Occupation Of Surface States Research Articles

Incorporation of indium into GaN layers in the context of MOVPE thermodynamics and growth – Ab initio studies

Indium incorporation on the GaN surface (0001) has been analyzed in the context of InGaN growth by Metal Organic Vapor Phase Epitaxy (MOVPE). To understand phenomena at the atomic level, calculations based on Density Functional Theory (DFT) and thermodynamic analysis of the surface processes have been performed. The considerations are limited to the terrace on GaN (0001) surface, covered fully by NH3/NH2 species. The ab initio results indicate that the adsorption energy strongly depends on the electronic properties of the surface. The adsorption energy varies from EadsDFT=-10.0eV for fully NH2 covered to EadsDFT=-3.0eV for mixed 0.69 NH2 + 0.31 NH3 covered surface. This change is related to Fermi level position moved from within valence band, across the bandgap, up to within conduction band. The shift in Fermi energy impacts the energy gained during adsorption due to electron occupation of surface states. Furthermore, a comprehensive thermodynamic analysis was conducted, revealing an additional, albeit smaller, contribution from the vibrational degrees of freedom to free energy. The combined effect of these factors leads to an increase in the equilibrium pressures of indium related to the increase in hydrogen pressure, which correlates with high coverage by ammonia molecules. The results demonstrate that the significant reduction in indium incorporation into InGaN layers during MOVPE growth under conditions of low hydrogen concentration in the vapor can be attributed to the influence of hydrogen on In adsorption processes.

Interface inductive currents and carrier injection in hybrid perovskite single crystals

Interfaces between the absorbing perovskite and transporting layers are gaining attention as the key locus that governs solar cell operation and long term performance. The interplay of ionic and electronic processes, along with the asymmetrical architecture of any solar cell, makes the interpretation of electrical measurements always inconclusive. A strategy to progress in relating electric responses, operating mechanisms, and device architecture relies upon simplifying the probing structure. Macroscopic CH3NH3PbBr3 single crystals with symmetrical contacts are tested by means of long-time current transient and impedance spectroscopy. It is observed that interfaces govern carrier injection to (and extraction from) perovskite layers through an inductive (negative capacitance) mechanism with a response time in the range of ∼1–100 s under dark conditions and inert atmosphere. Current transient exhibits a slow recovering after the occurrence of an undershoot, signaling a complex carrier dynamics which involves changes in surface state occupancy.

Photocarrier Radiometry Investigation of Light-Induced Degradation of Boron-Doped Czochralski-Grown Silicon Without Surface Passivation

Light-induced degradation (LID) effects of boron-doped Cz silicon wafers without surface passivation are investigated in details by photocarrier radiometry (PCR). The resistivity of all samples is in the range of 0.006 Omega.cm to 38 Omega.cm. It is found that light-induced changes in surface state occupation have a great effect on LID under illumination. With the increasing contribution of light-induced changes in surface state occupation, the generation rate of the defect decreases. The light-induced changes in surface state occupation and light-induced degradation dominate the temporal behaviors of the excess carrier density of high- and low-resistivity Si wafers, respectively. Moreover, the temporal behaviors of PCR signals of these samples under laser illumination with different powers, energy of photons, and multiple illuminations were also analyzed to understand the light-induced change of material properties. Based on the nonlinear dependence of PCR signal on the excitation power, a theoretical model taking into account both light-induced changes in surface state occupation and LID processes was proposed to explain those temporal behaviors.

The role of relative rate constants in determining surface state phenomena at semiconductor-liquid interfaces.

In this work, we present a theoretical study of surface state occupation statistics at semiconductor-liquid interfaces, as it pertains to the evolution of H2 and O2 through water splitting. Our approach combines semiclassical charge transport and electrostatics at the semiconductor-liquid junction, with a master rate equation describing surface state mediated electron/hole transfer. As a model system we have studied the TiO2-water junction in the absence of illumination, where it is shown that surface states might not always equilibrate with the semiconductor. Non-trivial electrostatics, for example including a shifting of the Mott-Schottky plateau in capacitive measurements, are explored when deep-level surface states partially equilibrate with the liquid. We also endeavor to explain observations of non-linearity present in Mott-Schottky plots, as they pertain to surface state occupation statistics. In general, it is intended that the results of this work will serve to further the use and development of quantitative device modeling techniques in the description of H2 evolution at semiconductor-liquid junctions.

Reversed oxygen sensing using colloidal quantum wells towards highly emissive photoresponsive varnishes.

Colloidal quantum wells combine the advantages of size-tunable electronic properties with vast reactive surfaces that could allow one to realize highly emissive luminescent-sensing varnishes capable of detecting chemical agents through their reversible emission response, with great potential impact on life sciences, environmental monitoring, defence and aerospace engineering. Here we combine spectroelectrochemical measurements and spectroscopic studies in a controlled atmosphere to demonstrate the ‘reversed oxygen-sensing’ capability of CdSe colloidal quantum wells, that is, the exposure to oxygen reversibly increases their luminescence efficiency. Spectroelectrochemical experiments allow us to directly relate the sensing response to the occupancy of surface states. Magneto-optical measurements demonstrate that, under vacuum, heterostructured CdSe/CdS colloidal quantum wells stabilize in their negative trion state. The high starting emission efficiency provides a possible means to enhance the oxygen sensitivity by partially de-passivating the particle surfaces, thereby enhancing the density of unsaturated sites with a minimal cost in term of luminescence losses.

Factors that determine and limit the resistivity of high-quality individual ZnO nanowires

Knowing and controlling the resistivity of an individual nanowire (NW) is crucial for the production of new sensors and devices. For ZnO NWs this is poorly understood; a 108 variation in resistivity has previously been reported, making the production of reproducible devices almost impossible. Here, we provide accurate resistivity measurements of individual NWs, using a four-probe scanning tunnelling microscope (STM), revealing a dependence on the NW dimensions. To correctly interpret this behaviour, an atomic level transmission electron microscopy technique was employed to study the structural properties of the NWs in relation to three growth techniques: hydrothermal, catalytic and non-catalytic vapour phase. All NWs were found to be defect free and structurally equivalent; those grown with a metallic catalyst were free from Au contamination. The resistivity measurements showed a distinct increase with decreasing NW diameter, independent of growth technique. The increasing resistivity at small NW diameters was attributed to the dominance of surface states removing electrons from the bulk. However, a fundamental variance in resistivity (102) was observed and attributed to changes in occupied surface state density, an effect which is not seen with other NW materials such as Si. This is examined by a model to predict the effect of surface state occupancy on the measured resistivity and is confirmed with measurements after passivating the ZnO surface. Our results provide an understanding of the primary influence of the reactive nature of the surface and its dramatic effect on the electrical properties of ZnO NWs.

Local capacitance analysis using a modified deep level spectrometer

Abstract A bimorph-based xz scanner and an amplifier, increasing the capacitance and current measurement sensitivities 250-times and 1000-times, respectively, have been built into the cryostat of a deep level transient spectrometer. The setup renders point by point local capacitance-voltage (C–V) at 1 MHz and dc currentvoltage (I–V) measurements using a sharp tip placed into tunnelling distance from the surface of analysed semiconductor sample. The C–V measurements revealed a strong dependence on the probe/sample separation, ranging from MOS-type at small tunnelling currents to Schottky-type at currents exceeding approximately 10 pA. Marked hysteresis was observed, indicating changes of surface state occupancy. These slow states are otherwise hardly detected, since they are absent in semiconductor/metal contacts and in MOS structures they would become mostly passivated interface states. The setup enables calibrated, fixed zero level capacitance measurement. The stray capacitance is the dominant component of the measured capacitance but it can be easily discarded.

Resistive Switching in Au/TiO2/Pt Thin Film Structures

The paper reports on the conditions of, and the mechanisms involved in pretreatment by strong electric field (“forming”) and subsequent resistive switching in Au/TiO 2 /Pt thin film structures on silicon. The thin TiO 2 films in these structures were prepared by different methods, namely, vacuum evaporation of metallic titanium followed by its thermal oxidation in air, and RF cathode sputtering of titanium dioxide from a powder target. Current-voltage and voltage-capacitance characteristics of the structures, as well as the relations connecting their conductivity with hold time under dc voltage of different polarities and temperature have been measured. The data obtained permit a conclusion that the physical mechanism underlying the forming consists in a sharp increase of the density of surface states in TiO 2 films initiated by electric breakdown of the Schottky barrier at the contact with the platinum electrode, while the resistive switching of the structures is governed by the variation of the occupation of surface states in the TiO 2 band gap and/or of defect concentration in the barrier region under the action of electric field.

Lateral tunneling injection and peripheral dynamic charging in nanometer-scale Schottky gates on AlGaN/GaN hetrosturucture transistors

The gate leakage and gate control characteristics of AlGaN/GaN heterostructure field effect transistors (HFETs) were systematically investigated in an attempt to clarify possible effects of surface states. The experiments were compared to rigorous computer simulations. We observed large amounts of leakage currents in the Schottky diodes fabricated on the AlGaN epitaxial layers. By the calculation based on a thin surface barrier model in which the effects of surface defect donor were taken into account, this large leakage was well explained by enhancement of tunneling transport processes due to the barrier thinning associated with ionization of surface-defect donor. On the other hand, the analysis on the current-voltage characteristics for the nanometer-scale Schottky contacts on AlGaN/GaN HFETs, indicated additional lateral leakage components. The comparison of the gate control characteristics between experiment and calculation clearly showed that the effective lateral expansion of gate length significantly impeded the gm enhancement by the reduction of geometrical gate length. This can be explained by the lateral electron tunneling process at the AlGaN surface stimulated by the pronounced gate leakage currents. Due to frequent tunneling transfer at the gate periphery, surface state occupancy near the gate becomes governed by the metal Fermi level, causing the dynamic surface state charging effects. This resulted in effective widening of the gate length, leading to degradation of gate control performance in AlGaN/GaN HFETs.

Photoinduced bulk-surface dynamics: time resolved two photon photoemission signals at semiconductor surfaces

A free particle theory of photoinduced bulk-surface dynamics at semiconductor surfaces is developed wherein relaxation processes arising from electron–electron and electron–phonon scattering are treated phenomenologically. The role played by bulk-surface dynamics in the thermalization and cooling processes of the bulk and the complementary issue of how bulk dynamics influences the surface state occupancy are both studied. Time resolved 2PPE spectra is analysed both in the context of pure bulk as well as combined bulk-surface dynamics and its relation to the time dependent populations in the conduction band and surface states is discussed.

Photoinduced bulk-surface dynamics: time resolved two photon photoemission signals at semiconductor surfaces

A free particle theory of photoinduced bulk-surface dynamics at semiconductor surfaces is developed wherein relaxation processes arising from electron–electron and electron–phonon scattering are treated phenomenologically. The role played by bulk-surface dynamics in the thermalization and cooling processes of the bulk and the complementary issue of how bulk dynamics influences the surface state occupancy are both studied. Time resolved 2PPE spectra is analysed both in the context of pure bulk as well as combined bulk-surface dynamics and its relation to the time dependent populations in the conduction band and surface states is discussed.

Have We Found The Final Explanation For The Onset Of Rapid Interlayer Mass Transport?

ABSTRACTIn recent STM experiments on multilayer island decay on Cu(111) in UHV a novel rapid interlayer mass transport was observed. This new transport mechanism may accelerate island stack decay by several orders of magnitude. Initial explanations for the onset of rapid decay events were based on the correlation between the occupation of surface states and the presence of a step edge barrier for adatom hopping over the island edges. We present new experimental data measured for Ag(111) which are in contradiction to the former interpretation of rapid interlayer mass transport. We show that all experimental results may be explained by considering an exchange mechanism when island edges in a stack are in close proximity.

Effects of gap states on scanning tunneling spectra observed on (110)- and (001)-oriented clean surfaces and ultrathin Si layer covered surfaces of GaAs prepared by molecular beam epitaxy

In an attempt to understand and control Fermi level pinning on GaAs surfaces, an ultrahigh vacuum (UHV) scanning tunneling spectroscopy (STS) study was made on (110) and (001) clean surfaces and Si covered (001) surfaces of n-type GaAs prepared by molecular beam epitaxy. Normal STS spectra showing conductance gaps corresponding to GaAs energy gap and anomalous spectra showing much larger gaps coexisted on all samples. The rate of finding normal spectra was very low on the initial surfaces, but it greatly increased after Si deposition particularly on the c(4×4) surface. A previous explanation of the gap anomaly by tip-induced electrostatic bend bending change is invalid. A new model based on a band bending change due to tip-induced local charging of surface states is presented where tunneling proximity makes occupancy of surface states in equilibrium with the scanning tunneling microscopy (STM) tip. Spots with anomalous spectra correspond to Fermi level pinning centers where the tip directly “writes” or “erases” single or a few electrons to and from the surface states. Away from the pinning center, such charge transfer does not take place, and normal STS spectra are obtained with Fermi level positions consistent with macroscopic band bending measured by x-ray photoelectron spectroscopy and by an UHV contactless capacitance–voltage system. No direct one-to-one correlation existed between the pinning center and any specific visual STM defect features such as vacancies, dimer-desorbed holes, dimer kinks, step etc. Pinning centers make up inhomogeneous distributions of spatially extended pinning areas of universal nature surrounding any kind of structural disorder. Si deposition is shown to be very effective in reducing the number, spatial extension and state density of such pinning areas, particularly on the initially c(4×4) reconstructed surface.

Interlayer Mass Transport and Quantum Confinement of Electronic States

A numerical analysis of the ripening of multilayer islands on Cu(111) shows that the Ehrlich-Schwoebel barrier for interlayer mass transport is independent of the terrace width w as long as w>wc=14  2 A, but vanishes for w<wc. The critical width wc corresponds exactly to the terrace width below which the surface state is pushed above the Fermi-level due to quantum confinement. The Ehrlich-Schwoebel barrier is therefore correlated with the occupation of surface states.

Electronic Surface States and the Hydrogen Dissociation Barrier

Surface states contribute significantly to the charge density in front of metal surfaces. Owing to the Pauli principle, this yields a repulsive contribution in the physisorption potential for closed-shell adsorbate particles, such as rare-gases or H2. In the case of rare-gas adsorption the surface state induced repulsion can be directly measured by means of high-resolution photoemission. The additional repulsive contribution influences also the barrier height in dissociative adsorption. This is demonstrated for f.c.c. (111) and f.c.c. (110) surfaces by adjusting the surface state occupancy with a variety of suitable adsorbates and monitoring the associated changes in the dissociation probability of H2.

Current-voltage characteristics of Schottky barrier diodes with dynamic interfacial defect state occupancy

Schottky barrier diode theory conventionally assumes that the barrier height remains constant with the applied bias. However, high ideality factors and certain anomalous capacitance-voltage characteristics can only be explained by letting the barrier height vary with the applied voltage. Changes in the occupation of surface states have been inferred to be the cause of this change in barrier height, although the precise nature of these surface states has not yet been determined. This article examines the role of monoenergetic interfacial defect states whose occupancy is dynamically controlled by the local carrier densities, rather than by equilibrium conditions. Certain types of surface donor and acceptor states are shown to have a strong influence on the resulting current-voltage characteristics. The Al/GaAs(100) system is discussed in detail and experimental results are compared to this analytic model.

Influence of monoenergetic surface state occupation on Fermi level pinning of metal–GaAs interfaces

Fermi level pinning at metal–GaAs interfaces is conventionally attributed to a high density of surface states. This article points out that not only the density and energy level of the surface states, but more importantly their electron occupancy determines the location and degree of pinning. For device applications, Fermi level pinning is most often based upon the Cowley–Sze model, which postulates a thin, electrically transparent interfacial layer, as would be expected for metalizations on GaAs which form arsenide compounds such as aluminum. However, this model is very difficult to apply to the more common surface spectroscopies (x‐ray and ultraviolet photoemission spectroscopies) because of its assumption of a uniform distribution of surface states with energy. This article presents a modified model which is based upon any multiplicity of monoenergetic donorlike or acceptorlike surface states and which therefore can be better correlated to the measured surface density of states. This model allows the full electronic structure of the surface states to be included, and it determines the degree and location of Fermi level pinning as a function of this defect structure alone. This eliminates the need for any empirically determined surface neutral energy, while still encompassing both the ideal Schottky and Bardeen limits of the Cowley–Sze model. An important result is that a high density of surface states is not by itself sufficient to cause pinning. The electrostatic restrictions of Gauss’ law require a surface charge on the interfacial layer in order to support any band bending within the semiconductor. This surface charge can arise only from sufficient occupancy of the surface states, e.g., the ionized surface acceptor states associated with arsenic vacancies or antisite defects in n‐type GaAs.

Surface recombination at semiconductor electrodes: a single level study

The recombination and the faradaic fluxes are shown to be sensitive to the location of a single level recombination center, when it is located near the band edges. As the surface level is shifted deeper into the band gap from either of the band edges, the back emission terms are dominated by electron capture and hole capture terms, and the occupancy of the surface level is no longer determined by its location in the band gap. However, when one of the back emission terms determines the surface state occupancy, it is shown that there exists a simple relation between the value of the surface level and the recombination and the faradaic fluxes respectively. Expressions to this effect are derived and verified in the case of the recombination flux, which characterized by the potential at which it attains its maximum value. For the faradaic flux the results are qualitative.

Defect-state occupation, Fermi-level pinning, and illumination effects on free semiconductor surfaces.

A comprehensive physical model for equilibrium and illuminated free semiconductor surfaces interacting with an arbitrary number of monoenergetic surface and bulk defect states is presented. The position of the surface Fermi energy is shown to be determined by the overall balance of surface charge and surface electric field, and is not merely set by alignment to the energy of the highest-density surface state. Boundary conditions are derived for nonequilibrium device analysis, which includes the full electrostatic and recombinative details of the surface states. An approximate analytical solution is given for the semiconductor transport equations with these boundary conditions and this model is used to describe illumination effects on the free surface. The solution can be represented in a graphical form, which allows the most significant contributions to be easily visualized. Fermi-level pinning on free surfaces is then classified according to the primary contributions to the charge balance and also according to the sensitivity of the surface potential to charge perturbations. The occupation factors for the surface states under illumination conditions are derived from the analytic model and are shown to both shift and saturate with increasing illumination intensity. This allows the origin of free-surface photovoltage, depletion-edge contraction, and band flattening to be reinterpreted as arising from a change in surface-state occupation rather than from a quenching of the depletion-region field by free electrons and holes.

Auger Decay of Surface Rydberg State

Decay of the first Rydberg surface state on a metal surface is calculated in terms of Auger transitions between the electronic surface states. Using a three band model to describe the relevant surface band structure, the line width due to this surface Auger process is found to be about 8 times larger on the Ni(111) surface than on the Cu surface. This significant difference results from the difference in surface state occupancy. Comparison with recent experimental data from two photon photoemission demonstrates that these surface states are essential for the decay of the excited electron at surface