High-Performance Thin Film Transistor with an Neodymium-Doped Indium Zinc Oxide/Al₂O₃ Nanolaminate Structure Processed at Room Temperature.

In this work, a high-performance thin film transistor with an neodymium-doped indium zinc oxide (Nd:IZO) semiconductor via a room temperature approach and adopting the Nd:IZO/Al₂O₃ nanolaminate structure was investigated. The effects of the ultrathin Al₂O₃ layer and the thickness of Nd:IZO layer in the nanolaminate structure on the improvement of electrical performance and stability of thin film transistors (TFTs) were systematically studied. Besides the carrier movement confined along the near-channel region, driven by the Al₂O₃ layer under an electrical field, the high performance of the TFT is also attributed to the high quality of the 8-nm-thick Nd:IZO layer and the corresponding optimal Nd:IZO/Al₂O₃ interface, which reduce the scattering effect and charge trapping with strong M⁻O bonds in bulk and the back-channel surface of Nd:IZO, according to the X-ray reflectivity (XRR), X-ray photoelectron spectroscopy (XPS), and micro-wave photo conductivity decay (μ-PCD) results. As a result, the Nd:IZO/Al₂O₃ TFT exhibits an outstanding performance, with a high μsat of 32.7 cm²·V-1·s-1, an Ion/Ioff of 1.9 × 10⁸, and a low subthreshold swing (SS) value of 0.33 V·dec-1, which shows great potential for the room temperature fabrication of TFTs in high-resolution or high-frame-rate displays by a scalable, simple, and feasible approach.

1. Introduction

Great interest in metal oxide semiconductors (MOS) for thin film transistors (TFTs) has grown dramatically in recent years, due to their visible-light transparency, good uniformity, compatibility with different substrates, fewer limitations on processing temperature, and the high mobility to drive flexible Active Matrix Organic Light Emitting Diode (AMOLEDs) or high-resolution displays [1,2]. As the most widely-studied MOS material in TFTs, indium gallium zinc oxide (IGZO), the saturation mobility of which is usually 10~20 cm2·V−1·s−1 [3,4], is limited by its natural structure, especially when the processing temperature is strictly controlled for some device applications on flexible plastic substrates or nanopapers. As reported by our previous works [5,6], we have successfully promoted the mobility of a-IGZO TFTs to 25 cm2·V−1·s−1 under room temperature conditions by using an a-IGZO/Al2O3 nanolaminate structure. However, it is difficult to have a higher breakthrough on device performance for the a-IGZO TFT, which is still not sufficient for the high-resolution or high-framerate displays requiring mobility of over 30 cm2·V−1·s−1 [7]. Therefore, efforts to improve the performance of MOS TFTs by using neodymium-doped indium zinc oxide (Nd:IZO) will be demonstrated in this research. Compared to a Ga–O (354 kJ/mol) bond [8], the bonding strength of Nd–O (703 kJ/mol) [9] is much stronger, so it is more efficient to suppress the formation of oxygen vacancy-related defects and improve the mobility of carriers. In addition, the electronegativity of Nd is as low as 1.1, so the carrier concentration can be controlled. In this work, we demonstrate a high-performance TFT (mobility > 30 cm2·V−1·s−1) with an Nd:IZO semiconductor processed by a room temperature approach, and investigate the effects of a sputtered Nd:IZO/Al2O3 nanolaminate structure. Moreover, the thickness of the Nd:IZO layer in Nd:IZO/Al2O3 structure also showed a great impact on the device performance, due to the nature of film growth during the sputtering process.

2. Materials and Methods

Figure 1a,c shows the schematic diagrams of a single Nd:IZO TFT and an Nd:IZO/Al2O3 TFT, respectively. Besides the insertion of an ultrathin Al2O3 layer with a thickness of 3 nm for the Nd:IZO/Al2O3 TFT, the processes for the fabrication of both devices are consistent. Firstly, a 300-nm-thick Nd-doped aluminum was deposited as a gate metal on glass by direct-current (DC) magnetron sputtering and patterned by photolithography. Then, a 200-nm-thick Nd:AlO was formed on the surface of the gate metal by anodic oxidation. The introduction of the Nd element was also reported to contribute to a smooth surface of the Al electrode and high dielectric properties of the AlO gate insulator [10]. The Nd:IZO semiconductor layer was deposited by radio-frequency (RF) magnetron sputtering with a power of 60 W and a pressure of 3 mTorr in a mixed atmosphere (Ar:O2 = 20:1) using the Nd:IZO target with an Nd2O3:In2O3:ZnO ratio of 1:62.5:36.5 wt.%. For the Nd:IZO/Al2O3 TFT, the 3-nm ultrathin Al2O3 layer was then prepared by RF magnetron sputtering, with a power of 120 W and a pure Ar pressure of 1 mTorr onto the surface of the Nd:IZO layer for 120 s. Finally, the 150-nm-thick Al source/drain (S/D) electrodes were prepared by DC magnetron sputtering and patterned by a shadow mask. The width/length ratio (W/L) of all devices was 500/100 μm·μm−1. Besides the baking process for the photoresist, carried out on thermal platform under 150 °C for 1 min, no further thermal treatment was adopted during the whole process. The electrical characteristics of the TFTs were measured by a semiconductor parameter analyzer (Agilent 4155 C, Santa Clara, CA, USA) under ambient conditions. X-ray photoelectron spectroscopy (XPS) analysis was carried out to investigate the chemical changes in the oxide films by using a THERMO ESCALAB250Xi (Thermo Fisher Scientific, Waltham, MA, USA) with an Al Ka (hν = 1486.6 eV) 15 kW beam spot source. The thickness (±0.1 nm), roughness and density of the Nd:IZO films were determined by X-ray reflectivity measurements (XRR; PANalytical EMPYREAN, Almelo, The Netherlands) using a Cu-Ka X-ray source at 40 kV and 40 mA. Microwave photo conductivity decay (μ-PCD; KOBELCO, Kobe, Japan) was used to characterize the relative carrier concentrations of the Nd:IZO/Al2O3 films and their relationship with the electrical performance of the TFT devices.

Figure 1

The schematic diagrams and output/transfer characteristics of (a,b) a single neodymium-doped indium zinc oxide (Nd:IZO) thin film transistor (TFT) and (c,d) an Nd:IZO/Al2O3 TFT.

3. Results and Discussion

Figure 1b,d shows the output and transfer characteristics of the single Nd:IZO TFT and the Nd:IZO/Al2O3 TFT—therein, the thicknesses of Nd:IZO layers are both 8 nm. The output characteristics of TFTs were obtained with VG = 0~20 V in steps of 5 V, and the transfer curves were measured with VD = 20.1 V (saturation regime). The single Nd:IZO TFT could already exhibit an electrical performance with a saturation mobility (μsat) of 4.1 cm2·V−1·s−1 and an on-to-off current ratio (Ion/Ioff) of 1.0 × 107, which implies an acceptable concentration of carriers for the room-temperature-prepared Nd:IZO layer. However, the mobility and on-state current (Ion) is still low, due to the high density of defects in the as-deposited oxide thin film, especially near the back-channel region. By inserting an ultrathin Al2O3 film upon the Nd:IZO layer, as shown in Figure 1d, an obvious enhancement on the Ion can be observed, the corresponding saturation mobility of the Nd:IZO/Al2O3 TFT can be remarkably improved to 32.7 cm2·V−1·s−1, and the Ion/Ioff reaches 1.9 × 108. Moreover, the hysteretic phenomenon reduces in the transfer curves of Nd:IZO/Al2O3 TFT. According to our previous study [11] and other reports [12,13], the Al2O3 acts as an electrical controller layer, inducing high-flux electron movement in the bulk and near-channel regions of the semiconductor layer by the carrier confinement effect under an electrical field, which avoids the scatting and trapping effect by the back-channel defects, and hence efficiently improves the device’s mobility. Meanwhile, in the Nd:IZO/Al2O3 system, the Al2O3 layer should have a specific role in the modification of the Nd:IZO semiconductor layer, which will be further discussed in this paper.

3.1. Effect of the Ultrathin Al2O3 Layer

3.1.1. Chemical Structure

To further investigate the effect of ultrathin Al2O3 on Nd:IZO layer, X-ray photoelectron spectra (XPS) measurements on the Nd:IZO/Al2O3 (8 nm/3 nm) multilayered films and a single Nd:IZO film (8 nm) were carried out. As shown in Figure 2a, the analysing depth for an XPS measurement on oxide films can reach ~10 nm, thus allowing chemical analysis for the Nd:IZO film covered by a 3-nm ultrathin Al2O3 layer. Herein, it is difficult to judge the difference between oxygen vacancies in the Nd:IZO by the O 1s core level spectra, because of the existence of Al2O3. Therefore, the XPS results for In 3d5/2, Zn 2p3/2, and Nd 3d5/2 core level spectra of the both samples were acquired as shown in Figure 2b–d. Compared to the single Nd:IZO film, all of the peaks of Nd:IZO with Al2O3 exhibit the positive shifts towards higher energy direction, indicating that the introduction of the Al2O3 layer can prevent oxygen decomposing from the metal–oxygen (M–O) bonds near the back-channel surface of the Nd:IZO film, which reduces the density of defects and thus enhances carrier mobility. Moreover, the decomposition of the Nd 3d5/2 lines through nearly Gaussian fitting shows that the Nd:IZO film covered by Al2O3 layer has a lower content of the Nd 3d5/24f4 configuration, which is considered to be associated with scattering centers or charge traps [14,15].

Figure 2

The (a) schematic illustration and (b) In 3d5/2, (c) Zn 2p3/2, and (d) Nd 3d5/2 core level spectra of the single Nd:IZO film and the Nd:IZO/Al2O3 stacked films.

3.1.2. Electrical Stabilities

Figure 3a,b show the electrical stabilities of a single Nd:IZO TFT and a Nd:IZO/Al2O3 TFT under negative/positive bias stress (NBS/PBS) in 1 h, respectively. Both of the TFTs exhibit favorable stabilities under NBS after the first 900 s, with slight shifts of Von, indicating that the introduction of the Nd element can actually prevent the ionized oxygen vacancies from migrating to the semiconductor–insulator interface under the negative bias field [16,17]. However, the PBS result with a positive shift of Von for the single Nd:IZO TFT shows that the diffusion of absorbed water or oxygen molecules can efficiently affect the device stability [18], due to the existence of back-channel defects in the sputtered Nd:IZO layer; with a thickness of only 8 nm, the △Von can reach 3.2 V after 1 h under PBS. Therefore, it is essential to introduce an ultrathin Al2O3 film, which also acts as a good passivation layer. As a result, the Nd:IZO/Al2O3 TFT exhibits an outstanding stability under PBS, so that no obvious change is observed on the device transfer characteristics.

Figure 3

The electrical stabilities of (a) a single Nd:IZO TFT and (b) a Nd:IZO/Al2O3 TFT under NBS (VG = −10 V) and PBS (VG = +10 V) for 1 h.

3.2. Effect of the Thickness of the Nd:IZO Layer in an Nd:IZO/Al2O3 Structure

3.2.1. Device Performance and Film Qualities

Figure 4a,b shows the output and transfer characteristics of the Nd:IZO/Al2O3 TFTs with different thickness (3, 5, 8, and 10 nm) of the Nd:IZO layers, and the corresponding electrical parameters are summarized in Table 1. The thickness of the Nd:IZO layer in an Nd:IZO/Al2O3 structure shows a great impact on the device performance. A significant trend can be found that the device performance increased at first, but then degraded as the Nd:IZO thickness increased from 3 to 10 nm. As a result, the TFT with an Nd:IZO thickness of 8 nm exhibits the best electrical properties, with an Ion of 4.5 × 10−4 A, a μsat of 32.7 cm2·V−1·s−1, an Ion/Ioff of 1.9 × 108, and an SS value of 0.33 V·dec−1. The XRR measurements on the 3, 5, 8, and 10-nm-thick Nd:IZO films were adopted, and the information for film density and roughness was acquired, as shown in Figure 4c. The density of Nd:IZO reaches 6.74 g·cm−3, with the optimal thickness of 8 nm, which contributes to fewer structural defects and consequently a higher device performance. Meanwhile, as the thickness increases, the roughness of the Nd:IZO increases continuously, and reaches 1.2 nm when the thickness is 10 nm, which is due to the nature of film growth during the sputtering process. The Nd:IZO/Al2O3 interfaces are sensitive to the surface roughness of over 1 nm for the Nd:IZO layers, since the thickness of upper Al2O3 layers are only 3 nm. Therefore, the rough back-channel surface of the Nd:IZO layer would enhance the carrier scattering effect and deteriorate the interface between Nd:IZO and Al2O3, consequently weakening the electrical properties of the Nd:IZO/Al2O3 TFTs.

Figure 4

The (a) output and (b) transfer characteristics of the Nd:IZO/Al2O3 TFTs with different thicknesses of the Nd:IZO layers; (c) the film density and roughness for the corresponding Nd:IZO layers.

Table 1

A summary of the parameters obtained by the results of electrical measurements and chemical analysis for the Nd:IZO/Al2O3 films and their corresponding TFT devices.

TFTs/Films	3 nmNd:IZO/Al2O3	5 nmNd:IZO/Al2O3	8 nmNd:IZO/Al2O3	10 nmNd:IZO/Al2O3
μsat (cm2·V−1·s−1)	2.1	17.1	32.7	22.5
Ion/Ioff	2.7 × 106	8.4 × 107	1.9 × 108	5.6 × 106
SS (V·dec−1)	0.32	0.41	0.33	0.63
Von (V)	1.6	1.8	1.0	−2.9
Nd:IZO density (g·cm−3)	6.37	6.41	6.74	6.46
Nd:IZO roughness (nm)	0.61	0.66	0.72	1.06
μ-PCD peak mean (mV)	9.0	65.6	205.6	97.4
Nd/[Nd + In + Zn] (at.%)	12.64	14.87	15.37	14.50
Al 2p Eb (eV)	74.11	74.22	74.35	74.19

3.2.2. The Nd:IZO/Al2O3 Interfaces

Microwave photoconductivity decay (μ-PCD) mapping scan measurements were carried out for the above-mentioned Nd:IZO/Al2O3 stacks on glass with a size of 10 × 10 mm2, in order to further investigate the effect of Nd:IZO thickness on the Nd:IZO/Al2O3 interfaces, as shown in Figure 5a–d. The Nd:IZO/Al2O3 stacks with an Nd:IZO thickness of 8 nm exhibit the highest peak mean value of the μ-PCD signal, which indicates the highest relative concentration and mobility of carriers, as well as the lowest density of shallow localized defects [19,20,21] that lead to the best electrical performance of the Nd:IZO/Al2O3 TFT. Associated with the XRR results, the growth of the Nd:IZO film during sputtering process can be simply demonstrated by Figure 5e. As the initial state of formation for continuous film, the 3-nm-thick Nd:IZO film with high defect density and low carrier concentration results in the poor performance of the TFT device. With the increase of sputtering time and film thickness, the Nd:IZO films become dense and the number of conductive carriers increases, so that the current driving capability of the corresponding TFTs are enhanced. However, as mentioned above, the roughness of the Nd:IZO increases as well, which leads to an increase of the back-channel defects in the Nd:IZO layer and thus deteriorates the Nd:IZO/Al2O3 interface, according to the μ-PCD results, reducing the mobility of the carriers by scattering and trapping effects.

Figure 5

The microwave photo conductivity decay (μ-PCD) peak value mapping scan results for the Nd:IZO/Al2O3 stacks with an Nd:IZO thickness of (a) 3 nm, (b) 5 nm, (c) 8 nm, and (d) 10 nm; and (e) the schematic diagrams for the growth process of these films.

3.2.3. Effect of Neodymium Concentrations

Furthermore, the XPS measurements on the Nd:IZO/Al2O3 stacks can also unveil the effect of varying Nd:IZO thicknesses. As shown in Figure 6a,b, a good agreement between variations in the binding energy of the Al 2p peak and the ratio of (Nd/(Nd + In + Zn) at.%) implies that the higher concentration of Nd can also contribute to a stronger formation of Al–O bonds (reflected by the positive shift of the Al 2p peak) [22], forming a tighter interface between Nd:IZO and Al2O3. The variation in Nd concentrations of the Nd:IZO films with varying thickness can be attributed to the different depositing rate of Nd/In/Zn atoms during the sputtering process. As the sputtering proceeds, the atomic ratio of each element should first gradually change, and then become stable as soon as the film reaches a certain thickness. According to the XRR/μ-PCD/XPS results above, the thickness of 8 nm is optimal for the high quality of the Nd:IZO film and a favorable Nd:IZO/Al2O3 interface, leading to the high carrier mobility and consequently, the high electrical performance of Nd:IZO/Al2O3 TFT.

Figure 6

The (a) Al 2p core level spectra of the Nd:IZO/Al2O3 stacked films with different Nd:IZO thickness, and (b) its relationship with the Nd concentrations in Nd:IZO films and corresponding device mobility.

4. Conclusions

In conclusion, we demonstrated a high-performance TFT with an Nd:IZO semiconductor via a room temperature approach, by adopting an Nd:IZO/Al2O3 nanolaminate structure. The Al2O3 layer is considered to induce high-flux electron movement in the bulk and near-channel region of the Nd:IZO layer, prevent oxygen decomposing from the M–O bonds near the back-channel surface of Nd:IZO, and act as a good passivation layer, so that the Nd:IZO/Al2O3 TFT remains highly stable under both NBS and PBS. Meanwhile, the thickness of the Nd:IZO layer has a great impact on the electrical performance of Nd:IZO/Al2O3 TFTs, due to the nature of the sputtering method. The film quality and carrier concentration are optimal when the thickness of the Nd:IZO layer reaches 8 nm, and the high concentration of Nd also benefits the formation of a tight Nd:IZO/Al2O3 interface with strong Al–O bonds, reducing the scattering centers and charge traps near the back-channel surface. As a result, the Nd:IZO/Al2O3 TFT exhibits an outstanding performance with a high μsat of 32.7 cm2·V−1·s−1, an Ion/Ioff of 1.9 × 108, and a low SS value of 0.33 V·dec−1, which shows great potential for the room temperature fabrication of TFTs in high-resolution or high-framerate displays by a scalable, simple, and feasible approach.