that exist within a material. XPS spectra are obtained by irradiating a material with a beam of X-rays while simultaneously measuring the kinetic energyand number of electrons that escape from the top 1 to 10 nm of the material being analyzed in ultra high vacuum (UHV) conditions. XPS is a surface chemical analysis technique that can be used to analyze the surface chemistry of a material in its "as received" state, or after some treatment, for example: fracturing, cutting or scraping in air or UHV to expose the bulk chemistry, ion beam etching to clean off some of the surface contamination, exposure to heat to study the changes due to heating, exposure to reactive gases or solutions, exposure to ion beam implant, exposure to ultraviolet light.
Fig. 2.6 is schematic diagram of XPS.
2-2.5 Scanning electron microscopy (SEM)
The principle of SEM imaging is due to using the electron beam emitted by heating tungsten wire. Then, the electron beam focused together to form a small electron beam by the anode of the accelerating voltage after two to three electromagnetic lens composed of electro-optical system. Finally, the beam focused on the specimen for two-dimensional scanning. When the electron beam scans the specimen, high-energy electron beam interact with matter.
Electronic elastic collision and inelastic collision effects result a secondary electron and backscatter electron etc. Signal amplified after the specimen's surface
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morphology, will be able to simultaneously screen imaging. Fig. 2.7 is schematic diagram of SEM.
2-3 Methods of device parameters extraction
In this section, the extractions of the device parameters are discussed in details.
Turn on voltage (Von), threshold voltage(Vth),the field effect mobility, the sub-threshold swing (S.S), and the on/off current ratio (Ion/Ioff) are extracted and assessed, respectively.
2-3.1 Turn-on voltage (V
on)
Von can directly characterizes the gate voltage required to fully “turn on” the transistor in a switching application. Turn-on voltage (Von) is identified as the gate voltage at which the drain current begins to increase in a transfer curve.
2-3.2 Threshold voltage (V
th)
Threshold voltage is related to the operation voltage and power consumptions of TFTs. We extract the threshold voltage from equation (2.1), the intercept point of the square-root of drain current versus voltage when devices operate in saturation mode √ID = √2LWμCOX(VG− VTH) (2.1)
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2-3.3 Mobility
Mobility is a parameter of how fast carriers can move in material. A higher magnitude of mobility allows for a faster switching time, i.e., the time it takes for the device switching from the off state to on state. In the off state, few current flows through the device. In the on state, large amount of currents flow through the device.
A large mobility means the device can conduct more current. The mobility in this study was extracted from the saturation region. The device was operated at drain-voltage of 20V, since the threshold voltage was much lower than 20V. The
saturation mobility is determined from the transconductance, define by gm= [∂√I∂VD
G]
VD=const (2.2) The drift component of drain current is
ID =12μCoxWL(VGS− VTH)2 (2.3) When the mobility is determined, the transconductance is usually taken to be
gm= √WμC2Lox (2.4) When this expression is solved for the mobility, it is known as the saturation mobility μsat = [W2gm2
LCox]
saturation
(2.5)
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2-3.4 I
on/I
offcurrent ratio
The Ion/Ioff (on/off) ratio represents large turn-on current and small off current. It is an indicator of how well a device will work as a switch. A large on/off current ration means there are enough turn-on current to drive the pixel and low off current to maintain in low consumption.
2-3.5 Sub-threshold swing (S.S)
It is a measurement of how rapidly the device switches from off state to on state.
Moreover, the sub-threshold swing also represents the interface quality and the defect density.
S = [∂(logI∂VG
D)]
VD=const (2.6)
If we want to have a better performance TFTs, we need to lower the sub-threshold swing.
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Figure in Chapter 2
Fig. 2.1 The process flow of the DG- (Dual-gate with nano-dot structure by Ar plasma treatment) a-IGZO TFT.
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Fig. 2.2 The process flow of the standard DG- (Dual-gate without nano-dot structure by Ar plasma treatment) a-IGZO TFT.
Fig. 2.3 Schematic device structures. (a) and (b)is dual-gate (DG) of a-IGZO TFTs without and with NDD by Ar plasma treatment.
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Fig. 2.4 RF-power sputtering deposition system.
Fig. 2.5 The molecular structure of PVP and PMF
Ground Vacuum Sputtering gas inlet
Heater Electrode
Wafer
Argon plasma Electrode/Target Match
network
RF Generator
RF Input
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Fig. 2.6 Schematic diagram of XPS system.
Fig. 2.7 Schematic diagram of SEM system.
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Chapter 3
Results and Discussions
With a high mobility (>10 cm2/Vs) and a low threshold voltage (< 5 V) under low temperature process, amorphous In-Ga-Zn-O thin-film transistors (a-IGZO TFTs) draw a lot of attentions. However, when a-IGZO TFTs are developed for a low-power high-frequency circuit, improved electron mobility and a low parasitic capacitance are required. In this section, we measure the capacitance of PVP and SiNx as top gate insulator and bottom gate insulator, discuss the influence of a-IGZO film under Ar plasma treatment and the influence of drain current enhancement in dual gate measurement . In this study, we used a novel, simple process to improve the mobility of the a-IGZO TFTs to obtain a high carrier mobility of a-IGZO TFTs, and proposed a efficient manufacturing method, called “nano-dot doping”. And Fig. 3.1 is Schematic device structures. (a) is standard(STD) dual gate a-IGZO TFT and (b) is dual gate (DG) of a-IGZO TFTs with NDD by Ar plasma treatment.
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