Charge carrier transport anisotropy in ultrananocrystalline diamond films

Diamond and Related Materials, 2006

In this paper, we discuss the transport mechanism in nitrogen-doped ultrananocrystalline (N-UNCD) and B-doped nanocrystalline (B-NCD) diamond thin films, which have recently attracted significant attention due to possible applications in electronics and bioelectronics. We present clear evidence that the transport in UNCD films at LHeT has low-dimensional quantum character and can be explained by a weak localisation (WL) model. Our model explains the negative magnetoresistance, observed in these films for the first time, and confirms the WL phenomena. For comparison, we have prepared thin B-NCD films, doped by B using TMB. Films with thickness of about 150 nm deposited on glass wafers are fully transparent and highly conductive. B-concentrations close to the Metal Insulating Transition (MIT) are confirmed by Raman measurements. We discuss the positive magnetoresistance data observed also for the first time in B-NCD films and compare the transport mechanism with UNCD films. D 2006 Published by Elsevier B.V.

Physical Review B, 2007

Further progress in the development of the remarkable electrochemical, electron field emission, hightemperature diode, and optical properties of n-type ultrananocrystalline diamond films requires a better understanding of electron transport in this material. Of particular interest is the origin of the transition to the metallic regime observed when about 10% by volume of nitrogen has been added to the synthesis gas. Here, we present data showing that the transition to the metallic state is due to the formation of partially oriented diamond nanowires surrounded by an sp 2-bonded carbon sheath. These have been characterized by scanning electron microscopy, transmission electron microscopy techniques ͑high-resolution mode, selected area electron diffraction, and electron-energy-loss spectroscopy͒, Raman spectroscopy, and small-angle neutron scattering. The nanowires are 80-100 nm in length and consist of ϳ5 nm wide and 6-10 nm long segments of diamond crystallites exhibiting atomically sharp interfaces. Each nanowire is enveloped in a sheath of sp 2-bonded carbon that provides the conductive path for electrons. Raman spectroscopy on the films coupled with a consideration of plasma chemical and physical processes reveals that the sheath is likely composed of a nanocarbon material resembling in some respects a polymer-like mixture of polyacetylene and polynitrile. The complex interactions governing the simultaneous growth of the diamond core and the sp 2 sheath responsible for electrical conductivity are discussed as are attempts at a better theoretical understanding of the transport mechanism.

Nanomaterials

Diamond is one of the fascinating films appropriate for optoelectronic applications due to its wide bandgap (5.45 eV), high thermal conductivity (3320 W m−1·K−1), and strong chemical stability. In this report, we synthesized a type of diamond film called nanocrystalline diamond (NCD) by employing a physical vapor deposition method. The synthesis process was performed in different ratios of nitrogen and hydrogen mixed gas atmospheres to form nitrogen-doped (n-type) NCD films. A high-resolution scanning electron microscope confirmed the nature of the deposited films to contain diamond nanograins embedded into the amorphous carbon matrix. Sensitive spectroscopic investigations, including X-ray photoemission (XPS) and near-edge X-ray absorption fine structure (NEXAFS), were performed using a synchrotron beam. XPS spectra indicated that the nitrogen content in the film increased with the inflow ratio of nitrogen and hydrogen gas (IN/H). NEXAFS spectra revealed that the σ*C–C peak weakene...

Japanese Journal of Applied Physics, 1998

Chemical-vapor-deposited (CVD) diamond films with intentional nitrogen doping have been characterized by various standard techniques. Electrical resistance measurements demonstrate that the nitrogen doping significantly varies the surface conductivity of as-grown diamond films; the surface resistance of N-doped diamond films can reach as high as 1011 Ω, which is about six orders of magnitude higher than that of an undoped one. Such high surface resistance remains stable even after 8 hours of exposure to hydrogen plasma. It is also found that the photoemission threshold energy of N-doped diamond films is about 0.55 eV less than the diamond band-gap energy, which implies the existence of compensated surface gap states and possibly, negative electron affinity in the as-grown N-doped diamond films. The particular properties observed in the N-doped diamond films are discussed in relation to the fabrication of diode-type diamond electron emitters.

Applied Physics Letters, 2009

ABSTRACT

Applied Surface Science, 2017

Chemical, structural, morphological and micro-/macro-electrical properties of undoped and nitrogen-(N-)doped diamond films are determined by X-ray photoelectron spectroscopy, Raman and photoluminescence spectroscopies, field emission scanning electron microscopy, atomic force microscopy, scanning capacitance microscopy (SCM) and two points technique for I-V characteristics, respectively. The characterization results are very useful to examine and understand the relationship among these properties. The effect of the nitrogen incorporation in diamond films is investigated through the evolution of the chemical, structural, morphological and topographical features and of the electrical behavior. The distribution of the electrical current is first assessed at millimeter scale on the surface of diamond films and then at micrometer scale on small regions in order to establish the sites where the carriers preferentially move. Specifically, the SCM images indicate a non-uniform distribution of carriers on the morphological structures mainly located along the grain boundaries. A good agreement is found by comparing the electrical currents at the micro-and macro-scale. This work aims to highlight phenomena such as photo-and thermionic emission from N-doped diamond useful for microelectronic engineering.

Diamond and Related Materials, 2000

UV and visible photoconductivity and electrical features of undoped diamond thin films grown by microwave plasma-assisted Ž . chemical vapour deposition MP-CVD on silicon and copper substrates are studied. The results are correlated with morphology Ž . properties analysed by atomic force microscopy AFM and micro-Raman. The photoconductivity presents several bands from 1.8 to 3.8 eV that are dependent on the substrate used to grow the samples in spite of some common bands observed. The J᎐V curve Ž . Ž . in DC in samples grown on Si has a rectifier behaviour Schottky emission in opposition to the samples grown on Cu that have Ž . no rectification SCLC conduction . With these results we can conclude that diamond based optoelectronic devices behaviour is controlled by two kinds of structural defects localized in microcrystal and in its boundaries. A general structure model for the optoelectronic behaviour is discussed. ᮊ

2011

Deposition of Ultrananocrystalline Diamond (UNCD) films have been carried out and dielectric properties of nitrogen incorporated UNCD films were studied using spectroscopic ellipsometry (SE), UV/VIS spectroscopy, and reflectance spectroscopy. Dielectric functions of the films were correlated with their nanostructure, elemental concentration, and growth conditions. The films were grown in a 915 MHz microwave plasma chemical vapor deposition system with 0%, 10%, and 20% N 2 gas diluted into Ar/CH 4 /H 2 plasma. Samples were deposited on 6-inch Si substrates. For UV/VIS spectroscopy studies, samples were deposited on quartz substrates. The bonding structure was investigated by Raman spectroscopy and the surface morphology of the films was characterized by atomic force microscopy (AFM) and scanning electron microscopy (SEM). Determination of the elemental composition of the deposited films was carried out by ion beam analysis (IBA) measurements. To obtain the precise concentration of carbon, hydrogen, nitrogen, and impurities incorporated in the film, Rutherford backscattering spectrometry (RBS), non-Rutherford backscattering spectrometry (NRBS), elastic recoil detection analysis

Journal of Nanoscience and Nanotechnology, 2016

Nitrogen-doped ultrananocrystalline diamond/hydrogenated amorphous carbon composite (UNCD/a-C:H) films were deposited on p-type Si substrates by coaxial arc plasma deposition. The deposited films possessed n-type conduction, and evidently formed pn heterojunctions with p-type Si substrates. The heterojunction devices showed typical rectification properties similar to those observed for conventional abrupt pn heterojunctions. The conduction mechanisms that govern current transport in these devices were analyzed using dark current-voltage measurements in the temperature range from 300 K to 80 K. The results revealed that a trap-assisted multi-step tunneling process is a dominant mechanism at lower temperatures and low forward bias. At least two defect levels with activation energies of 42 and 24 meV appear to activate this process. At moderate forward bias, the current followed a power-law dependence, attributable to a space-charge-limited current. This junction behavior might be owing to a large number of grain boundaries in the UNCD/a-C:H film that provide active centers for carrier recombination-tunneling processes at the junction interface.

Journal of Applied Physics, 2022

The quantum efficiency and mean tranvserse energy of electrons emitted from a cathode determine the quality of beams generated from photoinjectors. The nitrogen-incorporated ultrananocrystalline diamond, (N)UNCD, is a new class of robust semiconductor photocathodes which has been considered in photoinjectors for high peak current extraction. In this work, we measure the spectral response in quantum efficiency, photoemission energy spectra and mean tranvserse energy of the (N)UNCD photocathode using a photoemission electron microscope. The observed quantum efficiency was comparable to that of copper photocathodes. Photoemission spectra showed the evidence of scattering of electrons before emission. This relaxation of electrons due to scattering is also observed in the spectral response of the mean tranvserse energy. The mean tranvserse energy is limited to ∼ 70 meV at the threshold. We attribute this to the physical and chemical roughness of the (N)UNCD photocathode and hence, smoother films will be required to further reduce the mean transverse energy obtained from the (N)UNCD photocathode.

Journal of Applied Physics, 2019

In the nitrogen-incorporated ultrananocrystalline diamond ((N)UNCD) films, representing an n-type highly conductive two-phase material comprised of sp 3 diamond grains and sp 2rich graphitic grain boundaries, the current is carried by a high concentration of mobile electrons within the large-volume grain boundary networks. Fabricated in a simple thinfilm planar form, (N)UNCD was found to be an efficient field emitter capable of emitting a significant amount of charge starting at the applied electric field as low as a few V/µm which makes it a promising material for designing electron sources. Despite the semimetallic conduction, field emission (FE) characteristics of this material demonstrate a strong deviation from the Fowler-Nordheim law in a high-current-density regime when (N)UNCD field emitters switch from a diode-like to resistor-like behavior. Such phenomenon resembles the currentdensity saturation effect in conventional semiconductors. In the present paper, we adapt the formalism developed for conventional semiconductors to study current-density saturation in (N)UNCD field emitters. We provide a comprehensive theoretical investigation of (i) the influence of partial penetration of the electric field into the material, (ii) transport effects (such as electric-field-dependent mobility), and (iii) features of a complex density-of-states structure (position and shape of π−π * bands, controlling the concentration of charge carriers) on the FE characteristics of (N)UNCD. We show that the formation of the current-density saturation plateau can be explained by the limited supply of electrons within the impurity π − π * bands and decreasing electron mobility in high electric field. Theoretical calculations are consistent with experiment.

Diamond and Related Materials, 2011

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− 5mA/cm2 to 1× 10E− 2mA/ ...

Diamond and Related Materials, 2008

Nitrogen (N) ion implantation induced modification on structural and electron field emission (EFE) properties of ultrananocrystalline diamond (UNCD) films were reported. Low dose ion implantation slightly improved the EFE properties of UNCD films mainly due to the formation of defects and annealing brought the EFE parameters back to original state by eliminating the defects. Conversely, high dose ion implantation markedly enhanced the EFE properties for UNCD films possibly due to the induction of amorphous carbons for the UNCD films. The annealing process converts the amorphous phase into a more stable graphitic one such that the EFE properties persisted even after the annealing process.

Journal of Applied Physics, 2014

Copper ion implantation and subsequent annealing at 600 C achieved high electrical conductivity of 95.0 (Xcm) À1 for ultrananocrystalline diamond (UNCD) films with carrier concentration of 2.8 Â 10 18 cm À2 and mobility of 6.8 Â 10 2 cm 2 /V s. Transmission electron microscopy examinations reveal that the implanted Cu ions first formed Cu nanoclusters in UNCD films, which induced the formation of nanographitic grain boundary phases during annealing process. From current imaging tunneling spectroscopy and local current-voltage curves of scanning tunneling spectroscopic measurements, it is observed that the electrons are dominantly emitted from the grain boundaries. Consequently, the nanographitic phases presence in the grain boundaries formed conduction channels for efficient electron transport, ensuing in excellent electron field emission (EFE) properties for copper ion implanted/annealed UNCD films with low turn-on field of 4.80 V/lm and high EFE current density of 3.60 mA/cm 2 at an applied field of 8.0 V/lm. V

Diamond and Related Materials, 2007

In the preparation of high power diamond photoswitches, thick (more than 100 μm) lightly nitrogen-doped single crystals were grown at LIMHP, for which Differential Interference Contrast Microscopy, Raman spectroscopy, photoluminescence, and cathodoluminescence have confirmed good morphology and very low but well-controlled impurity doping level. In order to evaluate the effect of nitrogen incorporation on the electronic properties of these films, photoconductivity