Plasma-Assisted Atomic Layer Deposition of High-Density Ni Nanoparticles for Amorphous In-Ga-Zn-O Thin Film Transistor Memory
© The Author(s). 2017
Received: 31 December 2016
Accepted: 15 February 2017
Published: 21 February 2017
For the first time, the growth of Ni nanoparticles (NPs) was explored by plasma-assisted atomic layer deposition (ALD) technique using NiCp2 and NH3 precursors. Influences of substrate temperature and deposition cycles on ALD Ni NPs were studied by field emission scanning electron microscope and X-ray photoelectron spectroscopy. By optimizing the process parameters, high-density and uniform Ni NPs were achieved in the case of 280 °C substrate temperature and 50 deposition cycles, exhibiting a density of ~1.5 × 1012 cm−2 and a small size of 3~4 nm. Further, the above Ni NPs were used as charge storage medium of amorphous indium-gallium-zinc oxide (a-IGZO) thin film transistor (TFT) memory, demonstrating a high storage capacity for electrons. In particular, the nonvolatile memory exhibited an excellent programming characteristic, e.g., a large threshold voltage shift of 8.03 V was obtained after being programmed at 17 V for 5 ms.
Nickel (Ni) nanoparticles (NPs) have been intensively explored due to their various potential applications, such as magnetic materials [1–3] and steam reforming catalyst [4, 5]. Moreover, Ni NPs have also been investigated as a charge storage medium for nonvolatile memory devices [6–12], which could be attributed to some potential advantages of Ni NPs, including high density of states around the Fermi level, strong charge confinement, a high work function of 5.2 eV, and low diffusivity. Therefore, the employment of Ni NPs as the charge storage medium can obtain a deep potential well in nonvolatile memory devices by selecting appropriate insulators, hereby ensuring good data retention. To obtain high-density Ni NPs, most researchers performed rapid thermal annealing of ultra-thin Ni films [7–10]. However, this technique usually requires a high annealing temperature (e.g., 600~900 °C), which undoubtedly causes crystallization for most of high dielectric constant insulators, and also exhibits temperature incompatibility with the standard thin film transistor (TFT) process. For example, Tan et al. prepared the Ni NPs with an average size of 15 nm and a relatively low density of ~1011 cm−2 under the maximum process temperature of 700 °C . In recent years, the technique of atomic layer deposition (ALD), which is based on sequential self-limited and complementary surface chemisorption reactions, has been considered as a perfect thin film deposition method because of its outstanding advantages such as relatively low process temperature, stoichiometric composition, large-area uniformity, precise thickness control, and high conformability [13, 14]. Furthermore, ALD is also a very promising method for preparing high-density metal NPs (e.g., Pt, Ir, and Ru) at relatively low temperature of <350 °C [15–17]. As an example, Liu et al. obtained Ir NPs with a high density of 0.6 × 1012 cm−2 and an average size of 4.9 nm by ALD at 300 °C for nonvolatile memory applications . Ding et al. also reported that two-dimensional high-density Pt NPs (2 × 1012 cm−2) were self-assembled on the Al2O3 film at 300 °C by ALD, which demonstrated noticeable electron trapping capacity in Si-based nonvolatile memory .
On the other hand, to develop a fully functional transparent system on a panel, nonvolatile amorphous In-Ga-Zn-O (a-IGZO) TFT memory has been proposed due to easy integration into displays and flexible electronic devices [18–20]. This is attributed to some merits of a-IGZO, such as high electron mobility, good visible-light transparency, excellent uniformity, and low process temperature [21, 22]. Therefore, it is indispensable to explore the electrical information storage ability of a-IGZO TFT memory with innovative material and process. Considering excellent process temperature compatibility between ALD of metal NPs and fabrication of a-IGZO TFT devices, the a-IGZO TFT memory with the charge storage medium of ALD Ni NPs was investigated in this article. Firstly, the ALD growth of Ni NPs on the Al2O3 film was studied, including influences of substrate temperature and deposition cycles on ALD Ni NPs. Further, by using ALD high-density Ni NPs as charge storage medium, the a-IGZO TFT memory was fabricated and the memory characteristics were measured.
Firstly, an around 10-nm Al2O3 film was deposited on a cleaned p-type silicon wafer by ALD. Subsequently, Ni NPs were grown by plasma-assisted ALD on the surface of the Al2O3 film using NiCp2 and NH3 precursors. Herein, the NiCp2 precursor was kept at 80 °C; the vapor of NiCp2 was pulsed into the reaction chamber with a N2 carrier gas. The NH3 plasma was generated under a power of 3000 W with a flow rate of 180 sccm. During the ALD process, the working pressure in the deposition chamber was maintained at ~1200 Pa, and the NiCp2 and NH3 plasma pulse times were fixed at 2 and 20 s, respectively. To obtain the optimal process conditions, the growth of Ni NPs was investigated as a function of substrate temperature and reaction cycles, respectively.
To fabricate the a-IGZO TFT memory device, the cleaned p-type Si (100) wafer with a resistivity of 0.001~0.005 Ω cm was used as the starting substrate serving as the back gate of the device. Then, a 35-nm Al2O3 film, Ni NPs, and a 7-nm Al2O3 film were successively deposited by ALD in the same chamber, which were used as the blocking layer, charge storage medium, and tunneling layer of the memory, respectively. After that, a 40-nm a-IGZO film was deposited by radio frequency (RF) magnetron sputtering at room temperature using an InGaZnO4 target under the following conditions: Ar = 50 sccm, RF power = 110 W, and working pressure = 0.87 Pa. Subsequently, the active channel was defined by photolithography and wet etching (diluted HCl acid). Source and drain contacts of Ti/Au (30 nm/70 nm) layers were formed by e-beam evaporation and a lift-off technique. Finally, the fabricated device was annealed at 250 °C in O2 for 5 min. The control TFT device without Ni NPs was also fabricated for comparison.
The morphologies and compositions of ALD Ni NPs were characterized by field emission scanning electron microscope (FE-SEM) (JSM-6700F, JEOL, Tokyo, Japan) and X-ray photoelectron spectroscopy (XPS) (Kratos Axis Ultra DLD), respectively. The thicknesses of Al2O3 and IGZO films were measured with an ellipsometer (Sopra GES-SE, France). The electrical measurements were performed on the devices with a channel length (L) of 10 μm and a channel width (W) of 100 μm using a semiconductor device analyzer (Agilent B1500A) at room temperature in a dark box. The threshold voltage (V th) is defined as the gate voltage at which the drain current equals the W/L × 10−9 A when V ds is fixed at 0.1 V.
Results and Discussion
The growth of Ni NPs on the Al2O3 film has been investigated by plasma-assisted ALD using NiCp2 and NH3 precursors. By optimizing substrate temperature and deposition cycles, high-density (~1.5 × 1012 cm−2) and small size (3~4 nm) Ni NPs have been achieved at 280 °C in the case of 50 cycles. By using the above Ni NPs as the charge storage layer, the a-IGZO TFT memory was fabricated with the ALD gate stack of Al2O3/Ni NPs/Al2O3 assembled in the same chamber. The memory exhibits a high programming efficiency and a large programming window, which should be attributed to high-density Ni NPs. Therefore, plasma-assisted ALD Ni NPs provide a feasible approach to prepare large-area and high-density Ni NPs at relatively low temperatures for nonvolatile TFT memory applications.
The authors thank the National Natural Science Foundation of China (Grant Nos. 61474027 and 61274088) and the National Key Technologies R&D Program of China (2015ZX02102-003) for the financial support.
SBQ carried out the main part of the experimental and analytical works and drafted the manuscript. YPW carried out the early investigation of ALD Ni. YS and WJL gave their suggestions on the experimental design. SJD guided the whole work and revised the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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