Control Of Carrier Density Research Articles

Room-temperature modulation of microwave conductivity in ferroelectric-gated correlated oxides

We report the nonvolatile modulation of microwave conductivity in ferroelectric PbZr0.2Ti0.8O3-gated ultrathin LaNiO3/La0.67Sr0.33MnO3 correlated oxide channel visualized by microwave impedance microscopy. Polarization switching is obtained by applying a tip bias above the coercive voltage of the ferroelectric layer. The microwave conductivity of the correlated channel underneath the up- and down-polarized domains has been quantified by finite-element analysis of the tip-sample admittance. At room temperature, a resistance on/off ratio above 100 between the two polarization states is sustained at frequencies up to 1 GHz, which starts to drop at higher frequencies. The frequence-dependence suggests that the conductance modulation originates from ferroelectric field-effect control of carrier density. The modulation is nonvolatile, remaining stable after 6 months of domain writing. Our work is significant for potential applications of oxide-based ferroelectric field-effect transistors in high-frequency nanoelectronics and spintronics.

Carrier density control of Sb-doped rutile-type SnO2 thin films and fabrication of a vertical Schottky barrier diode

We report on the control of carrier density in r-SnO2 thin films grown on isostructural r-TiO2 substrates by doping with Sb aiming for power-electronics applications. The carrier density was tuned within a range of 3 × 1016–2 × 1019 cm−3. Two types of donors with different activation energies, attributed to Sb at Sn sites and oxygen vacancies, are present in the thin films. Both activation energies decrease as the concentration of Sb increases. A vertical Schottky barrier diode employing a Sb:r-SnO2/Nb:r-TiO2 exhibits a clear rectifying property with a rectification ratio of 103 at ±1 V.

Controllable Modulation of the Electronic Properties of a Two-Dimensional Ambipolar Semiconductor by Interface Ferroelectric Polarization.

Precise control of charge carrier type and density of two-dimensional (2D) ambipolar semiconductors is the prerequisite for their applications in next-generation integrated circuits and electronic devices. Here, by fabricating a heterointerface between a 2D ambipolar semiconductor (hydrogenated germanene, GeH) and a ferroelectric substrate (PbMg1/3Nb2/3O3-PbTiO3, PMN-PT), fine-tuning of charge carrier type and density of GeH is achieved. Due to ambipolar properties, proper band gap, and high carrier mobility of GeH, by applying the opposite local bias (±8 V), a lateral polarization in GeH is constructed with a change of work function by 0.6 eV. Besides, the built-in polarization in GeH nanoflake could promote the separation of photoexcited electron-hole pairs, which lead to 4 times enhancement of the photoconductivity after poling by 200 V. In addition, a gradient regulation of the work function of GeH from 4.94 to 5.21 eV by adjusting the local substrate polarization is demonstrated, which could be used for data storage at the micrometer size by forming p-n homojunctions. This work of constructing such heterointerfaces provides a pathway for applying 2D ambipolar semiconductors in nonvolatile memory devices, photoelectronic devices, and next-generation integrated circuit.

Controllable Carrier Doping in Two-Dimensional Materials Using Electron-Beam Irradiation and Scalable Oxide Dielectrics.

Two-dimensional (2D) materials, characterized by their atomically thin nature and exceptional properties, hold significant promise for future nano-electronic applications. The precise control of carrier density in these 2D materials is essential for enhancing performance and enabling complex device functionalities. In this study, we present an electron-beam (e-beam) doping approach to achieve controllable carrier doping effects in graphene and MoS2 field-effect transistors (FETs) by leveraging charge-trapping oxide dielectrics. By adding an atomic layer deposition (ALD)-grown Al2O3 dielectric layer on top of the SiO2/Si substrate, we demonstrate that controllable and reversible carrier doping effects can be effectively induced in graphene and MoS2 FETs through e-beam doping. This new device configuration establishes an oxide interface that enhances charge-trapping capabilities, enabling the effective induction of electron and hole doping beyond the SiO2 breakdown limit using high-energy e-beam irradiation. Importantly, these high doping effects exhibit non-volatility and robust stability in both vacuum and air environments for graphene FET devices. This methodology enhances carrier modulation capabilities in 2D materials and holds great potential for advancing the development of scalable 2D nano-devices.

Compact plasmon modulator based on the spatial control of carrier density in indium tin oxide.

To keep pace with the demands of semiconductor integration technology, a semiconductor device should offer a small footprint. Here, we demonstrate a compact electro-optic modulator by controlling the spatial distribution of carrier density in indium tin oxide (ITO). The proposed structure is mainly composed of a symmetrical metal electrode layer, calcium fluoride dielectric layer, and an ITO propagating layer. The carrier density on the surface of the ITO exhibits a periodical distribution when the voltage is applied on the electrode, which greatly enhances the interaction between the surface plasmon polaritons (SPPs) and the ITO. This structure can not only effectively improve the modulation depth of the modulator, but also can further reduce the device size. The numerical results indicate that when the length, width, and height of the device are 14µm, 5µm, and 8µm, respectively, the modulation depth can reach 37.1dB at a wavelength of 3.66µm. The structure can realize a broadband modulation in theory only if we select a different period of the electrode corresponding to the propagating wavelength of SPPs because the modulator is based on the scattering effect principle. This structure could potentially have high applicability for optoelectronic integration, optical communications, and optical sensors in the future.

A Large Bandgap Organic Salt Dopant for Sn‐Based Perovskite Thin‐Film Transistor

AbstractMetal halide perovskite optoelectronic devices have made significant progress over the past few years, but precise control of charge carrier density through doping is essential for optimizing these devices. In this study, the potential of using an organic salt, N,N‐dimethylanilinium tetrakis(pentafluorophenyl)borate, as a dopant for Sn‐based perovskite devices, is explored. Under optimized conditions, the thin film transistors based on the doped 2D/3D perovskite PEAFASnI3 demonstrate remarkable improvement in hole mobility, reaching 7.45 cm2V−1s−1 with a low subthreshold swing and the smallest sweep hysteresis (ΔVhysteresis = 2.27 V) and exceptional bias stability with the lowest contact resistance (2.2 kΩ cm). The bulky chemical structure of the dopant prevents it from penetrating the perovskite lattice and also surface passivation against Sn oxidation due to its hydrophobic nature surface. This improvement is attributed to the bifunctional effect of the dopant, which simultaneously passivates defects and improves crystal orientation. These findings provide new insights into potential molecular dopants that can be used in metal halide perovskite devices.

ANTARES: Space-resolved electronic structure

The spatially resolved ARPES (nanoARPES) is a development of conventional ARPES technique achieved with the focusing of light on the sample into the spot with submicron sizes. This development is used in research of essentially small samples, for example, heterostructures build of flakes of 2D materials, micro-crystals or polycrystalline samples, different crystal termination, domains in electronic structure. ANTARES is delivering the nanoARPES technique to user community since 2010, up to now the instrument was used with samples of various types, and that was useful to accumulate the specific experience. In this paper we report the current layout and actual performance of the ANTARES instrument at Synchrotron SOLEIL, as well as the most typical application areas. The most important and recent upgrades include focusing optics and in-operando setup. The new optical units deliver the increased flux on sample as well as the option to vary the photon energy. The in-operando setup offers the option of electrical connection to the sample being studied with nanoARPES, that promises various applications, where the most demanded now is the control of charge carrier density with applied voltage. This “gate-doping” is often applied to the heterostructures of 2D materials, that could be designed and build on purpose. The 2D heterostructures is probably the most typical field of application of the instrument, with or without in-operando option. At the same time the use case of the ANTARES instrument is still in the development by the realisation of new kinds of samples and of the new types of the experiments.

Dissociative Host-Dopant Bonding Facilitates Molecular Doping in Halide Perovskites

Molecular doping is a promising strategy to fine-tune the electronic properties of halide perovskites and accelerate their implementation in next-generation optoelectronics. However, a deeper understanding of the role of host-dopant interactions in these systems is needed to fully exploit the potential of this avenue. Herein, we demonstrate a surface post-treatment strategy employing n-type molecular dopant n-DMBI-H to modulate free hole density in p-type CH3NH3Sn0.75Pb0.25I3 films. We show that the adsorption of n-DMBI-H on surface Sn atoms, followed by the dissociation of an electron-donating hydride from the dopant, facilitates charge transfer to the perovskite and hole trapping at the dissociated hydride. We identify this mechanism as a key factor dictating doping compensation in perovskites, allowing carrier density control within nearly 1 order of magnitude via the dissociated molecular dopant located at film surfaces and grain boundaries. We then exploit n-DMBI-H in perovskite/transport layer junctions, achieving reduced carrier losses and improved contact selectivity and performance in p-i-n, Sn-rich perovskite solar cells. We expect this work to provide carrier density tuning guidelines for a broad range of tin-based perovskite applications.

Coupling Ferroelectric to colloidal Nanocrystals as a Generic Strategy to Engineer the Carrier Density Landscape

AbstractThe design of infrared nanocrystals‐based (NCs) photodiodes faces a major challenge related to the identification of barriers with a well‐suited band alignment or strategy to finely control the carrier density. Here, this study explores a general complementary approach where the carrier density control is achieved by coupling an NC layer to a ferroelectric material. The up‐and‐down change in ferroelectric polarization directly impacts the NC electronic structure, resulting in the formation of a lateral pn junction. This effect is uncovered directly using nano X‐ray photoemission spectroscopy, which shows a relative energy shift of 115 meV of the NC photoemission signal over the two different up‐ and down‐polarized ferroelectric regions, a shift as large as the open circuit value obtained in the diode stack. The performance of this pn junction reveals enhanced responsivity and reduced noise that lead to a factor 40 increase in the detectivity value.

Lithium-Ion Glass Gating of HgTe Nanocrystal Film with Designed Light-Matter Coupling.

Nanocrystals' (NCs) band gap can be easily tuned over the infrared range, making them appealing for the design of cost-effective sensors. Though their growth has reached a high level of maturity, their doping remains a poorly controlled parameter, raising the need for post-synthesis tuning strategies. As a result, phototransistor device geometry offers an interesting alternative to photoconductors, allowing carrier density control. Phototransistors based on NCs that target integrated infrared sensing have to (i) be compatible with low-temperature operation, (ii) avoid liquid handling, and (iii) enable large carrier density tuning. These constraints drive the search for innovative gate technologies beyond traditional dielectric or conventional liquid and ion gel electrolytes. Here, we explore lithium-ion glass gating and apply it to channels made of HgTe narrow band gap NCs. We demonstrate that this all-solid gate strategy is compatible with large capacitance up to 2 µF·cm-2 and can be operated over a broad range of temperatures (130-300 K). Finally, we tackle an issue often faced by NC-based phototransistors:their low absorption; from a metallic grating structure, we combined two resonances and achieved high responsivity (10 A·W-1 or an external quantum efficiency of 500%) over a broadband spectral range.

Flat-Band-Induced Many-Body Interactions and Exciton Complexes in a Layered Semiconductor.

Interactions among a collection of particles generate many-body effects in solids that result in striking modifications of material properties. The heavy carrier mass that yields strong interactions and gate control of carrier density over a wide range makes two-dimensional semiconductors an exciting playground to explore many-body physics. The family of III-VI metal monochalcogenides emerges as a new platform for this purpose because of its excellent optical properties and the flat valence band dispersion. In this work, we present a complete study of charge-tunable excitons in few-layer InSe by photoluminescence spectroscopy. From the optical spectra, we establish that free excitons in InSe are more likely to be captured by ionized donors leading to the formation of bound exciton complexes. Surprisingly, a pronounced red shift of the exciton energy accompanied by a decrease of the exciton binding energy upon hole-doping reveals a significant band gap renormalization induced by the presence of the Fermi reservoir.

Reduced interfacial recombination in perovskite solar cells by structural engineering simulation

This theoretical study performed configurational optimization of heterojunction perovskite solar cells to minimize internal recombination through simulation. Interfacial recombination at the absorber-electron transport layer (ETL) junction is one of the prime sources of recombination in perovskite solar cell devices. Carrier density control in the vicinity of interfaces across absorber/ETL junction lowers the interfacial recombination. We explore various design alterations to achieve this condition, such as (a) restricting the majority carrier at the interface or asymmetric doping at perovskite/ETL interfaces, (b) widening the absorber bandgap at the interface, (c) donor interfacial defect at perovskite/ETL junction, (d) high rear doping of hole transport layer at back contact. We investigated the feasibility of these structural optimizations for lowering the overall internal recombination through the device. We achieved an optimized device by incorporating all these methods, which have improved efficiency, fill factor, and V OC by 38.61%, 5.5% and 21.69%, respectively, over the benchmark device. The optimized perovskite structure may provide valuable guidelines to experimentalists for achieving the high efficiency of the perovskite solar cells.

Unintentional doping effect in Si-doped MOCVD β-Ga2O3 films: Shallow donor states

High-quality β-Ga2O3 films were epitaxially grown by using metalorganic chemical vapor deposition (MOCVD) with different donor concentrations, and their shallow donor states were investigated by the temperature-dependent Hall measurement and secondary ion mass spectroscopy (SIMS) analysis. Two donor levels with ionization energies of ∼36 and ∼140 meV were extracted. It is found that the unintentional doping (UID) effects in MOCVD contribute substantially to both these two levels: the first donor level may come not only from silicon doping but also from unintentional substitution of carbon for Ga sites, and the second donor level probably comes from doubly charged defects related to unintentional H doping. By analyzing the relationship between the growth conditions and donor states, combined with density functional theory calculations, it is found that reducing the oxygen partial pressure during the growth might be a feasible way to reduce the UID effect. This work paves the way to the precise control of carrier density in Si-doped MOCVD β-Ga2O3 films.

WSe2 as Transparent Top Gate for Infrared Near-Field Microscopy.

Independent control of carrier density and out-of-plane displacement field is essential for accessing novel phenomena in two-dimensional (2D) material heterostructures. While this is achieved with independent top and bottom metallic gate electrodes in transport experiments, it remains a challenge for near-field optical studies as the top electrode interferes with the optical path. Here, we characterize the requirements for a material to be used as the top-gate electrode and demonstrate experimentally that few-layer WSe2 can be used as a transparent, ambipolar top-gate electrode in infrared near-field microscopy. We carry out nanoimaging of plasmons in a bilayer graphene heterostructure tuning the plasmon wavelength using a trilayer WSe2 gate, achieving a density modulation amplitude exceeding 2 × 1012 cm-2. The observed ambipolar gate-voltage response allows us to extract the energy gap of WSe2, yielding a value of 1.05 eV. Our results provide an additional tuning knob to cryogenic near-field experiments on emerging phenomena in 2D materials and moiré heterostructures.

Identifying Defect-Induced Trion in Monolayer WS2 via Carrier Screening Engineering

Unusually high exciton binding energies (BEs), as much as ∼1 eV in monolayer transition-metal dichalcogenides, provide opportunities for exploring exotic and stable excitonic many-body effects. These include many-body neutral excitons, trions, biexcitons, and defect-induced excitons at room temperature, rarely realized in bulk materials. Nevertheless, the defect-induced trions correlated with charge screening have never been observed, and the corresponding BEs remain unknown. Here we report defect-induced A-trions and B-trions in monolayer tungsten disulfide (WS2) via carrier screening engineering with photogenerated carrier modulation, external doping, and substrate scattering. Defect-induced trions strongly couple with inherent SiO2 hole traps under high photocarrier densities and become more prominent in rhenium-doped WS2. The absence of defect-induced trion peaks was confirmed using a trap-free hexagonal boron nitride substrate, regardless of power density. Moreover, many-body excitonic charge states and their BEs were compared via carrier screening engineering at room temperature. The highest BE was observed in the defect-induced A-trion state (∼214 meV), comparably higher than the trion (209 meV) and neutral exciton (174 meV), and further tuned by external photoinduced carrier density control. This investigation allows us to demonstrate defect-induced trion BE localization via spatial BE mapping in the monolayer WS2 midflake regions distinctive from the flake edges.

Two-dimensional lateral surface superlattices in GaAs heterostructures with independent control of carrier density and modulation potential

We present a double-layer design for two-dimensional lateral surface superlattice systems in GaAs-AlGaAs heterostructures. Unlike previous studies, our device (1) uses an in situ gate, which allows a very short period superlattice in high mobility, shallow heterostructures and (2) enables independent control of the carrier density and the superlattice modulation potential amplitude over a wide range. We characterize this device design using low-temperature magneto-transport measurements and show that the fabrication process caused minimal damage to the system. We demonstrate the tuning of potential modulation from weak (much smaller than Fermi energy) to strong (larger than the Fermi energy) regimes.

Reversible Superconductor-Insulator Transition in (Li, Fe)OHFeSe Flakes Visualized by Gate-Tunable Scanning Tunneling Spectroscopy.

Electric field control of charge carrier density provides a key in situ technology to continuously tune the ground states and map out the phase diagram of correlated electron systems in one device. This technique is highly expected to be combined with the modern state-of-the art spectroscopic probes, such as angle-resolved photoemission spectroscopy and scanning tunneling microscopy/spectroscopy (STM/S), to efficiently address these states and the underlying physics. However, it is extremely difficult and not successful so far, mainly because the fabrication process of such devices makes them prohibitive for surface probes. Here, by using a solid Li-ion conductor (SIC) as gate dielectric, we have successfully developed gate-tunable STM/S and visualized the superconductor-insulator transition (SIT) in a thin flake of single crystal (Li, Fe)OHFeSe at the nanoscale. The gate-controlled Li-ion injection first enhances the superconductivity and then drives the flake into an inhomogeneous insulating state, where superconductivity is totally suppressed. This process can be reversed by applying an opposite gate voltage. Importantly, the atomically resolved images allow us to identify the critical role that the injected Li ions play in the tuning process. Our results not only provide clear evidence of the microscopic mechanism of the tunable superconductivity and SIT in the SIC-based (Li, Fe)OHFeSe devices, but also establish SIC-gating STM as a powerful tool for investigating the complicated phase diagram of correlated electron system spectroscopically in a single sample with the field-effect approach.

Sheet Resistance Analysis of Interface-Engineered Multilayer Graphene: Mobility Versus Sheet Carrier Concentration.

Both interlayer-undoped and interlayer-doped multilayer graphenes were prepared by the multiple transfers of graphene layers with multiple Cu etching (either dopant-free or doped during etching) and transfer, and the effect of interface properties on the electrical properties of multilayer graphene was investigated by varying the number of layers from 1 to 12. In both the cases, the sheet resistance decreased with increasing number of layers from 700 to 104 Ω/sq for the interlayer-undoped graphene and from 280 to 25 Ω/sq for the interlayer-doped graphene. Further, Hall measurements revealed that the origins of the sheet resistance reduction in the two cases are different. In the interlayer-undoped graphene, the sheet resistance decreased because of the increase in mobility with the addition of inner layers, which has a low carrier density and a high carrier mobility. On the other hand, it decreased because of the increase in sheet carrier density in the interlayer-doped multilayer graphene. The mobility and carrier density variations in both the cases were confirmed by fitting with the model of Hall effect in the heterojunction. In addition, we found that surface property modification by the doping of the top layer and the formation of double-layer graphene with different partial coverages allow the separate control of carrier density and mobility. Our study provides an effective approach for controlling the properties of multilayer graphene for electronic applications.

Tuning the carrier density in SrTiO3/LaTiO3/SrTiO3 quantum wells

We discuss methods of built-in carrier density control in SrTiO3/LaTiO3/SrTiO3 heterostructures that exhibit quasi-two-dimensional carrier confinement in an interfacial quantum well. Unlike the electronically similar LaAlO3/SrTiO3 heterostructures, where the polar discontinuity at the interface defines the accumulated carrier density, the LaTiO3 heterostructures offer two means of carrier density control—changing the La doping level and utilizing the effect of surface depletion through the change in the SrTiO3 capping layer thickness. Dynamic carrier tuning over a limited range is possible by the application of a back-gate bias, which primarily affects the depth distribution of carriers. We find that small changes in the pre-annealing conditions of a SrTiO3 substrate can have a dramatic effect on the low-temperature sheet resistance of the heterostructures.

Tailoring of electrical properties of TiO2 decorated CVD grown single-layer graphene by HNO3 molecular doping

Control of carrier density and shifting of Fermi level provides an approach to tune the physical properties of two-dimensional graphene. Here, we report the modulation in electrical properties of chemical vapor deposition (CVD) grown single-layer graphene (SLG) and titanium dioxide (TiO2) nanoparticles (NPs) decorated SLG by nitric acid (HNO3) molecules. The Raman spectroscopy and charge transport measurement confirmed HNO3 treatment leads to p-type doping in both i.e. pristine graphene and TiO2 coated SLG. The shift and relative intensity ratio of G and 2D peaks are analyzed after HNO3 treatment. For HNO3 doped without and with TiO2 NPs coated CVD grown SLG, a substantial Dirac point shift is observed in transfer characteristic, reveal p-type doping of CVD grown SLG through charge transfer and HNO3 molecule interaction. The change in carrier density and Fermi level is also calculated after HNO3 treatment for all the samples. Our results demonstrate chemical modification is an effective approach to modulate the electrical properties CVD grown SLG for diverse applications.