Interactions at tetraphenyl-porphyrin/InP interfaces observed by surface photovoltage spectroscopy

Illumination induced charge separation processes at tetraphenyl-porphyrin (H₂TPP)/InP interfaces are characterized. The results indicate that upon illumination an electronic transition takes place between the organic highest occupied molecular orbital (HOMO) and the InP conduction band. A 70 meV blue shift in the characteristic modulated surface photovoltage spectrum of a 50 nm H₂TPP film was observed when n-InP substrate was used. Such blue shift was not observed when Au, SnO₂:F or p-InP substrates were used. The shift may suggest a presence of an ordered interfacial sub-layer in an almost lying orientation, due to chemical or physical interactions at the interface. The results indicate that the Fermi level is unpinned at the interface. An exciton diffusion barrier in close proximity to the H₂TPP/n-InP interface has been demonstrated.     

An exchange bias observed in Tb/Cr/FeCo trilayers with ultrathin Cr layer at low temperature

Positive exchange bias field (H_e) is observed in Tb/Cr (t_Cr)/FeCo trilayers at 5 K without cooling field, and negative H_e for Tb/FeCo bilayer. The negative H_e of Tb/FeCo implies the FM coupling at the interface due to Co and Fe dominate in the magnetization of the ferrimagnetic interlayer alloy of FeCo and Tb. With the inserting of Cr layer, this situation is broken, and the positive H_e implies the antiferromagnetic interlayer coupling. A peak of H_e = 6.0 mT for trilayers with t_Cr = 1.5 nm is corresponding to the minimum value of coercivity as a function of t_Cr at 5 K, which is used to study the effect of the cooling field (H_fc) on H_e as a function of temperature. It is found that H_fc of 100 mT triggers H_e from positive to negative at T ≤ 15 K. The magnetoresistance results also confirm the coexistence of multiple MR mechanisms in these trilayers.     

Modification of the electronic structure and formation of an accumulation layer in ultrathin Ba/n-GaN and Ba/n-AlGaN interfaces

The electronic structure of the n-GaN(0001) and Al _x Ga_{1 ? x} N(0001) (x = 0.16, 0.42) surfaces and the Ba/n-GaN and Ba/AlGaN interfaces is subjected to in situ photoemission investigations in the submonolayer Ba coverage range. The photoemission spectra of the valence band and the spectra of the surface states and the core 3d level of Ga, the 2p level of Al, and the 4d and 5p levels of Ba are studied during synchrotron excitation in the photon energy range 50–400 eV. A spectrum of the surface states in Al _x Ga_{1 ? x} N (x = 0.16, 0.42) is found. The electronic structure of the surface and the near-surface region is found to undergo substantial changes during the formation of the Ba/n-GaN and Ba/AlGaN interfaces. The effect of narrowing the photoemission spectrum in the valence band region from 10 to 2 eV is detected, and surface eigenstates are suppressed. The Ba adsorption is found to induce the appearance of a new photoemission peak in the bandgap at the Fermi level in the Ba/n-GaN and Ba/n-Al_0.16Ga_0.84N interfaces. The nature of this peak is found to be related to the creation of an accumulation layer due to a change in the near-surface potential and enriching band bending. The energy parameters of the potential well of the accumulation layer are shown to be controlled by the Ba coverage.     

Characterization of In/Pd and Pd/In/Pd thin films by ellipsometric, XRD and AES methods

In/Pd and Pd/In/Pd thin films were prepared by thermal evaporation on the SiO₂ substrate in a vacuum. The structural and optical properties of the films were investigated by means of X-ray diffractometry (XRD), Auger electron spectroscopy (AES) and spectroscopic ellipsometry (SE). Auger depth profile studies were performed in order to determine the composition of elements in the Pd-In systems. Interdiffusion of metals was detected at room temperature. Optical properties of Pd-In composite layers formed due to the interdiffusion were derived from ellipsometric quantities Ψ and Δ measured in the photon energy range 0.75-6.50 eV at different angles of incidence. The effective optical spectra show absorption peaks dependent on the composition of nonuniform films. The XRD patterns indicated formation of Pd_1−xIn_x intermetallic phases in the samples.     

Crystallization behaviors of ultrathin Al-doped HfO_2 amorphous films grown by atomic layer deposition

In this work, ultrathin pure HfO_2 and Al-doped HfO_2films(about 4-nm thick) are prepared by atomic layer deposition and the crystallinities of these films before and after annealing at temperatures ranging from 550℃ to 750℃ are analyzed by grazing incidence x-ray diffraction. The as-deposited pure HfO_2 and Al-doped HfO_2 films are both amorphous. After550-℃ annealing, a multiphase consisting of a few orthorhombic, monoclinic and tetragonal phases can be observed in the pure HfO_2 film while the Al-doped HfO_2 film remains amorphous. After annealing at 650℃ and above, a great number of HfO_2 tetragonal phases, a high-temperature phase with higher dielectric constant, can be stabilized in the Al-doped HfO_2 film. As a result, the dielectric constant is enhanced up to about 35. The physical mechanism of the phase transition behavior is discussed from the viewpoint of thermodynamics and kinetics.     

Intrinsic point defects in niobium: Trapping at100Pd and111In impurities observed by PAC

Radiation-induced lattice defects in high-purity niobium have been investigated in the temperature range of 30K to 540 K by means of - perturbed angular correlation (PAC) measurements using the radioactive probes¹⁰⁰Pd/¹⁰⁰Rh and¹¹¹In/¹¹¹Cd. Both probes were produced within the niobium samples by means of heavy-ion nuclear reactions. At the Pd impurities trapping of defects occurred during heavy-ion irradiation at about 30 K in two defined configurations: defect 1(Pd) withv _Q1=e ² qQ/h=42(±2) MHz, ₁=0 and defect 2 (Pd) withv _Q2=(±2) MHz, ₂=1. Two defects were observed at the In impurities in annealing stage III (around 250 K) after heavy-ion as well as electron irradiations: defect 1(In) withv _Q1=87(±1) MHz, ₁=0 and defect 2(In) withv _Q2=105(±2) MHz, ₂=0.65(±0.02). A third defect (defect 3(In):v _Q3=177(±2) MHz, ₃0.2) appeared above 260 K after heavy-ion irradiation only. The data are interpreted in terms of interstitial trapping at the Pd impurities and vacancy trapping at the In impurities. Information on the microscopic structure of defect 1(In) and 2(In) is obtained from a PAC-single-crystal experiment. For defect 1(In) axial 111-symmetry is found, which leads us to identify this defect with a monovacancy as nearest neighbor with respect to the In probe. Defect 2(In) is the trapped divacancy for which an orientation is found that is consistent with both vacancies being nearest neighbor to the probe but second nearest neighbors to each other.     

Surface triple points and multiple-layer transitions observed by tuning the surface field at smectic liquid-crystal-water interfaces

We present an ellipsometric study of the interface between a smectic liquid crystal and water in the presence of a nonionic surfactant. The surfactant concentration serves as a handle to tune the surface field. For sufficiently large surfactant concentrations, a smectic phase is present at the interface in the temperature range above the smectic-A-isotropic bulk transition; when the bulk transition is approached, the thickness of this surface phase grows via a series of layer-by-layer transitions at which single smectic layers are formed. At lower surfactant concentrations, transitions appear at which the thickness of the surface phase jumps by multiple smectic layers, thereby implying the existence of triple points at which surface phases with different smectic layer numbers coexist. This is the first experimental demonstration of such surface triple points which are predicted by theoretical models.     

Formation of amorphous films by solid-state reaction in an immiscible Y-W system

Received: 26 November 1996/Accepted: 7 February 1997     

In0.2Ga0.8As/GaAs interface revealed by an AlAs marker layer

A detailed analysis of the microstructure of layered semiconductor heterostructures is only possible if the interface between the various layers can be accurately located. Microstructural features whose characterisation is dependent on the location of the interface include the planarity and width of the layers, the composition profile and the geometry of misfit dislocations (in the particular case of lattice mismatched heterostructures). We report on an investigation to image, at high resolution in the transmission electron microscope, the interface in an In_0.2Ga_0.8As/GaAs structure grown by molecular beam epitaxy. The difficulty in locating the interface, due to similarity in electron optical behaviour of GaAs and In_0.2Ga_0.8As when imaged in a 110 direction, was overcome by incorporating into the structure a marker layer of AlAs two unit cells thick (11.4 Å in total) between the GaAs and the In_0.2Ga_0.8As layers. Lattice fringe images of an In_0.2Ga_0.8As/AlAs/GaAs structure are presented which show, at near atomic resolution, the location of the misfit dislocations relative to the In_0.2Ga_0.8As/GaAs interface.     

Anomalous absorption of TM electromagnetic waves by an ultrathin layer: optical analog of liquid friction

A strong maximum of absorption (as much as 100%) of bulk TM electromagnetic waves by an ultrathin film with imaginary dielectric permittivity is shown to exist at some optimal film thickness. This typical thickness is usually much smaller than the wavelength and the wave penetration depth in the material of the film. The absorptivity maximum increases and the typical thickness decreases with increasing dielectric permittivity of the layer. An optical analog of linear (liquid) friction is discussed. A frictional contact approximation for TM electromagnetic waves is analyzed, and relevant boundary conditions with an optical coefficient of friction are derived.     

Electronic and magnetic properties of ultrathin films and interfaces investigated by dichroic core-level photoelectron spectroscopy

The electronic and magnetic structures of ultrathin films made of a ferromagnetic and a nonmagnetic material are theoretically investigated by means of magnetic dichroism in spin- and angle-resolved core-level photoelectron spectroscopy. How these properties manifest themselves in the photoemission intensities is analyzed with a focus on the interface between film and substrate. The dependence on both exchange and spin–orbit splitting, magnetic ordering, core-level shift and on the thickness of the covering layer are investigated in detail. Ultrathin films of Fe and Pd serve as prototypical systems because of their large exchange and spin–orbit splittings, respectively.     

Effects of Cr and V impurities on cohesion properties of Pd/TiAl interfaces

First principles calculations reveal that for Pd/TiAl interfaces the substitution of interface Ti atoms with Cr or V atoms is energetically favorable with negative heat of formation, and could bring about a very small increase of interface bond strength, while the interstitial Cr and V atoms should be unfeasible with highly positive heat of formation and would increase the bond lengths of interface Pd–Al and Pd–Ti bonds. Calculations also demonstrate that both Cr and V would induce an increase of interface energy, suggesting that the impurity atoms of Cr or V should be reduced to a minimum level, in order to get a thermally stable Pd/TiAl interface with a longer lifetime. In addition, it is found that the substitution of V at the Pd/TiAl interfaces should be much easier than that of Cr, which is in good agreement with similar experimental observations in the literature.     

Structural analysis of In/Ag, In/Cu and In/Pd thin films on tungsten by ellipsometric, XRD and AES methods

Compositional, microstructural and optical properties of In/Cu, In/Ag and In/Pd thin films evaporated on W substrate in a vacuum were investigated by means of Auger electron spectroscopy, X-ray diffractometry, scanning electron microscopy, and spectroscopic ellipsometry methods. Thicknesses of deposited pure metal layers were adjusted to atomic concentration ratios In:Ag = 1:2, In:Cu = 2:1 and In:Pd = 3:1. Interdiffusion of metals and creation of intermetallic compounds AgIn₂, Ag₂In and CuIn₂were detected at room temperature. Phase transformation and changes in the surface morphology due to annealing of samples at 393 K for 60 min were revealed. Combined investigations indicated a layered structure of In/Ag films. A tendency of island-like structure formation was stronger for In/Cu and In/Pd films. The complex dielectric functions of In/Ag, In/Cu and In/Pd composite layers were determined from spectroellipsometric data.     

Photoluminescence and capacitance–voltage characterization of GaAs surface passivated by an ultrathin GaN interface control layer

A novel surface passivation technique for GaAs using an ultrathin GaN interface control layer (GaN ICL) formed by surface nitridation was characterized by ultrahigh vacuum (UHV) photoluminescence (PL) and capacitance–voltage (C–V) measurements. The PL quantum efficiency was dramatically enhanced after being passivated by the GaN ICL structure, reaching as high as 30 times of the initial clean GaAs surface. Further analysis of PL data was done by the PL surface state spectroscopy (PLS³) simulation technique. PL and C–V results are in good agreement indicating that ultrathin GaN ICL reduces the gap states and unpins the Fermi level, realizing a wide movement of Fermi level within the midgap region and reduction of the effective surface recombination velocity by a factor of 1/60. GaN layer also introduced a large negative surface fixed charge of about 10¹² cm⁻². A further improvement took place by depositing a Si₃N₄ layer on GaN ICL/GaAs structure.     

Magnetic anisotropy of transition-metal interfaces from a local perspective: reorientation transitions and spin-canted phases in Pd capped Co films on Pd(111)

Layer-resolved self-consistent electronic calculations of magnetic anisotropy energy (MAE) provide new insight to the off-plane magnetization observed in Pd capped Co films on Pd(111). We demonstrate that the transition from perpendicular to in-plane phases with increasing film thickness involves an intermediate spin-canted phase. The interfaces responsible for the stability of the off-plane easy axes are characterized microscopically. A local analysis of the MAEs reveals an unexpected internal magnetic structure of the Co-Pd interfaces in which the magnetic moments and spin-orbit interactions at the Pd atoms play a crucial role.