Transfer Doping Research Articles

Carrier Modulation of Ambipolar Few‐Layer MoTe2 Transistors by MgO Surface Charge Transfer Doping

AbstractSemiconducting molybdenum ditelluride (2H‐MoTe2), a fast‐emerging 2D material with an appropriate band gap and decent carrier mobility, is configured as field‐effect transistors and is the focus of substantial research interest, showing hole‐dominated ambipolar characteristics. Here, carrier modulation of ambipolar few‐layer MoTe2 transistors is demonstrated utilizing magnesium oxide (MgO) surface charge transfer doping. By carefully adjusting the thickness of MgO film and the number of MoTe2 layers, the carrier polarity of MoTe2 transistors from p‐type to n‐type can be reversely controlled. The electron mobility of MoTe2 is significantly enhanced from 0.1 to 20 cm2 V−1 s−1 after 37 nm MgO film doping, indicating a greatly improved electron transport. The effective carrier modulation enables to achieve high‐performance complementary inverters with high DC gain of >25 and photodetectors based on few‐layer MoTe2 flakes. The results present an important advance toward the realization of electronic and optoelectronic devices based on 2D transition‐metal dichalcogenide semiconductors.

Al2O3 dielectric layers on H-terminated diamond: Controlling surface conductivity

This study investigates how the surface conductivity of H-terminated diamond can be preserved and stabilized by using a dielectric layer with an in situ post-deposition treatment. Thin layers of Al2O3 were grown by plasma enhanced atomic layer deposition (PEALD) on H-terminated undoped diamond (100) surfaces. The changes of the hole accumulation layer were monitored by correlating the binding energy of the diamond C 1s core level with electrical measurements. The initial PEALD of 1 nm Al2O3 resulted in an increase of the C 1s core level binding energy consistent with a reduction of the surface hole accumulation and a reduction of the surface conductivity. A hydrogen plasma step restored the C 1s binding energy to the value of the conductive surface, and the resistance of the diamond surface was found to be within the range for surface transfer doping. Further, the PEALD growth did not appear to degrade the surface conductive layer according to the position of the C 1s core level and electrical measurements. This work provides insight into the approaches to establish and control the two-dimensional hole-accumulation layer of the H-terminated diamond and improve the stability and performance of H-terminated diamond electronic devices.

Charge Transfer Doping Induced Conformational Ordering of a Non-Crystalline Conjugated Polymer

Charge transfer doping of a nominally disordered conjugated polymer induces long-range conformational ordering (stiffening) of backbone segments. Addition of [2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) to dilute solutions of poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) results in quantitative charge transfer in the ground electronic state of the polymer. Following charge (hole) injection, greater MDMO-PPV monomer coplanarity is evident in electronic, Raman, and electron paramagnetic resonance (EPR) spectra over a broad range of dopant loadings. New transitions emerge at lower energies with spectral patterns distinct from pristine materials but closely resemble minority low energy conformers selectively that can be prepared by careful control of processing conditions. We further demonstrate that characteristic Raman patterns of PPV systems actually contain signatures of a minority ordered form that interacts preferentially with the dopant. Subsequent additions of dopant al...

Boosting surface charge-transfer doping efficiency and robustness of diamond with WO3 and ReO3

An advanced charge-transfer yield is demonstrated by employing single monolayers of transition-metal oxides—tungsten trioxide (WO3) and rhenium trioxide (ReO3)—deposited on the hydrogenated diamond surface, resulting in improved p-type sheet conductivity and thermal stability. Surface conductivities, as determined by Hall effect measurements as a function of temperature for WO3, yield a record sheet hole carrier concentration value of up to 2.52 × 1014 cm−2 at room temperature for only a few monolayers of coverage. Transfer doping with ReO3 exhibits a consistent narrow sheet carrier concentration value of around 3 × 1013 cm−2, exhibiting a thermal stability of up to 450 °C. This enhanced conductivity and temperature robustness exceed those reported for previously exposed surface electron acceptor materials used so far on a diamond surface. X-ray photoelectron spectroscopy measurements of the C1s core level shift as a function of WO3 and ReO3 layer thicknesses are used to determine the respective increase in surface band bending of the accumulation layers, leading to a different sub-surface two-dimensional hole gas formation efficiency in both cases. This substantial difference in charge-exchange efficiency is unexpected since both surface acceptors have very close work functions. Consequently, these results lead us to consider additional factors influencing the transfer doping mechanism. Transfer doping with WO3 reveals the highest yet reported transfer doping efficiency per minimal surface acceptor coverage. This improved surface conductivity performance and thermal stability will promote the realization of 2D diamond-based electronic devices facing process fabrication challenges.

Co-adsorption of water and oxygen on GaN: Effects of charge transfer and formation of electron depletion layer

Species from ambient atmosphere such as water and oxygen are known to affect electronic and optical properties of GaN, but the underlying mechanism is not clearly known. In this work, we show through careful measurement of electrical resistivity and photoluminescence intensity under various adsorbates that the presence of oxygen or water vapor alone is not sufficient to induce electron transfer to these species. Rather, the presence of both water and oxygen is necessary to induce electron transfer from GaN that leads to the formation of an electron depletion region on the surface. Exposure to acidic gases decreases n-type conductivity due to increased electron transfer from GaN, while basic gases increase n-type conductivity and PL intensity due to reduced charge transfer from GaN. These changes in the electrical and optical properties, as explained using a new electrochemical framework based on the phenomenon of surface transfer doping, suggest that gases interact with the semiconductor surface through electrochemical reactions occurring in an adsorbed water layer present on the surface.

(Invited) Solution-Processed Charge Transfer Doping of Transition-Metal Dichalcogenides (TMDs) with Redox-Active Molecules

The promise of flexible, stretchable, and transparent circuitry has driven intense research in carbon-based and two-dimensional (2D) material-based electronics, such as organic semiconductors, graphene and transition-metal dichalcogenides (TMDs). For integrated circuits applications consisting of CMOS, p-n diodes and other logic components, both n-type and p-type semiconductors are equally important. Controlled doping of two-dimensional semiconductors promises a powerful tool to tailor their electrical and optical properties toward next generation electronic and optoelectronic devices. In this study, solution-processed charge transfer doping of four transition metal dichalcogenides MX2, where M is either Mo or W and X either S or Se, is achieved using molecular reductants and oxidants. Sub-hundred-micron size (ca. 50 to 100 μm) of MX2 were exfoliated via gold-mediated transfer technique.1 The doping type and strength can be conveniently controlled by the redox potential of the metal-organic molecules, the concentration of dopant solutions and treatment time.2 The doping effects are observed through the evolution of electrical characteristics of MX2 field effect transistor devices. I-V characteristics of MX2 field effect transistor (FET) devices treated with both p- and n-dopants are measured, from which the charge carrier densities induced by the doping were estimated. Detailed physical characterizations including photoemission (UPS and XPS), Raman and photoluminescence (PL) spectroscopy are used to study the doping effect and to understand the underlying mechanism. The origin of the doping is attributed to the shift of Fermi level and work function through the charge transfer process between molecular dopants and 2D semiconductors. Overall, we have demonstrated a solution-based charge transfer doping using molecular oxidants and reductants to p- or n-dope several 2D semiconductors. This technique provides a simple yet effective route to tailor the band structure of the 2D materials, and control the resulting electrical and optical properties. References (1) Desai, S. B.; Madhvapathy, S. R.; Amani, M.; Kiriya, D.; Hettick, M.; Tosun, M.; Zhou, Y.; Dubey, M.; Ager, J. W.; Chrzan, D. Adv. Mater. 2016, 28, 4053. (2) Tarasov, A.; Zhang, S.; Tsai, M. Y.; Campbell, P. M.; Graham, S.; Barlow, S.; Marder, S. R.; Vogel, E. M. Adv. Mater. 2015, 27, 1175.

Tuning the Electronic and Optical Properties of Monolayers As, Sb, and Bi via Surface Charge Transfer Doping

The novel stable two-dimensional (2D) monolayers As, Sb, and Bi have attracted much attention for their peculiar semiconducting electronic properties. Tuning the electronic and optical properties of monolayers As, Sb, and Bi is essential to broaden their applications in electronic and optoelectronic devices. Herein, on the basis of the density functional theory (DFT) calculations, we proposed an effective surface charge transfer doping (SCTD) strategy to control the electronic and optical properties of monolayers As, Sb, and Bi. Two types of common p-/n-type surface dopants, that is, tetrafluoro-tetracyanoquinodimethane (F4-TCNQ) and benzyl viologen (BV), were chosen in this work. Our calculations revealed that F4-TCNQ is capable of enhancing the p-type conductivity of the monolayers As, Sb, and Bi because of the charge transfer from the monolayers to F4-TCNQ, while BV could transform the p-type monolayers As, Sb, and Bi into n-type semiconductors because of the injection of electrons from BV to the monol...

Charge transfer doping of graphene without degrading carrier mobility

Density functional calculations are used to analyze the charge transfer doping mechanism by molecules absorbed onto graphene. Typical dopants studied are AuCl3, FeCl3, SbF5, HNO3, MoO3, Cs2O, O2, and OH. The Fermi level shifts are correlated with the electron affinity or ionization potential of the dopants. We pay particular attention to whether the dopants form direct chemisorptive bonds which cause the underlying carbon atoms to pucker to form sp3 sites as these interrupt the π bonding of the basal plane, and cause carrier scattering and thus degrade the carrier mobility. Most species even those with high or low electronegativity do not cause puckering. In contrast, reactive radicals like -OH cause puckering of the basal plane, creating sp3 sites which degrade mobility.

Gaseous Reactions in Adsorbed Water Present on Transition Metal Oxides

Water from ambient atmosphere is known to adsorb naturally on the surfaces of many transition metal oxides, and this adsorbed water is known to affect electrical, optical, and many functional properties such as catalytic activities. However, the nature of this underlying interaction remains unknown. In this work, we report a new type of semiconductor-adsorbed water interaction in metal oxides that is known as “electrochemical surface transfer doping”, a phenomenon that has previously been observed in hydrogen-terminated diamond. Herein, the adsorbed water on the metal oxide allows dissolution of gaseous species and induces charge transfer processes at the oxide/adsorbed-water interface. We further elucidate, using in situ infrared photoluminescence, electrical resistance, and X-ray photoelectron spectroscopy, the role of vacancy defects in the catalytic process by directly monitoring the charge transfer process between gaseous species and vacancies in defective metal oxides such as p-type nickel oxide and...

Realizing Stable p-Type Transporting in Two-Dimensional WS2 Films.

Two-dimensional (2D) semiconductors have become promising candidates for nanoelectronics applications due to their unique layered structure and rich physical properties. However, the significant lack of reproducible p-type doping methods that can avoid the instability induced by the widely used charge transfer doping method greatly limits the applications of these semiconductors in complementary metal-oxide-semiconductor (CMOS) integrated digital circuits. This work presents a new scheme to realize stable p-type doping for WS2 with excellent layer controllability, wafer-level uniformity, and high reproducibility at the same time. The p-type WS2 was produced by introducing substitutional doping of sulfur with nitrogen atoms during the sulfurization of WOxNy film. Nitrogen atoms acted as acceptors moving the Fermi level of WS2 toward the valance band. Both experimental and theoretical investigations were designed to study the physical properties of the films fabricated. The WS2 based field-effect transistors exhibited a well-defined p-type behavior with a large on/off current ratio of ∼105 and a high hole mobility of ∼18.8 cm2 V-1 s-1. This opens up a promising method to realize stable p-type doping of 2D materials, which is very attractive for future large-scale 2D CMOS device applications.

A mechanistic study on the carrier properties of nitrogen-doped graphene derivatives using thermoelectric effect

For graphene to find wide applicability in various nanoelectronic applications, it must be synthesized in bulk with reliable n- and p-type electrical properties. Most nitrogen-doped graphenes show p-type character despite their high nitrogen contents, leading to a lack of the n-type graphene. Here, we demonstrate that the reason why the pristine n-type graphene derivatives exhibit p-type character is due to unintentional hole doping by water and oxygen molecules from surroundings, which was firstly confirmed by understanding the electrochemical potential of graphene and water/oxygen layer. Furthermore, we show that this unintentionally p-doped graphene derivative can be changed to air-stable n-type by the surface charge transfer doping using polyethylenimine with the electron donating groups. The carrier properties of graphene derivatives were investigated using thermoelectric effects. These results demonstrate that the mechanistic and systematic study proposed here for controlling the carrier properties of graphene derivatives has great potential for further development of graphene-based electronic and energy devices.

Graphene contacts to a HfSe2/SnS2 heterostructure.

Two-dimensional (2D) heterostructures and all-2D contacts are of high interest for electronic device applications, and the SnS2/HfSe2 bilayer heterostructure with graphene contacts has some unique, advantageous properties. The SnS2/HfSe2 heterostructure is interesting because of the strong intermixing of the two conduction bands and the large work function of the SnS2. The band lineup of the well separated materials indicates a type II heterostructure, but the conduction band minimum of the SnS2/HfSe2 bilayer is a coherent superposition of the orbitals from the two layers with a spectral weight of 60% on the SnS2 and 40% on the HfSe2 for AA stacking. These relative weights can be either increased or reversed by an applied vertical field. A 3×3 supercell of graphene and a 2×2 supercell of SnS2/HfSe2 have a lattice mismatch of 0.1% and both the SnS2/HfSe2 conduction band at M and the graphene Dirac point at K are zone-folded to Γ. Placing graphene on the SnS2/HfSe2 bilayer results in large n-type charge transfer doping of the SnS2/HfSe2 bilayer, on the order of 1013/cm2, and the charge transfer is accompanied by a negative Schottky barrier contact for electron injection from the graphene into the SnS2/HfSe2 bilayer conduction band. Binding energies and the anti-crossing gaps of the graphene and the SnS2/HfSe2 electronic bands both show that the coupling of graphene to the HfSe2 layer is significantly larger than its coupling to the SnS2 layer. A tunneling Hamiltonian estimate of the contact resistance of the graphene to the SnS2/HfSe2 heterostructure predicts an excellent low-resistance contact.

Charge Trapping Analysis of High Speed Diamond FETs

Charge carrier trapping in diamond surface conduction field effect transistors (FETs) has been analyzed. For these devices two methods were used to obtain a negative electron affinity diamond surface; either plasma hydrogenation or annealing in an H2 environment. In both cases the Al2O3 gate dielectric can trap both electrons and holes in deep energy levels with emission timescales of seconds, while the diamond – Al2O3 interface traps exhibit much shorter time scales in the microsecond range. Capacitance-Voltage (CV) analysis indicates that these interface traps exhibit acceptor-like characteristics. Correlation with CV based free hole density measurements indicates that the conductance based interface trap analysis provides a method to quantify surface characteristics that lead to surface conduction in hydrogenated diamond where atmospheric adsorbates provide the acceptor states for transfer doping of the surface.

A comparison of various surface charge transfer hole doping of graphene grown by chemical vapour deposition

Charge transfer doping is a renowned route to modify the electrical and electronic properties of graphene. Understanding the stability of potentially important charge-transfer materials for graphene doping is a crucial first step. Here we present a systematic comparison on the doping efficiency and stability of single layer graphene using molybdenum trioxide (MoO3), gold chloride (AuCl3), and bis(trifluoromethanesulfonyl)amide (TFSA). Chemical dopants proved to be very effective, but MoO3 offers better thermal stability and device fabrication compatibility. Single layer graphene films with sheet resistance values between 100 and 200ohm/square were consistently produced by implementing a two-step growth followed by doping without compromising the optical transmittance.

Significantly enhanced optoelectronic performance of tungsten diselenide phototransistor via surface functionalization

Two-dimensional (2D) layered transition metal dichalcogenides (TMDs) have attracted enormous research interests and efforts towards the development of versatile electronic and optical devices, owing to their extraordinary and unique fundamental properties and remarkable prospects in nanoelectronic applications. Among the TMDs, tungsten diselenide (WSe2) exhibits tunable ambipolar transport characteristics and superior optical properties such as high quantum efficiency. Herein, we demonstrate significant enhancement in the device performance of WSe2 phototransistor by in situ surface functionalization with cesium carbonate (Cs2CO3). WSe2 was found to be strongly doped with electrons after Cs2CO3 modification. The electron mobility of WSe2 increased by almost one order of magnitude after surface functionalization with 1.6-nm-thick Cs2CO3 decoration. Furthermore, the photocurrent of the WSe2-based phototransistor increased by nearly three orders of magnitude with the deposition of 1.6-nm-thick Cs2CO3. Characterizations by in situ photoelectron spectroscopy techniques confirmed the significant surface charge transfer occurring at the Cs2CO3/WSe2 interface. Our findings coupled with the tunable nature of the surface transfer doping method establish WSe2 as a promising candidate for future 2D materialsbased optoelectronic devices.

P-type surface transfer doping of oxidised silicon terminated (100) diamond

High-resolution core-level photoemission was used to examine the interaction between the oxidised silicon-terminated diamond (100) surface and the molecular acceptor MoO3. An observed downward shift in the Fermi level position, accompanied by the appearance of two distinct charge states of MoO3, indicates charge transfer from the surface into the MoO3 adlayer in the form of surface transfer doping with a concurrent accumulation of holes in the diamond.

Surface charge transfer doping induced inversion layer for high-performance graphene/silicon heterojunction solar cells

Surface charge transfer doping (SCTD) induced p-type inversion layer was implemented in the graphene/silicon heterojunction solar cells, leading to significant improvement of device efficiency.

Solvent basicity promotes the hydride-mediated electron transfer doping of carbon nanotubes.

This study investigates the hydride-mediated electron transfer doping of single-walled carbon nanotubes using absorption spectroscopy and thermoelectric measurements. Specific solvent basicity is found to be important for the efficient n-type doping of carbon nanotubes. This progress is an essential requirement for the future development of electronic and energy devices.

A laser induced forward transfer process for selective boron emitters

In this work, we present a novel technological approach to form highly boron-doped selective emitters. The selective emitters are formed by using a Laser Induced Forward Transfer Doping (DLIFT) process, which allows precise adjustment of the doping profile by tuning the laser parameters and choosing an appropriate doping source. Surface dopant concentrations of up to Ns≈1×1021cm−3 were achieved by using DLIFT. Subsequent BBr3 tube furnace diffusion was performed to form a low surface concentration (Ns≈1×1019cm−3) homogeneous emitter. In comparison to the conventionally applied BBr3 tube diffusion, an increase in open circuit voltage of up to 6mV is predicted based on the photoluminescence measurements performed after passivation, screen-printing and firing processes. Simulations suggest that this voltage gain is most possibly due to a lower emitter saturation current density (j0e) of the highly doped DLIFT samples compared to the BBr3 diffusion samples. Moreover, it is observed that the laser-induced defects are reduced successfully after a subsequent boron tribromide (BBr3) tube diffusion, which is used to form the homogeneous emitter after the DLIFT process. The micro Raman spectroscopy results indicated that the compressive stress of −200MPa in the silicon crystal lattice after DLIFT process is released by the subsequent BBr3 tube diffusion. These results demonstrate that the DLIFT process in combination with the conventionally applied BBr3 tube diffusion can be an effective approach to form selective boron emitters in high efficiency n-type crystalline silicon (c-Si) solar cells.

Restoration of thermally reduced graphene oxide by atomic-level selenium doping

The use of reduced graphene oxide (rGO) suffers from irreparable damage because of topological defects and residual heteroatoms, which degrade the inherent properties of graphene. To restore its electrical transport properties, charge-transfer chemical doping with d-electron-rich heteroatoms has been proposed. Herein, we report the effects of atomic-level selenium doping in rGO. Using first-principles calculations, we found that selenium atoms could be selectively bonded in particular locations, such as the pseudo-edge sites of hole-cluster defects in the basal plane and edge defect sites of graphene; however, we found that the intrinsic topological defects of the basal plane were unfavorable for bonding. Numerous selenium atoms were introduced on the fully amorphorized rGO surface, inducing a dramatic change of its electrical transport properties by electron doping. The large metallic regions formed by the selenium atoms on rGOs led to the enhancement of electrical conductivity by 210 S cm–1 at 300 K. Moreover, the temperature-dependent conductivities (σ)/σ20K of selenium-doped rGOs (Se-rGOs) were almost constant in the temperature range of 20–300 K, indicating that the carrier mobility of Se-rGOs becomes temperature-independent after selenium doping, similar to that of pure graphene. Atomic-thin layers of selenium can turn microscale graphene oxide sheets into enhanced energy-storage devices, reports a new study. Graphene's extraordinary conductivity has attracted interest for applications such as battery anodes, but manufacturers seeking sizeable quantities often use reduced graphene oxide – chemically synthesized carbon films containing oxygen and other defects that hinder charge transport. Hyoung-Joon Jin from Inha University in South Korea and colleagues now demonstrate that heating graphene oxide with elemental selenium returns metal-like conductivity to the carbon sheet through a process called surface transfer doping. Scanning electron microscopy and first-principles calculations revealed that selenium atoms attach to surface edge defect sites and transfer electrons to graphene oxide. These dopants enabled graphene oxide to achieve a similar battery capacity and longevity as pure graphene when incorporated in a prototype lithium-ion device. Selenium atoms were selectively introduced in particular locations such as the pseudo-edge sites of hole-cluster defects in the basal plane and edge defect sites of the fully amorphorized surface of reduced graphene oxide (rGO), inducing a dramatic change of electrical transport properties of rGO by electron doping.