Transfer Doping Research Articles

<italic>V</italic>2O5MISFETs on H-Terminated Diamond

We report for the first time on the dc and RF performance of novel MISFETs fabricated on hydrogen-terminated (H-terminated) single crystal diamond film using vanadium pentoxide (V2O5) as insulating material. The active devices were characterized in terms of static $I$ – $V$ characteristics and static transconductance as well as of S-parameters for the calculation of the maximum cutoff frequency and the maximum oscillation frequency. Time stability of the drain current was evaluated overnight observing a maximum fluctuation of 7%. Investigations on temperature dependence of diamond-based MISFET were also performed up to 130 °C. The experimental results were compared with the better established diamond MESFET technology. Finally, the surface transfer doping of H-terminated diamond by very thin V2O5 insulator was also investigated in terms of conductivity, stability in air, and resistance to high temperatures.

Surface Charge Transfer Doping: Surface Charge Transfer Doping of Low‐Dimensional Nanostructures toward High‐Performance Nanodevices (Adv. Mater. 47/2016)

Surface Charge Transfer Doping: Surface Charge Transfer Doping of Low‐Dimensional Nanostructures toward High‐Performance Nanodevices (Adv. Mater. 47/2016)

Surface charge transfer doping of monolayer molybdenum disulfide by black phosphorus quantum dots

Monolayer molybdenum disulfide (MoS2), consisting of covalently bonded S-Mo-S sandwiched layers, has high carrier mobility and a direct bandgap of 1.8 eV, offering properties for electronic and optoelectronic devices with high performance. Usually, it is essential to modulate the carrier concentrations and conductivities of monolayer MoS2 for practical applications. In this paper, black phosphorus (BP) quantum dots (QDs) were synthesized by a liquid exfoliation method successfully, and have a diameter of ∼5 nm as confirmed with a transmission electron microscope (TEM). BP QDs were utilized to decorate monolayer MoS2 grown by chemical vapor deposition (CVD). The Raman and PL spectra of the BP QD/MoS2 hybrid structure clearly indicate that BP QDs are an effective n-type doping scheme for monolayer MoS2. Back-gated monolayer MoS2 transistors were fabricated and show an improved source–drain current after BP QD modifications. A high electron concentration of ∼5.39 × 1012 cm−2 in monolayer MoS2 was achieved, which is beneficial for designing FETs and photodetector devices with novel functions.

Surface Charge Transfer Doping via Transition Metal Oxides for Efficient p-Type Doping of II-VI Nanostructures.

Wide band gap II-VI nanostructures are important building blocks for new-generation electronic and optoelectronic devices. However, the difficulty of realizing p-type conductivity in these materials via conventional doping methods has severely handicapped the fabrication of p-n homojunctions and complementary circuits, which are the fundamental components for high-performance devices. Herein, by using first-principles density functional theory calculations, we demonstrated a simple yet efficient way to achieve controlled p-type doping on II-VI nanostructures via surface charge transfer doping (SCTD) using high work function transition metal oxides such as MoO3, WO3, CrO3, and V2O5 as dopants. Our calculations revealed that these oxides were capable of drawing electrons from II-VI nanostructures, leading to accumulation of positive charges (holes injection) in the II-VI nanostructures. As a result, Fermi levels of the II-VI nanostructures were shifted toward the valence band regions after surface modifications, along with the large enhancement of work functions. In situ ultraviolet photoelectron spectroscopy and X-ray photoelectron spectroscopy characterizations verified the significant interfacial charge transfer between II-VI nanostructures and surface dopants. Both theoretical calculations and electrical transfer measurements on the II-VI nanostructure-based field-effect transistors clearly showed the p-type conductivity of the nanostructures after surface modifications. Strikingly, II-VI nanowires could undergo semiconductor-to-metal transition by further increasing the SCTD level. SCTD offers the possibility to create a variety of electronic and optoelectronic devices from the II-VI nanostructures via realization of complementary doping.

Surface Charge Transfer Doping of Low-Dimensional Nanostructures toward High-Performance Nanodevices.

Device applications of low-dimensional semiconductor nanostructures rely on the ability to rationally tune their electronic properties. However, the conventional doping method by introducing impurities into the nanostructures suffers from the low efficiency, poor reliability, and damage to the host lattices. Alternatively, surface charge transfer doping (SCTD) is emerging as a simple yet efficient technique to achieve reliable doping in a nondestructive manner, which can modulate the carrier concentration by injecting or extracting the carrier charges between the surface dopant and semiconductor due to the work-function difference. SCTD is particularly useful for low-dimensional nanostructures that possess high surface area and single-crystalline structure. The high reproducibility, as well as the high spatial selectivity, makes SCTD a promising technique to construct high-performance nanodevices based on low-dimensional nanostructures. Here, recent advances of SCTD are summarized systematically and critically, focusing on its potential applications in one- and two-dimensional nanostructures. Mechanisms as well as characterization techniques for the surface charge transfer are analyzed. We also highlight the progress in the construction of novel nanoelectronic and nano-optoelectronic devices via SCTD. Finally, the challenges and future research opportunities of the SCTD method are prospected.

Studies on chemical charge doping related optical properties in monolayer WS2

Thermal stability of quasi particles, i.e., exciton and trion, and a strong particle-particle interaction significantly tune the optical properties of atomically thin two dimensional (2D) metal dichalcogenides. The present work addresses the effect of inherent defects upon optical properties of chemical vapor deposition grown 1 L-WS2 and proposes the use of chemical transfer doping as a reversible and simple method for identification of the type of excess charge in the system. Photoluminescence (PL) studies in pristine 1 L-WS2 show that an additional band at ∼0.06 eV below trion (X±) PL band was evolved (at low temperature) which was associated to the bound exciton with charged/neutral defect. Using 7,7,8,8-Tetracyanoquinodimethane and 2,2-bis1,3-dithiolylidene as p and n-type dopants, respectively, we determined that the inherent defects/metal vacancies, which could be due to the presence of Tungsten metal deficiency, contributed in p-type nature of the pristine 1 L-WS2. Doping of 2D transition metal dichalcogenides materials with organic molecule via the surface charge transfer method is not only a way to provide a handy way to tailor the electronic and optical properties but also can be used as a tool to determine the nature of defects in the material.

Surface Chemical Tuning of Phonon and Electron Transport in Free-Standing Silicon Nanowire Arrays.

We report electronic and phononic transport measurements of monocrystalline batch-fabricated silicon nanowire (SiNW) arrays functionalized with different surface chemistries. We find that hydrogen-terminated SiNWs prepared by vapor HF etching of native-oxide-covered devices show increased electrical conductivity but decreased thermal conductivity. We used the kinetic Monte Carlo method to solve the Boltzmann transport equation and also numerically examine the effect of phonon boundary scattering. Surface transfer doping of the SiNWs by cobaltocene or decamethylcobaltocene drastically improves the electrical conductivity by 2 to 4 orders of magnitude without affecting the thermal conductivity. The results showcase surface chemical control of nanomaterials as a potent pathway that can complement device miniaturization efforts in the quest for more efficient thermoelectric materials and devices.

Turning the n-Type CdS to p-Type CdS Nanowires By Surface Charge Transfer Doping

One-dimensional semiconductor nanostructure has been attracted great attention in electronic and optoelectronic applications in recent years. N-type CdS nanowires (NWs) doped with gallium were synthesized via chemical vapor deposition method[ 1 ]. In contrast to have n-type doping for CdS, p-type doping is relatively difficult to achieve due to the strong self-compensation [ 2,3 ]. In this work, we emonstrated the p-type doping of CdS NWs can be achieved by coating ~ 100 nm MoO3 onto CdS surface. Owing to the large work function of MoO3, the electrons in CdS are transfer to MoO3 and CdS exhibits the p-type characteristics, this process is called surface charge transfer doping. The charge transfer process was analyzed by X-ray photoelectron spectroscopy. MoO3-coated CdS NWs were then confirmed to be p-type by the back-gate field-effect transistors (FETs). Fig. 1(a) shows the optical microscopy image of back-gate Ga-doped CdS NW FET device. Fig. 1(b) depicts drain current (Id) versus drain voltage (Vd) at gate voltages (Vg) ranging from -30 V to 30 V at room temperature, it reveals typical n-type behavior. Besides, the transfer characteristic curves (Id-Vg) of Ga-doped CdS NW and MoO3-coated CdS NW devices shown in Fig. 1(c) as black and red curves, respectively. It is obvious that Ga-doped CdS NW reveals p-type characteristic after MoO3 layer coating. Finally, the MoO3-coated CdS/CdS NW junction photovoltaic devices were fabricated and discussed. Reference Cai, Jiajun et al., Phys. Chem. Chem. Phys., 13, 14663-14667, 2011Xie, Chao et al., J. Mater. Chem. C, 3, 6307-6313, 2015Li, Fang-Ze et al., Adv. Energy Mater., 3, 579-583, 2013 Figure 1

Stress and charge transfer in uniaxially strained CVD graphene

Mechanical properties of graphene prepared by chemical vapour deposition (CVD) are not easily comparable to the properties of nearly perfect graphene prepared by mechanical cleavage. In this work, we attempt to investigate the mechanical performance of CVD graphene (simply supported or embedded in polymer matrix), transferred by two different techniques, under uniaxial loading with simultaneous in situ monitoring by Raman microspectroscopy. The level of charge transfer doping and strain is assessed using the vector analysis modified for uniaxial strain. The strain distribution across the samples varies significantly, owing to the growth and transfer process, which induces wrinkles and faults in the CVD graphene. In simply supported specimens, the stress transfer efficiency is generally very low and the changes in Raman spectra are dominated by variations in the charge transfer originating from the realignment of the domains on the substrate upon the application of strain. In contrast, samples covered with an additional polymer layer exhibit an improved stress transfer efficiency, and the alterations of charge doping levels are negligible. In fully embedded specimens, the variations in stress transfer efficiencies are caused by the size of the effective graphene domains defined by cracks, folds and/or wrinkles.

Comparative investigation of surface transfer doping of hydrogen terminated diamond by high electron affinity insulators

We report on a comparative study of transfer doping of hydrogenated single crystal diamond surface by insulators featured by high electron affinity, such as Nb2O5, WO3, V2O5, and MoO3. The low electron affinity Al2O3 was also investigated for comparison. Hole transport properties were evaluated in the passivated hydrogenated diamond films by Hall effect measurements, and were compared to un-passivated diamond films (air-induced doping). A drastic improvement was observed in passivated samples in terms of conductivity, stability with time, and resistance to high temperatures. The efficiency of the investigated insulators, as electron accepting materials in hydrogenated diamond surface, is consistent with their electronic structure. These surface acceptor materials generate a higher hole sheet concentration, up to 6.5 × 1013 cm−2, and a lower sheet resistance, down to 2.6 kΩ/sq, in comparison to the atmosphere-induced values of about 1 × 1013 cm−2 and 10 kΩ/sq, respectively. On the other hand, hole mobilities were reduced by using high electron affinity insulator dopants. Hole mobility as a function of hole concentration in a hydrogenated diamond layer was also investigated, showing a well-defined monotonically decreasing trend.

Origin of ambipolar graphene doping induced by the ordered Ge film intercalated on SiC(0001)

Transfer doping mechanism of the quasi-free-standing monolayer graphene (QFMLG), depending on the thickness as well as the atomic structures of the intercalated Ge film, has been revealed by combined investigations of scanning tunneling microscopy and synchrotron photoemission spectroscopy. The ordered Ge film has been formed between QFMLG and the n-type 6H-SiC(0001) substrate by depositing Ge on the (63×63)R30° zero layer and postannealing. At postannealing temperatures lower than 900 °C, the interfacial structures inducing p-type QFMLG, like 2×1.5n3 and 4×33, are formed. On the other hand, at those higher than 900 °C, the interfacial structures inducing n-type QFMLG, like 7×43 [expressed as a (5−376) matrix], 8×33 and 21×21 [expressed as a (5115) matrix], are formed as a result of partial deintercalation of Ge atoms. Such a transition from p-type to n-type doping turns out to be associated with an increase in the density of states of an upper Hubbard band originating from strong correlation of the electrons at the Ge atoms on the first and the second Ge interfacial layers.

Pressure‐Induced Charge Transfer Doping of Monolayer Graphene/MoS2 Heterostructure

A unique way of achieving controllable, pressure-induced charge transfer doping in the graphene/MoS2 heterostructure is proposed. The charge transfer causes an upward shift in the Dirac point with respect to Fermi level at a rate of 15.7 meV GPa(-1) as a function of applied hydrostatic pressure, leading to heavy p-type doping in graphene. The doping was confirmed by I2D /IG measurements.

Band Alignment and Minigaps in Monolayer MoS2-Graphene van der Waals Heterostructures

Two-dimensional layered MoS2 shows great potential for nanoelectronic and optoelectronic devices due to its high photosensitivity, which is the result of its indirect to direct band gap transition when the bulk dimension is reduced to a single monolayer. Here, we present an exhaustive study of the band alignment and relativistic properties of a van der Waals heterostructure formed between single layers of MoS2 and graphene. A sharp, high-quality MoS2-graphene interface was obtained and characterized by micro-Raman spectroscopy, high-resolution X-ray photoemission spectroscopy (HRXPS), and scanning high-resolution transmission electron microscopy (STEM/HRTEM). Moreover, direct band structure determination of the MoS2/graphene van der Waals heterostructure monolayer was carried out using angle-resolved photoemission spectroscopy (ARPES), shedding light on essential features such as doping, Fermi velocity, hybridization, and band-offset of the low energy electronic dynamics found at the interface. We show that, close to the Fermi level, graphene exhibits a robust, almost perfect, gapless, and n-doped Dirac cone and no significant charge transfer doping is detected from MoS2 to graphene. However, modification of the graphene band structure occurs at rather larger binding energies, as the opening of several miniband-gaps is observed. These miniband-gaps resulting from the overlay of MoS2 and the graphene layer lattice impose a superperiodic potential.

Doping mechanism of MoO3in 4,4′-Bis(N-carbazolyl)-1,1′-biphenyl: A photoelectron spectroscopic study

The doping mechanism of molybdenum oxide (MoO3) in the hole transport material 4,4′-Bis(N-carbazolyl)-1,1′-biphenyl (CBP) is explored extensively using photoelectron spectroscopy. Doping limit and doping efficiency are investigated in dependence on the MoO3 content. In addition, shape changes in Mo3d and C1s core level spectra of doped layers are compared to the respective emissions of the pristine materials. Using density functional theory (DFT) calculations, the additional feature in the C1s emission can be assigned to CBP+ cations and in the Mo3d emission a Mo5+ species can be identified. Therefore, charge transfer can not only be deduced indirectly from the movement of the Fermi level within the CBP energy gap, but also directly in the formation of cations of the matrix and anionic dopant species. Finally, the dopant morphology in dependence on the MoO3 content is derived indirectly from XPS measurements and is correlated to the observed doping efficiency. Because MoO3 precipitates form in the CBP matrix as evident from TEM measurements, the bilayer interface band alignment is analyzed by stepwise evaporation of CBP onto a MoO3 layer. The results of the interface experiment show that the CBP:MoO3 doping behavior can be well understood with the internal interface charge transfer doping model.

(Invited) 2D Semiconductor Electronics: Advances, Challenges and Opportunities

Two-dimensional (2-D) semiconductors exhibit excellent device characteristics, as well as novel optical, electrical, and optoelectronic characteristics. In this talk, I will present our recent advancements in defect passivation, contact engineering, surface charge transfer doping, and heterostructure devices of layered chalcogenides. We have developed a defect repair/passivation technique that allows for observation of near-unity quantum yield in monolayer MoS2. The work presents the first demonstration of an optoelectronically perfect monolayer. Forming Ohmic contacts for both electrons and holes is necessary in order to exploit the performance limits of enabled devices while shedding light on the intrinsic properties of a material system. In this regard, we have developed different strategies, including the use of surface charge transfer doping at the contacts to thin down the Schottky barriers, thereby, enabling efficient injection of electrons or holes. We have been able to show high performance n- and p-FETs with various 2D materials. Additionally, I will discuss the use of layered chalcogenides for various heterostructure device applications, exploiting charge transfer at the van der Waals heterointerfaces. I will also present progress towards achieving tunnel transistors using layered semiconductors.

Adsorption of TCNQ and F4-TCNQ molecules on hydrogen-terminated Si(1 1 1) surface: van der Waals interactions included DFT study of the molecular orientations

A study of the molecules TCNQ and F4-TCNQ surface doped on hydrogen-terminated Si(111) surface [H–Si(111)], with emphasis on the role of van der Waals interactions in stabilizing the molecular adsorption orientations and the binding energy, has been carried out using density functional theory (DFT). The dispersion energy in the adsorption process is estimated using the DFT-D2/D3 and vdW-DF methods. The most stable spatial configuration of the adsorbed molecules predicted with and without including the dispersion energy in a low concentration dopant system is same though the magnitude of the binding energy is much larger in the former case. On the other hand in the case of increased dopant concentration (doubling the number of molecules on the same surface) the adsorption orientations predicted with and without the inclusion of dispersion energy are different and so is the magnitude of the binding energy. We conclude that inclusion of dispersion energy term is important for correct prediction of the spatial configurations of the adsorbed molecules in surface transfer doping of semiconductors.

Self-Limiting Oxides on WSe2 as Controlled Surface Acceptors and Low-Resistance Hole Contacts.

Transition metal oxides show much promise as effective p-type contacts and dopants in electronics based on transition metal dichalcogenides. Here we report that atomically thin films of under-stoichiometric tungsten oxides (WOx with x < 3) grown on tungsten diselenide (WSe2) can be used as both controlled charge transfer dopants and low-barrier contacts for p-type WSe2 transistors. Exposure of atomically thin WSe2 transistors to ozone (O3) at 100 °C results in self-limiting oxidation of the WSe2 surfaces to conducting WOx films. WOx-covered WSe2 is highly hole-doped due to surface electron transfer from the underlying WSe2 to the high electron affinity WOx. The dopant concentration can be reduced by suppressing the electron affinity of WOx by air exposure, but exposure to O3 at room temperature leads to the recovery of the electron affinity. Hence, surface transfer doping with WOx is virtually controllable. Transistors based on WSe2 covered with WOx show only p-type conductions with orders of magnitude better on-current, on/off current ratio, and carrier mobility than without WOx, suggesting that the surface WOx serves as a p-type contact with a low hole Schottky barrier. Our findings point to a simple and effective strategy for creating p-type devices based on two-dimensional transition metal dichalcogenides with controlled dopant concentrations.

Enhanced surface transfer doping of diamond by V2O5 with improved thermal stability

Surface transfer doping of hydrogen-terminated diamond has been achieved utilising V2O5 as a surface electron accepting material. Contact between the oxide and diamond surface promotes the transfer of electrons from the diamond into the V2O5 as revealed by the synchrotron-based high resolution photoemission spectroscopy. Electrical characterization by Hall measurement performed before and after V2O5 deposition shows an increase in hole carrier concentration in the diamond from 3.0 × 1012 to 1.8 × 1013 cm−2 at room temperature. High temperature Hall measurements performed up to 300 °C in atmosphere reveal greatly enhanced thermal stability of the hole channel produced using V2O5 in comparison with an air-induced surface conduction channel. Transfer doping of hydrogen-terminated diamond using high electron affinity oxides such as V2O5 is a promising approach for achieving thermally stable, high performance diamond based devices in comparison with air-induced surface transfer doping.

Healing of oxygen defects on VO2 surface: F4TCNQ adsorption

Oxygen-defect vacancies that routinely exist in wet production of VO2 material or on the surface of VO2 single crystal after surface treatment have significant influence on the metal-insulator phase transition features mainly due to their enhanced effect of doping on V 3d electronic structure. The removal of the surface oxygen defects is highly desired for investigating the VO2 intrinsic electronic properties. In this work, we propose a charge transfer doping method by using strong electric affinity molecule tetrafluorotetracyanoquinodimethane (F4TCNQ) adsorption rather than the normal thermal annealing in oxygen atmosphere to heal the surface oxygen defects of VO2 crystalline film. The healing effect is probed by the electronic structure evolution at the F4TCNQ/VO2 interface. The VO2 crystalline film is grown by an oxygen plasma assisted molecular beam epitaxy method on an Al2O3(0001) substrate. Surface oxygen defects on VO2 film are produced after a mild sputtering with an ionic energy of 1 keV and a thermal annealing in vacuum at 100 ℃. The influence of F4TCNQ molecule adsorption on the electronic structure of the sputtered VO2 film is studied by using in-situ synchrotron-based photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). XPS and XAS results demonstrate convincingly that V3+ species of sputtered VO2 are oxidized into the V4+ and simultaneously negative molecular ions form at F4TCNQ/VO2 interface resulting from the electron transfer from VO2 to the F4TCNQ layer. The preferred adsorption on surface defects and the strong electron withdrawing function of F4TCNQ molecules may account for the effective elimination of the electron doping effect of oxygen defects on VO2 surface. This charge transfer effect at interface recovers the electronic properties of VO2. Compared with thermal annealing in oxygen environment, the healing of oxygen defects by the molecular adsorption can prevent the surface from over oxidating VO2 into V2O5, which opens a new route to surface defect healing.

Determining the Fermi level by absorption quenching of monolayer graphene by charge transfer doping.

While optical properties of graphene in the visible region are solely defined by the frequency-independent fine structure constant, an onset of absorption has been observed in the infrared region due to Pauli blocking of interband transitions. Here, we report a complete absorption quenching in the infrared region by coating graphene with bis(trifluoromethanesulfonyl)amine (TFSA), an optically transparent p-type chemical dopant. The Fermi level downshift due to TFSA doping results in enhanced transmission in the infrared region proportional to the doping concentration. An absorption quenching onset method, developed in our work, to extract the Fermi level shift in pristine and doped graphene agrees with values extracted from Raman G-band and 2D-band shifts, Hall measurements and the binding energy shift observed in X-ray photo-electron spectroscopy. Performing simple UV-visible transmittance spectroscopy to obtain the absorption quenching onset of graphene also allows detection of environmental and substrate effects via Fermi level shift. Our method opens up the practical implementation of this unique phenomenon of graphene in future optoelectronic devices.