We show here that n-type InAs/InGaSb superlattices can be electrically isolated from lightly doped n-type GaSb
substrates at much higher temperatures than from the more common undoped p-type GaSb substrates without the use of
a large band gap insulating buffer layer. Temperature dependent Hall effect measurements show superlattice conduction
up to near room temperature, which is significantly higher than the 20 K observed for p-type substrates. Multi-carrier
analysis of magnetic field dependent transport data demonstrate the absence of a substrate related conduction channel.
We argue that the isolation is due to the depletion layer at the p-n junction between the p-type buffer layer and the n-type
substrate.
In this study, we investigated the effect of substrates on the electrical properties of transferred graphene. A wide range of substrates such as silicon carbide (SiC), glass, kapton, photo-print paper, polydimethylsiloxane (PDMS) and Willow glass were selected based on their surface properties, flexibility and lattice match. Four monolayers of graphene were transferred onto each of these substrates. A comparative study of the electrical characteristics of the transferred graphene film only and graphene/guanine film on the different substrates was undertaken.
We experimentally demonstrated graphene plasmon resonant absorption in mid-IR by utilizing an
array of graphene nanoribbon resonators on SiO2 substrate. By tuning resonator width we probed
the graphene plasmons with λp ≤ λ0/100 and plasmon resonances as high as 0.240 eV (2100 cm-1) for 40 nm wide nanoresonators. Resonant absorption spectra revealed plasmon dispersion as well
as plasmon damping due to the interaction of graphene plasmons with the surface polar phonons in
SiO2 substrate and intrinsic graphene optical phonons. Graphene nanoribbons with varying
widths enabled us to identify the damping mechanisms of graphene plasmons and much reduced
damping was observed when the plasmon resonance frequencies were close to the substrate polar
phonon frequencies. Then, by direct ebeam exposure of graphene nanoresonators, we effectively
changed the carrier density and caused red-shift of the plasmon spectra. This work will provide
insight into light-sensitive, frequency-tunable photodetectors based on graphene’s plasmonic
excitations.
Electronic circuits alone cannot fully meet future requirements for speed, size, and weight of many sensor systems, such as digital radar technology and as a result, interest in integrated photonic circuits (IPCs) and the hybridization of electronics with photonics is growing. However, many IPC components such as photodetectors are not presently ideal, but germanium has many advantages to enable higher performance designs that can be better incorporated into an IPC. For example, Ge photodetectors offer an enormous responsivity to laser wavelengths near 1.55μm at high frequencies to 40GHz, and they can be easily fabricated as part of a planar silicon processing schedule. At the same time, germanium has enormous potential for enabling 1.55 micron lasers on silicon and for enhancing the performance of silicon modulators. Our new effort has begun by studying the deposition of germanium on silicon and beginning to develop methods for processing these films. In initial experiments comparing several common chemical solutions for selective etching under patterned positive photoresist, it was found that hydrogen peroxide (H2O2) at or below room temperature (20 C) produced the sharpest patterns in the Ge films; H2O2 at a higher temperature (50 C) resulted in the greatest lateral etching.
We report transmission spectroscopy results from the mid- to far-infrared on graphene, grown by chemical vapor
deposition (CVD) on Cu. Similar results have been reported by several groups and their substrates of choice were
thermal Si dioxide, quartz, or SiC, where strong phonon absorption results in transmission blocking bands in midinfrared.
Silicon wafers (thickness ~ 500 μm), on the other hand, have transmission extending out to about 100 cm-1
when the doping level is low. Therefore, we choose to use Si wafers as the carrier substrates for transferred CVD
graphene. The complex refractive index of the Si substrate is measured by infrared spectroscopic ellipsometry. As a
result, continuous spectra (without blocking bands) in the range of 400 to 4000 cm-1 are obtained and they are modeled
by free carrier absorption (the Drude model) and interband transitions (considering the Pauli blocking.) From these, the
carrier density, carrier mobility, sheet resistivity, intraband scattering rate, and graphene layer number can be inferred. In
the far-infrared range, the absorption is dominated by the intraband free carrier absorption and it mainly results from the
interband transition in the mid-infrared range. Having continuous spectra using the Si substrates gives us the advantage
to model the whole spectral region (from far-infrared to mid-infrared) accurately.
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