Chromium Contacts Research Articles

Enhanced Electrical Properties of Lithography-Free Fabricated MoS2 Field Effect Transistors with Chromium Contacts.

Molybdenum disulfide (MoS2) as a two-dimensional semiconductor material has been actively explored for field-effect-transistors (FETs). The current prevailing method for MoS2 FET fabrication involves multiple complex steps, including electron beam (e-beam) lithography, annealing, etc., which are time-consuming and require polymer resists. As a consequence, the MoS2 exposed to chemicals during the patterning process may be unfavorably affected by residues and the performance of the final FET could be impaired while the annealing limits materials for FETs. Therefore, there is an urgent need to free the fabrication of FETs from e-beam lithography and annealing. In this study, we introduce an e-beam lithography-free method to fabricate MoS2 FETs by employing maze-like source/drain electrodes. In addition, an ohmic contact in multilayer MoS2 FETs using chromium (Cr) as source/drain electrodes is achieved without annealing. The underlying mechanism for contact performance is studied, and the tightness of the contact and the type of metal are found to be responsible because they determine the contact resistance. Furthermore, the long-term device degradation is explored, in which the oxidation of metal dominates. The facile fabrication process and mechanism explanation in this work might provide a new platform for future electronic devices.

Optical absorption spectroscopy of metallic (Cr) vapor in a vacuum arc

The measurement of the metallic vapor density in a vacuum arc is crucial to acquire a better understanding of both the anodic activity and the dielectric recovery process in vacuum interrupters. The objective of this study was to measure the chromium vapor density and its axial distribution within the gap between the chromium contacts. Optical absorption spectroscopy (OAS) with a broadband light-source is adopted for this investigation. The results show that when the vacuum arc burns in the diffuse mode, the metallic vapor density maxima occur near the electrodes during the arcing period. At the peak current, the vapor density near the electrodes can be as high as 2.5 × 1018 m−3. With the decrease of the arc current, the metallic vapor density near the electrodes decreases as well, while the vapor density in the center of the gap remains nearly constant during the arcing period. At current zero, the metallic vapor in the gap has a nearly uniform distribution of about 3 × 1017 m−3. When the vacuum arc burns in the high-current mode, the metallic vapor density near the anode is lower than that in other areas until the vacuum arc becomes diffuse. Then, the evaporation process of the anodic molten region starts to play an important role and the metallic vapor density near the anode increases. At current zero, the metallic vapor has a density of about 4 × 1018 m−3 near the anode, which is much higher than anywhere else. Because the metallic vapor density at current zero is too low to cause a Townsend avalanche, extra factors are needed for initiating the breakdown in the post-arc phase. These factors could include a residual plasma within the gap and the behavior of the liquid metal in the molten anodic region.

Modeling Spots on Composite Copper–Chromium Contacts of Vacuum Arcs and their Stability

Cathode spots on copper–chromium contacts of vacuum interrupters are simulated by means of a self-consistent space-resolved numerical model of cathode spots in vacuum arcs developed on the basis of the COMSOL Multiphysics software. Attention is focused on spots attached to Cr grains in the Cu matrix in a wide range of values of the ratio of the grain radius to the radius of the spot. In the case where this ratio is close to unity, parameters of spot are strongly different from those operating on both pure-copper and pure-chromium cathodes; in particular, the spot is maintained by Joule heat generation in the cathode body and the net energy flux is directed from the cathode to the plasma and not the other way round. An investigation of stability has shown that stationary spots are stable if current controlled. However, under conditions of high-power circuit breakers, where the near-cathode voltage is not affected by ignition or extinction of separate spots, the spots are unstable and end up either in explosive-like behavior or in destruction by thermal conduction. On the other hand, spots live significantly longer-up to one order of magnitude-if the spot and grain sizes are close; else, typical spot lifetimes are of the order of 10 $\mu \text{s}$ . This result is very interesting theoretically and may explain the changes in grain size occurring in the beginning of the lifetime of contacts of high-power current breakers. A sensitivity study has shown that variations in different aspects of the simulation model produce quantitative changes but do not affect the results qualitatively.

Metastability in Undoped Microcrystalline Silicon Thin Films Deposited by HWCVD

ABSTRACTFilms deposited by the Hot Wire CVD method were studied by means of dark conductivity, FTIR, Hydrogen Evolution, SEM and AFM surface characterization. Three types of metastability were observed: a) long term irreversible degradation due to oxidization processes on the film surface, b) reversible degradation determined by uncontrolled water adsorption, c) fast field switching effect in the film bulk.Oxygen and hydrogen content and its bonding configurations have been analyzed by hydrogen evolution and infrared spectroscopy methods on the films deposited on glass substrates and silicon wafers subsequently. It has been found that metastable processes close to the film surface are stronger than in the bulk. The switching effect is the fast increase of charge carrier density observed on bottom chromium contacts under a condition of air admittance. We propose this effect is associated with morphology changes during film growth and electrical field induced by adsorbed atmospheric components on the film surface.

An Amorphous Silicon Photoconductor for UV Detection

AbstractAn amorphous silicon photoconductor to detect wavelengths between 180 nm to 550 nm without scintillator is presented. The photoconductor is based on a coplanar configuration of the electrodes, similar to measurement structures to determine material characteristics of amorphous layers, e.g. for the Constant Photocurrent Method (CPM). After passing through a thin transparent passivation layer, the incident radiation is directly absorbed in the intrinsic a-Si:H material. The carrier collecting electrical field is applied perpendicular to the incoming light. Test structures have been fabricated with 80 nm thick sputtered chromium contacts on top of a 60 nm carbonized hydrogenated i-layer and a SixNx passivation layer with a thickness of about 36 nm. The spacing between the Schottky contacts is varied between 3 μm and 100 μm. They are deposited on top or below the a-SiC:H layer. First experiments with this simple coplanar design show that with an increasing voltage a shift towards UV wavelengths can be observed. The new UV detector is applicable in the field of TFA image sensors (Thin Film on ASIC) and in the new Lab-on-a-Chip concept presently under development at the institute for microsystem technologies.

In situ fabrication of vertical tunnel junctions for SET devices

A modified fabrication technique for vertical metallic tunnel junctions is presented. This approach for metallic single electron transistors (SETs) is based on the in situ growth of Ti–TiO X –Ti layer stacks embedded between chromium and gold contacts. The sequence of process steps as well as complementary nanoanalytic investigations are presented and discussed within the frame of data obtained by electrical characterization.

Influence of ion-induced interfacial chemical reactivity on contact resistance

Contact resistance measurements of chromium contacts deposited by partially ionized beam deposition on transparent conducting indium tin oxide (ITO) were performed. These provide a direct experimental evidence of the influence of interfacial chemical interaction on the contact resistance. The interfacial reactivity is controlled by modifying the energy and flux of ionized chromium atoms deposited on ITO employing a specially designed partially ionized deposition system with very high ionization efficiency. The true contact resistivityρ c is obtained by iteratively correcting the experimentally measured values for the finite sheet resistance of the ITO layer.ρ c decreases linearly with the energy of the ionized chromium. Auger sputter profiling shows no structural modifications at the interface due to a change in the energy of the chromium atoms, confirming that the observed change in the contact resistivity is directly related to interfacial chemical bonding of the atoms with the oxygen atoms in the ITO leading to a local increase of carrier concentration and lower interfacial resistance.

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.

Effect of annealing on the electrical behaviour of chromium contacts to silicon at various temperatures

Contacts of chromium annealed to ⟨ 111 ⟩ n-type silicon at 450°C for periods of up to 2 hours have been studied through measurements of the forward current-voltage characteristics at temperatures T where 293K > T > 150K. The results seem to match the morphological model proposed in a previous work which was based on room temperature measurements. The three parameters of interest, i.e. the ideality factor n, the series resistance Rs and the barrier height ΦBn are calculated and the effect of annealing time and temperature is discussed with special emphasis on the first annealing phase of tann < 15 min.

Brief communciation.Effect of annealing on the electrical behaviour of chromium contacts to silicon

The effect of annealing chromium to (111) n-type silicon contacts at 450°C for periods of up to two hours has been studied by measurement of the room temperature forward bias characteristic. It was found that there is an optimal annealing time of 15 minutes to give the lowest ideality factor. The series resistance of devices has also been interpreted from the measurements and it was found that this parameter is also low for annealing periods of between 10 and 35 minutes. The contact barrier height showed considerable variation with annealing time, indicating a maximum value of 0–6 eV after 15 minutes annealing and a minimum value of 0–56eV after contacts had been annealed for 35 minutes.

Indium-tin-oxide-metal interfacial resistance and its implication for solar cells

Abstract External metal-ITO (indium tin oxide) contacts on solar cells have been found to be highly resistive. These high resistance values represent an intrinsic limitation of the contacts due to physico-chemical interactions at the interface. The contact resistivity R c for silver contacts on 10 Ω/□ ITO is 3.55 × 10 −2 Ω cm 2 and increases to 4.9 × 10 −1 Ω cm 2 for 450 Ω/□ ITO . Similar results are obtained for chromium and aluminium contacts. Such high R c values are shown to result in significant additional losses in solar cell performance, particularly for external contacts of small size.

Ion bombardment of gallium thin films

There has recently been much interest in the structural changes in solids during ion bombardment, including reports of the production of amorphous metallic alloys by implementation of the alloying element to high doses [1] (~ 1017 ions cm-2). Although it has been known for some time that helium ion bombardment of certain crystalline semiconductors (e.g. silicon [2]) can produce an amorphous phase, it is only recently [3] that evidence for the amorphization of a metallic element, gallium, has been seen. In this letter we describe bombardment and annealing experiments (first reported in [4]) carried out independently on gallium. Fig. 1 shows a schematic diagram of the free energy of a general system as a function of temperature. In addition to the stable solid phase (a), the liquid (1) and vapour (v) phases, the curves describing the free energy of various metastable phases (b, c, d) are shown. We see that to form a metastable phase by the cooling of a vapour or liquid it is necessary to supercool the system below the melting-point of the metastable phase (e.g. below Tmb for the b phase). In practice, the phase which then crystallizes from the undercooled system is decided by "kinetic" factors controlling nucleation and growth of the solid [5]. A system can be undercooled in two ways: by "static" or slow undercooling, usually of micronsized droplets [6], or by "dynamic" undercooling during the rapid quenching of a fluid. This latter process occurs during thin-film deposition or splatquenching of liquids. Both processes are known to be effective in producing metastable (crystalline and amorphous) phases. Although the dynamic processes occurring during ion bombardment are still not fully understood, it is clear that a system immediately after the impact of a high-energy ion is in a highly excited and thermodynamically unstable state. While it may be necessary to describe the damage "spike" as a hydrodynamic process (involving pressure and density changes), rather than a simple local melting or vapourization of the solid, it is likely that the system can explore one or more of the metastable structural states shown in Fig. 1 during its return to thermodynamic equilibrium. Thus systems such as gallium which show metastable phase formation during static or dynamic undercooling might also be expected to form metastable phases during ion bombardment. Bombardment experiments were carried out using gallium or argon ions produced on a 100 kV heavy-ion accelerator [7]. Gallium samples were evaporated in situ from alumina boats onto sapphire (for electrical measurements) and thinfilm carbon (for electron diffraction) substrates during the same evaporation. After deposition, both samples were bombarded in turn at 13K with the same ion dose. Electron diffraction, 4-probe a.c. and 2-probe d.c. electrical measurements (using pre-evaporated chromium contacts

Contact Resistance Measurements of Platinum-Silicide and Chromium Contacts to Highly Doped n and p Silicon

A comparison of contact resistivity for platinum silicide contacts and sputtered chromium contacts to heavily doped n and p silicon has been done, by a method which is suitable for contacts on a highly conductive surface layer on a less conductive base material. Good statistics was obtained by using 50 separate structures on each wafer.The results showed less spreading and lower specific contact resistance for PtSi contacts on both n+ and p+-Si, than for Cr contacts which on p+ showed a non-ohmic behaviour.

Sprayed zinc-cadmium sulfide films for Cu2S/ZnxCd1-xS heterojunction solar cells

Cu <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> S/Zn <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</inf> Cd <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</inf> S solar cells with stable open-circuit voltages up to 0.784 V were formed on zinc-cadmium sulfide films deposited by spray pyrolysis. x was varied between zero and 0.7 by varying the spray solution composition. Zn <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</inf> Cd <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</inf> S films formed Schottky diodes with evaporated chromium contacts; depletion layer width (in light) in the films increased with x, from 0.16 to 3.1 microns. Resistivity of the films increased exponentially with x. Optical band gap increased with x from 2.41 eV at x = 0 to 2.82 eV at x = 0.6.

Electron transport in hydrogenated amorphous silicon: drift mobility and junction capacitance

Time-of-flight measurements of the electron drift mobility in hydrogenated amorphous silicon prepared by glow discharge decomposition of silane are described. The effect of electric field amplitude, dielectric relaxation, and the intensity and absorption length of the excitation light on the shape of the photocurrent transients is discussed. The electron drift mobility is found to be 0.9 cm 2V −1 s −1 and the transport process is diffusive with an electron deep-trapping lifetime in excess of 1 μs. In the second part of the paper the frequency and illumination dependences of the junction capacitance of Schottky barrier solar cells are interpreted in terms of a time-dependent screening length which is controlled by the density and release rates of electron traps. Based on this interpretation of the capacitance, a new method is proposed for the interpretation of capacitance-voltage data. The technique is applied to chromium, palladium and platinum Schottky barrier contacts and used to infer the electric field distribution and the density of deep hole traps.