Electronic and optical properties of boron-doped nanocrystalline diamond films

Abstract

We report on the electronic and optical properties of boron-doped nanocrystalline diamond (NCD) thin films grown on quartz substrates by CH 4 /H 2 plasma chemical vapor deposition.Diamond thin films with a thickness below 350 nm and with boron concentration ranging from 10 17 cm -3 to 10 21 cm -3 have been investigated. UV Raman spectroscopy and AFM have been used to assess the quality and morphology of the diamond films. Hall effect measurements confirmed the expected p-type conductivity. At room temperature, the conductivity varies from 1.5x10 -8 Ω -1 cm -1 for a non-intentionally doped film up to 76 Ω -1 cm -1 for a heavily B-doped film. Increasing the doping level results in a higher carrier concentration while the mobility decreases from 1.8 cm 2 V -1 s -1 down to 0.2 cm 2 V -1 s -1 . For NCD films with low boron concentration, the conductivity strongly depends on temperature. However, the conductivity and the carrier concentration are no longer temperature-dependent for films with the highest boron doping, and the NCD films exhibit metallic properties. Highly doped films show superconducting properties with critical temperatures up to 2K. The critical boron concentration for the metal-insulator transition is in the range from 2x10 20 cm -3 up to 3x10 20 cm -3 . We discuss different transport mechanisms to explain the influence of the grain boundaries and boron doping on the electronic properties of NCD films. Valence band transport dominates at low boron concentration and high temperatures, 2 whereas hopping between boron acceptors is the dominant transport mechanism for boron doping concentration close to the Mott transition. Grain boundaries strongly reduce the mobility for low and very high doping levels. However, at intermediate doping levels where hopping transport is important, grain boundaries have a less pronounced effect on the mobility. The influence of boron and the effect of grain boundaries on the optoelectronic properties of the NCD films are examined using spectrally resolved photocurrent measurements and photothermal deflection spectroscopy. Major differences occur in the low energy range, between 0.5 -1.0 eV, where both Boron impurities and the sp 2 carbon phase in the grain boundaries govern the optical absorption.

Boron-doped ultrananocrystalline diamond synthesized with an H-rich/Ar-lean gas system

“…This was also observed in boron doped NCD and MCD, [32] and was explained as a decrease of hydrocarbon content or hydrogen termination. FerrariÕs assignment of the 1150 cm -1 and 1470 cm -1 to ν 1 and ν 3 modes of trans-polyacetylene, suggests that the amount of transpolyacetylene decreases as the boron doping level increases.…”

“…[36] As the B/C gas ratio increases, the center of the peak shifts from 1355 cm -1 (B/C = 0 ppm) to ~1300 cm -1 (B/C = 6000 ppm), similar to ref. [32]. Although the peak shift of the first order of sp3 has been explained well, the shift of the D band peak along with boron concentration has not yet been explained.…”

“…As a reference, the resistivity of undoped UNCD, produced by a reactor never used for boron doping, was measured higher at 10 6 ohm·cm. Since the authors did not have access to secondary ion mass spectrometry (SIMS) a comparative measurement on the conductivity (the inverse of resistivity) vs. B/C gas ratio was conducted between the data presented here and the data from the literature for boron doped NCD [32]. At a B/C gas ratio of 6660 ppm, the reported conductivity for the heaviest doping is 76 s/cm, while by contrast, the BD-UNCD conductivity measured here was 100 s/cm for the heaviest doping level at 12500 ppm, which is comparable with the literature value.…”

Fabrication and characterization of boron-doped nanocrystalline diamond-coated MEMS probes

“…Moreover, the calculated values of RMS roughness of topography obtained from AFM investigations were 17 and 63 nm for boron-doped and undoped films, respectively. Thus, we could consider both fabricated samples as the typical nanocrystalline diamond (NCD) as defined by previous authors [20,46,47].…”

“…Moreover, diamond-coated tips are attractive for nanosensing application because they have remarkable electrochemical features which consist of chemical stability [16], a wide electrochemical window [17] and high anodic stability [18]. B-NCD achieves semi-metallic conductivity using in situ boron doping in chemical vapour deposition (CVD) process [19,20]. Boron-doped diamond films are commonly used as an electrode material for utilization with hazardous organic compounds or sensing applications [18,19,21].…”