Change In Band Bending Research Articles

Effect of surface hydrogen on metal-diamond interface properties

Schottky barrier formation of metal contacts deposited on diamond (111) and (100) surfaces was investigated. Three different metals (Au, Al, and Ti) were studied because of their different chemical reactivity with C, i.e., Au being a nonreactive metal, Al a weak carbide-forming metal, and Ti a strong carbide former. Both fully H-terminated, unreconstruted (1×1) surfaces and H-desorbed, reconstructed (2×2)/(2×1) surfaces with higher density of surface states were examined. Surface structures were determined via low-energy electron diffraction, and the change of surface band bending (SBB) and the interface chemistry during the contact formation were monitored using x-ray photoelectron spectroscopy. On the reconstructed surfaces, the SBB was independent of metal thicknesses. This was attributed primarily to Fermi-level pinning by the high density of surface states on the reconstructed surfaces. On the other hand, the surface-state densities were much lower on the unreconstructed surfaces and thus the Fermi level was not as strongly pinned as that on the reconstructed surfaces. When the metal coverage became large (≳0.5 monolayer) on these unreconstructed surfaces, the charge transferred from the metal contacts to the diamond, the Fermi level moved upward in the band gap and the SBB increased. However, even on the unreconstructed surfaces where the density of pinning states were low, the SBB for thick metal contacts did not correlate to metal properties such as work function and/or electronegativity. Regardless of the state of the diamond surfaces, it was also important to consider the interface chemistry (chemical reactivity of the metal on diamond) to understand the change of the SBB after annealing the metal contacts.

Optical study of band bending and interface recombination at Sb, S and Se covered gallium arsenide surfaces

The modification of the density of surface/interface gap states due to Sb deposition, (NH4)2Sx and Se vapour treatment is investigated by means of photoreflectance (PR) and photoluminescence (PL) experiments. On sulfidized GaAs, the surface states are shifted away from the highly recombination-efficient mid-gap position towards the valence-band edge as deduced from significant changes of band bending ⊘b and surface recombination velocity vS at n- and p-type samples. The Se treatment of n-GaAs yields a decrease of ⊘b as well as vS. The system S/GaAs and Se/GaAs exhibit different interface reactivities proved by PL degradation transient measurements.

Signature of the Weak Bond-Dangling Bond Conversion Process in a-Si:H as Seen by Total Photoelectron Yield Spectroscopy

Photoelectron Yield Spectroscopy is one of the most direct methods to study the density of gap states of amorphous semiconductors. We have made use of it to investigate the transient changes of the density of gap states of a-Si:H upon illumination with above-band-gap light. An excitation density of 50 mW/cm2 of 532 nm light from a frequency doubled Nd-YAG laser was modulated with frequencies between 0.1 Hz and 10 KHz and the transient response of the photoelectron yield signal was monitored by gated electron counting or a lock-in amplifier. From this we deduce a reversible increase of the occupied density of defect states under illumination with a maximum Δg ∼ 1017 cm3 eV-1 at about 0.3 eV below the Fermi energy and a decrease below EF− 0.75 eV, i. e. in the region of deep tail states. The effect exhibits a wide spectrum of time constants involved. Based on additional transient surface photo voltage measurements as well as numerical simulations we argue that the observed redistribution of states is not due to a light induced change in surface band bending or due to a mere excitation of electrons from occupied to empty states. Instead, we interpret the observed effect as a conversion of weak to dangling bonds, i.e. a precursor of the Staebler-Wronski Effect, lacking however the final step of hydrogen stabilisation necessary for the creation of metastable defects.

Coadsorption of Na and Br 2 on WSe 2 (0001). Creating a surface redox couple?

The interface between UHV-cleaved n-WSe 2 (0001) and coadsorbed Br 2 and Na has been investigated as a model of a semi-conductor/electrolyte interface. Br 2 is molecularly adsorbed and leads to a strong inversion layer of n-WSe 2. Na coadsorption forms Br − on the surface with no pronounced change of semiconductor band bending for Na coverage <Br 2 coverage. Flat band conditions are reestablished for complete transformation of Br 2 to a NaBr surface species.

Spectral and temporal features of photoluminescence of gallium arsenide quantum-wire crystals

Optical properties of GaAs quantum-wire crystals are investigated by means of time-resolved photoluminescence measurements. Recombination dynamics of carriers in the quantum-wire crystals are characterized on the basis of a rate equation which includes surface recombination and effect of depletion layer. Surface treatment with sulphur solution decreases significantly the surface charge density and surface recombination. Reduction in radiative life time is observed after the surface treatment. This implies that the modification of spatial profile of carrier wave functions is caused by a change in band bending at the surface.

Light-induced oscillating reactions of silicon in ammonium fluoride solutions: Part 1. Simultaneous photocurrent and excess microwave reflectivity measurements

For the first time, simultaneous photocurrent, I ph, and excess microwave reflectivity, m w*, profiles are recorded during the light-induced oscillatory behaviour of n-Si(111) in aqueous ammonium fluoride solutions. Stationary oscillations in I ph and m w* are complementary to each other with maxima of I ph at minima of m w* and vice versa. At higher positive potentials, irregular oscillatory behaviour is found and the complementarity of the signals is not fully maintained. For the initial time regime a model is suggested tentatively to determine electronic interface parameters from the simultaneous measurement of I ph and m w*. For increased time, the oscillatory behaviour has also been evaluated with respect to surface recombination and charge transfer processes. In this time region, however, the application of the model suggested is problematic, as changes in band bending, trapping of charges and space charge layer recombination are not accounted for. For the observed lowest quantum yields > 0.2, it is assumed that the drift velocity is not the limiting factor.

Effects of initial silicon carbide surface composition on room temperature Au/β-SiC(001) interface formation

Au was deposited in situ on β-SiC(001) surfaces at room temperature after the substrates were cleaned by annealing, first in a Si flux and then in vacuum. By adjusting the time and temperature of the vacuum anneal, SiC surfaces of four compositions, ranging from Si-rich to Si-deficient, were obtained. Surfaces were monitored using Auger electron, electron energy loss, X-ray photoelectron, and ultraviolet photoelectron spectroscopies as Au films ranging in thickness from ∼ 1 to 37 Å were deposited. For Au on Si-terminated β-SiC surfaces, three distinct regions formed as the Au overlayer grew: interfacial Au-Si alloy, pure Au above the alloy, and a surface layer of Au-Si alloy. Interfacial alloy thickness and surface alloy Si concentration were both proportional to the amount of Si on the initial SiC surface. For Au depositions š 5 Å, bands bending change, dependent on the initial β-SiC surface composition, were observed. On Si-deficient β-SiC surfaces, the Au growth mode differed completely. Au clusters formed, similar to Au behavior on C substrates. No band bending changes were observed, consistent with weak film/substrate interaction. For the Au/β-SiC system, overlayer composition and morphology are controlled by composition changes in the top 1–2 monolayers of the SiC substrate.

Initial stages of Schottky-barrier formation of Bi/Si(111) and Bi/Si(100) interfaces

We have studied the development of Schottky barrier heights during the initial formation of Bi/Si(100) and Bi(111) interfaces. In the submonolayer regime we have followed the band bending changes using synchrotron radiation core-level spectroscopy. It has been observed that the annealing of the interface can influence the Schottky barrier height. We compare these results with recent measurements on Pb/Si(100) and Pb/Si(111) interfaces.

Surface damage and deposition on gallium arsenide resulting from low energy carbon ion bombardment

A mass-separated carbon ion beam in the energy range of 20–500 eV was used in ultrahigh vacuum to study the ion-surface interactions that occur in alkane based reactive ion etching of GaAs. The effects of ion bombardment and of subsequent damage-removal treatments were determined by measuring the surface chemistry and band-bending changes with in situ polar-angle-dependent X-ray photoelectron spectroscopy. The study showed that carbon ion bombardment on GaAs even at 20 eV resulted in carbon incorporation and the formation of carbide species. Initially, slight sputtering occurred with a higher sputter yield for arsenic than that for gallium, but further ion exposure led to the formation of an amorphous carbon overlayer. The degree of arsenic depletion and the thickness of the damaged and carbon-contaminated layer increased with the impact energy. The carbon and carbide residues were not removed by annealing at 520°C, or by exposure to ultraviolet light/ozone. Bombardment with 100 eV hydrogen ions did remove the damage induced by carbon ion bombardment.

Structure effects on Schottky barrier heights of Pb/Si and Bi/Si interfaces

We have studied the development of Schottky barrier heights during the initial formation of Pb,Bi/Si interfaces. In the sub-monolayer regime we have followed the band bending changes using synchrotron radiation core-level spectroscopy. In addition to a bulk-sensitive mode used to determine the band bending changes (hν = 108 eV), we recorded spectra in a surface-sensitive mode (hν = 130 eV) to better follow the surface peak changes associated with the different atomic arrangements. We compare the Schottky barrier heights obtained in the monolayer regime with recent measurements on in situ prepared Pb/Si(111) diodes.

Preparation and spectroscopic characterization of γ-Al 2O 3 thin films

The electronic structure and surface properties of γ-Al 2O 3 thin films are studied. We have prepared the films by oxydizing Al foils under controlled conditions and we characterize the γ-Al 2O 3 samples by means of XPS, UPS, and TEM and found no charging. Pronounced effects in temperature-dependent changes of the work function are observed which result from changes in band bending and electron affinities by reorganisation and migration of defects. Thereby the ability of these systems for prototype studies in catalysis and analysis of defects is demonstrated.

Surface-photovoltage effects on adsorbate-covered semiconductor surfaces at low temperatures

Surface photovoltage (SPV) needs to be considered in optical studies of electronic properties of semiconductor surfaces. We used a Kelvin probe (CPD), ultraviolet photoemission spectroscopy (UPS), and electric-field-induced Raman spectroscopy (EFIRS) to investigate the effect of SPV on adsorbate-induced variations of surface band bending at InP(110) and GaAs(110) surfaces at low temperatures. On InP(110) surfaces, which were exposed to Cs, H, and Cl2 at ≊170 K, the SPV was found to depend on the type of adsorbate and its coverage. After the light was switched off, the SPV was found to decay with time constants varying between some minutes up to a few hours. For GaAs(110) surfaces exposed to atomic hydrogen at 140 K, we observed the light of a discharge lamp to flatten the bands. By using EFIRS, on the other hand, the buildup of surface band bending was unambiguously detected as a function of exposure to atomic hydrogen. The H-induced changes of surface band bending determined from variations of the work function measured in the dark with a Kelvin probe and of the ionization energy evaluated from the widths of UPS spectra excellently agree with the EFIRS-derived data.

The influence of surface states on the photovoltage of real silicon surface

AbstractThe surface photovoltage caused by band‐to‐band generation within the semiconductor depends on light intensity. This dependence differs significantly in two limiting cases. When recombination via fast surface states is negligible, the slope of the photovoltage versus the logarithm of intensity is always less than kT/e and depends on the amount of semiconductor band bending in the dark. When recombination via fast surface states becomes strong, this slope is kT/e, independent of the band bending. These differences are experimentally observed using a structure with ferroelectric NaNO2 deposited on a real silicon surface. The switching of the direction of the spontaneous polarization of the ferroelectric results in significant changes of the semiconductor band bending in the dark. The charge exchange with slow surface states results in a continuous change of the band bending in the dark. The measurements reveal a pronounced minimum of the density of fast surface states near mid‐gap. The results compare well to those from measurements on a field effect transistor structure with NaNO2 as gate insulator.

Shallow donor in buried oxide Si-On-insulator structures revealed by γ-irradiation-induced electron spin resonance activation

Si-on-insulator structures formed by implantation of oxygen (SIMOX) in a single step to a dose ≃ 1.8 × 10 18 O + cm −2 into p-type (0 0 1) Si and high temperature annealing have been studied by K-band electron spin resonance (ESR) at 4.3–31 K. γ-irradiation to a dose of 1 Mrad (Si) is found to introduce a new anisotropic ESR signal originating from a shallow donor in Si. At 4.3 K it is characterized by g (magnetic induction B | [0 0 1]) = 1.999 89 ± 0.000 04 and g ⊥ = 1.999 71 ± 0.000 04 and spin density (S = 1 2 ≃ 1.1 × 10 12 cm −2 . It is tentatively assigned to an oxygen-related double donor intrinsic to SIMOX, which ionization state is tuned by changes in Si band bending resulting from modification of the net charge in adjacent SiO 2 by γ-irradiation. Possibly, this centre relates to the new donors recently suggested from transport measurements.

GaAs-oxide removal using an electron cyclotron resonance hydrogen plasma

The surface chemistry of GaAs-oxide removal with an electron cyclotron resonance (ECR) hydrogen plasma has been investigated with x-ray photoelectron spectroscopy. It is found that As oxide is efficiently removed at room temperature, and heating expedites the removal of Ga oxide. Band bending changes during ECR hydrogen-plasma oxide reduction are also discussed.

Scanning tunneling microscopy of photoexcited carriers at the Si(001) surface

A scanning tunneling microscope is used to probe changes in the surface potential created by photoexcited carriers. At small tunneling currents, the dependence of surface photovoltage on laser intensity agrees with a model based on the transport of majority carriers through the depletion layer. At larger values of the tunneling current, charging of the surface by the current modifies the surface photovoltage because of changes in band bending near the probe tip. Spatial variations in the surface photovoltage are observed on a length scale comparable to the width of the depletion layer. Variations on an atomic scale are also observed and are interpreted as an increase in surface charging at specific defects on the Si(001) surface.

Synchrotron radiation induced surface photovoltage at metal/GaAs interfaces

We use the Kelvin method to study the synchrotron radiation induced surface photovoltage (SPV) on GaAs(110) as a function of metal coverage and temperature. We find that varying the temperature alone does not induced significant change in band bending in the semiconductor, but that the combination of low temperature and synchrotron light illumination on lightly doped n-GaAs induces a large and quasi-permanent SPV. On lightly doped n-GaAs, the low-temperature SPV (0.55 eV) is equal to the quasi-totality of the band bending at submonolayer coverage and discharges with a time constant of the order of hours. Above a monolayer, the rate of discharging increases dramatically, emphasizing the role of charge leakage through the overlayer. The room temperature SPV is considerably smaller (0.2 eV). Finally, no significant SPV is detected on highly doped GaAs. The impact of this synchrotron radiation induced SPV on the photoemission study of metal/semiconductor interfaces is discussed in detail.

Effect of light soaking on the short-wavelength photoresponse in hydrogenated amorphous silicon

The spectrum of the values of ημτ, the product of quantum efficiency, lifetime, and mobility of photogenerated carriers, has been obtained in the wavelength range 300–900 nm, before and after light soaking. It has been shown that in the annealed state, these values are nearly independent of wavelength as well as film thickness. The values of ημτ have been found to decrease more drastically in the short-wavelength region after light soaking, similarly to that observed after moisture adsorption. Changes in band bending are not likely to be the cause of the decrease in response as the dark conductivity does not change on light soaking. The implications of the result are discussed.

Synchrotron-radiation-induced surface photovoltage on GaAs studied by contact-potential-difference measurements.

We used the Kelvin method to study the synchrotron-radiation-induced surface photovoltage (SPV) on GaAs(110) and its impact on the photoemission study of metal-semiconductor interfaces formed at low temperature. We find that varying the temperature alone does not induce significant change in band bending in the semiconductor, but that the combination of low temperature and synchrotron-light illumination on lightly doped n-type GaAs induces a large and quasipermanent SPV. It is also found that low-intensity stray light can induce measurable SPV on these samples. Highly doped low-temperature p-type GaAs does not exhibit significant SPV under synchrotron-radiation illumination.

I n s i t u x-ray photoelectron spectroscopic study of remote plasma enhanced chemical vapor deposition of silicon nitride on sulfide passivated InP

The changes in chemical composition and Fermi level position of n- and p-InP surfaces induced by reactions with sodium sulfide and ammonium sulfide solutions and with vapor from ammonium sulfide solution have been characterized by x-ray photoelectron spectroscopy (XPS). Further, in situ XPS analysis was used to study interfaces formed on sulfide passivated surfaces by remote plasma enhanced chemical vapor deposition of silicon nitride. We found that both indium and phosphorus sulfide phases were formed as a result of exposing InP to sulfide vapor at room temperature. However, most of the surface native sulfides dissolved in water and the resultant surface was then covered with about one monolayer of indium sulfide. Further, reduction in donor states in the upper band gap was evident by these sulfide treatments. However, heating the sulfide passivated sample caused an increase in surface states. Sulfur doping effect was also observed upon heating InP covered with the air oxidation products of the sulfide vapor. In the study of silicon nitride deposition, we found that the formation of native nitride could be avoided by optimizing the processing parameters. In addition, we also found that the surface native sulfide on InP reacted with the deposited silicon nitride. Capitance–voltage (C–V) data collected at 1 MHz indicate that a low interface state density could be obtained on silicon nitride/sulfide passivated InP. However, measurements of surface band bending of InP as a function of charging potential on a 10 nm silicon nitride on InP by XPS indicate that slow surface states were present which restricted changes in band bending to less than 0.3 eV when the applied voltage on the thin dielectric was −2 V.