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 [1]. The first technological devices employing micron sized electron emitting structures are currently being commercialised [2,3]. Especially field emission flat panel displays (FED's) seem to be very promising of becoming a strong competitor against the LCD displays [4], due to low power consumption, high viewing angle of 160 degrees 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 10 Acm2 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 mAcm2 where observed for applied electric fields below 5 V/micron.
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. By means of energy resolved field emission measurements, we were able to determine the local electric fields present at the emission site and the emitter work function. In the case of CVD diamond and DLC field emitter this enabled us to relate the low field electron emission from these materials to local field enhancement. In the case of CVD diamond we determined an emitter work function in the range of 5.7 eV and local fields in the range of 2500 V/micron (for 1 nA emission current) at applied electric fields of below 10 V/micron. This means that the field emission originated from local field enhancement with enhancement factors of 250 and more. For DLC emitters we found work functions around 5 eV and again local fields around 2500 V/micron. We will compare the work function values determined by field emission spectroscopy with results of photoelectron emission experiments. We will point out the importance of nanoscaled structures for the field emission properties of the two materials.
Due to their geometrical structure carbon nanotubes can generate large field enhancement at their apexes and are therefore also very interesting for field emission cathodes. We have investigated the field emission properties of nanotube thin films grown onto silicon (100) by a plasma enhanced CVD process. By means of energy resolved field emission we determined for multiwalled (MWNT) and for singlewalled (SWNT) nanotubes, that the field emitted electrons originate from a continuos band of electron states at the Fermi energy. By field emission spectroscopy we could determine the work function of MWNT at their apex to be around 5+/-0.3 eV, which agrees with the average work function of 4.8 eV measured by photoelectron emission spectroscopy. For SWNT's we found a lower work function of 3.7+/-0.3 eV (at the tube apex). We will present a model to explain the work function differences for MWNT's and SWNT's.
References
[1] International Vacuum Microelectronics Conference IVMC, Williamsburg,
Virginia July (1988)
[2] PixTech Inc., Avenue Olivier Perroy, 13790 Rousset (France)
[3] Motorola Inc. Res. Ftal Panel Display Division, East Elliot Road,
Tempe Arizona 85284 (USA)
[4] A. Sobel, Scientific American, May 1998, 48 (1998)