ABSTRACT: A method to incorporate polarization charges at heterojunctions in compact
models for transistors is presented. By including the polarization sheet charge
as a Dirac delta function, the Poisson equation is solved to yield a closed
equation for the surface potential. A compact model for transistors based on
the surface potential incorporating polarization charges describes the on-state
as well as the off-state regimes of device operation. The new method of
incorporating polarization charges in compact models helps make a direct
connection to the material properties of the transistor. The current-voltage
(I-V) curves generated by this model are in good agreement with the
experimental data for GaN HEMTs.
ABSTRACT: In this paper we propose and experimentally demonstrate arrays of graphene electro-absorption modulators as electrically reconfigurable patterns for terahertz cameras. The active element of these modulators consists of only single-atom-thick graphene, achieving a modulation of the THz wave reflectance > 50% with a potential modulation depth approaching 100%. Although the prototype presented here only contains 4x4 pixels, it reveals the possibility of developing reliable low-cost video-rate THz imaging systems employing single detector.
Optics Express 01/2013; 21(2):2324-30. · 3.59 Impact Factor
ABSTRACT: We report the realization of field-effect transistors (FETs) made with
chemically synthesized multilayer 2D crystal semiconductor MoS2. Electrical
properties such as the FET mobility, subthreshold swing, on/off ratio, and
contact resistance of chemically synthesized (s-) MoS2 are indistinguishable
from that of mechanically exfoliated (x-) MoS2, however flat-band voltages are
different, possibly due to polar chemical residues originating in the transfer
process. Electron diffraction studies and Raman spectroscopy show the
structural similarity of s-MoS2 to x-MoS2. This initial report on the behavior
and properties of s-MoS2 illustrates the feasibility of electronic devices
using synthetic layered 2D crystal semiconductors.
Solid-State Electronics 01/2013; 80:67 - 71. · 1.40 Impact Factor
ABSTRACT: Femtosecond transient absorption spectroscopy and microscopy were employed to study exciton dynamics in suspended and Si3N4 substrate-supported monolayer and few-layer MoS2 2D crystals. Exciton dynamics for the monolayer and few-layer structures were found to be remarkably different from those of thick crystals when probed at energies near that of the lowest energy direct exciton (A exciton). The intraband relaxation rate was enhanced by more than 40 fold in the monolayer in comparison to that observed in the thick crystals, which we attributed to defect assisted scattering. Faster electron-hole recombination was found in monolayer and few-layer structures due to quantum confinement effects that lead to an indirect-direct band-gap crossover. Nonradiative rather than radiative relaxation pathways dominate the dynamics in the monolayer and few-layer MoS2. Fast trapping of excitons by surface trap states was observed in monolayer and few-layer structures, pointing to the importance of controlling surface properties in atomically thin crystals such as MoS2 along with controlling their dimensions.
ACS Nano 12/2012; · 10.77 Impact Factor
ABSTRACT: We propose and discuss terahertz electro-absorption modulators based on
graphene plasmonic structures. The active device consists of a self-gated pair
of graphene layers, which are patterned to structures supporting THz plasmonic
resonances. These structures allow for efficient control of the effective THz
optical conductivity, thus absorption, even at frequencies much higher than the
Drude roll-off in graphene where most previously proposed graphene-based
devices become inefficient. Our analysis shows that reflectance-based device
configurations, engineered so that the electric field is enhanced in the active
graphene pair, could achieve very high modulation-depth, even ~100%, at any
frequency up to tens of THz.
Applied Physics Letters 12/2012; 101(26):261115. · 3.84 Impact Factor
ABSTRACT: The outstanding electrical and optical properties of graphene make it an
excellent alternative as a transparent electrode. Here we demonstrate the
application of graphene as collector material in internal photoemission (IPE)
spectroscopy; enabling the direct observation of both electron and hole
injections at a Si/Al2O3 interface and successfully overcoming the
long-standing difficulty of detecting holes injected from a semiconductor
emitter in IPE measurements. The observed electron and hole barrier heights are
3.5 eV and 4.1 eV, respectively. Thus the bandgap of Al2O3 can be further
deduced to be 6.5 eV, in close agreement with the valued obtained by vacuum
ultraviolet spectroscopic ellipsometry analysis. The detailed optical modeling
of a graphene/Al2O3/Si stack reveals that by using graphene in IPE measurements
the carrier injection from the emitter is significantly enhanced and the
contribution of carrier injection from the collector electrode is minimal. The
method can be readily extended to various IPE test structures for a complete
band alignment analysis and interface characterization.
ABSTRACT: Atomically thin two-dimensional molybdenum disulfide (MoS2) sheets have
attracted much attention due to their potential for future electronic
applications. They not only present the best planar electrostatic control in a
device, but also lend themselves readily for dielectric engineering. In this
work, we experimentally investigated the dielectric effect on the Raman and
photoluminescence (PL) spectra of monolayer MoS2 by comparing samples with and
without HfO2 on top by atomic layer deposition (ALD). Based on considerations
of the thermal, doping, strain and dielectric screening influences, it is found
that the red shift in the Raman spectrum largely stems from modulation doping
of MoS2 by the ALD HfO2, and the red shift in the PL spectrum is most likely
due to strain imparted on MoS2 by HfO2. Our work also suggests that due to the
intricate dependence of band structure of monolayer MoS2 on strain, one must be
cautious to interpret its Raman and PL spectroscopy.
ABSTRACT: Switchable metamaterials offer unique solutions for efficiently manipulating
electromagnetic waves, particularly for terahertz waves, which has been
difficult since naturally occurring materials rarely respond to terahertz
frequencies controllably. However, few terahertz modulators demonstrated to
date exhibit simultaneously low attenuation and high modulation depth. In this
letter we propose a new class of electrically-tunable terahertz metamaterial
modulators employing metallic frequency-selective-surfaces (FSS) in conjunction
with capacitively-tunable layers of electrons, promising near 100% modulation
depth and < 15% attenuation. The fundamental departure in our design from the
prior art is tuning enabled by self-gated electron layers that is independent
from the metallic FSS. Our proposal is applicable to all possible electrically
tunable elements including graphene, Si, MoS2, oxides etc, thus opening up
myriad opportunities for realizing high performance switchable metamaterials
over an ultra-wide terahertz frequency range.
Optics Express 10/2012; 20(27):28664. · 3.59 Impact Factor
ABSTRACT: We demonstrate a graphene-based electro-absorption modulator achieving extraordinary control of terahertz reflectance. By concentrating the electric field intensity in an active layer of graphene, an extraordinary modulation depth of 64% is achieved while simultaneously exhibiting low insertion loss (∼2 dB), which is remarkable since the active region of the device is atomically thin. This modulator performance, among the best reported to date, indicates the enormous potential of graphene for terahertz reconfigurable optoelectronic devices.
Nano Letters 08/2012; 12(9):4518-22. · 13.20 Impact Factor
ABSTRACT: Graphene nanoribbon (GNR) field-effect transistors (FETs) with widths down to
12 nm have been fabricated by electron beam lithography using a wafer-scale
chemical vapor deposition (CVD) process to form the graphene. The GNR FETs show
drain-current modulation of approximately 10 at 300 K, increasing to nearly 106
at 4 K. The strong temperature dependence of the minimum current indicates the
opening of a bandgap for CVD-grown GNR-FETs. The extracted bandgap is estimated
to be around 0.1 eV by differential conductance methods. This work highlights
the development of CVD-grown large-area graphene and demonstrates the opening
of a bandgap in nanoribbon transistors.
ABSTRACT: We report the realization of field-effect transistors (FETs) made with
chemically- synthesized layered two dimensional (2D) crystal semiconductor WS2.
The 2D Schottky-barrier FETs demonstrate ambipolar behavior and a high (~105x)
on/off current ratio at room temperature with current saturation. The behavior
is attributed to the presence of an energy bandgap in the 2D crystal material.
The FETs show clear photo response to visible light. The promising electronic
and optical characteristics of the devices combined with the layered 2D crystal
flexibility make WS2 attractive for future electronic and optical devices.
ABSTRACT: Compared to the intense research focus on the optical properties, the transport properties in non-polar and semi-polar III-nitride semiconductors remain relatively unexplored to date. The purpose of this paper is to discuss charge-transport properties in non-polar and semi-polar orientations of GaN in a comparative fashion to what is known for transport in polar orientations. A comprehensive approach is adopted, starting from an investigation of the differences in the electronic bandstructure along different polar orientations of GaN. The polarization fields along various orientations are then discussed, followed by the low-field electron and hole mobilities. A number of scattering mechanisms that are specific to non-polar and semi-polar GaN heterostructures are identified, and their effects are evaluated. Many of these scattering mechanisms originate due to the coupling of polarization with disorder and defects in various incarnations depending on the crystal orientation. The effect of polarization orientation on carrier injection into quantum-well light-emitting diodes is discussed. This paper ends with a discussion of orientation-dependent high-field charge-transport properties including velocity saturation, instabilities and tunneling transport. Possible open problems and opportunities are also discussed.
Semiconductor Science and Technology 01/2012; 27(2):024018. · 1.72 Impact Factor
ABSTRACT: Terahertz technology promises myriad applications including imaging, spectroscopy and communications. However, one major bottleneck at present for advancing this field is the lack of efficient devices to manipulate the terahertz electromagnetic waves. Here we demonstrate that exceptionally efficient broadband modulation of terahertz waves at room temperature can be realized using graphene with extremely low intrinsic signal attenuation. We experimentally achieved more than 2.5 times superior modulation than prior broadband intensity modulators, which is also the first demonstrated graphene-based device enabled solely by intraband transitions. The unique advantages of graphene in comparison to conventional semiconductors are the ease of integration and the extraordinary transport properties of holes, which are as good as those of electrons owing to the symmetric conical band structure of graphene. Given recent progress in graphene-based terahertz emitters and detectors, graphene may offer some interesting solutions for terahertz technologies.
Nature Communications 01/2012; 3:780. · 7.40 Impact Factor
ABSTRACT: Nitrogen-polar III-nitride heterostructures present unexplored advantages over Ga(metal)-polar crystals for optoelectronic devices. This work reports N-polar III-nitride quantum-well ultraviolet light-emitting diodes grown by plasma-assisted molecular beam epitaxy that integrate polarization-induced p-type doping by compositional grading from GaN to AlGaN along N-face. The graded AlGaN layer simultaneously acts as an electron blocking layer while facilitating smooth injection of holes into the active region, while the built-in electric field in the barriers improves carrier injection into quantum wells. The enhanced doping, carrier injection, and light extraction indicate that N-polar structures have the potential to exceed the performance of metal-polar ultraviolet light-emitting diodes.
Applied Physics Letters 10/2011; 99(17):171104-171104-3. · 3.84 Impact Factor
ABSTRACT: Monolayer of hexagonal boron nitride (h-BN), commonly known as "white
graphene" is a promising wide bandgap semiconducting material for
deep-ultaviolet optoelectronic devices. In this report, the light absorption of
a single layer hexagonal boron nitride is calculated using a tight-binding
Hamiltonian. The absorption is found to be monotonically decreasing function of
photon energy compared to graphene where absorption coefficient is independent
of photon energy and characterized by the effective fine-structure constant.
ABSTRACT: The modulation depth of 2-D electron gas (2DEG) based THz modulators using
AlGaAs/GaAs heterostructures with metal gates is inherently limited to < 30%.
The metal gate not only attenuates the THz signal (> 90%) but also severely
degrades the modulation depth. The metal losses can be significantly reduced
with an alternative material with tunable conductivity. Graphene presents a
unique solution to this problem due to its symmetric band structure and
extraordinarily high mobility of holes that is comparable to electron mobility
in conventional semiconductors. The hole conductivity in graphene can be
electrostatically tuned in the graphene-2DEG parallel capacitor configuration,
thus more efficiently tuning the THz transmission. In this work, we show that
it is possible to achieve a modulation depth of > 90% while simultaneously
minimizing signal attenuation to < 5% by tuning the Fermi level at the Dirac
point in graphene.
ABSTRACT: Correlated transient absorption and atomic force microscopy (AFM) measurements have been performed for monolayer graphene, both free-standing and supported on a glass substrate. The AFM images allow us to locate regions of the suspended graphene. The transient absorption traces show a fast instrument response limited decay, followed by a slower intensity dependent decay. The fast decay is assigned to a combination of coupling between the excited charge carriers and the optical phonon modes of graphene and the substrate, and diffusion of the charge carrier out of the probe region. The slow decay is due to the hot phonon effect and reflects the lifetime of the intrinsic optical phonons of graphene. The time constant for the slow decay is longer for suspended graphene compared to substrate-supported graphene. This is attributed to interactions between the excited charge carriers and the surface optical phonon modes of the substrate, which supplies an additional relaxation channel for supported graphene.
Nano Letters 06/2011; 11(8):3184-9. · 13.20 Impact Factor
Physica Status Solidi (A) Applications and Materials 06/2011; 208(7):1620 - 1622. · 1.46 Impact Factor
ABSTRACT: Indium gallium nitride nanowires show promise as being prime candidates for optical devices since they can be grown with band gaps spanning the visible spectra, while at the same time can be composed of stress free material. The goal of the work presented here was to obtain InGaN nanowires producing green emission at room temperature. Two growth recipes were found to yield InGaN nanowire growth on silicon substrates using plasma-assisted molecular beam epitaxy. At room temperature the photoluminescence (PL) of wire ensembles indeed peaked at 530 nm but, in addition, it was discovered that at low temperatures the emission often covered a broader (360–700 nm) spectrum. This broad optical range indicated indium content fluctuations in individual wires, wire-to-wire fluctuations, or a combination of the two. EDX measurements performed on single wires confirmed this hypothesis and correlated well with PL data. Low temperature PL studies of InGaN individual wires also revealed interwire and intrawire inhomogeneity of emission spectra stemming from a nonuniform indium distribution. The emission quantum yield for bright single wires was extracted to be more than 50% at 4 K. The findings suggest that the wire surfaces do not efficiently quench optical emission at low temperatures. These defect-free wires offer not only a potential path for green emitters, but also as integrated phosphors for broad spectral emission.
Journal of Applied Physics 04/2011; 109(8):084336-084336-10. · 2.17 Impact Factor