The geometrical and vibrational properties of the Rh(111) surface

Low-energy electron diffraction (LEED) data have been used to characterize the clean Rh(111) surface. The surface geometry, the degree of surface relaxation, and the Debye temperature have been determined. In the Debye temperature measurement, specular LEED beam intensities were monitored as a function of temperature over a range of electron energies from approximately 30 to 1000 eV. It was found that the bulk Debye temperature is 380 ± 23 K, and the normal component of the Debye temperature at the lowest electron energy used is 197 ± 12 K. The Rh(111) surface relaxation has been determined both by a convolution-transform analysis and by dynamical calculations. Within experimental error, neither expansion nor contraction of the topmost layer has been detected. The results of the convolution-transform analysis of specular beams at two angles of incidence and of a nonspecular beam at normal incidence suggest an expansion of the topmost layer of 3 ± 5% of the bulk layer spacing. In agreement with this, comparisons between the results of the dynamical calculation and experimental data for five nonspecular beams at normal incidence suggest that the surface layer relaxes by 0 ± 5%. In addition, the dynamical calculations indicate that the topmost layer maintains an fcc structure.     

Structure of the clean Ta(100) surface

The clean Ta(100) surface and some aspects of hydrogen adsorption have been studied by LEED and AES. The thorough examination of LEED patterns did not provide any evidence for an atomic reconstruction of the clean surface over the entire temperature range investigated, 150–600 K. The r-factor analysis used for comparison between measured and calculated I–V spectra yields a contraction of the topmost layer spacing of about 11% and an expansion of the second layer spacing of about 1% compared to the bulk value. The hydrogen adsorption does not induce any superstructures, but small hydrogen exposures lass then 1 L influence I–V spectra substantially.     

Structural determinations of the Rh(100) and Cu(111) surfaces using the reliability factor proposed for LEED by Zanazzi and Jona

The reliability factor (R) proposed for LEED by Zanazzi and Jona has been applied to experimental and calculated intensities for the (100) surface of rhodium and for the (111) surface of copper. The calculations used the dynamical perturbation programs of Van Hove and Tong. For each metal, phase shifts were calculated both from a band structure potential and from a potential calculated with a 13 atom cluster. For Cu(111) the I(E) curves from the two potentials were indistinguishable visually and gave similar minimum R values (0.132 and 0.136); the two potentials used for rhodium showed somewhat greater differences. The approach described by Zanazzi and Jona has been supplemented by a simple statistical analysis of the errors involved in the predicted geometries. This study indicates that the topmost interlayer spacing in Cu(111) is contracted by 4.1 ± 0.6% from the bulk value; in Rh(100) the top spacing equals the bulk value to within 3%.     

Electro-optical effects at the discrete and continuum exciton states in GaAs

We report electroreflectance spectra from a GaAs-Au Schottky barrier interface, in the region of the fundamental energy gap, at a temperature of ～ 1.8 K. Both the ground exciton level and the continuum of states are investigated separately, by operating in the small modulation and in the on-off field limit, respectively. The field profile is monitored by I–V, C-V, and photovoltage measurements. It is found that, even at ～ 1.8K, a high surface field is generally present. This moves the actual reflecting boundary of the exciton-polariton away from the surface and affects the ER lineshape through field-induced inhomogeneity effects and interference across the high field layer below the surface. The spectra corresponding to the states of the continuum are discussed in terms of the recent calculations of photon-assisted tunneling with coulomb interaction.     

Dynamical LEED analyses of the clean and the NO-adsorbed Ir(111) surface

The adsorption structure of nitric oxide (NO) on Ir(111) was studied by thermal desorption spectroscopy (TDS) and dynamical analyses of low-energy electron diffraction (LEED). At the saturation coverage at about 100 K, a 2 × 2 pattern was observed by LEED and two peaks appeared at 365 and 415 K in TDS. No change in the LEED I–V curves was observed by annealing at 280 K, which means that the NO-saturated surface was retained at this temperature. On the contrary, partial desorption and changes of the LEED I–V curves were observed by annealing at 360 K. Combined with previous vibrational studies, it is suggested that one adsorption species is not affected, while another species is partially desorbed and the rest of them are dissociated by annealing at 360 K. Dynamical analyses of LEED were performed for the 280 K-annealed and the 360 K-annealed surfaces, which correspond to the NO-saturated and the NO-dissociated Ir(111) surfaces, respectively. These revealed that NO occupies the atop, fcc-hollow and hcp-hollow sites (atop-NO + fcc-NO + hcp-NO) for the NO-saturated Ir(111) surface with the saturation coverage of 0.75 ML. For the 360 K-annealed surface, the atop-NO is not affected but the fcc-NO and the hcp-NO are partially desorbed as NO and partially dissociated to N and O, both of which occupy the fcc-hollow site on the surface.     

Initial growth process and surface structure of Ag on Si(111) studied by low-energy Ion-Scattering Spectroscopy (ISS) and LEED-AES

The growth process of silver on a Si(111) substrate has been studied in detail by low-energy ion-scattering spectroscopy (ISS) combined with LEED-AES. Neon ions of 500 eV were used as probe ions of ISS. The ISS experiments have revealed that the growth at room temperature and at high temperature are quite different from each other even in the submonolayer coverage range. The following growth models have been proposed for the respective temperatures. At room temperature, the deposited Ag forms a two-dimensional (2D) island at around 2/3 monolayer (ML) coverage, where the Ag atoms are packed commensurately with the Si(111)1 substrate. One third of the substrate Si surface remains uncovered there. Then it starts to develop into Ag crystal, and at a few ML coverage a 3D island of bulk Ag crystal grows directly on the substrate. An intermediate layer, which covers uniformly the whole surface before the growth of Ag crystal, does not exist. At high temperatures (>~200°C), the well-known Si(111)√3-Ag layer is formed as an intermediate layer, which consists of 2/3 ML of Ag atoms and covers the whole surface uniformly. These Ag atoms are embedded in the first double layer of the Si substrate. It is concluded that the formation of the √3 structure needs relatively high activation energy which may originate from the large displacement of Si atoms owing to the embedment of the Ag atoms, and does not proceed below about 200°C. The most stable state of the Ag atoms on the outermost Si layer is in the shape of an island, both for the Si(111) surface and for the Si(111)√3-Ag surface.     

Hydrogen-induced structural changes on NiAl(110)

LEED I-V analysis and surface X-ray scattering measurements have been used to determine the structural changes induced by the adsorption of atomic hydrogen on NiAl(110) at 130 K. The clean surface, ordered with 50at% Ni50at%Al, relaxes away from bulk truncation to exhibit a large ripple. At 130 K the rippling (Al out, Ni in) is 0.19 Å, as determined by LEED I-V. The adsorption of atomic hydrogen reduces this rippling by 16% at half of saturation coverage and at full saturation by 44%. Saturation coverage was measured to be one hydrogen atom per (1 × 1) surface unit cell (1 monolayer) using nuclear reaction analysis. This observation contradicts first principles calculations that predict 1 monolayer of H removes the surface rippling.     

Structure determination of the (100) surface of rhodium by low energy electron diffraction

Elastic low-energy electron diffraction intensity data have been measured as a function of energy for two directions of incidence for the (100) surface of rhodium. The dynamical perturbation programs of Van Hove and Tong have been used for analysing these new experimental data, and it is concluded that the normal face-centered cubic registry is maintained to the surface layer. A preliminary comparison between measured and calculated I(E) curves indicates the topmost interlayer spacing to be 1.96 ± 0.10 Å, and therefore possibly slightly expanded from the bulk interlayer spacing of 1.90 Å.     

Work function distribution for W-Ir mixed metal matrix cathodes

Mixed metal matrix cathodes have inherent non-uniformity and patchiness of emission due to the presence of two-alloy phase structure on the surface. I-V characteristics of cathode studied in a close spaced diode configuration is one of the easy and cost effective methods to estimate the variation of work function on the cathode surface. Tungsten iridium mixed metal matrix dispenser cathodes of Ø1.4 mm (80 wt.% W-20 wt.% Ir) have been fabricated in the laboratory and their I-V characteristics have been investigated in diode configuration. In this paper the model suggested by Tonnerre et al. has been used to find out the work function distribution of W-Ir cathodes from I-V characteristics. An attempt has been made to correlate the microstructure with the work function values.     

Quantum size effect in low energy electron diffraction of thin films

Low energy electron microscopy (LEEM) is used to study the quantum size effect (QSE) in electron reflectivity from thin films. Strong QSE interference peaks are seen below 20 eV for Cu and Ag films on the W(1 1 0) surface and Sb films on the Mo(0 0 1) surface. Simple inspection of QSE interference peaks reveals that all three metals grow atomic layer-by-atomic layer. Layer-specific I(V) spectra obtained with LEEM permit structural analysis by full dynamical multiple scattering LEED calculations for a layer-by-layer view of thin film structure.     

Adsorption of carbon monoxide,oxygen, hydrogen,nitrogen, ethylene and benzene on an iridium (110) surface; Correlation with other iridium crystal faces

The interaction of CO, O₂, H₂, N₂, C₂H₄ and C₆H₆ with an Ir(110) surface has been studied using LEED, Auger electron spectroscopy and flash desorption mass spectroscopy. Adsorption of oxygen at 30°C produces a (1× 2) structure, while a c(2 × 2) structure is formed at 400°C. Two peaks have been detected in the thermal desorption spectrum of oxygen following adsorption at 30°C. The heat of adsorption of hydrogen is slightly higher on Ir(110) than on Ir(111). Adsorption of carbon monoxide at 30°C produces a (2 × 1) surface structure. The main CO desorption peak is found around 230, while two other desorption peaks are observed around 340 and 160°C. At exposures between 250 and 500°C carbon monoxide adsorption yields a c(2 × 2) structure and a desorption peak around 600°C. Carbon monoxide is adsorbed on an Ir(110) surface partly covered with oxygen or carbon in a new binding state with a significantly higher desorption temperature than on the clean surface. Adsorption of nitrogen could not be detected on either clean or on carbon covered Ir(110) surfaces. The hydrocarbon molecules do not form ordered surface structures on Ir(110). The thermal desorption spectra obtained after adsorption of C₆H₆ or C₂H₄ are similar to those reported previously for Ir(111) consisting mostly of hydrogen. Heating the (110) surface above 700°C in the presence of C₆H₆ or C₂H₄ results in the formation of an ordered carbonaceous overlayer with (1 × 1) structure. The results are compared with those obtained previously on the Ir(111) and Ir(755) or stepped [6(111) × (100)] surfaces. The CO adsorption results are discussed in relation to data on similar surfaces of other Group VIII metals.     

Displacive surface phases formed by hydrogen chemisorption on W {001}

A detailed LEED study is reported of the surface phases stabilised by hydrogen chemisorption on W {001}, over the temperature range 170 to 400 K, correlated with absolute determinations of surface coverages and sticking probabilities. The saturation coverage at 300 K is 19(± 3) × 10¹⁴ atoms cm^?2, corresponding to a surface stoichiometry of WH₂, and the initial sticking probability for both H₂ and D₂ is 0.60 ± 0.03, independent of substrate temperature down to 170 K. Over the range 170 to 300 K six coverage-dependent temperature-independent phases are identified, and the transition coverages determined. As with the clean surface $\text{(}\text{2}\text{×}\text{2}\text{)R45°}$ displacive phase, the c(2 × 2)-H phase is inhibited by the presence of steps and impurities over large distances (～20 Å), again strongly indicative of CDW-PLD mechanisms for the formation of the H-stabilised phases. These phases are significantly more temperature stable than the clean $\text{(}\text{2}\text{×}\text{2}\text{)R45°}$, the most stable being a c(2 × 2)-H split half-order phase which is formed at domain stoichiometries between WH_0.3 and WH_0.5. LEED symmetry analysis, the dependence of half-order intensity and half-width on coverage, and I-V spectra indicate that the c(2 × 2)-H phase is a different displacive structure from that determined by Debe and King for the clean $\text{(}\text{2}\text{×}\text{2}\text{)R45°}$. LEED I-V spectra are consistent with an expansion of the surface-bulk interlayer spacing from 1.48 to 1.51 Å as the hydrogen coverage increases to ～4 × 10¹⁴ atoms cm^?2. The transition from the split half-order to a streaked half-order phase is found to be correlated with changes in a range of other physical properties previously reported for this system. As the surface stoichiometry increases from WH to WH₂ a gradual transition occurs between a phase devoid of long-range order to well-ordered (1 × 1)-H. Displacive structures are proposed for the various phases formed, based on the hypothesis that at any coverage the most stable phase is determined by the gain in stability produced by a combination of chemical bonding to form a local surface complex and electron-phonon coupling to produce a periodic lattice distortion. The sequence of commensurate, incommensurate and disordered structures are consistent with the wealth of data now available for this system. Finally, a simple structural model is suggested for the peak-splitting observed in desorption spectra.     

Effects of hydrogen treatment on ohmic contacts to p-type GaN films

This study investigated the effects of hydrogen (H₂) treatment on metal contacts to Mg-doped p-GaN films by Hall-effect measurement, current-voltage (I-V) analyzer and X-ray photoemission spectra (XPS). The interfacial oxide layer on the p-GaN surface was found to be the main reason for causing the nonlinear I-V behavior of the untreated p-GaN films. The increased nitrogen vacancy (V_N) density due to increased GaN decomposition rate at high-temperature hydrogen treatment is believed to form high density surface states on the surface of p-GaN films. Compared to untreated p-GaN films, the surface Fermi level determined by the Ga 2p core-level peak on 1000 °C H₂-treated p-GaN films lies about ∼2.1 eV closer to the conduction band edge (i.e., the surface inverted to n-type behavior). The reduction in barrier height due to the high surface state density pinned the surface Fermi level close to the conduction band edge, and allowed the electrons to easily flow over the barrier from the metal into the p-GaN films. Thus, a good ohmic contact was achieved on the p-GaN films by the surface inversion method.     

Low-energy electron diffraction,Auger electron spectroscopy,and thermal desorption studies of chemisorbed CO and O2 on the (111) and stepped [6(111) × (100)] iridium surfaces

The adsorption of CO, O₂, and H₂O was studied on both the (111) and [6(111) × (100)] crystal faces of iridium. The techniques used were LEED, AES, and thermal desorption. Marked differences were found in surface structures and heats of adsorption on these crystal faces. Oxygen is adsorbed in a single bonding state on the (111) face. On the stepped iridium surface an additional bonding state with a higher heat of adsorption was detected which can be attributed to oxygen adsorbed at steps. On both (111) and stepped iridium crystal faces the adsorption of oxygen at room temperature produced a (2 × 1) surface structure. Two surface structures were found for CO adsorbed on Ir(111); a (√3 × √3)R30° at an exposure of 1.5–2.5 L and a (2√3 × 2√3)R30° at higher coverage. No indication for ordering of adsorbed CO was found on the Ir(S)-[6(111) × (100)] surface. No significant differences in thermal desorption spectra of CO were found on these two faces. H₂O is not adsorbed at 300 K on either iridium crystal face. The reaction of CO with O₂ was studied on Ir(111) and the results are discussed. The influence of steps on the adsorption behaviour of CO and O₂ on iridium and the correlation with the results found previously on the same platinum crystal faces are discussed.     

Surface structure analysis of metal adsorbed Si(1 1 1) surfaces by Patterson function with LEED I-V curves

Wu and Tong proposed the calculation method of Patterson function obtained directly from the LEED I-V curves which shows the relative position of surface atoms as an image. We have made the calculation program of Patterson function and applied to the structural analysis of the Si(1 1 1)1 × 1-Fe surface. Surface structure was able to be expressed almost correctly by the Patterson function obtained from the theoretical I-V curves for the model structure. In the Patterson function obtained from the experimental I-V curves, the locational relation between the atoms of subsurface layer was in agreement with the CsCl type structure. More over, because the faint peak, by which we can distinguish the model, can be seen, it seems that the model B8 is preferable to the model A8. This result is consistent with the model shown by Walter et al.     

A LEED determination of the structures of Ru(001) and of CORu(001)−√3 × √3 R30°

The structures of Ru(001) and of the √3 × √3 R30° overlayer of CO on Ru(001) have been determined by LEED I–V measurements and comparison to calculations. Special attention was paid to accurate angular alignment, selection of a well-ordered portion of the surface, and avoidance of beam-induced changes of the CO layer. Five orders of reflexes over a range of 300 eV each were used for the clean surface and 7 orders over 200 eV each for the CO superstructure. For the clean surface, a slight contraction of the first layer spacing (by 2%) was found which gave r-factors of 0.04 (Zanazzi-Jona) and 0.16 (Pendry) for 5 non-degenerate beams. For the CO structure the most probable geometry is the on-top site with spacings $\text{d(}\text{RuC}\text{) = 2.0 ± 0.1}\text{A}\text{?}\text{and}\text{d(}\text{CO}\text{) = 1.10 ± 0.1}\text{A}\text{?}\text{(r}{}_{\mathrm{<mtext>ZJ</mtext>}}\text{= 0.21; r}{}_{\mathrm{<mtext>P</mtext>}}\text{= 0.51)}$. The two threefold hollow and the bridge sites can be clearly excluded.     

Scanning tunneling spectroscopy of Ni/W(110): bcc and fcc properties in the second atomic layer

Nickel islands are grown on W(110) at elevated temperatures. Islands with a thickness of two layers are investigated with scanning tunneling microscopy. Spectroscopic measurements reveal that nanometer sized areas of the islands exhibit distinctly different apparent heights and dI/dVspectra. Spin polarized and paramagnetic band structure calculations indicate that the spectral features are due to fcc(111) and bcc(110) orientations of the Ni film, respectively.     

Influence of surface cleaning effects on properties of Schottky diodes on 4H-SiC

Ir/4H-SiC and IrO₂/4H-SiC Schottky diodes are reported in terms of different methods of surface pretreatment before contact deposition. In order to find the effect of surface preparation processes on Schottky characteristics the SiC wafers were respectively cleaned using the following processes: (1) RCA method followed by buffered HF dip. Next, the surface was oxidized (5.5 nm oxide) using a rapid thermal processing reactor chamber and circular geometry windows were opened in the oxide layer before metallization deposition; (2) the same as sequence (1) but with an additional in situ sputter etching step before metallization deposition; (3) cleaning in organic solvents followed by buffered HF dip. The I-V characteristics of Schottky diodes were analyzed to find a correlation between extracted parameters and surface treatment. The best results were obtained for the sequence (1) taking into account theoretical value of Schottky barrier height. The contacts showed excellent Schottky behavior with ideality factors below 1.08 and barrier heights of 1.46 eV and 1.64 eV for Ir and IrO₂, respectively. Very promising results were obtained for samples prepared using the sequence (2) taking into account the total static power losses because the modified surface preparation results in a decrease in the forward voltage drop and reverse leakage current simultaneously. The contacts with ideality factor below 1.09 and barrier height of 1.02 eV were fabricated for Ir/4H-SiC diodes in sequence (2).     

Effect of annealing on the electronic parameters of Au/poly(ethylmethacrylate)/n-InP Schottky diode with organic interlayer

A thin poly(ethylmethacrylate) (PEMA) layer is deposited on n-InP as an interlayer for electronic modification of Au/n-InP Schottky structure. The electrical properties of Au/PEMA/n-InP Schottky diode have been investigated by current–voltage (I–V) and capacitance–voltage (C–V) measurements at different annealing temperatures. Experimental results show that Au/PEMA/n-InP structure exhibit a good rectifying behavior. An effective barrier height as high as 0.83 eV (I–V) and 1.09 eV (C–V) is achieved for the Au/PEMA/n-InP Schottky structure after annealing at 150 °C compared to the as-deposited and annealed at 100 and 200 °C. Modified Norde's functions and Cheung method are also employed to calculate the barrier height, series resistance and ideality factors. Results show that the barrier height increases upon annealing at 150 °C and then slightly decreases after annealing at 200 °C. The PEMA layer increases the effective barrier height of the structure as this layer creates a physical barrier between the Au metal and the n-InP. Terman's method is used to determine the interface state density and it is found to be 5.141 × 10¹² and 4.660 × 10¹² cm^?2 eV^?1 for the as-deposited and 200 °C annealed Au/PEMA/n-InP Schottky diodes. Finally, it is observed that the Schottky diode parameters change with increasing annealing temperature.     

Temperature dependence and the effect of hydrogen peroxide on ITO/poly-ZnO Schottky diodes fabricated by laser evaporation

Transparent and efficient poly-ZnO ultraviolet Schottky diodes grown at different temperatures with indium-tin-oxide (ITO) as the metallic contact layer were fabricated with hydrogen peroxide (H₂O₂) applied as a surface treatment at 70 °C for 20 min. Analysis via field-emission scanning electron microscopy (FESEM) and X-ray photoelectron spectroscopy (XPS) demonstrated that the ZnO films underwent gradual oxidation and that H₂O₂ treatment resulted in an interfacial ZnO₂ layer that covered the ZnO surface. I–V measurements indicated that the ideality factor and the Schottky barrier height improved with increasing shunt resistance, and the trade-off between film quality and the degree of oxidation revealed that films grown at 400 °C exhibited the best diode characteristics.