Determination Of Barrier Height Research Articles

Charge injection from polyaniline-poly (methylmethacrylate) blends into poly (p-phenylene vinylene)

Blends of polyaniline doped with camphorsulfonic acid, (PAni–CSA), and poly(methylmethacrylate) (PMMA) are important candidates for hole injection electrode material in organic light emitting devices. We present a quantitative analysis of the potential barrier heights at interfaces between the PAni–CSA/PMMA blend and poly-(p-phenylene vinylene) (PPV). The barrier height determination, which is of crucial importance for radiant efficiency control of organic light emitting diodes, is based on temperature dependent tunneling current measurements. The value of the [PAni–CSA/PMMA]/PPV potential barrier height is found to be independent of the PAni–CSA concentration in the blend, except near the percolation threshold, where an increase in the barrier height value is observed.

Barrier height determination for n-type 4H-SiC schottky contacts made using various metals

We have studied Schottky barrier contacts to n-type 4H-SiC with Cr, Mo, Ta, W, Au, and Ni. We have focused on effects of the metal work function, measurement technique and interface behavior on the Schottky barrier heights (SBHs). The contacts were prepared by metal deposition via high frequency cathodic sputtering on chemical vapor epitaxially grown epitaxial layers with low residual doping (2 × 1015 cm−3). Prior to deposition on Si-terminated surfaces, they were in-situ cleaned by Ar-ion sputtering for 5 and 10 min, respectively. The contacts have been characterized by means of current-voltage, capacitance-voltage (C-V), and internal photoemission (IPE) methods at room temperature. With deep level transient spectroscopy, interface deep electron traps have been detected with thermal ionization energies of 0.68, 0.77, and 1.04 eV, respectively. These traps have been attributed to structural defects formed during epitaxial growth termination. The diodes have relatively low reverse leakage current, but the ideality factor is larger than one. The SBHs have been determined from C-V and IPE measurements. It has been shown that the C-V data may contain errors resulting in a low SBH if electron deep traps are present in the interface region. In general, the SBH of various metals are influenced by the metal work function and also by the semiconductor surface preparation. The interface homogeneity is an important characteristic of the Schottky contacts, which may be improved by an optimized ion sputtering prior to metal deposition. We have considered the SBHs determined by IPE measurements as most reliable.

Determination of interface state density of PtSi/strained-Si1-xGex/Si Schottky diodes

PtSi/p-strained-Si1-xGex (x=0.19 and x=0.29) Schottky barrier diodes have been fabricated and characterized for the determination of barrier height, ideality factor and the interface state density distribution with energy. Diodes having an epitaxial layer thickness of 20 and 52 nm were modeled with emphasis on comparison with experiment. To achieve better agreement with experimental data, an interfacial layer and associated series resistance were included in the model. The capacitance–voltage (C–V) technique has been used for the determination of the energy distribution of interface state density. The interface state density distribution for the Si0.81Ge0.19 and Si0.71Ge0.29 diodes is found to decrease with increasing energy from the valence band edge.

Sputter deposition and characterization of TiB 2/TiSi 2 bilayer contact structure

A TiB 2/TiSi 2 bilayer was obtained by co-sputter deposition of TiB 2/Ti on n-type Si (100) and by vacuum annealing. Two specimens with 25 nm and 50 nm as deposited Ti were prepared. Only the specimen having 50 nm Ti resulted after where the sublayer annealing in the TiB 2/TiSi 2 structure. Specimens having 25 nm Ti gave a structure of TiB 2/(TiSi), contained Ti 5Si 3 in addition to TiSi 2 embedded in an amorphous layer. The lowest sheet resistance was obtained on specimens, having 50 nm as deposited Ti, after 800°C annealing which resulted in the TiB 2/TiSi 2 diffusion barrier configuration. This structure was then exposed to Cu metallization and the barrier properties against Cu were studied. The structure of the diffusion barrier and its properties were evaluated by X-ray diffraction (XRD) and high resolution cross-sectional TEM (XTEM). Barrier height determination was performed only on specimens with 25 nm as deposited Ti from current-voltage measurements. The calculated values of Φ B, the barrier height, are in the range of 0.58 – 0.66 V.

Temperature dependent tunnelling current at metal/polymer interfaces—potential barrier height determination

It has been demonstrated that the expression for the temperature dependent tunneling current through a triangular potential barrier at metal/polymer interfaces fits experimental data including the deviation from a straight line observed in Fowler–Nordheim plots. The fitting parameters can be used to calculate the potential barrier height at the metal/polymer interface, which is of crucial importance for the control of charge injection in organic light emitting diodes. The tunneling current density expression reproduces the observed dependence of the current with temperature.

Self-energy of a charged conducting droplet as an inversion problem

The determination of fission barrier heights of charged clusters of alkali atoms requires the calculation of the electrostatic self-energy of arbitrarily shaped, conducting droplets. For axially symmetric shapes we propose to determine a charge distribution on the axis giving rise to the same electric field outside the droplet as the actual surface charge. This leads to an ill-posed inversion problem. We discuss its regularization and demonstrate the usefulness of the method in some examples.

Microscopic and macroscopic investigation of electrical contacts to n type and p type ZnSe

Surface preparation methods were established using wet chemical etching of ZnSe epilayers and the stoichiometry of the resulting surfaces was studied using X-ray photoelectron spectroscopy. It is found that the stoichiometry of the surface is very sensitive to the chemical treatment and surfaces can be produced that are rich in either Se or Zn. Electrical contacts were fabricated by vacuum evaporation of Ag, Sb, or Au on to nearly stoichiometric surfaces. Electrical properties were studied using the current–voltage technique and it has been found that the Schottky barrier heights measured vary from contact to contact, although the processing and metal used are identical. Discrete barrier heights of 1·67, 1·48, 1·20, and 0·90 eV are observed for metal/n-ZnSe contacts using the three metals studied. The first two barrier heights, 1·67 and 1·48 eV, are identical to those reported in the literature. Aging effects of current–voltage characteristics were also monitored and it was observed that the barrier height varies with time, showing one of the four values mentioned above. Ballistic electron emission microscopy carried out on 1·67 eV barrier contacts confirms the accuracy of the current-voltage barrier height determinations. Electrical properties of metal/p-ZnSe contacts are also presented and their aging effects discussed.MST/3314

Determination of Barrier Height and Closed-Form Potential for the B 0+(3Π) State in BrCl

The potential energy curve for the B 0+(3Π) state in BrCl exhibits a barrier which has been attributed to the effects of an avoided crossing with another 0+ electronic state that correlates with two ground state atoms. Existing data for ν levels 2-8 in 79Br35Cl and 81Br35Cl are used to obtain closed-form representations of the B-state potential that are reliable to ∼ 0.01 cm−1. When the data for both isotopomers are fitted simultaneously, the variance rises by a factor of 2, providing an estimate of the extent of Born-Oppenheimer breakdown in this state. From among several satisfactory closed-form potentials, there are some which approximately reproduce the observed breaking off of the high-J lines in the 0-8 band of the E → B transition near 3290 Å. This tunneling resonance spectrum is modeled using bound-free spectral calculations, from which the B-state barrier is located 18 520 ± 10 cm−1 above the minimum of the ground state and 275 cm−1 above the first dissociation limit.

A method for the determination of barrier heights from the capacitance-voltage characteristics of a Schottky junction containing bulk deep traps

In Schottky barrier diodes, appreciable concentrations of deep levels in the space charge region influence high frequency capacitance measurements. The barrier height, calculated by the usual analysis of 1 C 2 vs V plot, exhibits large steps at temperatures corresponding to the relaxation of the traps occurring during the voltage sweeps. In this work we develop a model which takes into account the partial ionization of the deep levels, to determine the correct parameters of the Schottky barrier by temperature dependent capacitance-voltage measurements. The influence of two kinds of bulk traps, already detected by DLTS, is clearly evidenced in the 1 C 2 vs V intercept and taken into account for the calculation of the true barrier height and trap concentrations. The correlation coefficient of the linear regression is very significantly improved if this model is used. The analysis is applied to Ti GaAs Schottky diode prepared by ion beam sputtering.

A new technique for the determination of barrier height of Schottky barrier diodes

A new technique for the determination of barrier height of Schottky barrier diodes

Schottky barrier heights of InxAl1−xAs (0≤x≤0.35) epilayers on GaAs

The electrical characteristics of Al Schottky diodes on n-type InxAl1−xAs (0≤x≤0.35) were investigated in detail by current-voltage and capacitance-voltage measurements. These high-quality InAlAs epilayers were grown on GaAs using step-graded buffers under proper growth conditions. It was found that the Schottky barrier height of the epilayers increases with Al content as opposed to what was predicted previously. The effect of the interfacial oxide layer on the determination of Schottky barrier height is also presented.

On the current mechanism in reverse-biased amorphous-silicon Schottky contacts. I. Zero bias barrier heights and current transport mechanism

A study of the zero-bias barrier heights of hydrogenated amorphous-silicon-based Schottky diodes and the prevailing current transport mechanisms in these structures is made using electrical and electro-optical techniques. Several series of devices were made using Cr, Mo, W, and Pt as Schottky metals. The current-voltage characteristics of the devices were obtained with their temperature dependence. The barrier heights were determined independently using internal photoemission experiments at three temperatures between 270 and 380 K. In devices where the Schottky barrier is deposited on top of the semiconductor material, the saturation current density is found to be most likely determined by combined drift and diffusion of the carriers. In devices where the Schottky barrier is formed at the bottom of the diode, the transport mechanism tends towards thermionic (field) emission, but only slight effects of the prevailing transport mechanism on the electrical performance of these diodes were observed. Also, in these devices a relatively defect-rich a-Si:H layer at the bottom (Schottky) contact could be detected opto-electronically, which prohibited barrier height determination using internal photoemission. For the influence of the metal work function φm on the electron-barrier height φb we obtain φb=(0.10±0.02)φm+(0.48±0.12), which is in good agreement with a previous determination of the dependence of the barrier for holes on φm.

A study of hydrogen abstraction reactions by halogen atoms with HFCO and HClCO: determination of transition state structures, barrier heights, and vibrational frequencies

Theoretical barriers and reaction energetics were determined using ab initio calculations at the PMP4/6-311 + +G** level with UMP2/6-311G** optimized geometries for hydrogen abstraction reactions of HFCO and HClCO by fluorine and chlorine atoms. The largest barriers are predicted for fluorine and chlorine atom abstractions of hydrogen from HFCO. Fluorine and chlorine atom abstractions of hydrogen from HClCO are predicted to be barrierless. The predicted barrier for fluorine atom abstraction of hydrogen from HFCO is calculated to be 1.3 kcal mol−1 and is in good agreement with the experimental value of 1.8 ± 0.4 kcal mol−1.

Electrical Properties of Copper Silicide/Silicon Interfaces

ABSTRACTThe electrical properties of Cu/Si(100) and Cu3Si/Si(100) interfaces have been studied using both n- and p-type silicon samples. Current-voltage and capacitance-voltage measurements were performed in the temperature range 80-295 K in order to monitor Schottky barrier formation and electrical carrier concentration profiles. Deep-level transient spectroscopy was employed to observe Cu-related energy levels in the forbidden band gap of Si, and different ion beam analysis techniques were applied to study the interfacial reaction between Cu and Si. Emphasis is put on determination of Schottky barrier heights and their variation with temperature, dopant passivation by Cu atoms and interaction of Cu with irradiation-induced point defects in silicon.

Determination of Barrier Height and Doping Density of a Schottky Diode from Infrared Photoresponse Measurements

The impurity doping concentration of a semiconductor is commonly determined by measuring the C-V profile of a Schottky diode. In this work, an alternative method is utilised to determine the impurity doping density of a moderately acceptor-doped Schottky diode using infrared photoelectric measurements. Due to the image lowering effect, the barrier is lowered with the increasing field or reverse bias. Having determined the relationship between the reverse bias and the lowered barrier, the doping density and the zero bias barrier eight from the infrared photo response measurements can be accurately determined.

Analysis of I– V measurements on PtSi-Si Schottky structures in a wide temperature range

I– V Measurements on PtSi-Si Schottky structures in a wide temperature range from 90 to 350 K were carried out. The contributions of thermionic-emission current and various other current-transport mechanisms were assumed when evaluating the Schottky barrier height Φ 0. Thus the generation-recombination, tunneling and leak currents caused by inhomogeneities and defects at the metal-semiconductor interface were taken into account. Taking the above-mentioned mechanisms and their temperature dependence into consideration in the Schottky diode model, an outstanding agreement between theory and experiment was achieved in a wide temperature range. Excluding the secondary current-transport mechanisms from the total current, a more exact value of the thermionic-emission saturation current I te and thus a more accurate value of Φ b was reached. The barrier height Φ b and the modified Richardson constant A ∗∗ were calculated from the plot of thermionic-emission saturation current I te as a function of temperature too. The proposed method of finding Φ b is independent of the exact values of the metal-semiconductor contact area A and of the modified Richardson constant A ∗∗ . This fact can be used for determination of Φ b in new Schottky structures based on multicomponent semiconductor materials. Using the experimentally evaluated value A ∗∗ = 1.796 × 10 6 Am −2 K −2 for the barrier height determination from I– V characteristics the value of Φ b = 0.881 ± 0.002 eV was reached independent of temperature. The more exact value of barrier height Φ b is a relevant input parameter for Schottky diode computer-aided modeling and simulation, which provided a closer correlation between the experimental and theoretical characteristics.

Translational energy study of CH 3 photofragments following ( n, π*) excitation of acetone

Excitation of a molecular beam of acetone at 266 nm leads to methyl photofragments that have been examined by resonant multiphoton ionization/time-of-flight mass spectrometry. Analysis of the arrival time profile of CH 3 + reveals a broad translational energy distribution for vibrationless methyl photofragments, with mean energy E T = 45.8 ± 4.0 kJ mol −1. The CH 3 0 0 photofragments are ejected isotropically in space. The mean photofragment recoil energy for two-body dissociation agrees with a recent determination of the acetone dissociation barrier height. Results are discussed in the context of related experiments on α-cleavage dynamics of acetic acid.

Electrical Study of Metal/Gaas Interfaces

We have carried out a systematic study of the electrical properties of Schottky barriers formed on atomically-clean and contaminated n-type and p-type GaAs surfaces[1-11]. Diodes were fabricated by in-situ deposition on clean GaAs (110) surfaces prepared by cleavage in ultrahigh vacuum and on contaminated surfaces prepared by cleavage and exposure to the atmosphere[1-4]. The consistent and reproducible barrier height determinations from the electrical measurements of unannealed and annealed diodes, when combined with results of transmission electron microscopy (TEM)[5,6] and surface sensitive studies on identically prepared samples[7,8], are found to be a particularly critical test of models of Schottky barrier formation. A strong correlation between annealing-induced changes in the Schottky barrier height and the stoichiometry of the near interfacial GaAs is found.

Aging of Schottky diodes formed on air-exposed and atomically clean GaAs surfaces: An electrical study

In this report we show that the application of a large voltage and current stress can significantly change the barrier height of a given metal (Ag,Al,Au,Pd,Cr)/GaAs(110) contact. These changes of barrier height depend on the metal used, the method of fabrication of the diode (air exposed or UHV cleaved), and the intensity and direction of the potential and current during the electrical aging. In particular, the air-exposed Ag diodes exhibit the largest change in the barrier height (∼85 meV) upon aging, while the UHV-cleaved Ag diodes do not exhibit a significant change. In the case of Au, both UHV-cleaved and air-exposed diodes show a small change (∼20 meV). The barrier heights of Pd, Al, and Cr air-exposed diodes do not exhibit an appreciable change upon aging (i.e., <10 meV). The changes in barrier height are found to have a very consistent and characteristic logarithmic dependence upon time. Once the electrical aging is stopped, the barrier height is found to recover to near the unaged value. The long time constant of the aging process, the ability of the barrier height to recover after aging, the long time constant of the recovery process, and the acceleration of the recovery process by illumination suggest that the changes in the barrier height which occur upon electrical aging are due to the creation and/or annihilation of deep level traps near the interface. We also report a systematic study of a comparison of barrier height determinations for Ag, Al, Au, Cr, and Pd diodes formed on air-exposed and UHV-cleaved GaAs(110) surfaces.

Simultaneous determination of desorption parameters and barrier heights for release of previously absorbed tracer amounts of cesium and potassium from a platinum sample

Neutral and ionic desorption of cesium and potassium from a polycrystalline platinum surface was investigated with a field reversal apparatus in the temperature interval 1600–1900 K under UHV conditions. The desorbing atoms and ions were released from previously absorbed tracer amounts of alkali from a molecular beam. The Arrhenius desorption parameters (k = Aexp(− E/KBT)) found are E+ = 1.80 eV. E0 = 3.27eV, A+ = 4.9 × 1011s−1 and A0 = s−1 for cesium and E+ = 2.47 eV, E0 = 3.65 eV, A+ = 3.2 × 1013s−1 and A0 = 5.8 × 1013s−1 forpotassium. The subscripts + and 0 indicate ionic and neutral desorption respectively. A rate limiting step in the diffusion out to the surface was observed. The rate limiting barriers found were Eout = 6.68 eV for cesium and Eout = 6.87 eV for potassium.