AFM images for BC (left) and Ni(9)-BC (right) on Si substrates, showing no significant change in the surface quality. rms roughness values are 2.1 nm for BC and 2.5 nm for Ni(9)-BC. The z range is 15 nm.
XRD data for Ni(0, 5, 7, and 9)-BC films on Si substrates over a selected region (20°–50°) chosen to optimally display the BC peaks. The Si (111) and peaks are at 28.5° and 25.5°, respectively. Undoped BC and Ni(5)-BC show diffraction peaks at 23.5° and 43°. The peak at 43° in the undoped sample consists of two peaks at 42.7° and 43.0° with almost equal intensities. In the Ni(5)-BC sample, a single peak at 43.59° is seen. The relative intensities of the x-ray peaks are obtained from Gaussian fits shown. No peaks are seen for Ni(7)-BC and Ni(9)-BC.
Ni2p XPS spectrum (a) before and (b) after sputtering to remove surface contaminants. Measurements of the surface before sputtering indicate the presence of NiO. After sputtering, the peak shape is similar to that of metallic Ni. (c) Plot of the Ni2p peak area as a function of dilution ratio. The Ni peak intensity is normalized to the area under the corresponding B1s peak. Increasing the ratio of Ni in the gas phase leads to a corresponding increase in Ni concentration in the thin films. The inset shows the XANES data of a Ni(7)-BC thin film, showing the Ni edge close to the expected energy of 8333 eV. (d) C1s XPS spectrum after sputtering, showing the absence of graphitic carbon in both undoped and Ni doped boron carbide.
Current-voltage (I-V) curves of Ni(X)-BC (, 5, 7, and 9) on n-type Si [(a)–(e) left hand column] and on p-type Si [(f)–(j) right hand column] at room temperature, showing clear evidence for the trend toward n-type behavior at higher doping concentrations.
Energy band diagram of undoped BC, n-type Si, and highly Ni doped BC. The electron affinity for BC is unknown. The constraints on the positions of the Fermi level in the BC arise from the behavior of the I-V curves with increased doping. All quantities for the Si substrate are well known. For details see text.
The plots of built-in potential vs natural log of relative Ni concentration normalized by the electron carrier concentration of n- (or p)-type Si. The top scale shows the dilution ratio, increasing to the right. Since for the p-type Si substrate is much smaller than that for the n-type substrate, all data on the p-type Si are shifted to the right. The nonlinearity of these curves indicates that the carrier concentration n in the BC layer is not proportional to the dopant concentration .
Ar gas flow through the orthocarborane and nickelocene vials for all samples. The dilution ratio is defined as the ratio of Ar gas flow through the nickelocene vial to Ar gas flow through the orthocarborane vial. The Ni concentration in the gas phase increases with increasing dilution ratio.
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