48 research outputs found
Patterning of graphene on silicon-on-insulator waveguides through laser ablation and plasma etching
We present the use of femtosecond laser ablation for the removal of monolayer graphene from silicon-on-insulator (SOI) waveguides, and the use of oxygen plasma etching through a metal mask to peel off graphene from the grating couplers attached to the waveguides. Through Raman spectroscopy and atomic force microscopy, we show that the removal of graphene is successful with minimal damage to the underlying SOI waveguides. Finally, we employ both removal techniques to measure the contribution of graphene to the loss of grating-coupled graphene-covered SOI waveguides using the cut-back method. This loss contribution is measured to be 0.132 dB/μm
Graphene growth on Ge(100)/Si(100) substrates by CVD method
The successful integration of graphene into microelectronic devices is strongly dependent on the availability of direct deposition processes, which can provide uniform, large area and high quality graphene on nonmetallic substrates. As of today the dominant technology is based on Si and obtaining graphene with Si is treated as the most advantageous solution. However, the formation of carbide during the growth process makes manufacturing graphene on Si wafers extremely challenging. To overcome these difficulties and reach the set goals, we proposed growth of high quality graphene layers by the CVD method on Ge(100)/Si(100) wafers. In addition, a stochastic model was applied in order to describe the graphene growth process on the Ge(100)/Si(100) substrate and to determine the direction of further processes. As a result, high quality graphene was grown, which was proved by Raman spectroscopy results, showing uniform monolayer films with FWHM of the 2D band of 32 cm−1
Laser ablation- and plasma etching-based patterning of graphene on silicon-on-insulator waveguides
Robustness of momentum-indirect interlayer excitons in MoS2/WSe2 heterostructure against charge carrier doping
Monolayer transition-metal dichalcogenide (TMD) semiconductors exhibit strong
excitonic effects and hold promise for optical and optoelectronic applications.
Yet, electron doping of TMDs leads to the conversion of neutral excitons into
negative trions, which recombine predominantly non-radiatively at room
temperature. As a result, the photoluminescence (PL) intensity is quenched.
Here we study the optical and electronic properties of a MoS2/WSe2
heterostructure as a function of chemical doping by Cs atoms performed under
ultra-high vacuum conditions. By PL measurements we identify two interlayer
excitons and assign them to the momentum-indirect Q-Gamma and K-Gamma
transitions. The energies of these excitons are in a very good agreement with
ab initio calculations. We find that the Q-Gamma interlayer exciton is robust
to the electron doping and is present at room temperature even at a high charge
carrier concentration. Submicrometer angle-resolved photoemission spectroscopy
(micro-ARPES) reveals charge transfer from deposited Cs adatoms to both the
upper MoS2 and the lower WSe2 monolayer without changing the band alignment.
This leads to a small (10 meV) energy shift of interlayer excitons. Robustness
of the momentum-indirect interlayer exciton to charge doping opens up an
opportunity of using TMD heterostructures in light-emitting devices that can
work at room temperature at high densities of charge carriers
Residual Metallic Contamination of Transferred Chemical Vapor Deposited Graphene
Integration of graphene with Si microelectronics is very appealing by
offering potentially a broad range of new functionalities. New materials to be
integrated with Si platform must conform to stringent purity standards. Here,
we investigate graphene layers grown on copper foils by chemical vapor
deposition and transferred to silicon wafers by wet etch and electrochemical
delamination methods with respect to residual sub-monolayer metallic
contaminations. Regardless of the transfer method and associated cleaning
scheme, time-of-flight secondary ion mass spectrometry and total reflection
x-ray fluorescence measurements indicate that the graphene sheets are
contaminated with residual metals (copper, iron) with a concentration exceeding
10 atoms/cm. These metal impurities appear to be partly mobile
upon thermal treatment as shown by depth profiling and reduction of the
minority charge carrier diffusion length in the silicon substrate. As residual
metallic impurities can significantly alter electronic and electrochemical
properties of graphene and can severely impede the process of integration with
silicon microelectronics these results reveal that further progress in
synthesis, handling, and cleaning of graphene is required on the way to its
advanced electronic and optoelectronic applications.Comment: 26 pages, including supporting informatio
Case studies of electrical characterisation of graphene by terahertz time-domain spectroscopy
Graphene metrology needs to keep up with the fast pace of developments in graphene growth and transfer. Terahertz time-domain spectroscopy (THz-TDS) is a non-contact, fast, and non-destructive characterization technique for mapping the electrical properties of graphene. Here we show several case studies of graphene characterization on a range of different substrates that highlight the versatility of THz-TDS measurements and its relevance for process optimization in graphene production scenarios
Design of Material Structures for Heterostructure Barrier Varactors
The Heterostructure Barrier Varactor (HBV), first proposed by Kollberg et al. [1], has a symmetric C-V and an anti-symmetric I-V characteristic. Therefore it will only produce odd harmonics of the input frequency when used in mm- and submm-wave frequency multipliers, which greatly simplifies the multiplier design. The high-bandgap barriers prevent electron transport through the structure so that the depletion region of the modulation layers is controlled by the applied bias, thus modulating the capacitance of the device. By varying the number of barriers and the thickness and doping of the modulation layers, it is possible to tailor the HBV diode for various applications, e.g. power-handling capability and frequency of operation.We present four different InGaAs/InAlAs HBV materials on InP fabricated by MOVPE. Two are designed for high-power, mm-wave applications and the other two are optimised for high efficiency up to submm-wave frequencies. In order to handle high power levels, materials 1816 and 1817 have six barriers and a relatively low doping concentration which results in a high break-down voltage. The measured break-down voltage for 1817 of approximately 52 Volts for a current density of 0,1\ub5A/\ub5m2 is, to the best of our knowledge, the highest value reported for HBVs. Materials 1819 and 1820 have higher doping concentrations and shorter modulation layers to reduce losses and the effect of current saturation, see Table 1. Design methods and predicted RF-performance will be presented
Design of Material Structures for Heterostructure Barrier Varactors
The Heterostructure Barrier Varactor (HBV), first proposed by Kollberg et al. [1], has a symmetric C-V and an anti-symmetric I-V characteristic. Therefore it will only produce odd harmonics of the input frequency when used in mm- and submm-wave frequency multipliers, which greatly simplifies the multiplier design. The high-bandgap barriers prevent electron transport through the structure so that the depletion region of the modulation layers is controlled by the applied bias, thus modulating the capacitance of the device. By varying the number of barriers and the thickness and doping of the modulation layers, it is possible to tailor the HBV diode for various applications, e.g. power-handling capability and frequency of operation.We present four different InGaAs/InAlAs HBV materials on InP fabricated by MOVPE. Two are designed for high-power, mm-wave applications and the other two are optimised for high efficiency up to submm-wave frequencies. In order to handle high power levels, materials 1816 and 1817 have six barriers and a relatively low doping concentration which results in a high break-down voltage. The measured break-down voltage for 1817 of approximately 52 Volts for a current density of 0,1\ub5A/\ub5m2 is, to the best of our knowledge, the highest value reported for HBVs. Materials 1819 and 1820 have higher doping concentrations and shorter modulation layers to reduce losses and the effect of current saturation, see Table 1. Design methods and predicted RF-performance will be presented