Comparison of Field Emission Behaviors of Graphite, Vitreous Carbon and Diamond Powders S. H. Lee, K. R. Lee, K. Y. Eun Thin Film Technology Research Center, Korea Institute of Science and Technology P. O. Box 131, Cheongryang, Seoul, , Korea

Field Emission Devices  Applications  Vacuum microelectronics (Display, Microwave devices, etc)  Problems of cathode materials  Mo / Si Difficult to process High cost to make devices Change of effective work function by residual gas Decrease of life time by back sputtering  Others material for cold cathode  Diamond, Diamond-like carbon, Carbon Nano-tube, etc

Carbon Base Cathode Materials  Diamond  sp 3 bonds  Diamond-Like Carbon  Amorphous structure of sp 3, sp 2, and sp 1 bonds  Graphite  sp 2 bonds  Semi-metal (Free carrier : cm -3 )  High electrically conductive material  Carbon Nano-tube  Structure like rolled graphite sheet  Very high aspect ratio  Problem of high temperature growth  Vitreous Carbon  Amorphous structure of sp 2 bonds.  High electrically conductive material

Problems in Carbon Base Cathode  Limited understanding on field emission mechanism  Uniformity and stability of the emission still remain as the prerequisites for the applications.  It is difficult to compare field emission behaviors between carbon materials of various doping levels It is necessary that the difference of bonding structure is investigated

Purposes of the Present Work  Comparison of the field emission behaviors between Diamond, Vitreous Carbon and Graphite Powders.  Effect of sp 2 and sp 3 bonding  Effect of crystal and amorphous structure  Observation of emission light image to identigy the emission sites.

Raman Spectrum of Used Powders Graphite Diamond Vitreous Carbon Diamond : [Ω·cm] Graphite : < [Ω·cm] Vitreous Carbon : <10 -3 [Ω·cm] Electrical conductivity

Sample Preparation glass Screen printing method Mo layer glass Silver paste mixed with the carbon powder glass Curing 150  C for 60 min & 450  C for 60 min Sputtering

Surface Morphology 10  m 20  m 10  m 20  m 10  m 20  m GraphiteDiamondVitreous Carbon Grain size : ~ 20 µm Roughness : ~ 5 µm

Current-Voltage Measurement Keithley 6517 Electrometer SRS PS350 Power Supply x y z e Anode Cathode CCD Viewing System Computer Interface 3-D Manipulator Viewing System SRS PS350 Power Supply Keithley 6517 Electrometer Phosphor coated ITO-glass Cathode Spacer CCD e Visible Light 998 Ω  Annealing  Over 6 hours by halogen lamp in vacuum (about 150 ºC)  Out-gassing  at 1×10 -7 Torr over 2 hours  Distance Control  50 µm ~ 200 µm  Spacer  Alumina (Al 2 O 3 )  Thickness : 478 µm ⓔ

Current-Voltage Behavior Graphite Vitreous Carbon Diamond Typical current-voltage characters of field emission The lowest on-set electric field and the most stable emission behavior in graphite.

Results and Discussions  Effect of sp 2 and sp 3 bonding  We observed the lower electric field and higher current density in graphite than those of diamond or vitreous carbon.  For good field emission, it is necessary that the cathode materials has much content of sp 2 bonding.  The sp 2 bonding works as the electrical conductive channel and the source of electron supply.  Effect of crystal and amorphous structure  The crystal structure is favorable in the view point of stable field emission from the carbon materials.

Electron Emission Image of Graphite Init. Condition At 1000 V At 1200 V At 1500 V On-set electric field : about 2 ~ 3 V/ ㎛ The localized light emission was observed.

Emission Image for Fluctuation of Graphite At 0 sec At 1000 sec At 2000 sec At 3600 sec At 0 sec At 1000 sec At 2000 sec At 3600 sec The local emission sites are not static. The total current decreased and stabilized after long emission.

Results and Discussions  The light emission as applied voltage  We observed the light emitted locally and occasionally disappeared.  As the voltage increased, the light emitted more brightly but sometimes the emission site changed.  The electrically conductive channel forms locally and is varied depending on the applied voltage.  Possibility of the full area light emission  The full area light emission can be occurred by adding an insulating layer between the electrode and the cathode materials. The insulating layer seems to reduce the difference in electric conductivity in the cathode materials. S. H. Ahn et al., J. Kor. Vac. Soc., 9 (2000) 122.

Conclusions  The field emission behaviors of the carbon base cathode materials strongly depend on the electrical conductivity of the materials.  The structural requirements for a good cold cathode of carbon materials  Well developed crystal structure for emission stability  High content of sp 2 hybridization bond for the ease of electron supply.  Emission stability can be reduced by controlling the electrical contact between electrode and the cathode. However, localized emission still remains as a limitation of the carbon cathode materials.