Recently, field emission devices are attractive for replacing traditional cathode ray tubes in displays. There are two main approaches to fabricate field emission devices. One is carbon nano-tube (CNT) and the other is silicon microtip or nanotip structures [3.9]-[3.11]. There are two advantages for silicon microtips or nanotip structures to serve as field emitter. First, they are compatible to mature IC technology, so the process is easily controlled and the yield is high. Second, the silicon tip is rigid.
The sharp silicon microtip and nanotip structures have been attention for field emission devices because such structures improve field enhancement which provides high field emission current. The polycrystalline nanowires with sharp corners in our work are suitable for field emission devices. The process sequence is described below.
At First, a 300 nm SiO2 served as the sacrificial layer was deposited on the oxidized wafer by the low-pressure chemical vapor deposition furnace (LPCVD) at 700 ℃.
The sacrificial SiO2 layer was patterned as several strips by standard optical lithography and then etched anisotropically with 100 nm in-depth by the reactive ion etching (RIE) to form the steps. Next, a 100 nm a-Si layer was conformally deposited on sacrificed layer for active layer by LPCVD at 550 ℃. After that, the active region was patterned only on the source, drain and the end of strips by transformer-coupled plasma reactive ion etching (TCP-RIE). Due to the steps of strips, the spacers remained along the sidewalls of the strips after etching. A phosphorous ion implantation was performed with a dosage of 5×1015 cm-2 and energy of 30 keV. The source/drain activation was executed by capping a 10nm SiO2 at 700 ℃. After that, the silicon tips were formed by etching 50nm in-depth sacrificial SiO2 layer with 3:50 diluted HF. Figure 3-24 shows the cross-section schematic view of the silicon nanotips, while the layout is displayed in Fig. 3-25. The emission area is 0.042cm2. The related optical microscope images are shown in Figs. 3-26 and 3-27. The top, tilted, and cross-section views SEM images of Si nanotips are exhibited in Figs. 3-28, 3-29, and 3-30, respectively. The emission current can be analyzed by Fowler-Nordheim (F-N) tunneling model which describe below.
Fowler and Nordheim derive the famous F-N equation as follow:
( ) exp[
2( ) / ]
Typically, the field emission current I is measured as a function of the applied voltage V. Substituting relationships of J = I/α and E = βV into Eq. (3-1), where α is the emitting area and β is the local field enhancement factor at the emitting surface, the following equation can be obtained]
from Eq. (3-3), the slope of a Fowler-Nordheim (F-N) plot is given by
)
The parameter β can be evaluated from the slope S of the measured F-N plot if the work function
φ
was knownThe electron field emission characteristics of Si nanotips were measured in a high vacuum environment under a pressure of 5×10-6 Torr. A glass substrate coated with indium tin oxide (ITO) and P22 phosphor (ZnS: Cu, Al) was used as the anode plate, and the gap between the cathode and the anode plate was set to be 160 μm as shown in Fig 3-31. The source voltage and total emission current from the cathode was measured by Keithley 237.
Figure 3-32 demonstrates field emission I-V plot and the F-N plot is shown in Fig. 3-33. The negative slope region indicates the F-N tunneling region. The maximum current density is 2.07×10-4 A/cm2 at 4.96 V/μm. The F-N tunneling began at 4.11V/μm. The luminescent image is demonstrated in Fig. 3-34.
3-7 Summary
In this chapter, more electrical characteristics are studied. The NH3 plasma passivation issue will be discussed at first. The GAA-MNC TFTs exhibit better NH3
plasma passivation efficiency than conventional TFTs. The mobility of GAA-MNC TFTs increases from 19 to 33 cm2/V-s; while the mobility of conventional TFTs increases from 22 to 26 cm2/V-s. Better NH3 plasma passivation efficiency in GAA-MNC TFTs can be contributed to multiple nanowire channels which increase the exposed surface to NH3 plasma. In the second part, the BSG-MNC TFTs, GAA-MNC and conventional TFTs are compared. As compared to the GAA-MNC TFTs, the sidewall-MNC TFTs indicated poor electrical characteristics. This result is also from the better gate controllability and three sharp corners in the GAA-MNC TFTs which provide more inversion carriers in on-state. The BSG-MNC TFTs show better subthreshold swing than conventional ones. This result is due to better gate controllability in non-planar surface of spacer sidewall. As compared to the conventional TFTs, the mobility of BSG-MNC TFTs is a little lower owing to the surface damage during RIE etching. The BSG-MNC TFTs with non-planar and nano-scale channels show better electrical characteristics than conventional planar TFTs. The GAA-MNC TFTs display better performance than BSG-MNC TFTs because the nanowire channel is surrounded by gate. In the third part, the short-channels effects and narrow-width effect of GAA-MNC and conventional TFTs would take into consideration. The GAA-MNC TFTs exhibit suppressed short-channel effects, such as lower DIBL, lower kink effect and non-significant threshold voltage roll-off. The improvement of short-channel effects is attributed to