Electronic and surface properties of H-terminated diamond surface affected by NO2 gas

Hydrogen-terminated diamond surface exhibits p-type conductivity during its exposure to air. To investigate this phenomenon, we examined the influence of different gases on the surface conductivity. Exposure to NO₂ gas resulted in the biggest increase in conductivity, while H₂O vapor decreased the surface conductivity. Moreover, even very low concentrations of NO₂ molecules in air increased the hole sheet concentration, and with increasing NO₂ concentration, the hole sheet concentration increased up to 2.3 × 10¹⁴ cm^? 2 (at 300 ppm NO₂). This increase of hole sheet concentration was observed during exposure to NO₂ gas and simultaneous adsorption of NO₂ molecules on the diamond surface, while it decreased when the exposure stopped and NO₂ molecules desorbed from the surface. X-ray photoelectron spectroscopy investigation showed upward band bending and partial oxidation of the hydrogen-terminated surface after exposure to air and NO₂. FETs exposed to NO₂ gas exhibited lower source and drain resistances, which led to a 1.8-fold increase of maximum drain current, transconductance increased 1.5-fold and maximum frequency of oscillation increased 1.6-fold.     

Investigation of electronic properties of graphene/Si field-effect transistor

We report a high-performance graphene/Si field-effect transistor fabricated via rapid chemical vapor deposition. Oligolayered graphene with a large uniform surface acts as the local gate of the graphene transistors. The scaled transconductance, g_m, of the graphene transistors exceeds 3 mS/μm, and the ratio of the current switch, I_on/I_off, is up to 100. Moreover, the output properties of the graphene transistor show significant current saturation, and the graphene transistor can be modulated using the local graphene gate. These results clearly show that the device is well suited for analog applications.     

Structural and electrical properties of nanocrystalline diamond (NCD) heavily doped by nitrogen

Gas mixtures containing up to 40% nitrogen by volume and 1% CH₄ with the balance being argon have been used for the deposition of nitrogen doped nanocrystalline diamond (NCD) films by means of microwave plasma enhanced chemical vapour deposition (MPECVD). The CVD plasma was monitored by optical emission spectroscopy to reveal the plasma species, e.g., CN molecules, as a function of the nitrogen additive. Structural properties of the deposited NCD films were studied by FESEM and Raman spectroscopy. Effects of nitrogen doping on the electrical resistivity and electron field emission characteristics of the NCD films were measured. In this work, correlation between the structural and electrical properties of NCD films and the nitrogen additive to the CVD plasma will be presented and discussed.     

Temperature and gate voltage dependent electrical properties of graphene field-effect transistors

Temperature (T) and gate voltage sweep range (V_Gmax) dependent electrical properties of graphene field-effect transistors were investigated in the ambient atmosphere. With the increase of T from 300 to 380 K, the charge neutrality point (CNP) in the transfer characteristic curve (I_D–V_G) was shifted negatively from the positive voltage to near zero, which might be mainly due to the desorption of water adsorbates from the graphene surface and other thermally activated processes at the graphene/SiO₂ interface at high T. Accompanying the CNP shift is the semiconducting behavior of the temperature-dependent conductivity, observed not only in the low carrier density region near the CNP, but also in the high carrier density region. The variations in the position and magnitude of the CNP in the I_D–V_G hysteresis loops were linearly enhanced by increasing either T or V_Gmax, which are explained in terms of the trapping/detrapping process and the chemical redox reaction at the graphene–oxide interface, as well as the thermally activated mobile ions movement in the dielectric layer.     

Graphene nanoribbon field-effect transistor at high bias

Combination of high-mean free path and scaling ability makes graphene nanoribbon (GNR) attractive for application of field-effect transistors and subject of intense research. Here, we study its behaviour at high bias near and after electrical breakdown. Theoretical modelling, Monte Carlo simulation, and experimental approaches are used to calculate net generation rate, ionization coefficient, current, and finally breakdown voltage (BV). It is seen that a typical GNR field-effect transistor''s (GNRFET) breakdown voltage is in the range of 0.5 to 3 V for different channel lengths, and compared with silicon similar counterparts, it is less. Furthermore, the likely mechanism of breakdown is studied.     

DC and RF performance of surface channel MESFETs on H-terminated polycrystalline diamond

DC and RF performance of submicron gate-length metal–semiconductor field effect transistors (MESFETs) fabricated on hydrogen-terminated polycrystalline diamond is investigated in detail for different material electronic quality (grain size in the range 100–200 µm) and device geometry (drain-source channel length in the range 1–3 µm). DC characteristics appear almost independent of both properties, giving maximum drain-source current values in the range 120–140 mA/mm in MESFETs having same gate length (0.2 µm) and gate width (25 µm). The layer properties underneath the hydrogenated surface seem then to affect the DC behaviour to a lesser extent when the same hydrogenation procedure is used. At variance, the electronic quality of diamond layers employed for MESFETs realization largely affects the RF performance, resulting into a low oscillation frequency f_max for a MESFET realized by a self-aligned process (1 µm drain-source channel length) onto low quality diamond polycrystalline film. Such a performance improves to f_max = 35 GHz for devices realized onto large grain polycrystalline diamond, although fabricated without self-aligned gate procedure (3 µm drain-source channel length). These findings are discussed in terms of different roles played by surface hydrogenation, device geometry detail and electronic quality of the polycrystalline diamond substrate for MESFET realization.     

AlN as passivation for surface channel FETs on H-terminated diamond

Surface Channel MESFETs (SC-FET) have suffered from instabilities and drift in the past. To overcome these effects a suitable device passivation seems to be one of the key aspects. In this investigation Atomic Layer Deposition (ALD) of AlN has been applied to Surface Channel FETs on hydrogenated diamond. Despite a deposition temperature of 370 °C, where usually the FET channel is permanently degraded, transistor operation with 65% of the initial current level could be obtained.     

Fabrication and electrical properties of SrTiO3/diamond junctions

Strontium titanate (STO) films were directly deposited on Ib (100) single crystal diamond by r.f. magnetron sputtering. The as-deposited STO film was in amorphous state. On the other hand, the crystalline STO film was obtained under the optimized condition of a deposition temperature of 250 °C and a post-annealing temperature of 650 °C. STO/diamond junctions were fabricated on boron-doped homoepitaxial layers grown on p⁺-type single crystal diamond substrates. Electrical properties of the STO/diamond junction were investigated by changing the surface terminations of diamond with hydrogen or oxygen and the crystallinity of the STO film. It was found that the amorphous STO acted like a semi-insulator on H-diamond surface and that the amorphous STO/O-diamond junction behaved like a Schottky diode. The crystalline STO/O-diamond showed a complex rectifying behavior. The crystalline STO film possessed a higher dielectric constant as compared to that of the amorphous one.     

Structural and electrical properties of sintered zinc-titanate ceramics

The aim of this work was an investigation of structural and electrical properties of sintered zinc-titanate ceramics obtained by mechanical activation. Mixtures of ZnO and TiO₂ were mechanically activated in a planetary ball mill up to 90 min and sintered isothermally in air for 120 min at 1100 °C. The phase composition in the ZnO–TiO₂ system after milling and sintering was analyzed using the XRD method. Microstructure analyses were performed using SEM. The results of electric resistivity, capacitance and loss tangent of the sintered samples were obtained. The existence of zinc-titanate as a dielectric was proved (?_r = 12.5, Q = 386.1, tgδ = 0.0026, ρ = 1.02 Ωm).     

Structural and electrical properties of BZT-added BNLT ceramics

Lead free piezoelectric ceramics (1−x)BNLT−xBZT with x=0.00, 0.06, 0.09 and 0.12 were prepared using a two-step mixed oxide method. Dielectric, ferroelectric and piezoelectric properties of the ceramics were improved by the addition of the BZT. XRD results show tetragonal symmetry structure of the BNLT–BZT ceramics. It was found that the tetragonality increases with increasing BZT content. The optimum composition is x=0.09, where the maximum values of the piezoelectric constant d₃₃ (~126 pC/N) and dielectric constant (~2400) were obtained at room temperature. This BNLT–BZT system can be a promising candidate for lead-free piezoelectric ceramics.     

Potential applications of surface channel diamond field-effect transistors

In order to realize high frequency and high power diamond devices, diamond FETs on the hydrogen-terminated diamond surface conductive layer have been fabricated. The fabricated diamond MESFETs show high breakdown voltage and output capability of 1 W mm⁻¹. High transconductance diamond MESFET utilizing a self-aligned gate FET fabrication process has been operated in high frequency for the first time. In the 2 μm gate MESFETs, the obtained cut off frequency f_T and maximum frequency of oscillation f_max are 2.2 and 7 GHz, respectively. It is expected that the diamond MESFET with 0.5 μm gate length fabricated by self-aligned gate process shows 8 GHz of f_T and 30 GHz of f_max.     

Gate interfacial layer in hydrogen-terminated diamond field-effect transistors

Cross-sectional transmission microscopy images of hydrogen-terminated diamond field-effect transistor reveal interfacial layers between the metal and hydrogen-terminated diamond layer. Especially interesting is that an interfacial layer between Al and H-terminated diamond is clearly seen. This layer corresponds to the energy barrier, which we confirmed from an RF analysis of a diamond FET. During growth, an amorphous-like subsurface layer with vacancies has already formed on H-terminated diamond, and during subsequent metal evaporation, metal diffuses through vacancies into the subsurface layer, and eventually the interfacial layer forms.     

Structural and electrical properties of bismuth magnesium tantalate pyrochlores

The subsolidus cubic pyrochlore phases in the Bi₂O₃–MgO–Ta₂O₅ (BMT) system were prepared with the proposed formula, Bi_3+(5/2)xMg_2?xTa_3?(3/2)xO_14?x (0.12 ≤ x ≤ 0.22). Replacement of smaller cations, Mg²⁺ and Ta⁵⁺ by larger Bi³⁺ cations with considerable oxygen non-stoichiometry within structure was proposed. The synthesised samples were confirmed phase pure by X-ray powder diffraction and their refined lattice parameters were in the range of 10.5532(4)–10.5672(9) Å. The grain sizes of the samples determined by SEM analysis were in the range of 0.6–10.60 μm and their average relative densities were more than 80%. Five infrared-active modes were also observed in their FTIR spectra due to their metaloxygen bonds. The BMT pyrochlores were highly electrical resistive with high dielectric constants, ?′ in the range of ～70–85; dielectric losses, tan δ in the order of 10^?3 at frequency 1 MHz and a negative temperature coefficient of permittivities, TC?′ of ～?158 to ?328 ppm/°C.     

Nucleation and growth surface conductivity of H-terminated diamond films prepared by DC arc jet CVD

High-quality polycrystalline diamond film has been extremely attractive to many researchers, since the maximum transition frequency (f_T) and the maximum frequency of oscillation (f_max) of polycrystalline diamond electronic devices are comparable to those of single crystalline diamond devices. Besides large deposition area, DC arc jet CVD diamond films with high deposition rate and high quality are one choice for electronic device industrialization. Four inch free-standing diamond films were obtained by DC arc jet CVD using gas recycling mode with deposition rate of 14 μm/h. After treatment in hydrogen plasma under the same conditions for both the nucleation and growth sides, the conductivity difference between them was analyzed and clarified by characterizing the grain size, surface profile, crystalline quality and impurity content. The roughness of growth surface with the grain size about 400 nm increased from 0.869 nm to 8.406 nm after hydrogen plasma etching. As for the nucleation surface, the grain size was about 100 nm and the roughness increased from 0.31 nm to 3.739 nm. The XPS results showed that H-termination had been formed and energy band bent upwards. The nucleation and growth surfaces displayed the same magnitude of square resistance (R_s). The mobility and the sheet carrier concentration of the nucleation surface were 0.898 cm/V s and 10¹³/cm² order of magnitude, respectively; while for growth surface, they were 20.2 cm/V s and 9.97 × 10¹¹/cm², respectively. The small grain size and much non-diamond carbon at grain boundary resulted in lower carrier mobility on the nucleation surface. The high concentration of impurity nitrogen may explain the low sheet carrier concentration on the growth surface. The maximum drain current density and the maximum transconductance (g_m) for MESFET with gate length L_G of 2 μm on H-terminated diamond growth surface was 22.5 mA/mm and 4 mS/mm, respectively. The device performance can be further improved by using diamond films with larger grains and optimizing device fabrication techniques.     

Organic nanofibers integrated by transfer technique in field-effect transistor devices

The electrical properties of self-assembled organic crystalline nanofibers are studied by integrating these on field-effect transistor platforms using both top and bottom contact configurations. In the staggered geometries, where the nanofibers are sandwiched between the gate and the source-drain electrodes, a better electrical conduction is observed when compared to the coplanar geometry where the nanofibers are placed over the gate and the source-drain electrodes. Qualitatively different output characteristics were observed for top and bottom contact devices reflecting the significantly different contact resistances. Bottom contact devices are dominated by contact effects, while the top contact device characteristics are determined by the nanofiber bulk properties. It is found that the contact resistance is lower for crystalline nanofibers when compared to amorphous thin films. These results shed light on the charge injection and transport properties for such organic nanostructures and thus constitute a significant step forward toward a nanofiber-based light-emitting device.     

Structural and vibrational properties of the 6H diamond: First-principles study

The 6H diamond is the first successfully synthesized diamond polytype, which are not found in nature. We have performed the investigations on structural properties and vibrational properties of the 6H diamond with a first-principles method and we also have identified the vibration normal modes of the 6H diamond with group theory. The structural properties of the 6H diamond are quite similar to the cubic diamond (3C diamond) and the lonsdaleite (2H diamond) as expected. The vibrational properties of the 6H diamond are more complicated than the 3C and 2H diamonds. At Γ point of the first Brillouin zone of the 6H diamond, there are 15 Raman active modes with nine different frequencies, and six IR active modes with four different frequencies. However, there are no IR active modes in the 3C and 2H diamonds. Therefore, the IR active modes of the 6H diamond can provide the indication modes for distinguishing the 6H diamond from the 2H and 3C diamonds. For Raman active modes, the frequencies of the E_1g² and E_2g² modes of the 6H diamond are much smaller than the Raman frequencies of the 2H and 3C diamonds; consequently, it also can contribute to distinguishing the 6H diamond from the 2H and 3C diamonds. The frequencies of two Raman active modes of the 6H diamond–the A_1g² mode and the E_1g¹ mode–agree very well with the experimental results.     

Manual turbostratic stacked graphene transistor: A study on electrical properties and device potential

This study investigated the potential of turbostratic stacking of graphene few-layers produced using the consecutive electrochemical delamination method for electronic applications. The temperature dependence of electrical conductivity was measured to identify the transport mechanisms and band overlap. By lowering the temperature from 298 to 20 K, it was shown that these highly disordered structures follow nearest neighbor hopping through the variable range hopping mechanism. Variations in band overlap for samples versus carrier concentration were extracted and show that trilayer graphene has the best electrical properties at a mobility of about 1000 cm²/V s and the lowest band overlap at 28.5 meV which is promising structure for optoelectronic applications.     

Structural characteristics and electrical properties of copper doped manganese ferrite

A series of Mn_1?xCu_xFe₂O₄ ferrite samples with 0.2 ≤ x ≤ 0.5 were prepared using the co-precipitation method. X-ray analysis confirmed the formation of single phase cubic spinel structure for all concentrations. Rietveld refinement revealed that the Mn_1?xCu_xFe₂O₄ with all concentrations of x belongs to normal spinel structure. The lattice parameters decrease leading to the increase in the X-ray density with increasing the copper concentration and this may be due to the difference in the ionic radii between Mn²⁺ and Cu²⁺. The decrease in the crystallite size with increasing the copper content is attributed to the higher formation temperature. The IR absorption spectra analyses were used for the detection and confirmation of the chemical bonds in spinel ferrites. The AC electrical conductivity, real part of the dielectric constant and the loss tangent tan δ were studied as a function of the applied frequency and temperature. It was found that the AC electrical conductivity increased with increasing temperature, this increase may be related to the increase in the drift mobility of the charge carriers, which are localized at ions or vacant sites. The AC conductivity increases with increasing copper concentration which may be ascribed to the decrease in hopping length. The dielectric constant ?′ and dielectric loss showed a decrease with increasing frequency and increase with increasing temperature for all compositions. The dielectric behavior is explained by using the mechanism of polarization process.     

Piezoresistive,optical and electrical properties of diamond like carbon and carbon nitride films

In the present study diamond like carbon (DLC) and carbon nitride (a-CN_x:H) films were deposited by closed drift ion source from the acetylene and nitrogen gas mixture. The piezoresistive, electrical and optical properties of ion beam synthesized DLC films were investigated. Piezoresistive properties of the diamond like carbon and carbon nitride films were evaluated by four point bending test. The piezoresistors were fabricated on crystalline alumina substrates using Al-based interdigitated finger type electrodes. Effects of the nitrogen concentration on the piezoresistive gauge factor were investigated. The dependence of the resistance of the metal/a-CN_x:H/metal structures on temperature has been studied. Current–voltage (I–V) and capacitance–voltage characteristics were measured for a-CN_x:H/Si heterostructures. The main current transport mechanisms were analyzed. Optical parameters of the synthesized films such as optical bandgap and B parameter (slope of the linear part of the Tauc plot) were investigated to study possible correlation with the piezoresistive properties.     

Heat-spreading diamond films for GaN-based high-power transistor devices

We discuss the potential of heat-spreading films with respect to improving the performance of thermally limited high-power high-frequency GaN-FET devices and report on successful diamond deposition on GaN-FETs. Detailed conditions for process compatibility with GaN-FET technology are discussed and shown to be satisfied by the low-temperature deposition process developed.