P-type Surface Conductivity Research Articles

Enhanced sensitivity of partial O-terminated H-diamond for H2S detection at room temperature

The p-type surface conductivity of H-terminated diamond (HD) has opened new path ways to develop diamond based electronic devices, photo-catalysts, chemical and bio-sensors. Herein, we report on the room temperature H2S detection behaviour of pristine HD and the surface modified HD films with partial O-termination (OHD) through ozonation. The response of the pristine HD and partial OHD devices that are ozonated for 30, 60 and 90 s, is found to be ∼55, 1420, 810 and 95 % respectively, for exposing 900 ppb of H2S under ambient atmosphere at room temperature. Here, the optimally partial OHD sensor displays an enhanced sensitivity by about an order of magnitude due to the catalytic activity of the sparsely populated O-functional groups on HD surface. Moreover, the gas sensor response is found to be higher in wet background atmospheres such as N2 and synthetic air as compared to their respective dry atmospheres. Also, the response curve of these sensors exhibits a peculiar decrease in resistance immediately after exposure to H2S under wet background atmosphere while such oxidative behaviour is absent under dry atmospheres. Based on these observations, the plausible sensing mechanism of these H-diamond based sensors is proposed with the concept of humidity induced H2S hydrolyzation.

Hydrogen-Terminated Diamond MOSFETs Using Ultrathin Glassy Ga2O3 Dielectric Formed by Low-Temperature Liquid Metal Printing Method

The p-type surface conductivity of hydrogen-terminated diamond (H-diamond) provides a viable approach toward diamond-based wide-bandgap metal-oxide-semiconductor field-effect transistors (MOSFETs) for high-power and high-frequency electronics. A facile, low-cost, and low-temperature method to form gate dielectrics on diamond that also preserves the integrity of hydrogen-termination is highly desirable for high-performance diamond surface electronics with process flexibility and high yield. In this work, we demonstrate a p-channel diamond MOSFET with an ultrathin glassy Ga2O3 dielectric layer derived from liquid metal. A liquid metal printing method was employed to transfer an amorphous Ga2O3 layer over the desired active p-channel region of H-diamond at low temperature, allowing the protection and preservation the hydrogen-terminated surface while also forming an efficient gate dielectric. The results of this work suggest that the liquid metal method can provide an efficient, low-cost, and high-yield pathway to form high-quality dielectrics for diamond-based transistors.

A reactive molecular dynamics study of the hydrogenation of diamond surfaces

Hydrogenated diamond has been regarded as a promising material in electronic device applications, especially in field-effect transistors (FETs). However, the quality of diamond hydrogenation has not yet been established, nor has the specific orientation that would provide the optimum hydrogen coverage. In addition, most theoretical work in the literature use models with 100% hydrogenated diamond surfaces to study electronic properties, which could be unreachable experimentally. In this work, we have carried out a detailed study using fully atomistic reactive molecular dynamics (MD) simulations on low indices diamond surfaces i.e. (001), (013), (110), (113), and (111) to evaluate the quality and hydrogenation thresholds on different diamond surfaces and their possible effects on electronic properties. Our simulation results indicate that the 100% surface hydrogenation on these surfaces is hard to achieve because of the steric repulsion between the terminated hydrogen atoms. Among all the considered surfaces, the (001), (110), and (113) surfaces incorporate a larger number of hydrogen atoms and passivate the surface dangling bonds. Our results on hydrogen stability also suggest that these surfaces with optimum hydrogen coverage are robust under extreme conditions and could provide homogeneous p-type surface conductivity on the diamond surfaces, a key requirement for high-field, high-frequency device applications.

Dual Role of Adsorbent and Non-monotonic Transfer p-Doping of Diamond.

Exposure to oxygen is usually detrimental for materials and devices as it leads to undesirable surface oxidation or even deeper corrosion. However, experiments with hydrogen-terminated H-diamond show that oxygen adsorption plays an instrumental role in inducing the p-type surface conductivity. Using first-principles calculations, we explore how the surface-physisorbed molecular O2 serves as an electron acceptor in the transfer doping of diamond. On the other hand, calculations reveal that in a chemisorbed state, oxygen groups substitute H, which lowers the bands in diamond and inhibits the transfer doping. This explains the non-monotonic carrier density dependence on the exposure to oxygen (or, similarly, other adsorbent-acceptor). We further find that ozone can be more efficient for p-type doping of H-diamond due to O3 having lower LUMO energy levels than in molecular O2.

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.

Vacancy-Ordered Double Perovskite Cs2TeI6 Thin Films for Optoelectronics.

Alternatives to lead- and tin-based perovskites for photovoltaics and optoelectronics are sought that do not suffer from the disadvantages of toxicity and low device efficiency of present-day materials. Here we report a study of the double perovskite Cs2TeI6, which we have synthesized in the thin film form for the first time. Exhaustive trials concluded that spin coating CsI and TeI4 using an antisolvent method produced uniform films, confirmed as Cs2TeI6 by XRD with Rietveld analysis. They were stable up to 250 °C and had an optical band gap of ∼1.5 eV, absorption coefficients of ∼6 × 104 cm–1, carrier lifetimes of ∼2.6 ns (unpassivated 200 nm film), a work function of 4.95 eV, and a p-type surface conductivity. Vibrational modes probed by Raman and FTIR spectroscopy showed resonances qualitatively consistent with DFT Phonopy-calculated spectra, offering another route for phase confirmation. It was concluded that the material is a candidate for further study as a potential optoelectronic or photovoltaic material.

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.

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 comparative studies of HF CVD diamond films by Raman and XPS spectroscopies

One of the most attractive features of diamond is its robust p-type surface conductivity that develops under atmospheric conditions on hydrogen-terminated samples. The as-grown CVD diamond films are H-terminated and H concentration depends on the ratios of sp2/sp3 carbon bonding. It is well known that the sp2/sp3 ratios in CVD diamond films is the key factor determining their physical properties and especially their electrical properties therefore the proper determination of the phase composition of CVD diamond films appear as a very important problem.The article presents results for seven diamond synthesized by HF CVD method layers analyzed by Scanning Electron Microscopy (SEM), laser Raman spectroscopy (LRS) and X-ray photoelectron spectroscopy (XPS). Values for the ratio of sp2/sp3 bonded carbon in the various films were obtained using XPS and LRS methods.

(Invited) Surface Transfer Doping: A Novel Alternative to Classical Doping in Semiconductor Electronics

Electronic properties of semiconductors can be controlled by introduction of appropriate dopants into the bulk lattice. In a recently discovered phenomenon, known as “surface transfer doping,” free electrons or holes can be generated in undoped semiconductor surfaces, without the introduction of foreign atoms, by controlled deposition of thin layer of chemical moieties having appropriate electronic structure. In this type of doping, the nature of electrical conductivity and the direction of electron transfer depend crucially on the relative positions of the Fermi level of semiconductor and the chemical potential of electrons in the surface chemical moieties. This doping phenomenon was first observed in undoped hydrogenated diamond, which showed a p-type surface conductivity when exposed to ambient air.1 In this case, surface conductivity develops spontaneously as a result of electron transfer from diamond to an adsorbed water film on the surface.2 , 3 , 4 Since then, this effect has been exploited for other systems, such as diamond/C60,5 graphene/C60. In addition, the process has been shown to affect electrical, optical and electrochemical properties of several technologically important materials such as single-walled carbon nanotubes,6 graphene,7 gallium nitride and zinc oxide.8 This presentation will give an overview of the recent progress in this field and its impact on the emerging technologies. Gi, R. S., et al., Jpn. J. Appl. Phys. (1995) 34, 5550Maier, F., et al., Phys. Rev. Lett. (2000) 85 (16), 3472Chakrapani, V., et al., Science (2007) 318 (5855), 1424Foord, J. S., et al., Diam. Relat. Mater. (2002) 11 (3-6), 856Strobel, P., et al., Nature (2004) 430 (6998), 439Chakrapani, V., et al., ECS Solid State Lett. (2013) 2 (11), M57Chen, W., et al., J. Am. Chem. Soc. (2007) 129 (34), 10418Chakrapani, V., et al., J. Am. Chem. Soc. (2008) 130 (39), 12944

Kelvin probe force microscopy studies of the charge effects upon adsorption of carbon nanotubes and C60 fullerenes on hydrogen-terminated diamond

Hydrogen-terminated diamond is known for its unusually high surface conductivity that is ascribed to its negative electron affinity. In the presence of acceptor molecules, electrons are expected to transfer from the surface to the acceptor, resulting in p-type surface conductivity. Here, we present Kelvin probe force microscopy (KPFM) measurements on carbon nanotubes and C60 adsorbed onto a hydrogen-terminated diamond(001) surface. A clear reduction in the Kelvin signal is observed at the position of the carbon nanotubes and C60 molecules as compared with the bare, air-exposed surface. This result can be explained by the high positive electron affinity of carbon nanotubes and C60, resulting in electron transfer from the surface to the adsorbates. When an oxygen-terminated diamond(001) is used instead, no reduction in the Kelvin signal is obtained. While the presence of a charged adsorbate or a difference in work function could induce a change in the KPFM signal, a charge transfer effect of the hydrogen-terminated diamond surface, by the adsorption of the carbon nanotubes and the C60 fullerenes, is consistent with previous theoretical studies.

Hydrogenated ZnO thin film with p-type surface conductivity from plasma treatment

Fabrication of a ZnO p-n homojunction within a single structure by a simple process is a challenging task. In this work, an intrinsic p-type surface conductive layer of ZnO with a controlled concentration of holes over n-type conductive bulk was obtained by a one-step room-temperature process via hydrogen plasma treatment. Non-contact surface sensitive techniques, such as Kelvin probe force microscopy and conductive force atomic microscopy, confirmed the existence of surface p-type conductivity through analyzing the distribution and concentration of charge carriers on the topmost surface of hydrogenated ZnO. A theoretical framework was constructed to provide a rationale of the p-type surface conductivity and justify its relation to the treatment time. It is believed that this finding will open a new possibility for the fabrication of ZnO based p-n junction devices.

Electrochemical Transfer Doping: A Novel Phenomenon Seen in Diamond, Gallium Nitride, and Carbon Nanotubes

Intrinsic diamond with its band gap of 5.5 eV is an extremely good insulator. However, for undoped, hydrogen-terminated diamond exposed to air, a p-type conductive surface layer appears that is mediated by adsorbates from the ambient atmosphere.1 Since its discovery, several explanations of this effect have been proposed, none has received wide acceptance and the mechanism remained controversial. Recent studies have shown indisputably that the conductivity arises as result of electron exchange between atmospheric oxygen and diamond through an electrochemical redox reaction occurring in an adsorbed water layer: 4H+ + 4e- + O2 = 2H2O This results in the formation of a hole accumulation layer on the surface.2,3 This novel phenomenon, referred to as “surface transfer doping” depends only on the band line up of the semiconductor with respect to the ambient. For diamond, the Fermi level is higher than the electron chemical potential of oxygen redox couple, which results in electron transfer into water layer and generation of p-type surface conductivity. See Figure 1. The electron chemical potential (Fermi energy) of the redox couple, which can be estimated from the Nernst equation. Since water layers form spontaneously on surfaces exposed to humid air, electrochemical transfer doping is a very general phenomenon that has been unrecognized so far. Since its discovery in diamond, the effect has been observed in other technologically important materials such as in single-walled semiconducting carbon nanotubes, graphene, gallium nitride and zinc oxide.4 The process not only affects the electrical properties but also optical properties of the semiconductor. For example, in both GaN and ZnO, electron exchange between oxygen redox couple and mid-gap states results in dramatic changes in the ubiquitous yellow and green luminescence respectively. This phenomenon opens the door to novel and robust class of devices and processes. 1. M. I. Landstrass, and K. V. Ravi, Appl. Phys. Lett., 55 (1989) 975. 2. F. Maier, M. Riedel, B. Mantel, J. Ristein, L. Ley, Phys. Rev. Lett. 85, 3472 (2000) 3. V. Chakrapani, J. C. Angus, A. B. Anderson, S. D. Wolter, B. R. Stoner and G. U. Sumanasekera, Science 318, 1424 (2007) 4. V. Chakrapani, C. Pendyala, K. Kash, A. B. Anderson, M. K. Sunkara, J. C. Angus, J. Am. Chem. Soc. 130, 12944 (2008) Figure 1

DEPOSITION CARBON NANOSTRUCTURES BY SURFATRON GENERATED DISCHARGE

Carbon nanostructures were deposited by surface wave discharge using various Ar/CH<sub>4</sub>/ CO<sub>2</sub> gas mixture ratios. The morphology was controlled by adjusting of gas concentration and was investigated by scanning electron microscopy (SEM). Also, the influence of the low temperature plasma treatment and process time on the wettability of the diamond films has been studied. The results indicate that for hydrogen termination of diamond surface indicate that the temperature as low as 400°C and treatment time of 15 min is sufficient to attain the p-type surface conductivity of diamond.

Diamond surface conductivity: Properties, devices, and sensors

Abstract

Sensing of phosgene by a porous-like nanocrystalline diamond layer with buried metallic electrodes

Nanocrystalline diamond with a porous-like morphology was used as the functional part of a semiconductor gas sensor. The device function is based on the two-dimensional p-type surface conductivity of intrinsic diamond with a H-terminated surface. Metallic electrodes are buried beneath the diamond film. Therefore, these electrodes are protected from harmful substances, and the electronic connection is facilitated by grain boundaries. The gas sensing properties of the sensor structure were examined using oxidising gases (i.e., phosgene, humid air) at various operating temperatures. A pronounced and selective increase by two orders of magnitude was found in the surface conductivity after sensor exposure to phosgene gas (20ppm) at 140°C. Density functional theory calculations indicated no direct charge transfer between the phosgene molecule and diamond. We present a model in which phosgene indirectly yet efficiently increases the H3O+ concentration, which consequently leads to multiplied electron transfer and a pronounced sensor response.

Nanodiamond-gated diamond field-effect transistor for chemical sensing using hydrogen-induced transfer doping for channel formation

A method for attaching nanodiamonds (NDs) to H-terminated diamond devices displaying surface conductivity, configured as an ion-sensitive field-effect transistor and resistor sensor, is demonstrated. From Hall effect measurements, there was minimal sign of degradation of the p-type surface conductivity after ND coating (∼1013 carriers/cm2, ∼27 cm2/V s). In response to pH changes, the device showed an improved response to the as-hydrogenated sensor, from 19 mV/pH to a maximum of 37 mV/pH. Configured in resistor mode, exposure to 2,4-dinitrotoluene vapor gave rise to sensitive detection, while the uncoated H-terminated device exhibited reaction instability. The mechanisms behind these observations are discussed.

Tunable Electrical Properties of Silicon Nanowires via Surface-Ambient Chemistry

p-Type surface conductivity is a uniquely important property of hydrogen-terminated diamond surfaces. In this work, we report similar surface-dominated electrical properties in silicon nanowires (SiNWs). Significantly, we demonstrate tunable and reversible transition of p(+)-p-i-n-n(+) conductance in nominally intrinsic SiNWs via changing surface conditions, in sharp contrast to the only p-type conduction observed on diamond surfaces. On the basis of Si band energies and the electrochemical potentials of the ambient (pH value)-determined adsorbed aqueous layer, we propose an electron-transfer-dominated surface doping model, which can satisfactorily explain both diamond and silicon surface conductivity. The totality of our observations suggests that nanomaterials can be described as a core-shell structure due to their large surface-to-volume ratio. Consequently, controlling the surface or shell in the core-shell model represents a universal way to tune the properties of nanostructures, such as via surface-transfer doping, and is crucial for the development of nanostructure-based devices.