Spectrum Of Surface States Research Articles

Photoluminescent spectra of surface states in alkaline earth oxides

Photoluminescence has been observed from high surface area alkaline earth oxides with exciting light of a much lower frequency than that expected from the band gaps of the bulk oxides. This luminescence can be quenched by oxygen and hydrogen and the quenching is reversible under some conditions. It seems probable that both the excitation and the luminescence spectra observed arise from excitons in the surface region. The different behaviour towards oxygen and hydrogen has been used to develop a model in which the excitation site, where absorption of light occurs, is thought to be associated with anions on the surface in states of unusually low coordination such as might be found at step, edge or corner sites on the surface. The luminescence is thought to be associated with a cation on the surface in a similar state of coordination.

Reflectance spectra of surface states in strontium oxide and barium oxide

Reflectance spectra of SrO and BaO powders outgassed at temperatures up to 1073 K have been measured over the range from text-decoration:overlineν= 20 000 cm–1 to text-decoration:overlineν= 52 000 cm–1. Spectral features attributable to exciton states are described, and the effects of O2, N2O, CO2 and H2O on the spectra are reported. Fluorescence occurs when spectra are measured in vacuo. SrO-coated MgO and BaO-coated MgO have also been studied. The spectra show two regions separated by an absorption edge which is centred at 44 000 cm–1 for SrO and 31 500 cm–1 for BaO. On the low text-decoration:overlineν side of the edge the spectra are sensitive to the adsorption of the gases, but exposure to gases causes little or no change in the spectra on the high text-decoration:overlineν side (except for H2O vapour which reacts in depth). The absorption bands at low text-decoration:overlineν are ascribed to excitons bound at surface ions and those at high text-decoration:overlineν to excitons bound at ions in the bulk.The chemical interaction of the gases with the surface states increases in intensity in the order O2 < N2O < CO2 < H2O for SrO and N2O < O2 < CO2 < H2O for BaO. The results are correlated with those for MgO and CaO reported previously.

Comments on “surface-state spectra from thick-oxide MOS tunnel junction”

Comments on “surface-state spectra from thick-oxide MOS tunnel junction”

Application of a complex band structure to compute the surface states by the Green's function method

By using the analytical properties of the band energies and wave functions in the complex quasi-momentum domain, a procedure is constructed for the calculation of the Green's function\(G_{k_\parallel }\). The function\(G_{k_\parallel }\) is represented as the sum of components dependent on complex wave vectors. If such a representation is used for\(G_{k_\parallel }\) in the calculation of the surface Green's function [2], then the equation to determine the surface state spectrum in a model with a potential jump agrees with that obtained in [3] by the method of merging the wave functions.

Surface-state spectra from thick-oxide MOS tunnel junctions : T. P. Ma and R. C. Barker. Solid State Electron. 1974. Vol. 17. p. 913

Surface-state spectra from thick-oxide MOS tunnel junctions : T. P. Ma and R. C. Barker. Solid State Electron. 1974. Vol. 17. p. 913

The Si (100) surface: A theoretical study of the unreconstructed surface

The potential, charge density, and energy spectrum of an ideal unreconstructed Si (100) surface is obtained self-consistently. Two bands of surface states that arise from broken surface bonds are found in the energy gap between the valence and conduction bands. One band ($\ensuremath{\lesssim}1$ eV wide) has its electron density localized about a line through and normal to the surface atoms, with a node on the atoms; the other band (\ensuremath{\sim} 2.5 eV wide) has its charge located around a line through a row of surface atoms, again having nodes on the surface atoms. The role these surface states play in surface reconstruction as well as in understanding the surface-state spectrum of stepped Si (111)-surface bands is discussed. Additional surface states within the valence band are found and their significance discussed.

Reflectance spectra of surface states in magnesium oxide and calcium oxide

The ultraviolet and visible reflectance spectra (52 000–15 000 cm–1) of high surface area MgO (250–300 m2 g–1) and CaO (100–200 m2 g–1) have been studied in vacuo and in the presence of O2, N2O, CO2 and H2O. The effect of outgassing MgO and CaO at increasing temperatures (773–1073 K) is to develop u.v. absorption bands in the range 30 000–50 000 cm–1. The absorption can be progressively destroyed by chemisorption, and also by sintering. The bands are attributed to excitons bound in surface states. In vacuo the outgassed oxides exhibit a fluorescence which is readily quenched by exposure to a low pressure of oxygen. Absorption bands occur at 37 000 and 46 000 cm–1 for MgO and at 35 500 and 44 500 cm–1 for CaO. With each oxide the chemisorption of gases erodes the lower energy band preferentially, and a similar effect occurs on sintering. The two bands are ascribed to exciton absorption at surface oxide ions in different states of coordinative unsaturation, the lower energy exciton band corresponding to the lower coordination.The influence of gases on the exciton spectra of outgassed MgO and CaO is shown to depend on their chemical reactivity, increasing in the sequence O2, N2O, CO2 and H2O. The effects with each gas are reversible on outgassing. Exposure of CaO to O2 gives rise to an absorption band at 23 500 cm–1, tentatively attributed to an O–3 adsorbed species.The surface states observed on CaO can be produced on MgO by adsorbing Ca ions from solution and heating to 1073 K.

Spectrum and nature of surface states

With the help of an approximative E(k) relation in the forbidden gap N(E) distributions for localized states are calculated. A potential fluctuation model gives energy distributions of states in the gap which exhibit features of surface state spectra or spectra of disordered systems. Binding and nonbinding states are coupled to pairs by the E(k) relation and characteristic properties of surface and interface state spectra like symmetry, acceptordonor character, minimum of the N(E) distribution may be described. Dangling bonds of preferential 111 character are the proper chemical picture. Approximative energy calculations show a preferential energy range for isolated interface states at a distance of 0.1–0.2 eV distance from the band edges. Comparison with experimental termspectra shows a good agreement with the treated model. Common characteristics of clean, ordinary and oxidized surfaces and the metal semiconductor contact are revealed.

Surface-state spectra from thick-oxide MOS tunnel junctions

The conductance and capacitance of thick-oxide MOS tunnel junctions (SiO 2 thickness 40∼65 A ̊ )have been measured from 35 Hz to 210 kHz. It is demonstrated that the use of a thick-oxide MOS tunnel junction enables one to obtain the surface-state data throughout the whole silicon band-gap with better resolution and better sensitivity than the conventional MOS capacitance techniques. A slight departure from equilibrium may occur in the voltage range where large tunnel current flows. Corrections to the energy scale must be made in this voltage range. A method for the evaluation of the junction quality is discussed. The simplified equivalent circuits necessary for the calculation of surface-state data are constructed under various bias conditions by an approach different from a previously published work. The present work supports the model that at least some of the observed surface states are a consequence of the diffusion of contact metals into the oxide.

Adsorption on Semiconductors in Relation to Their Degree of Dispersity

The use of thin (\\leqslant1 µ) nearly single-crystal semiconductor films as model adsorbents is discussed. In such cases, a quartz-crystal microbalance combined with field effect measurements would allow simultaneously recording of both the adsorbed quantity and the energy spectrum of surface states. The possibilities of a set up of this kind are analysed. It will be suitable for elucidation of problems related to electronic mechanisms in chemisorption: specification of the bonding between adsorbed particle and semiconductor surface (local chemisorption level, adsorption on surface defects, mere change in the parameters of nonadsorptive surface states); evaluation of the amount of chemically and physically adsorbed particles; effect of dispersity on the adsorption properties of semiconductors.

Statistics of recombination through quasicontinuous spectrum of surface states

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Photovoltage inversion effect resulting from a continuous spectrum of surface states: ZnO

The surface photovoltage inversion effect has been observed in ZnO single crystals, manifested as a pronounced increase in the surface barrier height under sub-bandgap illumination. The magnitude of photovoltage inversion was found to increase with increasing incident photon energy. However, the photovoltage transient was essentially independent of the photon energy. These findings are consistent with a model involving photostimulated excitation of electrons from the valence band into the surface states, the fractional occupancy of which varies on the energy scale (continuous spectrum of surface states).

Investigation of the fast surface state spectrum of MIS structures by a differentialC–U method

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Temperature- and illumination-dependence of the work function of gallium arsenide

Contact potentials of vacuum-cleaved (110) surfaces of GaAs have been measured as functions of temperature (77 < T < 400 K) and illumination intensity by means of a transparent vibrating capacitor. Surface photovoltage as a function of wavelength and intensity of light, as well as relaxation times, have been measured at various temperatures. Samples of greater than 10 18 cm −3 doping exhibit photovoltages of 1–2 mV or less and little variation of work function with temperature. At a doping level of 10 17 cm −3 p-type samples display photovoltages of more than 50 mV at 77 K and up to 10 mV at room temperature. Moderately doped n-type samples (1.8 × 10 16 and 2.7 × 10 17) exhibit photovoltages up to 0.25 eV at room temperature. At low temperatures, their work function can have two apparently stable values: upon rapid cooling in total darkness, the work function decreases about 80 mV, but during illumination at 77 K, however weak, it decreases by an additional 0.5 V to a new value at which it remains in the dark. This behavior can be accounted for by surface photovoltage according to a simple model not requiring a temperature dependence of surface state spectra. Light is absorbed through interband transitions in the bulk, the surface charge is altered by surface recombination and the return to equilibrium occurs by thermal excitation and tunneling of carriers.

Investigations of Scattering and Trapping Processes of Carriers in Ultrapure Germanium Films

Films of about 100 μ thickness were grown by iodide-transport process in a closed tube, with the concentration of acceptor impurities below 1012 cm−3. Measurements were made on the samples, which had one natural surface of film growth and the other surface prepared by the usual polishing and etching procedure. The Hall coefficient and surface-conductivity measurements were made by a dc field-effect method in a temperature range between 100 and 300 K. Results of the measurements are expressed in terms of surface mobility and trapped charge curves. The scattering of electrons is a diffuse process for both conditions of the surface (natural-grown surface and treated surface of the film) and over the whole temperature range. The scattering of holes is diffuse on the treated surface and only partially diffuse on the natural surface of film. The probability of diffuse reflection for holes in this case is equal to 0.3–0.4 and does not depend on temperature. The dominant mechanism of the surface scattering is discussed for different cases. The trapped charge curves for both conditions of the surface correspond to the assumption of a continuous spectrum of surface states. The form of the spectrum is independent of temperature for the natural surface. For the treated surface it changes with temperature, owing to an adsorption process.

Field Effect‐Capacitance Analysis of Surface States on Silicon

AbstractThe capacitance of a space charge layer, induced on a semiconductor surface, is analyzed to obtain information on the charge of surface states. Capacitance measurements on silicon‐silicon oxide‐metal combinations are presented, and these indicate a quasi‐continuous spectrum of surface states with a density of approximately 1 × 1012 states/cm2 · eV in the range 0.2 to 0.8 eV above the valence band for both, p‐ and n‐type samples, and a total density of 1.7 × 1012 states/cm2. Advantages and limitations of the capacitance‐method are discussed.