Fabrication of heterojunction diodes comprising nitrogen-doped ultrananocrystalline diamond/hydrogenated amorphous carbo
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ay photoemission spectra, which were measured with a MgKα line (h = 1253:6 eV are shown in figure. 1. O 1s peaks originate from the film surface since they disappeared immediately after Ar-ion bombardment. On the other hand, the N 1s peak was hardly influenced
Figure 1. X-ray photoemission spectra of nitrogen-doped UNCD/a-C:H films deposited at IN/H of 1.50 and 0.25.
Figure 2. Temperature dependence of electrical conductivities of 3 and 8 at. % nitrogendoped UNCD/a-C:H films. by the ion bombardment etching. Thus, the nitrogen content was estimated without considering the contribution of O1s peak to the calculation [22]. N 1s peaks are strengthened with an increase in the IN/H. The nitrogen contents of the films deposited at IN/H of 0.25 and 1.50 were estimated to be 3 and 8 at.%, respectively.
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Figure 3. (a) Schematic diagram of heterojunction and (b) I-V characteristics of diodes comprising 8 and 3 at.% nitrogen-doped UNCD/a-C:H films and p-type silicon substrates.
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Figure 4. C–V characteristics of heterojunction diodes comprising p-type silicon substrates and (a) 8 at.% and (b) 3 at.% nitrogen-doped UNCD/a-C:H films. The inset shows plot of 1/C2 versus applied reverse bias. The electrical conductivity (σ) of the films measured in the temperature range from 250 to 500 K. as shown in figure 2. Both films exhibit semiconducting behaviors, and the n-type conduction was thermally confirmed. The electrical conductivity is obviously enhanced with the nitrogen content. The activation energy ranges from 100 to 150 meV. The transport of carriers might occurs in grain boundaries (GBs) in hopping [9].
Figure 3(a) and 3(b) show schematic diagram of the heterojunction diode and its I-V curves measured in the dark at room temperature, respectively. The heterojunction diodes comprising 3 and 8 at.% nitrogen-doped UNCD/a-C:H films exhibit typical rectification actions with rectification ratios of 2 and 4 orders of magnitudes at ±5 V, respectively. The heterojunctions certainly works as pn junctions, which evidently indicates that the nitrogendoped UNCD/a-C:H acts as an n-type semiconductor. In order to investigate the nature of depletion regions in the diodes, C-V measurements were done at frequency of 100 kHz. Figure 5(a) and 5(b) shows the C-V curves of the heterojunction diodes comprising 3 and 8 at.% nitrogen-doped UNCD/a-C:H films. The capacitance decreases with increasing reverse bias voltage, which indicates that depletion regions in the diodes certainly spread under reverse biases. The built-in potentials of the diodes were estimated from the extrapolation in the 1/C2 plot, as shown in figure 5. The experimental data was fitted with the following equation:
Here, and are the dielectric constants of UNCD/a-C:H and Si, the carrier concentrations of the UNCD/a-C:H and Si layers, and the build-in potential of the heterojunction, respectively. The built-in potential of the diodes increases from 0.48 to 0.71 eV with increasing nitrogen content from 3 to 8 at.%. The carrier concentrations were estimated t
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