1.
Introduction

In modern era, the display technology has been rapidly advanced that peoples’ lives are getting into the swing of commercial products from small/medium mobiles device to large TVs. Display market have been faced to significant factors like higher resolution, lighter weight, lower power consumption, thinner thickness, and a free designing-form. Especially, the customers have demanded that the next generation of innovation could be flexible paper-thin display, which is available to fold or roll from large screen size to small form factor in the coming display market^[1]. Besides, the flexible active-matrix organic light-emitting diode (AMOLED) displays exhibit suitable characteristics for mobile terminals or large sheet-shaped televisions (TVs), attribute to its lightweight, thin thickness, mechanically robust and high quality images^[2]. Afterwards, regarding a display operation, many researchers have paid attentions on flexible TFT fabrication processes, that limited by process temperature due to polymer substrates^[3]. In addition, the device performances on flexible substrates should be considered such as high mobility and on-off current modulation, low contact resistance and mechanical flexibility in the meantime^{[4, 5]}.



Amorphous silicon (a-Si) TFTs and polycrystalline silicon (poly-Si) TFTs have been mostly produced for commercial display devices^{[6, 7]}, while numerous organic and inorganic materials had also been extensively explored to replace the silicon semiconductors^{[8, 9]}. Among various semiconductor TFTs, a-Si and organic TFT exhibited a low mobility (< 1 cm²/(V·s)) and inferior bias stability^{[5, 10–12]}. Although poly-Si TFTs showed outstanding high mobility (> 80 cm²/(V·s)) and excellent stability, its process is limited due to high process temperature budget (> 450 °C) as well as expensive laser crystallization system^{[13, 14]}. As a result, oxide semiconductor TFTs have recently been considered as alternative solutions due to their high mobility and low process temperature, relative low cost and good transparency in visible light region^[15]. Among oxide semiconductor, amorphous indium–gallium–zinc oxide (a-InGaZnO) is well known as a potential material for high stability and low off current^{[16, 17]}. As relative low mobility (~10 cm²/(V·s)) limits various application of a-IGZO TFTs, researches started to focus on the high performance of oxide semiconductor TFTs through methods of element doping, crystallization and multi-channel layer^[18–20].



Very recently, many flexible TFTs have been fabricated on polyimide (PI) or PEN (polyethylene naphthalate) films, which usually covered with functional buffer layers due to anti-water diffusion and smooth surface morphology development^[21]. The characteristics of buffer layer, normally deposited by ALD or chemical vapor deposition (CVD), would probably influence the electrical performance of TFTs, regarding the mobility and stability^[22]. Moreover, many studies on flexible oxide TFTs have been reported for the optimized channel layer and gate insulators^{[23, 24]}. Channel layers for enhancing mobility and stability of oxide TFTs, have been investigated on various deposition methods to keep lower process temperature and higher stability under mechanical stress^{[25, 26]}. In terms of gate insulators in the TFTs, the conventional insulator materials, such as SiO₂ and Si₃N₄ deposited by CVD, showed high process temperature (> 300 °C), which is limiting to select polymer substrate.



The ALD is a conventional thin film deposition technique, based on self-limited surface reaction, to dose in the chamber with two or more separated gaseous reactant pulses. Fig. 1 shows that the ALD growth process consists of alternating pulses of gaseous chemical precursors that react with the substrate, which were called “half-reactions”. During each half-reaction, the precursor that pulsed into chamber with certain time fully reacts with substrate surface. Then, there will be no more than one monolayer at the reaction surface due to the self-limiting process. Subsequently, inert carrier gas is purged into chamber to remove by-products. The process of cycling half-reactions by pulse and purge then forms layers of the desired material^[27]. Thus, ALD is appropriate for the precise control of film thickness and composition, while achieving uniform step coverages at relatively low temperatures^[28]. Thus, ALD techniques can grow oxide semiconductors, dielectrics or relative hydride gate insulator layers for applying TFT fabrications^[29–31].






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Figure1.
(Color online) Schematic diagram of general growth process of ALD.




This review article focuses on the ALD materials and processes in flexible oxide semiconductor TFT, including the buffer, gate insulator and oxide semiconductor layers. Also, it consists of the mechanical evaluation methods on flexible TFTs, including various bending conditions and device degradation mechanisms. This review introduces proper solutions using functional ALD layers to solve critical issues of flexible oxide semiconductor TFTs.

2.
Functional ALD layers for flexible oxide TFTs

2.1
ALD buffer layer

Conventional buffer layer fabrication process on glass substrate is losing its attentions due to high water-vapor transmission rate (WVTR) of carrier glass that possesses weak ability of blocking water molecules from the ambient permeating though the flexible plastic substrate^[32–34]. Recent reports about ALD buffer layer are summarized in Table 1, SiO₂, SiN_x and Al₂O₃ are commonly used for buffer layers on flexible substrate^{[22, 35–38]}, where SiO₂ and SiN_x buffer layers are fabricated by plasma enhanced chemical vapor deposition (PECVD) process and Al₂O₃ buffer layer is mostly deposited by ALD. Compared to ALD deposited Al₂O₃^{[39, 40]}, PECVD produced SiO₂ and SiN_x, although widely used in a-Si TFTs, were reported containing extremely high hydrogen (H₂) concentration (~25%)^{[41, 42]} and hydrogen diffusion that would probably degrade TFT performance. Besides, it was reported that utilizing an organic/inorganic ALD buffer layer can improve flexibility of TFT under mechanical strain, which proved that the TFTs would not degraded even under the mechanical bending situation at a curvature radius of 3.3 mm and after the repetitive bending cycles^[43].

Substrate	Buffer layer	Method	Active layer	Mobility	Strain/radius	Bending cycle	Ref.
PI	SiOx, SiNx/Al2O3	PECVD/ALD	IGZO	14.88	15 mm	10000	[22]
Polyethylene naphthalate	Organic/Al2O3	ALD	IGZO	15.5	3.3 mm	10000	[43]
PI	SiNx/SiO2/Al2O3	ALD	InOx	15	5 mm (0.4% tens.)	10000	[44]
PI	Al2O3	ALD	a-IGZO	12.5	2 mm		[45]

Table1.
Summary of recent reports for ALD buffer layer based flexible oxide TFTs.



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Substrate	Buffer layer	Method	Active layer	Mobility	Strain/radius	Bending cycle	Ref.
PI	SiOx, SiNx/Al2O3	PECVD/ALD	IGZO	14.88	15 mm	10000	[22]
Polyethylene naphthalate	Organic/Al2O3	ALD	IGZO	15.5	3.3 mm	10000	[43]
PI	SiNx/SiO2/Al2O3	ALD	InOx	15	5 mm (0.4% tens.)	10000	[44]
PI	Al2O3	ALD	a-IGZO	12.5	2 mm		[45]

2.1.1
Single buffer layer

It was reported reactant species could influence the barrier characteristics for ALD Al₂O₃ buffer, including WVTR and H₂ passivation^[46]. As Fig. 2(a) shows, top gate bottom contact In–Ga?Zn?O (IGZO) TFT was fabricated on PI substrate covered with SiN_x/Al₂O₃ buffer layer, while Al₂O₃ was deposited by ALD using water or ozone reactant. The source/drain (S/D) electrodes and active are formed by depositing indium-tin oxide (ITO) and IGZO by sputtering and patterned by wet etch. The transfer curves exhibit in Figs. 2(b) and 2(c) shows stability performance of devices where TFT with Al₂O₃ buffer using ozone as reactant exhibited lower field-effect mobility (4.73 cm²/(V·s)) and poor stability ($ Delta $V_th = ?5.32 V) under negative bias temperature stress (NBTS) (V_G = ?20 V, temperature = 60 °C and time = 3000 s in air) than using water as reactant (mobility of 7.15 cm²/(V·s) and $ Delta$V_th = ?0.64 V). The device with Al₂O₃ (ozone) exhibits much more seriously degradation under air ambient than in vacuum that reveals barrier characteristics of ambient molecules would probably be the reason of transfer characteristics degradation under bias and temperature stress.






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Figure2.
(Color online) (a) Schematic diagram of top-gate and bottom contact a-IGZO TFT, transfer characteristics with different buffer fabrication process (detailed fabrication process in supporting information) that (b) using water/ozone as reactant, (c) under NBTS (V_G = ?20 V, temperature = 60 °C and time = 3000 s) in air ambient (relative humidity, RH = 30%), (d) device density of states of near-conduction band for TFTs with different reactant and measured in RH 30% and vacuum.




Further thin film analyses were produced to investigate the mechanism of different TFTs degradation for different ALD processed buffers. The result of X-ray reflectivity (XRR) indicated Al₂O₃ (H₂O) buffer had higher film density (3.14 g/cm³) and much smoother surface (0.49 nm) comparing to Al₂O₃ (ozone) (2.97 g/cm³ and 1.04 nm). Based on the density of state (DOS) analysis (shown in Fig. 2(d)), higher density state existed in Al₂O₃ (ozone) buffer both in vacuum and air ambient comparing to Al₂O₃ (H₂O), the authors suggested that relative low film density of Al₂O₃ (ozone) resulted in facility of ambient molecules penetrating, and rough surface contributed to the poor-transfer performance where rough interfaces would act as charge trap site. Thus, the characteristics of effective passivation or external ambient eliminating of ALD buffer layer was affected by ALD reactant, playing essential role to reduce the defect state and then, the stability of TFT operation.

2.1.2
Hybrid buffer layer

The flexible a-IGZO TFT fabricated on polyimide substrate with stack structure or organic/inorganic hybrid structure were also reported for flexibility development^{[22, 43]}. The structure of top-gate and bottom contact a-IGZO TFTs fabricated on PEN substrate was shown in Fig. 3(a) and corresponding transfer performances with different buffer layer were demonstrated in Fig. 3(b). The buffer layers were designed as SiN_x 50 nm (Device A), SiN_x 50 nm/Al₂O₃ 10nm (Device B) and SiN_x 50 nm/Al₂O₃ 100 nm (Device C), respectively. The organic barrier and inorganic Al₂O₃ barrier were prepared by spin-coating and ALD, respectively. And S/D electrode and active layer, ITO and IGZO, was deposited by sputtering and patterned by wet etch. Al₂O₃ was deposited by ALD as gate dielectric and patterned by wet etch. Finally, gate electrodes and S/D pads were formed by deposition and patterning of Al by thermal evaporation. As device A exhibited quite poor transfer performance (not shown, saturation mobility 3.31 cm²/(V·s), threshold voltage ?23.93 V and sub-threshold voltage 0.76 V/decade) compared to other devices, it was supposed that higher H₂ concentration existed in SiN_x buffer layer lead to degradation by H₂ penetrates during the process of TFT fabrication. In terms of device B and C that with different thickness of Al₂O₃, the higher sub-threshold voltage (0.84 V/decade) of device B than device C (0.24 V/decade) indicated that critical thickness of Al₂O₃ was necessary for effective hydrogen passivation.



As flexibility of conventional ALD inorganic buffer materials have their limitations due to their high Young’s moduli^[47], organic materials, with much lower Young’s modulus, started to be utilized for hybrid buffer layer owing to the difference in the modulus of elasticity between two kinds of materials^[43]. The a-IGZO TFTs with top-gate bottom-contact structure on the flexible polyethylene naphthalate (PEN) substrate is shown in Fig. 3(c) where a 3 mm-thick organic barrier (spin-coating) and a 50 nm-thick inorganic Al₂O₃ (ALD) barrier were deposited in order to obtain a smooth surface and a low moisture permeability, and a protection against mechanical damage. To compare the roles of each barrier layer, the performance of TFTs with no buffer, only with an organic buffer and a hybrid organic/inorganic buffer were researched. In terms of transfer performance (Table 2), the TFT with hybrid barrier exhibited marked improvements attributed to both of surface smoothness and ambient impermeability. The mechanical bending performance also found flexible IGZO TFTs with the hybrid organic/inorganic buffer layer exhibited excellent stable device characteristics under a mechanical stress even at a curvature radius of 3.3 mm (shown in Fig. 3(d)) or after 10000 repetitive bending cycles (5 mm). Thus, we prefer to keep looking forward to the further research on the hybrid organic/inorganic ALD buffer layer optimized for flexible oxide TFTs.

Device	Mobility (cm2/(V·s))	Threshold voltage (V)	Subthreshold swing (V/dec)	On/off ratio
No buffer	1.11	9.4	0.4	3.1 × 107
Organic buffer	14.4	2.8	0.4	1.5 × 109
Hybrid buffer	15.5	4.1	0.2	4.7 × 109

Table2.
Transfer performance parameters comparison for device with different buffer layers^[43].



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Device	Mobility (cm2/(V·s))	Threshold voltage (V)	Subthreshold swing (V/dec)	On/off ratio
No buffer	1.11	9.4	0.4	3.1 × 107
Organic buffer	14.4	2.8	0.4	1.5 × 109
Hybrid buffer	15.5	4.1	0.2	4.7 × 109

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Figure3.
(Color online) (a) Structure of a-IGZO TFT fabricated on polyimide substrate with different buffer materials and stack structures (detail fabrication process in supporting information) and (b) relative transfer performance. (c) Schematic cross-sectional view of the fabricated a-IGZO TFT on a flexible PEN substrate with inorganic/organic buffer layer. (d) I_DS–V_GS transfer curves of organic and inorganic/organic buffer layer based TFTs^{[22, 43]}.

2.2
Hybrid organic/inorganic gate insulator layer

The recent reports of ALD gate insulator based flexible oxide TFTs are summarized in Table 3. For the realization of mechanically flexible devices, the aforementioned mechanical neutral plane is researched, which places the active layer in the device at mechanical neutral plane to reduce the mechanical stress when the device is under strains^[48]. However, conventional inorganic dielectric layers such as SiO₂, Si₃N₄, are vulnerable to mechanical stress and fail easily by cracking or delamination during repetitive large mechanical deformation^[49].

Substrate	Gate insulator	Method	Ative layer	Mobility	Strain/radius	Bending cycle	Ref.
PI	Al2O3	ALD	a-IGZO	7.5	3.5 mm		[50]
PI	PVP/Al2O3	ALD	IGZO	8.39	10 mm	100000	[49]
PI	Al2O3	ALD	IGZO	10.5	6 mm		[51]
Polyethylene naphthalate	Al2O3	ALD	IGZO	15.5	3 mm	10000	[52]
PI	Al2O3	ALD	IZO	42.1	2 mm	5000	[53]
PI	Al2O3	ALD	InOx	9.7	5 mm	10000	[54]
PET	Al2O3	ALD	IZO	40.1	7.5 mm	5000	[55]
PI	Al2O3	ALD	ZnO		1.75 mm (0.14% comp.)	90000	[56]
PI	Al2O3	ALD	InOx	15	5 mm (0.4% tens.)	10000	[44]
PI	Al2O3	ALD	a-IGZO	12.5	2 mm		[45]

Table3.
Summary of recent reports for ALD gate insulator based flexible oxide TFTs.



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Substrate	Gate insulator	Method	Ative layer	Mobility	Strain/radius	Bending cycle	Ref.
PI	Al2O3	ALD	a-IGZO	7.5	3.5 mm		[50]
PI	PVP/Al2O3	ALD	IGZO	8.39	10 mm	100000	[49]
PI	Al2O3	ALD	IGZO	10.5	6 mm		[51]
Polyethylene naphthalate	Al2O3	ALD	IGZO	15.5	3 mm	10000	[52]
PI	Al2O3	ALD	IZO	42.1	2 mm	5000	[53]
PI	Al2O3	ALD	InOx	9.7	5 mm	10000	[54]
PET	Al2O3	ALD	IZO	40.1	7.5 mm	5000	[55]
PI	Al2O3	ALD	ZnO		1.75 mm (0.14% comp.)	90000	[56]
PI	Al2O3	ALD	InOx	15	5 mm (0.4% tens.)	10000	[44]
PI	Al2O3	ALD	a-IGZO	12.5	2 mm		[45]

Lee et al. reported an ultrathin ALD Al₂O₃ layer based organic (poly-4vinyl phenol (PVP))/inorganic hybrid gate dielectrics for flexibility improvement of IGZO TFT as shown in Fig. 4(a) where the thickness of ALD Al₂O₃ varies from 20 to 40 nm. The inverted-staggered top-contact IGZO TFTs were fabricated on a 125 lm-thick PI film. Then, a PVP layer of 1 lm was spin coated on the PI substrates to make a smooth surface. A Ni gate electrode of 100 nm was deposited on the bare PVP layer by the e-beam evaporator system using a shadow mask. The PVP and Al₂O₃ layers of different thicknesses were formed by spin coating and ALD at 200 ℃, respectively. A 50-nm-thick active layer was deposited by RF sputtering and Al S/D electrodes of 70 nm thickness were deposited by thermal evaporation using a shadow mask. Then, the fabricated devices were annealed on a hot plate at 200 ℃ for 1 h. The authors simulated the mechanical strains of applied to Al₂O₃ layer in hybrid gate dielectric along with the structural characterization by finite elements method (FEM) (Fig. 4(b)) that shows mechanical stability of 20 nm and 30-nm-thick Al₂O₃ films were better than 40 nm-thick Al₂O₃ film based on consideration of strain at the interface of Al₂O₃/IGZO and critical strain for the fracture of the Al₂O₃ layer. The thicker Al₂O₃ layer (40 nm) based TFT exhibited electrical performance degradation of IGZO TFT (Fig. 4(c)). The cross-sectional image of TFT with a hybrid gate dielectric after 10⁵ bending cycles by SEM was added in Fig. 4(b). The device showed delamination at the interface between the IGZO layer and source/drain Al electrodes. It seems the delamination between the active layer and source/drain electrodes degraded the device characteristics






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Figure4.
(Color online) (a) Schematic illustration of the a-IGZO TFT on polyimide substrate with bottom gate and top contact structures (detail fabrication process in supporting information). (b) Strain and stress applied to thin Al₂O₃ layer in organic/inorganic hybrid gate dielectric with varying Al₂O₃ thicknesses. (c) Cross-sectional FIB/SEM images of the a-IGZO TFT with the PVP (400 nm)/Al₂O₃ (40 nm) hybrid gate dielectric after 10⁵ bending cycles in tension mode. (d) Transfer characteristics of cyclically bent a-IGZO TFTs with PVP (400 nm)/Al₂O₃ (20 nm) hybrid gate dielectrics, PVP (400 nm)/Al₂O₃ (30 nm) hybrid gate dielectrics, and PVP (400 nm)/Al₂O₃ (40 nm) hybrid gate dielectrics^[49]. (copyright 2014 Elsevier B.V.)

2.3
Active ALD layer

In terms of active layer deposition, the conventional sputter or solution based oxide TFTs, such as IGZO, commonly need process temperatures above 300 °C in order to realize sufficiently reliable oxide TFTs with superior electrical performance, which in the meantime, can degrade TFT performance result from coefficient of thermal expansion (CTE) and temperature capability of plastic substrate^[57]. Otherwise, the traditional a-IGZO TFTs exhibited insufficient field-effect mobility for high-resolution AMOLED display as reported around 10 cm²/(V·s), such as 7 cm²/(V·s) by Ok et al.^[46], 10 cm²/(V·s) by Jeong et al.^[45], 7 cm²/(V·s) by Bak et al.^[58].

2.3.1
Active layer structure for oxide TFTs

Based on the growth mechanism of ALD, considering the growth of high quality inorganic films at temperatures that plastic substrates can withstand, ALD is at present most appropriate.



An amount of reports about oxide TFT based on ALD published recently, which showed comparable performance to sputter or solution methods, such as 20 cm²/(V·s) of ZnO TFT by Tsai et al.^[59], 13.4 cm²/(V·s) of ZnHfO TFT by H. N. Alshareef et al.^[60], 6 cm²/(V·s) of Al–Zn–O (AZO) TFT by Leeet al.^[61], and so on^[62–64]. To increase performance of TFT, some groups came up with method of post annealing, the mobility of devices increased to 21.3 cm²/(V·s) using O₂ annealing at 400 °C by David Wei Zhang et al.^[65] and 30.2 cm²/(V·s) using O₂ annealing at 350 °C by Cho et al.^[66], and so on^{[67, 68]}. Such high annealing temperature does not meet the standard of low process temperature requirement for flexible TFTs, which would possibly play an essential role of next generation display.



Based on the advancement of ALD, active layer structure promotion was reported by some groups. The homogeneous laminated active layer was most widely researched, where concept of super-cycle was introduced. Each super-cycle consists of several sub-cycles for each element (shown in Fig. 5(a)), for example, the IZO active layer was deposited by IZO super-cycles, containing one ZnO cycle and one In₂O₃ cycle reported as Park et al.’s paper^[53]. Thus, the composition of deposited thin film can be effectively adjusted by the number of sub-cycles, which contributed to mobility optimization. On the other side, ALD process is a more appropriate approach for fabricating bi-layer channel structures that include quite thin front channel layer, commonly thinner than 5 nm, compared to the sputtering. The bi-layer channel structure owns superior TFT mobility and stability than single channel due to charge trap density and carrier concentrations control for both of front and back channel^{[69, 70]}.



As Fig. 5(b) shows, ZnO-based bi-layer channel TFTs consisting of ZnO and AZO layers with different Al compositions and stacking sequences. The 30 nm ZnO single-layer and 5 nm AZO (5 or 14 at.% Al)/25 nm ZnO and 25 nm ZnO/5 nm AZO (5 or 14 at.% Al) bilayer channel structures were deposited on SiO₂ (200 nm)/p⁺-Si substrates by ALD at 200 °C. The thickness of the AZO layers was fixed at 5 nm, and the total thickness of the bi-layer channel was 30 nm. For the bottom-gate TFT structure, all the bi-layers were patterned by a conventional photolithography process and wet etching. The source and drain electrodes were defined by depositing Ti (30 nm)/Au (70 nm) by e-beam evaporation using a shadow mask. The width and length of the channels were fixed at 500 and 50 μm, respectively. The ZnO single-layer and ZnO-based bi-layer channel TFTs were annealed in a furnace at 250 °C for 2 h under O₂ ambient to reduce the carrier density of the channel layer and the contact resistance. The use of AZO thin layers as front channel reduced the charge trap density at the gate dielectric/channel interface and the back-channel effect on the channel surface, resulting in more stable TFTs under a gate bias stress. In addition, by control the composition of Al, the crystallization, which can affect mobility and stability, was adjusted by ALD process^[64]. Similar approach for TFT performance by ALD was reported as shown in Fig. 5(c), where step-composition gradient channels in oxide based TFTs was deposited and its effects on the performances and electrical stability of the TFT devices was investigated^[63]. By utilizing ALD, the Al step-composition gradient channel could form alignment of conduction band offset within the channel layers for higher mobility as well as stability.






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Figure5.
(Color online) (a) Homogeneous laminated active layer deposited by ALD, (b) bi-layer, and (c) gradient active layer by ALD for oxide semiconductor TFTs^[63–64] (detailed fabrication process in supplement information).

2.3.2
Optimizing materials/processes for flexible oxide TFTs

Previous researches of ALD flexible TFT reveal its potential for applying into flexible display technology. However, the relative high post treatment temperature still limits the application for flexible applications. Some groups started to focused on indium oxide based TFTs, which is known as a material to achieve high electron mobility due to single free electron-like band of In 5s states in the cubic bixbyite structure in linear conducting chains of edge-sharing octahedral^[71–73]. However, as-deposited InO_x generally exhibits metal-like conduction, in order to successfully implement this material as a semiconductor, the total carrier concentration must be reduced to convey switching properties to the fabricated TFTs, which could be controlled available by post treatment as Fig. 6(a) shows^[54]. The performance of ALD indium oxide TFT was affected in terms of N₂O plasma treatment time. By increasing N₂O plasma treatment from 600 to 2400 s, the indium oxide positive shifted and mobility decreased, which can be explained by oxidation at surface of active layer. The suffering of plasma treatment at indium oxide channel induced oxygen deficiency decreasing and carrier concentration dropping when N₂O plasma treatment time increasing. Fig. 6(b) shows bottom gate top contact thin film transistors (TFTs) based on IZO channels were fabricated on polyimide (PI) substrate by atomic layer deposition (ALD) at different growth temperature. The effect and its mechanism of growth temperature of IZO channel layer was found that IZO channel layer grow at increased temperature till 200 °C, exhibited best transfer and stability performance (saturation mobility 42.3 cm²/(V·s), threshold voltage 0.7 V, sub-threshold voltage 0.29 V/decade and hysteresis 0.21 V) comparing to 150 and 175 °C due to highest carrier concentration with increased oxygen vacancy and decreased electron trap states^[54].






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Figure6.
(Color online) High performance flexible ALD oxide TFT with active layer as (a) InO_x with N₂O plasma post treatment (copyright 2016, American Chemical Society) and (b) IZO that in term of growth temperature dependence (copyright 2016, American Chemical Society)^{[53, 54]}.

3.
Mechanical stress evaluation on flexible oxide TFTs

As flexible devices were sensitive under mechanical stress due to dimensional shape transformation, researchers started to pay attention on the effects of mechanical stress^{[22, 45, 49, 54, 60, 65, 74–81]}. It was reported performance of flexible oxide TFTs would be degraded under critical strain, repetitive bending cycles or different bending direction^{[45, 49, 54, 65, 77–80]}.

3.1
Repetitive bending cycles with mechanical strains

Table 4 exhibits the electrical performances of flexible ALD InO_x TFT, which was initially flat and then bent to a tensile bending radius from 15 to 5 mm as Fig. 7(a). The decrease in the threshold voltage for tensile bending can be explained by two reasons as reported in previous papers^{[54, 80, 82–85]}. One is the increase in the distance between the atom that cause an effective decrease in the energy level splitting ($ varDelta$E) of the bonding and anti-bonding orbitals between the atoms in the semiconducting layer^[86]. This changes the value of the Fermi function (more electrons are excited to the anti-bonding state for the semiconductor, causing an increase in the channel conductivity under tensile bending strain). The additional electrons caused an increase in the channel conductivity and, therefore, a negative shift of V_th. The other reason is reported previously that the charge transport of oxide semiconductors is related to the generation of carriers caused by native defects such as oxygen vacancies during process^{[87, 88]}. Thus the applied tensile strain might affect the oxygen vacancies, which could change the electron concentration under mechanical deformation. The decrease of saturation mobility is due to the larger spacing between molecules is caused by tensile strain^[31]. On the other side, the generation of defects in the interface of gate insulator and channel is revealed by slightly increase of subthreshold slop. Besides, the increase of hysteresis also can be explained as the result from generation of charge traps by defects caused by tensile stress.

Bending radius (mm)	Strain	Vth (V)	μsat (cm2/(V·s))	S.S. (V/decade)	Hysteresis (V)	ION/IOFF
0	0	?0.22	9.7	0.87	0.9	9.4 × 109
15	0.12%	?0.49	8.77	0.96	1.23	4.6 × 109
10	0.18%	?0.72	8.38	0.98	1.47	4.8 × 109
5	0.38%	?1.46	8.09	0.97	1.87	4.8 × 109

Table4.
Transfer performance parameters under mechanical stress by jig bars of different radius^[54].



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Bending radius (mm)	Strain	Vth (V)	μsat (cm2/(V·s))	S.S. (V/decade)	Hysteresis (V)	ION/IOFF
0	0	?0.22	9.7	0.87	0.9	9.4 × 109
15	0.12%	?0.49	8.77	0.96	1.23	4.6 × 109
10	0.18%	?0.72	8.38	0.98	1.47	4.8 × 109
5	0.38%	?1.46	8.09	0.97	1.87	4.8 × 109

The transfer performances of ALD IZO TFT on PI substrates was also evaluated under repetitive bending with various strains, parameter change (shown in Fig. 7(b)). The measurements were produced after 100, 300, 1000, 2000 and 5000 bending cycles. Here the bending radius was adjusted from 5, 2, and 1.5 nm, while the calculated strain of 0.34%, 0.57% and 1.12% was induced. The V_th shifts in the negative direction and the saturation mobility decreases as the number of bending cycles increases. It is generally reported in the literature that the degradation of electrical characteristics for amorphous oxide semiconductor is not significant when subjected to mechanical strain (under 1%), owing to their amorphous microstructure. Thus, for TFT bended smaller than 1% stress strain till 5000 repetitive cycles, a slight shift in V_th towards negative values (from ?2.3 to ?4.3 V) is observed. As the TFTs bended repeatedly under tensile strain, the number of electrons that excited to the anti-bonding state increased, and resulted in carrier concentration increasing in the semicondu ctor^[86]. An alternative effect would involve the presence of electron-donating oxygen vacancies, of which the density may increase with the application of tensile stress^[87–89]. The slight decrease in field effect mobility may be attributed to the increased interatomic spacing in the semiconductor induced by the tensile strain^[31]. However, when the stress strain was as large as 1.12%, the TFT lost its transfer characteristics at 5000 cycles and showed serious degradation compared to stress strain of 0.34%, 0.57%. This phenomenon may result from the electron trap sites and micro-cracks formation during repeated cycles under large bending strain.






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Figure7.
(Color online) (a) Flexible ALD InO_x TFTs bending test over bending jig with different bending radius from 15 to 5 mm, (b) Example picture of the flexible TFT measurement with bending jig. (c) Evolution of the transfer performance of flexible IZO TFT fabricated on polyimide substrates as a function of bending cycles and strains^{[53, 54]}.

3.2
Tensile/compressive & parallel/vertical strains

The performance degradation of flexible TFTs under mechanical stress is also influenced by bending directions. Fig. 8 (a) shows a-IGZO flexible TFT parameters shift under different types of stress (tensile/compressive) on the 50 μm PI substrate^[79]. For the tensile stress, as bending radius decreased, threshold voltage negative shift and saturation mobility decreased. On the other hand, for the compressive stress, the variation tendencies of TFT parameter are opposite. These phenomena reveal that energy level splitting in the internal semiconductor materials induced by change of atomic bonding distance^[90] and resulted in TFT parameter change by changing Fermi-level energy level. On the other hands, comparison of parallel/vertical stress to the current path in TFTs is under growing concern^{[54, 91]}. As shown in Fig. 8(b), the repeated mechanical bending stress was applied vertical or parallel to the ALD InO_x TFTs^[54]. The mobility decreases gradually while S.S. value undergoes relatively minor change for vertical bending while TFT performance degradation is much more drastically for parallel bending. Similar report is published^[91] recently, where technology computer aided design simulation was performed and found donor-like and acceptor-like states increase, respectively, by 2.5 × 10¹⁷ and 0.4 × 10¹⁷ cm^?3 for vertical and parallel bending after 10000 bending.






onerror="this.onerror=null;this.src='http://www.jos.ac.cn/fileBDTXB/journal/article/jos/2018/1/PIC/17100007-8.jpg'"
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Figure8.
(Color online) (a) a-IGZO TFT performance for different types of stress (tensile/compressive) on the 50 μm PI substrate, and (b) schematics of bent InO_x TFTs for a bending axis vertical to (for case I) and parallel to (for case II) device current, and transfer characteristics of InO_x TFTs under repeated bending cycles with different bending cases (copyright 2016, American Chemical Society)^{[54, 79]}.

4.
Conclusion

In this review, recent progresses of flexible oxide TFTs have been focused on various functional layers fabricated by ALD process, anticipating the improvement of device performances such as mobility, stability, and flexibility. The proper ALD buffer layer would provide excellent transfer performance and stable reliability for flexible oxide TFTs. In addition, inorganic/organic ALD based gate insulators also play an essential role as a mechanical neutral plane to strength the flexibility of oxide semiconductor TFTs. Among various ALD layers for flexible TFTs, the ALD oxide semiconductor act as an important role for operating flexible devices with high mobility and stability, since the deposition method could easily control atomic compositions in nano-structure layer, unlike the sputtered oxide semiconductors. However, the mechanical stresses may degrade mobility and stability of oxide TFTs, originated from the micro-crack generation. Thus, ALD techniques could offer possible solutions regarding functional films, along with high quality, low deposition temperature, and less film-stress for applying flexible oxide semiconductor TFTs.