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.     

Preparation of high quality transparent chemical vapor deposition diamond films by a DC arc plasma jet method

The effects of parameters such as chamber pressure, substrate temperature and methane concentration on the growth rate and the quality of diamond films deposited by DC arc plasma jet were studied. Free standing high quality transparent diamond thick films were produced at a growth rate up to 7–10 μm h⁻¹ under the optimal conditions. Supersaturated atomic hydrogen played an important role in the process, which was produced due to the extremely high temperature of DC arc plasma jet.     

Carbon transition efficiency and process cost in high-rate,large-area deposition of diamond films by DC arc plasma jet

A new DC plasma torch in which arc jet states and deposition parameters can be regulated over a wide range has been built. It showed advantages in producing stable plasma conditions at a small gas flow rate. Plasma jets with and without magnetically rotated arcs could be generated. With straight arc jet deposition, diamond films could be formed at a rate of 39 μm/h on Mo substrates of Φ 25 mm, and the conversion rate of carbon in CH₄ to diamond was less than 3%. Under magnetically rotated conditions, diamond films could be deposited uniformly in a range of Φ 40 mm at 30 μm/h, with a quite low total gas flow rate and high carbon conversion rate of over 11%. Mechanisms of rapid and uniform deposition of diamond films with low gas consumption and high carbon transition efficiency are discussed.     

Gas phase study and oriented self-standing diamond film fabrication in high power DC arc plasma jet CVD

Gas phase species were diagnosed by in situ spatially resolved OES in high power DC arc plasma jet CVD system. CH, C₂, H were found as main species in this deposition plasma environment. Concentration of species was studied with the variation of deposition parameters such as methane concentration, substrate temperature and gas flow rate. C₂ was found to be the most sensitive species to deposition parameters. The electron mean temperature was deduced from H_γ/H_β, and changed little no matter how the deposition parameters varied. Self-standing diamond films with (111) orientation were grown within the modified species atmosphere where the intensity ratio of CH/C₂ was higher than 0.41.     

High temperature thermal conductivity of free-standing diamond films prepared by DC arc plasma jet CVD

Free-standing diamond films with 1.68 mm in polished thickness have been prepared by DC arc plasma jet CVD. By means of simply changing the placing orientation of diamond films along the laser transmission direction while testing, the through-thickness thermal conductivity (κ_⊥) together with the in-plane (κ_//) thermal conductivity of free-standing diamond films were measured by laser flash technique over a wide temperature range. Results show that the thermal conductivity κ_⊥ and κ_// of free-standing diamond films are up to 1916 and 1739 Wm^− 1 K^− 1 at room temperature, respectively, showing small anisotropy (9%), and following the relationship κ ~ T^− n as temperature rises. The conductivity exhibits similar value compared to that of high-quality single crystal diamond above 500 K for both through-thickness and in-plane directions of CVD diamond films. The effects of impurities and grain boundaries on thermal conductivity of diamond films with increasing temperature were discussed.     

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.     

Deposition of large area high quality diamond wafers with high growth rate by DC arc plasma jet

We have successfully developed a system for deposition of large area diamond films by a DC arc plasma jet operated in gas recycling mode. In the present paper, the influence of substrate temperature, methane concentration, flow rate of feeding gas and the input power of the jet for diamond film deposition is presented. Deposition of a large area of uniform thickness high quality diamond wafer of Φ65 mm in diameter at a growth rate of 15 μm/h is reported. The thickness of the wafer is 0.7 mm and the thermal conductivity can be 18.1 W/cm K.     

Free-standing diamond films deposited by DC arc plasma jet on graphite substrates with a destroyable Ti interlayer

A destroyable Ti interlayer on graphite substrate was used for fabrication of crack-free free-standing diamond films by high-power DC Arc Plasma Jet. Ti interlayer was arc ion plated on the polycrystalline graphite substrate. The thickness, morphology and composite phase of the Ti interlayer were examined by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Titanium carbide (TiC) was detected in both sides of the interlayer, which played an important role with respect to reasonable adhesion and diamond nucleation. Semi-translucent and crack-free diamond films were obtained and characterized by SEM and Raman spectroscopy. It is shown that the diamond films so obtained have excellent Raman signature. The overall results suggest that plating Ti interlayer on graphite substrate is an effective way to obtain potentially crack-free free-standing diamond films.     

Effects of electrode arrangement and pressure on synthesis of diamond films by arc discharge plasma jet chemical vapor deposition

The setup and deposition conditions of electrode arrangement and pressure have been studied to synthesize diamond films at high growth rate on wide area efficiently by arc discharge plasma jet chemical vapor deposition. An apparatus has been used in which four plasma torches, one is used for cathode and the others for divided anodes, are arranged and the positions of these torches are changeable. Growth rate, deposition area and thickness of diamond films have increased with changing the electrode arrangements without improvement of thickness variation. Maximum growth rate of our apparatus has occurred at the pressure of 6.7 kPa and diamond films that have less variations of quality and surface roughness have been synthesized at lower pressure during deposition. Moreover, a high conversion rate, which is the ratio of carbon atoms that form diamond in supplied methane gas, of 16% has been obtained at the pressure of 6.7 kPa and methane concentration of 2%.     

Effects of nitrogen addition on morphology and mechanical property of DC arc plasma jet CVD diamond films

Nitrogen-doped diamond films have been synthesized by 100 KW DC arc plasma jet chemical vapor deposition using a CH₄/Ar/H₂ gas mixture. The effect of nitrogen addition into the feed gases on the growth and surface morphology and mechanical property of diamond film was investigated. The reactant gas composition was determined by the gas flow rates. At a constant flow rate of hydrogen (5000 sccm) and methane (100 sccm), the nitrogen to carbon ratio (N/C) were varied from 0.06 to 0.68. The films were grown under a constant pressure (4 KPa) and a constant substrate temperature (1073 K). The deposited films were characterized by scanning electron microscopy, Raman spectroscopy and X-ray diffraction. The fracture strength of diamond films was tested by three point bending method. The results have shown that nitrogen addition to CH₄/H₂/Ar mixtures had led to a significant change of film morphology, growth rate, crystalline orientation, nucleation density and fracture strength for free-standing diamond films prepared by DC arc plasma jet.     

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.     

直流电弧等离子体喷射法高速制备高质量纳米金刚石膜研究

利用直流电弧等离子体喷射法沉积装置在底径Ф65mm高5mm的Mo球面衬底上成功制备出纳米金刚石薄膜,文章研究了在稳定电弧状态下碳氢比对金刚石膜形貌的影响.通过扫描电子显微镜、原子力显微镜及Raman光谱对样品的晶粒尺寸及质量进行了表征. 研究结果表明: 在稳定电弧状态下,通过提高碳氢比可以在Mo球面衬底上的表面高速沉积出高质量的纳米金刚石薄膜, 晶粒尺寸大约为4～80nm,平均粒径27.4nm.     

Design of novel plasma reactor for diamond film deposition

Development of microwave plasma reactors is a key factor for improving microwave plasma chemical vapor deposition (MPCVD) techniques for producing high quality diamond films. In this paper, a new microwave plasma reactor operated at 2.45 GHz was proposed on the basis of numerical simulation. The Finite Element Method (FEM) was used to optimize the geometry, and the Finite Difference Time Domain (FDTD) method was employed to calculate the electric field and the plasma density. The proposed reactor works mainly at the TM₀₂₁ mode, and it has an excellent power handing capability. Preliminary experiment showed that high density hemispherical plasma could be ignited inside the reactor, and uniform diamond film could be deposited on substrates at high input microwave power.     

Deposition of diamond film on silicon and quartz substrates located near DC plasma

Deposition of diamond films on to both Si and quartz substrates was succeeded by means of locating the substrate near plasma, and their microstructures were investigated by using SEM and Raman spectroscopy. The plasma was generated by intermittent DC discharge in H₂–CH₄ gas mixture at high gas pressure. The deposition rate of the film was remarkably increased when distance (D) between the substrate and the plasma (discharge electrodes) decreased. When the gas pressure (P_g) was increased from 100 to 250 Torr, the deposition rate was extremely increased and the crystalline quality of the film was improved. The deposition rate, when P_g = 200 Torr and D = 5 mm, was about 2.1 and 1.7 μm/h for Si and quartz substrate, respectively.     

Field emission of polycrystalline diamond films grown by microwave plasma chemical vapor deposition. II. Effect of p-type doping in diamond

The effects of boron (B) doping on the field emission (FE) of diamond films grown by a microwave plasma chemical vapor deposition technique were studied. Raman scattering spectroscopic analysis revealed that B-doping significantly suppressed formation of non-diamond components in the diamond film. The B-doped p-type diamond films had low resistivity, ranging from 0.07 to 20 Ω cm, and various volume fractions of non-diamond components in the diamond films. The turn-on electric field, F_T, was independent of the resistivity, the film thickness, and the volume fraction of the non-diamond components. The lowest F_T value of 8 V μm⁻¹ and the highest emission current of 3×10⁻² A cm⁻² were obtained in the B-doped diamond films. The high efficiency of the electron emission in the B-doped diamond films was believed to be due to the increase in volume fraction of the conductive regions in the film and the high density of emission sites on the film surface.     

A compact ellipsoidal cavity type microwave plasma reactor for diamond film deposition

Electric field distribution in ellipsoidal microwave cavities with different sizes was modeled by the finite difference time-domain method (FDTD). The influence of varying size on the performance of the ellipsoidal cavities was studied. Through the simulations, eight series of resonant patterns were found. Based on this simulation result, a compact ellipsoidal cavity type microwave plasma chemical vapor deposition reactor has been proposed and its performance was predicted. It is shown that such a compact ellipsoidal cavity type reactor retains the same ability to concentrate microwave energy into its focus, facilitating both production of high density plasmas and deposition of diamond films.     

Magnetoresistance of boron-doped chemical vapor deposition polycrystalline diamond films

The electrical properties and magnetoresistance of boron-doped polycrystalline diamond films grown on p-typed Si (100) by hot filament chemical vapor deposition have been investigated. As the atomic boron concentration increases from 3×10¹⁷ to 3×10¹⁹ cm⁻³, and grain size from 5 to 15 μm, the quality of diamond is improved, which causes the carrier mobility μ and longitudinal magnetoresistance change rate Δρ_///ρ₀ to increase. For a magnetic field (B) of 20 tesla and temperature 300 K, the longitudinal resistance change rate Δρ_///ρ₀ is up to 20%. Meanwhile, Δρ_///ρ₀ is proportional to μ²B² in a low field and proportional to μ^1.5B in a high field. It is the first time that a result is obtained in a high field.     

Boron carbide thin film deposition using supersonic plasma jet with substrate biasing

Hard, microcrystalline boron carbide thin films have been deposited from the thermal dissociation of hydrogen, methane and boron trichloride in a supersonic plasma jet. The influence of negative and positive substrate bias on the film properties and morphology has been investigated. A continuous ion bombardment has been found to increase the film crystallinity, however, it has led to poor adhesion to the substrate. Pulsed d.c. positive biasing has been developed as a means to elevate the electron temperature and control the gas phase chemistry, while limiting the total current flowing in the secondary discharge. In this case, it has been found that the deposition rates increase with bias voltage as a function of the third power, without affecting the film hardness and morphology. Also, the boron-to-carbon atomic ratio of the films increases with increasing positive bias voltage, from carbon-rich to stoichiometric boron carbide. Correlations between deposition rates and gas species line emission indicate that atomic boron is the primary growth species. The pulsed d.c. biasing enhancement presented in this paper constitutes a novel approach to controlling the film composition and deposition rate.