Optimizing the Crystallinity of a ZrO2 Thin Film Insulator for InGaZnO‐Based Metal–Insulator–Semiconductor Capacitors

The utilization of a zirconium oxide (ZrO2) thin film as the insulator in a metal–insulator–semiconductor (MIS) capacitor to enhance the characteristics of thin‐film transistors is investigated. Although the high crystallinity of ZrO2 is favorable to achieving higher capacitance density in metal–insulator–metal capacitors with ZrO2 thin films, it decreases the capacitance with increasing applied bias in Mo/ZrO2/InGaZnO (IGZO)‐structured MIS capacitors. Through comprehensive physical, chemical, and electrical characterizations, this study investigated the mechanism underlying the decreasing capacitance with increasing the applied bias in the accumulation state of the Mo/ZrO2/IGZO‐structured MIS capacitor depending on the crystallinity of ZrO2. Furthermore, the investigation identifies the optimal crystal structure of ZrO2 thin films for IGZO‐based MIS capacitors, highlighting the importance of forming meso‐crystalline structures in high dielectric constant (k) materials to enhance the k value and mitigating the decrease in capacitance caused by the accumulated carrier loss through grain boundaries of ZrO2.


Introduction
Currently, thin-film transistors (TFT) based on the transparent oxide amorphous InGaZnO (IGZO) have been widely studied as DOI: 10.1002/admi.202300883switching devices for liquid-crystal displays and flexible active-matrix organic light-emitting diode displays. [1]The operation speed and dimensions of TFT should be enhanced to achieve highspeed and high-resolution displays. [2]In this regard, the application of gate insulators (GIs) with high-dielectric-constant (k) materials has attracted much interest; they include ZrO 2 , [3] HfO 2 , [4] TiO 2 , [5,6] TaO x , [7] and Al 2 O 3 . [8]Among the various candidate high-k materials for GI applications, ZrO 2 has a high k (≈25) and an adequate bandgap (≈5.8 eV) that are suitable for the low-voltage operation of TFTs. [9,10]owever, the use of high-k materials might degrade the TFT performance, which has not been considered in the use SiO 2 as the GI. [11,12]][20][21][22][23] Moreover, as ZrO 2 is a widely used high-k material in metal-insulator-metal (MIM) capacitor applications, several studies have been conducted on adopting ZrO 2 in the GIs of IGZO-based TFTs.However, for ZrO 2 , the crystallinity, which strongly influences the k value, also hinders the enhancement of TFT performance.
In this study, a metal-insulator-semiconductor (MIS) structure comprising Mo/ZrO 2 /IGZO was investigated.A ZrO 2 thin film was deposited as an insulator layer using atomic layer deposition (ALD), and its crystallinity was modulated using the deposition process temperature.In contrast to the electrical characteristics of ZrO 2 thin films in the MIM structure, the crystallization of ZrO 2 induced a degradation in the capacitance density of the MIS structure.Moreover, a distinctive decrease in capacitance density with increasing applied bias in the accumulation range was observed when employing a crystallized ZrO 2 thin film.Hence, a mechanism involved in the behavior of Mo/ZrO 2 /IGZO MIS structure was investigated using various physical, chemical, and electrical characterization methods.Finally, we demonstrated an optimized ZrO 2 ALD process for the MIS structure for next-generation TFT applications.

Results and Discussion
First, ALD of ZrO 2 using cyclopentadienyl tris(dimethylamino)zirconium (CpZr[N(CH 3 ) 2 ] 3 , CpZr) and O 3 on a Mo substrate was demonstrated.Owing to the high thermal stability of the Cp ligand in CpZr, no significant thermal decomposition in the temperature range of 150-300 °C was observed (Figure 1a). [24,25]The growth rate of ZrO 2 thin film gradually increased with increasing process temperature, which is typical behavior in the ALD process induced by increasing reactivity of O 3 , not by the thermal decomposition of CpZr, in given ALD sequence of CpZr feeding, Ar purge, O 3 feeding, and Ar purge of 20 (20), 20 (60), 5 (10), and 20 (30) s, respectively, at the deposition temperature of 200-300 (150) °C.The crystal structures of the 10-nm-thick ZrO 2 thin films were examined after post-deposition annealing (PDA) (Figure 1b) using GA-XRD measurements.Although the PDA was conducted under the same conditions, the crystallinity of the ZrO 2 thin films varied with the deposition temperature.At deposition temperatures of 150 and 200 °C, the thin films exhibited an amorphous phase, even when conducting the PDA process.The tetragonal-phased crystal structure was observed in the thin film deposited at 300 °C.[28] This result suggests that the crystallization of ZrO 2 thin film, deposited on Mo, requires the formation of a crystal nucleus through the thermal budget accumulated during the deposition process prior to the PDA process.
The k values of the ZrO 2 thin films in a Mo/ZrO 2 /TiN MIM capacitor and the Mo/ZrO 2 /IGZO MIS capacitors were investigated (Figure 2).A schematic diagram of the MIS capacitor is shown in the inset of Figure 2a.During the electrical measurements, the top Mo electrode was grounded, and a bias was applied to the bottom Mo electrode.When the DC bias was positive, electrons accumulated in the IGZO semiconductor layer.Therefore, the k value of ZrO 2 thin film was calculated using the capacitance density of the MIS capacitor obtained at an applied DC bias, AC level, and frequency of +2.5 V, 50 mV, and 100 kHz, respectively, in the accumulation region.In the case of the MIM capacitor, the k value was extracted from the capacitance density at a DC bias of 0 V.The change in k with respect to the deposition temperature is shown in Figure 2a.With increasing deposition temperature, k increased from 10.8 at 150 °C to 14.1 and 21.7 at 200 and 250 °C, respectively, which implied that the enhanced crystallinity of ZrO 2 thin films increases the k value.However, the k of thin films deposited at 300 °C exhibited a slightly lower value of 20.0 although the ZrO 2 thin film deposited at 300 °C had a fully crystallized structure (Figure 1b).In contrast, the dielectric constant extracted from the MIM structure exhibited a trend of gradual increase with increased deposition temperature of 300 °C, and achieved the highest value of 30.59 with the ZrO 2 thin film deposited at 300 °C.This difference between MIS and MIM capacitors at 300 °C implied that the decrease of k value at 300 °C was attributed to the IGZO.At the initial region of accumulation, at a DC bias of ≈+1.1 V, the k value of ZrO 2 deposited at 300 °C was 21.8, which was higher than that of ZrO 2 deposited at 250 °C (19.5) (Figure 2b).[31][32] Moreover, this decrease in capacitance density with increasing the DC bias in the MIS capacitor was only observed for using ZrO 2 deposited at 300 °C, implying that it is related to the property of ZrO 2 (insulator) not with Mo (metal) or IGZO (semiconductor) layers.In the accumulation state, the MIS capacitor can act as an MIM capacitor.However, the actual behavior would differ from the ideal behavior for several reasons, particularly those originating from the insulator and the interface of the insulator and metal, or the insulator and semiconductor. [33]n the MIM capacitor using ZrO 2 deposited via ALD, the degradation of the MIM capacitor properties originated from the interface of the ZrO 2 insulator and the bottom electrode.
For the ZrO 2 ALD, several mechanisms have been suggested to explain the degradation of the interface between the insulator and metal (bottom electrode), including the oxidation of the metal substrate by an oxygen source in the initial stage of ALD [34,35] and the oxygen scavenging effect of the metal substrate. [36]Both mechanisms are affected by the deposition process temperature; a higher process temperature facilitates the degradation of the interface.To clarify the origin that induced the capacitance density decreasing in the MIS capacitors using ZrO 2 deposited at 300 °C, the hysteresis and frequency dispersion characteristics of the MIS capacitors were investigated.The hysteresis was assessed by measuring the shift in DC bias values (Figure 3a), with the capacitance density reaching half of the difference between maximum and minimum capacitance density values (ΔV), during a cyclic DC bias sweep ranging from −2.5 to +2.5 V and back to −2.5 V (Figure 2b).As shown in Figure 3a, all the examined MIS capacitors exhibited positive values in the shift in the ΔV.During the positive bias applied on the bottom Mo electrode, electrons would be injected from IGZO to ZrO 2 and subsequently trapped within certain defects.This phenomenon leads to a reduction in the effective gate bias, ultimately resulting in a positive shift.However, the value did not have any trend regarding the process temperature.Since the IGZO layer was deposited on the ZrO 2 layer, the interface state would be regardless of the deposition temperature of ZrO 2 .In contrast, the interface of Mo electrode and ZrO 2 was strongly affected by the deposition temperature of ZrO 2 .Higher deposition temperature would induce damage to the substrate, especially the metal electrode.Moreover, the oxygen scavenging ef-fect of metal electrodes is attributed to the formation of oxygen vacancy in ZrO 2 , which would be severe with increasing the deposition temperature. [37]In this regard, a larger hysteresis indicates larger defects at the interface of Mo electrode and ZrO 2 , or bulk region of ZrO 2 .The defect in ZrO 2 related to inducing hysteresis was also not severely incorporated in the case of ZrO 2 deposited at 300 °C.Therefore, the defect inducing the hysteresis was not corresponding to the capacitance density decreasing.The frequency dispersion characteristics were evaluated from the ratio of capacitance density with frequencies of 10, and 50 to 100 kHz at DC bias of +2.5 V.As shown in C-V curves varied with frequency (inset of Figure 3c), the capacitance density was decreased with increasing the frequency (Figure 3b).In the MIM capacitor, the decreasing capacitance density was related to the defects in the insulator.During the capacitance measurement, a 50-mV-high AC bias with designated frequency was applied, and this applied AC bias induced charging/discharging on the electrode.However, by the applied AC bias, charging/discharging on the defects in the insulator also occurred simultaneously.The LCR meter cannot distinguish the contributions of charging/discharging on the electrode and the defects, in turn, the measured capacitance value is determined by the total amount of charging/discharging on the electrode as well as the defects.The charging/discharging on the defects is relatively slower than that on the electrode because the charging/discharging on the defects is accompanied by an additional process of injection of an electron from the electrode to the insulator through overcoming a certain barrier, hence, the charging/discharging on the defects would be suppressed as increasing the frequency of AC bias.This is because the capacitance density was decreased with increasing the frequency in the MIM capacitor.Therefore, an increased ratio of capacitance density at low frequency compared to high frequency indicates relative defect amount in the insulator qualitatively: an increased ratio implies relatively higher defect density.Figure 3c shows capacitance ratio values of frequency of 10 kHz calculated with capacitance density at 100 kHz varied with deposition process temperatures.For the process temperatures of 150, 200, and 250 °C, the ratio was slightly increased with increasing the process temperature.However, the ratio was significantly increased in the case of a process temperature of 300 °C.
From the above hysteresis and frequency dispersion results, the capacitance density decreasing might be attributed to relate with the defect inducing the frequency dispersion, not the hysteresis.However, since IGZO was formed by sputtering process on the already deposited ZrO 2 thin film, the deposition process temperature did not affect to the interface state.Oxygen vacancy (generally called ZrO 2-x , sub-oxide) and carbon impurity in ZrO 2 thin film, the most probable defects in the insulator, were decreased with increasing the process temperature (Figure 3d).In both XPS spectra of Zr 3d (Figure 3e-h) and O 1s (Figure 3i-l), contributions of the peak corresponding to the oxygen vacancy were decreased with increasing the deposition temperature.This is attributed to the increased reactivity of O 3 by increasing the process temperature.Consequently, the interface status or the defects related to the insulator deposition process were not related to the capacitance density decreasing.
Another mechanism of the non-ideal behavior in the MIS capacitor originates from the intrinsic property of the insulator.The critical difference of ZrO 2 deposited at 300 °C compared to the other samples is the crystallinity of ZrO 2 thin film: only ZrO 2 thin film deposited at 300 °C exhibited a distinctive diffraction peak by crystallization (Figure 1b).Therefore, only ZrO 2 thin film deposited at 300 °C had the grain boundary due to the result of crys-tallization.Moreover, enhanced crystallinity with larger grain size induced electric field concentration on the grain boundary. [38]onsequently, in the ZrO 2 , the grain boundary is a major carrier conduction path, [39] and the electric field concentration in the grain boundary would facilitate the carrier injection and conduction through the grain boundary. [38]In this regard, in the case of the MIS capacitor of ZrO 2 deposited at 300 °C, the carrier accumulated in IGZO could injected into the grain boundary of ZrO 2 .
Indeed, the estimated grain size of ZrO 2 calculated from the XRD peak of 2 of 30.4°usingScherrer's equation was 10.6 nm, which is comparable to the film thickness.This means the loss of accumulated carrier in the IGZO would be occurred through the grain boundary.As shown in Figure 4a, the dielectric loss of MIS capacitors was examined.The DC voltage at which the dielectric loss begins to increase coincides with the DC voltage at which the accumulation begins (Figure 2b).This implies that the dielectric loss in the accumulation region corresponded to the charge loss in the accumulation layer of IGZO.In the cases of ZrO 2 deposited at 150, 200, and 250 °C, the dielectric loss values were moderate.However, in the case of ZrO 2 deposited at 300 °C, the dielectric loss was almost identical to that of the other samples up to a DC voltage of +0.3 V, but it increased dramatically thereafter.This severe dielectric loss indicated the significant charge loss in the accumulation layer of IGZO by the applied DC voltage.Additionally, the frequency dependency also implies that the capacitance density decreasing was related to the loss of accumulated carrier in the IGZO (Figure 3b).In the case of frequency of 10 kHz, the capacitance density was linearly increased with increasing the deposition temperature.However, the increase gradually decreased with increasing frequency.In the low frequency, the time for accumulating carrier on the interface is abundant, and loss of carrier could be restored.Hence, the capacitance of the MIS capacitor was determined by only the insulator, which has higher k value due to crystallized ZrO 2 deposited on 300 °C.However, with increasing frequency, the time for the restoration of loss of carrier was decreased.Finally, the capacitance of the MIS capacitor would be decreased by the formation of the series capacitor of insulator and semiconductor.In current density versus applied bias (J-V) curves (Figure 4b), the MIS capacitor with ZrO 2 deposited at 300 °C exhibited a relatively low J value, but, the J value was abruptly increased and higher than the others at the V higher than ≈+0.5 V.This behavior indicated the grain boundary in the ZrO 2 deposited at 300 °C contributed to the current conduction by the electric field concentration effect at a certain applied bias.Therefore, the MIS capacitor with crystallized ZrO 2 insulator decreases the capacitance density, resulting in degradation of the capacitance density in the MIS capacitor using IGZO.

Conclusion
This investigation focused on an IGZO-based MIS capacitor employing ZrO 2 as the insulator, which is one of the most widely used high-k materials.The characteristics of a ZrO 2 thin film deposited via ALD and its crystal structure on a Mo electrode were examined.The crystallinity of the ZrO 2 thin film with a tetragonal phase increased with higher deposition process temperatures, and a distinct diffraction peak resulting from high crystallinity was observed for ZrO 2 deposited at 300 °C.However, despite the higher crystallinity, the k value of the MIS capacitor decreased at 300 °C compared with that at 250 °C.Furthermore, the MIS capacitor with ZrO 2 deposited at 300 °C exhibited a decrease in capacitance density, which decreased with increasing applied DC bias.Various physical, chemical, and electrical measurements revealed that the accumulated carrier loss in IGZO originated from grain boundary formation during crystallization in the ZrO 2 layer, which was not observed in the MIS capacitor using SiO 2 as the insulator.Consequently, when adopting highk materials as insulators in MIS capacitors, the formation of a meso-crystalline structure in high-k materials is favorable for enhancing the k value and simultaneously suppressing the depletion effect caused by grain boundaries.

Experimental Section
The bottom Mo electrode was deposited using RF sputtering with a Mo target and Ar gas glow rate of 30 sccm at a deposition temperature of 110 °C.ZrO 2 thin films were deposited on the Mo bottom electrode using ALD process (iOV dX1, iSAC Research) with cyclopentadienyl tris(dimethylamino)zirconium (CpZr[N(CH 3 ) 2 ] 3 , CpZr, SKtrichem) and O 3 (concentration of 200 g m −3 ) as Zr precursor and oxygen source, respectively.The canister of CpZr was maintained at 80 °C to obtain the appropriate vapor pressure.The deposition temperatures were set to 150, 200, 250, and 300 °C.The total thickness of the ZrO 2 thin films was kept to 10 nm.
To fabricate the MIS capacitor, the active layer was formed through the deposition of 50-nm-thick amorphous-IGZO via RF magnetron sputtering with IGZO target of In:Ga:Zn = 1:1:1 wt.%.The process pressure, RF power, and relative oxygen flow rate ([O 2 ]/[Ar+O 2 ]) were 5 mTorr, 100 W, and 0.03, respectively.Post-deposition annealing (PDA) was conducted at 300 °C for 1 h under a vacuum atmosphere using a furnace.To measure the electrical properties, a contact metal of 50-nm-thick Mo thin film (top Mo electrode) was deposited via RF sputtering defined by a metal shadow mask with a 300 μm diameter hole.
The film thickness of ZrO 2 was measured using a spectroscopic ellipsometer (ESM-300, J. A. Woollam).The crystal structures of the films were examined using glancing angle incident X-ray diffraction (GA-XRD, X' pert Pro, PANalytical) with incident angle of 0.5°.The electrical properties were evaluated by measuring the capacitance versus voltage, capacitance versus frequency, and dielectric loss using Agilent 4284 LCR meter, and the current versus voltage characteristics were measured using an Agilent 4155C semiconductor parameter analyzer.

Figure 1 .
Figure 1.a) Thermal decomposition of CpZr and growth rate of ZrO 2 thin film using CpZr and O 3 regarding the deposition process temperature.b) XRD patterns of ZrO 2 thin film deposited at 150, 200, 250, and 300 °C and followed by the PDA process.

Figure 2 .
Figure 2. a) k value of ZrO 2 thin film in the MIM and MIS capacitors calculated from the capacitance density of DC bias at 0 and +2.5 V, respectively (inset) schematic diagram of the MIS capacitor.b) capacitance density versus DC bias curves of the MIS capacitors of forward sweep (from −2.5 to +2.5 V depicted with closed symbol and solid line), and backward sweep (from +2.5 to −2.5 V depicted with opened symbol and dotted line).

Figure 3 .
Figure 3. a) Hysteresis (ΔV) of the MIS capacitor regarding to the ZrO 2 thin film deposition process temperature.b) Capacitance density of DC bias at +2.5 V with varied AC frequencies for the MIS capacitors.c) Frequency dispersion ratio calculated with capacitance density of 10 kHz over 100 kHz regarding the process temperature (inset) C-V curves of the MIS capacitor with ZrO 2 deposited at 300 °C with AC frequencies of 10, 30, 50, 70, and 100 kHz.d) Sub-oxide ratio and carbon concentration from the XPS measurement regarding to the deposition process temperature.e-h) Zr 3d and i-l) O 1s XPS spectra of the ZrO 2 thin film deposited at 150, 200, 250, and 300 °C.

Figure 4 .
Figure 4. a) Dielectric loss versus applied DC bias in C-V measurement and b) current density versus applied bias of the MIS capacitor regarding the ZrO 2 thin film deposition process temperature.