Distribution Of Schottky Barrier Height Research Articles

The barrier height distribution in identically prepared Al/p-Si Schottky diodes with the native interfacial insulator layer (SiO 2)

The Al/p-Si Schottky diodes (33 dots) with the native interfacial insulator layer (SiO 2) were fabricated on the same quarter Si wafer. The Schottky barrier height (SBH), ideality factor ( n), interface state densities ( N ss) and series resistance ( R s) of these diodes have been calculated from their experimental forward bias current–voltage ( I– V), reverse bias capacitance–voltage ( C– V) and conductance–voltage ( G/ ω– V) measurements. Even though they are identically performed on the same quarter Si wafer, the calculated values of SBH have ranged from 0.680 to 0.736 eV, and ideality factor n from 1.62 to 2.87, and interface state densities N ss from 0.395×10 13 to 1.2×10 13 eV −1 cm −2 and series resistance R s from 623 to 4900 Ω. It was found that the values of barrier height Φ B obtained from C– V characteristics is larger than that of the value from I– V characteristics. The energy distribution of surface state density profiles for the selected six samples Al/p-Si Schottky diodes was determined from forward bias I– V characteristics by taking the bias dependence of the effective barrier height into account at room temperature. The experimental values of SBHs distributions obtained from the I– V and C −2– V characteristics have been fitted by a Gaussian function, and their mean values of SBHs have been calculated to be 0.708 and 0.810 eV, respectively. Experimental results show that the interface states at a native insulator layer between metal and semiconductor play an important role in the value of the SBH, ideality factor, series resistance and the other main electrical parameters of Schottky diodes.

High spatial and energy resolution characterization of lateral inhomogeneous Schottky barriers by conductive atomic force microscopy

We present a novel approach based on conductive atomic force microscope (c-AFM) for nano-scale mapping in laterally inhomogeneous metal–semiconductors Schottky contacts. For ultra-thin (1–5 nm) metal films with resistivity exceeding by about two orders of magnitude the bulk value, the current localization under the tip (10–20 nm diameter) occurs. By spectroscopic analysis of the current–voltage characteristics for different tip positions, the 2D Schottky barrier height (SBH) distribution is obtained with ∼0.1 eV energy resolution. The method was applied to explain the experimentally observed SBH lowering in macroscopic Au/4H–SiC diodes, in the presence of a not uniform SiO 2 layer at the SiC/Au interface. Nano-scale mapping on the oxide free Au/4H–SiC contact demonstrates a SBH distribution peaked at 1.8 eV and with tails from 1.6 eV to 2.1 eV. When ∼2 nm not uniform SiO 2 layer is intentionally grown on SiC before contact formation, the Au/SiO 2/4H–SiC SBH distribution exhibits a 0.3 eV lowering in the peak and a broadening (tails from 1.1 eV to 2.1 eV), thus explaining the macroscopically observed average effect.

Current- and capacitance-voltage characteristics of Cd/p-GaTe Schottky barrier diodes under hydrostatic pressure

Schottky barrier diodes (SBDs) on p-type GaTe have been fabricated by Cd metallization and characterized by current–voltage (I–V) and capacitance–voltage (C–V) techniques as a function of hydrostatic pressure (0.0–7.0 kbar). The evaluation of the experimental data reveals a decrease of barrier height (Φb), ideality factor (n) and series resistance (Rs) with an increase in the hydrostatic pressure. The zero-bias barrier height, ideality factor and series resistance values for the Cd/p-GaTe SBD by I–V measurements have been in the range of 0.743–0.682 eV, 1.246–1.219 and 30.5–16.4 Ω for the 0.0–7.0 kbar pressure interval, respectively. C–V measurements at 1.0 MHz have resulted in higher barrier heights than those obtained from I–V measurements. The discrepancy between Schottky barrier heights (SBHs) obtained from I–V and C–V measurements is explained by the introduction of a spatial distribution of SBHs due to barrier height inhomogeneities that prevail in the metal/GaTe interface. The change of barrier height values (Φb(0) − Φb(P)) obtained from the I–V and C–V techniques turns out to have a mean linear pressure coefficient of −8.77 ± 0.10 meV kbar−1 (=−87.7 ± 1.0 eV GPa−1), approximately equal to that found for the band gap of GaTe. We have concluded that the variation of the barrier height due to the applied pressure should follow precisely the variation of the semiconductor band gap, accepting that the Fermi level is a reference level which is pinned to the conduction-band minimum (CBM) as a function of the pressure.

Nonideality of Au/Si and Au/GaAs Schottky barriers due to process-induced defects

A mechanism of local lowering of the Schottky barrier height (SBH) is proposed, which causes nonideality in nearly ideal Au/ n-Si and Au/ n-GaAs Schottky barriers. Positively ionized defects generated by the process very close to the interface induce electrons in the metal-induced gap states (MIGS) and lower the SBH locally. The spatial density distribution of the ionized defects obtained from the SBH distribution is determined by the unique interaction with the MIGS. The defects are considered to have the negative-U property and are neutralized at very close positions to the MIGS. The potential distributions close to the interface have a considerable potential drop due to the large defect density. These inhomogeneous potentials are coincident with the energy level scheme of the defect identified as the defect causing the nonideality. This defect is Si self-interstitial in Au/Si SB, and As antisite in Au/ n-GaAs SB. This MIGS with process-induced defect model supersedes the previously proposed two major Fermi level pinning models. The mystery of the T 0 effect is solved. The thermionic-field emission current taking place in the strong electric field has influence on the I– V characteristics at low temperatures. Regarding the C– V characteristics of Au/Si SB, the observed extra capacitance under the forward bias is an experimental evidence in accordance with the proposed model.

Investigation on the barrier height and phase transformation of nickel silicide Schottky contact

The Schottky barrier heights of the nickel silicide Schottky contacts formed at different annealing temperatures were independently investigated by I– V– T measurement and the infrared photocurrent measurement. It has been found that the Gaussian distribution of Schottky barrier height can be used to explain the experimental results very well. The Schottky barrier heights of the samples annealed at temperatures of 550 °C and 600 °C are larger than those of the samples annealed at the other temperatures. This may result from the phase transformation of nickel silicide at different annealing temperatures. The formation NiSi phase can be confirmed by Raman spectroscopy.

Ni/Si solid phase reaction studied by temperature-dependent current-voltage technique

The temperature-dependent current–voltage (I–V–T) technique has been used to study the Ni/Si solid phase reaction by measuring the Schottky barrier height (SBH) inhomogeneity of Ni-silicide/Si Schottky diodes. The experimental results show the strong dependence of SBH inhomogeneity on the Ni/Si solid phase reaction. The SBH distribution of the diodes annealed at 500 and 600 °C can be described by a single-Gaussian function and the diode annealed at 500 °C is found to have the best homogeneity and the smallest leakage current. The SBH distribution of the diodes annealed at 400, 700, and 800 °C can be described by a double-Gaussian function in which the mean value of the second Gaussian function is substantially smaller than that of the dominant Gaussian function. The variation of SBH inhomogeneity, an interface property, is related to the phase evolution process in the Ni/Si solid phase reaction, and verified by reverse I–V measurements. Our results indicate that the I–V–T technique may be developed as a wafer-level testing tool to monitor the silicidation process in the complementary metal–oxide–semiconductor device fabrication.

Double threshold behaviour of I - V characteristics of CoSi 2 /Si Schottky contacts

The forward current-voltage (I-V) characteristics of polycrystalline CoSi2/n-Si(100) Schottky contacts have been measured in a wide temperature range. At low temperatures (≤200K), a plateau-like section is observed in the I-V characteristics around 10-4Acm-2. The current in the small bias region significantly exceeds that expected by the model based on thermionic emission (TE) and a Gaussian distribution of Schottky barrier height (SBH). Such a double threshold behaviour can be explained by the barrier height inhomogeneity, i.e. at low temperatures the current through some patches with low SBH dominates at small bias region. With increasing bias voltage, the Ohmic effect becomes important and the current through the whole junction area exceeds the patch current, thus resulting in a plateau-like section in the I-V curves at moderate bias. For the polycrystalline CoSi2/Si contacts studied in this paper, the apparent ideality factor of the patch current is much larger than that calculated from the TE model taking the pinch-off effect into account. This suggests that the current flowing through these patches is of the tunnelling type, rather than the thermionic emission type. The experimental I-V characteristics can be fitted reasonably well in the whole temperature region using the model based on tunnelling and pinch-off.

Gaussian distribution of inhomogeneous barrier height in Si Schottky barriers

Deviations from the ideal characteristics in actual nearly ideal Schottky barriers (SBs) are due to inhomogeneous distributions of the Schottky barrier height (SBH) φ. By assuming the distribution as a Gaussian of the standard deviation σ from the ideal SBH ΦB, the SB characteristics have been qualitatively explained. However, the reason is unknown why the SBH distribution is a Gaussian. From the observed temperature dependence of σ, the distribution is found to exponentially decay more gradually than the Gaussian at large ΦB−φ values. According to our model of the local SBH lowering by ionized donors in Si SBs, the SBH distribution represents the spatial distribution of the ionized donors from the interface. The energy levels of the ionized donor decrease, become broad with approaching the interface, and are in ionization equilibrium with the Fermi level. The Gaussian distribution of the inhomogeneous SBH in Si SBs is consistently understood by our microscopic model.

Mechanism of nonideality in nearly ideal Si Schottky barriers

The origin of nonideality in an actual nearly ideal Schottky barrier is an inhomogeneous Schottky barrier height (SBH). A high density of point defects is generated in the neighborhood of the interface by the fabrication process of the metal/Si interface. Local SBH lowering by positively ionized defects close to the interface is considered the cause of inhomogeneity based on the property of the metal-induced gap states. Results of analysis by this mechanism are in excellent agreement with ballistic electron emission microscopy (BEEM) observation of low-SBH spots. A Gaussian distribution of inhomogeneous SBH explains the BEEM spectrum, as well as the temperature dependence of both effective SBH and ideality factor, i.e., the so-called T0 anomaly. The spatial distribution of the ionized donor and its variation under applied voltage are obtained. This result indicates that the origin of the ideality factor is preferential neutralization of the donor close to the interface in equilibrium with the Fermi level. Thus, the proposed mechanism explains the various properties of nearly ideal Si Schottky barriers.

Ballistic electron emission microscopy studies of the temperature dependence of Schottky barrier height distribution in CoSi 2/n-Si(1 0 0) diodes formed by solid phase reaction

Ballistic electron emission microscopy (BEEM) and ballistic electron emission spectroscopy have been performed on polycrystalline and epitaxial CoSi 2/n-Si(1 0 0) contacts at temperatures ranging from −144°C to −20°C. The ultra-thin CoSi 2 films (∼10 nm) were fabricated by solid state reaction of a single layer of Co (3 nm) or a multilayer of Ti (1 nm)/Co (3 nm)/amorphous-Si(1 nm)/Ti (1 nm) with a Si substrate, respectively. The spatial distribution of barrier height over the contact area obeys a Gaussian function at each temperature. The mean barrier height increases almost linearly with decreasing temperature with a coefficient of −0.23±0.02 meV/K for polycrystalline CoSi 2/Si diodes and −0.13±0.03 meV/K for epitaxial diodes. This is approximately equal to one or one-half of the temperature coefficient of the indirect energy gap in Si, respectively. It suggests that the Fermi level is pinned to different band positions of Si. The width of the Gaussian distribution is about 30–40 meV, without clear dependence on the temperature. The results obtained from conventional current–voltage and capacitance–voltage ( I– V/ C– V) measurements are compared to BEEM results.

A BEEM study of Schottky barrier height distributions of ultrathin CoSi2/n-Si(100) formed by solid phase epitaxy

The spatial distributions of the Schottky barrier heights of ultrathin CoSi2 films (~10 nm) on n-Si(100), obtained by multilayer solid state reaction of Co/Ti/n-Si, Co/a-Si/Ti/n-Si, Ti/Co/a-Si/Ti/n-Si and Co/n-Si systems, are studied by ballistic electron emission microscopy (BEEM) and spectroscopy (BEES) at low temperature (~-80 °C). The barrier heights determined from BEEM spectra range between 520 meV and 700 meV, with an approximate Gaussian distribution. The mean barrier heights of the epitaxial CoSi2/Si contacts are 0.60-0.61 eV, lower than the 0.64 eV for polycrystalline CoSi2/Si contacts. Adding a thin amorphous Si interlayer (1 nm) slightly increases the probability of higher barrier heights, while a thin Ti capping layer (1 nm) has no significant influence on the mean barrier height. The BEEM results are compared to those from I-V/C-V measurements.

A BEEM study of the temperature dependence of the barrier height distribution in PtSi/n-Si Schottky diodes

Schottky barrier height distributions of a PtSi film on n-Si(100) are directly measured by ballistic electron emission spectroscopy at temperatures ranging from 180 to 300 K. At any temperature, the measured barrier height distribution can be fitted by a Gaussian function quite well. The mean barrier height decreases with increasing temperature almost linearly with a coefficient of (−2.6±0.2)×10 −4 eV/K, which is approximately equal to the temperature dependence of the energy band gap of Si, thus suggesting that the Fermi level at the interface is pinned to the valence band edge. The width of the Gaussian barrier distribution is 32±4 meV, without clear dependence on temperature. The results are consistent with the values obtained from traditional C– V measurements.

A ballistic electron emission microscopy study of barrier height inhomogeneities introduced in Au/III-V semiconductor Schottky barrier contacts by chemical pretreatments

The distribution of Schottky barrier heights over the contact area in Au/III-V semiconductor (GaAs, InP, AlxGa1-xAs, InxGa1-xAs) diodes was determined using ballistic electron emission microscopy. Samples which received a chemical pretreatment in aqueous HF or HCl solutions showed changes in the barrier height distribution. In some cases, short rinses in deionized water could remove these effects. Additional XPS measurements and our former work on Si enabled us to propose a model wherein negatively charged species containing F or Cl at the interface are assumed to be responsible for these changes in barrier height distribution. However, in some cases, these effects were shadowed by more drastic influences due to the chemical processing such as changes in the stoichiometry of the surface region.

Ballistic electron emission microscopy study of barrier height inhomogeneities introduced in Au/n-Si Schottky contacts by a HF pretreatment

The distribution of Schottky barrier heights over the contact area in Au/n-Si diodes was determined by ballistic electron emission microscopy. For samples on which an aqueous HF pretreatment of the Si substrate was applied, the histogram contains several high barrier Gaussian distribution components. After a short rinse, in de-ionized water or methanol, it was mainly the most important lower Gaussian component which was left. Using additional x-ray photoemission spectroscopy and atomic force microscopy measurements allowed us to propose a model, wherein negatively charged species containing F at the interface, are thought to be responsible for the high barrier Gaussian components.

Metal-induced gap states model of nonideal Au/Si Schottky barrier with low defect density

Temperature dependence of current–voltage characteristics was investigated on Au/n-Si Schottky barrier. The experimental current density can be represented by a modified equation of the thermionic emission, in which nearly the same ideality factors n Φ and n v appear in the temperature dependent exponential term of the barrier height Φ B and in that of the applied voltage V, respectively. Origin of n Φ is considered to be spatially inhomogeneous Schottky barrier height distribution. Origin of n v is considered to be applied bias dependence of the effective barrier height. A microscopic model of the inhomogeneity based on the MIGS model is proposed. Positively charged defects close to interface but outside the evanescent tail of MIGS produce local lowering of barrier height due to induced charge density, which depends on a distance of the defect from interface. The local barrier height lowering is restored by disappearance of the defect charge under forward bias. This model is applicable to interfaces of defect density lower than 10 14 cm −2, which has been considered to be necessary for the Fermi level pinning.

The origin of the integral barrier height in inhomogeneous Au/Co/GaAs67P33-Schottky contacts: A ballistic electron emission microscopy study

In this work we investigated the relationship between the integral Schottky barrier height (SBH) obtained from conventional current–voltage (I–V) measurement and the distribution of the local SBH measured by ballistic electron emission microscopy (BEEM) on a nanometer scale length. For this purpose, we investigated inhomogeneous Au/Co/GaAs67P33-Schottky contacts. The samples were prepared by the deposition of a discontinuous Co film on the semiconductor followed by the deposition of a continuous Au film. This provided regions with local presence of one or the other metal (Au or Co) at the metal-semiconductor interface, resulting in mesoscopically extended SBH inhomogeneities. The local SBH distribution as well as the integral SBH depended on the preparation parameter of the Co layer, i.e., on the combination of the substrate temperature (300 or 500 K) and the nominal Co thickness (0, 0.25, 0.5, 0.8, 1.0 nm). For the different preparation parameters, statistical distributions of the local SBH were measured by BEEM. Treating these SBH distributions in terms of a parallel conduction model for the electron transport across the MS interface, we calculated for each preparation parameter an integral SBH and compared it with the measured integral SBH obtained from conventional I–V measurement. The calculated and measured integral SBH’s were in very good agreement, demonstrating clearly the strong influence of the low SBH regions on the electron transport across the interface and therefore on the integral SBH. The SBH values for homogeneous Au/GaAs67P33- and Co/GaAs67P33-Schottky contacts, i.e., with only one sort of metal at the interface, were determined to be ΦSBAu=1180±10 meV and ΦSBCo=1030±10 meV. As with regard to the inhomogeneous Schottky contacts the fraction of area of the MS interface covered by Co increased, the local SBH distributions as well as the integral SBH’s decreased gradually from the value of ΦSBAu to ΦSBCo.

Influence of low energy ballistic electron on the transmittance properties of Au/Si interface studied by ballistic-electron-emission microscope

The electronic properties of Au/Si interface are investigated with a homemade ballistic-electron-emission microscope. The spatial distribution of Schottky barrier height is detected. Irreversible changes of ballistic transmittance of local interface with typical size of a few hundred of angstroms in diameter are observed by raising the tip voltage temporarily. Ballistic-electron-emission spectroscopy indicates that the electron transmission probabilities are reduced significantly inside the modified region, which is attributed to the Au-Si intermixing and other chemical reactions occurring at the interfacial layer stimulated by the injected energetic hot electrons.

Inhomogeneous Reaction between Epitaxial Al and Si(111) Revealed by Scanning Internal Photoemission Microscopy (IPEM)

Aluminum is one of the most important materials for the Si LSI metallization. An epitaxial growth of Al films on Si can be achieved since the lattice constant ratio between Al and Si is approximately given by 3 to 4. It is expected that epitaxial Al films are highly stable against thermal annealing. However, epitaxial-Al/Si contacts have sometimes shown a significant change in Schottky barrier height (SBH) and interface structure upon annealing. The objective of this paper is to clarify the relation between the SBH inhomogeneity and the thermally induced reaction between Al and Si through grain boundaries. The microscopic SBH distribution was measured by using scanning internal- photoemission microscopy (IPEM).Epitaxial Al layers were deposited on n-Si(l11) by electron beam evaporation at substrate temperatures of 220°C and 250°C. In the as-deposited state, the Al film fabricated at 220°C consisted mainly of “type-A”- and partially of “type-”"-orientated interfaces (referred to as a mixed phase: MP-AS), while that deposited at 250°C consisted only of “type-B” (referred to as a single phase: SP-AS). The samples were annealed for 1 hour at temperatures between 450-550°C and then quenched.

Lateral distribution of Schottky barrier height: A theoretical approach

The effect of the metal, semiconductor and interface parameters on the Schottky barrier height is analysed in order to determine whether the lateral distribution of the barrier height obeys a normal or the lognormal law. It is found that the fluctuations of some parameters result in a normal lateral distribution, while the fluctuations of the others yield a lognormal lateral distribution of the Schottky barrier height. It is shown that the only available experimental barrier height distribution may be explained by a superposition of two lognormal distributions.

Beem and UHV-TEM Studies of PtSi/Si(001)

AbstractRecent work using ballistic electron emission microscopy (BEEM) and ultra-high vacuum transmission electron microscopy (UHV-TEM) to study structure-property correlations of PtSi/Si(001) contacts is described. The spatial uniformity of the Schottky barrier height (SBH) for diodes grown at room temperature and 350°C was compared with the interface microstructure using TEM. Narrow SBH distributions with standard deviations of 7–12 meV were obtained independent of the deposition system and temperature. Analysis of the distributions compared well with standard current-voltage and capacitance-voltage measurements on the same diode. These low temperature depositions produced fine grained PtSi films (10–20 nm) with an interface roughness dependent on the cleaning process. Probable orientation effects on the SBH distribution were apparently screened out because of the low doping of the substrates that were used. As the deposition temperature is increased the PtSi tends towards polycrystalline epitaxy with island growth beginning at 700°C. Grains are rectangular and develop a large shape anisotropy of great interest for future investigations.