Improved Retention Characteristics of Pd-Nanocrystal-Based Nonvolatile Memories by a Simple Timing Technique

By controlling the different initial time of Hf and Al precursor supply, the proposed HfxAlyOz tunneling layer with different Al/Hf ratios from 0.21 (top) to 0.06 (bottom) can be fabricated. Regarding Al/SiO2/Pd nanocrystals/proposed HfxAlyOz/Si, the proposed HfxAlyOz film can prevent stress from Pd nanocrystals and achieve a higher electron current during programming. Regarding the retention characteristics, the final memory windows of 2.5/2.05 V at 25/90◦C can be obtained, respectively. Programmed and erased with ±10 V/100 ms, the device shows a memory window of 2.4 V after 104 cycles for a 25◦C test. © The Author(s) 2015. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. [DOI: 10.1149/2.0071512ssl] All rights reserved.

2][3] However, the crystallization temperature of HfO 2 is very low and easily results in a higher leakage current.Previous research indicated the crystallization temperature of HfO 2 can be significantly increased by adding Al to the film. 4Moreover, adding Al to HfO 2 is found to increase the Si/dielectric interfacial stability. 5egarding memory application, Hf x Al y O z film was served in the storage layer of the nonvolatile memory (NVM) device and good memory characteristics were achieved. 6][9][10][11] However, previous study reported that a huge stress induced by metal NCs in the SiO 2 tunneling layer led to a worse retention characteristic and a small memory window because of many shallow traps in the SiO 2 tunneling layer.Therefore, to reduce the shallow traps induced by metal NCs was an important issue. 123] Instead of SiO 2 , the Hf x Al y O z was served as the tunneling layer, the retention characteristic in Pd NCs NVM device was improved. 12However, the induced shallow traps in the Hf x Al y O z tunneling layer still affect the retention characteristics.In this paper, less shallow traps in the proposed Hf x Al y O z tunneling layer could be achieved by a simple timing technique.The NVM device with proposed Hf x Al y O z film will show better memory characteristics as compared to previous tunneling layer.

Experimental
Hf x Al y O z films with different Al/Hf ratios (0, 0.12 and 0.278) were deposited on p-type Si substrate at 500 • C by using combinations of Hf[OC(CH 3 ) 3 ] 2 [C 5 H 11 O 2 ] 2 and Al[PCH(CH 3 ) 2 ] 3 precursors in metal organic chemical vapor deposition (MOCVD) system.Two precursors were injected into a pre-chamber, and then the precursors were vaporized and supplied into the main chamber by O 2 gas at 1500 sccm.Al/Hf ratio in Hf x Al y O z film was related to respective injected times into pre-chamber.The main chamber pressure was set at 5 mbar.As shown in Fig. 1a, an interfacial layer was obtained at Hf x Al y O z /Si interface.These deposited samples were called HfO 2 , HfAlO-1 and HfAlO-2, respectively.After Hf x Al y O z deposition, a thin Pd layer was set at 2 nm and deposited by using an E-gun system (3 × 10 6 Torr, 40 mA and 0.02∼0.03nm/sec).After Pd deposition, all samples were annealed with rapid thermal annealing at 500 • C for 30 sec.Then, these samples were followed by a 30-nm-thick SiO 2 blocking oxide deposition in plasma enhanced CVD system.The gases, power, pres-sure and temperature were SiH 4 /N 2 O/N 2 (8.5/710/161.5 sccm), 20 W, 1Torr and 300 • C, respectively.The photolithography process was conducted and Al (200 nm) was deposited as the top electrodes by a lift-off technology.There was a N 2 annealing at 400 • C after the deposition of the Al gate.In addition, some Al/Pd NCs/Hf x Al y O z /IL/Si structures without blocking oxide deposition were also manufactured to compare programming current densities with one another.The energy band diagram of Al/SiO 2 /Pd NCs/Hf x Al y O z /IL/Si structure during program operation is shown in Fig. 1b.In Results and discussion section, this energy band diagram will be used to explain the tunneling behavior.The capacitance-voltage (C-V) measurements were performed by a precision LCR meter of HP 4284A.In addition to LCR meter, the current measurement needs an HP 4155C parameter.The charge retention measurement adopts the C-t method. 14  Flatband Voltage (V) - low traps of the tunneling layer and shows a positive shift in C-V curve.When the voltage sweeps from −8 to 8 V, it is easily relaxed from the shallow traps and shows a negative shift in C-V curve. 15t is considered that more induced shallow traps in Hf x Al y O z film leads to a more obvious C-V hysteresis.Regarding C-V hysteresis ( V FB ), the summarized table is shown in the inset of Fig.

Conclusions
Different Al/Hf ratios from 0.21 (top) to 0.06 (bottom) in the proposed Hf x Al y O z tunneling layer can be fabricated.When used in NVM device, this film has less shallow traps induced by Pd NCs and provides a higher tunneling current.Regarding the proposed NVM device, the final memory windows of 2.5/2.05V at 25/90 • C can be achieved, respectively.When programmed and erased with ±10 V/100 ms, the proposed NVM device shows a memory window of 2.4 V after 10 4 cycles for a 25 • C test.

Figure 1 . 3 Figure 2 .
Figure 1.(a) Cross-sectional schematic of Al/SiO 2 /Pd NCs/Hf x Al y O z /IL/Si (b) Energy band diagram of the structure with an interfacial layer during program operation.

12 Figure 3 .
Figure 3.Chemical compositional analysis of the Hf x Al y O z film under the same supply time and (a) without time delay (b) with a 2 sec delay concerning Al[PCH(CH 3 ) 2 ] 3 precursor.

Figure 4 .
Figure 4. (a) Cross-sectional TEM image of NVM with proposed Hf x Al y O z film.(b) Retention characteristics of the structures with/without delay concerning Al[PCH(CH 3 ) 2 ] 3 precursor.
Hf x Al y O z film can increase the programming current density.Regarding HfAlO-1 film, it is fabricated under the same supply time (15 sec) and initial time of Hf and Al precursors.Based on these results and discussion, the original process for HfAlO-1 film with Al/Hf ratio of 0.11-0.12(in Fig.3a) should be modulated.Regarding the proposed Hf x Al y O z film, Hf precursor is supplied first, and then Al precursor is supplied with a 2 sec delay.As expected, Al/Hf ratio on the Hf x Al y O z surface is 0.21, but only 0.06 near Hf x Al y O z /IL/Si interfaces, as shown in Fig.3b.Regarding the proposed NVM device, TEM image is shown in Fig.4a.Owing to less induced traps in proposed Hf x Al y O z film, it shows a small drop at initial stage in the retention characteristic and a final memory window of 2.5 V at 25 • C.Moreover, a final memory window of 2.05 V at 90 • C is also observed, as shown in Fig.4b.Among 30 measured samples, the proposed NVM samples show that the memory windows vary from 2.3 to 2.6 V.In addition, the reliability of Al/SiO 2 /Pd NCs/proposed Hf x Al y O z /IL/Si structure is also measured.The data endurance of the proposed memory device retains an obvious memory window of 2.4 V after 10 4 cycles and write/erase voltage is +/−10 V for 100 ms.
5c.Therefore, Hf x Al y O z film with higher Al/Hf ratio has a better immunity to the stress from Pd NCs and a smaller C-V hysteresis is observed.For Al/Pd NCs/Hf x Al y O z /IL/Si, the summarized table of the current density at 0.8 V is shown in the inset of Fig.2d.It reveals that the programming current density is lower concerning the Hf x Al y O z film with a higher Al/Hf ratio.Due to introducing Al to the HfO 2 , it passivates the occupied V O gap states and reduces leakage current paths in the Hf x Al y O z film.5According to the discussion mentioned above, the NVM device with more shallow traps in HfO 2 film shows a rapid drop at initial stage in retention characteristic and the least shallow traps in HfAlO-2 leads to a smallest variation of flatband voltage, as shown in Fig.2e.Regarding initial/final memory windows, the summarized table is shown in the inset of Fig.2e.Therefore, the Hf x Al y O z film with a higher Al/Hf ratio can reduce shallow traps induced by Pd NCs to improve the retention characteristic and a lower Al/Hf ratio in the