Cr Contacts Research Articles

Study the Conduction Mechanism and the Electrical Response of Strained Nano-thin 3C-SiC Films on Si used as Surface Sensors

AbstractVarious superior properties of SiC such as high thermal conductivity, chemical and thermal stability and mechanical robustness provide the basis for electronic and MEMS devices of novel design [1]. This work evaluates heterostructures that consist of a few nanometers-thick 3C-SiC films on silicon substrates. Nano-thin SiC films differ significantly in their electrical behavior compared to the bulk material [2], a finding that gives rise to a potential use of these films as surface sensors. To gain a better understanding of the effect of surface states on the electrical response of these thin, strained films, several metal-semiconductor-metal heterostructures have been examined under variable conditions. The nano-thin, strained films were grown using gas source molecular beam epitaxy. Reflection high-energy electron diffraction patterns obtained from several 3C-SiC films indicate that these films are strained nearly 3% relative to the SiC lattice constant. Al, Cr and Pt contacts to a nano-thin film 3C-SiC were deposited and characterized. I-V measurements of the strained nano-thin films demonstrate metal-semiconductor-metal characteristics. Band offsets due to biaxial tensile strain introduced within the 3C-SiC films were calculated and band diagrams incorporating strain effects were simulated. Electron affinity of 3C-SiC has been extracted from experimental I-V curves and is in good agreement with the value that has been calculated for a strained 3C-SiC film [3]. On the basis of experimental and simulation results, an empirical model for the current transport has been proposed. Fabricated devices have been characterized in a controlled environment under hydrogen flow and also in a reactive ambient, while heating the sample and oxidizing the surface, to investigate the effects of the environment on the surface states. Observed changes in I-V characteristics suggest that these nano-thin films can be used as surface sensors.

Morphology and electric conductance of ultra-thin Cr contacts on 6H-SiC(0001): AFM and current-sensing AFM study

Early stages of Cr contact formation on 6H-SiC(0001) were investigated using the atomic force microscope (AFM) and current-sensing AFM with conducting tip. Cr layers were vapor deposited under ultrahigh vacuum onto samples cut out of a single crystal of n-type 6H-SiC(0001) that were ex situ hydrogen etched in a tubular flow reactor. Topography of the samples, their local conductance patterns and local current–voltage characteristics of the Cr/SiC contact were examined simultaneously as a function of Cr-adlayer thickness and annealing temperature. The growth of Cr follows the Stranski–Krastanov growth mode. The layers of mean thickness ⩾4.5 nm have a grainy structure. Differences in quality of the electric contact between the grains and the substrate as well as between the grains themselves enable to obtain a good contrast image of the local conductance of the layer. The contact of the as-deposited at room temperature Cr-layer of the thickness from 1.5 to 10 nm is typical of the rectifying junction. Annealing the Cr-SiC contact at temperatures up to 1800 K leads to coalescence of grains and dissolution of Cr in the substrate. The dissolution essentially disturbs the rectifying character of the electric contact Cr/SiC.

The Study of Thermal Treatment on Electrical Properties at Cr/p-GaN

The barrier height of as-deposited Cr contacts was found to be ΦB= 0.87eV with the ideality of η=1.51 and remained almost unchanged after further annealing at 500 oC for 5 minutes. The barrier height of diodes were increased drastically after annealed at 600 oC where ΦB=1.01eV with η=1.69. Upon annealing at 700 oC for 5 minutes, the ΦB decreased to 0.61eV and the Cr diodes were degraded for higher temperature in this annealing duration. The ΦB remained in 0.80- 0.90eV respectively at 800 oC for 2 minutes and 900-1000 oC for 1 minute.

Spatial and Temporal Variations in Electronic Transport Through a CdTe-Based Schottky Barrier

AbstractWe study the electric current through metal-semiconductor junctions of a type used in thin-film PV for back contacts. To concentrate on one type of junction we used the symmetric structures of rf-sputtered CdTe layer sandwiched between two Cr contacts. Along with the conventional measurements, the current-sensing contact mode AFM was employed to measure the current-voltage characteristics and current variations with time under fixed voltage. We found that (i) the electric current flow is laterally strongly nonuniform; (ii) it chaotically varies over time; (iii) this behavior did not correlate with surface topography. We interpret our observations in terms of defect assisted tunneling through time-dependent defect pathways.

Optical and electrical properties of 4H-SiC irradiated with fast neutrons and high-energy heavy ions

Photoluminescence and deep-level transient spectroscopy are used to study the effect of irradiation with fast neutrons and high-energy Kr (235 MeV) and Bi (710 MeV) ions on the optical and electrical properties of high-resistivity high-purity n-type 4H-SiC epitaxial layers grown by chemical vapor deposition. Electrical characteristics were studied using the barrier structures based on these epitaxial layers: Schottky barriers with Al and Cr contacts and p +-n-n + diodes fabricated by Al ion implantation. According to the experimental data obtained, neutrons and high-energy ions give rise to the same defect-related centers. The results show that, even for the extremely high ionization density (34 keV/nm) characteristic of Bi ions, the formation of the defect structure in SiC single crystals is governed by energy losses of particles due to elastic collisions.

The electrical resistance response of continuous carbon fibre composite laminates to mechanical strain

Abstract Measurements have been made of the effect of mechanical strain on potential distributions and resistance of unidirectional and multidirectional carbon fibre epoxy laminates. The effects of current flow direction and technique for current introduction on piezo-resistance have been studied. It was found that uniform current introduction at sample edges produced by sputtered Au–Cr contacts across the entire cross-section produced consistently low values of gauge factor of 1.75 for current flow parallel to the fibres and 2.7 for transverse current flow. Non-uniform current introduction, produced variously by local point introduction of current, or use of viscous adhesives producing intermittent contact, resulted in a wide range of apparent gauge factors ranging from 20.6 to −89. These anomalous values may be explained by a model in which the high anisotropy of resistance in unidirectional CFRP maintains initial non-uniform current throughout the sample. Under mechanical strain points of fibre contact will change, altering the distribution of current carrying fibres and leading to local changes in current. Thus changes in potential difference between two points produced by mechanical strain will not be exclusively caused by changes in local resistance. The presence of transverse plies in multidirectional laminates ensures that in plane non-uniform current distributions are largely eliminated, and the effect on piezo-resistance of non-uniform current introduction is minimised.

Sputter-deposited metal contacts for n-type GaN

Sputter-deposited Au, Pt, Cr, Ni and Cu contacts for n-type GaN films were studied using current–voltage (I–V) and capacitance–voltage (C–V) measurements. These films were grown by molecular beam epitaxy (MBE), heteroepitaxially on the basal plane of sapphire. The contacts were non-ideally rectifying in nature. Assuming that the non-ideality was due to effects of series resistance and recombination current, a computer curve fitting procedure was employed that enabled the separation of these effects from the thermionic emission current, thereby permitting the calculation of the barrier height. An analysis of the results indicates that the barrier heights for metal contacts on GaN are determined by the difference between the metal and the semiconductor electronegativities and substantially influenced by metal induced gap states (MIGS)/sputtering damage induced surface states (SDISS). The concentration of metal induced gap states/sputtering induced damage states was determined to be approximately 2.7 × 1013 states cm−2 eV−1.

Time-Resolved Switching Studies in a-Si:H and Related Films

AbstractSwitching in a-Si:H and a-Si:HNx layers is investigated by pulse current transient and Auger scanning microspectroscopy measurements. Switching in a-Si:H with Ag and Cr contacts exhibits 2 different regimes depending on the voltage pulse polarity. With a positive top Ag contact, switching occurs in nanoseconds after a certain latency time, which depends on voltage exponentially. For a negative Ag contact, there is no latency time provided the voltage exceeds a certain critical value. This might be related to interface effects on contact properties or field-assisted metal diffusion. Scanning Auger element micromaps reveal metallic filaments in the switched films. They contain both Ag and Cr throughout the film thickness. Two phases of the filament formation are suggested – a precursor phase and a post-switching phase characterized by local heating and atomic diffusion. Soft and hard switching are observed in a-Si:HNx films simultaneously and their rates depend strongly on the contact material and applied voltage. Soft switching might be related to the charge trapping in this wide bandgap material.

Getting the right answer to a key question concerning molecular wires

It is shown that the Cr5(tpda)4Cl2 molecule has alternating long and short (Cr···Cr) contacts in three compounds, including one in which an evenly spaced chain was recently claimed.

Space-Charge-Limited Currents in n-i-n Devices Incorporating Glow-Discharge and Hot-Wire Deposited a-Si:H

ABSTRACTSpace-charge-limited currents have been examined in a wide variety of n-i-n devices. If the devices were completely symmetric, the current-voltage characteristics would be identical for positive and negative bias, but in several devices differences between the two polarities were observed. In order to understand in which part of the device these differences originate, the influence of the contacts and interfaces on the JV characteristics were examined by using different metal top contacts, different n-layers and different i-layers. Ag and Al top contacts gave minor differences between the polarities, whereas with Cr contacts no differences were observed. Incorporation of a defect layer in the i-layer results in asymmetric JV curves. We have observed a small asymmetry in an experimental device, and a large asymmetry using AMPS modeling. N-i-n devices appear to be a sensitive probe for interface defects.

EXAFS study of graphite intercalated with metal chlorides

Synchrotron EXAFS spectra of transition metal chlorides intercalated into a graphite matrix (1) Gr ∗CrCl 3, (2) Gr ∗FeCl 3, (3) Gr ∗CoCl 2, (4) Gr ∗NiCl 2, (5) Gr ∗CuCl 2 and (6) chemically reduced Gr ∗Cr, and X-ray powder data confirm the structure of hexagonal layers of the ‘guest’ chlorides stacked in the graphite matrix. The two nearest coordination spheres of the metal atoms in (1)–(5) corresponding to MCl (2.32–2.46 Å) and M … M distances (3.3–3.6 Å) agree well with the crystal structures of pure MCl n . In (6), zerovalent Cr atoms are probably η 6-coordinated to graphite hexagons; no CrCr contacts were observed.

Electric strength of vacuum gap under various voltage conditions

Although the electric strength of vacuum gap under various voltage conditions has been discussed in many paper, research on this subject is still ongoing. The first page of our investigations was examining the prebreakdown state of vacuum gap under ac voltage excitation. Field electron emission in short vacuum gap (0–2 mm) and microdischarges in long vacuum gap (over 10mm) were analyzed. Field electron emission current was measured with an unbalanced hv bridge. Next, using voltage-current characteristics analysis, the parameters of field electron emission were obtained. The microdischarges phenomenon was monitored with a multichannel pulse-height analyzer. The relation between breakdown voltage and the parameters characterizing prebreakdown phenomena was then determined. Finally, the characteristics of electric strength under ac (50 Hz), switching impulse (250/2500 μs) and lighting impulse (1.2/50 μs) voltages were obtained. The investigations were carried out with CuCr contacts in a wide range of interelectrode distance (0.5–12 mm).

Metallizations of InP based on transition metals

Single element Ni and Cr metallizations to low doped n- and InP have been investigated to clarify how they affect electrical performance of advanced multilayer metallizations for ohmic contact and Schottky diode applications. It was found that the Ni and Cr contacts to n-InP show unstable ohmic behaviour at all annealing temperatures (300-500 °C), and that Ni and Cr metallizations to p-InP show diode behaviour. The electrical behaviour of the contacts were compared to phase formation paths of Ni-In-P and Cr-In-P ternary systems. From this it was found that amorphous ternary phases form first by interdiffusion of Cr and Ni into the InP crystal. Subsequently, at higher annealing temperatures, crystallization and eventually phase separation determines the electrical properties of the contacts. After annealing at 500 °C the Cr diodes to p-InP are almost ideal, as deduced from barrier height measurements using a combination of I-V and C-V methods. This indicates that the metallization forms a two-layer structure with Cr-P phases lying above pure In. The Ni diodes to p-InP annealed at 500 °C, on the other hand, are not ideal, indicating that Ni-In and Ni-(In)-P phases in parallel are determining the electrical properties of the diode.

Ligand steric bulk: A neglected factor in the formation of CrCr supershort contacts

The ability of three center chelating ligands to form dichromium units and to enforce short and very short CrCr contacts is examined in a consistent series of cyclohexyl amidinate chromium complexes, with the aim of understanding the factors intrinsic in the nature of the bridging ligands which are able to promote or disfavor dinuclear aggregation and to determine the extent of intermetallic separation. The present study describing the crystal structures of the dinuclear [(CyNC(H)NCy) 2Cr] 2·C 7H 8 ( 1) and mononuclear (CyNC(CH 3)NCy) 2Cr ( 2), {CyNC[Ph( o-CH 2NMe 2)]NCy} 2Cr·2THF ( 3) and [(Me 3Si)NC(Ph)N(SiMe 3)] 2Cr ( 4) indicates that the steric interaction between the lateral cyclohexyl and the central amidinate group is capable of determining the nuclearity (monomeric versus dimeric) and probably the extent of intermetallic separation. Crystal data are as follows. 1: C 52H 84N 8Cr 2·toluene, M=1017.41, monoclinic, P2 1/ c, a=13.682(5), b=17.446(3), c=24.521(3) Å, β=90.02(5)°, V=5853(3) Å 3, Z=4, T=−30 °C, Mo Kα, R=0.104, R w=0.077 for 4030 reflections out of 10319 unique; 2: C 28H 50N 4Cr, M=494.73, orthorhombic, Pnma, a=8.104(1), b=26.572(4), c=12.533(2) Å, V=2699(1) Å 3, Z=4, T=−157 °C, Mo Kα, R=0.054, R w=0.069 for 1580 reflections out of 2752 unique; 3: C 44H 68N 6Cr·2THF, M=853.16, triclinic, P 1 , a=12.293(5), b=20.437(5), c=11.071(4) Å, α=94.55(3), β=111.72(3), γ=73.86(3)°, V=2481(1) Å 3, Z=2, T=−157 °C, Mo Kα, R=0.093, R w=0.081 for 6492 reflections out of 8710 unique.

Properties of Cadmium Selenide Thin Films as a Function of Lateral Distance from Chromium Contacts

The electrical, structural, and compositional properties of thin films were examined as a function of lateral distance from Cr contacts. The local threshold voltage of transistor channels was large and negative near the contacts and eventually approached a maximum positive value. This result is consistent with heavy n‐type doping of the caused by lateral diffusion of chromium contact metal. Energy‐dispersive x‐ray analysis (EDAX) showed that chromium had diffused into the film up to a distance of 20 μm from the contacts. For films annealed with a maximum temperature of 370°C, the grain size was uniform across the channel. Samples annealed at 400°C for 4 h had an enhanced grain size near the contacts.

Metal‐Metal Bonding in Paramagnetic Chromium(III) Complexes – An MO‐Theoretical Case Study

AbstractThe electronic structures of the edge‐sharing bioctahedral chromium(III) dimers {C5H5Cr(CH3)(μ1‐Cl)}2 (2) and {Me5C5Cr(CH3)(μ1‐CH3)}2 (4* (Cp* derivative), 4 (Cp derivative)), the trinuclear complex {C5H5Cr(μ1‐Cl)}3(μ1‐CH) (3), and the face‐sharing bioctahedral chromium(III) compounds {Me5C5Cr}2(μ1‐CH2) (6*, 6) [{Me5C5Cr}2(μ1‐CH3)3]+ (7*, 7) have been studied MO‐theoretically by the extended Hückel method. Proceeding from 2 over 3 to 4*, or from 6* to 7* a 3‐center/4‐electron chloride or methylene bridge is replaced by a 3‐center/2‐electron methyl group (a methylidyne cap in the trimer). The 3c/2e bridges give rise to an increased metal – metal overlap population due to Cr – Cr bonding within the core levels. In the series of 2 – 3 – 4* this is accompanied by a decrease in the Cr – Cr distance (329 – 284 – 260 pm) and effective magnetic moment, μeff (2.53 – 2.05 – 1.53 μB per Cr at room temp.). The latter can be ascribed to an increased splitting within the chromium d‐block because of the shorter metal‐metal separation. The bioctahedra 6* and 7* feature even shorter Cr – Cr contacts than 4* (239 and 242 pm), primarily because of their face‐sharing geometry. However, their effective magnetic moments (per Cr) are 2.33 and 1.32 μB at room temp. The high magnetic moment for 6* can be understood from the orbital interactions in the frontier orbital metal d block: A face‐sharing bioctahedron shows one σ, two δ, and no π type overlap, while an edge‐sharing bioctahedron (4) has σ, π, and δ interaction between the metals. In the former this destabilizes only σ* sufficiently to prevent occupation by electrons (leaving five orbitals for six electrons, possible spin multiplicities S0, 1, 2) while for the latter both σ* and π* are destabilized beyond electron occupation (leaving four MOs for six electrons, possible spin multiplicities S0, 1). 4*, 6* and 7* are unusual examples of metal – metal bonded complexes containing octahedral CrIII ions.

Interfacial Structure of Metal Layer Films (Al/Cr) Using Transmission Electron Microscope

The microstructures in Schottky barrier samples of Al and Cr single layer metal contacts and Al-Cr bimetal contacts on p-type Si have been investigated using analytical transmission electron microscope (TEM). Al-Cr bimetal contacts were made using a layered structure with one thin layer and a thick layer on the top. The analytical TEM was used to determine the precise thickness and grain size of the coated materials as well as chemical compositions and morphologies. The present study proposes that the microstructures of the Schottky contact interfaces are important for the Schottky barrier heights of Al, Cr, and Al-Cr contacts.

Electrical transport properties of CdSe thin-film transistors with Cr contacts

Electrical transport properties of CdSe thin-film transistors have been examined as a function of lateral distance from Cr contacts. The results show that the channel conductivity is enhanced in the vicinity of the contacts and drops sharply at a critical distance. The magnitude of this distance is sensitive to the surface treatment prior to CdSe deposition. Hall-effect measurements show an increasing electron mobility with increasing gate voltage, consistent with gate-voltage-induced lowering of grain-boundary potential barriers.

Short and Supershort Cr–Cr Distances: A Vanishing Borderline Between Metal–Metal Bonds, Magnetic Couplings and Ligand Artifacts

Abstract The structural features of three classes of Cr(II) compounds (alkoxides, amides, aryls) are critically reviewed with the aim of investigating the role of ligand bridging interactions in the formation of short Cr–Cr quadruple bonds. A systematic failure of Cr–Cr bond formation is observed with either monodentate or bidentate ligands. Fairly short Cr–Cr contacts are easily elongated and dinuclear structures cleaved by simply coordinating a molecule of solvent either to chromium or to the alkali cation present in anionic chromates. Conversely, extremely short Cr–Cr contacts are almost invariably formed when particular bridging ligands with a three-center chelating geometry and the electronic configuration of an allylic system have been used. Comparison of the magnetic properties of two series of Cr(II) aryls and pyrrolyls, where the three center chelating geometries of the ligands have been progressively modified, indicated that three-center chelating ligands are able to determine the dimeric aggreg...

Metal contacts to p-type GaAs with large Schottky barrier heights

Metal Schottky barrier contacts to p-type (100) GaAs are described in which a large Schottky barrier height φB is attained by using a very thin Si or Ge interface layer to influence the interface Fermi energy EiF. The metals investigated are Au, Cr, Mn, Ni, and Al. Au, Cr, and Mn contacts have φB values from 0.7 to 0.9 eV when ∼15 Å Si and Ge interlayers are made heavily n type. Ni and Al contact φB values were limited to 0.6–0.7 eV. X-ray photoemission spectroscopy was used to obtain the EiF and interface composition during contact formation; the φB for the corresponding thick contacts was measured by current-voltage and capacitance-voltage methods.