various Al mole fractions and SL period thicknesses. We found that their sheet hole concentration depended strongly on the Al mole fraction and SL period thickness. We found the maximum spatial averaged hole concentrations for SLs with period thicknesses of 360 Å and 100 Å to be 3×1018 cm-3 at room temperature. The resistivities of the SLs were less temperature dependent than those of GaN and AlGaN, indicating that the enhanced hole generation in these SLs could be ascribed to the large energy shift of the valence band edge due to the piezoelectric field.

Optical Spectra of Single Quantum Dots: Influence of Impurities and Few-Particle Effects

The evolution of photoluminescence (PL) spectra of single GaAs/AlGaAs quantum dots (QD) is studied as a function of laser excitation power and temperature. At very low powers, where multi-exciton occupation of the QD can be excluded, an unexpected and pronounced spectral evolution is observed (large energy shifts and appearance of multiple emission lines). A similar evolution is observed at low excitation powers with increasing temperature. A model, taking into account the influence of the shallow, residual impurities in the environment of each QD, explains the observed spectral evolutions in terms of photo-depletion of the QD and hopping of impurity-bound carriers back into the QD. Theoretical calculations of the PL due to Nelectrons + 1 hole (Ne + 1h) QD states allow us to attribute the ≈︂2 meV spaced lines in the experimental spectra to the different charge states Ne + 1h, (N — 1) e + 1h, … of the QD.

Magnetoluminescence Studies of Highly Packed InGaAs Self-organized Quantum Dots on GaAs(311)B

We have fabricated a highly packed and ordered In0.4Ga0.6As quantum dots (QDs) array on GaAs(311)B substrate. In order to investigate the lateral coupling of QDs, we have carried out photoluminescence and magnetoluminescence measurement to evaluate the optical properties of these QDs. A large energy shift dependence on magnetic field was observed from a sample in which the QDs surface coverage was almost 100%. These results indicate a possibility that the QDs are coupled laterally, and the photo-generated carriers and excitons can move within the two-dimensional plane over distances larger than QDs size.

Field emission from diamond, diamond-like and nanostructured carbon films

We have deposited nanotube films on silicon via a chemical vapor deposition (CVD) growth process known from the deposition of diamond. We used a metallic catalyst which was deposited onto the silicon surface prior to the CVD deposition. The films are very pure, adhere well and are very well suited for electron field emission. We measured emission at 2.6 V/μm (for 1 nA emission current) and an emission site density reaching 10 4/cm 2 at 3–4 V/μm as measured on a phosphor screen. Electrons originate at the Fermi level and the high local fields at the emission site is produced by the geometry of the nanotube. The results obtained on these films are comparable to those from differently prepared CVD diamond films. So far, we have no evidence that electron injection occurs. The emission process is governed by field amplification at protrusions and tips. In a second experiment we have measured emission from a metallic micrometer sized grain fixed on a diamond (100) surface, with different surface termination (hydrogen, oxygen, sp 2 carbon). The field emitted electron energy distribution (FEED) spectra show large energy shifts which are due to the surface resistivity and not due to injection of electrons in the conduction band. Hence, energy shifts in FEED spectra do not necessarily reflect an injection mechanism.

Influence of GaAs capping on the optical properties of InGaAs/GaAs surface quantum dots with 1.5 μm emission

Uncapped InGaAs quantum dots (surface quantum dots) on a GaAs substrate emit photoluminescence at a long wavelength of 1.53 μm at room temperature. When the surface dots are covered by a GaAs cap layer, the emission energy of the dots increases by 287 meV. This large energy shift is mainly caused by inducing compressive stress from the cap layer. In segregation on the surface led to greater photoluminescence intensity in the surface quantum dots even at room temperature due to the suppression of nonradiative surface recombination.

Surface conductivity induced electron field emission from an indium cluster sitting on a diamond surface

Electron field emission has been performed from a 5 μm indium cluster sitting on a nitrogen doped diamond (100) surface (3×3×1 mm3). The motivation to study such a system arose from the fact that electron field emission from N-doped diamond was reported in literature. However, the emission mechanism is not really known and still the subject of an intense discussion in the diamond/amorphous carbon community. We have chosen this setup to conduct a model experiment with well-defined parameters. The emission properties were analyzed by energy resolved field emission (FEED). We found that electron emission takes place at the apex of the indium grain for a hydrogenated diamond surface. However, large energy shifts were observed in the FEED spectra due to a potential drop across the hydrogen saturated surface. An oxygen termination kills the emission. By sputtering the surface the emission reappears with much lower energy shifts in the FEED spectra due to the conductive sp2 layer. We present a model taking into account the ohmic surface resistivity of the diamond surface and discuss the impact on emission properties of nanoclustered films. The results presented in this article show that surface conduction (hydrogen terminated or sputtered surface) can lead to electron field emission with large shifts in the FEED spectra. While the emission stems from a metallic micron sized tip, the electron supply is guaranteed by surface conduction.

The Mechanism of the Anomalous Energy Shift between s-States in Mirror Nuclei with a Halo Structure

A mechanism of the anomalous energy shift between s-states in mirror nuclei is investi­ gated by using the complex scaling method (CSM). The s-states are described by a resonant core+p system and by a loosely bound core+n system, where the latter reproduces a neu­ tron halo structure. To understand the mechanism of the anomalous energy shift appearing in the resonant s-state of the core+p system, we pick out the complex expectation values of the Coulomb potential and kinetic energy operators from the Hamiltonian for resonant states, using the procedure recently developed in the framework of the CSM. We find that the kinetic energy reduction is large, in addition to the Coulomb energy reduction in the ordinary Thomas-Ehrman shift. As a result, a large energy shift is reasonably explained by the combination of two mechanisms of the kinetic energy reduction and the Coulomb energy reduction.

Dislocation-related photoluminescence peak shift due to atomic interdiffusion in SiGe/Si

Low-temperature photoluminescence (PL) spectroscopy was used to study electronic states associated with threading dislocations (D lines) in strain-relaxed Si1−xGex layers. The structures investigated were grown by ultrahigh vacuum chemical vapor deposition at 550 °C and consist of a Si(001) substrate, followed by a stepwise graded buffer layer, followed by a thick uniform composition Si1−xGex layer. The PL peak positions of the four D lines after isochronal annealing at temperatures between 600 and 800 °C were measured. We show that the large energy shift of the D1 line is due to a change in the local band gap energy at the dislocation core due to strain-driven diffusion of Ge atoms away from the dislocation core with an activation energy Ea, which varies with Ge mole fraction x.

Luminescence in selectively excited germanium microcrystallites

We have obtained the luminescence spectrum from Ge microcrystallites selectively excited. The spectrum has a broad feature with several structures, which are classified into two categories: a steplike structure near the excitation energy with an onset energy of 23 meV that is assigned to the phonon structure due to the LA$(L)$ phonon, and a feature with three peaks at larger energy shifts (220, 380, and 590 meV) from the excitation energy. The results suggested a close similarity of the luminescence mechanisms of the Ge microcrystallite system and porous Si.

Quantum Well Intermixing for Optoelectronic Applications

AbstractIon implantation induced intermixing of GaAs-AlGaAs quantum well structures with H, O and As ions is investigated by low temperature photoluminescence. Large energy shifts are observed in all the cases, though recovery of photoluminescence intensities in proton case is more significant than others. Energy shifts are linear with proton dose and no saturation was observed even up to a dose of ∼ 5 × 1016 cm−2. Saturation in energy shifts are seen for both As and 0 ions at high doses. Energy shifts are also found to be dependent on Al composition of the barriers. Wavelength shifted quantum well lasers are fabricated with energy shifts of about 5 nm with no changes in threshold current using proton implantation. Anodic oxide induced intermixing of GaAs-AlGaAs quantum wells is demonstrated.

Optical properties of ultrathin PbI 2 microcrystallite in polymer

We have prepared PbI 2 microcrystallites embedded in polymer, which have a layer structure and are ultrathin crystals consisting of two to nine monolayers. A very large energy shift of the exciton absorption band has been observed in these microcrystallites and interpreted in terms of size confinement of the translational motion of excitons in the c-direction perpendicular to the crystal surface. The simple effective mass approximation is broken down in 2, 3, 4 layer crystallites, because the crystal thickness is smaller compared to the exciton Bohr radius. Secondly, in the Raman spectrum where the excitation energy is resonant to the exciton energy, there appears a new line in the energy region below 20cm −1 which is characteristic of a ultrathin crystal. The Raman shift increases with decreasing the crystal thickness. This line is assigned as due to a longitudinal mode of a rigid-layer phonon. Thirdly, the confinement of the internal motion is studied by measuring a diamagnetic shift of the exciton energy in the magnetic field up to 150 T and a bound exciton luminescence in the ultrathin microcrystallites. Much larger binding energy of the exciton compared to the bulk value is estimated. This fact suggests that the envelope function of the exciton shrinks not only by a strong spacial restriction in the c-direction but also by the dielectric screening from the surrounding polymer. Fourthly, the exciton-phonon interaction is studied by the hole-burning measurement. From the temperature dependence of the hole width, it is suggested that the exciton is scattered by impurities or defects below 40 K and by a LO phonon above 40 K. Finally, the hydrostatic pressure dependence of the exciton energy and the resonant Raman scattering in the energy region of the optical phonons have been measured and the size effect on the atomic bonding between I and Pb is discussed. It is concluded that covalent bonding between Pb and I atoms changes to ionic bonding in the ultrathin crystallites.