Charge storage characteristics of Au nanocrystal memory improved by the oxygen vacancy-reduced HfO2 blocking layer
© Tang et al.; licensee Springer. 2013
Received: 2 May 2013
Accepted: 21 August 2013
Published: 28 August 2013
This study characterizes the charge storage characteristics of metal/HfO2/Au nanocrystals (NCs)/SiO2/Si and significantly improves memory performance and retention time by annealing the HfO2 blocking layer in O2 ambient at 400°C. Experimental evidence shows that the underlying mechanism can be effectively applied to reduce oxygen vacancy and suppress unwanted electron trap-assisted tunneling. A memory window of 1 V at an applied sweeping voltage of ±2 V is also shown. The low program/erase voltage (±2 V) and the promising retention performances indicate the potential application of NCs in low-voltage, non-volatile memory devices.
KeywordsMemory performance Oxygen deficiency Annealing Non-volatile memory 85.35.-p 61.72.Cc
Nanocrystal (NC) floating gate memory devices have recently attracted much attention as a strong candidate for non-volatile memories given their scalability, fast write/erase speeds, low operating voltages, and long retention times [1–4]. Numerous attempts have been made to develop non-volatile memory devices using metal NCs, such as Ni , Au , Ir , and Pt , because metal NCs have a higher density of states around the Fermi level, a wider range of available work functions, and smaller energy perturbation compared with their semiconductor counterparts . Further improvement in memory performance can be achieved through the integration of metal NCs with high-κ dielectric materials, such as HfO2 and Al2O3. The use of high-κ dielectric materials as blocking layers decreases the electric field at the top dielectric and program/erase (P/E) voltages, which also supports the demand for small effective oxide thickness . Au NCs with high work functions (5.1 eV) enable the creation of a deep potential well to trap charge carriers, such as HfO2, with high dielectric constants (20 to 25) and relatively high barrier heights (−5.7 eV). The structure of metal/HfO2/Au NCs/SiO2/Si shows a strong potential for application in non-volatile memory devices [13, 14].
Metal/HfO2/Au NCs/SiO2/Si is fabricated in this study. The capacitance-voltage (C-V) characteristics show that the main storage consists of holes. However, electron trapping is seldom achieved because of the HfO2 blocking layer. X-ray photoelectron spectroscopy (XPS) confirms that the oxygen deficiency within the HfO2 layer is caused by the presence of Hf-Hf bonding. The energy band diagram shows that electrons trapped in the NCs tend to leak into the gate electrode through trap-assisted tunneling, which is supported by the oxygen vacancy-related levels during programming. However, Hf-Hf bonding disappears after HfO2 is annealed at 400°C for 10 min in O2 ambient. The structure of metal/HfO2 (as-annealed)/Au NCs/SiO2/Si shows that both electrons and holes are stored. Given their memory window of 1 V at an applied sweeping voltage of ±2 V, low P/E voltage (±2 V), and promising retention performances, low-voltage NC memories have a strong potential for application in non-volatile memory devices.
A metal/HfO2/Au NCs/SiO2/Si (A1) structure was fabricated. P-type Si with a doping level of 8.33 × 1017 cm−3 was used as a substrate. A 3-nm-thick thermal SiO2 oxide was fabricated using a rapid thermal annealing (RTA) device after pre-gate cleaning. An Au film with a thickness of approximately 1 nm was sputtered using SCD005 (Balzers Union, Balzers, Liechtenstein) with a sputtering time of 2 s. The sample was then annealed in N2 ambient using the RTA device. Annealing was performed at 600°C for 10 s to form Au NCs. A 30-nm HfO2 film deposited by the electron beam (E-beam) evaporation system with a base pressure of 3.6 × 10−6 Torr served as the blocking layer. After depositing the TaN/Al metal gate electrode with thicknesses of 50/300 nm and the Cr/Au bottom electrode with thicknesses of 20/200 nm through magnetron sputtering, the capacitive structure of the NC memory device was finally completed. Metal/HfO2/SiO2/Si (A2), metal/SiO2/Au NCs/SiO2/Si (A3), and metal/HfO2 (PDA)/Au NCs/SiO2/Si (A4) were fabricated using the same process, with the exception of a 20-nm SiO2 film deposition using the E-beam for sample A3 and the annealing of HfO2 after deposition at 400°C for 10 min in the O2 ambient for sample A4. XPS with a 1,486.6-eV Al Kα source was used to obtain composition information about the as-deposited and annealed HfO2 film. The electrical characteristics of the NC memory devices were measured in the parallel mode using a Keithley 4200 semiconductor characterization system (Cleveland, OH, USA) and a Keithley 590 C-V analyzer at room temperature.
Results and discussion
Electrons trapped in Au NCs tend to tunnel into the gate electrode through the oxygen vacancy-related levels of the HfO2 blocking layer and tend to degrade memory performance because of the existence of oxygen vacancy. Annealing the HfO2 blocking layer at 400°C in O2 ambient decreases oxygen vacancy and suppresses unwanted electron trap-assisted tunneling. Given their memory window of 1 V at an applied sweeping voltage of ±2 V, low P/E voltage of ±2 V, and improved retention performances, low-voltage NC memories show promise for application in non-volatile memory devices.
High-resolution transmission electron microscopy
Rapid thermal annealing
X-ray photoelectron spectroscopy.
This work was supported by the National Basic Research Program of China under grant numbers 2011CB301905 and 2012CB933503; National Natural Science Foundation of China under grant numbers 61108064, 61036003, and 61176092; the Fundamental Research Funds for the Central Universities (2011120143); and Ph.D. Programs Foundation of Ministry of Education of China (20110121110025).
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