Gadolinium oxide nanocrystal nonvolatile memory with HfO2/Al2O3 nanostructure tunneling layers
© Wang et al; licensee Springer. 2012
Received: 28 November 2011
Accepted: 8 March 2012
Published: 8 March 2012
In this study, Gd2O3 nanocrystal (Gd2O3-NC) memories with nanostructure tunneling layers are fabricated to examine their performance. A higher programming speed for Gd2O3-NC memories with nanostructure tunneling layers is obtained when compared with that of memories using a single tunneling layer. A longer data retention (< 15% charge loss after 104 s) is also observed. This is due to the increased physical thickness of the nanostructure tunneling layer. The activation energy of charge loss at different temperatures is estimated. The higher activation energy value (0.13 to 0.17 eV) observed at the initial charge loss stage is attributed to the thermionic emission mechanism, while the lower one (0.07 to 0.08 eV) observed at the later charge loss stage is attributed to the direct tunneling mechanism. Gd2O3-NC memories with nanostructure tunneling layers can be operated without degradation over several operation cycles. Such NC structures could potentially be used in future nonvolatile memory applications.
KeywordsNVMs Gd2O3 nanocrystal nanostructure HfO2/Al2O3 tunneling layer
Nanocrystal (NC) memory has been widely studied as a possible solution to the scaling-down problem that traditional floating gate (FG) nonvolatile memories (NVMs) have faced. It is believed that NC memory is superior to FG memories because of either the lower leakage current from the NCs to the Si substrate or the lower lateral electron migration between NCs [1–3]. In this regard, the tunneling oxide thickness can be reduced due to the enhancement of immunity against local oxide defects, thereby allowing higher charge injection efficiency through the tunneling oxide to the charge trapping layer. The performance of NC memory depends on the densities, sizes, and shapes of the NCs. Several NC materials such as silicon (Si), germanium (Ge), gold (Au), and platinum (Pt) have been used in memory devices [4–7]. Several approaches have been investigated in order to fabricate NCs. Among these, a common method is the use of a thermal annealing process to induce crystalline phase separation (such as in HfO2-NC) or condensation effects (Au-NC formation) [8–12]. However, the method of HfO2-NC formation requires a dual sputtering process, i.e., the Si and Hf targets are loaded simultaneously in an ambient argon and oxygen mixture to form a HfSiO layer; this is followed by rapid thermal annealing (RTA) treatment [8, 9]. The Au-NC embedded in a SiO2 matrix is formed by annealing a Au thin film whose thickness is controlled to within 3 nm. The size and density of Au-NC are sensitive to the thickness of the Au thin film and the annealing temperature. This will lead to variations in the process control of Au-NC formation [10, 11]. In recent years, the use of gadolinium oxide (Gd2O3) has attracted considerable attention for application as high-k gate dielectrics in complementary metal-oxide-semiconductor (CMOS) technologies . Furthermore, the Gd2O3 was also demonstrated to be the potential candidate of III-V CMOS application because the trivalent oxide can be allowed to have a charge matching with the GaAs interface . In addition, a few studies have demonstrated a method of synthesizing Gd2O3-NC via a few chemical reaction steps . The simplest way to form Gd2O3-NC is the use of RTA treatment on an amorphous Gd2O3 (a-Gd2O3) thin film prepared by sputtering [16, 17]. This method has been applied in memory fabrication; large memory windows and good data retention can be achieved by using optimized RTA temperatures . Some parts of a-Gd2O3 will transform into a nanostructure crystalline phase after RTA treatment, while other parts remain in the amorphous phase. This procedure can natively form Gd2O3-NC embedded in an a-Gd2O3 thin film. Here, the smaller bandgap of Gd2O3-NC, which is surrounded by the larger bandgap of a-Gd2O3, could be responsible for the charge storage mechanism due to the bandgap offset [16, 18].
Another solution to the scaling-down problem of NVMs is to substitute band-engineering silicon-oxide-nitride-oxide-silicon (BE-SONOS) for FG memories [19–21]. A Si3N4 film is treated as the charge-trapping layer in the BE-SONOS structure due to the presence of a large amount of discrete trap distributions, while the SiO2/SiNx/SiO2 layer is treated as the tunneling layer by exploiting the unique band structure and the increased physical thickness . It has been demonstrated that BE-SONOS memories exhibit a good performance in terms of programming and erasing (P/E) speed and data retention. Further, high-k materials such as HfO2 have been applied to the tunneling oxide layer of NC memory because of their lower capacitance-equivalent thickness and lower band offset with Si substrates . In this study, a nanostructure using a-Gd2O3/HfO2/Al2O3 as the tunneling layer is applied to Gd2O3-NC memories, in which a-Gd2O3 is a part of the Gd2O3 thin film. The HfO2 and Al2O3 layers were prepared by atomic layer deposition and radio frequency (RF) sputtering system, respectively. Data retention can be improved due to the increased physical thickness of the tunneling layer, and the P/E speed can be improved due to band alignment in the programming and erasing states.
Results and discussion
Here, Qloss denotes the charge loss from the shallow-trap and deep-trap electron loss for the Gd2O3-NC memories, Ea represents the activation energy for charge loss, kB denotes the Boltzmann constant, and T denotes the absolute temperature. The Ea of the shallow-trap charge loss (0.13 to 0.17 eV) is higher than that of the deep-trap charge loss (0.07 to 0.08 eV). This indicates that the charge loss mechanism in the shallow trap is thermionic emission (which has higher dependence on temperature) while the charge loss mechanism in the deep trap is direct tunneling (lower temperature dependence). The charge loss mechanism in this case is identical with that reported before [23, 25].
In this study, we examined the Gd2O3-NC memories with a nanostructure tunneling layer comprising HfO2/Al2O3/SiO2. When compared with devices comprising a single tunneling layer, these NC memories with a nanostructure tunneling layer exhibit a larger VFB shift and greater data retention because of the band alignment and the increased physical thickness of the tunneling layer. From the retention characteristics, it is observed that the activation energy is 0.13 to 0.17 eV for shallow-trap charge loss and 0.07 to 0.08 eV for deep-trap charge loss. Because the charge loss mechanism for the shallow trap is dominated by thermionic emission, the activation energy is higher than that for the charge loss mechanism of the deep trap, which is dominated by direct tunneling. A band diagram was proposed to completely explain the programming and retention characteristics. In contrast, the endurance characteristics are not influenced by the nanostructure tunneling layer. The Gd2O3-NC memories with nanostructure tunneling layers could potentially be used in future NVM applications.
The authors wish to thank the National Science Council and Chang Gung University, Republic of China, for their financial support under contracts NSC100-2221-E-182-012 and UERPD2A0041, respectively.
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