Hussain Alsalman

van der Waals epitaxial growth of graphene on sapphire by chemical vapor deposition without a metal catalyst.

van der Waals epitaxial growth of graphene on c-plane (0001) sapphire by CVD without a metal catalyst is presented. The effects of CH(4) partial pressure, growth temperature, and H(2)/CH(4) ratio were investigated and growth conditions optimized. The formation of monolayer graphene was shown by Raman spectroscopy, optical transmission, grazing incidence X-ray diffraction (GIXRD), and low voltage transmission electron microscopy (LVTEM). Electrical analysis revealed that a room temperature Hall mobility above 2000 cm(2)/V·s was achieved, and the mobility and carrier type were correlated to growth conditions. Both GIXRD and LVTEM studies confirm a dominant crystal orientation (principally graphene [10-10] || sapphire [11-20]) for about 80-90% of the material concomitant with epitaxial growth. The initial phase of the nucleation and the lateral growth from the nucleation seeds were observed using atomic force microscopy. The initial nuclei density was ~24 μm(-2), and a lateral growth rate of ~82 nm/min was determined. Density functional theory calculations reveal that the binding between graphene and sapphire is dominated by weak dispersion interactions and indicate that the epitaxial relation as observed by GIXRD is due to preferential binding of small molecules on sapphire during early stages of graphene formation.

Electrical Characteristics of Multilayer MoS2 FET’s with MoS2/Graphene Heterojunction Contacts

The electrical properties of multilayer MoS2/graphene heterojunction transistors are investigated. Temperature-dependent I-V measurements indicate the concentration of unintentional donors in exfoliated MoS2 to be 3.57 × 10(11) cm(-2), while the ionized donor concentration is determined as 3.61 × 10(10) cm(-2). The temperature-dependent measurements also reveal two dominant donor levels, one at 0.27 eV below the conduction band and another located at 0.05 eV below the conduction band. The I-V characteristics are asymmetric with drain bias voltage and dependent on the junction used for the source or drain contact. I-V characteristics of the device are consistent with a long channel one-dimensional field-effect transistor model with Schottky contact. Utilizing devices, which have both graphene/MoS2 and Ti/MoS2 contacts, the Schottky barrier heights of both interfaces are measured. The charge transport mechanism in both junctions was determined to be either thermionic-field emission or field emission depending on bias voltage and temperature. On the basis of a thermionic field emission model, the barrier height at the graphene/MoS2 interface was determined to be 0.23 eV, while the barrier height at the Ti/MoS2 interface was 0.40 eV. The value of Ti/MoS2 barrier is higher than previously reported values, which did not include the effects of thermionic field emission.

Chemical Vapor Deposition Growth of Large Single-Crystal Mono-, Bi-, Tri-Layer Hexagonal Boron Nitride and Their Interlayer Stacking

Two-dimensional hexagonal boron nitride (h-BN) is a wide bandgap material which has promising mechanical and optical properties. Here we report the realization of an initial nucleation density of h-BN <1 per mm2 using low-pressure chemical vapor deposition (CVD) on polycrystalline copper. This enabled wafer-scale CVD growth of single-crystal monolayer h-BN with a lateral size up to ∼300 μm, bilayer h-BN with a lateral size up to ∼60 μm, and trilayer h-BN with a lateral size up to ∼35 μm. Based on the large single-crystal monolayer h-BN domain, the sizes of the as-grown bi- and trilayer h-BN grains are 2 orders of magnitude larger than typical h-BN multilayer domains. In addition, we achieved coalesced h-BN films with an average grain size ∼100 μm. Various flake morphologies and their interlayer stacking configurations of bi- and trilayer h-BN domains were studied. Raman signatures of mono- and multilayer h-BN were investigated side by side in the same film. It was found that the Raman peak intensity can be used as a marker for the number of layers.

Long wavelength optical response of graphene-MoS2 heterojunction

The optical response of graphene-MoS2 heterojunctions is investigated. Spatial resolution photoresponse maps obtained using multiple bias conditions are measured and analyzed by exciting the graphene-MoS2 heterojunction area, MoS2, and Ti-MoS2 junction on the same device with an 800 nm wavelength Ti-Sapphire raster scanning laser. It is found that a large photothermal electric (PTE) effect is the dominant mechanism for photoresponse in a graphene-MoS2 heterojunction. Responsivities of 0.139 mA/W and 0.019 mA/W on the graphene-MoS2 heterojunction area and 0.457 mA/W and 0.032 mA/W on the Ti-MoS2 junction area are observed with and without a bias, respectively, using a 430 μW laser. Current enhancement due to laser illumination is observed as far as 14 μm from the edge of the graphene-MoS2 heterojunction. Voltage generated by the PTE effect lowers the Schottky barrier junction, enabling more current flow during laser excitation. Photothermal-generated voltages of 0.22–0.47 mV and 31.8–37.9 mV are estimated ...

A high response MoS 2 -graphene hetero-junction photodetector with broad spectral range

In this paper, we demonstrate a novel MoS2-graphene hetero junction photodetector spanning a wide spectral response from visible to infrared with a photoconduction responsivity of 24.2 mA/W at 520 nm and 2.1 mA/W at 1.01 μm.

Microwave noise in epitaxial graphene on SiC

High frequency noise was measured for epitaxial graphene on SiC. The noise measurements were carried out in the frequency range from 200 MHz up to 10 GHz in the direction parallel to the applied electric field in the graphene layer. Nanosecond voltage pulses were applied to minimize the effect of graphene layer self-heating. The measured spectral density of current fluctuations within 200 MHz–2.5 GHz frequency range can be approximated with a generation-recombination noise and a white noise contribution. Shot noise dominates at 10 GHz. Fano factor of suppressed shot noise was estimated. The shot noise was possibly associated with electron jumps across the potential barriers located in the graphene layer. Current-voltage characteristics were measured up to 17 kV/cm electric field and the maximum drift velocity was estimated as ∼5.2×107 cm/s.

Photothermal electric effect of multilayer MoS 2 -graphene heterojunction

Due to the exceptional electrical and optical properties of graphene, public interest has grown rapidly since the material was exfoliated onto SiO2 in 2004 [1-3]. Other two dimensional (2D) crystal materials such as MoS2, hBN, and WSe2, individually or in combination with graphene, also have shown intriguing properties. One of the interesting applications is using the 2D heterojunction as a photodetector. There have been literatures reported on graphene-MoS2 heterojunction photodetectors, claiming the photoresponse mechanism as a photovoltaic effect (PV), which is due to the separation of electron-hole pairs created in MoS2 at less than 700 nm wavelengths [4, 5]. In this paper, we report photothermal electric effect (PTE) of a multilayer MoS2-graphene heterojunction photodetector based on the large differences of Seebeck coefficients between MoS2 (-700 μV/K) [6] and graphene (-50 μV/K) [7].

Growth of 2D heterostructures of graphene/BN

Metal free direct growth of graphene on h-BN using a high temperature (~1550°C) chemical vapor deposition technique was done under Ar environment. Growth temperature, methane partial pressure, hydrogen/methane flow ratio, and growth time were varied and optimized. Raman spectroscopy clearly showed the signature of graphene with G- (~1580cm-1) and 2D-mode (~2700cm-1). The smallest width of G- and 2D-peak was 30 and 55cm-1, respectively, and the Raman I2D/IG ratio varied between 0.7 and 1.8. Raman D-peak (~1350cm-1) shows a strong dependence on growth temperature with the smallest ID/IG value of 0.15 at 1550°C. In the case of long growth, nitrogen and boron doping were detected by x-ray photoelectron spectroscopy with a small Raman D’-peak. A continuous graphene film with the rms roughness (1×1 μm2 area) of 0.32nm was shown by atomic force microscopy. Early stage of growth revealed circular shaped nucleation islands, the density and heights of which are ~15/μm2 and 1-2 graphene monolayer (ML), respectively. The hydrogen/methane flow ratio was found to be a critical parameter to obtain smooth 2D growth. Growth of h-BN is performed with ammonia borane, hydrogen and Ar. The growth is found to be critically dependent on the conditions of the ammonia boran precursor. Reproducible continuous films of h-BN are reported.