Hysteresis Characteristics - Results and Discussion

Chapter 3 Results and Discussion

3.5 Hysteresis Characteristics

Several studies on the hysteresis phenomenon have been reported. In this thesis such an issue is also addressed. In the following analysis, the forward sweep (FS) refers to the sweeping of gate voltage from a negative value to a positive one, and the reverse sweeping (RS) refers to the sweeping in the opposite direction. Difference between the transfer characteristics of the two sweeping modes indicates the extent and effect of the hysteresis. In addition, it has been shown that the kind of gate dielectric used in devices may have effects on the hysteresis [54-55]. In this study, we explore the characteristics of devices with PECVD TEOS SiO₂/α-IGZO channel. Figs.

3.29(a), (b), (c), and (d) show the results of measurements performed on devices with the α-IGZO channel deposited with RF power of 100, 125, 150, and 200W, respectively. The O₂ flow is fixed at 1sccm for these devices. Figs. 3.30(a) and (b), show the results of measurements performed on devices with the α-IGZO channel deposited with O₂ flow of 3 and 5sccm, respectively. The RF power is 100W for the devices. In the figures, the hysteresis loops are clockwise.

The above results are presumably due to the electron trapping in the gate oxide which occurs mainly during the FS so that the Vth shifts positively in the RS curve.

The trapped electrons are injected from the channel into the gate oxide. The process parameters obviously play a role in affecting the hysteresis. The hysteresis window is defined as

Hysteresis Window V_1W RS# V_1W FS#, (Eq. 3-7) where Vth is defined as the value of Vg when Id equals 1nA ×

under Vd of 0.1V.

Fig. 3.31(a) shows the hysteresis window extracted from Figs. 3.29(a) ~(d) as a function of deposition power. It is seen that the window size shrinks with increasing deposition power. Fig. 3.31(b) shows that the hysteresis window extracted from Figs.

3.29(a), 3.30(a), and (b) as a function of oxygen flow with a fixed deposition power (100W). The hysteresis effect becomes more severe as oxygen flow is used.

During FS, the free electrons are accumulated near the interface by high electric field exerted by the gate voltage, and some of the electrons are injected into the gate dielectric and trapped therein. During RS and even subsequent FS, not all of the trapped electrons would de-trap. V_th thus becomes larger. The de-trap rate of the trapped electrons in the gate oxide depends on the trap level and distance from the channel. These properties are affected by the deposition conditions of the α-IGZO channel [55]. More efforts are needed for making these phenomena more clear.

Chapter 4

Conclusion

4.1 Conclusion

In this thesis, we have successfully fabricated and characterized α-IGZO TFTs with the α-IGZO channel layer deposited under various sputtering parameters. As the film was deposited with a higher RF power but a fixed oxygen flow conditions, we observed that the transfer curves are negatively shifted while µFE is improved without deteriorating the SS. It was confirmed by XPS measurements that the value of In/(In+Ga+Zn) ratio of the deposited films increases with a higher deposition power, presumably the reason that µFE is significantly enhanced. Moreover, by comparing the characteristics of devices deposited with different oxygen flow and a fixed deposition power, the carrier concentration of the α-IGZO channel is found to be reduced with increasing oxygen flow during sputtering. As a result, the transfer curves are positively shifted and µFE is significantly decreased. From the electrical results of the α-IGZO TFTs, the deposition power of 150W with oxygen flow of 1sccm was chosen as the sputtering conditions for preparing the α-IGZO channels in this study.

We’ve also investigated the effects of an n⁺ insertion layer between S/D metal and α-IGZO channel layer. In this study, α-IGZO TFTs with the n⁺ insertion layer exhibit less S/D parasitic resistance than the ones without n⁺ insertion layer, which in turn enhances the on-current. Nonetheless, origins for the accompanying degradation in the off-current and the negative shift of V_th remain unclear at this stage. Besides,

have been studied as well. It was observed that the resistivity of the α-IGZO channels receiving the post annealing treatment is higher than the control without the treatment, resulting in an increased V_th and mobility degradation.

Temperature-dependent sub-sthreshold characteristics were also observed for the fabricated α-IGZO TFTs. The increase in sub-threshold current in α-IGZO TFTs is well described by the thermally activated electrons. The thermally activated Arrhenius model proposes that the activated electrons are released from traps into the conduction band and may increase the current by orders of magnitude. However, the transfer characteristics are degraded as temperature rises above 75^oC due to the dominance of phonon scattering. The differences in hysteresis characteristics of theα-IGZO TFTs are mainly from the contribution of trapped electrons in or near the gate dielectric and trapping/detrapping in α-IGZO channel layer.

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Figure Captions

Table 1.1 Comparison of silicon based semiconductor TFT and metal-oxide based semiconductor TFT.

α-Si TFT Poly-Si Metal Oxide TFT

Phase Amorphous Polycrystalline Amorphous or

Polycrystalline Channel Mobility 1 cm²/V⁻¹·s⁻¹ ~200 cm²/V⁻¹·s⁻¹ 10~40 cm²/V⁻¹·s⁻¹

Switching Characteristics

0.4~0.5 V/decade 0.2~0.3 V/decade 0.09~0.6 V/decade Source/ Drain

Leakage Current

~10^-13 A ~10^-12A ~10^-13A

Uniformity Good Not Good Good

Long-term TFT reliability

Low High Unknown

Maximum Process Temperature

~250°C ~400-500°C RT to 350°C

Manufacturing Cost (Number of

mask)

Low (4~6) High (7~11) Very low (4~6)

Application Display

LCD LCD, OLED LCD, OLED

Fig.1.1. The elements of heavy metal cations with on the Periodic Table.

he elements of heavy metal cations with (n-1)d¹⁰ns⁰electronic configuration electronic configuration

(a) (b)

Fig. 1.2. Schematic drawings for the carrier transport paths (that is, conduction band bottoms) in the crystalline and amorphous semiconductor. (a) Covalent semiconductor, for example, silicon. (b) Post transition metal oxide semiconductors. [5]

Fig. 1.3. Illustration of the image of percolation conduction over distributed potential barriers. The drawn surface shows a potential isosurface for electron transport. (a) High temperature case. An electron chooses a shorter but higher barrier path. (b) Low temperature case. An electron chooses a longer, but lower barrier path. [32]

(a)

(b) (c)

Fig. 1.4. The bumpy surface represents the conduction band edge and flat layer represents the Fermi level. The model is simplified by assuming that the temperature is close to 0 K. (a) when the Fermi level is low, there is not enough electron to induce above the conduction band edge. (b)As the Fermi level increases, electrons trickle through potential valley (percolation conduction) (c) when the Fermi level is high enough, almost all potential barriers are immersed under the Fermi level so that electrons more almost unhindered. [37]

Table 1.2. The common deposition methods for manufacturing ZnO and α-IGZO film as active channel of transparent TFTs.

Deposition Technology

Advantages Disadvantages

Sol-gel method Easy Processing

Low Cost of Equipment

High manufacture temperature

Spray Easy Processing Good Uniformity of Large Scale Good Adhesion

The damaged sputter target Low Deposition Rate

Evaporation Fast Deposition Rate Difficult to control the deposited film

Pulse Laser Deposition (PLD)

Good Surface Roughness Easy to Make Multilayer Fast Deposition Rate

The deposited film reserves the stoichiometry of sputter target

Contamination (particle attached) Low productivity

High cost of equipment Not easy to make large scale deposition

Table 2.1. Major deposition parameters for preparing the α-IGZO films. In (a) the deposition power was varied to study its impacts, while in (b) the oxygen flow was varied to study its impacts.

(a)

(b)

System base pressure （Torr） 3 × 10^-6 Working pressure （mTorr） 5 Deposition temperature （℃） RT

Ar gas flow（sccm） 50

O2 gas flow (sccm) 1

RF deposition power （W） 100、125、150、200

System base pressure （Torr） 3 × 10^-6 Working pressure （mTorr） 5 Deposition temperature （℃） RT

Ar gas flow（sccm） 50

O2 gas flow (sccm) 1、3、5、10

RF deposition power （W） 100

Deposition Power (W)

80 100 120 140 160 180 200 220

Deposition Rate ( nm/s)

0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10

(a)

Oxygen Flow ( sccm)

0 2 4 6 8 10 12

Deposition Rate (nm/s)

0.005 0.010 0.015 0.020 0.025 0.030 0.035 0.040

(b)

Fig 2.1. (a) The deposition rate as a function of power at constant oxygen flow (1sccm) and (b) the deposition rate as a function of oxygen flow at fixed deposition power

(a) Fig. 2.2. (a) Gate definition

(c)

Fig. 2.2. (c) 50 nm α-IGZO channel film was deposition by RF sputter. (d) PR coating for defining the source/drain regions.

(e) (f)

Fig. 2.2. (e) Formation of source/drain metal deposition with the lift Coating of a PR layer.

(b)

definitionand (b) deposition of a 100 nm TEOS oxide by PECVD.

IGZO channel film was deposition by RF sputter. (d) PR coating for defining the source/drain regions.

(e) (f)

Fig. 2.2. (e) Formation of source/drain metal deposition with the

lift-and (b) deposition of a 100 nm TEOS oxide by PECVD.

IGZO channel film was deposition by RF sputter. (d) PR

-off method. (f)

(g) Fig 2.2. (g) Exposure and (h)

(i) Fig 2.2. (i) Cross-sectional view of the with n⁺ layer.

(h)

Fig 2.2. (g) Exposure and (h) Patterning of the active region by wet etching.

(j)

sectional view of the α-IGZO TFTs device without n t etching.

IGZO TFTs device without n⁺ layer and (j)

48 Fixed Oxygen Flow: 1 sccm W/L=400/100µµµµm Fixed Oxygen Flow: 1 sccm W/L=400/100µµµµm

Gate Voltage (V) Fixed Oxygen Flow: 1 sccm W/L=400/100µµµµm Fixed Oxygen Flow: 1 sccm W/L=400/100µµµµm

V_D=0.1V Fresh 14 Samples

(d)

Fig. 3.1. Transfer characteristics of α-IGZO devices deposited with deposition power of (a) 100W, (b) 125W, (c) 150W and (d) 200W at a constant oxygen flow of 1sccm.

Gate Voltage (V)

-6 -4 -2 0 2 4 6 8 10 12

D ra in C u r re n t (A )

10

^-15

10

^-14

10

^-13

10

^-12

10

^-11

10

^-10

10

^-9

10

^-8

10

^-7

10

^-6

10

^-5

100 W 125 W 150 W 200 W Fixed Oxygen Flow: 1 sccm

W/L=400/100 µµµµm V_D=0.1V

Fresh

Fig 3.2. Typical transfer characteristics of α-IGZO devices deposited with different deposition power.

Channel Length (µµµµm)

Fig. 3.3. (a) Extracted threshold voltage and (b) sub-threshold swing as a function of channel length for devices deposited with various rf powers.

Channel Length ( µµµµ m)

0 20 40 60 80 100 120

O n C u rr en t (A )

0.00000 0.00005 0.00010 0.00015 0.00020 0.00025 0.00030

100W 125W 150W 200W Fixed Oxygen Flow: 1 sccm

W=400µµµµm V_G-V_th=8V V_D=10V Fresh

(c)

Fig. 3.3. (c) Extracted on-current measured at VG-Vth=8V and VD=10V as a function of channel length for devices deposited with various rf powers.

Table 3.1. Summary of material analysis for α-IGZO films with different deposition power.

XPS RT AFM

Power (W)

Flow

(sccm) In(%) Ga(%) Zn(%) O(%)

RMS

(nm) In/(In+Ga+Zn)

50 1 20.03 11.76 8.11 60.1 0.589 0.502

100 1 19.43 10.26 7.84 62.47 0.763 0.518

125 1 21.23 10.61 9.28 58.88 0.612 0.516

150 1 21.29 10.07 9.17 59.47 0.581 0.525

200 1 20.73 9.14 8.09 62.04 0.574 0.546

Channel Length ( µµµµ m)

0 20 40 60 80 100 120

F ie ld E ffe ct M o b il ity (c m

/V s)

6 8 10 12 14 16

18 100W

125W 150W 200W Fixed Oxygen Flow: 1sccm

W=400µµµµm V_D=0.1V Fresh

Fig. 3.4. Field-effect mobility for devices deposited at different rf powers as a function of channel length.

Channel Length (µµµµm) a function of gate overdrive for a device with W/L of 400/100µm.

Gate Overdrive (V)

3 4 5 6 7 8 9

R

total

, R

( ΩΩΩΩ )

0 10000 20000 30000 40000 50000 60000

R_total (ΩΩΩΩ)

R

_SD

(Ω Ω Ω Ω)

R_ch (ΩΩΩΩ) Fixed Deposition Power: 200W

Fixed Oxygen Flow: 1sccm W/L=400/10µµµµm V_D=0.1 V

Fig. 3.6. Extracted R_total, R_ch, and R_SD as a function of gate overdrive for a device with W/L of 400/10µm.

V_G-V_th (V)

Fig. 3.7. (a) Extracted channel resistance per micrometer and (b) effective mobility as a function of gate overdrive.

Drain Voltage (V)

60 Fixed Oxygen Flow: 3 sccm W/L=400/100µµµµm Fixed Oxygen Flow: 5 sccm W/L=400/100µµµµm

V_D=0.1V Fresh 10 Samples

(b)

Fig. 3.9. Transfers characteristics of several α-IGZO devices with oxygen flow of (a) 3sccm and (b) 5sccm, respectively.

Gate Voltage (V)

-4 -2 0 2 4 6 8 10 12

D ra in C u r re n t (A )

10^-15 10^-14 10^-13 10^-12 10^-11 10^-10 10^-9 10^-8 10^-7 10^-6 10^-5

Oxygen Flow = 1sccm Oxygen Flow = 3sccm Oxygen Flow = 5sccm Deposition Power:100W

W/L=400/100µµµµm V_D=0.1V

Fresh

Fig 3.10. Transfer characteristics of α-IGZO devices deposited with in different oxygen flow.

Fig. 3.11. (a) Extracted threshold voltage and (b) sub-threshold swing as a function of the channel length.

Channel Length (µµµµm)

Fig. 3.11. (c) Extracted on-current measured at VD=10V and (d) field-effect mobility as a function of the channel length.

Fig. 3.12. Extracted (a) channel resistance per micrometer and (b) effective mobility as a function of gate overdrive.

Drain Voltage (V) oxygen flow of (a) 3sccm and (b) 5sccm.

Table 3.2 Summary of electrical characteristics of α-IGZO TFTs with different sputtering deposition parameters. W/L=400µm/100µm.

Deposition Conditions

100W O₂=1 sccm

100W O₂=3 sccm

100W O₂=5 sccm

125W O₂=1 sccm

150W O₂=1 sccm

200W O₂=1 sccm V_th (V) 3.23±0.15 5.67±0.17 6.70±0.28 2.13±0.28 0.66±0.15 0.97±0.07 µFE

(cm²/Vs) 7.40±0.18 6.71±0.16 5.31±0.26 7.89±0.59 10.22±0.36 14.17±0.08 SS

(V/decade) 0.429±0.033 0.432±0.037 0.388±0.039 0.477±0.044 0.476±0.023 0.472±0.035 On current

×10^-5 (A) 1.62±0.04 1.08±0.04 0.91±0.08 1.51±0.66 2.10±0.03 2.47±0.11

Drain Voltage (V)

在文檔中 a-IGZO 薄膜電晶體的製作與特性分析 (頁 36-0)