Hydrogenated Diamond Surface Research Articles

Growth of synthetic diamond films and their electrophysical properties

The possibilities of formation of single-crystal homoepitaxial diamond structures grown by the method of vapor-phase chemical deposition are investigated. Optimal technological conditions for the formation of structures are suggested. The layer with p-type conductivity formation process depending on the temperature and time is determined. It was shown that applied thermal treatment method in hydrogen can be an alternative to the conventional method of H layer formation in microwave hydrogen plasma due to simpler and more reproducible. The profile distribution of boron atoms in a diamond crystal was determined under different implantation modes. The temperature dependence of specific conductivity in the temperature range 80–700 K was studied, and the activation energy was calculated. The results of Hall measurements of the electrophysical parameters of implanted samples are presented, and the effect of nitrogen concentration on the electrophysical parameters is revealed. The electrophysical parameters of the structures obtained under various modes of ion implantation of boron in a crystal and subsequent annealing are presented. The possibility of creating a field effect transistor on a hydrogenated diamond surface is shown. The current-voltage characteristic of the manufactured sample was studied and it was shown that it demonstrates low leakage through the gate.

High performance ionic-liquid-gated air doped diamond field-effect transistors

We report successful fabrication of high performance ion-gated field-effect transistors (FETs) on hydrogenated diamond surface. Investigations on the hydrogen (H)-terminated diamond by Hall effect measurements shows Hall mobility as high as ∼200 cm2 V−1 s−1. In addition we demonstrate a rapid fabrication scheme for achieving stable high performance devices useful for determining optimal growth and fabrication conditions. We achieved H-termination using hydrogen plasma treatment with a sheet resistivity as low as ∼1.3 kΩ/sq. Conductivity through the FET channel is studied as a function of bias voltage on the liquid ion-gated electrode from −3.0 to 1.5 V. Stability of the H-terminated diamond surface was studied by varying the substrate temperature up to 350 °C. It was demonstrated that the sheet resistance and carrier densities remain stable over 3 weeks in ambient air atmosphere even at substrate temperatures up to 350 °C, whereas increasing temperature beyond this limit has effected hydrogenation. This study opens new avenues for carrying out fundamental research on diamond FET devices with ease of fabrication and high throughput.

Water desorption effects on the surface electrical resistance of air-exposed hydrogenated diamond

The air-exposed hydrogenated diamond surface has been studied by carrier transport measurements and density functional theory. Our results have allowed us to understand the close relationship between the physisorbed water molecules and the electrical resistance. We have therefore been able to show that the evolution of the resistance over time and temperature can be related to the extent of the hole accumulation layer originating at the surface of the hydrogenated diamond. The method has allowed us to estimate the desorption energy of a single water molecule by means of resistance measurements alone.

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.

Interactions between hydrogenated diamond surface and adsorbates with different concentration of NO2 molecules: A first‐principles study

To elucidate the effects of NO2 and H2O molecules on the surface conductivity of hydrogenated diamond film, models of various adsorbates containing different molecular ratio of NO2 and H2O on hydrogenated diamond (100) surfaces were constructed. The adsorption energies, equilibrium geometries of adsorbates, density of states, and atomic Mulliken populations were studied by using first‐principles method. The results showed that H2O molecule in the adsorbate could weaken the interactions between the adsorbates and hydrogenated diamond surface significantly. Compared with H2O molecule, NO2 molecule relaxes more dramatically when adsorbed on hydrogenated diamond surface. In addition, density of states for C(100):H–2NO2, C(100):H–NO2, and C(100):H–NO2 + H2O systems are very similar to each other, which indicates an obvious peak at valence band maximum level for all the three samples. It can be attributed to mainly single occupied molecule orbital of NO2 molecule and slightly C–H bond of C(100):H substrate. When the adsorbates contain one NO2 and two H2O molecules, the peak shifts slightly into valence band, but its intensity increases significantly. All the samples exhibit p‐type surface conductivity when adsorbed with pure NO2 molecules, and the surface conductivity remains as H2O molecules added into the NO2 adsorbate layer. However, for oxygenated diamond surface, very week interactions generate between diamond surface and various adsorbates. All the oxygenated diamond (100) surfaces with various adsorbates containing different NO2 and H2O molecules on it exhibit an insulating property.

Band offset and electronic properties at semipolar plane AlN()/diamond heterointerface**Project supported by the Scholarship Council of China (Grant No. 201508340047), the Postdoctoral Science Foundation of China (Grant No. 2016M601993), the Postdoctoral Science Foundation of Anhui Province, China (Grant No. 2017B215), and the Anhui Province University Outstanding Talent Cultivation Program, China (Grant No. gxfxZD2016077).

Tailoring the electronic states of the AlN/diamond interface is critical to the development of the next-generation semiconductor devices such as the deep-ultraviolet light-emitting diode, photodetector, and high-power high-frequency field-effect transistor. In this work, we investigate the electronic properties of the semipolar plane AlN()/diamond heterointerfaces by using the first-principles method with regard to different terminated planes of AlN and surface structures of diamond (100) plane. A large number of gap states exist at semi-polar plane AlN()/diamond heterointerface, which results from the N 2p and C 2s2p orbital states. Besides, the charge transfer at the interface strongly depends on the surface termination of diamond, on which hydrogen suppresses the charge exchange at the interface. The band alignments of semi-polar plane AlN()/diamond show a typical electronic character of the type-II staggered band configuration. The hydrogen-termination of diamond markedly increases the band offset with a maximum valence band offset of 2.0 eV and a conduction band offset of 1.3 eV for the semi-polar plane N–AlN()/hydrogenated diamond surface. The unique band alignment of this Type-II staggered system with the higher CBO and VBO of the semi-polar AlN/HC(100) heterostructure provides an avenue to the development of robust high-power high-frequency power devices.

Thermally Stable, High Performance Transfer Doping of Diamond using Transition Metal Oxides

We report on optimisation of the environmental stability and high temperature operation of surface transfer doping in hydrogen-terminated diamond using MoO3 and V2O5 surface acceptor layers. In-situ annealing of the hydrogenated diamond surface at 400 °C was found to be crucial to enhance long-term doping stability. High temperature sheet resistance measurements up to 300 °C were performed to examine doping thermal stability. Exposure of MoO3 and V2O5 transfer-doped hydrogen-terminated diamond samples up to a temperature of 300 °C in ambient air showed significant and irreversible loss in surface conductivity. Thermal stability was found to improve dramatically however when similar thermal treatment was performed in vacuum or in ambient air when the oxide layers were encapsulated with a protective layer of hydrogen silsesquioxane (HSQ). Inspection of the films by X-ray diffraction revealed greater crystallisation of the MoO3 layers following thermal treatment in ambient air compared to the V2O5 films which appeared to remain amorphous. These results suggest that proper encapsulation and passivation of these oxide materials as surface acceptor layers on hydrogen-terminated diamond is essential to maximise their environmental and thermal stability.

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.

Vacancies in the C(1 0 0)-(2 × 1) diamond surface layers

Our study provides the results of quantum-chemical calculations of the vacancy defect states on a clean and hydrogenated diamond surface C(100)-(2×1) with their positions varying in the first to sixth surface layers. It was found that the most stable defect is the monovacancy in the third layer under the top bilayer dimer row of the clean surface. The formation energy for this vacancy is 0.55eV. In the hydrogenated surface, the vacancy in the third layer becomes metastable, the ground state is the vacancy in the topmost surface layer. It is shown that a vacancy in the third layer does not lead to further spontaneous graphitization of the surface.

Surface properties of hydrogenated diamond in the presence of adsorbates: A hybrid functional DFT study

We use an ab initio B3LYP approximation to perform a quantitative analysis of the electronic and structural properties of hydrogen terminated (100), (110), and (111) diamond surfaces. We also study the charge transfer produced by the adsorption of a number of molecular species, including NO2, NO, and O3. Our results shed light on the understanding of hydrogenated diamond surface conductivity and demonstrate that combination of surface termination and molecular species is key to controlling hole doping on diamond. This study, which does not rely on any empirical parameters, also provides guidelines for the choice of adsorbate molecules that yield desired hole injection into the hydrogenated diamond surfaces.

Comparative investigation of surface transfer doping of hydrogen terminated diamond by high electron affinity insulators

We report on a comparative study of transfer doping of hydrogenated single crystal diamond surface by insulators featured by high electron affinity, such as Nb2O5, WO3, V2O5, and MoO3. The low electron affinity Al2O3 was also investigated for comparison. Hole transport properties were evaluated in the passivated hydrogenated diamond films by Hall effect measurements, and were compared to un-passivated diamond films (air-induced doping). A drastic improvement was observed in passivated samples in terms of conductivity, stability with time, and resistance to high temperatures. The efficiency of the investigated insulators, as electron accepting materials in hydrogenated diamond surface, is consistent with their electronic structure. These surface acceptor materials generate a higher hole sheet concentration, up to 6.5 × 1013 cm−2, and a lower sheet resistance, down to 2.6 kΩ/sq, in comparison to the atmosphere-induced values of about 1 × 1013 cm−2 and 10 kΩ/sq, respectively. On the other hand, hole mobilities were reduced by using high electron affinity insulator dopants. Hole mobility as a function of hole concentration in a hydrogenated diamond layer was also investigated, showing a well-defined monotonically decreasing trend.

The effect of surface termination on glucose oxidation using Ni‐modified diamond electrodes

AbstractAbstractauthoren This study describes the decoration of hydrogen‐ and oxygen‐terminated boron‐doped diamond electrodes (BDD) with three different loadings of Ni. The Ni was deposited electrochemically for 600, 400, and 100 s on both hydrogen‐terminated BDD (BDDH) and oxygen‐terminated BDD (BDDO). Scanning electron microscopy (SEM) showed that all Ni particles were roughly spherical in shape, but distribution and size varied with electrode termination: a uniform coverage with a particle size dependent on deposition duration was achieved on BDDH, but on BDDO the particles are deposited primarily along surface ridges and had similar sizes for all three deposition times. The performance of the samples was then tested using glucose sensing as an exemplar application. It was found that glucose oxidation varies greatly between electrodes at concentrations similar to those found in human blood. The variability between the samples was attributed to surface differences between electrodes and the difference in particle location. Amperometry using BDDs decorated with the 100 s Ni deposition gave a stable current response with respect to glucose concentration in the range 0.1–13 mM with much higher glucose oxidation currents being observed for the Ni nanoparticles deposited on the hydrogenated diamond surface.

A first principles study on the active adsorbates on the hydrogenated diamond surface

Hydrogenated diamond film exhibits a high surface conductivity, which is very suitable for many in-plane microelectronic and microelectrochemical devices. However, the surface conductivity mechanism of hydrogenated diamond film remains unclear up to now. It inevitably retards its further applications. This work is to elucidate the effects of active adsorbate and water molecule on surface conductivity of hydrogenated diamond film. By the first principles method based on density functional theory, several models corresponding to hydrogenated and oxygenated diamond (100) surfaces physisorbed with various active adsorbates are built up. The adsorbed species include H3O+ ion mixed with H2O molecules with different concentrations. The adsorption energy, equilibrium geometry and density of states corresponding to the adsorption system are investigated. At the same time, the electron populations for different atoms of the physisorbed adsorbates are studied. The results show that the equilibrium geometry of H3O+ ion relaxes significantly after adsorption on hydrogenated diamond (100) surface. In addition, its adsorption energy increases dramatically compared with the system of individual H2O molecule adsorbed on hydrogenated diamond (100) surface. It follows that the strong interactions occur between H3O+ ion and hydrogenated diamond surface. With the concentration of the adsorbed H2O molecules increasing, the adsorption energy between the adsorbate and hydrogenated diamond (100) surface decreases gradually. It indicates that the interactions between H3O+ ion and the substrate weaken as the water molecule concentration increases. Concerning the electronic structure of H3O+ ion adsorbed on hydrogenated diamond (100) surface, shallow acceptors appear near Fermi level, which arises from charge transfer from hydrogenated diamond surface to adsorbed H3O+ ion. Therefore, hydrogenated diamond surface exhibits a p-type conductivity. With regard to the mixed adsorptions of H3O+ ion and H2O molecule, no significant effect on its conductivity is detected, though its surface energy band structure changes. At the same time, the electron transfers from hydrogenated diamond (100) surfaces to the adsorbates are also similar for all the systems with the adsorbates including one H3O+ ion and different H2O molecules. Thus, the adsorbed H2O molecule concentration in this work has no effect on the surface conductivity of hydrogenated diamond surface. However, the adsorbates containing H2O molecules and H3O+ ion physisorbed on oxygenated diamond (100) surfaces do not exist stably. The H3O+ ion will decompose into one H2O molecule and one H atom, which form HO bond with one O atom of oxygenated diamond surface. All the oxygenated diamond surfaces with various adsorbates exhibit an electric insulativity.

Self-assembling diacetylene molecules on atomically flat insulators.

Single crystal sapphire and diamond surfaces are used as planar, atomically flat insulating surfaces, for the deposition of the diacetylene compound 10,12-nonacosadiynoic acid. The surface assembly is compared with results on hexagonal boron nitride (h-BN), highly oriented pyrolytic graphite (HOPG) and MoS2 surfaces. A perfectly flat-lying monolayer of 10,12-nonacosadiynoic acid self-assembles on h-BN like on HOPG and MoS2. On sapphire and oxidized diamond surfaces, we observed assemblies of standing-up molecular layers. Surface assembly is driven by surface electrostatic dipoles. Surface polarity is partially controlled using a hydrogenated diamond surface or totally screened by the deposition of a graphene layer on the sapphire surface. This results in a perfectly flat and organized SAM on graphene, which is ready for on-surface polymerization of long and isolated molecular wires under ambient conditions.

Impedance analysis of Al2O3/H-terminated diamond metal-oxide-semiconductor structures

Impedance spectroscopy (IS) analysis is carried out to investigate the electrical properties of the metal-oxide-semiconductor (MOS) structure fabricated on hydrogen-terminated single crystal diamond. The low-temperature atomic layer deposition Al2O3 is employed as the insulator in the MOS structure. By numerically analysing the impedance of the MOS structure at various biases, the equivalent circuit of the diamond MOS structure is derived, which is composed of two parallel capacitive and resistance pairs, in series connection with both resistance and inductance. The two capacitive components are resulted from the insulator, the hydrogenated-diamond surface, and their interface. The physical parameters such as the insulator capacitance are obtained, circumventing the series resistance and inductance effect. By comparing the IS and capacitance-voltage measurements, the frequency dispersion of the capacitance-voltage characteristic is discussed.

Direct observation of electron emission from the grain boundaries of chemical vapour deposition diamond films by tunneling atomic force microscopy

The emission of electrons from diamond in vacuum occurs readily as a result of the negative electron affinity of the hydrogenated surface due to features with nanoscale dimensions, which can concentrate electric fields high enough to induce electron emission from them. Electrons can be emitted as a result of an applied electric field (field emission) with possible uses in displays or cold-cathode devices. Alternatively, electrons can be emitted simply by heating the diamond in vacuum to temperatures as low as 350 °C (thermionic emission), and this may find applications in solar energy generation or energy harvesting devices. Electron emission studies usually use doped polycrystalline diamond films deposited onto Si or metallic substrates by chemical vapor deposition, and these films have a rough, faceted morphology on the micron or nanometer scale. Electron emission is often improved by patterning the diamond surface into sharp points or needles, the idea being that the field lines concentrate at the points lowering the barrier for electron emission. However, there is little direct evidence that electrons are emitted from these sharp tips. The few reports in the literature that have studied the emission sites suggested that emission came from the grain boundaries and not the protruding regions. We now present direct observation of the emission sites over a large area of polycrystalline diamond using tunneling atomic force microscopy. We confirm that the emission current comes mostly from the grain boundaries, which is consistent with a model for emission in which the non-diamond phase is the source of electrons with a threshold that is determined by the surrounding hydrogenated diamond surface.

FET on hydrogenated diamond surface

It is demonstrated that a FET can be created on a hydrogenated diamond surface. A technology for production and properties of conducting hydrogenated diamond surface are considered. The technology is used to create a FET on the hydrogenated (110) surface of the 2A natural diamond.

Water adsorbate mediated accumulation gas sensing at hydrogenated diamond surfaces

Hydrogenated diamond (HD) specimens feature a peculiar form of gas response which occurs at room temperature and which is mediated by a thin layer of water that tends to accumulate on HD surfaces as these are exposed to normal atmospheric conditions. HD specimens are known to be sensitive to gases and vapours that are capable of forming acid–base reactions in liquid water. In this paper we show that HD sensors feature a novel kind of accumulating gas response which – under favourable conditions – comes close to an integrator-like behaviour. In this integrator-limit of operation, the HD gas response becomes proportional to the total flow of reactive gases that had been interacting with the water adsorbate layer on top of the HD sensors.

Refractory two-dimensional hole gas on hydrogenated diamond surface

Use of two-dimensional hole gas (2DHG), induced on a hydrogenated diamond surface, is a solution to overcoming one of demerits of diamond, i.e., deep energy levels of impurities. This 2DHG is affected by its environment and accordingly needs a passivation film to get a stable device operation especially at high temperature. In response to this requirement, we achieved the high-reliability passivation forming an Al2O3 film on the diamond surface using an atomic-layer-deposition (ALD) method with an H2O oxidant at 450 °C. The 2DHG thus protected survived air annealing at 550 °C for an hour, establishing a stable high-temperature operation of 2DHG devices in air. In part, this achievement is based on high stability of C-H bonds up to 870 °C in vacuum and above 450 °C in an H2O-containing environment as in the ALD. Chemically, this stability is supported by the fact that both the thermal decomposition of C-H bonds and reaction between C-H bonds and H2O are endothermic processes. It makes a stark contrast to the instability of Si-H bonds, which decompose even at room temperature being exposed to atomic hydrogen. In this respect, the diamond 2DHG devices are also promising as power devices expectedly being free from many instability phenomena, such as hot carrier effect and negative-bias temperature instability, associated with Si devices. As to adsorbate, which is the other prerequisite for 2DHG, it desorbed in vacuum below 250 °C, and accordingly some new adsorbates should have adsorbed during the ALD at 450 °C. As a clue to this question, we certainly confirmed that some adsorbates, other than those at room temperature, adsorbed in air above 100 °C and remained at least up to 290 °C. The identification of these adsorbates is open for further investigation.

Controlling the Electrical Behavior of Semiconducting Carbon Nanotubes via Tube Contact

The electromechanical behavior of single-walled carbon nanotubes (SWNTs) in contact with different materials is investigated by scanning probe microscopy. An anomalous diamond/semiconducting nanotube behavior is observed, which is consistent with ab initio calculations: the formation of a broken-gap heterojunction between semiconducting SWNTs and a hydrogenated diamond surface results in a metallic response for such SWNTs.