Vibrational properties of nitrogen-doped ultrananocrystalline diamond films grown by microwave plasma CVD

Ultrananocrystalline diamond (UNCD) films grown in an argon-rich Ar/CH₄/H₂ microwave plasma with nitrogen gas added in amounts of 0%–20% were studied by Raman spectroscopy with multiple excitation wavelengths in the range of 244–647 nm and by optical absorption in UV–visible. The Raman spectra have demonstrated the presence of diamond, amorphous carbon and polyacetylene in the UNCD films. Analysis of vibrational and optical properties of amorphous carbon phase proves that nitrogen stimulates the transition from amorphous carbon into an ordered graphite-like structure with narrowed optical band gap, which is supposed to be responsible for the high electrical conductivity of the N-doped UNCD.     

Determination of trap density-of-states distribution of nitrogen-doped ultrananocrystalline diamond/hydrogenated amorphous carbon composite films

Thin films comprising nitrogen-doped ultrananocrystalline diamond/hydrogenated amorphous-carbon(UNCD/a-C∶H)composite films were experimentally investigated.The prepared films were grown on Si substrates by the coaxial arc plasma de-position method.They were characterized by temperature-dependent capacitance-frequency measurements in the temperat-ure and frequency ranges of 300-400 K and 50 kHz-2 MHz,respectively.The energy distribution of trap density of states in the films was extracted using a simple technique utilizing the measured capacitance-frequency characteristics.In the measured tem-perature range,the energy-distributed traps exhibited Gaussian-distributed states with peak values lie in the range:2.84×1016-2.73×1017 eV-1 cm-3 and centered at energies of 120-233 meV below the conduction band.These states are generated due to a large amount of sp2-C and π-bond states,localized in GBs of the UNCD/a-C∶H film.The attained defect parameters are accommodating to understand basic electrical properties of UNCD/a-C∶H composite and can be adopted to suppress defects in the UNCD-based materials.     

Defect structure for the ultra-nanocrystalline diamond films synthesized in H2-containing Ar/CH4 plasma

The modification on the microstructure of diamond films due to the addition of H₂ species into the Ar/CH₄ plasma was investigated. While the Ar/CH₄ plasma produced UNCD films with equiaxed grains (about 5 nm in size), the (Ar-H₂)/CH₄ plasma produced acicular-shaped grains (about 5 × 20 nm in size). Transmission electron microscopy studies indicate that these acicular-shaped grains actually are agglomerates of diamond flakes, which contain stacking faults lying on the (111) lattice plane. Presumably, the incorporation of H₂ species in the plasma leads to partial etching of hydrocarbons adhered onto the diamond clusters, such that the C₂- (or active carbon) species contained in the plasma can attach to the diamond surface anisotropically, leading to diamond flakes. The incorporation of H₂ in Ar plasma can also suppress the formation of i-carbons, an allotropic phase of diamonds. The critical proportion of H₂ in Ar plasma for inducing the changes in the granular structure is around 0.03%. The proportion of grain boundaries was thus reduced and the electron field emission properties of the materials were thus degraded. However, the suppression of the film electrical conductivity without sacrificing the smooth surface characteristic has the applications as high-thermal-conductivity heat spreaders and substrates for surface-acoustic-wave devices.     

Improvement of field emission performance on nitrogen ion implanted ultrananocrystalline diamond films through visualization of structure modifications

The relationship between the electron field emission properties and structure of ultra-nanocrystalline diamond (UNCD) films implanted by nitrogen ions or carbon ions was investigated. The electron field emission properties of nitrogen-implanted UNCD films and carbon-implanted UNCD films were pronouncedly improved with respect to those of as-grown UNCD films, that is, the turn-on field decreased from 23.2 V/μm to 12.5 V/μm and the electron field emission current density increased from 10E−5 mA/cm² to 1 × 10E−2 mA/cm². The formation of a graphitic phase in the nitrogen-implanted UNCD films was demonstrated by Raman microscopy and cross-sectional high-resolution transmission electron microscopy. The possible mechanism is presumed to be that the nitrogen ion irradiation induces the structure modification (converting sp³-bonded carbons into sp²-bonded ones) in UNCD films.     

Enhancing electron field emission properties of UNCD films through nitrogen incorporation at high substrate temperature

The electron field emission (EFE) and electrochemical (EC) properties of N₂(10%)-incorporated ultra-nanocrystalline diamond (N₂-UNCD) films were investigated. Microstructure examination using TEM indicates that incorporating the N₂ species without the substrate heating induced the presence of stacking faults, which can be effectively suppressed by growing the films at elevated temperature. While the synthesis of N₂-UNCD without substrate heating can efficiently enhance the EC properties (large potential window with smaller background current) of the films, the EFE behavior of the films can be improved only when the films were grown at an elevated temperature. Moreover, coating the conducting N₂-UNCD on Si-tips can further enhance the EFE and CV behaviors, viz. (E₀)_tip = 5.0 V/μm with (J_e)_tip = 0.28 mA/cm² at 15 V/μm applied field and ΔE_p = 0.5 V with redox peak 170 μA were achieved.     

论UNCD的应用与研发方向（工业金刚石战略发展思考之三）

对UNCD的研发背景、特性、系列制品的用途以及研究与发展方向作了翔实阐述。系列制品包括：Na Dia Probes,Conductive Na Dia Probe,UNCD Faces and Seals,UNCD Horigon。文章从医疗器具、人工器官及医学研究;MEMS生物传感器;污水净化与清除有毒物质;表面声波与体声波器件;旋转机械设备中的机械密封与流体动力学轴承;电子频率基准仪以及下一代化学机械抛光垫修整器与其它三维形状金刚石制品等潜在应用领域论述UNCD的研究与发展方向。     

Zero-biased and visible-blind UV photodetectors based on nitrogen-doped ultrananocrystalline diamond nanowires

Taking advantage of the superflat surface of ultrananocrystalline diamond (UNCD), highly precise UNCD nanowire (NW) arrays were fabricated to develop high-performance UV photodetectors. The large surface-to-volume ratio of a nanowire significantly increases the number of surface trap states, and the reduced dimensionality effectively confines the active area of the charge carrier and shortens the transit time, which results in an enhanced photoconductivity and response speed. In this paper, the zero-biased UV photodetectors based on nitrogen-incorporated ultrananocrystalline diamond nanowire arrays have been demonstrated and characterized. The estimated responsivity was 2.0 A/W for 350 nm incident light when the device operated at room temperature. The UVA and UVB photocurrent signals from this visible blind photodetector were well defined with a rise and decay time of less than 1 s.     

Pre-nucleation techniques for enhancing nucleation density and adhesion of low temperature deposited ultra-nanocrystalline diamond

Effect of pre-nucleation techniques on enhancing nucleation density and the adhesion of ultra-nanocrystalline diamond (UNCD) deposited on the Si substrates at low temperature were investigated. Four different pre-nucleation techniques were used for depositing UNCD films: (i) bias-enhanced nucleation (BEN); (ii) pre-carburized and then ultrasonicated with diamond powder solution (PC-U); (iii) ultrasonicated with diamond and Ti mixed powder solution (U-m); (iv) ultrasonicated with diamond powder solution (U). The nucleation density is lowest for UNCD/U-substrate films ( 10⁸ grains/cm²), which results in roughest surface and poorest film-to-substrate adhesion. The UNCD/PC-U-substrate films show largest nucleation density ( 1 × 10¹¹ grains/cm²) and most smooth surface (8.81 nm-rms), whereas the UNCD/BEN-substrate films exhibit the strongest adhesion to the Si substrates (critical loads =  67 mN). Such a phenomenon can be ascribed to the high kinetic energy of the carbon species, which easily form covalent bonding, Si–C, and bond strongly to both the Si and diamond.     

UNCD薄膜的研究进展与应用

UNCD薄膜系其晶粒尺寸小于5 nm为主要特征高性能金钢石薄膜,有的研究者称为超纳米膜,具有诸多优异的物理和化学性能,在工业应用中得到广泛的青睐。UNCD不仅具备普通金刚石的各项性能,而且因其极小的晶粒尺寸,使得UNCD薄膜表面极其光滑,大大降低了摩擦系数,不需要进行抛光处理就可以直接进行工业应用,从而大大节省时间和成本。本文综述了近年来通过提高形核密度、气体掺杂、调节衬底温度等方法制备出高质量的UNCD膜,指出随着Ar掺入量的增加,二次形核率提高,晶粒尺寸细化。讨论了UNCD膜在MEMS器件、耐磨材料、表面声波和体声波器件以及生物医学应用领域的优势,并介绍了美国阿贡实验室和ADT公司在UNCD膜领域的研究进展。     

Formation of ultrananocrystalline diamond films with nitrogen addition

Nitrogen-doped ultrananocrystalline diamond (UNCD) films have been prepared by the microwave plasma jet chemical vapor deposition system (MPJCVD) using a gas mixture of Ar-1%CH₄-10%H₂ and addition of 0.5-7% nitrogen. This growth process by MPJCVD with 10% hydrogen addition that yields UNCD films compared with those UNCD films produced by MPCVD with a high Ar/CH₄ ratio due to the focused microwave plasma jet greatly enhanced the enough dissociation of react gases and formed C₂ species with an energetic state at lower argon concentration. The surface morphologies were changed drastically from continuous to rough granular surface with increasing the nitrogen content due to the great rise of CN species in the plasma. The width of grain boundaries composed of sp²-bonded carbon increased with increasing nitrogen content in the films. Moreover, the seldom defects in the UNCD films induced by the addition of nitrogen in the plasma were identified and investigated by using a scanning transmission electron microscope (STEM). The highest nitrogen-doped benefit with a N/C atomic ratio of 3.25% in UNCD films was reached by addition of only 3% N₂ in plasma (Ar-1%CH₄-10%H₂-3%N₂), showing the MPJCVD can greatly reduce the used amount of nitrogen in the synthesis of nitrogen-doped UNCD films.