Variability of electrical contact properties in multilayer MoS2 thin-film transistors

We report the variability of electrical properties of Ti contacts in back-gated multilayer MoS₂ thin-film transistors based on mechanically exfoliated flakes. By measuring current–voltage characteristics from room temperature to 240 °C, we demonstrate the formation of both ohmic and Schottky contacts at the Ti–MoS₂ junctions of MoS₂ transistors fabricated using identical electrode materials under the same conditions. While MoS₂ transistors with ohmic contacts exhibit a typical signature of band transport, those with Schottky contacts indicate thermally activated transport behavior for the given temperature range. These results provide the experimental evidence of the variability of Ti metal contacts on MoS₂, highlighting the importance of understanding the variability of electronic properties of naturally occurring MoS₂ for further investigation.     

Effect of multiple tips on light emission induced by STM from gold nanostructures

Light emission induced by scanning tunneling microscope on gold islands grown on MoS₂ surfaces has been investigated. Surface geometry and roughness show that different apexes of the same tip can modify the energy of photons emitted in the tunneling junction. Comparisons of topography and photon map are used to locate islands imaged twice and to represent approximately the tip shape used. Light emission spectroscopy on the same island with two apexes of the multiple tip reveals variations of emission properties according to the apex used, showing the importance of tip geometry in the emission process induced by tip induced plasmon modes.     

Formation of thermally stable low-resistance Ti/W/Au ohmic contacts on n-type GaN

A Ti(12 nm)/W(20 nm)/Au(50 nm) metallization scheme has been investigated for obtaining thermally stable low-resistance ohmic contacts to n-type GaN (4.0×10¹⁸ cm^-3). It is shown that the current–voltage (I–V) characteristics of the samples are abnormally dependent on the annealing temperature. For example, the samples that were annealed at temperatures below 750 °C for 1 min in a N₂ ambient show rectifying behavior. However, annealing the samples at temperatures in excess of 850 °C results in linear I–V characteristics. The contact produces a specific contact resistance as low as 8.4×10^-6 Ω cm² when annealed at 900 °C. It is further shown that the contacts are fairly thermally stable even after annealing at 900 °C; annealing the samples at 900 °C for 30 min causes insignificant degradation of the electrical and structural properties. Based on glancing angle X-ray diffraction and Auger electron microscopy results, the abnormal temperature dependence of the ohmic behavior is described and discussed. PACS 72.80.Ey; 73.40.Cg; 73.20.At; 79.60Bm; 73.40.Gk     

ZrB2/Pt/Au Ohmic contacts on bulk, single-crystal ZnO

There is a strong interest in developing thermally stable metallization schemes for ZnO and boride-based contact stacks are expected to have potential because of their thermodynamic stability. The contact characteristics on bulk single-crystal n-ZnO of a ZrB₂/Pt/Au metallization scheme deposited by sputtering are reported as a function of annealing temperature in the range 300-800°C. The contacts were rectifying for anneal temperatures <500 °C but exhibited Ohmic behavior at higher temperatures and exhibit a minimum specific contact resistivity of 9 × 10⁻³ Ω cm after 700 °C anneals. The contact stack reverts to rectifying behavior after annealing above 800 °C, coincident with a degraded surface morphology and intermixing of the Au, Pt and ZrB₂. The boride-based contacts exhibit higher thermal stability but poorer specific contact resistivity than conventional Ti/Au metal stacks on ZnO.     

Scanning tunneling microscope study of nanosized metal–semiconductor contacts between ErSi2 nanoislands and Si(0 0 1) substrate

The current–voltage (I–V) characteristics of the nanosized metal–semiconductor contacts formed between the epitaxially grown ErSi₂ islands and p-Si(0 0 1) substrate are measured in situ by the scanning tunneling microscope. Experimental results show that the current densities passing through the nanocontacts are five orders of magnitude larger than that of the macroscopic ones and have an obvious dependence on the contact area. Especially, it is found that I–V characteristics of the contacts are sensitive to the sample surface adsorption. Our investigations indicate that surface conduction plays an important role in the electrical transport process from ErSi₂ islands to the Si(0 0 1) substrate. Furthermore, for the nanocontacts with surface currents suppressed effectively, the ideality factor and the effective Schottky barrier height are estimated by using the standard thermionic emission model. Our analysis suggests that the current through the interface between ErSi₂ nanoislands and the p-Si(0 0 1) substrate is enhanced due to the effects of tunneling and image force lowering.     

The work function of hydrothermally synthesized UO2 and the implications for semiconductor device fabrication

The photoelectric work function of nearly stoichiometric (111) and (100) hydrothermally grown UO₂ was measured to be 6.28 ± 0.36 eV and 5.80 ± 0.36 eV, respectively. Candidate metals for electrical contacts are identified for both rectifying and non‐rectifying contacts based on work function, lattice compatibility, and electrical conductivity.     

Patterned horizontal growth of ZnO nanowires on SiO2 surface

We report patterned horizontal growth of ZnO nanowires on SiO₂ surface for the study of electrical and luminescent characteristics of individual nanowires and for device applications. Patterns of gold catalytic seed islands with barrier layers which suppress vertical growth were employed to facilitate horizontal growth on SiO₂ surface. After the growth, ZnO nanowire devices are fabricated by patterning electrodes aligned over the seed islands and their device characteristics are investigated. We could also investigate history of synthesis conditions by obtaining local luminescence characteristics along individual nanowires.     

High pressure electrical transport measurements of Cs2MoS4 and KTbP2Se6 and X-ray diffraction study of Cs2MoS4

We report the results of electrical resistance measurements at high pressures on Cs₂MoS₄ and KTbP₂Se₆. The results of high pressure X-ray diffraction study of Cs₂MoS₄ are also presented. Interestingly, in the case of Cs₂MoS₄ the resistance vs. pressure follows the behavior of the absorption edge vs. pressure obtained from our optical measurements lending further support to a direct-indirect band crossing. In the case of KTbP₂Se₆,the phase transition at about 9.2 GPa is reflected in a sharp drop of the resistance. In addition we report the pressure dependence of the lattice constants as well as the equation of state of Cs₂MoS₄.     

Evidence for a two-band behavior of MgB2 from point-contact and tunneling spectroscopy

The break-junction tunneling has been systematically investigated in MgB₂. Two types of the break-junction contacts have been exploited on the same samples, which demonstrated tunnel contact like (SIS) and point contact like (SnS) behavior. Both of them have shown the existence of the two distinct energy gaps. We have also observed peculiarities on the I(V)- characteristics related to Leggett's collective mode assisted tunneling.     

Surface electromigration of Au ultrathin film on MoS2

The mass transport of Au ultrathin film on a semiconductor MoS₂ was investigated by atomic force microscopy (AFM) and scanning Auger microscopy (SAM). The surface electromigration of the Au film was found when a dc current was passed through the MoS₂ substrate. The Au ultrathin film on MoS₂ grew in a typical Volmer–Weber (V–W) growth mode, The AFM measurements indicated that the distribution of the Au islands exhibited clearly a preferential lateral spread towards the cathode, that is, the surface electromigration took place. The direction of the surface electromigration on MoS₂ is opposite to that of the Au electromigration on Si.     

用扫描隧道显微镜针尖操纵金岛

用扫描隧道显微镜，在小隧道阻抗的条件下（小偏压和大隧道电流），通过移动针尖，实现了在室温下对真空蒸镀在高定向石墨上的、由几万个原子组成的纳米尺度金岛的操纵．在大隧道阻抗的情形下，用同一个针尖可对操纵的结果进行观察，而不会对金岛产生扰动．这种可控的操纵是通过当钨针尖与金岛间距离很近时形成的金属间黏附力大于金岛与石墨间的摩擦力而实现的 关键词：     

Electrodeposited nickel-cobalt composite coating containing MoS2

Ni-Co/MoS₂ composite coatings were prepared by electrodeposition in a Ni-Co plating bath containing nano-sized MoS₂ particles to be co-deposited. The polarization behavior of the composite plating bath was examined on a PAR-273A potentiostat/galvanostat device. The friction and wear behaviors of the Ni-Co/MoS₂ composite coatings were evaluated with UMT-2MT test rig in a ball-on-disk contact mode. The morphologies of the original and worn surfaces of the composite coatings were observed on scanning electron microscope (SEM). It was found that the introduction of MoS₂ nano-particulates in the electrolyte caused the shift towards larger negatives of the reduction potential of the Ni-Co alloy coating, and the co-deposited MoS₂ showed no significant effect on the electrodeposition process of the Ni-Co alloy coating. However, the co-deposited MoS₂ led to changes in the surface morphology and structure of the composite coating as well. Namely, the peak width of the Ni-Co solid solution for the composite coating is broader as compared to that of the Ni-Co alloy coating. The co-deposited MoS₂ particulates were uniformly distributed in the Ni-Co matrix and contributed to increase tribological properties of the Ni-Co alloy coating.     

Sr/Si界面沉积SrTiO3初始生长阶段的扫描遂道显微术研究

本文工作利用脉冲激光沉积术(PLD)和超高真空扫描隧道显微术(UHV-STM),研究了在Sr/Si(001)-(2×1)衬底表面上真空室温沉积几个单层SrTiO₃薄膜的初始生长过程.经660 ℃退火处理后,Sr/Si衬底表面上形成了纳米岛状结构.经分析,这些纳米小岛为C49-TiSi₂和 C54-TiSi₂.实验结果表明,在没有氧气的情况下退火,Sr/Si界面无法有效阻止SrTiO₃薄膜与Si衬底之间的相互作用. 关键词： 脉冲激光沉积术(PLD) 扫描隧道显微镜(STM) 3')" href="#">SrTiO₃ 2')" href="#">C54-TiSi₂     

Electrical stability of multilayer MoS2 field‐effect transistor under negative bias stress at various temperatures

The electrical stability of molybdenum disulfide (MoS₂) transistors is crucial for their use in various applications. However, it is tricky to evaluate the inherent stability of MoS₂ transistors because it is highly dependent on environmental conditions during measurement such as humidity, light, and electrical factors. We studied the threshold voltage instability under negative bias stress at a variety of temperatures in a vacuum and in the dark to eliminate any environmental effects. In particular, the measurement of transfer curves under stress is minimized in order to study the inherent instability of MoS₂ transistors, even though the measurement of transfer curves is normally indispensable to check for the evolution of electrical instability. MoS₂ transistors have high average effective energy when compared to conventional amorphous Si and oxide semiconductor transistors, which allows for adequate operation at high temperatures. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Adsorption of Cs and O2 on MoS2

Adsorption of Cs on basal planes of MoS₂ has been studied with LEED, Auger and work function measurements. LEED observations show that in the 200–300 K range Cs is adsorbed as amorphous layers on MoS₂. Correlation of Auger and work function measurements indicates that the work function, sticking coefficient and the maximum density of Cs that can be deposited on the MoS₂ surface depend strongly on substrate temperature. Cesium is deposited on MoS₂ in two adsorption states. Although MoS₂ is extremely inert to O₂ adsorption, the presence of Cs causes a drastic increase in the adsorption of oxygen which in turn increases the amount of Cs that can be deposited on the surface. Lastly, it has been found that part of the Cs adatoms are diffused into the bulk of MoS₂.     

Characteristics of a type-II n-MoS2/p-Ge van der Waals heterojunction

A few-atomic-layer molybdenum disulfide (MoS₂) film on Si/SiO₂ substrates grown by metal-organic chemical vapor deposition was investigated. The few-atomic-layer MoS₂ film was subsequently transferred onto a (100) p-Ge substrate to build a van der Waals n-p heterojunction. The as-grown few-atomic-layer MoS₂ film and the MoS₂/Ge heterostructure were characterized atomic force microscopy, spectroscopic ellipsometry, high-resolution scanning transmission electron microscopy, Raman spectroscopy analyses, photoluminescence (PL) measurements at room temperature (RT, 300 K), and type-II band alignment of the heterostructure determined by ultraviolet photoelectron spectroscopy. The RT-PL measurements showed dominant peaks at 1.96 and 1.8 eV for the as-grown MoS₂ and red-shifted PL peaks for that transferred onto Ge. We examined the electrical characteristics of the few-atomic-layer MoS₂ by forming a type-II band alignment van der Waals heterojunction with a highly doped p-Ge. The heterojunction solar cell exhibited an open-circuit voltage of 0.15 V and a short-circuit current density of 45.26 μA/cm². The external quantum efficiency measurements showed a spectral response up to approximately 500 nm owing to the absorption by the few-atomic-layer MoS₂ film.     

Single atom doping in 2D layered MoS2 from a periodic table perspective

Molybdenum Disulfide (MoS₂) is a well-known transition metal dichalcogenide with a hexagonal structure arrangement analogous to graphene. Two dimensional (2D) MoS₂ has attracted wide attention in various applications such as energy storage, catalysis, sensing, energy conversion and optoelectronics due to its unique properties including tunable bandgap, substantial carrier mobility, outstanding mechanical strength and dangling-bond free basal surface. Moreover, MoS₂ has shown an excellent capability to be a host for foreign atoms which tune its physicochemical properties. Herein, currently known structural changes in the MoS₂ crystals introduced by various single atom dopants coming from all over the chemical table of elements are reviewed. Accompanying electrical, optical and magnetic properties of such structures are discussed in detail. Potential applications of the doped MoS₂ are introduced briefly as well. The review concentrates on the recent state-of-the-art results obtained mostly by the high resolution scanning transmission electron microscopy (STEM), such as high angle annular dark field (HAADF) imaging as well as scanning probe microscopy (SPM) such as scanning tunneling microscopy (STM). These techniques have been used to decipher dopant positions and other sub-atomic structural changes introduced to the MoS₂ structure by isolated dopants.     

A study of the behavior of Fe and Ni on MoS2

A correlation of AES and work function measurements of Fe and Ni on MoS₂ indicated that, near room temperature of MoS₂ and low coverage, Fe and Ni formed islands. With increasing coverage the Fe-islands were developed to 3-D small particles, while Ni-islands needed higher temperature for this development. The average size and density of the particles depended on the substrate temperature. Beyond a certain size the Fe-particles began to act catalytically on an interaction of the subsequently deposited Fe with S atoms of the top layer of MoS₂. The Ni particles remained clean up to 800 K and did not interact with S.     

Ta/Ni/Ta multilayered ohmic contacts on n-type SiC

The interface formation, electrical properties and the surface morphology of multilayered Ta/Ni/Ta/SiC contacts were reported in this study. It was found that the conducting behavior of the contacts so fabricated is much dependent on the metal layer thickness and the subsequent annealing temperature. Auger electron spectroscopy (AES) and X-ray diffraction analyses revealed that Ni₂Si and TaC formed as a result of the annealing. The Ni atoms diffused downward to metal/SiC interface and converted into Ni₂Si layer in adjacent to the SiC substrate. The released carbon atoms reacted with Ta atoms to form TaC layer. Ohmic contacts with specific contact resistivity as low as 3 × 10⁻⁴ Ω cm² have been achieved after thermal annealing. The formation of carbon vacancies at the Ni₂Si/SiC interface, probably created by dissociation of SiC and formation of TaC during thermal annealing, should be responsible for the ohmic formation of the annealed Ta/Ni/Ta contacts. The addition of Ta into the Ni metallization scheme to n-SiC restricted the accumulation of carbon atoms left behind during Ni₂Si formation, improving the electrical and microstructure properties.     

Adsorption studies of Pd on MoS2

Adsorption of Pd on the basal plane of MoS₂ layer compound has been studied in an UHV system with AES, LEED and WF measurements. For substrate temperatures at or below 500 K the deposited Pd formed initially 2D islands. Upon further Pd deposition a number of the 2D islands, depending on temperature, changed to 3D particles. At 700 K Pd grew to 3D particles from the early stages of its deposition. Heating of Pd-covered MoS₂ at RT caused a change of the 2D islands to 3D particles which coalesce to larger and fewer. The Pd adatoms did not interact with the surface S atoms as Fe does. The Pd particles thus remain clean on MoS₂ which is promising in heterogeneous catalysis. At 1200 K a small amount of Pd atoms remained about uniformly distributed and submerged under the S atoms of the outmost layer of MoS₂.