Chapter 1
Introduction
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1.1 General Background
Thin-film transistors (TFTs) are fundamental building blocks and play a domination role for state-of-the-art microelectronics, such as flat-panel displays and system-on-glass.
Furthermore, the fabrication of low-temperature TFTs will allow flexible large-area electronic devices to be developed. As shown in Fig.1-1,these devices are flexible, lightweight, shock resistant and potentially affordable—properties that are necessary for large, economic, high-resolution displays, wearable computers and paper displays [1]. Research on amorphous semiconductors started in 1950s to seek appropriate materials. And hydrogenated amorphous silicon (a-Si:H) is the first material which can control carrier concentration by impurity dop-ing as in crystalline. Nowadays, the a-Si:H which is used widely in active-matrix flat-panel display (AMFPD) circuits. However, the growth of high-end commercial market with high-resolution such as ultra definition (2000x4000), high frame rate(>120Hz), and size larger than 50 in. necessitates the development of high-performance transistors(with field-effect mobility >3cm2 /V.s).[2]
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Hydrogeneated amorphous silicon thin film transistor (a-Si:H TFT) is not an appropriate device for next generation displays on account of its low mobility (< 1 cm V-1 s-1 ), photo sen-sitivity (low band gap about 1.7ev) and rather high deposition temperature(~400℃). On the
other hand, polycrystalline silicon (poly-Si) TFT which has high mobility (> 100 cm V-1 s-1 ) suffers from a problem with the variation of electrical properties due to grain ry[ 3-4].The mobility of a-Si:H (~1 cm2(Vs)-1) is much smaller than that of single crystalline Si (~200 cm2(Vs)-1) due to the intrinsic chemical bonding nature. The average carrier trans-portation paths in covalent semiconductors, such as a-Si:H, consist of strongly directive sp3 orbital. The bond angle fluctuation significantly alters the electronic levels, causing high density of deep tail-states, as shown in Fig.1-2 [ 5]. In contrast, degenerate band conduction
and large mobility (>10cm2/V.s) are possible in amorphous oxide semiconductors (AOSs) containing post-transition-metal cations. These features are completely different from those of the covalent semiconductors. Figure 1-2 illustrates the carrier transport paths in AOSs. The bottom of the conduction band in the oxide semiconductors that has high ionicity is primarily composed of spatially spread metal ns orbitals with isotropic shape (here n is the principal quantum number), and direct overlap among the neighbouring metal ns orbitals is possible.
The magnitude of this overlap is insensitive to distorted metal–oxygen–metal (M–O–M) chemical bonds that intrinsically exist in amorphous materials. Therefore, AOSs exhibit Hall-effect mobilities similar to those of the corresponding crystalline phase, even if they are
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formed at room temperature. These carrier transport properties are unique to oxide semicon-ductors, and are not seen in covalent amorphous semiconductors such as a-Si:H[1].
1.2 Amorphous Oxide Semiconduvtors
1.2.1 Introduction of Amorphous Oxide Semiconductors
Amorphous oxide semiconductors (AOSs) are expected as new channel materials in thin-film transistors (TFTs) for large-area and/or flexible flat-panel displays and other
gi-ant-microelectronics devices. So far, many prototype displays have been demonstrated in these four years since the first report of AOS TFT. The most prominent feature of AOS TFTs
is that they operate with good performances even if they are fabricated at low temperatures without a defect passivation treatment. The TFT mobilities exceed 10 cm2/V.s, which are more than ten times larger than those of conventional amorphous semiconductor devices.[6]
1.2.2 Material properties of zinc oxide (ZnO)
Thin film transistors (TFTs) made with amorphous and poly silicon have become in the last 10
years the key point of the electronic flat panel display industry, just as silicon chips were ear-lier called the staple of the electronic computer revolution.[7] Very recently ZnO-based thin-film transistors (TFTs) have been reported to be drawing interest from researchers. It is because those TFTs may achieve the following important goals: depositing channel layer on a
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flexible substrate through low temperature processes, realizing transparent TFTs and
achiev-ing extra functions such as photodetections usachiev-ing ZnO channel. Moreover, since deposited ZnO usually maintains a crystalline phase, although the deposition process is carried out even
at room temperature, ZnO-based TFT probably exceeds a conventional amorphous Si TFT in terms of field effect mobility.[8-9] The combination of transparency, high mobility, and room
temperature processing makes the ZnO-TFT a very promising low-cost optoelectronic device for the next generation of invisible and flexible electronics, such as switching for addressing AMOLED.[10]
1.3 Motivation
Transparent conducting oxides (TCOs) have both good optical transparency (in thin films) and high electronic conductivity ( S/cm).[11] Nevertheless, it is noted that the ZnO thin
film can be very easily crystallized during the deposition process, leading to the formation of grain boundary defects. In 2004, Hosono et al explored a new class of amorphous oxide sem-iconductors based on InGaZnO (indium gallium zinc oxide, IGZO) and demonstrated that high-performance transistors (μFE ∼ 8.3 cm2 (Vs)−1) can be fabricated using the IGZO thin film deposited even at room temperature. And now, a-IGZO is one of the most popular mate-rials on active layer for flat panel display. [12] The mobility of the IGZO TFT is around 10cm2v-1s-1, further improvement of the mobility is required for super high-definition display
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or circuit integration in the displays.[13] On the purpose of mobility enhancement and rare element replacement,we investigatedthe InSnZnO as one candidate for achieving high μ ox-ide TFTs and with no rare element (Ga). Amorphous InSnZnO (a-IZTO) is a promising can-didate for substituting the indium tin oxide (ITO) due to its high optical transparency, good conductivity and high work function.[27] In former studies, a-IZTO films were generally an-alyzed with different deposition and annealing condition, few research has concerned the in-fluence of oxygen incorporation on a-IZTO thin film. In our experiment, we investigated the effect of oxygen incorporation in a-IZTO thin film, changing the oxygen flow rate in order to improve the thin film quality.
1.4 Thesis Organization
This thesis is divided into four chapters. In Chapter 2, the measurement and extraction of electrical parameters are introduced. The fabrication flow of a-IZTO thin film transistors is also described. In Chapter 3, the electrical and material properties of a-IZTO films are pre-sented. First, the oxygen effects and different thickness on a-IZTO TFTs is discussed. Second, Reliability analysis of a-IZTO TFTs with different oxygen flow rate are also presented. Third, photosensitivity characteristic of a-IZTO TFTs with different oxygen flow rate is investigated.
Finally, conclusions and future works are summarized in Chapter 4.
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K. Nomura, H. Ohta, A. Takaji, T. Kamlya, M. Hirano, H. Hosono, Nature 432 488 (2004)
Fig.1- 1 Flexible transparent TFTs (TIFTs) (a) A photograph of the flexible TIFT sheet bent at R = 30 mm.
The TIFT sheet is fully transparent in the visible light range (b) a photograph of the flexible TIFT sheet.
The transparent TFT devices are made visible by adjusting the angle of the illumination
K. Nomura, H. Ohta, A. Takaji, T. Kamlya, M. Hirano, H. Hosono, Nature 432 488 (2004) Fig.1- 2 Schematic orbital drawing of electron pathway (conduction band bottom) in conventional
silicon-base semiconductor and ionic oxide semiconductor.
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Chapter 2
Experimental Procedures
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2.1 Fabrication flow of thin film transistors
Fig.2-1 is the flow path in my experiment. We did both material and electric analysis to analyze our a-IZTO TFTs. Because of the innovation of sputter, the active layer with several channel thickness were formed by RF reactive sputter at room temperature in a mixture of ar-gon(Ar) and oxygen (O2) with target composed of In2O3,ZnO,SnO2 . And then, after the film deposition, we use the fixed annealing condition to define our device in order to figure out the effect of the difference of oxygen flow rate to a-IZTO TFTs. In our present work, the fabrica-tion of bottom gate inverted staggered thin-film transistors using an n-type a-IZTO film as active layer will be described. Our IZTO-based TFT, as shown schematically illustrated cross-sectional and planed views in Fig.2-2 were fabricated on SiO
2/n-Si substrates. Thermal oxide was chosen as the gate insulator. After patterning the a-IZTO films which are deposited by RF sputtering system at room temperature, source and drain electrodes were deposited and defined by a shadow mask. The film thickness was measured by AFM. The a-IZTO film was controlled at 10nm, 30nm, 50nm, ect and the electrodes of source/drain regions were Indium
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Tin Oxide (ITO) with 50nm thick deposited upside of the a-IZTO channel by RF sputtering
system. The width and the length range from 200um to 1000um, and the width/length ratio of our TFTs was about 0.2 to 5. Prior to the deposition of a-AZTO films, 100-nm-thick SiO
2 lay-ers were thermally grown by 650℃ in Horizontal Furnace on n-type Si (100) substrates. This
process was done in Class 10 in National Nano Device Laboratories. The active layer-a-IZTO film were deposited by sputtering IZTO target (In2O3: ZnO: SnO2 = 2:1:4 mol%) in RF power sputtering system at room temperature. Then, devices were thermally-annealed by a tube fur-nace in the nitrogen atmosphere for defect elimination. On the other hand, another part of a-IZTO film were deposited straight on silicon wafer in order to get the material analysis.
2.2 Electrical Measurement and Parameter Extraction Method
The devices electrical properties were measured by Keithley 4200 IV analyzer in a light-isolated probe station at room temperature. In IDS-VGS measurement, the typical drain-to-source bias was swept from VGS = -30V to VGS = 40V. In IDS-VDS measurement, the typical drain-to-source bias was swept from VDS = 0 V to VDS = 40 V.
In this section, we describe the methods of typical parameters extraction such as threshold voltage (Vth), subthreshold swing (SS), On/Off current ratio (Ion/Ioff) and field effect mobility
(μFE) from device characteristics.
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2.2.1 Determination of the Vth
Threshold voltage (Vth) was defined from the gate to source voltage at which carrier conduction happens in TFT channel. Vth is related to the gate insulator thickness and the flat band voltage. Plenty of methods are available to determine Vth which is one of the most im-portant parameters of semiconductor devices. This thesis adopts the constant drain current method, which is, the voltage at a specific drain current NID is taken as Vth, that is, Vth = VG
(NID) where Vth is threshold voltage and NID stands for normalized drain current. Constant current method is adopted in most studies of TFTs. It provides a Vth close to that obtained by the complex linear extrapolation method. Generally, the threshold current NID = ID/(W/L) is specified at 1 nA in linear region and at 10 nA in saturation region; W and L represent for TFT channel length and width, respectively.
2.2.2 Determination of the Subthreshold Swing
Subthreshold swing (S.S. , V / decade.) is a typical parameter to describe the control
ability of gate toward channel which is the speed of turning the device on and off. It is defined as the amount of gate voltage required to increase and decrease drain current by one order of magnitude. S.S. is related to the process, and is irrelevant to device dimensions. S.S. can be lessened by substrate bias since it is affected by the total trap density including interfacial trap density and bulk density. In this study, S.S. was defined as one-half of the gate voltage
re-10
quired to decrease the threshold current by two orders of magnitude (from 10-8A to 10-10A).
The threshold current was specified to be the drain current when the gate voltage is equal to Vth.
2.2.3 Determination of the field effect mobility
Typically, the field-effect mobility (μFE) is determined from the transconductance (gm)
COX is the gate oxide capacitance per unit area,
W is channel width, L is channel length, Vth is the threshold voltage.
If VD is much smaller than VG – VTH (i.e. VD << VG – Vth) and VG > Vth, the drain current can The transconductance is defined as:
VD
Similarly, we get mobility in the saturation region as
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(2-5)
2.2.4 Determination of On/Off Current Ratio
Drain on/off current ratio is another important factor of TFTs. High on/off current ratio represents not only the large turn-on current but also the small off current (leakage current). It affects AMLCD gray levels (the bright to dark state number) directly.
There are many methods to specify the on and off current. The easiest one is to define the maximum current as on current and the minimum leakage current as off current while drain voltage equal to 0.1V.
DSMinoff
off I
on IDSMax on
/ when VDS=0.1V (2-6)
2.2.5 Extraction of Optical Band Gap
The optical band-gap (Eg), which can be theoretically obtained according to Tauc model [14], and the Davis and Mott model in the high absorbance region, as followed：
(αhν)n=D*(h ν-Eg) (2-7)
where
α is the absorption coefficient,
hν is the photon energy,
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Eg is the optical band gap, and D is a constant.
The constant n is usually equal to 2 for amorphous semiconductors since it gives the best linear curve in the ban-edge region. The absorption coefficient (α) can be obtained from the transmittance data by using the equation of α =(1/d)*ln(1/T), where d and T are the thickness and the transmittance of the a-IZTO films with different oxygen flow rate.
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Fig.2- 1 The experimental flow path
Fig.2- 2 a-IZTO TFT device structure
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Chapter 3
Results and Discussion
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3.1 The effects of oxygen flow rate and channel thickness on a-IZTO TFTs
3.1.1 Characteristic analysis of a-IZTO TFTs with different RF sputtering power
The a-IZTO thin film was deposited on Si wafer and glass substrate together with the
devices for material analysis. After a-IZTO thin film deposited, all of them were annealed at 300oC for a half hours. Then we choose film thickness 30nm to have material analysis.
Atomic force microscope (AFM), scanning electron microscope (SEM), and X-ray diffraction (XRD) were used to analyze the surface properties of thin film with different oxygen flow rate deposited after annealing process.
The AFM results of a-IZTO film roughness are shown in Fig.3-1(a), (b), (c), (d), and (e) . The thin film’s roughness for oxygen flow rate 0, 0.1, 0.5, 1, and 1.5 after annealing process is
0.21, 0.35, 0.29, 0.25, and 0.19nm, respectively. Therefore, the film deposition with different oxygen flow rate incorporation after annealing process does not affect the morphology of a-IZTO thin film.
The SEM results of a-IZTO thin film’s surface image are shown in Fig.3-2(a), (b), (c), (d), and (e). The images provide the short-range structures for a-IZTO thin film with different
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oxygen flow rate and there is no grain boundary. Therefore, the thin films were still amor-phous phase with different oxygen flow rate incorporation after annealing process.
Fig.3-3 shows structure characterizations of a-IZTO thin film. In XRD patterns, a-IZTO thin film with different oxygen flow rate incorporation do not exhibit sharp diffraction peaks assignable to crystalline phase or poly phase. As a result, the thin films with different oxygen flow rate remain the amorphous phase after annealing process. Table 3-1 shows the film ele-mental composition by XPS analysis, it reveals that oxygen incorporation doesn’t have sig-nificant influence in elemental composition since the oxygen concentration in Table 3-1 has no obvious tendency.
The transfer characteristics and other parameters of a-IZTO TFTs fabricated by three different RF sputter power with the same thickness 30nm and consistent oxygen flow rate (0.5 sccm) were shown in Fig.3-4 and Table 3-2. From the Table 3-1, TFT with power 70w has the relatively good characteristic, as a result, we choose the power 70w as our experiment RF sputtering power.
Fig.3-5 shows the relation between deposition rate (nm/min) and oxygen flow rate(sccm) with power 70w. The film thicknesses were measured by AFM system in order to get the dep-osition rate. From the figure, we found that depdep-osition rate is clearly deceases when the oxy-gen flow rate increases.
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3.1.2 Characteristic analysis of a-IZTO TFTs with different oxygen flow rate
The a-IZTO TFTs using bottom gate staggered structure were fabricated with different oxygen flow rate and different thickness. The electrical performance shows in Fig.3-6(a), (b) and (c). Error bar in Fig.3-7 presents the overall characteristic of different oxygen flow rate with different thickness. The performance with different thickness has the same tendency for the mobility, threshold voltage, and sub-threshold swing. Mobility decreases as the oxygen flow rate increase. Threshold voltage increase as the oxygen flow rate increase, and sub-threshold swing increase when the oxygen flow rate increase as well. For the discussion of variation, devices with thickness 10nm have the lowest variation whether on mobility, threshold voltage, or sub-threshold swing. Also, devices with thickness 10nm have higher mobility and lower sub-threshold swing rather than the others.
When it comes to the same tendency of threshold voltage in three different thickness, from the literature, as the n-type oxide semiconductor, a well-known mechanism
for doping is that the oxygen vacancy generates two free electrons in the conductor band and works as a shallow donor. The higher quantity of oxygen incorporation may decrease the ox-ygen vacancy in the a-IZTO film.[15,16] In our experiment, we also found that threshold voltage increases when the oxygen flow rate increases. From this result, we can infer that due to the increasing oxygen flow rate, oxygen vacancy decrease which results in the increasing threshold voltage.
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On the other hand, the trap densities of a-IZTO film (Nt) can be estimated from
sub-threshold swing value with the following formula.[17,18]
S. S. = ln ×𝑘𝑇
𝑒 × [ +𝑒(𝑡𝑁𝑡+𝐷𝑖𝑡)
𝐶𝑜𝑥 ] (3-1) where
k is Boltzmann constant T is temperature(K)
t is the channel thickness(nm) Cox is the gate capacitance per area e is elementary electric charge Nt is the shallow trap density Dit is the interface trap density
Assuming the trap tNt dominated and Dit is negligible, we can calculate the Nt of different oxygen flow rate. From the Fig.3-8 and Table 3-3, take 10nm a-IZTO for example, we can find that trap states increase as oxygen flow rate increases. Moreover, we also discovered that when the oxygen flow rate increases from 0 sccm to 0.1 sccm, trap states apparently decrease.
This result also indicates that the a-IZTO thin film requires proper quantity of oxygen to re-pair its defects and optimize the characteristic, but too much oxygen incorporation may
con-trarily result in increasing defect.
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3.1.3 Characteristic analysis of a-IZTO TFTs with different channel thickness
Fig.3-9(a) shows the transfer characteristic of a-IZTO TFTs with no oxygen
incorporation on three different channel thickness (10nm,30nm,50nm). From the figure, the device with no oxygen incorporation at 10nm shows the normal thin film transistor characteristic, however, the device with 30nm is more conductive than 10nm device with ex-tra negative threshold voltage and 50nm has even more conductive characteristic than the others. Through the result, we can infer that the quantity of carriers increase when the channel thickness increases. When it compares with the device deposited with 0.1 sccm oxygen flow rate as shown in Fig.3-9(b), the result reveals the oxygen incorporation suppresses the carrier concentration that even 50nm device has thin film transistor characteristic although the threshold voltage are getting negative as the channel thickness increases.
When the oxygen flow rate increases to 0.5 sccm, three different thickness(10nm,30nm,50nm) devices are all have similar characteristic, and then, we fabricated thicker a-IZTO devices (70nm,90nm,110nm) as shown in Fig.3-10. The result indicates that the oxygen flow rate 0.5 sccm is not enough to suppress the carrier concentration of thicker a-IZTO thin film.
3.1.4 Summary
According to X-ray diffraction (XRD) pattern, we know that the structure of IZTO film
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is still in amorphous phase with different oxygen flow rate incorporation after 300oC anneal-ing process. The increasanneal-ing oxygen flow rate cause (1) the mobility decreases (μ↓), (2) the
threshold voltage positively shifts (Vth↑), (3) sub-threshold swing increases (S.S ↓). On the other hand, the trap density increases when oxygen flow rate increases.
3.2 Reliability Analysis of a-IZTO TFTs
3.2.1 Electrical reliability analysis under gate bias stress
As shown in Fig.3-7, the result shows that devices with thickness10nm have the lowest variation whether on mobility, threshold voltage, or sub-threshold swing. For the discussion of the effect on different oxygen flow rate, we uses the a-IZTO TFTs with thickness10nm to have reliability analysis in vacuum in order to isolate from ambiance influence.
Fig.3-11(a),(b) shows the threshold voltage (Vth) shift of a-IZTO TFT devices under
Fig.3-11(a),(b) shows the threshold voltage (Vth) shift of a-IZTO TFT devices under