Transfer Doping Research Articles

2D hole gas mobility at diamond/insulator interface

The hole mobility of two-dimensional (2D) gas at (001) and (111) diamond/insulator interfaces is investigated theoretically and compared with experimental data from the literature. It is shown that the surface impurity scattering is the limiting mechanism at room temperature in most of the H-terminated diamond field effect transistors, where the negative charges created by transfer doping are in the vicinity of the 2D gas. By repelling the negative charges at the metal/insulator interface, as recently reported for the (111) h-BN/diamond interface, we demonstrate that it is possible to achieve high mobility values of the order of 3000 cm2/V s when a pure phonon scattering occurs. This work confirms the potential of two-dimensional hole gas diamond field effect transistors for high power and high frequency applications.

Simulation Study of Surface Transfer Doping of Hydrogenated Diamond by MoO3 and V2O5 Metal Oxides.

In this work, we investigate the surface transfer doping process that is induced between hydrogen-terminated (100) diamond and the metal oxides, MoO3 and V2O5, through simulation using a semi-empirical Density Functional Theory (DFT) method. DFT was used to calculate the band structure and charge transfer process between these oxide materials and hydrogen terminated diamond. Analysis of the band structures, density of states, Mulliken charges, adsorption energies and position of the Valence Band Minima (VBM) and Conduction Band Minima (CBM) energy levels shows that both oxides act as electron acceptors and inject holes into the diamond structure. Hence, those metal oxides can be described as p-type doping materials for the diamond. Additionally, our work suggests that by depositing appropriate metal oxides in an oxygen rich atmosphere or using metal oxides with high stochiometric ration between oxygen and metal atoms could lead to an increase of the charge transfer between the diamond and oxide, leading to enhanced surface transfer doping.

High-electron-affinity oxide V2O5 enhances surface transfer doping on hydrogen-terminated diamond

Diamond exhibits many desirable properties that could benefit the development of future carbon-based electronic devices. Its hydrogen-terminated surface, in conjunction with a suitable surface acceptor, develops a two-dimensional (2D) p-type surface conductivity through the surface transfer doping mechanism which can then be harvested for constructing functional devices. In this study, we have revisited the surface transfer doping of diamond by a high electron affinity (EA) transition metal oxide, V2O5. Through a combination of in-situ electrical measurements, Hall effect measurements and first-principles density functional theory (DFT) calculations, we explicitly show the intrinsic surface transfer doping behavior of V2O5, with doping performance superior to other competing TMOs such as MoO3. The metallic surface conduction of diamond induced by V2O5 is persistent down to 250 mK; this when coupled with the high hole density exceeding 7 × 1013 cm−2 offers a promising platform for the development of advanced diamond surface electronics exploiting many interesting quantum transport properties of the 2D hole layer of diamond.

Strong spin-orbit interaction induced by transition metal oxides at the surface of hydrogen-terminated diamond

Hydrogen-terminated diamond possesses an intriguing p-type surface conductivity which is induced via thermodynamically driven electron transfer from the diamond surface into surface acceptors such as atmospheric adsorbates, a process called surface transfer doping. High electron affinity transition metal oxides (TMOs) including MoO3 and V2O5 have been shown to be highly effective solid-state surface acceptors for diamond, giving rise to a sub-surface two-dimensional (2D) hole layer with metallic conduction. In this work, low temperature magnetotransport is used as a tool to show the presence of a Rashba-type spin-orbit interaction with a high spin-orbit coupling of 19.9 meV for MoO3 doping and 22.9 meV for V2O5 doping, respectively, through the observation of a transition in the phase-coherent backscattering transport from weak localization to weak antilocalization at low temperature. Surface transfer doping of diamond with TMOs provides a 2D hole system with spin-orbit coupling that is over two times larger than that reported for diamond surfaces with atmospheric acceptors, opening up possibilities to study and engineer spin transport in a carbon material system.

Palladium forms Ohmic contact on hydrogen-terminated diamond down to 4 K

A hydrogen-terminated diamond (H-terminated diamond) surface supports a two-dimensional (2D) p-type surface conductivity when exposed to the atmosphere, as a result of the surface transfer doping process. The formation of reliable Ohmic contacts that persist to cryogenic temperature is essential for the exploration of quantum transport in the diamond 2D conducting channel. Herein, the contact properties of Pd on H-terminated diamond have been fully investigated down to 4 K using transmission line method measurements. Pd is shown to form an Ohmic contact on H-terminated diamond with linear I–V characteristics and low specific contact resistance in the range of (8.4 ± 1) ×10−4 Ω·cm2 to (1.3 ± 0.2) ×10−3 Ω·cm2 for the temperature range of 300 K–4 K. This is in stark contrast to reference devices with Au/Pt/Ti contacts, which exhibit a significant temperature dependence and non-Ohmic behavior at low temperature. Using 2D thermionic emission theory, a negative Schottky barrier height (SBH), − 23 ± 1 meV, between Pd and H-terminated diamond has been determined, in comparison to a positive SBH of 42 ± 1 meV for the Au/Pt/Ti/H-terminated diamond interface. These results show that Pd serves as an excellent candidate for forming reliable Ohmic contacts on H-terminated diamond for enabling precise electrical transport measurements at cryogenic temperature.

H-Terminated Diamond Surface Band Bending Characterization by Angle-Resolved XPS

Concerning diamond-based electronic devices, the H-terminated diamond surface is one of the most used terminations as it can be obtained directly by using H2 plasma, which also is a key step for diamond growth by chemical vapour deposition (CVD). The resultant surfaces present a p-type surface conductive layer with interest in power electronic applications. However, the mechanism for this behavior is still under discussion. Upward band bending due to surface transfer doping is the most accepted model, but has not been experimentally probed as of yet. Recently, a downward band bending very near the surface due to shallow acceptors has been proposed to coexist with surface transfer doping, explaining most of the observed phenomena. In this work, a new approach to the measurement of band bending by angle-resolved X-ray photoelectron spectroscopy (ARXPS) is proposed. Based on this new interpretation, a downward band bending of 0.67 eV extended over 0.5 nm was evidenced on a (100) H-terminated diamond surface.

Surface charge doping induced carrier type reversal in spin coated CdS/rGO layered nanohybrid films

The growth of reduced graphene oxide (rGO) based semiconductor nanohybrids through simple, scalable, additive-free and cost-effective route has fascinated significant attention of researchers for both fundamental research areas and its commercial applications. In the present work, cadmium sulphide/reduced graphene oxide (CdS/rGO) films were sequentially or layer-by-layer (LBL) deposited by spin coating method. The samples were characterized by using x-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), UV-visible spectroscopy, Raman spectra and Hall measurements. The cubic phase of the CdS and formation of rGO was confirmed by XRD measurements. The FESEM micrographs elucidate a change in morphological features with rGO content in CdS/rGO nanohybrid films. The Raman spectra indicate the characteristic features for nanostructured CdS and rGO in these samples. Optical transmission is found to increase along with a decrease in optical gap with rGO content. Hall measurements showed the change in the majority carrier concentration with rGO content or the observation of surface charge transfer doping (SCTD) in these nanocomposites. The change in dispersion of refractive index with rGO content for CdS/rGO nanocomposites in terahertz (THz) spectral region has been observed. These results are very important for the development of new functionality of nanohybrid materials for emerging technologies.

Tuning Band Gaps and Photoelectrochemical Properties of Electrodeposited CuO Films by Annealing in Different Atmospheres

The band gaps and photoelectrochemical properties of electrodeposited CuO films were tuned through annealing as-electrodeposited films in different atmospheres at various temperatures. The direct band gaps of the CuO film descended from 1.80 eV for as-electrodeposited film to 1.76 eV for the film annealed in O2 at 300 °C or 1.64 eV for the film annealed in N2 at 300 °C. The stable photocurrent density for CuO electrode annealed in O2 at 300 °C measured at −0.4 V vs Ag/AgCl under a simulated sun light reached to 0.44 mA cm−2 while the value was only 0.29 mA cm−2 for the electrode annealed in N2 in a solution with 10 mM potassium ferricyanide, 10 mM potassium ferrocyanide, and 0.5 M Na2SO4. The mechanism for photoresponse enhancement was studied and could be due to faster charge transfer and higher doping density for the CuO film annealed at 300 °C in O2.

High temperature (300 °C) ALD grown Al2O3 on hydrogen terminated diamond: Band offset and electrical properties of the MOSFETs

Hydrogen-terminated diamond (H-diamond) metal-oxide-semiconductor field effect transistors (MOSFETs) were fabricated on a polycrystalline diamond substrate. The device has a gate length of 2 μm and uses Al2O3 grown by atomic layer deposition at 300 °C as a gate dielectric and passivation layer. The Al2O3/H-diamond interfacial band configuration was investigated by X-ray photoelectron spectroscopy, and a large valence band offset (3.28 eV) that is very suitable for p-channel H-diamond FETs was observed. Meanwhile, the measured O/Al ratio hints that there are Oi or VAl defects in the Al2O3 dielectric, which can work as an acceptorlike transfer doping material on a H-diamond surface. The device delivers the maximum saturation drain current of over 200 mA/mm, which is the highest for 2-μm H-diamond MOSFETs with the gate dielectric or passivation layer grown at 300 °C or higher temperature. The ultrahigh on/off ratio of 1010 and ultralow gate leakage current of below 10−12 A have been achieved. The high device performance is ascribed to the ultrahigh carrier density, good interface characteristics, and device processes. In addition, the transient drain current response of the device can follow the gate voltage switching on/off pulse at a frequency from 100 kHz to 1 MHz, which indicates the potential of the H-diamond FETs in power switch applications.

Doping molecular organic semiconductors by diffusion from the vapor phase

Spiro-OMeTAD can be doped by infiltration of F4TCNQ from the vapor phase without causing crystallization. Optical spectroscopy and thermopower measurements examined the question of the formation of dication states by charge transfer doping.

Surface charge transfer doping and effective passivation of black phosphorus field effect transistors

A metal oxide/h-BN/BP structure was built to realize electron doping and air stability for BPFETs.

Charge Transfer Doping of Conjugated Polymers with Large Vibrational Activities: Insights into the Regime of Partial Charge Transfer

Effective charge transfer (CT) doping of conjugated polymers depends on electronic and structural factors alike, though the former receives the most attention in design and mechanistic consideratio...

Role of adsorbed water in inducing electron accumulation in InN

Nominally undoped indium nitride (InN) is known to have an electron accumulation layer on its surface, and prior studies have shown this layer to be sensitive to chemical species. However, the exact roles of these species and the underlying mechanism of e− accumulation layer formation are not clear. In this work, it is shown that ambient adsorbed water on the InN surface strongly enhances the e− accumulation layer formed due to intrinsic surface states. Desorption of ambient physisorbates leads to a decrease in band bending, an increase in work function in undoped InN, and the observation of a p-type Mott-Schottky behavior in Mg:doped InN. The underlying mechanism of this surface-adsorbed water interaction may be through a process called “surface transfer doping,” which has previously been reported in hydrogenated diamond and other semiconductors such as GaN and ZnO.

MoO3 induces p-type surface conductivity by surface transfer doping in diamond

Surface transfer doping of diamond using high electron affinity transition metal oxides (TMOs), such as MoO3, has emerged as a key enabling technology for the development of diamond-based surface two-dimensional (2D) electronics. However, the omission of a critical pre-annealing step in the device fabrication process to remove atmospheric adsorbates prior to TMO deposition, as seen in numerous studies, makes the role of TMOs ambiguous in light of air-induced surface conductivity, preventing a full understanding of the intrinsic surface transfer doping behavior of TMO-based surface acceptors. Here, using in-situ four-probe electrical measurements we explicitly show the insulating-to-conducting transition in diamond surface driven by MoO3 induced surface transfer doping. Variable-temperature Hall-effect measurements reveal weak temperature-dependence down to 250 mK, evidencing the 2D Fermi liquid nature of the resulting surface conducting channel on diamond. Using first-principles calculations, we confirm the interfacial charge exchange upon MoO3 adsorption leading to a degenerate hole conducting layer on diamond. This work provides significant insights into understanding the surface transfer doping of diamond induced by TMOs, and paves the way for the investigations of many interesting quantum transport properties in the resulting 2D hole conducting layer, such as phase-coherent magnetotransport, on diamond surface.

Low-Power Complementary Logic Circuit Using Polymer-Electrolyte-Gated Graphene Switching Devices.

The modulation of the electrical properties of graphene and its device configurations for low-power consumption are important in developing graphene-based logic electronics. Here, we demonstrate the change in the charge transport in graphene from ambipolar to unipolar using surface charge transfer doping of the polymer electrolyte. Unipolar graphene field-effect transistors (GFETs) were obtained by the surface treatment of poly(acrylic acid) (PAA) for p-type and poly(ethyleneimine) (PEI) for n-type as polymer-electrolyte gates. In addition, lithium perchlorate (LiClO4) in a polymer matrix can be used for the low-gate voltage operation of GFETs (less than ±3 V) because of its high gating efficiency. Using polymer-electrolyte-gated GFETs, complementary graphene inverters were fabricated with a voltage swing of 57% and maximum voltage gain (Vgain) of 1.1 at a low supply voltage (VDD = 1 V). This is expected to facilitate the development of graphene-based logic devices with low-cost, low-power, and flexible electronics.

Reduction Treatment of Molecular-Doped Polymer Semiconductors for High-Performance N-Channel Organic Field-Effect Transistors

Here, we study an efficient reduction treatment for improving the electron transport properties in n-type organic field-effect transistors (OFETs) based on solution-processed polymer semiconductors. If complementary-type printed electronic circuits are to be enabled, equivalently high-performance p-type and n-type organic semiconductors have to develop. Most organic semiconductors have p-type performance superior to the n-type performance. Therefore, high-performance n-type organic semiconductors and their doping process are essential. Reduced-state viologen is known to be a powerful molecular dopant via donating electrons to the conjugated polymer semiconductors. However, the reduced-state viologen readily converts to its initial ionic species under ambient conditions, which leads to insufficient n-doping performance, especially for low electron affinity organic semiconductors. N-doped polymer semiconductors using pristine viologen molecular dopants show inadequate n-channel OFET behavior. For enhancing the electron transfer doping process, solution-based reduction treatment is used to improve remarkably the performance of n-channel OFETs based on initially p-type dominant donor-acceptor-type ambipolar polymer semiconductors. This solution-processed organic active channel layer can be produced at low cost and can be applied to flexible electronic and optoelectronic devices by using various graphic art printing techniques.

(Bi0.2Sb0.8)2Te3 based dynamic synapses with programmable spatio-temporal dynamics

Neuromorphic computing has recently emerged as a promising paradigm to overcome the von-Neumann bottleneck and enable orders of magnitude improvement in bandwidth and energy efficiency. However, existing complementary metal-oxide-semiconductor (CMOS) digital devices, the building block of our computing system, are fundamentally different from the analog synapses, the building block of the biological neural network—rendering the hardware implementation of the artificial neural networks (ANNs) not scalable in terms of area and power, with existing CMOS devices. In addition, the spatiotemporal dynamic, a crucial component for cognitive functions in the neural network, has been difficult to replicate with CMOS devices. Here, we present the first topological insulator (TI) based electrochemical synapse with programmable spatiotemporal dynamics, where long-term and short-term plasticity in the TI synapse are achieved through the charge transfer doping and ionic gating effects, respectively. We also demonstrate basic neuronal functions such as potentiation/depression and paired-pulse facilitation with high precision (>500 states per device), as well as a linear and symmetric weight update. We envision that the dynamic TI synapse, which shows promising scaling potential in terms of energy and speed, can lead to the hardware acceleration of truly neurorealistic ANNs with superior cognitive capabilities and excellent energy efficiency.

(Invited) Investigation of Interfacial Charge Transfer Doping of 2D MoS2

Layered two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDCs) have attracted a great deal of attention because of their exciting electrical and optical properties and their potential applications in electronic and optoelectronic devices. Because of ultra low volume and chemically inert surface of 2D materials, it is challenging to achieve controlled doping of 2D materials. Recently, interfacial charge transfer induced doping of 2D materials have gained significant attention. In this talk, I will present electrical transport studies of 2D molybdenum disulfide (MoS2) field effect transistors decorated with zero dimensional (0D) gold (Au) nanoislands. I will discuss how the Au nanoisland coverage affects the interfacial charge transfer based doping and tailor the electrical properties of the host material. From the detailed low temperature electron transport measurements, I will elucidate the role of 2D-0D interface on the transport properties of MoS2 FETs.

A DFT study of the surface charge transfer doping of diamond by chromium trioxide

In this study, a density functional theory method is employed to investigate the surface charge transfer doping of diamond by chromium trioxide (CrO3) with high electron affinity. Superior surface charge transfer of the hydrogenated diamond surface is demonstrated using CrO3 as an electron acceptor. The charge density difference and Bader charge analysis reveal that the electrons are transferred from the diamond surface to CrO3 molecule, leading to the formation of two-dimensional hole gas, and the holes left in the diamond surface increase the conductivity of the diamond surface. The analysis of electronic structure indicates that areal hole density as large as 9.85 × 1013cm−2 for CrO3-doped diamond surface can be achieved. Besides, the optical absorption near infrared region of the hydrogenated diamond surface is greatly enhanced upon CrO3 doping, which implies that this CrO3-doped diamond surface is a promising candidate for optoelectronic materials. The present study provides an in-depth theoretical understanding of the formation of two-dimensional hole gas on diamond surface induced by a new transition metal oxide, and predicts that the CrO3-doped diamond surface may have important implications in electronic and optoelectronic devices.

Convection-Flow-Assisted Preparation of a Strong Electron Dopant, Benzyl Viologen, for Surface-Charge Transfer Doping of Molybdenum Disulfide.

Transition metal dichalcogenides (TMDCs) have received attention as atomically thin post‐silicon semiconducting materials. Tuning the carrier concentrations of the TMDCs is important, but their thin structure requires a non‐destructive modulation method. Recently, a surface‐charge transfer doping method was developed based on contacting molecules on TMDCs, and the method succeeded in achieving a large modulation of the electronic structures. The successful dopant is a neutral benzyl viologen (BV0); however, the problem remains of how to effectively prepare the BV0 molecules. A reduction process with NaBH4 in water has been proposed as a preparation method, but the NaBH4 simultaneously reacts vigorously with the water. Here, a simple method is developed, in which the reaction vial is placed on a hotplate and a fragment of air‐stable metal is used instead of NaBH4 to prepare the BV0 dopant molecules. The prepared BV0 molecules show a strong doping ability in terms of achieving a degenerate situation of a TMDC, MoS2. A key finding in this preparation method is that a convection flow in the vial effectively transports the produced BV0 to a collection solvent. This method is simple and safe and facilitates the tuning of the optoelectronic properties of nanomaterials by the easily‐handled dopant molecules.