Low Barrier Regions Research Articles

Analysis of temperature-dependent forward and leakage conduction mechanisms in organic thin film heterojunction diode with fluorine-based PCBM blend

The forward and reversed biased current–voltage behaviors of the organic diode were detailed in a wide range of temperatures. In this diode, a donor–acceptor-conjugated copolymer system was constructed with poly((9,9-dioctylfluorene)-2,7-diyl-(2-dodecyl-benzo[1,2,3]triazole)) as a partner of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). Two-order of magnitude rectification ratio was achieved, and the temperature-dependent values of saturation current, zero-bias barrier height, and ideality factor were extracted according to the thermionic emission model. The temperature responses of these diode parameters showed an existence of inhomogeneity in the barrier height formation. As a result, the observed non-ideal behavior was explained by Gaussian distribution of barrier height where low-barrier regions are effective in the forward biased conduction mechanism at low temperatures. Together with this analysis, series resistances were evaluated using Cheung’s functions and also density of interface states were investigated. On the other hand, reverse biased current flow was found under the dominant effect of Poole–Frenkel effects associated with these interfacial traps. The reverse current conduction mechanism was detailed by calculating characteristic field-lowering coefficients and barrier height values in the emission process from the trapped state in the range of temperatures of interest.

Radiation resistance of nano-crystalline iron: Coupling of the fundamental segregation process and the annihilation of interstitials and vacancies near the grain boundaries

Finding novel nuclear materials with high radiation resistance is very important for the nuclear industry and requires the understanding of the self-healing of radiation damage in such novel materials as nano-crystalline iron. Combining molecular dynamics simulations, molecular statics calculations and the object kinetic Monte Carlo method, we found that the self-healing capability of nano-crystalline iron is closely related to the coupling of the individual fundamental segregation and annihilation processes of vacancies and interstitials near the grain boundary (GB). Statically, both near the GB and at the GB, a low-energy-barrier/barrier-free region forms around the interstitial which promotes the annihilation of vacancies. The annihilation process was found to always involve the collective motion of multiple atoms due to the recovery of the strained atoms around the interstitial. Dynamically, the annihilation involves two coupled processes. Before segregating into the GB, the interstitial annihilates lots of vacancies near the GB as it diffuses near the GB together with the low-barrier region. In addition, although the interstitial is tightly bound to the GB after segregation, it efficiently removes the vacancies near the GB while moving along the GB, with the low-barrier region extending into the neighborhood of the GB and even into the grain interior. These two mechanisms were found to work at low temperatures, even temperatures where the vacancy was immobile. This study revealed the interaction of the major radiation defects at different scales and thereby uncovered the origin of the high radiation resistance of nano-crystalline iron.

Electrical Characteristics of an Ag/n-InP Schottky Diode Based on Temperature-Dependent Current–Voltage and Capacitance–Voltage Measurements

The rectifying junction properties of an Ag/n-InP Schottky diode are investigated in a wide temperature range from 10 K to 300 K (−263 °C to 27 °C). The electronic structure of the junction is analyzed by the techniques of current–voltage I–V and capacitance–voltage C–V measurement as a function of temperature. The electrical parameters are characterized with the standard thermionic emission theory. The main electrical characteristics including the values of apparent barrier height $$ \phi_{\text{e}} $$ and ideality factor n are found to be 0.414 eV and 1.008 at 300 K (27 °C), respectively, even though the value of barrier height $$ \phi_{\text{b}} $$ at 300 K (27 °C) from C–V data is 0.417 eV. The $$ \phi_{\text{e}} $$ , n, and Richardson plot demonstrate strong temperature dependency; that is, the $$ \phi_{\text{e}} $$ decreases, n increases, and the Richardson plot deviates with decreasing temperature. Such behaviors are attributed to Schottky barrier anomalies, which are explained by assuming the existence of a Gaussian distribution of nanometer-sized patches with low barrier height at the interface. The accurate theoretical models such as Tung’s lateral inhomogeneity and multi-Gaussian distribution to comment the barrier inhomogeneity on the electron transport across the interface are applied, and the comparisons between these approaches for the present experimental results are carried out. According to the multi-Gaussian distribution approach, the double-Gaussian nature of Ag/n-InP/In is commented by the values of the weighting coefficients, standard deviations, and mean barrier height calculated for each distribution. The total effective area of the patches $$ NA_{\text{e}} $$ is calculated for high and low temperatures, and as a result, it is found that the low barrier regions influence significantly the electron transport at the interface of the junction. The discrepancy between I–V and C–V barrier heights is discussed based on a Gaussian approach. From the linear relationship between $$ \phi_{\text{e}} $$ and n, the homogeneous barrier height $$ \phi_{\text{b}} $$ is noted to be 0.418 eV. The values of $$ A^{*} $$ (effective Richardson constant) and $$ \phi_{\text{b}} $$ are determined from classic modified Richardson plot as $$ A^{*} $$ : 8.08 A cm−2 K−2 and $$ \bar{\phi }_{\text{b}} $$ : 0.416 eV and from Tung’s model as $$ A^{*} $$ : 9.35 A cm−2 K−2 and $$ \mathop {\phi_{\text{b}} }\limits^{\_} $$ : 0.418 eV, which demonstrates an excellent agreement with the theoretical value (9.4 A cm−2 K−2) of n-InP. As a result, in order to obtain the more reliable values of $$ A^{*} $$ and $$ \mathop {\phi_{\text{b}} }\limits^{\_} $$ , it could be reported that Tung’s lateral inhomogeneity approach is more meaningful; taking into account the effective patch area, which is significantly lower than the whole geometric area of the diode.

Temperature dependence of current-and capacitance–voltage characteristics of an Au/4H-SiC Schottky diode

In this study, the current–voltage (I–V) and capacitance–voltage (C–V) measurements of an Au/4H-SiC Schottky diode are characterized as a function of the temperature in 50–300K temperature range. The experimental parameters such as ideality factor and apparent barrier height presents to be strongly temperature dependent, that is, the ideality factor increases and the apparent barrier height decreases with decreasing temperature, whereas the barrier height values increase with the temperature for C–V data. Likewise, the Richardson plot deviates at low temperatures. These anomaly behaviors observed for Au/4H-SiC are attributed to Schottky barrier inhomogeneities. The barrier anomaly which relates to interface of Au/4H-SiC is also confirmed by the C–V measurements versus the frequency measured in 300K and it is interpreted by both Tung’s lateral inhomogeneity model and multi-Gaussian distribution approach. The values of the weighting coefficients, standard deviations and mean barrier height are calculated for each distribution region of Au/4H-SiC using the multi-Gaussian distribution approach. In addition, the total effective area of the patches NAe is obtained at separate temperatures and as a result, it is expressed that the low barrier regions influence meaningfully to the current transport at the junction. The homogeneous barrier height value is calculated from the correlation between the ideality factor and barrier height and it is noted that the values of standard deviation from ideality factor versus q/3kT curve are in close agreement with the values obtained from the barrier height versus q/2kT variation. As a result, it can be concluded that the temperature dependent electrical characteristics of Au/4H-SiC can be successfully commented on the basis of the thermionic emission theory with both models.

Effect of KOH treatment on the Schottky barrier inhomogeneity in Ni/n-GaN

Abstract The effect of KOH treatment on the Schottky barrier inhomogeneity in Ni/n-GaN Schottky diodes was investigated. It was observed that both the barrier heights and ideality factors varied from diode to diode with a linear relationship between barrier height and ideality factor, indicating the presence of a lateral inhomogeneity in the Schottky barrier. Simple extrapolation of the straight line obtained from the linear fitting to the barrier height versus ideality factor plot to the image-force controlled ideality factor produced the lateral homogeneous barrier heights, which were higher than those from current–voltage measurements. Furthermore, Gaussian fitting to the distribution of barrier heights exhibited the increased barrier height with the smaller standard deviation after KOH treatment, implying the improved barrier homogeneity. A possible explanation for this behavior can be an improvement of the Ni/n-GaN interface intrinsic properties, e.g., through a reduction of the surface states acting as low barrier region.

Inhomogeneities in Ni∕4H-SiC Schottky barriers: Localized Fermi-level pinning by defect states

We investigated arrays of Ni, Pt, or Ti Schottky diodes on n-type 4H-SiC epitaxial layers using current-voltage (I-V) measurements, electron beam induced current (EBIC), polarized light microscopy, x-ray topography, and depth-resolved cathodoluminescence spectroscopy. A significant percentage of diodes (∼7%–30% depending on epitaxial growth method and diode size) displayed “nonideal” or inhomogeneous barrier height characteristics. We used a thermionic emission model based on two parallel diodes to determine the barrier heights and ideality factors of high- and low-barrier regions within individual nonideal diodes. Whereas high-barrier barrier heights increased with metal work function, low-barrier barrier heights remained constant at ∼0.60, 0.85, and 1.05eV. The sources of these nonidealities were investigated with a variety of spectroscopic and imaging techniques to determine the nature and energy levels of the defects. EBIC indicated that clusters of defects occurred in all inhomogeneous diodes. Cathodoluminescence spectra revealed additional peaks in the nonideal diodes at 2.65, 2.40, and 2.20eV, which complement the low-barrier barrier heights. It is proposed that defect clusters act to locally pin the Fermi level, creating localized low-barrier patches, which account for the inhomogeneous electrical characteristics.

Determination of the characteristic parameters of Sn/n-GaAs/Al–Ge Schottky diodes by a barrier height inhomogeneity model

A study on parameters of the Sn/n-GaAs Schottky barrier diode (SBD) fabricated on an n-type GaAs substrate has been made. The Sn/n-GaAs SBD has shown a nearly ideal behaviour with ideality factor and barrier height (BH) values of 1.081 and 0.642 eV, respectively, from the experimental forward-bias current–voltage (I–V) characteristics. A BH value of 0.724 eV has been obtained from the experimental reverse-bias capacitance–voltage (C–V) characteristics. An accurate theoretical modelling of the effect of the presence of inhomogeneities on the electron transport across the metal–semiconductor interface has been applied. This model attempts to explain abnormal experimental results obtained on ‘real’ Schottky diodes. Our results clearly demonstrate that the electron transport at the metal–semiconductor interface is significantly affected by low-barrier regions (patches). When the experimental data are described by the thermionic emission theory of inhomogeneous Schottky contacts, it has been concluded that both the experimental forward and reverse I–V characteristics and the difference between the values of the experimental I–V and C–V SBHs should be considered. An experimental BH difference of Δ = 0.082 V has been obtained for the Sn/n-GaAs SBD that is less than the critical value; therefore, it has been seen that the potential in front of the patch is not pinched off.

Barrier inhomogeneities and electrical characteristics of Ni/Ti bilayer Schottky contacts on 4H–SiC after high temperature treatments

AbstractThe analysis of “nonideal” behaviour in current–voltage characteristics of fabricated Schottky diodes on 4H–SiC is carried out. An accurate theoretical modelling of the effect of the presence of inhomogeneities on the electron transport across the metal‐semiconductor interface is applied. This model trends to explain a large series of abnormal experimental results obtained on “real” Schottky diodes. Our results clearly demonstrate that the electron transport at the metal–semiconductor interface is significantly affected by low barrier regions. It is suggested that the commonly observed larger‐than‐unity ideality factors, underestimation of the Richardson constant and Schottky Barrier Height correlation with ideality factor can be attributed to an effective area involved in the current transport which may be significantly lower than the geometric area of the diode contact. It is also proved by comparing different types of Schottky diodes that in order to properly characterise the interface uniformity, not only the absolute value of the ideality factor is important but also its temperature dependence and its correlation with barrier height have to be taken into account. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electrical Characterization of Inhomogeneous Ni<sub>2</sub>/Si/SiC Schottky Contacts

Abstract. The electrical characterization of nickel silicide (Ni 2 Si) Schottky contacts on 4H-SiC is reported in this work. In spite of the nearly ideal behaviour observed at room temperature ( n =1.05), a deviation from the ideality was observed at lower temperatures, thus suggesting that an inhomogeneous Schottky barrier has actually formed. The experimental results were described by the Tung’s model on inhomogeneous Schottky barriers, which considers low barrier regions embedded in a uniform high barrier Ni 2 Si contact. The inhomogeneity of the barrier is responsible of the commonly observed discrepancy between the experimental values of the Richardson’s constant A ** (i.e. 2.6 A/cm 2 K 2 in our contacts) from its theoretical value of 146 A/cm 2 K 2 in 4H-SiC. Introduction Silicon carbide Schottky diodes are very attractive devices because of their capability to reach high switching frequencies under high voltage operation and with a low on-resistance when compared to analogous silicon devices [1]. In spite of the progresses already achieved in the technology of SiC Schottky diodes, obtaining a barrier with nearly ideal electrical behaviour may represent an outstanding problem. In fact, the control of the surface preparation before metal deposition affects the uniformity of the Schottky barrier height Φ

Current transport at the p-InP|poly(pyrrole) interface

The interface between the inorganic semiconductor p-type InP and the conjugated polymer poly(pyrrole) exhibits the electrical characteristics of a Schottky diode. Capacitance–voltage measurements yield an average barrier height of 0.62 ± 0.01 eV at temperature T =298 K. At the same temperature, the empirical quality factor, extracted from current–voltage measurements, is near unity. However, the current–voltage measurements show a deviation from thermionic emission theory as the temperature is reduced, as witnessed by the increase of the quality factor and the curvature in the Richardson plot. Such deviation is best explained by the barrier inhomogeneity model, in which the barrier becomes voltage dependent due to the interaction of a small low-barrier region with a higher surrounding potential, termed the “pinch-off” effect. Traditional current–voltage models, including image force lowering or an interfacial layer, cannot predict the temperature dependence of the current–voltage data, although thermionic field emission may facilitate current transport in the interfaces with a higher doped InP substrate. Furthermore, the probability of sufficiently energetic incident charge carriers crossing the interface, termed the transmission coefficient, is smaller than that observed in metal Schottky diodes.

Electron transport at Au/InP interface with nanoscopic exclusions

We present an investigation of electron transport at the Au/InP metal semiconductor (MS) interface in the presence of nanoscopic barrier inhomogeneities. In particular, we focus on the transport regime wherein the low barrier inhomogeneous regions are pinched off by the depletion potential of the surrounding higher barrier region. To realize this, we have fabricated a composite MS structure comprising of a known density of nanometer sized Ag aerosol particles on InP overlayed by a uniform Au film. Temperature dependent current–voltage (I–V) measurements were performed to characterize the electron transport at the composite MS interface. The experimental observations are explained using a simple model for the MS junction current in which the Ag/InP regions are assumed to be identical and noninteracting. Our results clearly demonstrate that the electron transport at the MS interface is significantly affected by low barrier regions even in the pinch-off regime. In addition, the influence of the particle density and the Ag/InP barrier heights on the diode characteristics is also investigated. It is suggested that Schottky barrier inhomogeneities could be the main source of the usually observed larger-than-unity ideality factors in diodes. Furthermore, our results indicate possibilities of using such composites to explore the physics and applications of nanoinjecting contacts.

Schottky barrier height measurements of type-A and type-B NiSi2 epilayers on Si

Schottky barrier heights of single-crystal type-A and type-B NiSi2 epilayers on nondegenerate n-(111) Si have been measured by photoresponse and forward I–V methods. High-quality molecular beam epitaxy grown NiSi2 layers of thicknesses ranging from 70 to 600 Å on sputter-cleaned, P-doped Si subtrates (∼1.5×1015 cm−3) were studied. The type-A and type-B orientations consistently yield photoresponse barrier heights which differ by greater than 0.1 eV. We observe the value φBn=0.62±0.01 eV for all type-A structures from both photoresponse and I–V measurements. However, we obtain a discrepancy between barrier heights measured by I–V (φBn=0.69±0.01 eV) and photoresponse (φBn=0.77±0.05 eV) methods, and in addition consistently observe an unusual bowing of the type-B photoresponse curves at low photon energies. We show that both the detailed shape of the type-B photoresponse curves and the discrepancy between I–V and photoresponse-measured barrier heights can be accounted for by modeling the type-B barrier as a mixture of high and low barrier regions. Quantitative agreement with experiment is obtained for the values φhi =0.81±0.01 eV and φlo 0.64±0.01 eV, with effective fractional area coverages of 91% and 9% for high- and low-barrier regions, respectively.

Size dependence of ’’effective’’ barrier heights of mixed-phase contacts

The literature currently abounds with experimental studies of Schottky barrier heights of various metals upon many semiconductors. Unfortunately, these studies present some puzzling aspects: (1) Commonly, barriers determined by C–V studies are larger than barriers determined by I–V studies, and (2) Results obtained by different workers under apparently identical conditions are not always similar. A possible explanation for such effects is simply that many/most contacts experimentally achieved are in fact multiphase; these different barrier-height regions could result from variations in the metallurgical reactions assumed by many current models of Schottky barrier energetics. The different barrier heights measured by different techniques follow directly from the functional form of the relevant probes (e.g., I–V would more heavily weight a low-barrier region). The lack of reproducibility would follow from kinetic aspects of the relevant metallurgical interactions. A recent publication discusses the functional form for I–V and C–V ’’effective’’ barrier heights from mixed-phase contacts isolated from one another. These results apply directly to mixed-phase contacts only if the linear dimensions of all contact regions are large compared to the Debye length of the substrate (≊0.1 μ for 1015 silicon). In this paper, we examine the effects of contact dimensions upon equilibrium potentials (e.g., band bending) as well as transport studies to infer ’’effective’’ barrier heights for truly mixed-phase contacts of varying dimensions but fixed area ratios.