55 research outputs found
Growth-Induced Strain in Chemical Vapor Deposited Monolayer MoS2: Experimental and Theoretical Investigation
Monolayer molybdenum disulphide (MoS) is a promising two-dimensional (2D)
material for nanoelectronic and optoelectronic applications. The large-area
growth of MoS has been demonstrated using chemical vapor deposition (CVD)
in a wide range of deposition temperatures from 600 {\deg}C to 1000 {\deg}C.
However, a direct comparison of growth parameters and resulting material
properties has not been made so far. Here, we present a systematic experimental
and theoretical investigation of optical properties of monolayer MoS grown
at different temperatures. Micro-Raman and photoluminescence (PL) studies
reveal observable inhomogeneities in optical properties of the as-grown single
crystalline grains of MoS. Close examination of the Raman and PL features
clearly indicate that growth-induced strain is the main source of distinct
optical properties. We carry out density functional theory calculations to
describe the interaction of growing MoS layers with the growth substrate as
the origin of strain. Our work explains the variation of band gap energies of
CVD-grown monolayer MoS, extracted using PL spectroscopy, as a function of
deposition temperature. The methodology has general applicability to model and
predict the influence of growth conditions on strain in 2D materials.Comment: 37 pages, 6 figures, 10 figures in supporting informatio
Non-invasive Scanning Raman Spectroscopy and Tomography for Graphene Membrane Characterization
Graphene has extraordinary mechanical and electronic properties, making it a
promising material for membrane based nanoelectromechanical systems (NEMS).
Here, chemical-vapor-deposited graphene is transferred onto target substrates
to suspend it over cavities and trenches for pressure-sensor applications. The
development of such devices requires suitable metrology methods, i.e.,
large-scale characterization techniques, to confirm and analyze successful
graphene transfer with intact suspended graphene membranes. We propose fast and
noninvasive Raman spectroscopy mapping to distinguish between freestanding and
substrate-supported graphene, utilizing the different strain and doping levels.
The technique is expanded to combine two-dimensional area scans with
cross-sectional Raman spectroscopy, resulting in three-dimensional Raman
tomography of membrane-based graphene NEMS. The potential of Raman tomography
for in-line monitoring is further demonstrated with a methodology for automated
data analysis to spatially resolve the material composition in micrometer-scale
integrated devices, including free-standing and substrate-supported graphene.
Raman tomography may be applied to devices composed of other two-dimensional
materials as well as silicon micro- and nanoelectromechanical systems.Comment: 23 pages, 5 figure
Contact Resistance Study of Various Metal Electrodes with CVD Graphene
In this study, the contact resistance of various metals to chemical vapour
deposited (CVD) monolayer graphene is investigated. Transfer length method
(TLM) structures with varying widths and separation between contacts have been
fabricated and electrically characterized in ambient air and vacuum condition.
Electrical contacts are made with five metals: gold, nickel, nickel/gold,
palladium and platinum/gold. The lowest value of 92 {\Omega}{\mu}m is observed
for the contact resistance between graphene and gold, extracted from back-gated
devices at an applied back-gate bias of -40 V. Measurements carried out under
vacuum show larger contact resistance values when compared with measurements
carried out in ambient conditions. Post processing annealing at 450{\deg}C for
1 hour in argon-95% / hydrogen-5% atmosphere results in lowering the contact
resistance value which is attributed to the enhancement of the adhesion between
metal and graphene. The results presented in this work provide an overview for
potential contact engineering for high performance graphene-based electronic
devices
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Electron Transport across Vertical Silicon/MoS2/Graphene Heterostructures: Towards Efficient Emitter Diodes for Graphene Base Hot Electron Transistors
Heterostructures comprising silicon, molybdenum disulfide (MoS2), and graphene are investigated with respect to the vertical current conduction mechanism. The measured current-voltage (I-V) characteristics exhibit temperature-dependent asymmetric current, indicating thermally activated charge carrier transport. The data are compared and fitted to a current transport model that confirms thermionic emission as the responsible transport mechanism across devices. Theoretical calculations in combination with the experimental data suggest that the heterojunction barrier from Si to MoS2 is linearly temperature-dependent for T = 200-300 K with a positive temperature coefficient. The temperature dependence may be attributed to a change in band gap difference between Si and MoS2, strain at the Si/MoS2 interface, or different electron effective masses in Si and MoS2, leading to a possible entropy change stemming from variation in density of states as electrons move from Si to MoS2. The low barrier formed between Si and MoS2 and the resultant thermionic emission demonstrated here make the present devices potential candidates as the emitter diode of graphene base hot electron transistors for future high-speed electronics. Copyright © 2020 American Chemical Society
All CVD Boron Nitride Encapsulated Graphene FETs with CMOS Compatible Metal Edge Contacts
We report on the fabrication and characterization of field effect transistors
(FETs) based on chemical vapor deposited (CVD) graphene encapsulated between
few layer CVD boron nitride (BN) sheets with complementary metal oxide
semiconductor (CMOS) compatible nickel edge contacts. Non-contact Tera-hertz
time domain spectroscopy (THz-TDS) of large-area BN/graphene/BN (BN/G/BN)
stacks reveals average sheet conductivity >1 mS/sq and average mobility of 2500
cm/Vs. Improved output conductance is observed in direct current (DC)
measurements under ambient conditions, indicating potential for radio-frequency
(RF) applications. Moreover, we report a maximum voltage gain of 6 dB from a
low frequency signal amplifier circuit. RF characterization of the GFETs yields
an f x L product of 2.64 GHzm and an f x L
product of 5.88 GHzm. This study presents for the first time THz-TDS usage
in combination with other characterization methods for device performance
assessment on BN/G/BN stacks. The results serve as a step towards scalable, all
CVD 2D material-based FETs for CMOS compatible future nanoelectronic circuit
architectures.Comment: 6 page
Electron Transport across Vertical Silicon/MoS2/Graphene Heterostructures: Towards Efficient Emitter Diodes for Graphene Base Hot Electron Transistors
Heterostructures comprising silicon, molybdenum disulfide (MoS2), and graphene are investigated with respect to the vertical current conduction mechanism. The measured current-voltage (I-V) characteristics exhibit temperature-dependent asymmetric current, indicating thermally activated charge carrier transport. The data are compared and fitted to a current transport model that confirms thermionic emission as the responsible transport mechanism across devices. Theoretical calculations in combination with the experimental data suggest that the heterojunction barrier from Si to MoS2 is linearly temperature-dependent for T = 200-300 K with a positive temperature coefficient. The temperature dependence may be attributed to a change in band gap difference between Si and MoS2, strain at the Si/MoS2 interface, or different electron effective masses in Si and MoS2, leading to a possible entropy change stemming from variation in density of states as electrons move from Si to MoS2. The low barrier formed between Si and MoS2 and the resultant thermionic emission demonstrated here make the present devices potential candidates as the emitter diode of graphene base hot electron transistors for future high-speed electronics. Copyright © 2020 American Chemical Society
Assessment of wafer-level transfer techniques of graphene with respect to semiconductor industry requirements
Graphene is a promising candidate for future electronic applications.
Manufacturing graphene-based electronic devices typically requires graphene
transfer from its growth substrate to another desired substrate. This key step
for device integration must be applicable at the wafer level and meet the
stringent requirements of semiconductor fabrication lines. In this work, wet
and semidry transfer (i.e. wafer bonding) are evaluated regarding wafer
scalability, handling, potential for automation, yield, contamination and
electrical performance. A wafer scale tool was developed to transfer graphene
from 150 mm copper foils to 200 mm silicon wafers with-out adhesive
intermediate polymers. The transferred graphene coverage ranged from 97.9% to
99.2% for wet transfer and from 17.2% to 90.8% for semidry transfer, with
average cop-per contaminations of 4.7x10 (wet) and 8.2x10
atoms/cm (semidry). The corresponding electrical sheet resistance extracted
from terahertz time-domain spectroscopy varied from 450 to 550
for wet transfer and from 1000 to 1650 for semidry transfer.
Although wet transfer is superior in terms of yield, carbon contamination level
and electrical quality, wafer bonding yields lower copper contamination levels
and provides scalability due to existing in-dustrial tools and processes. Our
conclusions can be generalized to all two-dimensional (2D) materials
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