Gate Leakage in Conventional and Insulated Gate Structure Triode

In a conventional triode, gate leakage current would increase as the gate voltage

increase. It would cause the anode current decreasing [Fig.3-3]. A measurement of gate voltage and current was established at anode voltage 450 V and gate voltage swept from 0 to 100 V. The result was revealed in Fig.3-7. The gate leakage current almost remains constant and is not a function of gate voltage. Comparison with the conventional structure, it not only avoid the short circuit problem between gate and emitters but also reduce the gate leakage current to improve the anode current performance.

3.3.3 Field Emission Current Stability

A emission current stability test was performed on the insulated gate structure CNT triode. An average emission current (I0) of 2.64 µA was established at the anode voltage of 450 V, gate voltage of 30 V and cathode voltage grounded. The emission current reliability over a short term period of 1500 seconds is shown in Fig.3-8. No obvious degradation of emission current was observed and the fluctuation was about ±5.5%.

3.4 Summary and conclusions

Based on the selective growth of CNTs via the MPECVD, the insulated gate structure was fabricated. The distance between polysilicon gate and the CNTs emitter was determined by the wet etching process. Thus, the interelectrode gap is easily formed in good uniformity and reproducibility with dimensions below 1 µm. The turn–on voltage of the fabricated device with interelectrode gap of 1.2 µm is 18 volt, and the emission current density is 0.1 mA/cm² at gate voltage 136 volt. The emission current fluctuation

is about ± 5.5% for 1500 seconds.

Chapter 4

Conclusions and Future Prospects

Conclusions

The modification of surface morphology of CNTs has been achieved by O2 + Ar plasma post treatments. The SEM micrographs revealed the surface distribution of CNTs after plasma post treatments. For the generated plasma power of 200W and 300W, parts of CNTs were shortened and the remaining CNTs protruded from the surface of CNTs films. When the generated plasma power increased to 400W, most of the CNTs were shortened and less protruding CNTs were observed. The field emission characteristics confirmed the improvement of field emission properties under suitable PPT conditions, the field emission current density increased to 2.38 mA/cm² at the electric field of 0.8 V/µm and the turn-on electric field decreased from 0.9 V/µm untreated to 0.19V/µm for PPT conditions of generated power of 300Wand etching time of 60s . The experimental results reveal that improved emission properties can be achieved by optimizing the density and the length variation of CNTs under proper plasma treatment conditions.

The center of packed CNTs were affected by weaker electric field distribution and the electron would be screened out. The increase of total edge length improve the field emission characteristics. It is the reason that nearly CNTs in the edge would be thought the efficient emitting area. In this method, we can lower the turn-on voltage without extra process like plasma post treatment. By proper configuration designs, the turn-on voltage and current density would be improved.

Based on the selective growth of CNTs via the MPECVD, the insulated gate structure was fabricated. The distance between polysilicon gate and the CNTs emitter was determined by the wet etching process. Thus, the interelectrode gap is easily formed in good uniformity and reproducibility with dimensions below 1 µm. The turn–on voltage of the fabricated device with interelectrode gap of 1.2 µm is 18 volt, and the emission current density is 0.1 mA/cm² at gate voltage 136 volt. The emission current fluctuation is about ± 5.5% for 1500 seconds.

Future Prospects

For the synthesize of carbon nanotubes for field emission devices, the further research topics are proposed as follows:

(1) Low temperature (below 450 ^oC) growth of CNTs.

(2) Pretreatment of the catalyst for reduced density growth of CNTs.

(3) Post treatment of CNTs such as rapid thermal treatment, plasma treatment or ion bombardment to reduce the density of CNTs.

(4) Surface coating with ultra-thin metals to enhance the field emission property of CNTs.

For the field emission property investigation of CNTs (1) The long-term reliability should be investigated.

(2) The field emission behavior in different ambient (e.g. different gas, different pressure

conditions) should be discussed.

For the CNT triodes, the further research topics are proposed as follows:

(1) The gate-to-emitter gap can be further reduced to lower the turn-on voltage.

(2) Optimal gate structure or insulated gate surface for the CNT triodes should be developed to reduce the gate current.

(3) The focus gate can be applied to the CNT triodes to improve the emission characteristics.

(4) To demonstrate a prototype of CNT FED.

Finally, for the applications of CNTs in vacuum microelectronics, the further research topics are proposed as follows:

(1) Fabrication of CNTs lateral field emission device for high frequency and high power circuit applications.

(2) Fabrication of vacuum sensors or gas sensors based on CNTs

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Chapter 2

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Chapter 3

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X1= 0 X2

Bottom of conduction band

Metal Vacuum

(a)

(b)

Fig. 1-1 Energy diagrams of vacuum-metal boundary: (a)without external electric field; and (b) with an external electric field.

(a)

(b)

Fig. 1-2 The schematic diagram of (a) conventional CRT, (b) FED.

(a)

(b)

Fig. 1-3 The SEM micrograph of (a)Spindt type triodes array, (b) Spindt type field emission triode

(a)

(b)

(c)

(d)

Fig. 1-4 The FED products based on Spindt type field emitters, (a) motorola 5.6”

color FED, (b) Pixtech 5.6” color FED,(c) Futaba 7” color FED and (d) Sony/Candescent 13.2” color FED.

(a)

(b)

Fig. 1-5 (a) Si tip formed by isotropic etching and (b) Si tip field emission triodes array formed by CMP.

(a) (b)

Fig. 1-6 (a) SEM image of CNT cathode from Samsung’s FED. (b) Demonstration of a 4.5-inch FED from Samsung. The emitting image of fully sealed SWNT-FED at color mode with red, green, and blue phosphor columns.

Table 1-1 Comparison between vacuum microelectronics and solid-state electronics.

Items Solid State

Microelectronics

Vacuum Microelectronics

Current Density 10⁴ – 10⁵ (A/cm²) similar

Turn-on Voltage 0.1 – 0.7 V 5 – 300 V

Structure solid/solid interface solid/vacuum interface

Electron Transport in solid in vacuum

Electron Velocity 3×10⁷ (cm/sec) 3×10¹⁰ (cm/sec) Flicker Noise due to interface due to emission

Thermal & Short Noise comparable comparable

Electron Energy < 0.3 eV a few to 1000 eV Cut-off Frequency < 20 GHz (Si) &

100 GHz (GaAs)

< 100 – 1000 GHz

Power small – medium medium – large

Radiation Hardness poor excellent

Temperature Effect -30 – 50 °C < 500 °C

Fabrication & Materials well established (Si) &

fairly well (GaAs)

not well established

Fig. 2-1 The relationship of current density and distances between emitters.

Fig. 2-2 Microwave Plasma Enhanced Chemical Vapor Deposition (MPECVD) system.

Gas inlet

Plasma

Sample Pyrometer

ASTeX-5000 Microwave power supply To

pump

Thermocouple

Substrate

Substrate

Substrate

Substrate

Fig. 2-3 Growth mechanism of carbon nanotubes by MPECVD.

(a)

(b)

(c)

Fig. 2-4 SEM micrographs of the tip- rowth mode of CNTs by MPECVD. g

(a)

(b)

(c)

(d)

(e)

(f)

Fig. 2-5 Procedures of CNTs treated with plasma post treatments.

Emission Current (Ia)

Fig. 2-6 High vacuum measurement system.

Applied Voltage (Va) Vacuum Chamber

Anode-to-Cathod Distance = 500 µm

Ground Plate

Anode Plate

Spacer

Si Wafer

(a)

(b)

Fig.2-7 (a)Top-view (b)Cross-section of patterned CNTs.

(a)

(b)

Fig. 2-8 (a)Vertically aligned and high density of nanotubes (b)Diameter distribution of carbon nanotubes.

(a)non-PPT

(b)200W

(c)300W

(d)400W

Fig. 2-9 Plasma post treatment (a)Non-PPT (b)200W,60s (c)300W,60s (d)400W,60s.

(a)

(b)

(c)

(d)

Fig. 2-10 Cross section (a)Non-PPT (b)200W,60s (c)300W,60s (d)400W,60s.

-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Fig. 2-11 (a)J-E curve (b)F-N plot of different treatment time.

(a)

(b)

(c)

Fig. 2-12 Plasma post treatment (a)300W,30s (b)300W,60s (c)300W,90s.

(a)

(b)

(c)

Fig. 2-13 Cross section (a)300W,30s (b)300W,60s (c)300W,90s.

-0.2 0.0 0.2 0.4 0.6 0.8 1.0

Fig.2-14 (a)J-E curve (b)F-N plot of different treatment time at 300W.

(a)

(b)

(c)

Fig.2-15 Plasma post treatment (a)200W,30s (b)200W,60s (c)200W,90s.

(a)

(b)

(c)

Fig.2-16 Plasma post treatment (a)400W,30s (b)400W,60s (c)400W,90s.

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

Fig. 2-17 J-E curve of all treatment conditions.

Fig. 2-18 Simulation of screening effect.

(a)1000 µm

(b)100 µm

(c) 10µm

Fig. 2-19 Pattern dimension (a)1000 µm (b)100 µm (c)10 µm.

0.0 0.2 0.4 0.6 0.8 1.0

Fig. 2-20 (a)J-E curve and (b)F-N plot of different pattern dimension.

Table 2-1 Parameters of different plasma post treatments.

30s 60s 90s

200W A B C

300W D E F

400W G H I

Table 2-2 Field emission characteristics of CNTs with different plasma post treatments.

Turn-on field (V/µm) Current Density at 0.8V/µm

Non-PPT 0.9 -

200W,30s 0.44 4.94mA/cm²

200W,60s 0.36 15mA/cm²

200W,90s 0.20 5.89mA/cm²

300W,30s 0.50 3.42mA/cm²

300W,60s 0.19 2.38mA/cm²

300W,90s 0.71 0.029mA/cm²

400W,30s 0.88 -

400W,60s 1.39 -

400W,90s 1.88 -

(a)

(b)

(c)

(d)

(e)

(f)

Fig. 3-1 Fabrication procedure of the carbon nanotubes insulated gate structure field emission device.

(a)

0 2 4 6 8 10

0.000 0.005 0.010 0.015 0.020 0.025

Gate Current (A)

Vg )

short circuit

(V

(b)

Fig.3-2 (a)SEM of conventional triode (b)short circuit problem between gate and emitters.

0 5 10 15 20 25 30

Fig. 3-3 Gate voltage versus (a)anode current (b)gate current in triode structure.

(a)

(b)

Fig. 3-4 SEM of insulated gate structure eld emission triode (a)top view (b)cross fi section.

-20 0 20 40 60 80 100 120 140 160

Fig. 3-5 (a) The field emission current versus gate voltage, the anode was set at 450 volt, (b) The F-N plot of the fabricated device, the linearity clarifies the

field emission phenomenon.

0 100 200 300 400 500 600 -0.2

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

Anode Current (uA)

Anode Voltage (V) Vg=10

Vg=20 Vg=30

Fig. 3-6 The field emission current versus anode voltage with different gate bias.

0 20 40 60 80 100 -0.000020

-0.000015 -0.000010 -0.000005 0.000000 0.000005 0.000010 0.000015 0.000020

Gate Current (A)

Gate Voltage (V)

Fig. 3-7 Gate voltage versus gate leakage current.

-200 0 200 400 600 800 1000 1200 1400 1600 0.000003

0.000004 0.000005 0.000006 0.000007 0.000008

Anode Current (uA/cm2 )

Time (s)

Fig. 3-8 Emission current stability of the insulated gate structure CNTs triode over a period of 1500 seconds.

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