Surface Transfer Doping Research Articles

Alkali and alkaline earth metals induced n-type surface transfer doping of diamond: A first-principles calculation

The advancement of diamond-based electronic devices is hindered by the n-type doping in diamond, which is a major challenge for diamond. Surface transfer doping is an effective way for modulating the surface conductivity. In this study, we propose and select eight kinds of alkali and alkaline earth metals with low electronegativities as n-type surface dopants on F-terminated diamond using first-principles calculations. Our results reveal n-type doping is realized on diamond surface, and a negative correlation between the electronegativity of metal dopants and areal electron density. High areal electron densities of 2.975 × 1013 and 3.102 × 1013cm-2 and room-temperature electron mobilities of 1102 and 1152 cm2 V-1 s-1 are obtained on Rb and Cs doped diamond surfaces, respectively. Our findings provide a way for designing of n-type diamond, and it is helpful for the understanding of the surface doping mechanism for semiconductors.

In situ photoemission spectroscopies to reveal surface transfer doping on hydrogenated milled nanodiamonds

Nanodiamonds (ND) exhibit exceptional chemical, electronic, thermal, and optical properties, making them valuable for applications in nanomedicine, energy, quantum technologies, advanced lubricants, and polymer composites. Surface analysis techniques such as X-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS) and reflection electron energy loss spectroscopy (REELS) are critical for understanding the surface properties of nanomaterials, including nanodiamonds. This study investigates hydrogenated milled nanodiamonds (H-MND) by integrating UPS and XPS measurements with REELS. Through in situ annealing within an ultra-high vacuum (UHV) chamber, we examine the impact of surface termination on surface conductivity, focusing on the role of adsorbates. Our findings reveal that a surface transfer doping mechanism, akin to that observed in bulk diamond, governs a pseudo p-type conductivity in H-MND. The conductivity dependence on ambient exposure, water, and subsequent annealing demonstrates its reversibility. The study also discusses the nature of electron acceptors and the influence of ND facets on conductivity.

Origin of two-dimensional hole gas at the hydrogen-terminated diamond surfaces: Negative interface valence-induced upward band bending

The surface transfer doping model has been extensively adopted as a mechanism to account for the generation of hole accumulation layers below hydrogen-terminated diamond (H-diamond) surfaces. To achieve effective surface transfer doping, surface electron acceptor materials with high electron affinity (EA) are required to produce a high density of two-dimensional hole gas (2DHG) on the H-diamond subsurface. We have established ingenious theoretical models to demonstrate that even if these solid materials do not have a high EA value, they remain capable of absorbing electrons from the H-diamond surface by forming a negatively charged interface to act as a surface electron acceptor in the surface transfer doping model. Our calculations, particularly for the local density of states, provide compelling evidence that the effect of an interface with negative charges induces an upward band bending on the H-diamond side. Furthermore, the valence band maximum of the diamond atoms at the interface crosses the Fermi level, giving rise to strong surface transfer p-type doping. These results give a strong theoretical interpretation of the origin of 2DHG on H-diamond surfaces. The proposed guidelines contribute to further improvements in the performance of 2DHG H-diamond field effect transistors.

Surface Transfer Doping in MoO3-x/Hydrogenated Diamond Heterostructure.

Surface transfer doping is proposed to be a potential solution for doping diamond, which is hard to dope for applications in high-power electronics. While MoO3 is found to be an effective surface electron acceptor for hydrogen-terminated diamond with a negative electron affinity, the effects of commonly existing oxygen vacancies remain elusive. We have performed reactive molecular dynamics simulations to study the deposition of MoO3-x on a hydrogenated diamond (111) surface and used first-principles calculations based on density functional theory to investigate the electronic structures and charge transfer mechanisms. We find that MoO3-x is an effective surface electron acceptor and the spatial extent of doped holes in hydrogenated diamond is extended, promoting excellent transport properties. Charge transfer is found to monotonically decrease with the level of oxygen vacancy, providing guidance for engineering of the surface transfer doping process.

N-type doping of diamond surface by potassium

For diamond with exceptional physical and chemical properties, it is a challenging task to develop an effective n-type dopant with satisfactory electronic properties. In this work, using the first-principles calculation, we report the n-type doping of diamond surface (areal electron density of 2.508 × 1013–4.090 × 1013 cm−2 and carrier mobility of 189–1186 cm2 V−1 s−1 at 298 K) by surface transfer doping using potassium (K). Electrons are transferred from the K atoms to the diamond surface, then electrons accumulation appears on the diamond surface. In addition, the areal electron density and electron affinity values increase with increasing K concentration. This work provides a new strategy for producing n-type diamond-based devices.

Faraday effect sensing of single-molecules by graphene-based layered structures

To investigate the sensing properties of the Faraday effect in graphene in terahertz (THz), we consider the modulations of Dirac gaps in graphene when gas molecules are adsorbed to its surface. Here, the effect of the associated bandgap opening in graphene’s spectrum on the sensing properties of the Faraday rotation (FR) in a 1D photonic graphene-based sensor with a microcavity defect channel covered by two graphene layers. Our proposed model introduces an optical contact-free mechanism for detecting low-concentration molecules attached to graphene’s surface. We also show that FR angles could reveal reversal signs for a different amount of surface transfer doping. Finally, the effect of increasing the temperature on the Faraday effect at a certain opening gap in graphene’s spectrum is investigated. Interestingly, stronger FR effect it was observed at higher temperatures. Our results could open the path for development of contact-free sensing applications in THz region.

Tuning of electrical properties of CVD grown graphene by surface doping with organic molecules

Tailoring the charge carriers of two-dimensional (2D) materials is essential for high performance optoelectronic devices. The surface transfer doping by adsorption of molecules on 2D crystals is an attractive technique to tune the properties. Here, we study the change in the electronic transport properties of monolayer graphene (MLG) by surface doping with two different types of molecules. An effect of methyl isobutyl ketone (MIBK) and chlorobenzene molecular doping on the carrier concentration and electrical conductivity of chemical vapor deposition(CVD)-grown MLG was carried out by Raman spectroscopy and electrical transport measurement. The shifting of Raman peaks toward higher wave number and shifting of Dirac points toward positive gate voltage confirmed that the surface doping of graphene with MIBK and chlorobenzene molecules induced holes doping effect. The molecular doping approach significantly improved the carrier concentration of CVD grown MLG, which is a promising result. Our study will be useful for understanding as well as improvement of graphene based electronic device research.

(Invited) Universal Alignment of Surface and Bulk Oxygen Levels in Semiconductors

Lattices of most semiconductors have a large density of intrinsic and extrinsic defects which gives rise to effects such as surface Fermi level (EF) pinning, dopant compensation, asymmetrical p versus n-doping, and device degradation. Oxygen and hydrogen are the two most important impurities in semiconductors because of their ubiquitous presence in growth and device processing environments and consequently their incorporation strongly influences electronic and electrical properties. Therefore, a deeper understanding of the interaction of these species with the semiconductor surface and bulk defects is necessary for enabling the development of devices based on them such as photovoltaic and photocatalytic systems, fuel cells, and many others. Analysis of the reported surface work function values and the substitutional bulk O-defect energies show that the surface Fermi level of position in a broad range of group IV, III-V, II-VI, and I-III-VI2 semiconductors with physisorbed O2 lies universally at approximately -5.1 eV below the vacuum level. Similarly, results show that the energy of the bulk substitutional O-related amphoteric defects incorporated during crystal growth also has a universal energy of ~ -5.0 eV with respect to the vacuum level for most of the investigated semiconductors. It is shown that the process of ‘surface transfer doping’ that involves an adsorbed nanoscale water film on the semiconductor surface is likely responsible for the universal alignment of oxygen levels. Figure 1

Degenerate p-Type and n-Type Doping of Diamane by Molecular Adsorption

Motivated by the successful synthesis of two-dimensional diamane [Nat. Nanotechnol. 2020, 15, 59-66], in this work, the electronic and optical properties of diamane with molecular adsorption are investigated by first-principles calculation. Based on the surface transfer doping mechanism, we report the degenerate p-type and n-type doping for hydrogenated diamane (H-diamane) and fluorinated diamane (F-diamane), respectively. Hole accumulation on H-diamane and degenerate levels in the density of states (DOS) are found when organic molecules (tetracyanoethylene, tetracyanoquinodimethane, and tetrafluorotetracyanoquinodimethane) and transition-metal oxides (MoO3, CrO3, WO3, V2O5, and ReO3) are chosen as acceptors on H-diamane. Conversely, F-diamane shows electron accumulation and degenerate levels in DOS when organic molecules (decamethylcobaltocene and cobaltocene2) are adsorbed. The carrier concentration values of H-diamane and F-diamane are 1.91 × 1013 to 3.96 × 1013 cm–2 and 1.96 × 1013 to 3.38 × 1013 cm–2, respectively. After adsorption, the optical absorption increases significantly in the visible-light region. Our findings would provide a feasible route to modulate the electronic and optical properties of diamane.

Surface transfer doping of oxidised silicon-terminated (111) diamond using MoO3

Surface transfer doping of oxidised silicon-terminated (111) diamond using MoO3

Surface transfer doped diamond diodes with metal oxide passivation and field-plate

Surface transfer-doping, involving hydrogen terminated diamond surfaces, has been an effective method for producing diamond devices for some years but suffered from poor device longevity and reproducibility. The emergence of metal oxides as an encapsulant has begun to change this situation. Here, HfO2 encapsulated surface transfer doped diamond Schottky diodes with stable device characteristics have been demonstrated. Ideality factor and Schottky barrier heights of the devices did not vary considerably across extended periods of use (up to 39 days). The devices showed excellent blocking capabilities, demonstrating no catastrophic breakdown under the maximum field applied and only a slight increase in leakage current at the reverse bias and field strength of 200 V and 0.167 MV cm−1, respectively. Indeed, a large rectification ratio of up to 108 and a very low leakage current of ≈10−9 A cm−1 were maintained at this reverse bias (200 V). Furthermore, multiple devices were compared across a single substrate, something rarely reported previously for surface transfer doped diamond diodes. Leakage currents and rectification ratios were similar for all of the devices.

Two-dimensional ambipolar carriers of giant density at the diamond/cubic-BN(111) interfaces: toward complementary logic and quantum applications.

The extremely difficult ambipolar doping activation greatly hinders the outstanding performance of diamond for electronic devices. The main concern has been devoted to surface conduction by two-dimensional (2D) carriers. 2D hole gas (2DHG) in the diamond is induced by surface transfer doping dominated by the adsorbate's status and faces stability issues. Meanwhile, a feasible way to generate the other essential ambipolar carrier-2D electron gas (2DEG) is still lacking. We propose that the well-lattice-matched diamond/cBN(111) interfaces can spontaneously induce 2D ambipolar carriers with a giant density of 4.17 × 1014 cm-2, an order higher than other competitors. 2DEG and 2DHG can be separately achieved near the hetero-interfaces consisting of C-N and C-B bonds, respectively. Interestingly, the robust 2D charges are derived from a novel bulk-induced polarization-discontinuity at the interfaces, which can be attributed to an unexpected non-zero formal polarization of centrosymmetric cBN along the [111] direction. The existence of 2D ambipolar carriers at the diamond/cBN(111) interfaces has resolved the missing n-type conduction in diamond, thus opening up possibilities for complementary logic applications. Additionally, the high density of quantum-confined 2D ambipolar carriers provides an excellent platform for strongly correlated systems, which could lead to novel quantum information processing applications.

Efficient n-Type Surface Doping in Diamond

Conventional doping in diamond is challenging for its application in electronic devices, which could be overcome by an alternative method of surface-transfer doping. In this study, efficient surface n-type conductivity is obtained on the N-terminated diamond surface doped with decamethylcobaltocene (DMC) and cobaltocene (CoCp2) molecules having low ionization energy values by first-principles calculation. By the surface-transfer doping mechanism, electrons are transferred from the dopants to diamond, and then electron accumulation occurs on the diamond surface. For DMC- and CoCp2-doped diamond surfaces, the areal electron density values are 4.822 × 1013 and 4.519 × 1013 cm–2, and the carrier mobility values are 357 and 108 cm2 V–1 s–1 at 298 K, respectively. Moreover, after DMC and CoCp2 molecular adsorption, the optical absorption coefficient increases in the visible region. This study will promote investigations for two-dimensional electron gas on the diamond surface.

Theoretical study of the interface engineering for H-diamond field effect transistors with h-BN gate dielectric and graphite gate

Diamond has compelling advantages in power devices as an ultrawide-bandgap semiconductor. Using first-principles calculations, we systematically investigate the structural and electronic properties of hydrogen-terminated diamond (H-diamond) (111) van der Waals (vdW) heterostructures with graphite and hexagonal boron nitride (h-BN) layers. The graphite/H-diamond heterostructure forms a p-type ohmic contact and the p-type Schottky barrier decreases as the number of graphite layers increases. In contrast, the h-BN/H-diamond heterostructure exhibits semiconducting properties and a tunable type-II band alignment. Moreover, the charge transfer is concentrated at the interface with a large amount of charge accumulating on the C–H bonds on the H-diamond (111) surface, indicating the formation of a highly conductive two-dimensional hole gas (2DHG) layer. In a similar vein, the promising structural and electronic properties of graphite, h-BN, and H-diamond (111) in the graphite/h-BN/H-diamond (111) vdW heterostructure are well preserved upon their contact, while such heterostructure exhibits flexible band offset and Schottky contacts. These studies of interface engineering for H-diamond heterostructures are expected to advance the application of 2D materials in H-diamond field effect transistors, which is an important development in the design of surface transfer doping for 2DHG H-diamond devices.

P-Type Ohmic Contact to Monolayer WSe2 Field-Effect Transistors Using High-Electron Affinity Amorphous MoO3

Monolayer tungsten diselenide (1L-WSe2) has been widely used for studying emergent physics due to the unique properties of its valence bands. However, electrical transport studies have been impeded by the lack of a reliable method to realize Ohmic hole-conducting contacts to 1L-WSe2 especially at low carrier densities and low temperatures. Here, we report low-temperature p-type Ohmic contact to 1L-WSe2 field-effect transistors at carrier densities (n) below n = 1 × 1012 cm–2 with negligible temperature dependence down to the lowest measured temperature (10 K). The non-rectifying barrier is achieved between 1L-WSe2 and molybdenum trioxide (MoO3), where 1L-WSe2 underneath MoO3 is heavily hole-doped through surface transfer doping. Electrical transport measurements reveal linear current–voltage relations at a temperature of 10 K and carrier densities from n = 7.7 × 1011 cm–2 to below the threshold. The finding is also supported by nearly temperature-independent output curves up to room temperature and a negligible contact barrier down to the subthreshold regime. Furthermore, the contact resistivity of MoO3-contacted 1L-WSe2 FET is 30.2–64.8 kΩ μm at n = 1.5 × 1012 cm–2, which is the lowest reported for 1L-WSe2 FETs at such low carrier density. Realizing robust p-type Ohmic contact to a 2D transition metal dichalcogenide semiconductor will enable direct electronic measurements of quantum transport in correlated phases in the valence bands of monolayer semiconductors.

Correlative trends between tribological and electronic properties of dry lubricants: Influence of humidity and dopants

Ambient gases and water vapor are known to impact, either positively or negatively, the coefficient of sliding friction (COF) of many solid lubricants. However, the underlying mechanism remains poorly understood. This study reports on the analysis of prior results on the tribological and electronic properties of several classes of dry lubricants. The results show a direct correlation between these two properties of the sliding surfaces, with the ambient humidity strongly influencing both through the surface transfer doping process that occurs in the nanoscale adsorbed water film. The resulting space charge layer (hole accumulation versus electron depletion) strongly influences the COF, with low values achieved when a high density of charge carriers are present on both the sliding surfaces.

WO3 Passivation of Access Regions in Diamond MOSFETs

We study the impact of access region passivation on the electrical characteristics of hydrogen-terminated diamond MOSFETs with tungsten Carbide (WC) edge contacts. Our experiments reveal a significant improvement to both contact and extrinsic channel sheet resistance once the access regions are passivated with WO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> , indicating that WO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> is an effective surface transfer-doping agent. We analyze a peculiar bump that appears in the subthreshold characteristics of the devices that prevents their effective turn-off. The bump is found to be mitigated when the access regions are passivated by WO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> . Poisson–Schrödinger (P–S) simulations suggest that this bump arises from the field-effect action by the gate over the access region immediately adjoining the gate. This parasitic field effect arises when the surface is unpinned and with a light hole concentration. Owing to its increased surface transfer doping, the use of WO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> as surface passivation is effective in delivering a sharp device turn-off.

Temperature Dependence of the Fano Resonance in Nanodiamonds Synthesized at High Static Pressures

Temperature dependence of Fano resonance, recently discovered in infra-red (IR spectra of nanodiamonds synthesized from chloroadamantane at high static pressures, is investigated. For the first time, marked variations of the resonance parameteres are observed. On heating, the shape of the Fano resonance changes considerably; the effect completely disappears above 350 C, but is recovered after cooling to ambient conditions. Such behaviour implies that assignment of the Fano effect to the surface transfer doping mechanism is not very plausible for the studied samples. The resonance shape varies due to strong temperature dependence of the difference of frequencies of IR-active "bright" and Raman-active "dark" modes of nanodiamond. The frequency of the Raman "dark" mode is only weakly temperature-dependent.

Surface transfer doping of MoO3 on hydrogen terminated diamond with an Al2O3 interfacial layer

A thin layer of Al2O3 was employed as an interfacial layer between surface conductive hydrogen-terminated (H-terminated) diamond and MoO3 to increase the distance between the hole accumulation layer in diamond and negatively charged states in the acceptor layer and, thus, reduce the Coulomb scattering and increase the hole mobility. The valence band offsets are found to be 2.7 and 3.1 eV for Al2O3/H-terminated diamond and MoO3/H-terminated diamond, respectively. Compared to the MoO3/H-terminated diamond structure, a higher hole mobility was achieved with Al2O3 inserted as an interface layer. This work provides a strategy to achieve increased hole mobility of surface conductive diamond by using optimal interlayer along with high high electron affinity surface acceptor materials.

High-mobility p-channel wide-bandgap transistors based on hydrogen-terminated diamond/hexagonal boron nitride heterostructures

Field-effect transistors made of wide-bandgap semiconductors can operate at high voltages, temperatures and frequencies with low energy losses, and are important for power and high-frequency electronics. However, wide-bandgap p-channel transistors perform poorly compared with n-channel ones, making complimentary circuits difficult to achieve. Hydrogen-terminated diamond is a potential p-type material for such devices, but surface transfer doping—thought to be required to generate conductivity—limits performance because it requires ionized surface acceptors that can lead to hole scattering. Here we show that p-channel wide-bandgap heterojunction field-effect transistors can be created, without surface transfer doping, using a hydrogen-terminated diamond channel and hexagonal boron nitride gate insulator. Despite having a reduced density of surface acceptors, the transistors have a low sheet resistance (1.4 kΩ) and large ON current (1,600 μm mA mm−1) compared with other p-channel wide-bandgap transistors, due to a high room-temperature Hall mobility (680 cm2 V−1 s−1). The transistors also exhibit normally OFF behaviour with an ON/OFF ratio of 108. Wide-bandgap transistors with room-temperature hole mobility of 680 cm2 V−1 s−1 can be created without surface doping using hydrogen-terminated diamond/hexagonal boron nitride heterostructures.