Diamond-like carbon (DLC) films have been grown on silicon substrates by pulsed Nd-YAG laser ablation using an oscillating graphite target. Compared with unpatterned flat DLC films, excellent field emission characteristics were obtained in micro-patterned DLC films. Moreover columnar DLC structure showed higher area density of emission sites and better field emission uniformity. These findings indicate that the patterned and particle-filtered DLC film has high potential as a cold cathode electron source for field emission display. Main reason for these improvements seems to be associated with the increaes of emission sites due to the increase of area density of the edge-shaped surface and isolation among the DLC columns.
In chapter 3. field emission characteristics and electrical resistivity of the DLC films after annealing at high temperature were discussed together with the analyses of Raman spectroscopy.
The DLC films annealed at high temperature showed almost no emission characteristics and the electrical resistivity of the films showed almost metal like behavior. Moreover, the DLC films deposited on the ＄LN_2＄-cooled substrates which have higher ＄sp^3＄ fraction showed slightly worse emission characteristics than those deposited on the room temperature substrates. These results suggest that existence of ＄sp^3＄ bond is necessary for the emission characteristics but fraction of ＄sp^3＄ bond in the film is not linearly proportional to the emission characteristics. Also, this implies that not only ＄sp^3＄ bond fraction but also distribution of ＄sp^2＄ bond cluster in the films is an important parameter for emission characteristics.
In chapter 4, effects of nitrogen doping on field emission characteristics of patterned Diamond-like Carbon (DLC) films were studied. Nitrogen-doping in DLC film was carried out by introducing ＄N_2＄ gas into the vacuum chamber during deposition. Higher emission current density of 0.3∼0.4 mA/㎠ at the applied field of 8.5 MV/m was observed for the films with 6 at % nitrogen compared to that of 0.1∼0.2mA/㎠ in the undoped films but the emission current density decreased with further increase of nitrogen content up to 11 at % range.
At 11 at % nitrogen content, emission current was not detected or emission current reduced to zero with increasing emission time. As nitrogen content increases further, substantial emission current was detected again.
Electrical resistivity and optical band gap were measured for DLC films with wide range of nitrogen content. The resistivity and optical band gap showed rapid decrease with increasing nitrogen content and a minimum value around 11 at % nitrogen. With further increase of nitrogen content, electrical resistivity and optical band gap increased. Lower electrical resistivity, small optical band gap and no emission current at 11 at % N film strongly suggest that fraction of ＄sp^2＄ bond region of graphitization is increased in this range.
In order to understand change of bonding characteristics with varying nitrogen content in DLC films, such analysis tools as Raman spectroscopy, x-ray photoemission spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR), electron energy loss spectroscopy (EELS) and high resolution transmission electron microscopy (HRTEM) were used.
At 6 at % nitrogen, electrical resistivity was greatly reduced but EELS results showed ＄sp^3＄/(＄sp^2＄+＄sp^3＄) ratio of about 47 % while that of undoped specimen showed 52 %, Moreover, carrier concentration was increased andXPS results also support this phenomenon. These results means some of nitrogen are simply substitute carbon atoms in ＄sp^3＄ bonds and rest of nitrogen form ＄sp^2＄ -type bonds with carbon as analyzed by FT-IR. The highest emission current density was observed in this range.
As nitrogen content increases to 11 at %, volume fraction of ＄sp^2＄ bond rapidly increases and ＄sp^3＄/(＄sp^2＄+＄sp^3＄) ration becomes 20 % range. In this nitrogen content range, ＄sp^2＄-type C=N bonds formation were proved by FT-IR and XPS results. The change of electrical resistivity and optical band gap are also consistent with this interpretation.
When nitrogen content increases beyond 11 at % range, electrical resistivity and optical band increases again and carrier mobility drops. In this range, ＄sp^3＄/(＄sp^2＄+＄sp^3＄) ration increases again becomes 37 % at 24 at % N. The tendency of C-N ＄sp^3＄ bond formation was observed by deconvolted XPS spectra and EELS analyses. At 24 at % N specimen, we could observe substantial emission behavior again. Also, more enhanced C-N ＄sp^3＄ bond formation at a given nitrogen content was observed by 550℃ annealing.
It is concluded that high ＄sp^3＄ bond fraction and electron-conducting path are needed to obtain good emission characteristics. An adequate nitrogen doping is very effective to control ＄sp^3＄ bond fraction and electron conduction in the DLC films without sacrifice of emission stability.
To analyze the degradation of field emission characteristics of DLC films, edge-shaped DLC emitters were fabricated by lithographic technique. In the case of 11 at % nitrogen-doped DLC, the degradation of emission current was much faster than the case of 6 at % nitrogen-doped DLC. Eletrical resistance of 11 at % nitrogen-doped DLC was drastically decreased during field emission test. The graphitization may easily occur in DLC with lower fraction of ＄sp^3＄ sites and easily degrade field emission characteristics.
Consequently, micro-patterning improves electrical field-gain and optimum nitrogen doping in DLC films greatly improves field emission characteristics.
