Effect of high current density on the admittance response of interface states in ultrathin MIS tunnel junctions.

International audienceThe effect of a high current density on the measured admittance of ultrathin Metal-Insulator-Semiconductor (MIS) tunnel junctions is investigated to obtain a reliable energy distribution of the density, D-S(E), of defects localized at the semiconductor interface. The behavior of admittance Y(V, T, omega) and current density J(V, T) characteristics is illustrated by rectifying Hg//C12H25-Si junctions incorporating n-alkyl molecular layers (1.45 nm thick) covalently bonded to n-type Si(111). Modeling the forward bias admittance of a nonequilibrium tunnel junction reveals several regimes which can be observed either in C(omega approximate to 0) vs. (J) plots of the low frequency capacitance over six decades in current or in M ''(omega) plots of the electrical modulus over eight decades in frequency. At low current density, the response of interface states above mid-gap is unaffected and a good agreement is found between the interface states densities derived from the modeling of device response time tau(R)(V) and from the low-high frequency capacitance method valid for thick MIS devices; the low defect density near mid-gap (D-S 1 mA cm(-2)), the admittance depends strongly on both the density of localized states and the dc current density, so that the excess capacitance method overestimates D-S. For very high current densities > 10 mA cm(-2)), the observation of a linear C(omega approximate to 0) vs. (J) dependence could indicate some Fermi level pinning in a high interface density of states located near the Si conduction band. The temperature-independent excess capacitance C(omega approximate to 0) - C(1 MHz) observed at very small J, not predicted by the admittance model, is attributed to some dipolar relaxation in the molecular junction

Barrier height distribution and dipolar relaxation in metal-insulator-semiconductor junctions with molecular insulator: Ageing effects

International audienceElectrical transport through molecular monolayers being very sensitive to disorder effects, admittance and current density characteristics of Hg // C12H25 – n Si junctions incorporating covalently bonded n-alkyl molecular layers, were investigated at low temperature (150-300 K), in the as-grafted state and after ageing at the ambient. This comparison reveals local oxidation effects both at the submicron scale in the effective barrier height distribution and at the molecular scale in the dipolar relaxation. In the bias range dominated by thermionic emission and modified by the tunnel barrier attenuation, , where is the thickness of the molecular tunnel barrier and is the inverse attenuation length at zero applied bias, some excess current is attributed to a distribution of low barrier height patches. Complementary methods are used to analyze the current density J(V, T) characteristics of Metal-Insulator-Semiconductor tunnel diodes. Assuming a Gaussian distribution of barrier heights centered at provides an analytical expression of the effective barrier height, ; this allows fitting of the distribution standard deviation and tunnel parameter over a wide temperature range. In a more realistic modeling including the voltage dependence of barrier height and circular patch area, the so-called “pinch-off” effect is described by a distribution of parameter which combines interface potential modulation and patch area variations. An arbitrary distribution of values, fitted to low-temperature J(V) data, is equally well described by Gaussian or exponential functions. Ageing in air also increases the interface oxidation of Si substrate and affects the density of localized states near mid gap, which typically rises to the high 1011 eV-1.cm-2 range, as compared with < 1011 eV-1.cm-2 in the as-grafted state. The bias-independent relaxation observed near 1 kHz at low temperature may be attributed either to dipoles in the alkyl chain induced by the strong permanent dipoles of interface silicon oxide or to a local relaxation of water molecules trapped at the OML / silicon interface. The respective roles of SiO2 formation and water physisorption on the decrease of patch barrier height are also discussed

Temperature dependence of current density and admittance in metal-insulator-semiconductor junctions with molecular insulator

International audienceElectrical transport in ultrathin Metal-insulator-semiconductor (MIS) tunnel junctions is analyzed using the temperature dependence of current density and admittance characteristics, as illustrated by Hg//C12H25 - n Si junctions incorporating n-alkyl molecular layers (1.45 nm thick) covalently bonded to Si(111). The voltage partition is obtained from J(V, T) characteristics, over eight decades in current. In the low forward bias regime (0.2-0.4 V) governed by thermionic emission, the observed linear T-dependence of the effective barrier height, qΦEFF(T) = qΦB+(kT)β0dT, provides the tunnel barrier attenuation, exp(-β0dT), with β0= 0.93 Å−1 and the thermionic emission barrier height, ΦB = 0.53 eV. In the high-forward-bias regime (0.5-2.0 V), the bias dependence of the tunnel barrier transparency, approximated by a modified Simmons model for a rectangular tunnel barrier, provides the tunnel barrier height, ΦT = 0.5 eV; the fitted prefactor value, G0 = 10−10 Ω−1, is four decades smaller than the theoretical Simmons prefactor for MIM structures. The density distribution of defects localized at the C12H25 - n Si interface is deduced from admittance data (low-high frequency method) and from a simulation of the response time τR(V) using Gomila's model for a non equilibrium tunnel junction. The low density of electrically active defects near mid-gap (DS < 2 × 1011 eV−1.cm−2) indicates a good passivation of dangling bonds at the dodecyl - n Si (111) interface

Étude des propriétés électroniques des monocouches moléculaires greffées sur des substrats nSi(111)

Le transport électrique dans les jonctions tunnel minces de type MIS a été analysé en utilisant la dépendance en température du courant et de l admittance en fonction de la tension sur des jonctions Hg//ML nSi. Le taux de couverture et l épaisseur des couches moléculaires (-CnH2n+1, -C10H20-COOH) formant une liaison covalente avec le Si(111), avant et après fixation de clusters (Se8Re6(TBP)4(OH)2, Mo6Br8F6), sont obtenus par XPS et par ellipsométrie. Les caractéristiques électriques ont été interprétées en utilisant un nouveau modèle de transport dans les jonctions MIS de très faible épaisseur. Nous avons observé que les groupements fonctionnels (acide et clusters) modifient la structure électronique de la jonction et par conséquent la caractéristique I(V). Aux faibles tensions en direct, dominées par l émission thermoïonique, on observe une dépendance linéaire de la hauteur de barrière thermoïonique en fonction de la température. Aux fortes tensions, le modèle de Simmons est utilisé pour décrire la dépendance de la transparence de la barrière tunnel en fonction de la tension. La distribution des défauts localisés à l interface est déduite des mesures de l admittance (basse fréquence) et de la modélisation du temps de réponse R(V) en utilisant un modèle de jonction tunnel hors équilibre. La faible densité de défauts près du milieu de gap indique une bonne passivation des liaisons pendantes à l interface ML / Si de la jonction.Electrical transport in ultrathin Metal-Insulator-Semiconductor (MIS) tunnel junctions is analyzed using the temperature dependence of current voltage and admittance spectroscopy measurements applied to Hg // ML n Si junctions. The coverage and thickness of molecular layers (-CnH2n+1, -C10H20-COOH) covalently bonded to Si(111), before and after capping with clusters (Se8Re6(TBP)4(OH)2, Mo6Br8F6), are deduced from XPS and ellipsometry. The I(V) characteristics are described using a new model for transport in ultrathin MIS junctions. We observe that functional groups (acid, clusters) modify the electronic structure of the junction and consequently the I(V) characteristics. In the low forward bias regime governed by thermionic emission, the observed linear T-dependence of the effective barrier height provides the thermionic emission barrier height and the tunnel barrier attenuation. In the high-forward-bias regime, the bias dependence of the tunnel barrier transparency is approximated by a modified Simmons model for a rectangular tunnel barrier. The density distribution of defects localized at the ML / Si interface is deduced from admittance data (low-high frequency method) and from a simulation of the response time R(V) using a model for a non equilibrium tunnel junction. The low density of electrically active defects near mid-gap indicates a good passivation of dangling bonds at the ML / Si interface.RENNES1-BU Sciences Philo (352382102) / SudocSudocFranceF

Identification of dipolar relaxation and interface states response in the low temperature admittance of MIS tunnel junctions with molecular monolayer insulator : Hg // C12H25 – n-Si(111)

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Tunnel barrier parameters derivation from normalized differential conductance in Hg/organic monomolecular layer-Si junctions

International audienceThe shape of tunnel barrier junctions is derived from experimental current density versus bias, J(V), using the normalized differential conductance, NDC=d log J/d log V, to discriminate barrier height, ΦT, and barrier width, dT, effects. Parameterization of the Simmons model for a rectangular tunnel barrier, with NDC≈dTV/(ΦT-qV)1/2, provides physical (dT,ΦT) values for Hg||monomolecular layer--n Si(111) junctions incorporating functionalized n-alkyl layers covalently bonded to silicon

Elaboration, Characterizations, and Energetics of Robust Mo6 Cluster-Terminated Silicon-Bound Molecular Junctions

International audienceReaction of a [Mo6Br8F6]2- octahedral cluster unit with functional carboxylic acid leads to the formation of the corresponding carboxylate along with the elimination of HF and the formation of a Mo-O bond between the cluster and the carboxylate. This offers an easy way to prep. Mo6 cluster functionalized surfaces by simple reaction of carboxylic acid-modified n-type Si(111) surfaces in a soln. of [Mo6Br8F6]2- cluster units. The surface coverage of metallic clusters has been controlled in the range 0.3-5.8 × 1013 cm-2 by diln. of the carboxylic acid-terminated org. chains with inert n-dodecyl chains. The homogeneity of the grafted films on the oxide-free Si surface was followed by combined XPS and STM analyses whereas the electronic gap corresponding to the difference between the conduction band min. (CBM) and the valence band max. (VBM) of a single immobilized cluster was detd. from tunneling spectroscopy data obtained from a surface with a low cluster coverage. The energy position of the VBM of the functional surface was detd. by UPS for a surface with a high cluster coverage and charge transport characteristics through Hg//[Mo6Br8F6]-terminated org. monolayer (OML)//n-Si junctions were measured using the mercury drop technique. These studies allow the detn. of the electronic band diagram at the cluster-org. layer-Si(111) interfaces. Results are discussed and compared with other covalent assemblies of metallic clusters onto Si surfaces differing by the nature of cluster cores, [Re6Sei8]2+ or [Mo6Ii8]4+, and linker functionality, carboxylic acid or pyridine

Covalent attachment of metallic cluster cores (ReSe, MoBr) on silicon (111) surfaces via alkyl chains: surface characterization and electronic properties

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