The 8-inch free-standing CVD diamond wafer fabricated by DC-PACVD

We report the fabrication of the 8-inch free-standing CVD diamond wafers by DC-PACVD process with the diode-type electrode configuration. Methane–hydrogen gas mixture was used as the precursor gas. The methane volume % in hydrogen, the gas flow rate and the chamber pressure were 5～12%, 400 sccm and 100～130 Torr, respectively. The discharge voltage and the discharge current were 840 ～ 910 V and 90～110 A, respectively. The substrate temperature was 1200～1300 °C. The thermal conductivity, crystallinity and microstructure were characterized by the converging thermal wave technique, Raman spectroscopy, optical microscopy and SEM, respectively. The maximum growth rate was 9 μm/h for thermal grade 8-inch wafer. The deviation of thickness and the thermal conductivity over the 8-inch wafer was around 10% of the respective averaged values. The distribution of FWHM of Raman diamond peak over the wafer surface also showed excellent uniformity. The extremely simple scale-up of the present deposition technology was demonstrated.     

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.     

Device scaling of pseudo-vertical diamond power Schottky barrier diodes

Device size scaling of pseudo-vertical diamond Schottky barrier diodes (SBDs) has been characterized for high-power device applications based on the control of doping concentration and thickness of the p^? CVD diamond layer. Decreasing parasitic resistance on the p⁺ layer utilising lithography and etching realises a constant specific on-resistance of less than 20 mΩ cm² with increasing device size up to 200 µm. However, the leakage current under low reverse bias conditions is increased markedly. Due to the increase in the leakage current, the reverse operation limit is decreased from 2.4 to 1.3 MV/cm when the device size is increased from 30 to 150 µm. If defects induce an increase in leakage current under the reverse conditions, the density of the defects can be estimated to be 10⁴–10⁵/cm². This value is 5–10 times larger than the density of dislocations in single crystal diamond Ib substrate.     

Edge termination techniques for p-type diamond Schottky barrier diodes

The superior material properties of diamond power semiconductor devices make them a crucial technology. For high-voltage applications, optimized structures such as electric field edge termination are required for devices. In this paper, we have investigated the optimization of electric field relaxation techniques in oxygen-terminated p-type diamond Schottky barrier diodes and made comparisons with regard to electric field crowding and breakdown in oxides. Due to the low dielectric constant of diamond, Al₂O₃ is appropriate for the fabrication of field plate structures in diamond power devices.     

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.     

Leakage current analysis of diamond Schottky barrier diodes by defect imaging

The leakage current of pseudo-vertical-type diamond Schottky barrier diodes (SBDs) was analyzed using a defect visualization technique. Even under a low electrical field, 50% of the fabricated diamond SBDs exhibited a high leakage current that cannot be explained by any of the carrier transport mechanisms through the Schottky barrier. The SBDs with high leakage current were confirmed to contain a high density of dislocations that are revealed as deep etch pits by H₂/CO₂ plasma treatment. The maximum operation voltage of the SBDs is clearly dependent on the number of deep etch pits.     

Design and optimization of planar mesa termination for diamond Schottky barrier diodes

In this paper, we present planar mesa termination structure with high k dielectric Al₂O₃ for high-voltage diamond Schottky barrier diode. Analysis, design, and optimization are carried out by simulations using finite element technology computer-aided design (TCAD) Sentaurus Device software. The performances of planar mesa termination structure are compared to those of conventional field plate termination structure. It is found that optimum geometry of planar mesa terminated diode requires shorter metal plate extension (1/3 of the field plate terminated diode). Consequently, planar mesa terminated diode can be designed with bigger Schottky contact to increase its current carrying capability. Breakdown performance of field plate termination structure is limited at 1480 V due to peak electric field at the corner of Schottky contact (no oxide breakdown occurs). In contrast, peak electric field in planar mesa termination structure only occurs in the field oxide such that its breakdown performance is highly dependent on the oxide material. Due to Al₂O₃ breakdown, planar mesa termination structure suffers premature breakdown at 1440 V. Considering no oxide breakdown occurs, planar mesa termination structure can realize higher breakdown voltage of 1751 V. Therefore, to fully realize the potential of planar mesa terminated diode, it is important to choose suitable high k dielectric material with sufficient breakdown electric field for the field oxide.     

On-state behaviour of diamond Schottky diodes

Synthetic diamond power Schottky structures have the capability of sustaining significant reverse voltages with rather thin drift layers. Diamond Schottky barrier diodes (SBDs) also exhibit promising on-state behaviour. This paper looks at the influence of temperature and drift doping on the forward characteristics of these devices. Physical explanations for the variation of the forward current with the forward voltage, together with equations which aim to model this dependence, are also presented.     

UV Schottky photodiode on boron-doped CVD diamond films

We report on experimental study of photosensitivity and Q-DLTS spectra of polycrystalline CVD diamond UV photodetectors. The measured characteristics of Schottky photodiode on boron-doped diamond films are compared with those obtained for planar photoconductive structures (photoresistor type) based on undoped CVD diamond. The Schottky photodiode exhibited a sharp cut-off in photoresponse with spectral discrimination ratio (between wavelengths of 190 nm and 700 nm) as high as 5 · 10⁵ at zero bias voltage (at zero dark current). The photodiode showed the maximum of photoresponse at wavelength < 190 nm, and a low density of trapping and recombination centers as evaluated with the Q-DLTS technique. The devices demonstrated the photoresponsivity at 190 nm from 0.03 to 0.1 A/W with quantum yield of 0.20 to 0.67 in closed circuit, while the photovoltage ≥ 1.6 V was measured in open circuit regime. Another type of UV detector, the planar photoconductive structures with interdigitizing ohmic electrodes fabricated on undoped diamond film and operated under a bias voltage, revealed a higher density of (surface) defect centers and the maximum photoresponse at  210 nm wavelength. A strong influence of UV light illumination on the Q-DLTS spectra of the planar photoconductive structures was observed. This effect can be used for development of new UV detectors and dosimeters based on the Q-DLTS signal measurements.     

Two levels of the Schottky barrier in CVD diamond/silicon heterojunctions at low temperature

The I–V characteristics of some CVD diamond/silicon heterojunctions measured in the temperature range of 129–295 K showed an abnormal increase in current at low voltage and temperature. The devices show a bend in reverse and forward currents due to extra current, suggesting that there are two independent contributions to the thermionic current, corresponding to two levels of the Schottky barrier. When the temperature increases, the influence of the Schottky barrier height at small voltages decreases and normal current behaviour is observed at high temperatures. Conductance curves measured at a test frequency of 200 Hz also present a bend for low voltage and temperature. To explain the atypical current behaviour, an electrical equivalent circuit model composed of two diodes in parallel with different barrier heights was proposed. The results show that lowering of the barrier height in specific areas explains the excess current at low voltage and the origin of this effect is possibly related to non-uniformity in specific regions of the material.     

Electrical AC behaviour of MPCVD diamond Schottky diodes

The present work reports some experimental results on the electrical AC behaviour of metal–undoped diamond Schottky diodes fabricated with a free-standing MPCVD diamond film (5 μm thick). The metals are gold for the ohmic contact and aluminium for the rectifier. The capacitance and loss tangent vs. frequency shows that capacitance presents a relaxation maximum at frequencies near 10 kHz at room temperature. Although the simple model (small equivalent circuit) can justify the values for the relaxation, it cannot justify the departure from the Debye model, also verified in the Cole–Cole plot. Taking into account the existence of traps in the depletion region, a best fit to the experimental results was obtained. The difference between the Fermi level and the band edge of 0.2–0.3 eV is in agreement with the activation energy found from the loss tangent analysis. The capacitance with applied voltage (Mott–Schottky plots) gives a defect density of 10¹⁶ cm⁻³ with contact potentials near 0.5 V and the profile of defect density obtained shows a major density (approx. 10¹⁷ cm⁻³) in a layer with a thickness less than 50 nm from the junction, decreasing by one order of magnitude with increasing distance. Finally a structural model is proposed to explain the AC behaviour found.     

High-temperature characteristics of Ag and Ni/diamond Schottky diodes

The high-temperature characteristics of diamond Schottky diodes fabricated using Ag or Ni on in-situ boron-doped diamond were examined. Up to 600 °C, Ag Schottky diodes exhibited a high rectification ratio of the order of 10⁴. Even at ~ 750 °C, their rectification ratio was about 10, indicating that diamond field effect transistors with Ag Schottky diodes can operate at this temperature. In contrast, Ni Schottky diodes did not show clear rectification above 600 °C. An analysis of the I–V curves indicated that the Ag Schottky diodes have a higher rectification ratio than the Ni Schottky diodes at high temperatures due to their higher barrier heights (ϕ_B = ~ 2.0 and ~ 0.7 eV for Ag and Ni, respectively).     

Simulation and design of junction termination structures for diamond Schottky diodes

In this paper, the first step of the design of a junction termination structure usable on diamond Schottky diodes is introduced. Through the collaboration of AMPERE and LAAS laboratories, a study of junction termination structures using field plates and semi-resistive materials was carried out. Several results from simulations of p-type Schottky diodes protected by MESA etching and coated with several layers of dielectric materials are shown in this paper. The analysis of those simulations, conducted on pseudo-vertical diodes protected by a field plate on a semi-resistive layer deposited on top of a dielectric, shows a great efficiency of such junction termination structures.     

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.     

Ramp oxide termination structure using high-k dielectrics for high voltage diamond Schottky diodes

The promising theoretical properties of diamond, together with the recent advances in producing high-quality single crystal diamond substrates, have increased the interest in using diamond in power electronic devices. This paper presents numerical and experimental off-state results for a diamond Schottky barrier diode (SBD), one of most studied unipolar devices in diamond. Finding a suitable termination structure is an essential step towards designing a high voltage diamond device. The ramp oxide structure shows very encouraging electronic performance when used to terminate diamond SBDs. High-k dielectrics are also considered in order to further improve the reliability and electrical performance of the structure.     

Characterization of d.c.jet CVD diamond films on molybdenum

Diamond films grown using a thermal plasma technique are characterized using a variety of techniques. The relationships between the chemistry, morphology, and mechanical properties are explored using microscopy, Raman spectroscopy, Auger electron spectroscopy, and X-ray photoelectron spectroscopy. The characteristics of films grown using two different nucleation enhancement techniques are shown. Films grown using high methane concentrations at the beginning of growth produce large grained columnar films, whereas films grown on substrates which have been treated with a diamond polishing step show nanocrystalline structures. Variations in sp³ and sp² bonding and peak shifts are tracked through the thickness of the film, corresponding to variations in the methane concentration during growth. Stresses are measured using peak shifts and beam bending techniques. Adhesion is tested using indentations, and is shown to increase both as growth temperatures and surface roughness increase.     

Electrical characteristics of Al/polyindole Schottky barrier diodes. I. Temperature dependence

In this study, the forward and reverse bias current–voltage (I–V), capacitance–voltage (C–V), and conductance–voltage (G/ω–V) characteristics of Al/polyindole (Al/PIN) Schottky barrier diodes (SBDs) were studied over a wide temperature range of 140–400 K. Zero‐bias barrier height Φ_B0(I–V), ideality factor (n), ac electrical conductivity (σ_ac), and activation energy (E_a), determined by using thermionic emission (TE) theory, were shown fairly large temperature dispersion especially at lower temperatures due to surface states and series resistance of Al/PIN SBD. I–V characteristics of the Al/PIN SBDs showed an almost rectification behavior, but the reverse bias saturation current (I₀) and n were observed to be high. This high value of n has been attributed to the particular distribution of barrier heights due to barrier height inhomogeneities and interface states that present at the Al/PIN interface. The conductivity data obtained from G/ωV measurements over a wide temperature range were fitted to the Arrhenius and Mott equations and observed linear behaviors for σ_ac vs. 1/T and ln σ_ac vs. 1/T^1/4 graphs, respectively. The Mott parameters of T₀ and K₀ values were determined from the slope and intercept of the straight line as 3.8 × 10⁷ and 1.08 × 10⁷ Scm^?1K^1/2, respectively. Assuming a value of 6 × 10¹² s^?1 for ν₀, the decay length α^?1 and the density states at the Fermi energy level, N(E_F) are estimated to be 8.74 Å and 1.27 × 10²⁰ eV^?1cm^?3, respectively. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Characterization of substrate off-angle effects for high-quality homoepitaxial CVD diamond films

We have studied the substrate off-angle effects for the crystalline quality of the homoepitaxial diamond films mainly by using steady-state cathodoluminescence (CL) and time-resolved photoluminescence (PL) measurements. By means of the microwave plasma chemical vapor deposition method under high-power microwave power with high methane concentrations, the homoepitaxial diamond films were grown on the high-pressure/high-temperature-synthesized (HPHT) Ib (001) substrates inclined along either <110> or <100> direction by different off-angles ranging from 2° to 5°. In spite of high growth rates, we have succeeded in improving crystalline quality by employing the HPHT substrates with considerably large off-angles. Both steady-state CL and time-resolved PL measurements clearly indicate that larger off-angles lead to better crystalline quality of the homoepitaxial film, suggesting that further improvements in crystalline quality can be expected when using substrates having even larger off-angles.     

Effect of arc characteristics on the properties of large size diamond wafer prepared by DC arc plasma jet CVD

Diamond wafer with 100 mm in diameter and 1 mm in thickness was prepared by DC arc plasma jet CVD. The area arc distribution above the surface space of substrate, consisting of arc center, arc main and arc edge, directly determined the quality and homogeneity of diamond wafer. As the distance to the central area increased, the crystal size of diamond increased firstly and then decreased. Meanwhile, “dark feature” texture quantity of polished diamond film corresponding to arc main was lower than that corresponding to arc center and arc edge. XRD results showed the ratio of I₍₂₂₀₎/I₍₁₁₁₎ of diamond films increased from edge to center gradually. The intrinsic stress of diamond film corresponding to arc main was greater than the other two areas according to the Raman spectrum, and the highest transmittance in the 10.6 micron wavelength was obtained in the area corresponding to arc main by Fourier infrared. The difference of fracture strength in three areas was less than 20 MPa, which reveals the arc characteristics have less effect on fracture strength.