Characterization of leakage current on diamond Schottky barrier diodes using thermionic-field emission modeling

A diamond vertical Schottky barrier diode (SBD) with nonepitaxial crystallites (NCs) exhibits high leakage current in both its forward and its reverse characteristics. A shunt path current through the grain boundary of the NCs is the dominant mechanism. The defectless device shows a low leakage current of less than 10^− 11 A/cm², and the device yield corresponds to the density of the NCs. The reverse leakage current of the defectless device increases with the reverse field. The leakage current of the diamond SBD is in good agreement with the tunneling model described by thermionic-field emission (TFE) rather than the conventionally used barrier-lowering model. The TFE current dominates when the reverse electric field is larger than 1.2 MV/cm, and current density reaches 10^− 6 A/cm², even at 1.6 MV/cm, which is lower than the avalanche limit.     

Characterization of Schottky barrier diodes on a 0.5-inch single-crystalline CVD diamond wafer

In this study, Schottky barrier diodes were fabricated on a 0.5-inch single-crystalline diamond wafer, and the quality of the wafer as well as the performance of the devices were characterized. A rocking curve map indicated that the FWHM of the central 8 × 8-mm region was 10–50 arc sec, which is similar to that of high-quality HPHT single-crystalline diamond. The fabricated pVSBDs on the p^?/p⁺ stacked layer showed a high operation limit for the electrical field, with the mean value of this limit being higher than 2.5 MV/cm when the electrode was smaller than 300 µm. The performance of the devices seemed to be associated with the quality of the wafer. This indicates that the leakage current of a device is determined by the quality of the diamond wafer on which it is fabricated.     

Vertical structure Schottky barrier diode fabrication using insulating diamond substrate

To obtain high current operation of the diamond SBDs, the device should be designed in a vertical type structure in order to minimize the device on-resistance. In this research, we have designed and developed the technology for fabrication of diamond vertical structure Schottky barrier diodes (vSBD) by utilizing Inductively Coupled Plasma etching technique. Free standing CVD grown epilayers (p⁺/p^? = 100 μm/5 μm) were obtained by removing the base Ib substrate on which the epi-layers were grown, using ICP etching process. After ICP etching, ohmic contact (Ti/Pt/Au) was made at the bottom of p⁺ layer, and Schottky contact (Mo) was made at top side on oxidized surface of p^? layer, to realize Diamond/Mo vSBDs and were analyzed for their electrical characteristics. The SBDs showed a reproducible ideality factor close to 1.0, and a barrier height of 1.4 eV, with a small standard deviation of 0.06 and 0.12 eV respectively. Diodes in the vertical structure exhibited R_on with a battery uniformity irrespective of their location on the wafer, compared the diodes in a pseudo-vertical structure. Room temperature I–V analysis of the fabricated vSBDs (70 μm size) exhibited a high forward current density of 2980 A/cm² (= 0.115 A) with a low R_onS of 8 mΩ cm², which could be attained due to the vertical geometry of the diodes. At the high temperature operation, still higher current density could be obtained. Satisfactory reverse blocking characteristics also could be achieved with a breakdown field of 2.7 MV/cm for small size diodes.     

Power high-voltage and fast response Schottky barrier diamond diodes

We developed and investigated a set of packaged vertical diamond Schottky barrier diodes (SBDs) with a large crystal area of up to 25 mm². All devices show forward current above 5 A and the blocking voltage over 1000 V in the temperature range from 20 °C to 250 °C. Due to the large crystal area and finite thermal resistance of the crystal-case interface the forward current self-heating effect results in a good diamond SBDs performance not only at elevated temperatures but also at normal conditions. As a result we measured about 4 V forward voltage drop, 35 mΩ × cm² specific on-resistance and 100 nA/cm² leakage current for the diode case at room temperature. At a case temperature of 250 °C the forward voltage drop was less than 2.5 V, the specific on-resistance about 40 mΩ × cm² and the leakage current about 100 μA/cm². The Baliga's figure of merit was 25–30 MW/cm² in the temperature range of 20-250 °C. The typical value of the reverse recovery time less than 10 ns while switching from 2 A forward current to 100 V blocking voltage meets the requirements for practical use of diamond SBDs in effective switch-mode power converters operating at frequencies higher than 1 MHz. Further device design optimization and the diamond epitaxial layer quality improvement will help to reduce the power losses in on-state and make diamond SBDs competitive with SiC diodes even at room temperature.     

CVD diamond Schottky barrier diode,carrying out and characterization

A p-type diamond Schottky barrier diode (SBD) on homoepitaxial CVD diamond is presented.The technologic steps required to carry out the experimental device are described in this paper. The B-acceptors concentration and the barrier height have been extracted from C–V measurements leading to Ns ～ 1.2 × 10¹⁷ cm^? 3 and Φ_Β = 1.8 eV respectively. The current–voltage experimental measurements performed at room temperature have shown high current density in the range of 900 A/cm². However, the breakdown voltage of the device was limited to 25 V, this low reverse value suggest the presence of a large defect density in the bulk.     

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.     

Reduction of dislocation densities in single crystal CVD diamond by using self-assembled metallic masks

The development of diamond-based electronic devices designed to operate at high power is strongly hampered by the lack of low dislocation single crystal material. Dislocations in Chemically Vapor Deposited (CVD) diamond are indeed generally responsible for leakage current, seriously deteriorating the performance of the devices. They can be due to defects such as polishing damage or contamination found at the substrate's surface, or they can directly originate from existing bulk defects that extend into the homoepitaxial layer. Although significant improvements have been achieved by using adapted surface treatments, dislocations found in CVD diamond grown on standard quality single crystal substrates are still typically in the range 10⁵–10⁶ cm^− 2. In this work, we report on a new growth strategy aiming at preventing threading dislocations from propagating into CVD diamond layers. It is based on the selective masking of existing defects revealed at the surface of the substrates by Pt nanoparticles. The interaction of dislocations with such embedded particles has been assessed and critical remarks are given as to the use of this technique in order to reduce dislocation densities in synthetic diamond.     

Homoepitaxial diamond p–n+ junction with low specific on-resistance and ideal built-in potential

We have realized low specific on-resistance and ideal built-in potential simultaneously for a (111)-oriented homoepitaxial diamond p–n⁺ junction. As the p–n⁺ junction, the heavily phosphorus doped n⁺-type layer, which shows variable range hopping conduction, was formed on the (111)-oriented boron doped p-type one. By using this hopping conduction, the resistivity of the n⁺-type layer becomes lower by three orders of magnitude than that of a lightly P-doped layer. Current density–voltage characteristics showed a rectification ratio of 10⁶ at ± 15 V at room temperature. The current density and the specific on-resistance at forward bias voltage of 15 V at room temperature are over 100 A/cm² and 8 × 10^− 2 Ωcm², respectively. This low specific on-resistance corresponds to the lower resistivity of the n⁺-type layer by three orders of magnitude than that of conventional lightly P-doped n-type layer. The existence of the space-charge layer at the vicinity of the p–n⁺ junction was confirmed from capacitance–voltage (C–V) characteristics. From C⁻²–V characteristics at 200 °C, the built-in potential was estimated as approximately 4.4 eV, which is identical to that of conventional diamond p–n junction.     

Growth of a three-dimensional complex carbon nanoneedle electron emitter for fabrication of field emission device

A three-dimensional complex carbon nanoneedle electron emitter film with high emission current density at low electric field has been developed by a direct current plasma chemical vapor deposition system. Sample grown on stainless wire substrate pretreated with the mixing powders of diamond and molybdenum exhibited novel film morphology. The scanning electron microscopy image taken from this film indicated a three-dimensional complex nanostructure emitter, the center of which was a carbon nanoneedle, and many small carbon nanowalls growing from the needle. The density of unique nanostructure emitters was about 5 × 10⁷/cm². The I–V characteristic addressed an emission current density of 251 mA/cm² at the electric field of 2.2 V/μm, and the field emission current was stable, making it possibly suitable for developing field emission device.     

Synthesis of diamond using ultra-nanocrystalline diamonds as seeding layer and their electron field emission properties

A modified nucleation and growth process was adopted so as to improve the electron field emission (EFE) properties of diamonds films. In this process, a thin layer of ultra-nanocrystalline diamonds (UNCD), instead of bias-enhanced-nuclei, were used as nucleation layer for growing diamond films in H₂-plasma. The morphology of the grains changes profoundly due to such a modified CVD process. The geometry of the grains transform from faceted to roundish and the surface of grains changes from clear to spotty. The Raman spectroscopies and SEM micrographs imply that such a modified diamond films consist of UNCD clusters (~ 10–20 nm in size) on top of sp³-bonded diamond grains (~ 100 nm in size). Increasing the total pressure in CVD chamber deteriorated the Raman structure and hence degraded the EFE properties of the films, whereas either increasing the methane content in the H₂-based plasma or prolonged the growth time improved markedly the Raman structure and thereafter enhanced the EFE properties of diamond films. The EFE properties for the modified diamond films can be turned on at E₀ = 11.1 V/μm, achieving EFE current density as large as (J_e) = 0.7 mA/cm² at 25 V/μm applied field.     

785 nm Raman spectroscopy of CVD diamond films

Raman spectroscopy is a powerful technique often used to study CVD diamond films, however, very little work has been reported for the Raman study of CVD diamond films using near-infrared (785 nm) excitation. Here, we report that when using 785 nm excitation with 1 µm spot size, the Raman spectra from thin polycrystalline diamond films exhibit a multitude of peaks (over 30) ranging from 400–3000 cm^− 1. These features are too sharp to be photoluminescence, and are a function of film thickness. For films > 30 µm thick, freestanding films, and for films grown in diamond substrates the Raman peaks disappear. This suggests that the laser is probing the vibrations of molecular units at the grain boundaries of the disordered crystallites present at the interface between the diamond and substrate.     

Characterization of vertical Mo/diamond Schottky barrier diode from non-ideal I–V and C–V measurements based on MIS model

In this study, we investigated non-ideal characteristics of a diamond Schottky barrier diode with Molybdenum (Mo) Schottky metal fabricated by Microwave Plasma Chemical Vapour Deposition (MPCVD) technique. Extraction from forward bias I–V and reverse bias C^− 2–V measurements yields ideality factor of 1.3, Schottky barrier height of 1.872 eV, and on-resistance of 32.63 mΩ·cm². The deviation of extracted Schottky barrier height from an ideal value of 2.24 eV (considering Mo workfunction of 4.53 eV) indicates Fermi level pinning at the interface. We attributed such non-ideal behavior to the existence of thin interfacial layer and interface states between metal and diamond which forms Metal-Interfacial layer-Semiconductor (MIS) structure. Oxygen surface treatment during fabrication process might have induced them. From forward bias C–V characteristics, the minimum thickness of the interfacial layer is approximately 0.248 nm. Energy distribution profile of the interface state density is then evaluated from the forward bias I–V characteristics based on the MIS model. The interface state density is found to be uniformly distributed with values around 10¹³ eV^− 1·cm^− 2.     

Electrical and light-emitting properties from (111)-oriented homoepitaxial diamond p–i–n junctions

We have succeeded in fabricating a (111)-oriented diamond p–i–n junction with high crystalline quality intrinsic layer and with low series resistance. The series resistance of this diamond p–i–n junction was improved by decreasing the resistivity and specific contact resistance of n-type layer, which is allowed to inject higher current while maintaining lower junction temperature. Current density–voltage characteristics showed a rectification ratio of 10⁶ at ± 15 V at room temperature. A clear ultraviolet emission at around 235 nm due to free exciton recombination was observed at a forward current, while the broad visible light emission from deep levels was significantly suppressed. Moreover, stronger excitonic emission by two orders of magnitude than that of (001)-oriented diamond p–i–n junctions with high series resistance was realized.     

Homoepitaxial single crystal diamond growth at different gas pressures and MPACVD reactor configurations

Homoepitaxial growth of single crystal diamond by microwave plasma chemical vapor deposition in a 2.45 GHz reactor was investigated at high microwave power density varied from 80 W/cm³ to 200 W/cm³. Two methods of achieving high microwave power densities were used (1) working at relatively high gas pressures without local increase of electric field and (2) using local increase of electric field by changing the reactor geometry (substrate holder configuration) at moderate gas pressures. The CVD diamond layers with thickness of 100–300µm were deposited in H₂–CH₄ gas mixture varying methane concentration, gas pressure and substrate temperature. The (100) HPHT single crystal diamond seeds 2.5 × 2.5 × 0.3 mm (type Ib) were used as substrates. The high microwave power density conditions allowed the achievement of the growth rate of high quality single crystal diamond up to 20 µm/h. Differences in single crystal diamond growth at the same microwave power density 200 W/cm³ for two process conditions—gas pressure 210 Torr (flat holder) and 145 Torr (trapezoid holder)—were studied. For understanding of growth process measurements of the gas temperature and the concentration of atomic hydrogen in plasma were made.     

Investigation of the nucleation and growth mechanisms of nanocrystalline diamond/amorphous carbon nanocomposite films

Nucleation and growth, but especially the development of the morphology of nanocrystalline diamond/amorphous carbon (NCD/a-C) nanocomposite films have been investigated by systematic variation of three important parameters, namely the deposition time, the growth rate, and the substrate pre-treatment used to enhance the nucleation density. The films have been characterized, among others, by scanning electron microscopy, atomic force microscopy, and Fourier transform infrared spectroscopy. It is shown that, by successive addition of ultradispersive diamond powder to the suspension of nanocrystalline diamond powder in n-pentane used for the ultrasonic pre-treatment, the nucleation density can be enhanced by two orders of magnitude from 1 · 10⁸ cm^− 2 to > 1 · 10¹⁰ cm^− 2. This reduces the thickness required to achieve closed films from 1 µm to 100 nm. However, once coalescence of the individual nodules emerging from the nucleation sites has taken place the films loose “memory” of the nucleation step and start to develop the typical NCD morphology consisting of larger features with diameters of some hundreds of nm which are in turn composed of much smaller features. Irrespective of the feature size and of the parameters used, the films of this investigation possess AFM rms roughnesses of 9–13 nm, indicating that rms values are not sufficient to characterize NCD surfaces.     

Microstructure,optical and electrical properties of sputtered HfTiO high-k gate dielectric thin films

The microstructure, optical and electrical properties of HfTiO high-k gate dielectric thin films deposited on Si substrate and quartz substrate by RF magnetron sputtering have been investigated. Based on analysis from x-ray diffraction (XRD) measurements, it has been found that the as-deposited HfTiO films remain amorphous regardless of the working gas pressure. Meanwhile, combined with characterization of ultraviolet-visible spectroscopy (UV–vis) and spectroscopy ellipsometry (SE), the deposition rate, band gap and optical properties of sputtered HfTiO gate dielectrics were determined. Besides, by means of the characteristic curves of high frequency capacitance–voltage (C–V) and leakage current density–voltage (J–V), the electrical parameters, such as permittivity, total positive charge density, border trap charge density, and leakage current density, have been obtained. The leakage current mechanisms are also discussed. The energy band gap of 3.70 eV, leakage current density of 1.39×10⁻⁵ A/cm² at bias voltage of 2 V, and total positive charge density and border trap charge density of 9.16×10¹¹ cm⁻² and 1.3×10¹¹ cm⁻², respectively render HfTiO thin films deposited at 0.6 Pa, potential high-k gate dielectrics in future CMOS devices.     

Nitrogen-doped diamond films selected-area deposition by the plasma-enhanced chemical vapor deposition process

The nitrogen-doped diamond films have been successfully synthesized by using urea as the nitrogen source. Selected-area deposition of diamond nuclei was formed by using a SiO₂ layer as the masking material. Diamond pads, around 9 μm in diameter, were obtained when the N-doped diamond films were deposited on these patterned diamond nuclei using the chemical vapor deposition process. An emission current density as high as 200 μA/cm², with a turn-on field of around 8 V/μm, was obtained. However, the diamond emitters broke down easily, which is ascribed to the localized melting of the substrate materials surrounding the diamond pads.     

Electrical properties and energy-storage performance of (Pb0.92Ba0.05La0.02)(Zr0.68Sn0.27Ti0.05)O3 antiferroelectric thick films prepared by tape-casing method

The energy-storage performance and dielectric properties of tape-cast (Pb_0.92Ba_0.05La_0.02)(Zr_0.68Sn_0.27Ti_0.05)O₃ (PBLZST) antiferroelectric (AFE) thick films with different thicknesses were systematically studied. As the thickness of the thick films increased from 40 to 80 µm, the dielectric constant and saturation polarization (P_s) of the thick films were gradually increased, while their corresponding breakdown strength (BDS) was decreased. A maximum recoverable energy-storage density of 6.8 J/cm³, companied by an efficiency of 61.2%, was achieved in the PBLZST AFE thick film with a thickness of 40 µm at room temperature. Moreover, the energy density of the PBLZST AFE thick films also displayed good thermal stability over 25–200 °C. In addition, all the samples had a low leakage current density of ~10⁻⁶ A/cm² at room temperature. These findings demonstrated that the PBLZST thick films should be a promising candidate for applications in high energy-storage capacitors.     

Effect of (Zn,Mn) co-doping on the structure and ferroelectric properties of BiFeO3 thin films

BiFe_1-2xZn_xMn_xO₃ (BFZMO, with x = 0–0.05) thin films were synthesized via sol–gel method. Effects of (Zn, Mn) co-doping on the structure, ferroelectric, dielectric, and optical properties of BiFeO₃ (BFO) films were investigated. BFZMO thin films exhibit rhombohedral structure. Scanning electron microscopy (SEM) images indicate that co-doping leads to a decrease in grain size and number of defects. Leakage current density (4.60 × 10^?6 A/cm²) of BFZMO film with x = 0.02 was found to be two orders of magnitude lower than that of pristine BFO film. Owing to decreased leakage current density, saturated P–E curves were obtained. Maximum double remnant polarization of 413.2 μC/cm² was observed for BFZMO thin film with x = 0.02, while that for the BFO film was found to be 199.68 μC/cm². The reason for improved ferroelectric properties is partial substitution of Fe ions with Zn and Mn ions, which resulted in a reduction in the effect of oxygen vacancy defects. In addition, co-doping was found to decrease optical bandgap of BFO film, opening several possible routes for novel applications of these (Zn, Mn) co-doped BFO thin films.     

Mechanistic study of ion-induced diamond nucleation

The effect of low-energy ion bombardment of silicon on diamond nucleation was investigated. By bombarding 100 eV ions of methane and hydrogen on a silicon substrate prior to diamond growth by chemical vapor deposition, diamond nucleation can be immensely enhanced. The ion beam treatment deposited a layer of nano-crystalline graphitic carbon embedded with amorphous SiC. Diamond then nucleated on the graphite overlayer; the nucleation density increased with increasing ion dose. At 1×10¹⁹ ions cm⁻², a nuclei density of 4×10⁸ cm⁻² was obtained. These results show that ion bombardment of the substrate enhances diamond nucleation.