Two-dimensional Carrier Research Articles

Characteristics of AlGaN/GaN high electron mobility transistor temperature sensor

Semiconductor temperature sensors have been widely used in medical, industrial, aviation and civil fields due to their advantages such as high sensitivity, small size, low power consumption and strong anti-interference ability. However, most Si-based temperature sensors are not suitable for the application in high-temperature environments. The new AlGaN/GaN heterojunction material not only has a wide band gap, but also has a high two-dimensional electron gas concentration and carrier mobility. Therefore, the device made with it not only has good electrical properties, but also can be applied in ultra-high environments. In this paper, a temperature sensor based on gateless AlGaN/GaN high electron mobility transistor structure was fabricated and its temperature-dependent electrical properties were characterized. The temperature dependence of current-voltage characteristics of the device were tested from 50 to 400 °C. The sensitivity of the device was studied as a function of the channel aspect ratio of the device. The stability of electrical properties was characterized after heating in air and nitrogen at 300—500 °C for 1 hour. The theoretical and experimental results show that as the aspect ratio of the device increases, the sensitivity of the device increases. At a fixed current of 0.01 A, the average sensitivity of the device voltage with temperature changes is 44.5 mV/°C. Meanwhile, the good high temperature retention stability is shown during stability experiments.

Improved electron capture capability of field-assisted exponential-doping GaN nanowire array photocathode

The exponential-doping GaN nanowire arrays (GaN NWAs) photocathode has a “light-trapping effect”, and the built-in electric field can promote the concentration of the photogenerated carrier center to the top surface of the nanowire. However, in the preparation of actual NWAs photocathodes, the problem that photons emitted from the sides of the nanowires cannot be effectively collected has been encountered. Our proposed field-assisted exponential-doping GaN NWAs can bend the motion trajectory of the emitted electrons toward the collecting side. In this study, the quantum efficiency (QE) and collection efficiency (CE) of the external field-assisted exponential-doping GaN NWAs photocathode are derived based on the two-dimensional carrier diffusion equation and the initial energy and angular distribution, respectively. For a field-assisted exponential-doping GaN NWAs with a width d = 200 nm and a height H = 400 nm, the optimal structural parameters are obtained: the incident angle θ = 50° and the nanowire spacing is L = 335.6 nm. On this basis, the field intensity of 0.5 V/μm can maximize the CE of the NWAs. All the results show that the field-assisted approach does contribute to the collection of emitted electrons, which can provide theoretical guidance for high-performance electron sources based on exponential-doping GaN NWAs photocathodes. And field-assisted exponential-doping GaN NWAs cathode is expected to be verified by the experimental results in the future.

Role of Plasmon Modes on the Optical Reflectivity of Graphene-Metallic Structures: A Theoretical Approach

In recent years, the tunable plasmon modes in the terahertz region of a multilayer graphene structure interacting with a metallic film substrate have attracted significant interest motivated by the graphene´s unique optical and electronic properties and the possibility to enhance light-matter interaction. In this work, the plasmon waves in graphene layered systems on a conducting thin film are investigated, the hybrid graphene-metal metamaterialis surrounded by two semi-infinite materials with different dielectric constants ε1andε2, respectively. The dispersion relations of electronic collective excitations are calculated by the zeros of an effective dielectric constant obtained from a recursive relation for the amplitudes associated with the electric field between graphene layers in the metamaterial. Long-range Coulomb interactions based on the hybrid layered graphene-metal structure lead new set spectra of collective excitations. At long wavelength (q®0) the optical modes (w~q1/2)depend on the two-dimensional carrier density, the metallic thickness, the metallic substrate plasmon frequency, the number of the graphene layers and the dielectric constants in which the hybrid graphene-metal structure is embedded. This latter plays an important role in a wide range of applications such as a surface plasmon resonance biological sensors and terahertz surface plasmons in optically pumped graphene metamaterials.

Artificial two-dimensional polar metal by charge transfer to a ferroelectric insulator

Integrating multiple properties in a single system is crucial for the continuous developments in electronic devices. However, some physical properties are mutually exclusive in nature. Here, we report the coexistence of two seemingly mutually exclusive properties-polarity and two-dimensional conductivity-in ferroelectric Ba0.2Sr0.8TiO3 thin films at the LaAlO3/Ba0.2Sr0.8TiO3 interface at room temperature. The polarity of a ∼3.2 nm Ba0.2Sr0.8TiO3 thin film is preserved with a two-dimensional mobile carrier density of ∼0.05 electron per unit cell. We show that the electronic reconstruction resulting from the competition between the built-in electric field of LaAlO3 and the polarization of Ba0.2Sr0.8TiO3 is responsible for this unusual two-dimensional conducting polar phase. The general concept of exploiting mutually exclusive properties at oxide interfaces via electronic reconstruction may be applicable to other strongly-correlated oxide interfaces, thus opening windows to new functional nanoscale materials for applications in novel nanoelectronics.

One-pot synthesis of a CoS-AC electrode in a redox electrolyte for high-performance supercapacitors

Due to their unique physical and chemical properties, nanostructured metal sulfide materials have shown excellent electrochemical performance. Among them, the natural semiconductor cobalt disulfide (CoS) is a promising electrode material for supercapacitors owing to its two-dimensional layered structure and fast carrier transmission. However, the curling of the CoS layers and the structural stability of CoS materials should be improved. In this context, we developed a facile solvothermal method to fabricate CoS-AC composite to solve the inherent defects of individual materials. The AC both increased the electrical conductivity of the electrode and supported the active species, thus improving the stability of the composite during the charge–discharge processes. Furthermore, the addition of K3Fe(CN)6 to the electrolyte to form a redox electrolyte largely enhanced the pseudocapacitance. The CoS-AC composite exhibited a high specific capacitance of 797.79 F g−1 at a high current density of 10 A g−1. The as-prepared CoS-AC//AC asymmetric supercapacitor device showed a superior cycling stability (77.53% maintained after 2000 cycles). The synergy between the electrode and the redox electrolyte was crucial for improving the supercapacitor performance. The CoS-AC composites were simply synthesized using solvothermal approach with enhanced electrochemical properties.

Comparison of atomic layer deposited Al2O3 and (Ta2O5)0.12(Al2O3)0.88 gate dielectrics on the characteristics of GaN-capped AlGaN/GaN metal-oxide-semiconductor high electron mobility transistors

The current research investigates the potential advantages of replacing Al2O3 with (Ta2O5)0.12(Al2O3)0.88 as a higher dielectric constant (κ) gate dielectric for GaN-based metal-oxide-semiconductor high electron mobility transistors (MOS-HEMTs). The electrical characteristics of GaN-capped AlGaN/GaN MOS-HEMT devices with (Ta2O5)0.12(Al2O3)0.88 as the gate dielectric are compared to devices with Al2O3 gate dielectric and devices without any gate dielectric (Schottky HEMTs). Compared to the Al2O3 MOS-HEMT, the (Ta2O5)0.12(Al2O3)0.88 MOS-HEMT achieves a larger capacitance and a smaller absolute threshold voltage, together with a higher two-dimensional electron gas carrier concentration. This results in a superior improvement of the output characteristics with respect to the Schottky HEMT, with higher maximum and saturation drain current values observed from DC current-voltage measurements. Gate transfer measurements also show a higher transconductance for the (Ta2O5)0.12(Al2O3)0.88 MOS-HEMT. Furthermore, from OFF-state measurements, the (Ta2O5)0.12(Al2O3)0.88 MOS-HEMT shows a larger reduction of the gate leakage current in comparison to the Al2O3 MOS-HEMT. These results demonstrate that the increase in κ of (Ta2O5)0.12(Al2O3)0.88 compared with Al2O3 leads to enhanced device performance when the ternary phase is used as a gate dielectric in the GaN-based MOS-HEMT.

Ultrafast Carrier Redistribution in Single InAs Quantum Dots Mediated by Wetting-Layer Dynamics

Individual epitaxial semiconductor quantum dots (QDs) have been extensively considered as an ``artificial atom'' platform for quantum optics and photonics applications. The QD carrier dynamics responsible for ultimate device performance is indeed complex, due in part to rich interaction with the wetting layer's two-dimensional carrier reservoir. The authors investigate this interaction with time-resolved experiments and rate-equation modeling, showing that these analyses are important for understanding the limitations of single-photon photoluminescence emission, improving lasers and fast optical modulators, and developing next-generation ultrafast all-optical switches.

Electron transport in N-polar GaN-based heterostructures

Electron transport in N-polar GaN-based high-electron-mobility transistor (HEMT) structures with a combination of In0.18Al0.82N-AlN as the barrier was studied via temperature-dependent van der Pauw Hall and Shubnikov de Haas measurements. In contrast to Ga-polar HEMT structures, no persistent photoconductivity could be detected. In a sample with 10 nm thick InAlN, only one oscillation frequency was observed, demonstrating that a single sublevel is present. From the oscillations, a two-dimensional electron gas carrier density of 8.54 × 1012 cm−2 and a mobility of 4970 cm2/V s were extracted at 1.7 K. This sample was further investigated using ionic liquid gating. The charge density was varied from 7.5 × 1012 cm−2 to 9.6 × 1012 cm−2. The electron mobility significantly declined with decreasing charge density. This is in contrast to Ga-polar HEMT structures, where the electron mobility typically increases slightly as the charge density decreases.

Two-dimensional charge carrier distribution in MoS2 monolayer and multilayers

Control of majority carrier type and concentration in transition metal dichalcogenides (TMDs) is an important goal for engineering and improving TMD-based devices. Monolayer and few-layer molybdenum disulphide (MoS2) is an n-type semiconductor due to the presence of electron-donating native defects whose distribution is strongly dependent on the processing history and ambient environment. However, the spatial heterogeneity of the charge carrier concentration has not yet been studied in MoS2 when implemented in devices such as field-effect transistors (FETs). Here, we present a method to extract the spatial distribution of charge carriers using Kelvin probe force microscopy of MoS2 FETs in operando. The carrier concentration in monolayer MoS2 exfoliated on SiO2/Si ranges from 1.2×1012 cm−2 to 2.3×1012 cm−2, corresponding to a three-dimensional concentration of 1018 cm−3 to 2.5×1018 cm−3. A comparable carrier concentration is obtained for few-layer MoS2, while for thicker MoS2 (>50 nm) it is an order of magnitude lower (2×1017 cm−3–4×1017 cm−3). This finding is consistent with an increased concentration of electron-donating sulfur vacancies at surfaces compared to the bulk. Thus, the reported method for measuring the carrier concentration may advance strategies for doping and improve understanding of devices and defects in 2D materials.

Spatial Mapping of Local Density Variations in Two-dimensional Electron Systems Using Scanning Photoluminescence.

We have developed a scanning photoluminescence technique that can directly map out the local two-dimensional electron density with a relative accuracy of ∼2.2 × 108 cm-2. The validity of this approach is confirmed by the observation of the expected density gradient in a high-quality GaAs quantum well sample that was not rotated during the molecular beam epitaxy of its spacer layer. In addition to this global variation in electron density, we observe local density fluctuations across the sample. These random density fluctuations are also seen in samples that were continuously rotated during growth, and we attribute them to residual space charges at the substrate-epitaxy interface. This is corroborated by the fact that the average magnitude of density fluctuations is increased to ∼9 × 109 cm-2 from ∼1.2 × 109 cm-2 when the buffer layer between the substrate and the quantum well is decreased by a factor of 7. Our data provide direct evidence for local density inhomogeneities even in very high-quality two-dimensional carrier systems.

An Explicit Unified Drain Current Model for Silicon-Nanotube-Based Ultrathin Double Gate-All-Around <sc> mosfet</sc>s

A unified drain current model for silicon-nanotube-based ultra-thin double gate-all-around (DGAA) mosfet is presented in this work. Physics-based, analytical expressions for surface potential and charge density are derived utilizing Fermi-Dirac statistic, one-dimensional density of states (DOS) and Gauss’ law in the ultrathin channel region of the device. Using the derived expressions of surface potential and charge density, a closed-form expression for the drain current under drift-diffusive transport regime is formulated within the purview of gradual channel approximation. The proposed drain current model takes account for two-dimensional carriers’ confinement, which becomes prominent in ultrathin channel devices. The derived model, which is valid for all regions of operation, provides a physics-based description of the surface potential, charge density, and drain current. Output and transfer characteristics of the proposed drain current model have been verified by comparing against results obtained from the three-dimensional numerical simulator for different device parameters. The performance of the proposed drain current model has been further validated with the data presented in reported results. The present drain current model is explicit, analytical, and does not employ any numerical schemes and thus can be readily used in the efficient exploration of any circuit design and simulations.

Design of Anti-Distortion Two-Dimensional Code on Prism and Cylinder Combination Surface based on Pre-Stretching

Two-dimensional code has shown its impressive performance on mobile payment and advertising marketing. To ensure that the two-dimensional code is correctly identified by recognition devices, the traditional two-dimensional code must be attached to the surface of plane. However, if the code attached to the surface of cylinder or prism carrier, the image of the normal two-dimensional code will be distortion because of image projection, which will lead to identification problem. To this end, we propose a novel approach of design and identification of anti-distortion two-dimensional code on prism and cylinder combination surface based on pre-stretching, to address the challenge of identifying a two-dimensional code on prism and cylinder combination carrier. According to the spatial geometric relation of the two-dimensional code carrier, pre-stretching the normal two-dimensional code image via inverse mapping to obtain the pre-deformed two-dimensional code that will be printed and then pasted on the surface of the carrier. Specifically, the image of the pre-stretched two-dimensional code in the projection plane of the acquisition device is just as the normal two-dimensional code, which will be correctly identified. Experimental results show that the two-dimensional code after pre-stretching could be identified by normal recognition devices.

Excitation-induced transition to indirect band gaps in atomically thin transition-metal dichalcogenide semiconductors

Monolayers of transition metal dichalcogenides (TMDCs) exhibit an exceptionally strong Coulomb interaction between charge carriers due to the two-dimensional carrier confinement in connection with weak dielectric screening. High densities of excited charge carriers in the various band-structure valleys cause strong many-body renormalizations that influence both the electronic properties and the optical response of the material. We investigate electronic and optical properties of the typical monolayer TMDCs MoS$_2$, MoSe$_2$, WS$_2$ and WSe$_2$ in the presence of excited carriers by solving semiconductor Bloch equations on the full Brillouin zone. With increasing carrier density, we systematically find a reduction of the exciton binding energies due to Coulomb screening and Pauli blocking. Together with excitation-induced band-gap shrinkage this leads to redshifts of excitonic resonances up to the dissociation of excitons. As a central result, we predict for all investigated monolayer TMDCs that the $\Sigma$-valley shifts stronger than the K-valley. Two of the materials undergo a transition from direct to indirect band gaps under carrier excitation similar to well-known strain-induced effects. Our findings have strong implications for the filling of conduction-band valleys with excited carriers and are relevant to transport and optical applications as well as the emergence of phonon-driven superconductivity.

Frequency-dependent substrate screening of excitons in atomically thin transition metal dichalcogenide semiconductors

Atomically thin layers of transition metal dichalcogenides (TMDCs) exhibit exceptionally strong Coulomb interaction between charge carriers due to the two-dimensional carrier confinement in connection with weak dielectric screening. The van der Waals nature of interlayer coupling makes it easy to integrate TMDC layers into heterostructures with different dielectric or metallic substrates. This allows to tailor electronic and optical properties of these materials, as Coulomb interaction inside atomically thin layers is very susceptible to screening by the environment. Here we theoretically investigate dynamical screening effects in TMDCs due to bulk substrates doped with carriers over a large density range, thereby offering three-dimensional plasmons as tunable degree of freedom. We report a wide compensation of renormalization effects leading to a spectrally more stable exciton than predicted for static substrate screening, even if plasmons and excitons are in resonance. We also find a nontrivial dependence of the single-particle band gap on substrate doping density due to dynamical screening. Our investigation provides microscopic insight into the mechanisms that allow for manipulations of TMDC excitons by means of arbitrary plasmonic environments on the nanoscale.

Thermooptical evidence of carrier-stabilized ferroelectricity in ultrathin electrodeless films

Ferroelectric films may lose polarization as their thicknesses decrease to a few nanometers because of the depolarizing field that opposes the polarization therein. The depolarizing field is minimized when electrons or ions in the electrodes or the surface/interface layers screen the polarization charge or when peculiar domain configuration is formed. Here, we demonstrate ferroelectric phase transitions using thermooptical studies in ∼5-nm-thick epitaxial Pb0.5Sr0.5TiO3 films grown on different insulating substrates. By comparing theoretical modeling and experimental observations, we show that ferroelectricity is stabilized through redistribution of charge carriers (electrons or holes) inside ultrathin films. The related high-density of screening carriers is confined within a few-nanometers-thick layer in the vicinity of the insulator, thus resembling a two-dimensional carrier gas.

Absence of confinement in (SrTiO3)/(SrTi0.8Nb0.2O3) superlattices

The reduction of dimensionality is considered an efficient pathway to boost the performances of thermoelectric materials. Quantum confinement of the carriers is expected to induce large Seebeck coefficients $(S)$ and it also suppresses the thermal conductivity by increasing the phonon scattering processes. However, quantum confinement in superlattices is not always easy to achieve and needs to be carefully validated. In the past decade, large values of $S$ have been measured in $({\mathrm{SrTiO}}_{3})$/$({\mathrm{SrTi}}_{0.8}{\mathrm{Nb}}_{0.2}{\mathrm{O}}_{3})$ superlattices [H. Ohta et al., Nat. Mater. 6, 129 (2007); Y. Mune et al., Appl. Phys. Lett. 91, 192105 (2007)]. In the $\ensuremath{\delta}$-doped compound, the reported $S$ was almost six times larger than that of the bulk material. This huge increase has been attributed to the two-dimensional carrier confinement in the doped regions. Here, we demonstrate that the experimental data are well explained quantitatively assuming delocalized electrons in both in-plane and growth directions. Moreover, we rule out the confined electron hypothesis whose signature would be the suppression of the Seebeck coefficient. This strongly suggests that the presupposed confinement picture in these superlattices is unlikely.

Non-equilibrium hole capture to excited acceptor states in quantum wells due to optical scattering

The probability of optical-phonon-assisted scattering of non-equilibrium holes to excited acceptor states in GaAs/AlGaAs quantum wells is considered. In the model we used, a well-known expression for the probability of intersubband optical scattering of two-dimensional carriers was expanded to the case when the final hole state is the excited state of an acceptor center. The temperature dependences of the probability of hole capture to excited acceptor states with simultaneous optical phonon emission are calculated. The result allows one to estimate the contribution of optical scattering to the experimentally observed terahertz electroluminescence due to intracenter carrier transitions.

In-plane Anisotropy of Quantum Transport in Artificial Two-dimensional Au Lattices.

We report an experimental observation and direct control of quantum transport in artificial two-dimensional Au lattices. Combining the advanced techniques of low-temperature deposition and newly developed double-probe scanning tunneling spectroscopy, we display a two-dimensional carrier transport and demonstrate a strong in-plane transport modulation in the two-dimensional Au lattices. In well-ordered Au lattices, we observe the carrier transport behavior manifesting as a band-like feature with an energy gap. Furthermore, controlled structural modification performed by constructing coupled "stadiums" enables a transition of system dynamics in the lattices, which in turn establishes tunable resonant transport throughout a wide energy range. Our findings open the possibility of the construction and transport engineering of artificial lattices by the geometrical arrangement of scatterers and quantum chaotic dynamics.

Relation between film thickness and surface doping of MoS2 based field effect transistors

Ultra-thin MoS2 film doping through surface functionalization with physically adsorbed species is of great interest due to its ability to dope the film without reduction in the carrier mobility. However, there is a need for understanding how the thickness of the MoS2 film is related to the induced surface doping for improved electrical performance. In this work, we report on the relation of MoS2 film thickness with the doping effect induced by the n-dopant adsorbate poly(vinyl-alcohol). Field effect transistors built using MoS2 films of different thicknesses were electrically characterized, and it was observed that the ION/OFF ratio after doping in thin films is more than four orders of magnitudes greater when compared with thick films. Additionally, a semi-classical model tuned with the experimental devices was used to understand the spatial distribution of charge in the channel and explain the observed behavior. From the simulation results, it was revealed that the two-dimensional carrier density induced by the adsorbate is distributed rather uniformly along the complete channel for thin films (<5.2 nm) contrary to what happens for thicker films.

High vertical carrier mobility in the nanofiber films of a phthalocyanine derivative and its application to vertical-type transistors

Abstract A self-assembled nanofiber film of octabutoxy-phthalocyanine (H2PCOC4) was prepared by a simple spin-coating method with a mixed solvent. This film indicated a high vertical hole mobility up to 0.12 cm2 V−1 s−1 when the nanofiber structure is formed from a mixed solvent of chloroform and o-dichlorobenzene. The high vertical mobility is attributed to the unique crystal structure of H2PCOC4 and molecular arrangement in the fiber film. Quantum chemical calculations reveal that the crystal structure of H2PCOC4 enables two-dimensional carrier transport that is advantageous for the vertical transport. The nanofiber films were applied to vertical-type organic transistors, resulting in higher output current compared to a non-structured film.