Electrical conductivity of nitride carbon films with different nitrogen content

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Abstract

The electrical conductivity of nitride carbon (DLC: N) films has been studied. It is found that the electrical conductivity of the deposited films increases slowly with increasing nitrogen content, however, it decreases after the nitrogen content in the film reaches a certain value of 12.8 at%. Thermal treatment results show that the electrical conductivity of the lowly nitrogen doped DLC film increases rapidly, while that of the heavily doped film decreases after annealing at 300 °C for 30 min. Raman and XPS spectra results show that when the nitrogen content in the films reaches a certain value, there appears nonconductive phases. Therefore the electrical conductivity of the heavily doped films decreases. FTIR spectra analysis results show that the nitrogen atom as an impurity center undergoes an ‘activation’ process during the thermal treatment, which leads to the increase of the electrical conductivity. Therefore, the nitrogen in these two kinds of films has different effects on the electrical conductivity.

Introduction

α-Si/H film can be made into semiconductor by doping, and then be used in the field of microelectronics extensively. Compared with α-Si/H film, DLC film has sp2 and sp1 bonding fractions but the sp3 bonding fraction. They, respectively, form diamond, graphite and the linear or annular carbon chain. So, the chemical bond structure of DLC film is more complicated than that of α-Si/H film. The electrical conduction of DLC film by doping is not sure because there exists three bonding fractions [1]. Take the nitrogen atom for example, if there still exists an unpaired electron in nitrogen atom after the incorporation of nitrogen into the DLC film, the nitrogen can contribute to the electrical conductivity; however, if the nitrogen atom has only nonbonding lone pair electrons remained, the DLC: N film cannot show the doping effect. Silva et al. [2] reported that there were nine binding modes of nitrogen in the DLC: N film, but only three of them contribute to the electrical conductivity, shown in Fig. 1.

On the other hand, there will appear α-CNx phase in the DLC: N film if we increase the nitrogen content [3], [4]. α-CNx has some properties similar to that of β-C3N4. Someone has predicted that the mechanical properties of β-C3N4 is superior to that of diamond, which arouses the study interest [5], [6], [7], [8]. Many groups reported their research of the mechanical property and application of α-CNx film [9], [10], [11], [12]. In this paper, we focus on the electrical conductivity of nitride carbon film.

Section snippets

Experimental detail

The DLC: N films were deposited on the n-type (001) silicon wafer by the RFPCVD method. The thickness of specimen is about 300 nm. The Au electrode was evaporated on the DLC: N film, while the Al electrode was evaporated on the back of the silicon wafer. The structure of samples used for electrical measurement shows in Fig. 2. The specimens were annealed separately at the temperature of 300 and 400 °C at the N2 atmosphere for 30 min. The I–V curve was measured by the HP 4140B microcurrent setup.

Discussion

From Fig. 3, we knew the electrical conductivity of films was increased by 1.5–2 orders of magnitude with nitrogen increasing from 0 to 10.4 at%. However, when the nitrogen content was 12.8 at%, the electrical conductivity was decreased by 0.5 orders of magnitude. It is due to the appearance of α-CNx phase which is nonconductive in the films when the nitrogen content reaches certain concentration, which decreases the concentration of effective donor nitrogen in the films. Therefore, the

Conclusion

From the experiment and its analysis, we observe two effects of nitrogen on the electrical conductivity:

  • (1)

    If the nitrogen content in the films is low, the valence electrons of nitrogen as the impurity center can be saturated easily by H, the doping effect is not obvious; after suitable annealing, the N–H bond can be decomposed, which activates the impurity center, then the electrical conductivity increases.

  • (2)

    If the nitrogen content in the films is high, there will appear nonconductive carbon

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