N-type Diamond Films Research Articles

N-Type doping of diamond by hot-filament chemical vapor deposition growth with phosphorus incorporation

Epitaxial growth of n-type semiconductor diamond films on (111)-oriented diamond has been achieved by hot-filament chemical vapor deposition (HFCVD) using a methane source and a trimethylphosphine dopant source. Secondary-ion mass spectrometry showed that the phosphorus atoms are incorporated into the films in the concentration range of 1018–1019 cm−3 from the vapor phase. Hall-effect measurements confirmed n-type conductivity in a wide temperature range up to 873 K. Electrons are thermally activated from a phosphorus donor level of approximately 0.57 eV as dominant carriers under the presence of tungsten atoms with concentrations of around 1018 cm−3 from filaments. These results indicate that HFCVD has the potential to be applied to an n-type doping process for fabricating diamond electronic devices in the phosphorus concentration range of not lower than 1018 cm−3.

Theoretical evaluation of electronic density-of-states and transport effects on field emission from n-type ultrananocrystalline diamond films

In the nitrogen-incorporated ultrananocrystalline diamond [(N)UNCD] films, representing an n-type highly conductive two-phase material comprised of sp3 diamond grains and sp2-rich graphitic grain boundaries, current is carried by a high concentration of mobile electrons within large-volume grain-boundary networks. Fabricated in a simple thin-film 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 volts per micrometer, which makes it a promising material for designing electron sources. Despite 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 diodelike to a resistorlike behavior. Such a phenomenon resembles the current-density 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) 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 a high electric field. Theoretical calculations are consistent with the experiment.

Microstructural and electrochemical properties of sulfur ion implanted nanocrystalline diamond films

Nanocrystalline diamond (NCD) films have a composite structure composed of diamond grains and amorphous carbon grain boundaries. Compared with microcrystalline diamond (MCD) films, the NCD film grain boundaries are rich in a large number of π bonds, thus providing conductive channels. Its conductivity is 3−7 orders of magnitude higher than that of MCD, and the surface of NCD film is uniform and dense, and the roughness is lower, so the NCD film is a promising electrode material. In our previous study, microwave plasma chemical vapor deposition was successfully used to prepare n-type sulfur-doped diamond films with good electrical properties. However, the electrochemical properties of sulfur-doped nanocrystalline diamond films have not been studied till now. In the present work, the nanocrystalline diamond films are prepared by the hot-wire chemical vapor deposition. The films are subjected to ion implantation and vacuum annealing. The effects of annealing temperature on the microstructure and electrochemical properties of the films are investigated. The results show that the sulfur ion implantation is beneficial to the improvement of the electrochemical reversibility of the film. When annealed at 800 °C and below, the amorphous carbon phase at the grain boundary in the film gradually changes into the trans-acetylene phase, resulting in a gradual deterioration of electrochemical performance. When the annealing temperature rises to 900 °C, Raman spectrum and TEM results show that the film has more diamond phase content and better lattice quality, and the trans-polyacetylene in the grain boundary is cracked; XPS results indicate that the CO bond at this time, C=O bond, and π—π* content increase significantly; Hall test shows that the film mobility and carrier concentration are significantly higher than those of unannealed film. The redox peak in the electrolyte is highly symmetrical, the peak potential difference is reduced to 0.20 V, the electrochemical active area is increased to 0.64 mC/cm<sup>2</sup>, and the electrochemical reversibility is much better thanthose of samples annealed at 600 °C, 700 °C, and 800 °C, respectively.

Electrical contacts to nanocrystalline diamond films studied at high temperatures

Electrical contacts of Ni, NiSi, Cu, Au, Al, and Ti electrodes to an n-type nanocrystalline diamond film are studied at temperatures between room temperature and 500 °C in a vacuum by the transmission line measurement. Direct current-voltage characteristics measured between pairs of electrodes on the film show almost straight lines, typical of ohmic contacts, for all kinds of electrode materials. The measured series resistance is divided into resistance of the film, resistance of the electrode, and the contact resistance between the electrode and film. The Ni electrode has the lowest contact resistance, which decreases from about 380 to 200 mΩ cm2 with temperature. The contact resistance accounts for a large portion of the total resistance at low temperatures. The results confirm that the contact resistance has a close relation with the work function of electrodes such that the larger the work function, the lower the contact resistance.

N-type control of single-crystal diamond films by ultra-lightly phosphorus doping

A wide impurity doping range of p- and n-type diamond semiconductors will facilitate the development of various electronics. This study focused on producing n-type diamond with ultra-lightly impurity doping concentrations. N-type single-crystal diamond films were grown on (111)-oriented diamond substrates by phosphorus doping using the optimized doping conditions based on microwave plasma-enhanced chemical vapor deposition with a high magnetron output power of 3600 W. The surface morphology was investigated by an optical microscopy using the Nomarski prism and confocal laser microscopy, and the phosphorus concentration was estimated by a secondary ion mass spectrometry. The phosphorus concentration was reproducibly controlled to between 2 × 1015 and 3 × 1017 cm−3 using a standard mass flow controller, and the average incorporation efficiency was around 0.1%. The electrical properties of the films were characterized by the Hall effect measurements as a function of temperature over a wide range from 220 to 900 K. N-type conductivity with thermal activation from a phosphorus donor level at around 0.57 eV was clearly observed for all the phosphorus-doped diamond films. The electron mobility of the film with a phosphorus concentration of 2 × 1015 cm−3 was recorded at 1060 cm2/V s at 300 K and 1500 cm2/V s at 225 K.

Carrier transport in homoepitaxial diamond films with heavy phosphorus doping

Carrier transport in heavily phosphorus (P)-doped n-type (n+-type) diamond films as functions of P concentration and temperature was investigated. It was found that the carrier transport in these diamond films could be explained by summing the band conduction and a nearest-neighbor hopping (NNH) conduction at localized donor levels, even at a high temperature of approximately 500 K. In particular, heavily P-doped samples larger than 7.5 × 1019 cm−3 showed a high conductivity of over 9.8 × 10−2 Ω−1 cm−1 at 300 K by NNH conduction, which can be used for device application.

Rectification properties of n-type nanocrystalline diamond heterojunctions to p-type silicon carbide at high temperatures

Highly rectifying heterojunctions of n-type nanocrystalline diamond (NCD) films to p-type 4H-SiC substrates are fabricated to develop p-n junction diodes operable at high temperatures. In reverse bias condition, a potential barrier for holes at the interface prevents the injection of reverse leakage current from the NCD into the SiC and achieves the high rectification ratios of the order of 107 at room temperature and 104 even at 570 K. The mechanism of the forward current injection is described with the upward shift of the defect energy levels in the NCD to the conduction band of the SiC by forward biasing. The forward current shows different behavior from typical SiC Schottky diodes at high temperatures.

Fabrication of 4H-SiC/Nanocrystalline Diamond PN Junctions

Nitrogen-incorporated, n-type nanocrystalline diamond (NCD) films are deposited on p-type Si(001) and 4H-SiC(0001) substrates by moderate-pressure, microwave plasma-enhanced chemical vapor deposition using a mixture of 1%CH4-30%N2-69%Ar. X-ray diffraction and visible Raman spectroscopy reveal that the structure of the NCD films is identical independent of the substrate materials, such that diamond nanoparticles with apparent crystal sizes of 5-8 nm are embedded in amorphous sp2 carbon matrix. For p-Si/n-NCD heterojunctions in a diode configuration, the rectifying behavior in current-voltage curves depends upon the substrate temperature for film deposition, and the rectification ratio reaches a maximum of about 300 when the film is deposited at 830 °C. For p-4H-SiC/n-NCD heterojunctions, the rectification ratio increases greatly to about 10000 when the film is deposited at 830 °C due exclusively to suppression of the reverse leakage current.

Spatially resolved electron energy loss spectroscopy on n-type ultrananocrystalline diamond films

The addition of nitrogen to the synthesis gas during synthesis of ultrananocrystalline-diamond (UNCD) films results in films uniquely exhibiting very high n-type electrical conductivity even at ambient temperatures. This result is due to the formation of nanowires having elongated diamond core nanostructures and a sp2-bonded C sheath surrounding the core. The work presented here provides detailed confirmation of this important result through spatially resolved-electron energy loss spectroscopy. The direct observation of nitrogen incorporated in the sheath has been enabled. The incorporation of this nitrogen provides strong support to a plausible mechanism for the n-type conduction characteristic of the UNCD films.

Characterization of phosphorus-doped homoepitaxial (1 0 0) diamond films grown using high-power-density MWPCVD method with a conventional quartz-tube chamber

Phosphorus-doped n-type homoepitaxial diamond films have been successfully grown at high substrate temperatures (>1000 °C) on high-pressure/high-temperature-synthesized type-Ib single-crystalline diamond (1 0 0) substrates, by using a conventional microwave plasma chemical-vapor-deposition (CVD) system with high power densities. The deposition system employed in this work had an easily exchangeable 36 mm inner-diameter quartz-tube growth chamber. The homoepitaxial diamond films thus grown were characterized by means of Hall-effect measurements with an AC magnetic field, atomic force microscope observations and secondary ion mass spectrometry techniques. The dependences of the substrate temperature (≤1300 °C) and the P/C ratio in the source gas (≤9900 ppm) on the specimen features were investigated. The optimum substrate temperature deduced was ≈1160 °C, which was also applicable to the CVD growth of undoped homoepitaxial diamond layers. The n-type conductions with an activation energy ≈0.6 eV were observed for the specimens with amounts of the P atoms incorporated to ≈1.5 × 10 18 cm −3 whereas the doping efficiencies changed from ≈0.06% to ≈0.92% with the growth condition. Possible origins for these results are discussed in relation to the growth mechanism.

Electron emission suppression from hydrogen-terminated n-type diamond

By total photoelectron yield spectroscopy (TPYS), we have found a thermal instability of a negative electron affinity in hydrogen-terminated n-type diamond films, which has never been observed for intrinsic and p-type diamond films. Experimentally, we have succeeded in detecting surface defect states and surface valence band extended states on the same n-type sample after soft annealing from 100 to 300 °C, which allowed us to evaluate the surface Fermi level and surface band bending using phosphorus doping parameters. Our results show that “quasi-positive” electron affinity prevents electron emission from an n-type diamond film even though its surface has a negative electron affinity.

Localized emission from flat diamond cathodes

We analyze steady-state field emission from an n-type semiconductor under the assumption that its surface presents no barrier for electron emission from the conduction band into the vacuum. We construct a classical Lagrangian field theory for the coupled electric and quasi-fermi potentials and use it to show that uniform emission is unstable against laterally nonuniform perturbations. We use a two-parameter model of normal emission current to show that the Lagrangian of the linearized system is minimal when all electrons are emitted from a single site. In addition to this intensely localized emission, we show the normal electric field at the surface is moderately enhanced at the solitary emission site even though the surface itself is planar. We use the result to explain the isolated emission sites observed in nanocrystalline n-type diamond films.

N-type conductivity of phosphorus-doped homoepitaxial single crystal diamond on (001) substrate

The macroscopic surface morphology and crystallinity of (001) n-type diamond films, which have been quite recently achieved by P-doping using plasma-enhanced chemical vapor deposition technique, were studied. The observation of diffraction spots, streaks, and Kikuchi patterns in reflection high energy electron diffraction analysis indicated that the surface smoothness and the crystallinity were fine. Regarding the electrical properties of (001) n-type diamond films, Hall-effect measurements over a wide temperature range from 260 to 1000 K were investigated. The conduction band transport without the effect of hopping transport was confirmed within this experimental temperature range.

Sulfur-doped n-type diamond: Preparation and electrochemical properties

Films of sulfur-doped synthetic diamond are chemical-vapor-deposited using codoping with sulfur and boron. The sulfur in diamond is detected with particle-induced x-ray emission (PIXE), x-ray photoelectron spectroscopy (XPS), and secondary ion mass spectroscopy (SIMS). Electrochemical and thermoelectric measurements show that the sulfur-containing diamond films grown in gas phase lean in boron are n-type, whereas those grown with higher boron content are overcompensated, hence, p-type. Electrochemical properties of the n-type diamond films are studied for the first time using methods of electrochemical impedance, open-circuit photopotential, and voltammetric curves in Fe(CN)63-/4- solutions. A mechanism of boron-stimulated sulfur incorporation into diamond and the nature of donors thus formed is discussed.

Electron Spin Resonance of Phosphorus in n-Type Diamond

Electron spin resonance (ESR) of electrically active centers in n-type epitaxial diamond films has been measured by electrically detected magnetic resonance (EDMR). In the regime of hopping conductivity, 31P-related centers could be separated from a large number of P1 nitrogen centers in the substrate crystal and from carbon dangling bond defects close to the electrical contacts. One of these centers is characterized by g⊥ = 2.0026, g‖ = 2.0042, Aiso = 17.6 G, and Aaniso = 1.8 G, which is incompatible with the predictions for hydrogenic donor states of phosphorus on substitutional sites. These predictions are better fulfilled by a second spurious center with approximately g = 2.006, Aiso = 380 G, and Aaniso = 15 G.

Electrically detected magnetic resonance studies of phosphorus doped diamond

Phosphorus doped n-type epitaxial diamond films have been studied by electron spin resonance (ESR) and electrically detected magnetic resonance (EDMR). At low electric field, the dominant defects influencing the electronic transport are carbon dangling bonds, while at higher fields the anisotropic spin resonance signal of a new phosphorus-related center with g ⊥=2.0026, g ||=2.0042, A iso=17.6 G, and A aniso=1.8 G is observed. These results indicate that room temperature conductivity in this film is dominated by hopping via phosphorus-related defect centers rather than via hydrogenic donor states of phosphorus atoms on substitutional sites.

Phonon-assisted electronic transitions in phosphorus-doped n-type chemical vapor deposition diamond films

One year ago we published the first results on the electronic structure of the P-level in 1000 ppm PH 3/CH 4 doped {111}-oriented n-type diamond films, using the quasi-steady-state photocurrent technique (PC) and photothermal ionization spectroscopy (PTIS). In this work we have extended our measurements at various temperatures (4.2–77.4 K) to samples with various doping levels (100, 500 and 1000 ppm PH 3/CH 4). This allowed us to obtain more precise results for the electronic structure of the phosphorus defect in homoepitaxial n-type CVD diamond films, making use of the 155 meV LO-phonon to explain the oscillatory photoconductivity. These results are confirmed by the PTIS maxima and Fourier transform infra-red (FTIR) data. In addition we present first measurements on a 2000-ppm doped {100}-oriented sample.

Sulfur-doped homoepitaxial (001) diamond with n-type semiconductive properties

We have investigated the growth of sulfur-doped (S-doped) diamond crystals via microwave plasma assisted chemical vapor deposition by introducing hydrogen sulfide (H2S) into the gas phase. Adding a small amount of H2S effectively improved the crystal quality, and the sulfur was successfully incorporated into the crystal. No evidence of hydrogen incorporation was observed in the crystal. The S-doped homoepitaxial (001) diamond showed n-type conduction by Hall effect measurements in the temperature range 250–550 K. The mobility of electrons at room temperature was 597 cm2 V−1s−1; this value suggests that there could be many practical uses. The crystallinity of the S-doped n-type diamond films were characterized by various techniques which demonstrated the high quality of the films.

Ga Ohmic contact for n-type diamond by ion implantation

Electric contacts formed on an n-type diamond film by means of the Ga ion implantation were studied. The implanted Ga contacts revealed Ohmic property at room temperature. Contact resistivity for the implanted Ga contacts formed on the diamond film with donor density of 3×1018 cm−3 was 4.8×106 Ω cm2. The value is more than one order smaller than that for conventional vacuum-deposited metal contacts on the same diamond film. This contact has enabled us to measure electric properties for relatively lightly doped n-type diamond films in a temperature range between room temperature and 600 °C.

Origin, state of the art and some prospects of the diamond CVD

A short review on the diamond CVD origin, together with its state of the art and some prospects was given. New hybrid methods of the diamond CVD permit to gain 1.2 to 6 times of growth rate in comparison with ordinary diamond CVD's. Recent results on n-type diamond film synthesis through phosphorus doping in the course of the CVD process are briefly discussed. In comparison with high-pressure diamond synthesis, the CVD processes open new facets of the diamond as ultimate crystal for science and technology evolution. It was stressed that, mainly on the basis of new CVDs of diamond, the properties of natural diamond are not only reproduced, but can be surpassed. As examples, mechanical (fracture resistance), physical (thermal conductivity), and chemical (oxidation stability) properties are mentioned. Some present issues in the field are considered.