Impact of cation compositions on the performance of thin-film transistors with amorphous indium gallium zinc oxide grown through atomic layer deposition

ABSTRACT This paper reports the effect of the cation composition on the electrical properties of amorphous indium gallium zinc oxide (a-IGZO) thin-film transistors (TFTs) where atomic layer deposition (ALD) was used to deposit an a-IGZO channel layer. The In0.38Ga0.18Zn0.44O transistors at a 200°C annealing temperature exhibited 39.4 cm2/V·s field effect mobility (µFE), −0.12 V threshold voltage (VTH), 0.40 V/decade subthreshold gate swing (SS), and >107 ION/OFF ratio, corresponding to the state-of-the-art characteristics of transistors with a sputtered IGZO channel. Further enhancement of the μFE value was observed for the devices with a higher In fraction: the In0.45Ga0.15Zn0.40O transistor had a higher μFE value of 48.3 cm2/V·s, −4.06 V VTH, 0.45 V/decade SS, and >107 ION/OFF ratio. The cation composition dependence on the performance of the a-IGZO TFTs was explained by analysing the density-of-state (DOS) distribution for the corresponding devices using the experimental independent variable (IV) and theoretical Technology Computer-aided Design (TCAD) simulation.


Introduction
The demand for thin-film transistors (TFTs) with high performance has been increasing due to the need for low power consumption, ultra-high pixel density, good form factor, and interactive functionality. In particular, the research on metal oxide semiconductors that are applicable to high-end active-matrix displays is in the spotlight because of their merits, including high mobility, good uniformity over large areas, low process temperature, optical transparency, and cost-effective fabrication process [1][2][3]. Metal oxides consisting of ternary or quaternary metal compounds (e.g. InZnO, ZnSnO, InGaZnO) have been investigated for use in TFTs, and their electrical properties can be adjusted by varying the relative atomic ratios of the metal ions [4][5]. Among them, amorphous indium gallium zinc oxide (a-IGZO) has become an industry-standard channel layer due to its extremely low leakage current, good subthreshold gate swing (SS), reasonable mobility, and good long-term reliability.
Magnetron sputtering is currently mainly used for depositing the a-IGZO channel layer because it allows a low-temperature process and good productivity of a facile a-IGZO film with a cation composition close to that of the sputtering target material. It is difficult, however, to control the cation composition in the a-IGZO film. Also, the presence of high-energy radicals in the plasma chamber can damage the dielectric/semiconductor interface and the growing film itself, which may act as unwanted electrical trap states. Solution-based deposition processes have been developed as alternatives to reduce the fabrication cost of a-IGZO films. The device performance and electrical reliability of solution-based oxide transistors, however, have turned out to be inferior to those of sputtering-based devices. Atomic layer deposition (ALD) has gained attention of late as an advanced process for growing an a-IGZO channel layer because of its intriguing features, such as its accurate thickness/cation composition controllability, good uniformity, and good step coverage. Some of the previous reports showed better electrical performance compared to the sputtering and solution-based process [6][7][8][9][10].
In this work, TFTs with an a-IGZO channel layer grown via ALD were examined. In particular, the cation composition of the a-IGZO film was tailored by adjusting the subcycle duty and consequence. The a-IGZO TFTs with an optimized cation composition exhibited a high field effect mobility (μ FE ) of 48.3 cm 2 /Vs, −4.06 V threshold voltage (V TH ), 0.45 V/decade SS, and > 10 7 I ON/OFF ratio. The cation-composition-dependent performances of the a-IGZO TFTs were explained by analyzing the distribution of the density of states (DOS) in a band gap, which were extracted by measuring the temperature-dependent independent variable (IV) characteristics based on the Meyer-Neldel rule (MN rule). Such experimental DOS profiles for the various IGZO TFTs were confirmed via Technology Computer-aided Design (TCAD) simulation.

Materials and device preparation
Bottom-gate, top-contact-structure a-IGZO TFTs were fabricated on a p + -Si substrate. A heavily doped, p-type Si wafer was used as the gate electrode, and a thermally oxidized 100-nm-thick SiO 2 layer was used as the gate dielectric. 13-nm-thick a-IGZO active layers were deposited in a traveling-wave-type ALD apparatus (CN1 Co., Ltd., South Korea). The proposed source canisters are bypass-type: metal precursors were injected directly into the source line, and 50 standard cubic centimeter per minute (sccm) N 2 gas was used as the carrier gas for the precursor delivery. The N 2 gas flow rate was controlled by a mass flow controller. The canister containing the In precursor was maintained at 80°C to provide sufficient vapor pressure and dose, while the canisters containing the Ga and Zn precursors were kept at room temperature because they have sufficient vapor pressure at room temperature. O 3 was used as a reactant. A 970 sccm O 2 -30 sccm N 2 gas mixture was introduced into the O 3 generator, which produces O 3 gas at a 250 g/m 3 concentration. An a-IGZO active layer was grown in one cycle, which consists of several sequential steps in a sequence of In 2 O 3 , ZnO, and Ga 2 O 3 subcycles. By adjusting the number of subcycles of each binary metal oxide, a-IGZO layers with different cation compositions were obtained. A rather long purge time (10 s for each metal precursor and O 3 purge) was adopted to prevent the mixing of the precursors and the reactant. The substrate temperature was set at 250°C, at which simultaneous self-limiting behaviors of the In 2 O 3 , Ga 2 O 3 , and ZnO films in terms of the substrate temperature were observed. The a-IGZO active layer was patterned via photolithography and wet etching. After that, post-deposition annealing (PDA) was performed in the air for 1 h, at 200°C, for the a-IGZO active layer. An indium tin oxide (ITO) film was deposited as a source/drain (S/D) electrode via DC magnetron sputtering, and was patterned through the lift-off method. The  In  50  38  54  42  57  45  Ga  17  18  14  15  14  15  Zn  33  44  32  43  29  40 channel width (W) and length (L) of the a-IGZO TFTs were 10 and 50 μm, respectively. The completed devices were subjected to contact annealing in the air for 1 h, at 250°C.

Characterization
The chemical compositions of the a-IGZO films were examined via X-ray fluorescence (XRF; Thermo Scientific) spectroscopy, for which the atomic concentration was calibrated via proton-induced X-ray emission. The weight percentages (wt. %) of the cation compositions obtained via XRF analysis for the a-IGZO thin films are summarized in Table 1, where the relative cation percentage (at. %) of each film was calculated by considering its molecular weight. The thicknesses of the In 2 O 3 , ZnO, Ga 2 O 3 , and a-IGZO films were measured via spectroscopic ellipsometry (SE). The chemical states of the a-IGZO films were analyzed via X-ray photoelectron spectroscopy (XPS; SIGMA PROBE, ThermoVG). The back surface region ( ∼ 5 nm) of the a-IGZO films was etched via plasma in-situ etching in the XPS chamber to remove the surface contamination. In this study, the O 1s XP spectra were deconvoluted into three asymmetric Lorentzian-Gaussian peaks with a Shirley background. The net electron density (n e ) and mobility of the a-IGZO films were evaluated through Hall effect measurement using the van der Pauw configuration. The electrical characteristics of the a-IGZO TFTs were measured in the air, at room temperature, using a Keithley 2636 source meter. Figure 1 shows the variations in the thicknesses of the In 2 O 3 , Ga 2 O 3 , and ZnO thin films as a function of the number of ALD cycles. A linear relationship was observed between the number of ALD cycles and the film thickness, which is expected in the typical ALD process. Figure 2 shows the representative transfer and output characteristics of the a-IGZO TFTs with different cation compositions, which were fabricated through 200°C PDA. The μ FE was calculated from the linear   Table 2). It is noted that these mobility metrics are superior to those of sputtering-based a-IGZO devices. As the In/Ga ratio increased in the IGZO channel layer, the μ FE values of the resulting devices were improved. Thus, the In 0.45 Ga 0.15 Zn 0.40 O transistors exhibited a high μ FE value of 48.3 ± 1.7 cm 2 /Vs, −4.06 ± 0.42 V V TH , and 0.46 ± 0.04 V/decade SS without any I ON/OFF ratio ( > 10 7 ) compromise. The output characteristics for all the IGZO TFTs showed clear pinchoff, which indicates that the electron transport in the ALD-derived a-IGZO transistor was controlled by the V GS and V DS . To obtain an insight into the cation-compositiondependent mobility variations for the IGZO TFTs, the chemical states of the a-IGZO films with different cation compositions were examined via XPS. Figure 3 shows the O 1s peak XP spectra of the a-IGZO films with different cation compositions. The photoelectron binding energies were calibrated to the C 1s peak for the C-C bonds at 284.5 eV. The O 1s peak was deconvoluted into three peaks at 530.5, 531.5, and 532.5 eV. The O 1s peaks centered at 530.5 and 531.5 eV were assigned to the oxygen in the oxide lattices without and with an oxygen vacancy (V O ), respectively [11]. The peak at 532.5 eV was identified as impurity-related oxygen, such as hydroxyl groups.  Table 2. On the other hand, the relative areas of the hydroxyl-group-related peaks were nearly independent of the cation composition (Table 3).    as on a face or an edge of an a-IGZO film, acts as a beneficial shallow donor, which is favored in a denser network. Thus, it is obvious that the largest V O density was obtained for In 0.45 Ga 0.15 Zn 0.40 O (in the O 1s XP spectra), which is the reason for its highest n e value (in the Hall effect data). In addition, the highest mobility for the In 0.45 Ga 0.15 Zn 0.40 O TFT can be attributed to not only the smallest percolation path formation but also the percolation-boosting effect based on the proportionality between n e and μ FE [1].

Results and discussion
To obtain an insight into the cation-compositiondependent performance of a-IGZO TFTs, the DOS profiles in a forbidden band gap of the a-IGZO semiconductor were extracted from the measurement-temperaturedependent IV characteristics based on the MN rule. There are many previous studies that extracted the DOS of the metal oxide TFT by applying the MN rule [13,14]. Following the MN theory, for a thermally activated process (A), a prefactor A 0 can be defined in the following Arrhenius equation: where k is the Boltzmann constant, T is the temperature, and E A is the activation energy of the process. Also, for disordered systems, A 0 and E A have a correlation that can be described using the following equation: where E MN and A 00 are the MN energy and prefactor for A 0 , respectively. These relationships between A 0 and E A are observed for the current of conductance in various types of TFTs, whether organic or inorganic. Thus, the thermally activated I DS of the TFTs can be described by the following equations: where I DS0 and I DS00 are the prefactors, respectively, for I DS . From these MN relations, the total DOS distribution for a-IGZO TFT devices with different cation compositions with 200°C PDA was calculated. Figure 5(a)-(c) show the relationship between 1/kT and I DS as a function of V GS for the different a-IGZO TFTs with 200°C PDA, where a common crossing point was clearly observed. For each dataset, E A and I DS0 can be determined using the minimum linear square method. Figure 5(d)-(f) show the calculated DOS distribution as a function of the energy (E-E C ) for the a-IGZO TFTs. The total DOS value at a specific energy level is the summation of N it and N SS because both trap states hinder the moving up of the E F level at the interface with increasing V GS ( > V FB ). The total DOS distributions were found to increase when the compositions of the In and Ga cations increased and decreased, respectively. A portion of this V O defect can act as a carrier trap center located in the tailing states below the aforementioned CB edge. This explanation is consistent with the above experimental observation: the increasing DOS distributions with increasing In fraction in the ALD-derived a-IGZO transistors. TCAD simulation was performed to double-check the MN-rule-based extraction of the DOS distributions of the given a-IGZO TFTs. Figure 6 shows that the measured transfer characteristics of the a-IGZO TFTs can be well fitted by the model obtained from the TCAD simulation incorporating the extracted DOS as a parameter. In the simulation, the free-electron densities n free = 3.  Figure 7 shows the subgap DOS distribution of the a-IGZO TFTs with different cation compositions obtained from the MN rule and the TCAD simulation. The extracted acceptor-like DOS g(E) close to E C was empirically modeled to be a superposition of deep and tail states in exponential form, as follows: with N TA = 1.18 × 10 18 , 1.35 × 10 18 , and 2.68 × 10 18 cm −3 , respectively, and kT TA = 0.06, 0.055, and 0.05 eV, for the tail states from the conduction band. Also,  N DA = 3.0 × 10 16 , 2.0 × 10 16 , and 1.5 × 10 17 cm −3 , respectively, and kT DA = 0.27, 0.50, and 0.15 eV, for the deep states from the conduction band. The comparative result from the TCAD simulation is the well-fitted result of the data obtained from the MN rule. Finally, it was noted that the gate bias thermal stability of ALD-derived IGZO TFTs is important for their implementation in commercial products. This reliability is being investigated, and the result of the investigation will be reported elsewhere.

Conclusion
In this work, the effect of the cation composition on the performance of amorphous indium gallium zinc oxide (a-IGZO) thin-film transistors (TFTs) where an IGZO channel layer had been deposited via atomic layer deposition (ALD) was investigated. The cation composition ratio of the a-IGZO channel layer was controlled by the changing number of injection subcycles of binary In 2 O 3 , Ga 2 O 3 , and ZnO during ALD growth. It was found that the carrier mobility of the fabricated transistors increased with increasing In fraction in the a-IGZO film. At the same time, the V TH value showed a negative displacement with increasing In fraction whereas the I ON/OFF ratio was maintained irrespective of the In fraction. This behavior can be well explained by the role of In cation as a carrier generator and an oxygen vacancy (V O ) creator. Thus, the In 0.45 Ga 0.15 Zn 0.40 O transistor exhibited a respectable high μ FE value of 48.3 cm 2 /Vs, −4.06 threshold voltage (V TH ), 0.45 V/decade subthreshold gate swing (SS), and a good I ON/OFF ratio of > 10 7 . The Meyer-Neldel rule (MN rule)-and Technology Computer-aided Design (TCAD) simulation-based DOS distribution analysis for the different a-IGZO transistors revealed that the acceptor-like tail states below the CB edge increased with increasing In fraction in the a-IGZO channel, which is responsible for the higher SS factor for the In 0.45 Ga 0.15 Zn 0.40 O transistor. The highperformance ALD-derived a-IGZO semiconductor can be implemented in the various emerging devices for the high-end next-generation active-matrix organic lightemitting diode (AMOLED) and flexible and stretchable display.
Kwon-Shik Park received his Ph.D. Material Science & Engineering degree from Korea Advanced Institute of Science and Technology (KAIST), South Korea in 1999. Since 2000, he has been with the R&D Center of LG Display Co., Ltd., South Korea, where he has been researching on LCD and OLED device and process technologies as the vice president and the head of the Device Process Research Division. His research has focused on the device technology for advanced displays, the design of integrated circuits based on various backplane technologies, and new materials and processes for future displays. In 2009, he joined Inha University as an assistant professor. Since September 2015, he has been a professor at the Department of Electronic Engineering of Hanyang University. He has authored/co-authored more than 140 international journal papers and has 112 international patents.