Guangjun Cheng

Ultraviolet/ozone treatment to reduce metal-graphene contact resistance

We report reduced contact resistance of single-layer graphene devices by using ultraviolet ozone treatment to modify the metal/graphene contact interface. The devices were fabricated from mechanically transferred, chemical vapor deposition grown single layer graphene. Ultraviolet ozone treatment of graphene in the contact regions as defined by photolithography and prior to metal deposition was found to reduce interface contamination originating from incomplete removal of poly(methyl-methacrylate) and photoresist. Our control experiment shows that exposure times up to 10 min did not introduce significant disorder in the graphene as characterized by Raman spectroscopy. By using the described approach, contact resistance of less than 200 Ω μm was achieved for 25 min ultraviolet ozone treatment, while not significantly altering the electrical properties of the graphene channel region of devices.

Influence of Metal–MoS2 Interface on MoS2 Transistor Performance: Comparison of Ag and Ti Contacts

In this work, we compare the electrical characteristics of MoS2 field-effect transistors (FETs) with Ag source/drain contacts with those with Ti and demonstrate that the metal-MoS2 interface is crucial to the device performance. MoS2 FETs with Ag contacts show more than 60 times higher ON-state current than those with Ti contacts. In order to better understand the mechanism of the better performance with Ag contacts, 5 nm Au/5 nm Ag (contact layer) or 5 nm Au/5 nm Ti film was deposited onto MoS2 monolayers and few layers, and the topography of metal films was characterized using scanning electron microscopy and atomic force microscopy. The surface morphology shows that, while there exist pinholes in Au/Ti film on MoS2, Au/Ag forms a smoother and denser film. Raman spectroscopy was carried out to investigate the metal-MoS2 interface. The Raman spectra from MoS2 covered with Au/Ag or Au/Ti film reveal that Ag or Ti is in direct contact with MoS2. Our findings show that the smoother and denser Au/Ag contacts lead to higher carrier transport efficiency.

Highly reproducible and reliable metal/graphene contact by ultraviolet-ozone treatment

Resist residue from the device fabrication process is a significant source of contamination at the metal/graphene contact interface. Ultraviolet Ozone (UVO) treatment is proven here, by X-ray photoelectron spectroscopy and Raman measurement, to be an effective way of cleaning the metal/graphene interface. Electrical measurements of devices that were fabricated by using UVO treatment of the metal/graphene contact region show that stable and reproducible low resistance metal/graphene contacts are obtained and the electrical properties of the graphene channel remain unaffected.

Epitaxial graphene homogeneity and quantum Hall effect in millimeter-scale devices

Quantized magnetotransport is observed in 5.6 × 5.6 mm2 epitaxial graphene devices, grown using highly constrained sublimation on the Si-face of SiC(0001) at high temperature (1900 °C). The precise quantized Hall resistance of [Formula: see text] is maintained up to record level of critical current Ixx = 0.72 mA at T = 3.1 K and 9 T in a device where Raman microscopy reveals low and homogeneous strain. Adsorption-induced molecular doping in a second device reduced the carrier concentration close to the Dirac point (n ≈ 1010 cm-2), where mobility of 18760 cm2/V is measured over an area of 10 mm2. Atomic force, confocal optical, and Raman microscopies are used to characterize the large-scale devices, and reveal improved SiC terrace topography and the structure of the graphene layer. Our results show that the structural uniformity of epitaxial graphene produced by face-to-graphite processing contributes to millimeter-scale transport homogeneity, and will prove useful for scientific and commercial applications.

Field effects of current crowding in metal-MoS2 contacts

Gate assisted contact-end Kelvin test structures and gate assisted four-probe structures have been designed and fabricated to measure the field effects of current crowding at the source/drain contacts of top-gate MoS2 field effect transistors. The transistors exhibited n-type transistor characteristics. The source/drain contact resistance was measured by using both gate-assisted Kelvin and gate-assisted four-probe structures. The values of contact resistance measured by these two test structures are significantly different. The contact-front contact resistance obtained from the four-probe structure is strongly influenced by field effects on current crowding, while the contact-end resistance obtained from the Kelvin test structure is not. The metal-MoS2 contact current transfer length, LT, can be determined from the comparison between these two measurements. LT was observed to increase linearly with increasing gate voltage. This work indicates that the contact characteristics can be more precisely measured when both gate-assisted test structures are used.

Toward clean suspended CVD graphene

The application of suspended graphene as electron transparent supporting media in electron microscopy, vacuum electronics, and micromechanical devices requires the least destructive and maximally clean transfer from their original growth substrate to the target of interest. Here, we use thermally evaporated anthracene films as the sacrificial layer for graphene transfer onto an arbitrary substrate. We show that clean suspended graphene can be achieved via desorbing the anthracene layer at temperatures in the 100 °C to 150 °C range, followed by two sequential annealing steps for the final cleaning, using Pt catalyst and activated carbon. The cleanliness of the suspended graphene membranes was analyzed employing the high surface sensitivity of low energy scanning electron microscopy and x-ray photoelectron spectroscopy. A quantitative comparison with two other commonly used transfer methods revealed the superiority of the anthracene approach to obtain larger area of clean, suspended CVD graphene. Our graphene transfer method based on anthracene paves the way for integrating cleaner graphene in various types of complex devices, including the ones that are heat and humidity sensitive.

The Influence of Temperature on the Magnetic Behavior of Colloidal Cobalt Nanoparticles

Applications of magnetic nanoparticles, including hyperthermia for cancer treatments, require knowledge of how the colloidal environment affects the magnetic properties of the nanoparticles. Here, 10 nm diameter cobalt nanoparticles synthesized by thermodecomposition in 1,2-dichlorobenzene (DCB) are used to study the effect of the colloidal environment on the magnetic behavior of such materials. The magnetic properties are investigated by magnetization (M) versus temperature (T) measurements and vector magnetometry performed on the samples under zero-field-cooled conditions. Of particular interest in the M versus T data is a continuous rise in the magnetization observed around the DCB melting point during sample heating and a discontinuous drop around the DCB supercooling point during sample cooling. Vector magnetometer measurements quantify the portion of the sample that does not respond to the applied field. The magnitude of this unreversed component doubles with decreasing temperature as the temperature cools through the supercooling point in DCB. There is also an increase in the uniaxial anisotropy of the sample from 61.1(7)times10-7 J to 104.2(9)times10-7 J as the liquid-to-solid transition is traversed.

Band offset and electron affinity of MBE-grown SnSe2

SnSe2 is currently considered a potential two-dimensional material that can form a near-broken gap heterojunction in a tunnel field-effect transistor due to its large electron affinity which is experimentally confirmed in this letter. With the results from internal photoemission and angle-resolved photoemission spectroscopy performed on Al/Al2O3/SnSe2/GaAs and SnSe2/GaAs test structures where SnSe2 is grown on GaAs by molecular beam epitaxy, we ascertain a (5.2 ± 0.1) eV electron affinity of SnSe2. The band offset from the SnSe2 Fermi level to the Al2O3 conduction band minimum is found to be (3.3 ± 0.05) eV and SnSe2 is seen to have a high level of intrinsic electron (n-type) doping with the Fermi level positioned at about 0.2 eV above its conduction band minimum. It is concluded that the electron affinity of SnSe2 is larger than that of most semiconductors and can be combined with other appropriate semiconductors to form near broken-gap heterojunctions for the tunnel field-effect transistor that can pote...

Millimeter-sized graphene quantum hall devices for resistance standards

We present a repeatable process to produce millimeter-sized graphene device for quantum Hall resistance (QHR) standard. Our large-area, homogeneous monolayer graphene is grown on silicon carbide (SiC) substrate at high temperature by an optimized epitaxial method. Well-developed quantum Hall plateaus have been observed in devices fabricated from samples annealed at 1900 oC by a clean lithography process. Metrological accuracy of better than 5 × 10-9 has been observed in a 27 mm2, octagonal device at a record current level of 0.72 mA for temperatures between 1.6 K and 3.1 K.

Modified RCA clean transfer of graphene and all-carbon electronic devices fabrication

Graphene is regarded as a promising material that could be the basis for future generations of low-power, faster, and smaller electronics [1,2]. Currently, chemical vapor deposition (CVD) growth method is the only way that can produce large area monolayer graphene up to tens of inches with high quality [3,4], which makes it the most promising graphene producing method for large scale device applications. The first step necessary in fabricating devices from CVD-grown graphene, is to transfer the graphene from the metal growth substrate onto a device-compatible substrate (typically an insulator). It is crucial to device performance, yield, and uniformity that the quality of the graphene is not degraded during this transfer process.