Existence Of Surface States Research Articles

Surface optical phonon and A1(LO) in ZnO submicron crystals probed by Raman scattering: Effects of morphology and dielectric coating

The authors studied the surface optical phonon and A1(LO) in ZnO submicron crystals as well as ZnO single crystals with submicron surface structures by employing ultraviolet Raman scattering. The small variation in Raman frequency of A1(LO), when the growth condition is changed, is mainly caused by the crystal imperfection via two mechanisms: one is the frequency lowering by native defects and the other is the frequency increasing due to the tilt of c-axis of ZnO crystals away from the surface normal direction that leads to the mixing of A1(LO) and E1(LO). The evolution of Raman features, before and after dielectric coatings, in conjunction with the photoluminescence, provides clear-cut evidence for the observation of a surface optical phonon. The frequency of the observed surface mode is lowered by the existence of surface states, and the surface states can be effectively reduced by the dielectric coatings.

Surface States in a Square Lattice under Magnetic Field Applied in the Surface Region

Electronic surface states localized on the edge of a semi-inflnite square lattice are studied in the tight binding approximation. We examined the existence of surface states in the presence of magnetic fleld applied in the surface region of a 2D lattice. The applied fleld is perpendicular to the lattice and conflned to a stripe near the surface. We also included a surface site perturbation caused by the presence of the surface. The magnetic fleld is introduced into the model by the Peierls substitution.

Growth and optical properties of ZnO microwells by chemical vapor deposition method

Abstract The well-like ZnO nanostructures were obtained by chemical vapor deposition method. The uniform and dense ZnO slim nano-columns were grown along the circle to form a microwell. The growth mechanisms, such as 1D linear, 2D screw dislocation and step growth are discussed. These observations provide some insight into the growth kinetics in vapor–solid growth process. The fabrication of ZnO microwell morphology provided a direct experimental evidence for explaining the 1D growth mechanism based on the axial screw dislocation. Photoluminescence (PL) microscopy showed the surface-related optical properties. The green light emission enhancement revealed that the ZnO microwells have waveguide properties. The abnormal enhancement of integrated PL intensity of deep-level emission with temperature increase showed abundant surface state existence.

Semimetal–semiconductor transition in thin Bi films

The temperature dependences of the Hall coefficient of thin bismuth films with thicknesses d = 8 to 215 nm prepared by thermal evaporation in vacuum on mica substrates were obtained in the temperature range 80–300 K. It was established that at thicknesses smaller than ∼ 25–30 nm, semimetallic conductivity is not observed. It was suggested that in this thickness range a semimetal–semiconductor transition occurs. Using this assumption and taking into account the existence of surface states, it was shown that the gap between the valence and conduction bands in the semiconductor region increases with decreasing d.

Mobility of conduction electrons in ultrathin Fe and Cu films on Si(111)

Using in situ transmission spectroscopy in the middle infrared, an enhancement of the Drude-type plasma frequency with respect to the bulk value was found for ultrathin Fe films in ⟨111⟩ orientation grown on $\mathrm{Si}(111)7\ifmmode\times\else\texttimes\fi{}7$. That finding is in good agreement with ab initio calculations that, in particular, take into account the existence of surface states and surface resonances. These calculations also predict an increase of the plasma frequency with decreasing thickness for Cu(111) films, but the experimental proof for Cu was hampered by an islandlike growth. However, in experiment and in theory the bulk value for the plasma frequency of copper is already reached at $3\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ thickness.

Surface resonant states and superlensing in acoustic metamaterials

We report that the negative material responses of acoustic metamaterials can lead to a plethora of surface resonant states. We determine that negative effective-mass density is the necessary condition for the existence of surface states on acoustic metamaterials. We offer the microscopic picture of these unique surface states; in addition, we find that these surface excitations enhance the transmission of evanescent pressure fields across the metamaterial. The evanescent pressure fields scattered from an object can be resonantly coupled and enhanced at the surface of the acoustic metamaterial, resulting in an image with resolution below the diffraction limit. This concept of acoustic superlens opens exciting opportunities to design acoustic metamaterials for ultrasonic imaging.

Spin‐polarized surface states in semiconductor superlattices

Electronic surface states in Al0.1Ga0.9As|GaAs superlattice are calculated for a system with a ZnMnSe barrier in a surface cell. In the absence of applied magnetic field the ZnMnSe barrier has the same height as the Al0.1Ga0.9As barriers in the bulk cells. When a magnetic field is applied perpendicularly to the superlattice layers, the surface cell magnetic barrier height is found to vary with spin orientation. Hence, each spin polarization involves different conditions of surface state existence. Our computations show a split of Shockley states into two levels (corresponding to different spin polarizations) and appearance of Tamm states (with specific spin polarization), induced by sufficiently strong magnetic fields. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Surface matter waves caused by the absorption of nonrelativistic particles

The existence of surface states of nonrelativistic particles at the interface between a vacuum and a layer of a material on a reflecting substrate, which are caused by the absorption in the layer rather than the presence of discrete levels, is established. The wave functions of such states (surface matter waves) are found and the domains of their existence in the complex plane of the “optical” potential (both attractive and repulsive) of the layer are determined. Threshold effects of the count of microparticles at the intersection of the boundaries of these domains are revealed. The spatial characteristics of the surface matter waves are calculated. An example of the layer composition providing the existence of such waves is given for ultracold neutrons. The increase in the loss of trapped ultracold neutrons owing to the excitation of the surface matter waves of these particles in the trap walls is discussed.

Second harmonic generation at the GaAs(1 1 1)-B|solution interface

Second harmonic generation from the interface between GaAs(1 1 1)-B and an aqueous electrolyte has been investigated at a long wavelength (1064 nm) and at two shorter ones (865 and 815 nm). The signal showed the symmetry of the crystal; the isotropic amplitude was found to vary substantially with the electrode potential, indicating that a major part of the signal is generated near the interface. The potential dependence was stronger at shorter wavelengths. Impedance spectroscopy revealed the existence of surface states, which may affect the magnitude of the signal.

Photoluminescence upconversion in colloidal CdTe quantum dots

Photoluminescence ~PL! up-conversion, the observation of luminescence with energies higher than those of the excitation photons, has been widely observed in the bulk 1 and heterostructures 2‐5 of semiconductors, quantum wells, 6 porous silicon, 7 self-assembled quantum dots ~QDs!, 8 and colloidal QDs. 9,10 It was proposed that the existence of intermediate states with energies resonant with or lower than those of the excitation photons is a prerequisite for these upconverted photoluminescence ~UCPL! processes. Carriers must be excited to these intermediate states by the first excitation photon and further to the higher energy states through various underlying mechanisms such as Auger excitation, 7 two-step two-photon absorption ~TS TPA!, 1‐3,5,6 or the thermal effect. 9,10 Most of these intermediate states are ‘‘observable,’’ 2‐7 i.e., in addition to the UCPL emission, one can also see the normal PL emission from these states with energies lower than those of the excitation photons. However, in some cases of self-assembled and colloidal QDs, these intermediate states can hardly be observed since carriers cannot efficiently populate them by the above-band-gap excitation of normal PL. 8 In a way, UCPL can provide a unique method for detecting the existence of these ‘‘defect’’ states in semiconductor quantum nanostructures and investigating their properties, which are usually different from those of the band-gap states. Semiconductor colloidal QDs are characterized by a large surface-to-volume ratio due to their small particle sizes, as well as the surface complexity originating from the passivation ligands used to neutralize the surface dangling bonds. Surface states have long been invoked to be responsible for some complex phenomena that could not be explained by using the concept of band-gap states only. For example, the almost universal occurrence of a biexponential distribution in the radiative lifetime strongly implies the existence of surface states in colloidal QDs. 11,12 In this sense, time-resolved measurements can provide one, but only indirect, way to probe the surface states in colloidal QDs, and consequently the existence of them is still controversial. 13 However, if one can observe a UCPL signal from colloidal QDs, this problem will be easily resolved since the demand of intermediate states can only be fulfilled by the existence of surface states. Recently, UCPL signals were observed in colloidal InP, 9 CdSe, 9,10 and CdTe ~Ref. 10! QDs, which have provided a strong proof of the existence of surface states in colloidal QDs and opened up a new opportunity for the systematic study of these surface states. Here, we report on our optical studies of efficient UCPL in colloidal CdTe QDs with an energy gain of as high as 360 meV. Compared with the normal PL, the UCPL signal shows a redshift of about 80 meV in its peak energy, while the excitation spectroscopy of the UCPL ~UCPLE! measurement at this peak energy showed a broad excitation band with energies below those of the band gap. Time-resolved measurements were utilized to study the time-resolved dynamics of this UCPL, whose radiative lifetime now becomes nearly twice as long as that of the normal PL. Finally this UCPL is attributed to the recombination of carriers from the band-gap states and surface states mainly through a thermal excitation process.

Conditions of coexistence of Tamm and Shockley states in a superlattice with a perturbed surface

Conditions of electron surface-state existence are studied by means of the transfer matrix method in a semi-infinite superlattice with alloy concentration changing either abruptly (square potential) or gradually (sinusoidal potential). A small surface perturbation is introduced into the model by modifying the shape of surface potential well. The effect of this surface perturbation on Shockley (S) states is studied, with special attention paid to the threshold perturbation which destroys an existing S surface state by shifting it from the gap to a band; for Tamm (T) states, we search for the minimum perturbation value inducing a T surface state to appear. The results obtained show coexistence of Tamm and Shockley states within a precisely specified range of the model parameter values. The theory is applied to investigate S and T surface states coexistence in Al x Ga 1 - x As superlattice.

Atomic Scale Observation of Domain Boundaries on c(2×2) Fe(001) Thin Film Surfaces

Scanning tunneling microscopy (STM) measurements reveal that Fe thin films epitaxially grown on a MgO(001) substrate under appropriate conditions have an atomic step-terrace surface with a c(2×2) surface reconstruction, and that this surface is often accompanied by line-like patterns along the Fe<110 > direction. We have investigated the electronic properties of the patterns in detail by atomically resolved STM. We found that the pattern is a domain phase boundary of the c(2×2) domains, and that the domain boundaries consist of atoms exhibiting a (1×1) structure. The STM image shows remarkable bias-voltage dependency as indicated by the contrast inversion between the patterns and c(2×2) domains as the polarity of the applied bias-voltage is changed, and the contrast achieved its maximum value at +0.2 V. This bias-voltage dependency of the patterns can be explained by the existence of surface states which are observed at around 0.2 eV above the Fermi level on the Fe(001) (1×1) surface and 0.4 eV on the c(2×2) domains.

Surface states in one-dimensional photonic band gap structures

We study, both theoretically and experimentally, the existence and behavior of surface localized modes in a one-dimensional (1D) comblike photonic band gap structure. The structure is composed of a waveguide, along which dangling side branches are grafted periodically. We demonstrate a theoretical rule about the existence of surface states in such 1D periodic structures; namely, when one considers together two semi-infinite structures obtained by cutting an infinite one along a plane perpendicular to the axis of the structure, one always has as many localized surface states as minigaps. We propose an original method to observe the surface modes, namely, by measuring the transmission through a guide along which a finite comb is attached vertically. The surface modes of the comb are obtained as maxima or minima of the transmission spectrum. The experiments are performed by using coaxial cables in the frequency range of a few hundred MHz.

Understanding of surface states in a correlated electron system

The aim of this work is to find a simple analytic model to explain some principal aspects of the behavior of surface states in correlated electron systems. We start from the analytic expression for the Green function of the semi-infinite tight binding linear chain. This Green function in case of modification of the center of gravity of the first atom and the change in coupling between the first and the second atom is evaluated as an exact analytic expression. Conditions for the existence and classification of surface states are given. The spectral weight of surface states and the local density of states are evaluated. The method is applied to a s.c. (100) surface of a local moment crystal. Conditions for the existence of surface states are derived and their locations in the Brillouin zone are predicted. It is shown that it is possible to include correlation effects within the framework of the discussed model. The comparison with former numerical results is performed.

On the origin of surface states in a correlated local-moment film

The electronic quasiparticle structure of a ferromagnetic local moment film is investigated within the framework of the s-f model. For the special case of a single electron in an otherwise empty energy band being exchange coupled to a fully ordered localised spin system the problem can be solved exactly and, for the spin-down electron, some marked correlation effects can be found. We extend our model to incorporate the influence of the surface on the electronic structure. Therefore we modify the hopping integrals in the vicinity of the surface. This leads to the existence of surface states, both for the spin-up and the spin-down spectral density of states. The interplay between the modification of the hopping integrals and the existence of surface states and correlation effects is discussed in detail.

ダイヤモンド薄膜表面のカソードルミネッセンスによる評価

Cathodoluminescence (CL) obtained from the surface of chemical vapor deposited diamond is studied. The foreign adsorbed atoms of diamond surfaces vary not only the electrical properties but also CL properties. CL intensity of oxidized surface is lower than that of hydrogenated surface because the existence of surface states promote the surface recombination of electrons and holes. However, it is also shown that fluorination of the oxidized surface recovers the CL intensity of diamond surface through reduction of surface states.

Effective-mass superlattice: surface states and zero energy gaps

The electronic structure of a terminated effective-mass superlattice (EMSL) is investigated taking into account the effect of the EMSL surface (i.e. the EMSL/substrate interface). Special attention is paid to the conditions under which two neighbouring miniband edges cross each other, which results in the vanishing of a corresponding mini-gap for certain EMSL bulk parameters (the so-called zero energy gap conditions). This is shown to influence considerably the surface electronic structure of EMSL and, in particular, to affect the existence of surface states. It is found that usually a surface state appears within one of the mini-gaps opening at the bulk band crossing point, so this mini-gap does not effectively exist, while the other mini-gap exhibits no surfacerelated features and, therefore, is well defined. The importance of the obtained results for the device applications of EMSL is pointed out.

Surface electronic structure of zinc-blende-type GaN(111)

The electronic structure (layer resolved density of states) has been calculated for the gallium nitride (111) oriented slab, using the linear combination of atomic orbitals procedure. The existence of surface states on both Ga- and N-terminated surfaces has been shown. The origin and localization properties of these states have also been analysed.

Electronic surface states in superlattice with complex basis

The electronic structure of a semi-infinite superlattice (SL) with a complex basis consisting of two identical wells coupled via two different barriers is investigated. The possibility of the surface state existence for a periodic termination of the SL potential (i.e., without perturbing the SL potential at the surface) is shown for such a two-barrier basis, in contrast to the conventional one of single well and barrier. This main conclusion is based on analytical considerations, however, some numerical results for the GaAs/AlAs SL are also presented.

Continuous photoluminescence, time resolved photoluminescence and optically detected magnetic resonance measurements of PbI 2 nanometer-sized particles, embedded in SiO 2 films

The nanometer sized particles of PbI 2 were embedded in SiO 2 films. X-ray diffraction and the TEM pictures showed the preservation of the bulk layered structure and symmetry. The PL spectrum of the nano-particles exhibited a pronounced blue shift of the exciton band due to quantum size effect. The Lead Iodide represents an exceptionally small exciton Bohr radius ( a B = 19 A ̊ ) and a special case in which m e ⪢ m h . The prepared samples contained particles with mean radii, a, in the range a B < a < 3 a B . Within this limit (with m e ⪢ m h ), the experimental results suggest that the electron is localized nearly at the center of the particle, enabling the hole to move around it. Thus, the size confinement permits the creation of an acceptor-like exciton. The PL spectrum revealed additional states, associated with stoichiometric defects either at the interior or surface sites of the nano-particles. These defects act either as donor or acceptor states. The dynamics of the various recombination processes has been investigated by measuring the time resolved PL spectra. The results show a multiexponential behavior of the various recombination emission bands, indicating the occurrence of trapping and detrapping processes. Analysis of these results suggests that the existence of surface states give rise to these complex radiative decay processes. The correlation between donor-acceptor recombination emission bands in the aforementioned samples and lattice imperfections was examined, utilizing optically detected magnetic resonance (ODMR) spectroscopy. The results identified the following imperfection sites: an acceptor site associated with an isotropic Lead vacancy defect, [ V −] pb 2+ and a donor site, associated with an anisotropic Iodine vacancy, [ V 0] Iodine .