Energy Distribution Of Surface States Research Articles

DETERMINATION OF MAIN ELECTRICAL PARAMETERS OF Au–4H-n-SiC (MS) AND Au–Al2O3–4H-n-SiC (MIS) DEVICES

In this study, Au–4H-[Formula: see text]-SiC metal–semiconductor (MS) and Au–Al2O3–4H-[Formula: see text]-SiC metal–insulator–semiconductor (MIS) devices were fabricated to examine the effects on the performance of electronic devices of interfacial insulating materials. In order to determine the dielectric properties, capacitance/conductance–voltage ([Formula: see text]–[Formula: see text] measurements were realized in a wide range of voltages ([Formula: see text]3.0 V)–(11.0 V). Current–voltage ([Formula: see text]–[Formula: see text] measurements to obtain the electric properties were realized at [Formula: see text][Formula: see text]V. Moreover, both the energy distributions of surface states ([Formula: see text] and series resistance ([Formula: see text] were obtained from the [Formula: see text]–[Formula: see text] data. Obtained results provided that series resistance originating from interfacial layer (Al2O[Formula: see text] was more effective on the [Formula: see text]–[Formula: see text] and [Formula: see text]–[Formula: see text] characteristics which must be taken into account in the calculation of main electrical parameters. The rectification ratio (RR) and shunt resistance ([Formula: see text] of the MIS device were almost 103 times greater than those of the MS structure. Using Al2O3 between Au and 4H-[Formula: see text]-SiC also led to an increase in the value of barrier height (BH) and a decrease in the value of ideality factor ([Formula: see text]. These results confirmed that Al2O3 layer leads to an increase in the performance of MS device with respect to low values of [Formula: see text], reverse saturation current ([Formula: see text] and [Formula: see text] and high values of RR, [Formula: see text] and BH.

Analysis of surface states and series resistance in Au/n-Si Schottky diodes with insulator layer using current–voltage and admittance–voltage characteristics

In order to good interpret the experimentally observed Au/n-Si (metal–semiconductor) Schottky diodes with thin insulator layer (18 Å) parameters such as the zero-bias barrier height ( Φ bo), ideality factor ( n), series resistance ( R s) and surface states have been investigated using current–voltage ( I– V), capacitance–frequency ( C– f) and conductance–frequency ( G– f) techniques. The forward and reverse bias I– V characteristics of Au/n-Si (MS) Schottky diode were measured at room temperature. In addition, C– f and G– f characteristics were measured in the frequency range of 1 kHz–1 MHz. The higher values of C and G at low frequencies were attributed to the insulator layer and surface states. Under intermediate forward bias, the semi-logarithmic Ln ( I)– V plot shows a good linear region. From this region, the slope and the intercept of this plot on the current axis allow to determine the ideality factor ( n), the zero-barrier height ( Φ bo) and the saturation current ( I S) evaluated to 2.878, 0.652 and 3.61 × 10 −7 A, respectively. The diode shows non-ideal I– V behavior with ideality factor greater than unity. This behavior can be attributed to the interfacial insulator layer, the surface states, series resistance and the formation barrier inhomogeneity at metal–semiconductor interface. From the C– f and G– f characteristics, the energy distribution of surface states ( N ss) and their relaxation time ( τ) have been determined in the energy range of ( E c − 0.493 E v)–( E c − 0.610) eV taking into account the forward bias I– V data. The values of N ss and τ change from 9.35 × 10 13 eV −1 cm −2 to 2.73 × 10 13 eV −1 cm −2 and 1.75 × 10 −5 s to 4.50 × 10 −4 s, respectively.

Nanoscale measurement of the energy distribution of semiconductor surface states

We report on a method to quantitatively measure the local energy distribution of surface states density within the band gap of semiconductors using Kelvin probe force microscopy. The method is based on measuring the contact potential difference of a cross-sectional $pn$ junction; as the tip scans the junction surface, the surface states population changes, thus modifying the measured local surface band bending. The energy distribution is then obtained by fitting the calculated surface potential to the measured contact potential difference. The method is applied to ${p}^{++}n$ and ${n}^{++}p$ Si (110) surfaces and ${p}^{++}n$ InP (110) surfaces measured under different environments. The results are compared to data obtained by theoretical models and other experimental methods.

Effect of pressure on the properties of a silicon-glass interface

The effect of hydrostatic pressure on the energy distribution of surface states localized at the boundary between silicon (Si) and a lead borosilicate glass (PbO-SiO2-B2O3-Al2O3-Ta2O5) has been studied. At a pressure of 8 kbar, donor centers are formed in a layer of glass adjacent to the silicon-glass interface, which are capable of participating in electron exchange with the semiconductor.

Molecular Control over Semiconductor Surface Electronic Properties: Dicarboxylic Acids on CdTe, CdSe, GaAs, and InP

We present “design rules” for the selection of molecules to achieve electronic control over semiconductor surfaces, using a simple molecular orbital model. The performance of most electronic devices depends critically on their surface electronic properties, i.e., surface band-bending and surface recombination velocity. For semiconductors, these properties depend on the density and energy distribution of surface states. The model is based on a surface state-molecule, HOMO-LUMO-like interaction between molecule and semiconductor. We test it by using a combination of contact potential difference, surface photovoltage spectroscopy, and time- and intensity-resolved photoluminescence measurements. With these, we characterize the interaction of two types of bifunctional dicarboxylic acids, the frontier orbital energy levels of which can be changed systematically, with air-exposed CdTe, CdSe, InP, and GaAs surfaces. The molecules are chemisorbed as monolayers onto the semiconductors. This model explains the widel...

Energy distribution of surface states in the Si band-gap for MOS diodes obtained from XPS measurements under biases

The energy distribution of surface states in the Si band-gap for MOS diodes has been obtained from X-ray photoelectron spectroscopic (XPS) measurements under biases. The XPS measurements are performed for the 〈3 nm thick Pt/2.5 nm thick native silicon oxide/n-Si(100)〉 MOS diodes. The binding energy of the substrate Si 2p peak, E Si(2p), with respect to that of the Pt 4f peak of the metal overlayer, E Pt(4f), is decreased by applying a forward bias, while it is increased by applying a reverse bias. These shifts are completely reversible, and attributed to the band-edge shifts caused by accumulation of electric charges in the surface states. By analyzing the amount of the bias-induced shift in E Si(2p) − E Pt(4f), measured as a function of the bias voltage, the energy distribution of the surface states present at the native oxide Si interface is obtained for the first time. The density of the surface states is high near the Fermi level (0.49 eV above the valence band maximum) and decreases toward the valence and conduction bands.

New method for determination of energy distribution of surface states in the semiconductor band-gap: XPS measurements under biases

The energy distribution of surface states in the semiconductor bandgap has been obtained spectroscopically from measurements of X-ray photoelectron spectra (XPS) under biases. The XPS measurements are performed for [ca. 30 A ̊ - thick Pt ca. 25 A ̊ - thick oxide/p- InP (1 0 0)] MIS diodes. The difference in binding energy between the In(3 d) and Pt(4 f) peaks, E In E Pt, is changes by applying biases, because of accumulation of charges in the surface states. The energy distribution of the surface states is determined by analyzing the amount of the biasinduced shift in E In E Pt. The density of the surface states is high near the mid-gap, near the conduction band minimum and near the valence band maximum.

Semiconductor analysis using organic-on-inorganic contact barriers. I. Theory of the effects of surface states on diode potential and ac admittance

A new method for measuring the density of states at semiconductor surfaces using organic-on-inorganic (OI) semiconductor contact barriers is suggested. This work is an extension of previous models of OI diode behavior which includes the ac admittance characteristics, and which considers the range of validity of approximations to OI diode capacitance used in previous experiments. The theory describes the potential distribution across the device. We consider the case of ideal OI diodes, as well as diodes with significant densities of states at the inorganic semiconductor surface. This analysis leads to a technique whereby the low-frequency conductance and capacitance characteristics can be used to obtain information about the magnitude and the energy distribution of surface states in the inorganic semiconductor bandgap. Also, the carrier concentration profiles of the substrate can be conveniently obtained. Due to the noninvasive nature of the organic/inorganic contact, information about the density of states at relatively undisturbed semiconductor surfaces obtained via this technique may prove useful in determining the processes involved in Schottky barrier formation and metal-insulator-semiconductor diode surface properties. This theory has been applied to the investigation of surfaces of III-V alloy semiconductor-based OI diodes, and the experimental results will be presented in a subsequent paper. In addition, an expression for the OI diode n value obtained from the forward current-voltage characteristics is derived. It is found that the n value depends on surface states which are in equilibrium with the organic as well as the inorganic materials. Estimates of the surface state densities obtained from previously reported n values are consistent with expectations for the semiconductors under test.

An analytical formulation for the J( V) characteristics of a photoelectrolytical cell

An analytical formulation is given in the present work for the study of the electrical characteristics of an ideal photoelectrolytical cell utilizing an n-type, wide band gap semiconducting electrode. Recombinations both at the surface and in the depletion layer are included and a model is suggested that accounts for the statistical effects in the transfer process of electrical charges at the semiconductor-electrolyte interface. An equation is obtained for the total hole current in which the different contributions from the various recombination processes are easily recognizable and the energy distribution of surface states and recombination levels are explicitly taken into account. A detailed numerical analysis of equations shows the correlation between the photocurrent near its onset and the slope of the J( V) curves with the parameters that characterize each one. A possible dependence of photocurrent on light radiation intensity is given and tunnel currents at the semiconductor-electrolyte interface are included in the model.

Electronic properties and chemical interactions at III–V compound semiconductor surfaces: Germanium and oxygen on GaAs(110) and InP(110) cleaved surfaces

The adsorption of foreign atoms at semiconductor surfaces may alter the surface band-bending as well as the ionization energy by changing the energy distribution of surface states within the bulk band gap and the surface dipole, respectively. Well-cleaved GaAs and InP surfaces exhibit no surface states in the bulk band gap but both the condensation of Ge and the uptake of oxygen are introducing acceptor- as well as donor-type surface states in the bulk band gaps of both semiconductors. Up to a monolayer coverage of Ge the energy distributions of the adsorption-introduced surface states remains unchanged on GaAs(110) as well as on InP(110). Oxygen chemisorption, however, initially pins the Fermi level at the same positions observed with adsorbed Ge but then drives the Fermi level closer to the conduction-band edge with the onset of the main uptake. The Ge adsorption introduces dipoles on both semiconductors while no such effect is observed with oxygen chemisorbed on GaAs(110) as well as on InP(110). The consequences of these findings are discussed with regard to the surface geometry of oxygen chemisorbed on the two semiconductors considered.

Determination of capture cross section and surface-states concentration profile using the surface-acoustic-wave convolver

Energy distribution of surface states and majority-carriers capture cross section is determined using the surface-acoustic-wave (SAW) convolver. The semiconductor is placed a small distance above the SAW delay line, with a uniform air gap between the two media. A fast rise time dc pulse is applied across the semiconductor-delay line structure, and the resulting change in the SAW propagation loss is observed. The transient response of the SAW propagation loss represents the emission or trapping of majority carriers from surface states. From this transient response, the capture cross section and the concentration profile of the surface states are determined. The results obtained agree with the already known distribution of fast surface states; it is constant at the middle of the gap and increases towards the conduction band, whereas the capture cross section is constant in the middle of the gap and decreases toward the conduction band. This new SAW technique is simple, sensitive, and requires no contact to the semiconductor surface.

On capacity measurements and the energy distribution of surface states at the electrolyte-GaAs interface

The frequency-dependent values for the capacities obtained for GaAs, GaAsP and GaP were interpreted by means of the usual equivalent circuit with the capacity of the surface states and that of the depletion layer in parallel. Consistent results are obtained only assuming a continuous energetic distribution of surface states in the forbidden gap. Adsorption of a dye and Ru3+ leads to an altered distribution function, the maximum appearing can be accounted for by superimposing a Lorentz function on the cosh type of function. The dye adsorption was determined radiochemically and obeys a Freundlich isotherm. Ellipsometric measurements under the same conditions suggest the formation of non-passivating surface films in the electrolytes used.

On capacity measurements and the energy distribution of surface states at the electrolyte-GaAs interface

The frequency-dependent values for the capacities obtained for GaAs, GaAsP and GaP were interpreted by means of the usual equivalent circuit with the capacity of the surface states and that of the depletion layer in parallel. Consistent results are obtained only assuming a continuous energetic distribution of surface states in the forbidden gap. Adsorption of a dye and Ru 3+ leads to an altered distribution function, the maximum appearing can be accounted for by superimposing a Lorentz function on the cosh type of function. The dye adsorption was determined radiochemically and obeys a Freundlich isotherm. Ellipsometric measurements under the same conditions suggest the formation of non-passivating surface films in the electrolytes used.

Measurements of interface state density in MNOS structures

A technique is described for measuring the density and energy distribution of surface states at the Si-SiO2 interface in MNOS capacitors. The effects of charge injection into the insulator are taken into account by making use of a fast voltage ramp to monitor the instantaneous values of the surface potential during a quasistatic capacitance-voltage sweep. The technique has been used to investigate the effect of charges trapped in the nitride on the interface-state density.

Surface states and barrier heights of metal-amorphous silicon schottky barriers

The barrier heights, φ B, of Schottky barriers formed between Al, Ni, Cr, Pd, Au, Rh, and Pt metal films and undoped discharge-produced amorphous silicon (a-Si) have been measured. The dependence of φ B on the metal work function, φ M, has been characterized for values of φ M between ≈4.0 and 5.5 eV. Experiments were performed to compare the effects of surface states on φ B of both the undoped (n-type) amorphous and n-type single crystal Si. These effects of surface states are very similar, indicating similar densities and energy distributions of surface states. The densities of surface states on a-Si are found to be ≳10 13cm −2eV −1 which result in the pinning of φ B to values ≳0.7 eV even with φ M as low as 4.0 eV.

Interface state density in Au- nGaAs Schottky diodes

A method for determining the surface state density in Schottky diodes taking into account both I–V and C–V data while considering the presence of a deep donor level is presented. The model assumes that the barrier height is controlled by the energy distribution of surface states in equilibrium with the metal and the applied potential and does not include, explicitly, an interfacial layer. The model was applied to extract interface state densities of Au- nGaAs guarded Schottky diodes fabricated from bulk and VPE (100) GaAs with carrier conentrations between 3 × 10 15 and 8 × 10 16 cm −3. These diodes exhibited ideality ( n) factors of approximately 1.02 and room temperature saturation current densities ∼10 −8 A/cm 2. This model is in substantial agreement with forward bias measurements over the 77–360°K temperature range investigated, in that a temperature-independent energy distribution of interface states was obtained. In reverse bias the interface state model is most valid with the higher carrier concentration material and at high temperature and low bias voltage. Typical interface state densities from 0.07 eV above the zero bias Fermi level to 0.01 eV below the Fermi level were 2 × 10 13 cm −2 eV −1. The validity of the model under reverse bias is restricted by a non-thermionic reverse current, thought to be enhance field emission from traps.

Surface States in Tunnelable MOS Structures

The bias dependence of the tunneling current, low frequency capacitance and differential conductance of MOS structures with very thin SiO2 films on p-type Si substrates have been investigated over the temperature range from 151 K to 324 K. The surface state density of about ∼8×1011 states/cm2 is obtained for p–Si(100) orientation and of a little higher value for p–Si(111), but the energy distribution of surface states near the valuence band depends strongly on the crystallographic orientation. The surface state capture cross-sections for p–Si(111) and p–Si (100) MOS are 2.0×10-16 cm2 and 3.1×10-16 cm2, respectively. Experimental evidence of the tunneling current component from metal to surface states is seen in DC conductance and surface state density measurements.

Surface states associated with acid sites on solids

It is shown that present models of Lewis acidity or basicity and of surface states on ionic solids have substantial overlap, although the former is designed to describe chemical interactions at the surface site, and the latter electron capture. The site requirements for a Lewis acid are compared to the requirements for an acceptor surface state to suggest under what conditions a site should exhibit both strong acidity and a deep surface state: The expected influence of electronic properties of the solid on acid strength, and the influence of adsorbed water, acids or bases on surface state energies are discussed. Experimental measurements are reported where both chemical acidity measurements and electrical surface state measurements are made on a series of Lewis acids. A positive correlation is found. Where there is correlation, it is concluded that chemical interaction measurements should provide a valuable tool to describe the energy distribution of surface states on a semiconductor surface. Also, the use of the surface state models and measurements from semi-conductor physics should help in understanding and classifying acid and basic sites on ionic semiconductors and insulators.

Surface states in the conduction band region of Si

The energy distribution of surface states within the conduction band has been determined analizing the mobility ratio μ H/μ eff. A shallow trap model was used to describe the surface states.

Properties of silicon-anodic oxide interfaces

The study of the complex capacitance C*mos of MOS capacitors has shown the important influence of the moisture on the properties of anodic oxide MOS structures. Two behaviours have been pointed out: the “dry” type and the “wet” type. The unambiguous determination of surface state parameters from the variations of C*mos as a function of frequency, temperature, and applied bias is discussed. A method of characterizing the distribution of surface states is proposed. The utilization of this method permits to explain some apparently contradictory results published in the literature, and shows that: 1. whatever the oxide (“wet” or “dry”) and silicon (n or p) types may be, the silicon-oxide interfaces are characterized by the same energy distribution of surface states, whose density is high (about 1013 eV−1 cm−2) and nearly symmetrical with respect to the middle of the gap, 2, the mobile protons contained in “wet” oxide have no influence on the distribution of surface states, 3. in order to reduce the density of surface states by thermal treatments at low temperatures (about 150 °C) the oxide must be of “wet” type.