Characteristics of Surface-Emitting Cold Cathode Based on Porous Polysilicon
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Mat. Res. Soc. Symp. Proc. Vol. 509 0 1998 Materials Research Society
Experimental Details The samples under investigation were prepared by the anodisation of polysilicon layer grown on (100) n-type 1DMcm silicon wafer. Non-doped polysilicon layers were grown by LPCVD to a thickness of 1.5 pam. Subsequently, polysilicon layers were anodised in a solution of HF(50%):ethanol=l:l at a current density of 1OmA/cm2 for 30 seconds under the illumination by a 500W tungsten lamp from a distance of 20 cm. The final thickness of PPS layer was 1 pm. After anodisation, PPS layers were rapidly oxidized by rapid thermal oxidation (RTO) furnace at a temperature of 700 9C for 60 minutes. Subsequently, a semitransparent 10 nm of Au film was deposited onto the PPS layer. The area of the Au film was about 20 mm2 . An ohmic electrode was fabricated at the back of the silicon wafer. The prepared samples were mounted into the ultra high vacuum system (about 10-5 Pa) for the measurement of the emission characteristics at room temperature. Figure 1 shows the schematic of the measurement system and circuits. A dc bias voltage Vp, was applied to the top Au electrode with respect to the back electrode. The anode electrode was set above the Au electrode and kept at a positive potential (V,) of IOOV to collect the emitted electrons. The distance between the electron emitting surface of the diode and the collection anode was 10 mm. The diode current (Ip,) and the corresponding emission current (I.) were measured as a function of VPS. Anode Electrode
1&
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ro emitting elect Au electrode •
_
___
_
PPS layer
V ps
polysilicon layer back electrode--
n-silicon substrate ,.-
Figure 1: Schematic diagram of the measurement system of emission current. This system was installed into the high vacuum (about 10- Pa) chamber. Vp, was applied to the PPS diode and V, was applied to the anode electrode. I. is emission current and Ip,is the diode current. Results and discussion Figure 2 shows the current-voltage relationship of the fabricated diode. In the reverse bias region, there was no emission current. When positive bias was applied to the diode, emission current was observed at the diode voltage (Vp,) of 8 V. Emission current was rapidly increased as Vp, was
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increased. Emission efficiency as IA~p was about 0.01% at VP, - 15V. Figure 3 shows the Fowler-Nordheim (FN) plot of this diode. It is evident that the FN plot for this sample follows a linear behaviour. This indicates that field-induced tunnelling process occurs during the electrons emission from PPS diode. The experimental results shown above confirm the mechanism for electron emission from the diode is due to tunnelling effect. The porous layer is composed of a number of silicon nanocrystallites. It is assumed that these nanocrystallites are surrounded by a thin SiO 2 layer which is formed during the photo-anodisation and subsequent RTO process. When a bias voltage is applied to the diode, major potential drop is produced in the PPS layer, especially in the region near the PPS
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