Investigating an abnormal hump phenomenon in top gate a-InGaZnO thin-film transistors due to mobile sodium diffusion

Top gate a-InGaZnO (IGZO) thin-film transistors (TFTs) annealed at high temperature show excellent initial current–voltage (I–V) characteristics. However, when they are exposed to positive gate bias for a long time, hump can occur in the subthreshold region. This abnormal hump is accelerated at a higher positive gate voltage and mitigate by a negative gate voltage. While the strength of the hump is irrelevant to a change in channel width, it relies significantly on channel length. This phenomenon might be due to mobile Na ions diffused from a glass substrate migrating toward the back and edge side of the IGZO semiconductor by a vertical gate electric field. When a layer of Al2O3 is formed between the IGZO semiconductor and the glass substrate, the hump phenomenon could be successfully solved by serving as a barrier for Na ions moving into the IGZO.


Methods
A borosilicate glass (Schott, Borofloat®, 0.7 mm, UK) was used as a substrate to fabricate a IGZO TFT device.Indium Tin Oxide (ITO; 20 nm) was deposited and patterned to form source and drain (S/D) electrodes on top of the glass substrate.IGZO (In: Ga: Zn = 1: 1: 1 atomic%, 30 nm) was then deposited with a radio frequency (RF)-magnetron sputtering system.An RF power source (13.56 MHz, 100 W) was employed for depositing IGZO layers.During deposition, the chamber working pressure was set to be 0.67 Pa with argon (Ar): oxygen gas (O 2 ) flow rate ratio of 35: 5 standard cubic centimeters per minute at room temperature.IGZO films were photo-patterned and wet etched with a buffered oxide etchant (200:1, Mattech Resource).Al 2 O 3 as a gate insulator layer was deposited by plasma enhanced atomic layer deposition (PEALD) using trimethyl aluminum (TMA) as a metal source and O 2 as a reactant.Plasma power was set to be 100 W at 250 °C.One cycle of atomic layer deposition (ALD) was performed with the following sequence: TMA source injection (0.5 s)-argon (Ar) purge (3.0 s)-O 2 injection (1.0 s)-RF-plasma (0.5 s)-Ar purge (5 s).The thickness of Al 2 O 3 layers was set to be 150 nm.After S/D contact holes were formed on the Al 2 O 3 dielectric layer by photolithography, Mo (50 nm) gate electrode was sputter-deposited and patterned to afford IGZO TFTs with a top-gate staggered configuration.These IGZO TFTs were post-annealed at 350 °C for 1 h.Meanwhile, an IGZO TFT with a barrier layer on the glass substrate was fabricated as a comparative TFT device.Before depositing ITO S/D, an Al 2 O 3 barrier layer (50 nm) was formed on the substrate using PEALD under the same deposition conditions (250 °C, 100 W) as a gate insulator.All other TFT processes except for the barrier layer were carried out in the same way.I-V characteristics of a-IGZO TFTs were then measured with gate voltage (V g ) sweep of − 20 V to + 20 V at a constant drain voltage (V d ) of + 10 V using a semiconductor device analyzer (B1500A, Agilent) at room temperature under dark air ambient condition.Time of flight-secondary ion mass spectrometry (TOF-SIMS; M6, IONTOF GmbH) was conducted to determine the amount of each component such as In, Ga, Zn, H, OH, and Cl ions in IGZO films.A cesium (Cs + ) primary ion beam was used at a current of 120 nA and a raster size of 200 × 200 mm 2 .Chemical states of oxygen atoms in Al 2 O 3 /IGZO/glass stack and Al 2 O 3 /IGZO/Al 2 O 3 /glass stack were examined by X-ray photoelectron spectroscopy (K-Alpha + , Thermo Fisher Scientific).

Results and discussion
Electrical characteristics and bias stress stability of top gate a-IGZO TFTs. ) after annealing at 350 °C for 1 h, respectively.These a-IGZO TFTs exhibited excellent electrical performances and uniformity with field effect mobility of 7.3 ± 0.92 cm 2 V −1 s −1 , V th of 0.25 ± 0.33 V, subthreshold slope (SS), and 112 ± 44 mVdec −1 in the above-described range of channel width by length (W/L).To examine electrical reliabilities of these a-IGZO TFTs, changes in their I-V curves were observed after they were stressed under positive gate bias condition applying gate voltages of + 10 V, + 20 V, and + 30 V at a fixed drain voltage of + 0.1 V for 10,000 s, respectively.As the gate voltage of TFTs increased, the V th of transfer curves shifted more to the positive direction as shown in Fig. 2.This could be attributed to an increase in charge trapping at the channel interface of the IGZO semiconductor by a prolonged positive gate bias 21,22 .Meanwhile, when the stress time reached 10,000 s under positive gate bias stress (PBS), a weak hump appeared in subthreshold region of the I-V curve (Fig. 2a).As shown in Fig. 2b,c, considering that the hump was significantly increased with increasing gate voltage to + 20 V and + 30 V, it was found that this hump phenomenon was generated obviously by a gate bias stress.We noted the occurrence of this abnormal hump at long-term direct current (DC) gate bias stress.To determine whether the same hump would appear in negative gate field, a sustained negative gate bias stress (NBS) to the a-IGZO TFT was applied under the condition of a gate voltage of − 20 V and a drain voltage of + 10 V for 10,000 s.As shown in Fig. 3a, there was little change in the I-V curve up to 3,000 s at NBS.Although the V th shifted slightly negatively (ΔV th = -0.98V) after 10,000 s, no hump occurred.As shown in Fig. 3b, we applied NBS again to the TFT device where the hump was generated by PBS to observe how the hump changed.The hump gradually decreased as NBS continued.It completely disappeared after 10,000 s.Therefore, it was clearly found that the hump phenomenon in the a-IGZO TFTs occurred only by positive gate bias.Meanwhile, according to previous studies on reliabilities of a-IGZO TFTs, impact ionization by strong lateral electric field in drain region of a-IGZO semiconductors can cause hump 20,23,24 .The electrical reliability in terms of hump against high drain field stress in the a-IGZO TFT was then investigated by applying continuous DC stress under a drain voltage of + 25 V and a gate voltage of + 5 V (V dg = + 20 V) for 10,000 s.As shown in Fig. 4, there was only a slight increase in drain current after drain current stress.No hump phenomenon appeared.Therefore, the hump phenomenon in our a-IGZO TFTs was not related to the lateral electrical field induced by the high drain voltage.Instead, it was caused by the vertical electrical field formed by gate voltage.

Characterization and analysis for hump generation in top gate a-IGZO TFTs.
Several studies have been conducted on hump due to a gate bias stress of a-IGZO TFTs [15][16][17][18][19][20] .Hump phenomena are known to occur when shallow donor species such as ionized vacancies, metal interstitials, and hydrogen interstitials in a-IGZO semiconductors are trapped in the channel interface or a-IGZO edge region, or when they are trapped in the back channel.We observed changes in I-V curves after applying PBS by 10,000 s to respective TFT devices with various channel widths and channel lengths to understand the generation mechanism of hump caused by gate bias applied to a-IGZO TFTs. Figure 5a shows initial I-V curves of a-IGZO TFTs with a channel length fixed at 20 μm and different channel widths of 20, 40, 80, and 160 μm and I-V curves of those after PBS of 10,000 s. Figure 5b shows I-V curves of IGZO TFTs with a channel width fixed at 160 μm and different channel lengths of 5, 10, 20, 40, 80, and 160 μm before and after PBS.Dashed lines are initial I-V characteristics of a-IGZO TFTs and solid lines are I-V characteristics of those after PBS 10,000 s.As shown in Fig. 5a, the size of the hump was irrelevant to the change in channel width of TFTs.On the other hand, as shown in Fig. 5b, the channel length had an obvious effect.The current level of the hump increased as the channel length was shortened.Considering that the hump phenomenon depended only on channel length, it suggested that a parallel type of parasitic transistor was created in the edge region of a-IGZO active island by continuous PBS, resulting in a hump.We thought that it would not be favorable to explain the gradual hump generation using a shallow donor model related to charged  defects such as ionized vacancies, metal interstitials, or hydrogen interstitials created in a-IGZO by PBS.This is because shallow donors in the a-IGZO semiconductor can have more stable formation energy when the Fermi level (E F ) is moved away from the conduction band edge by negative gate bias 25,26 .In other words, it is difficult to suggest that these shallow donors could directly affect the hump because they must be reduced within the IGZO bulk in PBS.In addition, the device characteristics were compared by varying barrier thicknesses such as 2 nm, 5 nm, and 50 nm to compare whether SiO 2 serves as a diffusion blocking layer or is more effective than Al 2 O 3 layer.Figure S1a-c show IGZO TFTs with Al 2 O 3 barrier of 50 nm, 5 nm, and 2 nm, respectively.In Al 2 O 3 barrier, hump occurred at 2 nm, but hump did not occur at 5 nm.On the other hand, Fig. S2a-c show IGZO TFTs with SiO 2 barrier of 50 nm, 5 nm, and 2 nm, respectively.In SiO 2 barrier, the size of hump decreased as the thickness increased, but hump still occurred even at 50 nm.Therefore, it was confirmed that Al2O3 ALD thin film acts as an excellent barrier for Na, but the SiO 2 ALD thin film is difficult to use as a good barrier.
On the other hand, this hump phenomenon might be caused by ionic chemical species that have moved from the glass substrate.To prevent ionic chemical species that could affect electrical properties of the TFT coming across from the glass substrate, an Al 2 O 3 barrier layer (50 nm) was formed on top of the glass substrate using PEALD.All other processes were done under the same conditions to fabricate the a-IGZO TFT. Figure 6 shows I-V curves a-IGZO TFTs with Al 2 O 3 barrier under PBS with gate voltage of + 10 V, + 20 V, and + 30 V, respectively.Interestingly, there was no hump in these TFTs at all.From the fact that Al 2 O 3 barrier can solve this problem, the cause of the hump could be diffusion of some cation species to the a-IGZO layer from the glass substrate during a high temperature post-annealing of a-IGZO TFT.
TOF-SIMS analysis was conducted to determine which ionic impurity diffused into the a-IGZO layer affected the hump.With respect to analysis targets, proton, hydroxyl ions, Na ions, and Al ions were considered as candidates of ion species that might affect electrical properties of TFTs by diffusion into IGZO from a glass substrate.Figure 7a shows TOF-SIMS data for a-IGZO thin film without Al 2 O 3 barrier and Fig. 7b shows data for a-IGZO thin film with Al 2 O 3 barrier.Comparing these two SIMS data, it was found that numbers of proton and hydroxyl  ions were as small as being negligible (only a few counts for H and OH).Thus, their changes in IGZO according to the presence or absence of Al 2 O 3 barrier might be insignificant, making it difficult to affect the hump in the a-IGZO TFT.It was also found that Na ions diffused a lot inside a-IGZO film when there was no Al 2 O 3 barrier and that Al ions diffused a lot inside IGZO film when there was an Al 2 O 3 barrier.Since the hump phenomenon was only found in a-IGZO TFTs without barrier layer, Al diffusion was not the cause of the hump.Thus, it could be concluded that Na ions should be the chemical species that caused the hump.In addition, XPS analysis was conducted to investigate the basic film quality of a-IGZO on a glass, a-IGZO on Al 2 O 3 , and Al 2 O 3 .Figure 8 shows O1s spectra of Al 2 O 3 /a-IGZO/glass stack and 2 O 3 /a-IGZO/Al 2 O 3 /glass stack.The binding energy of O1s peak of Al 2 O 3 bulk layer was 531.7 eV in both stacks.a-IGZO (531.1 eV) on glass and a-IGZO (531.0 eV) on Al 2 O 3 showed similar O1s spectra in the IGZO bulk film layers.However, the O1s spectrum was quite different in the region close to the rear interface of a-IGZO.a-IGZO (531.0 eV)/glass interface showed relatively higher binding energy than IGZO (530.7 eV)/Al 2 O 3 interface.This showed that the chemical bonding environment of the IGZO rear interfaces might vary considerably due to the difference of inorganic layers such as SiO 2 and Al 2 O 3 despite the same binding energy in the a-IGZO bulk regions of the two stacks.Meanwhile, we fabricated a-IGZO TFTs using a Si wafer including SiO 2 as an alkali-free substrate.Figure S3 shows the I-V characteristics of a-IGZOTFT on the Si wafer substrate according to the PBS time, but hump did not occur at all.Therefore, by using a Si wafer as an alkali-free substrate, it was possible to reconfirm that Na + in the substrate was the cause of time-delayed hump in the I-V curve.
Interpretation of hump generation mechanism in top gate a-IGZO TFTs.We examined how Na ions diffused into IGZO could cause a hump.Figure 9 shows 3D-device structures and cross-sectional views of the cross-section passing through the center of the gate in the plan view of the IGZO TFTs. Figure 9a,b show the TFTs without Al 2 O 3 barrier and with Al 2 O 3 barrier, respectively.Especially, Fig. 9a illustrated the process of creating parasitic transistors on edge regions of a-IGZO active island that could cause a hump.To understand the origin of the hump characteristics, TOF-SIMS analysis data before and after post-annealing of 350 °C were compared as shown in Fig. S4.Although post-annealing obviously increased the diffusion of Na ions, Na ions  were identified within a-IGZO on glass even before post-annealing.It is believed that the diffused Na ions were detected in a-IGZO even before annealing due to a heat treatment effect by the high temperature deposition process (250 °C) of the Al 2 O 3 gate insulator.Therefore, rather than Na + diffusion being the direct cause of hump, Na + migration (drift) due to an electric field by gate bias might be the cause of hump.Na + could be migrated to the interface of glass and a-IGZO due to a vertical electrical formed under PBS.Figure 10 shows an energy band diagram to explain the mechanism of hump generation due to Na + diffused in the IGZO film by the gate electric field.Figure 10a shows an initial flat-band state of a-IGZO TFT during high-temperature annealing.Due to diffused Na ions from the glass substrate, the energy band would be slightly bent behind the a-IGZO layer.Figure 10b,c show a-IGZO band bending by Na + migration into the back of the IGZO by the electric field under long-term PBS, presenting the mechanism of hump generation accordingly.Regarding edge regions, both sidewalls of the IGZO are thin.The electric field on the IGZO edge regions would be stronger than that on the a-IGZO bulk layer.Na ions would be quickly trapped, and conduction band lowering might occur first at the edge region.Eventually, the channel of parallel transistors created in the edge regions might open early, causing a hump.In other words, even if the same gate voltage was applied, a-IGZO on the edge region might reach an accumulation state first.IGZO channel on the edge region could be formed earlier than that on the bulk region, leading to the occurrence of a hump.On the other hand, as shown in Fig. 10d,e, when NBS was applied, the trapped Na ions would be de-trapped from the IGZO interface and migrated in the direction of the front channel.Therefore, electrons accumulated at the edge would be depleted and the hump disappeared.

Conclusion
Top gate a-IGZO TFTs post-annealed at high temperature of 350 °C showed good field effect mobility with uniform V th and SS at their I-V characteristics according to different channel widths and lengths.However, hump occurred in the subthreshold region when they were exposed to positive gate DC bias stress for a long time (more than 3600 s).This stress-induced hump accelerated at higher positive gate voltage but disappeared by negative gate voltage.In particular, the current level of the hump increased as the channel length was shortened, while it was independent on channel width.It was thought that parallel parasitic transistors might have been created in the edge region of a-IGZO active island during sustained PBS.This could be caused by ionic chemical species moving from the glass substrate.TOF-SIMS analysis confirmed that Na ions were significantly diffused into a-IGZO from glass substrate where there was no Al 2 O 3 barrier layer.To prevent ionic chemical species that could affect electrical properties of the TFT from coming across from the glass substrate, an Al 2 O 3 barrier layer was formed between a-IGZO TFT and glass substrate, leading to no hump in the resultant TFT at all.Mobile Figure 1a,b show I-V characteristics of top gate a-IGZO TFTs with different channel widths (W = 20, 40, 80, 160 μm with a fixed L of 20 μm) and different channel lengths (L = 5, 10, 20, 40, 80, 160 μm with a fixed W of 160 μm

Figure 3 .
Figure 3. I-V characteristics for a-IGZO TFTs according to stress time under negative gate bias condition of V g = − 20 V and a drain voltage of + 10 V (a) from an initial state of a pristine TFT without stress and (b) those from the hump state after stress under a positive gate bias for 10,000 s.

Figure 4 .
Figure 4. I-V characteristics of a-IGZO TFTs under high drain bias stress of V d = + 25 V and gate voltage of + 5 V (V dg = + 20 V).

Figure 5 .
Figure 5. I-V characteristics for a-IGZO TFTs of initial states and after PBS of 10,000 s with (a) different channel widths and (b) different channel lengths.

Figure 6 .
Figure 6.I-V characteristics of a-IGZO TFTs with Al 2 O 3 buffer layer on glass substrate according to positive gate bias stress of (a) V g = + 10 V, (b) V g = + 20 V, and (c) V g = + 30 V.

Figure 7 .
Figure 7. TOF-SIMS data of (a) an a-IGZO film without Al 2 O 3 barrier layer and (b) an a-IGZO film with Al 2 O 3 barrier layer.

Figure 9 .
Figure 9. 3D-device structures and cross-sectional views of the cross-section passing through the center of the gate in the plan view of the IGZO TFTs; (a) the TFTs without Al 2 O 3 barrier and (b) the TFTs with Al 2 O 3 barrier.

Figure 10 .
Figure 10.Energy band diagrams of a-IGZO TFTs (a) at initial state after high-temperature annealed, (b) at bulk region under long-term PBS condition, (c) for generation of parasitic transistors causing a hump on the edge region under long-term PBS condition (Na ions would be quickly trapped, and conduction band lowering might occur first at the edge region.),(d) at bulk region under long-term NBS condition, and (e) on the edge region under long-term NBS condition (The trapped Na ions would be de-trapped from the IGZO interface and migrated in the direction of the front channel.).