Surface Transfer Doping Of Diamond Research Articles

Surface transfer doping of diamond using solution-processed molybdenum trioxide

The surface of hydrogen-terminated diamond (H-terminated diamond) supports a p-type surface conductivity when interfaced with high electron-affinity surface acceptors through the surface transfer doping process. High electron-affinity transition metal oxides (TMOs), such as MoO3, have been regarded as superior candidates in surface transfer doping of diamond, holding great promise for enabling practical diamond electronic devices. In this work, a simple, solution-processed method is demonstrated to deposit a molybdenum trioxide (MoO3) layer on H-terminated diamond surface to achieve surface transfer doping of diamond. The surface of diamond following the deposition of solution-processed MoO3 (sp-MoO3) experienced significant reduction in sheet resistivity, corresponding to an increase in hole density, compared with the pristine air-doped diamond. This hole accumulation layer induced by sp-MoO3 is thermally stable up to 450 °C, and demonstrates metallic conductivity down to 250 mK with a strong spin-orbit interaction (SOI) in the induced two-dimensional (2D) surface conducting layer. Our study demonstrates that sp-MoO3 is comparable with thermally-evaporated MoO3 in doping performance but with great advantage as a simple, inexpensive and highly flexible fabrication method for developing diamond electronics.

A hybrid self-aligned MIS-MESFET architecture for improved diamond-based transistors

Diamond is a promising electronic semiconductor candidate that has recently attracted intense interest in the implementation of its superior physical properties in electronic devices. In particular, attention has been focused on the surface transfer doping of diamond, in which the hydrogen-terminated diamond (diamond:H) benefices of a unique conductive two-dimensional hole gas (2DHG) layer at the diamond's sub-surface upon coverage with a suitable surface acceptor. Several diamond:H transistors have been developed. However, their inherent architecture dependence on the diamond:H conductive surface sensitivity to harsh processing environments has been a major barrier to the realization of high-performance devices. Here, we report on a diamond:H transistor structure that incorporates a mutual diamond:H active channel into the hybrid model of a metal-semiconductor field-effect transistor (MESFET) and a self-aligned metal-insulator-semiconductor FET (MISFET) with a common gate connection. The resulting diamond:H hybrid transistor exhibits a beneficial symbiosis that includes the advantages of both the MISFET (a high ON current of 0.8 μA/μm and a low OFF current of ∼10−9 μA/μm) and MESFET (almost an ideal subthreshold swing of 67 mV/dec) performance operations in the same multilayered device.

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.

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.

The role of hydrogen plasma power on surface roughness and carrier transport in transfer-doped H-diamond

Surface transfer doping of diamond fundamentally requires termination of the diamond surface with a species such as hydrogen to allow the interfacial charge exchange required to establish surface conductivity. Here we show the effects of varied hydrogen plasma power on the roughness and conductivity of the (100) diamond surface. Prior to hydrogen termination, substrates were etched using tailored Cl2 + Ar and O2 + Ar chemistries to produce a very smooth surface of ~0.2 nm roughness average while also removing ~3.4 μm from the top surface as measured by Atomic Force Microscopy (AFM). Use of etching post polishing provides an effective means of producing smoother diamond surfaces with reduced crystal damage as opposed to scaife polishing alone. By producing nominally identical etched surfaces, a relationship between surface conductivity and hydrogen termination plasma power was observed. Using MoO3 as a surface acceptor material, Hall measurements were performed to examine sheet resistance, carrier density and mobility within the diamond. Increased surface conductivity due to enhanced hole mobility was observed at higher hydrogen plasma power conditions, despite an associated increase in roughness of the diamond surface.

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.

Superior Surface Transfer Doping of Diamond with MoO3

A conductive diamond surface with highest yet reported sheet conductivity and unsurpassed thermal stability is shown to be due to surface transfer doping of hydrogen terminated diamond by a molybdenum trioxide (MoO3). Surface conductivities, as determined by Hall Effect measurements as function of temperature for different MoO3 thicknesses, have yielded total areal hole densities ranging from 7 × 1013 cm−2 to 1 × 1014 cm−2, exceeding those reported for H2O transfer doped diamond.

Surface transfer doping of diamond by MoO3: A combined spectroscopic and Hall measurement study

Surface transfer doping of diamond has been demonstrated using MoO3 as a surface electron acceptor material. Synchrotron-based high resolution photoemission spectroscopy reveals that electrons are transferred from the diamond surface to MoO3, leading to the formation of a sub-surface quasi 2-dimensional hole gas within the diamond. Ex-situ electrical characterization demonstrated an increase in hole carrier concentration from 1.00 × 1013/cm2 for the air-exposed hydrogen-terminated diamond surface to 2.16 × 1013/cm2 following MoO3 deposition. This demonstrates the potential to improve the stability and performance of hydrogen-terminated diamond electronic devices through the incorporation of high electron affinity transition metal oxides.

Surface transfer doping of diamond with a molecular heterojunction

Surface conductivity and C1s core level measurements were employed to show that surface transfer doping of hydrogen-terminated diamond C(100) can be achieved with a molecular heterojunction formed with C60F48 and an intralayer of zinc-tetraphenylporphyrin. Measurement of the shift in the diamond Fermi energy shows that the zinc-tetraphenylporphyrin (ZnTPP) layer modifies the C60F48–diamond interaction, modulating the extent of charge transfer between the diamond and the fluorofullerene. In contrast to the case of C60F48 acceptors, the presence of a ZnTPP layer prevents the formation of air-induced surface conductivity, showing that the intralayer acts to selectively separate these two doping channels.

Surface Transfer Doping of Diamond (100) by Tetrafluoro-tetracyanoquinodimethane

Surface transfer doping is demonstrated on hydrogenated diamond (100) by the electron acceptor, tetrafluoro-tetracyanoquinodimethane (F4-TCNQ). Synchrotron-based photoemission spectroscopy reveals that electrons are transferred from diamond to adsorbed F4-TCNQ, leaving a diamond surface p-type doped with a high areal density of holes of about 1.6 × 1013 cm-2.

Surface transfer doping of diamond by fullerene

Fifteen years after Landstrass and Ravi [1] [I.M. Landstrass, K.V. Ravi, App. Phys. Lett, 55 (1989) 975] reported a hydrogen induced surface conductivity of diamond, it has now been established that the phenomenon is based on a sub-surface hole accumulation layer. A quantitative model for the origin of this layer has been proposed by our group earlier and is based on the charge exchange between the diamond surface and solvated ions in a water layer that forms spontaneously on all surfaces in air. The process relies on the particularly low ionization energy of the hydrogenated diamond surface [2] [F. Maier, M. Riedel, B. Mantel, J. Ristein, and L. Ley, Phys. Rev. Lett. 85 (2000) 3472]. Here, we report on fullerenes (C 60) and fluorinated fullerenes (C 60F 48) as alternative adsorbate systems. They induce surface conductivity on a level comparable (C 60) or even higher (C 60F 48) than that observed under atmospheric conditions. Whereas in the case of C 60 efficient doping requires the formation of the van-der Waals solid on top of the surface of diamond, C 60F 48 acts as surface acceptor even as isolated molecule. In both cases, the doping efficiency can be modeled by considering the electrochemical equilibrium between the diamond and the adsorbate layer.

Surface Transfer Doping of Diamond.

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Surface transfer doping of diamond.

The electronic properties of many materials can be controlled by introducing appropriate impurities into the bulk crystal lattice in a process known as doping. In this way, diamond (a well-known insulator) can be transformed into a semiconductor, and recent progress in thin-film diamond synthesis has sparked interest in the potential applications of semiconducting diamond. However, the high dopant activation energies (in excess of 0.36 eV) and the limitation of donor incorporation to (111) growth facets only have hampered the development of diamond-based devices. Here we report a doping mechanism for diamond, using a method that does not require the introduction of foreign atoms into the diamond lattice. Instead, C60 molecules are evaporated onto the hydrogen-terminated diamond surface, where they induce a subsurface hole accumulation and a significant rise in two-dimensional conductivity. Our observations bear a resemblance to the so-called surface conductivity of diamond seen when hydrogenated diamond surfaces are exposed to air, and support an electrochemical model in which the reduction of hydrated protons in an aqueous surface layer gives rise to a hole accumulation layer. We expect that transfer doping by C60 will open a broad vista of possible semiconductor applications for diamond.