The nonlinear response of electrons (holes) in doped graphene on ac pumping is considered theoretically for the frequency region above the energy relaxation rate but below the momentum and carrier-carrier scattering rates. Temporally-dependent heating of electrons by a strong ac field, which is described within the energy balance approach, leads to an effective generation of high-order harmonics. The efficiency of up-conversion of the 1 mm radiation into the third harmonic by a single-layer graphene is about 10^{-7} at pumping level ~100 kW/cm^2, room temperature, and concentration ~5 x 10^{11} cm^{-2}.
Using density functional theory, we present a comparative study of the electronic properties of BN-doped graphene monolayer, bilayer, trilayer, and multilayer systems. In addition, we address a superlattice of pristine and BN-doped graphene. Five doping concentrations between 12.5% and 75% are considered, for which we obtain band gaps from 0.02 eV to 2.43 eV. We show that the effective mass varies between 0.007 and 0.209 free electron masses, resembling a high mobility of the charge carriers.
A topological insulator is a new state of matter, possessing gapless spin-locking surface states across the bulk band gap which has created new opportunities from novel electronics to energy conversion. However, the large concentration of bulk residual carriers has been a major challenge for revealing the property of the topological surface state via electron transport measurement. Here we report surface state dominated transport in Sb-doped Bi2Se3 nanoribbons with very low bulk electron concentrations. In the nanoribbons with sub-10nm thickness protected by a ZnO layer, we demonstrate complet…
Time- and angle-resolved photoemission measurements on two doped graphene samples displaying different doping levels reveal remarkable differences in the ultrafast dynamics of the hot carriers in the Dirac cone. In the more strongly ($n$-)doped graphene, we observe larger carrier multiplication factors ($>$ 3) and a significantly faster phonon-mediated cooling of the carriers back to equilibrium compared to in the less ($p$-)doped graphene. These results suggest that a careful tuning of the doping level allows for an effective manipulation of graphene's dynamical response to a photoexcitation.
We investigated negative photoconductivity in graphene using ultrafast terahertz techniques. Infrared transmission was used to determine the Fermi energy, carrier density and mobility of p-type CVD graphene samples. Time-resolved terahertz photoconductivity measurements using a tunable mid-infrared pump probed these samples at photon energies between 0.35eV to 1.55eV, approximately one half to three times the Fermi energy of the samples. Although interband optical transitions in graphene are blocked for pump photon energies less than twice the Fermi energy, we observe negative photoconductivit…
A 2D electron gas system in an oxide heterostructure serves as an important playground for novel phenomena. Here, we show that, by using fractional delta-doping to control the interface's composition in LaxSr1-xTiO3/SrTiO3 artificial oxide superlattices, the filling-controlled 2D insulator-metal transition can be realized. The atomic-scale control of d-electron band filling, which in turn contributes to the tuning of effective mass and density of the charge carriers, is found to be a fascinating route to substantially enhanced carrier mobilities.
The rate at which excited charge carriers relax to their equilibrium state affects many aspects of the performance of nanoscale devices, including switching speed, carrier mobility and luminescent efficiency. Better understanding of the processes that govern carrier relaxation therefore has important technological implications. A significant increase in carrier-carrier interactions caused by strong spatial confinement of electronic excitations in semiconductor nanostructures leads to a considerable enhancement of Auger effects, which can further result in unusual, Auger-process-controlled reco…
A significant advance toward achieving practical applications of graphene as a two-dimensional material in nanoelectronics would be provided by successful synthesis of both n-type and p-type doped graphene. However reliable doping and a thorough understanding of carrier transport in the presence of charged impurities governed by ionized donors or acceptors in the graphene lattice are still lacking. Here we report experimental realization of few-layer nitrogen-doped (N-doped) graphene sheets by chemical vapor deposition of organic molecule 1, 3, 5-triazine on Cu metal catalyst. By reducing the …
The double carrier transport has been observed in thin film black phosphorus (BP) field effect transistor (FET) devices in highly electron doped region. BP thin films with typical thickness of 15 nm were encapsulated by hexagonal boron nitride (h-BN) thin films to avoid degradation by air exposure. Their Hall mobility has reached 5300 cm2/Vs and 5400 cm2/Vs at 4.2 K in the hole and electron doped regions, respectively. The gate voltage dependence of conductivity exhibits an anomalous shoulder structure in electron doped region. In addition, at gate voltages above the shoulder, the magnetoresis…
We modulate the atomic structure of bilayer graphene by driving its lattice at resonance with the in-plane E1u lattice vibration at 6.3um. Using time- and angle-resolved photoemission spectroscopy (tr-ARPES) with extreme ultra-violet (XUV) pulses, we measure the response of the Dirac electrons near the K-point. We observe that lattice modulation causes anomalous carrier dynamics, with the Dirac electrons reaching lower peak temperatures and relaxing at faster rate compared to when the excitation is applied away from the phonon resonance or in monolayer samples. Frozen phonon calculations predi…
Carrier-dopant exchange interactions in Mn-doped PbS colloidal quantum dots were studied by circularly polarized magneto-photoluminescence. Mn substitutional doping leads to paramagnetic behavior down to 5 K. While undoped quantum dots show negative circular polarization, Mn doping changes its sign to positive. A circular polarization value of 40% was achieved at T=7 K and B=7 tesla. The results are interpreted in terms of Zeeman splitting of the band edge states in the presence of carrier-dopant exchange interactions that are qualitatively different from the s,p-d exchange interactions in II-…
The quantum capacitance model is applied to obtain an exact solution for the space-resolved carrier density in a multigated doped graphene sheet at zero temperature, with quantum correction arising from the finite electron capacity of the graphene itself taken into account. The exact solution is demonstrated to be equivalent to the self-consistent Poisson-Dirac iteration method by showing an illustrative example, where multiple gates with irregular shapes and a nonuniform dopant concentration are considered. The solution therefore provides a fast and accurate way to compute spatially varying c…
Graphene/silicon heterostructures have attracted tremendous interest as a new platform for diverse electronic and photonic devices such as barristors, solar cells, optical modulators, and chemical sensors. The studies to date largely focus on junctions between graphene and lightly-doped silicon, where a Schottky barrier is believed to dominate the carrier transport process. Here we report a systematic investigation of carrier transport across the heterojunctions formed between graphene and highly-doped silicon. By varying the silicon doping level and the measurement temperature, we show that t…
In nanoscale junctions the interaction between charge carriers and the local vibrations results in renormalization, damping and heating of the vibrational modes. We here formulate a nonequilibrium Green's functions based theory to describe such effects. Studying a generic junction model with an off-resonant electronic level, we find a strong bias dependence of the frequency renormalization and vibrational damping accompanied by pronounced nonlinear vibrational heating in junctions with intermediate values of the coupling to the leads. Combining our theory with ab-initio calculations we further…
Carrier doping by the electric field effect has emerged recently as an ideal route for monitoring many-body physics in two-dimensional (2D) materials where the Fermi level is tuned in a way that -- indirectly -- the strength of the interactions can also be scanned. The possibility of systematic doping in combination with high resolution photoemission has allowed to uncover a genuinely many-body electron spectrum in single-layer MoS2 transition metal dichalcogenide, resolving three clear quasi-particle states, where only one state should be expected if the electron-phonon interaction vanished. …
Graphene is a promising material for ultrafast and broadband photodetection. Earlier studies addressed the general operation of graphene-based photo-thermoelectric devices, and the switching speed, which is limited by the charge carrier cooling time, on the order of picoseconds. However, the generation of the photovoltage could occur at a much faster time scale, as it is associated with the carrier heating time. Here, we measure the photovoltage generation time and find it to be faster than 50 femtoseconds. As a proof-of-principle application of this ultrafast photodetector, we use graphene to…
There has been much interest in the injection and detection of spin polarized carriers in semiconductors for the purposes of developing novel spintronic devices. Here we report the electrical injection and detection of spin-polarized carriers into Nb-doped strontium titanate (STO) single crystals and La-doped STO epitaxial thin films using MgO tunnel barriers and the three-terminal Hanle technique. Spin lifetimes of up to ~100 ps are measured at room temperature and vary little as the temperature is decreased to low temperatures. However, the mobility of the STO has a strong temperature depend…
The modulation of the transmitted (reflected) radiation due to change of interband transitions under variation of carriers concentration by the gate voltage is studied theoretically. The calculations were performed for strongly doped graphene on high-K (Al_2O_3, HfO_2, AlN, and ZrO_2) or SiO_2 substrates under normal propagation of radiation. We have obtained the modulation depth above 10% depending on wavelength, gate voltage (i.e. carriers concentration), and parameters of substrate. The graphene - dielectric substrate - doped Si (as gate) structures can be used as an effective electrooptica…
The controlled modification of the electronic properties of ZnO nanorods via transition metal doping is reported. A series of ZnO nanorods were synthesized by chemical bath growth with varying Co content from 0 to 20 atomic % in the growth solution. Optoelectronic behavior was probed using cathodoluminescence, time-resolved luminescence, transient absorbance spectroscopy, and the incident photon-to-current conversion efficiency (IPCE). Analysis indicates the crucial role of surface defects in determining the electronic behavior. Significantly, Co-doping extends the light absorption of the nano…
We present a microscopic study on the impact of doping on the carrier dynamics in graphene, in particular focusing on its influence on the technologically relevant carrier multiplication in realistic, doped graphene samples. Treating the time- and momentum-resolved carrier-light, carrier-carrier, and carrier-phonon interactions on the same microscopic footing, the appearance of Auger-induced carrier multiplication up to a Fermi level of 300 meV is revealed. Furthermore, we show that doping favors the so-called hot carrier multiplication occurring within one band. Our results are directly compa…
Phosphorus (P) in germanium (Ge) delta-doped layers are fabricated in ultra-high vacuum by adsorption of phosphine molecules onto an atomically flat clean Ge(001) surface followed by thermal incorporation of P into the lattice and epitaxial Ge overgrowth by molecular beam epitaxy. Structural and electrical characterizations show that P atoms are confined, with minimal diffusion, into an ultra-narrow 2-nm-wide layer with an electrically-active sheet carrier concentration of 4x10^13 cm-2 at 4.2 K. These results open up the possibility of ultra-narrow source/drain regions with unprecedented carri…
For most optoelectronic applications of graphene a thorough understanding of the processes that govern energy relaxation of photoexcited carriers is essential. The ultrafast energy relaxation in graphene occurs through two competing pathways: carrier-carrier scattering -- creating an elevated carrier temperature -- and optical phonon emission. At present, it is not clear what determines the dominating relaxation pathway. Here we reach a unifying picture of the ultrafast energy relaxation by investigating the terahertz photoconductivity, while varying the Fermi energy, photon energy, and fluenc…
We have calculated, within the framework of the Boltzmann equation, a dc electric current and emf in a two-dimensional system induced by the high-frequency field of an electromagnetic wave or the electric field of a plasmon wave. It is established that the generated current consists of two contributions, one of which is proportional to the real part of the wave vector projection of the exciting wave onto the interface plane and represents the electron drag effect, and the other contribution is proportional to the extinction coefficient of the wave in the interface plane. It is shown that the m…
Considering scattering by unintentional background charged impurities and by charged dopants in the modulation doping layer as well as by GaAs acoustic phonons, we theoretically consider the practical intrinsic (phonons) and extrinsic (background and dopants) limits to carrier mobility in modulation doped AlGaAs-GaAs 2D semiconductor structures. We find that reducing background impurity density to $10^{12}$ cm$^{-3}$ along with a modulation doping separation of 1000 Åor above will achieve a mobility of $100 \times 10^6$ cm$^2$/Vs at a carrier density of $3\times 10^{11}$ cm$^{-2}$ for T=1K. At…
We use magneto-conductivity to study the magnetic proximity effect on surface states of doped topological insulators. Our bilayers consist of a layer of Fe$_7$Se$_8$, which is a metallic ferrimagnet and a layer of Bi$_{0.8}$Sb$_{1.2}$Te$_{3}$ which is a highly hole-doped topological insulator. Using transport measurements and a modified Hikami-Larkin-Nagaoka model, we show that the ferromagnet shortens significantly the effective coherence length of the surface states, suggesting that a gap is opened at the Dirac point. We show that the magnetically induced gap persists on surface states which…
In the present theoretical work we have considered impurities, either boron or phosphorous, located at different substitutional sites in silicon quantum dots (Si-QDs) with diameters around 1.5\,nm, embedded in a SiO2 matrix. Formation energy calculations reveal that the most energetically-favored doping sites are inside the QD and at the Si/SiO2 interface for P and B impurities, respectively. Furthermore, electron and hole transport calculations show in all the cases a strong reduction of the minimum voltage threshold, and a corresponding increase of the total current in the low-voltage regime…
Minority carrier diffusion lengths in low-doped n-InGaAs using InP/InGaAs double-heterostructures are reported using a simple electrical technique. The contributions from heavy and light holes are also extracted using this methodology, including minority carrier mobilities and lifetimes. Heavy holes are shown to initially dominate the transport due to their higher valence band density of states, but at large diffusion distances, the light holes begin to dominate due to their larger diffusion length. It is found that heavy holes have a diffusion length of 54.5 +/- 0.6 microns for an n-InGaAs do…
A theory is developed for the density and temperature dependent carrier conductivity in doped three-dimensional (3D) Dirac materials focusing on resistive scattering from screened Coulomb disorder due to random charged impurities (e.g., dopant ions and unintentional background impurities). The theory applies both in the undoped intrinsic ("high-temperature", $T \gg T_F$) and the doped extrinsic ("low-temperature", $T \ll T_F$) limit with analytical scaling properties for the carrier conductivity obtained in both regimes, where $T_F$ is the Fermi temperature corresponding to the doped free carr…
We present the results of Monte-Carlo simulations of the charge carrier transport in a disordered molecular system containing spatial and energetic disorders using the dipolar glass model. Model parameters of the material were chosen to fit a typical polar organic photoconductor polycarbonate doped with 30% of aromatic hydrazone, whose transport properties are well documented in literature. Simulated carrier mobility demonstrates a usual Poole-Frenkel field dependence and its slope is very close to the experimental value without using any adjustable parameter. At room temperature transients ar…
Recent advances in the growth of III-V semiconductor nanowires (NWs) hold great promise for nanoscale optoelectronic device applications. Recently, it was found that a small amount of nitrogen (N) incorporation in III-V semiconductor NWs can effectively red-shift their wavelength of operation and tailor their electronic properties for specific applications. However, understanding the impact of N incorporation on non-equilibrium charge carrier dynamics and transport in semiconducting NWs is critical in achieving efficient semiconducting NW devices. In this work, ultrafast optical pump-terahertz…
Understanding the interplay between illumination and the electron distribution in metallic nanostructures is a crucial step towards developing applications such as plasmonic photo-catalysis for green fuels, nano-scale photo-detection and more. Elucidating this interplay is challenging, as it requires taking into account all channels of energy flow in the electronic system. Here, we develop such a theory, which is based on a coupled Boltzmann-heat equations and requires only energy conservation and basic thermodynamics, where the electron distribution, and the electron and phonon (lattice) temp…
Graphene is known to show a significant photo-thermoelectric effect that can exceed its photovoltaic contribution. Here, by utilizing this effect, we demonstrate a photovoltage measurement of cyclotron resonance in a double-back-gated h-BN/graphene/h-BN two-terminal device. A graphite local bottom gate was fabricated in addition to a p-doped Si global back gate. By tuning the two gate voltages, an in-plane graphene junction having an asymmetric carrier-doping profile was created. With the help of this asymmetric structure, the photo-thermoelectric voltage generated in the vicinity of the metal…
Inducing an inversion layer in organic semiconductors is a highly nontrivial, but critical, achievement for producing organic field-effect transistor (OFET) devices, which rely on the generation of inversion, accumulation, and depletion regimes for successful operation. In recent years, a major milestone was reached: an OFET was made to successfully operate in the inversion-mode for the first time. Here, we develop a pulsed bias technique to characterize the dopant type of any organic material system, without prior knowledge or characterization of the material in question. We use this techniqu…
Bi2Se3 is an important semiconductor thermoelectric material and a prototype topological insulator. Here we report observation of Shubnikov-de Hass (SdH) oscillations accompanied by quantized Hall resistances (Rxy) in highly-doped n-type Bi2Se3 with bulk carrier concentrations of few 10^19 cm^-3. Measurements under tilted magnetic fields show that the magnetotransport is 2D-like, where only the c-axis component of the magnetic field controls the Landau level formation. The quantized step size in 1/Rxy is found to scale with the sample thickness, and average ~e2/h per quintuple layer (QL). We s…
The authors of preprint arXiv:1810.00565 and paper in Light: Science & Applications (2019), Y. Dubi and Y. Sivan, made several wrong and inconsistent comments on several of our papers. In addition, the paper in Faraday Discuss. 214, 215-233 (2018) by the same authors also contains several wrong statements in relationship with our work. Moreover, the authors address in their comments features that were in fact not present in our work. In what follows we present correction to a number of specific points in which they either misrepresented or erroneously interpreted our published work.
The single-band current-dipole Kubo formula for the dynamical conductivity of heavily doped graphene from Kupčić [Phys. Rev. B 91, 205428 (2015)] is extended to a two-band model for conduction $π$ electrons in lightly doped graphene. Using a posteriori relaxation-time approximation in the two-band quantum transport equations, with two different relaxation rates and one quasi-particle lifetime, we explain a seemingly inconsistent dependence of the dc conductivity $σ^{\rm dc}_{αα}$ of ultraclean and dirty lightly doped graphene samples on electron doping, in a way consistent with the charge cont…
We find, through first-principles calculations, that hole doping induces a ferromagnetic phase transition in monolayer GaSe. Upon increasing hole density, the average spin magnetic moment per carrier increases and reaches a plateau near 1.0 $μ_{\rm{B}}$/carrier in a range of $3\times 10^{13}$/cm$^{2}$-$1\times 10^{14}$/cm$^{2}$ with the system in a half-metal state before the moment starts to descend abruptly. The predicted magnetism originates from an exchange splitting of electronic states at the top of the valence band where the density of states exhibits a sharp van Hove singularity in thi…
We investigated the dynamics of the interaction between spin-polarized photo-created carriers and Mn ions on InGaAs/GaAs:Mn structures. The carriers are confined in an InGaAs quantum well and the Mn ions come from a Mn delta-layer grown at the GaAs barrier close to the well. Even though the carriers and the Mn ions are spatially separated, the interaction between them is demonstrated by time-resolved spin-polarized photoluminescence measurements. Using a pre-pulse laser excitation with an opposite circular-polarization clearly reduces the polarization degree of the quantum-well emission for sa…
In an ideal bulk topological-insulator (TI) conducting surface states protected by time reversal symmetry enfold an insulating crystal. However, the archetypical TI, Bi2Se3, is actually never insulating; it is in fact a relatively good metal. Nevertheless, it is the most studied system among all the TIs, mainly due to its simple band-structure and large spin-orbit gap. Recently it was shown that copper intercalated Bi2Se3 becomes superconducting and it was suggested as a realization of a topological superconductor (TSC). Here we use a combination of techniques that are sensitive to the shape o…
The notion of artifical atom relies on the capability to change the number of carriers one by one in semiconductor quantum dots, and the resulting changes in their electronic structure. Organic molecules with transition metal atoms that have a net magnetic moment and display hysteretic behaviour are known as single molecule magnets (SMM). The fabrication of CdTe quantum dots chemically doped with a controlled number of Mn atoms and with a number of carriers controlled either electrically or optically paves the way towards a new concept in nanomagnetism: the artificial single molecule magnet. H…
We show how a proper radial modulation of the composition of core-multi-shell nanowires critically enhances the control of the free-carrier density in the high-mobility core with respect to core-single-shell structures, thus overcoming the technological difficulty of fine tuning the remote doping density. We calculate the electron population of the different nanowire layers as a function of the doping density and of several geometrical parameters by means of a self-consistent Schrödinger-Poisson approach: Free carriers tend to localize in the outer shell and screen the core from the electric f…
We study theoretically wave-vector and frequency dispersion of the complex dynamic conductivity tensor (DCT), $σ_{lm}(\mathbf{k}, ω)$, of doped monolayer graphene under a strong dc electric field. For a general analysis, we consider the weak ac field of arbitrary configuration given by two independent vectors, the ac field polarization and the wave vector $\mathbf{k}$. The high-field transport and linear response to the ac field are described on the base of the Boltzmann kinetic equation. We show that the real part of DCT, calculated in the collisionless regime, is not zero due to dissipation …
We generalize the topological response theory of three-dimensional topological insulators (TI) to metallic systems-specifically, doped TI with finite bulk carrier density and a time-reversal symmetry breaking field near the surface. We show that there is an inhomogeneity-induced Berry phase contribution to the surface Hall conductivity that is completely determined by the occupied states and is independent of other details such as band dispersion and impurities. In the limit of zero bulk carrier density, this intrinsic surface Hall conductivity reduces to the half-integer quantized surface Hal…
