Studies of phosphorus doped diamond-like carbon films
Introduction
DLC films have been studied as a possible material for use in cold cathode devices and field emission displays (FEDs) because of their low surface work function, low temperature deposition and compatibility with existing microelectronic processing technology [1]. Doping of DLC films with nitrogen has been suggested as a method of improving their electronic properties [2] allowing control of film conductivity and optimisation of field emission. Most previous studies have concentrated on N as a dopant [3], and films with ≤40% N content have been reported [4]. N-doping has been variously reported to improve electrical performance giving field emission thresholds as low as 0.5 V μm−1 [5] but, conversely, some reports show that N-doping can degrade field emission performance [6]. A possible alternative dopant to nitrogen is phosphorus. Additions of PH3 into the traditional CH4/H2 gas mixture [7] have been used to produce n-type chemical vapour deposited (CVD) diamond films [8] but, to date, there have been few reports of attempts to do the same with DLC films [9], [10], [11], [12], [13]. Veersamy et al. [10] incorporated up to 1% P into DLC (or tetrahedral amorphous carbon) films produced by a filtered cathodic arc method using a red phosphorus-doped carbon cathode, and found that P addition can reduce the resistivity of the films by 6–7 orders of magnitude, with no apparent change to the amorphous nature of the carbon films. Capacitively-coupled radio frequency (RF) plasma deposition has been used to produce P-doped DLC films [[9], [11]] using PH3 as a dopant gas giving films with P contents estimated to be ∼11 at.%. These films, too, showed increases in room temperature conductivity of nearly five orders of magnitude, with resistivities which were strongly dependent upon the deposition temperature. Golzan et al. [12] showed that when films were doped with ∼3% P, the dopant destabilised the tetrahedral network in favour of an sp2 bonded network.
In this work, we have investigated the effect of a wide range of P content on the growth rate and electronic properties of DLC films. In particular, we present the results of the first field emission studies from P-doped DLC films and their variation with P content.
Section snippets
Experimental
Film deposition was carried out in a 13.56 MHz capacitively-coupled RF parallel plate plasma reactor. The gas feedstock was a mixture of CH4 (maintained at a constant flow of 30 sccm) and PH3 (0 to 8 sccm) at a process pressure of 60 mTorr. 35–60 W of applied RF power was used to maintain a constant DC self-bias on the powered electrode of −150 V during deposition. The substrates were placed on this powered electrode. These substrates were 1 cm2 mirror polished single crystal Si (100) — or quartz when
Results and discussion
The P-doped DLC films were smooth on a nanometer scale, uniform in thickness and exhibited a variety of colours dependent on their thickness. Using a simple mechanical scratch test we found that, in general, the films with high P doping levels were softer than undoped films grown with pure CH4. Films with P:C ratios <0.05 could not be scratched with metal tweezers, whereas those with P:C>0.05 could be scratched easily.
The compositions of various DLC films grown with different PH3/CH4 gas mixing
Conclusions
The P:C films deposited in these experiments appear to fall into three different regimes, corresponding to different P content. The first regime is for films having a P:C ratio of <0.025, which behave like poor quality DLC films. The P does not seem to be incorporated in an electronically active form and in fact degrades the electrical properties, presumably by damaging some of the conducting pathways within the film. Compared to the undoped DLC films, these films have higher field emission
Acknowledgements
The authors would like to thank J. Hayes and Professor J.W. Steeds of the Bristol University Physics Department for Raman spectroscopy analysis, and J. Eccles of Millbrook Instruments for SIMS analysis. PWM also wishes to thank the Royal Society for funding and the award of a University Research Fellowship.
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