The technological advances in the field of microelectronic fabrication techniques have triggered great interest in vacuum microelectronics. In contrast to solid state microelectronics, which refers to scattering dominated electron transport in semiconducting solids, vacuum microelectronics relies on the scattering free, ballistic motion of electrons in vacuum. Since the first international conference on vacuum microelectronics the progress in this field has increased substantially . The first technological devices employing micron sized electron emitting structures are currently being commercialized [2,3]. Especially field emission flat panel displays (FED's) seem to be very promising of becoming a strong competitor against the LCD displays , due to low power consumption, high viewing angle of 160° and high brilliance.
One of the key processes in vacuum microelectronics is the generation of free electrons in vacuum. Most commonly field emission electron sources are considered the best choice because of their high emission current density of up to 107 Acm-2 and relatively low energy spread of the emitted electrons of less than 500 meV. Carbon thin films have proven to be very interesting for being used as electron emitting cathodes. For a wide range of different carbon materials, such as diamondlike carbon (DLC), chemical vapour deposited (CVD) diamond, nanocoralline mostly sp2-bonded carbon and nanotube thin films, field emission current densities of up to 1 mAcm-2 where observed for applied electric fields below 5 Vmm-1.
We have investigated the field emission properties of different carbon thin films, such as activated and non activated CVD diamond films, filtered arc discharge deposited DLC films and nanotube thin films. As can be seen in Fig. 1 for a nanotube thin film emitter emission currents in the mA range are observed at applied fields of 3 Vmm-1.
Fig. 1. I-V characteristic of a nanotube thin film emitter, following the Fowler-Nordheim relation for many orders of magnitude in emission current
Fig. 2. Emission site density at different applied fields of a nanotube thin film emitter made visible by a phosphorus screen.
Similar results can be obtained on CVD diamond and DLC thin film emitters. In the case of a flat metal surface, with a typical work function of 5 eV fields in the order of 4000 Vmm-1 are required to get the same emission currents. Where as the field emission mechanism for nanotubes seems to be governed by their field enhancing properties due to the high aspect ration of the tubes, the mechanism for DLC and CVD diamond films is far less clear and under very intense investigation. In the case of diamond field emitters the field emission is often correlated with the negative electron affinity (NEA). Yet we observe a strong influence of nanoscaled structures on the diamond and DLC films on the field emission properties.
Fig. 3. Upper part: SEM image of PE-CVD (Plasma Enhanced-CVD) grown nanotube exhibiting good field emission properties. Lower part: SEM image of a good emitting CVD diamond film exhibiting a nanocrystalline structure.
Using a combined method of measuring the field emitted electron energy distribution and the I-V characteristic of an emitter simultaneously, we were able to determine independently the work function and field enhancement factors of different field emitters. This enabled us to get an estimation of the local electric fields present at the emission site. In all cases we have investigated, we found that the observed low field electron emission is due to local field enhancement on the sample surface. In the case of multiwalled nanotube emitters (MWNT's) we found work function values around 5 eV and field enhancement factors ranging around 500. In the case of CVD diamond films we have observed work functions around 5.6 eV and field enhancement factors ranging from 250 to 1800. From the FEED spectrum we could extract the occupied density of states of the emission site. We could observe, that the field emission in the case of nanotube as well as diamond emitters originates from continuum states near the Fermi energy. In the case of singlewalled nanotubes we could observe a significantly lower work function of 3.7 eV, compared to the MWNT's. We attribute this effect to an electrostatic effect lowering the work function in the case of small spherical objects (R <10 nm).
In order to get more insight into the emission mechanism we have performed simultaneous field- and ultra violet photoelectron emission spectroscopy (UPS). This method allows us to compare the work function of the emitter, with the average work function of the sample.
Fig. 4a shows the electron spectrum of a nanotube thin film emitter irradiated with He I (21.2 eV) UV light and an applied field of 4.6 Vmm-1 (the sample bias of 230 V was subtracted from the energy scale). One can see two features in the spectrum, firstly a sharp peak at 0 eV showing the field emitted electrons and secondly the broad emission spectrum of the excited electrons above 5 eV. Fig. 4b shows the detail spectrum of the field emitted electrons, which can be fitted in excellent agreement using classical tunnelling theory (solid line). From the shape of the field emission spectrum and the I-V characteristics of this emitter we determined a work function of 5±0.3 eV and field enhancement factor of 765. This means that the local field present at the emission site is around 3500 Vmm-1. In Fig. 4a the dotted line denotes the position of the vacuum level at the emission site of 5 eV as determined by the field emission spectrum. One can see that this level corresponds very nicely with the low energy cut-off of the photoemission spectrum indicating the average work function of the nanotube thin film.
Fig. 4. Simultaneous field-and photoelectron emission spectrum (a). Detail spectrum of the field emitted electron energy distribution (b).
Our results indicate that the low field electron emission in the case of nanotube, diamond and DLC thin film emitters has the same origin of field enhancement. We will discuss different models explaining this field enhancement properties for the different materials. We will also address the possibilities and difficulties regarding the realization of patterned emission structures.
 International Vacuum Microelectronics Conference IVMC,
Williamsburg, Virginia July (1988)