High-density NiSi nanocrystals embedded in Al2O3/SiO2 double-barrier for robust retention of nonvolatile memory

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Abstract

NiSi nanocrystals of high density and good uniformity were synthesized by vapor–solid–solid growth in a gas source molecular beam epitaxy system using Si2H6 as Si precursor and Ni as catalyst. A metal–oxide–semiconductor memory device with NiSi nanocrystal–Al2O3/SiO2 double-barrier structure was fabricated. Large memory window and excellent retention at both room temperature and high temperature of 85 °C were demonstrated.

Highlights

► NiSi nanocrystals of high density and good uniformity were synthesized. ► MOS memory with band-engineered NC-double-barrier structure as floating gate was fabricated. ► Excellent retention at both room temperature and 85 °C were demonstrated.

Introduction

Nonvolatile memories with discrete charge storage nodes have been investigated extensively during the past decade. Since it was pioneered by Tiwari in 1995 [1], Si nanocrystal (NC) floating gate memory has been nominated as a promising replacement of conventional flash memory thanks to its immunity to defect-related charge leakage and potential to exceed flash scaling limit. Both academia and industry have also invested tremendous efforts into research of other NC memories, exploring new materials and novel gate structures for future flash memory [2], [3], [4], [5], [6], [7], [8], [9]. Metal silicide NCs, with high density of states and robust thermal stability, have attracted much attention since they were proposed as good candidates to improve NC memory performance [10] and much work has been done to explore this material for nonvolatile memory application [11], [12], [13], [14], [15], [16], [17], [18]. As an alternative way of improving memory performance, NC core–shell structure with additional barrier layer as floating gate has been developed and adopted by researchers. For example, metal and semiconductor NC core with oxide shell synthesized by various methods such as laser irradiation induced native oxidation [19], micelle dipping [20], chemical vapor deposition and annealing [21] and pulsed laser deposition [22] were reported.

In this paper, we report a metal–oxide–semiconductor capacitor memory device with an engineered floating gate similar to core–shell structures. The floating-gate structure consists of a layer of high-density vapor–solid–solid (VSS) induced NiSi NCs by gas source molecular beam epitaxy (GSMBE) embedded in-between two Al2O3 thin barriers deposited by atomic layer deposition (ALD). Fig. 1a shows a schematic diagram of the device structure. The Al2O3/NiSi NC/Al2O3 floating gate is sandwiched by a control oxide layer and a tunnel oxide layer. Fig. 1b shows the flat energy band diagram of the memory device. The Fermi-level of NiSi NC with a work function of 4.7 eV [23] is aligned within the mid-gap of bulk Si. The conduction band offset between NiSi and Al2O3 (electron affinity 1.35 eV, Ref. [24]) is as high as 3.35 eV. The benefit of using additional Al2O3 barrier layers is two folds. First, it is to minimize diffusion of Ni metal atoms into SiO2 tunneling layer during high temperature process to reduce the charge leakage paths for prolonged retention. Second, it is to maintain programming efficiency and improve retention performance. Energy band diagrams of programming and retention states are illustrated in Fig. 1c. During programming, gate bias is applied so that electrons can be pulled into NCs by Fowler–Nordheim tunneling. Because of the high-K property of Al2O3, electric field concentration effect [25] makes most of the voltage drop on SiO2 layer. In addition, since the barrier height of Al2O3 layer is lower than that of SiO2, electrons do not actually have to go through the barrier of Al2O3 but only the thin SiO2 tunnel barrier to reach the NC. Hence, it is believed that this structure has the ability to maintain the efficiency of programming operation compared to the structure without Al2O3. On the other hand, in retention state, electrons are kept in the deep quantum well formed by Al2O3/NiSi NC/Al2O3 structure. In this case, electrons see a barrier of both Al2O3 and SiO2 and the total barrier thickness is increased. Therefore, robust retention characteristics are expected for this Al2O3/NiSi NCs/Al2O3 floating-gate memory device combining metallic silicide NCs with double-barrier structure.

Section snippets

Device fabrication

Device fabrication starts with a pre-cleaned p-type Si (1 0 0) substrate. A thin thermal oxide of 3 nm was grown on the substrate at 850 °C. This was followed by 2.5 nm Al2O3 deposition using ALD. A very thin layer of Ni was coated on the sample by room temperature electron-beam evaporation as catalyst and the sample was immediately transferred into a custom-built GSMBE system for subsequent silicide NC synthesis. Disilane (Si2H6) was used as the Si precursor to perform VSS growth at 600 °C, which is

Results and discussion

Fig. 2a shows a scanning electron microscopy (SEM) image of the as-grown NCs on Al2O3 surface. The average size of uniformly distributed NCs over the whole sample surface is about 4.5 nm and the density is around 1.5 × 1012 cm−2. X-ray photoelectron spectroscopy (XPS) was utilized to determine the chemical nature of the NCs. Fig. 2b shows the XPS result of Ni 2p3/2 for the sample before top Al2O3 coverage. The binding energy peak found at 853.9 eV indicates that the nature of NC is NiSi [27]. Fig. 2

Conclusions

NiSi NCs of high density and good uniformity were synthesized by VSS growth in a GSMBE system using Si2H6 as Si precursor, based on which a nonvolatile memory with Al2O3/NiSi NC/Al2O3structure as floating gate was fabricated and characterized. The memory device exhibits large memory window, robust retention at both room temperature and high temperature of 85 °C, and good endurance. Further device geometry optimization of using thinner control oxide and varied Al2O3 thickness may be carried out

Acknowledgements

This material is based on research sponsored by DARPA/Defense Microelectronics Activity (DMEA) under agreement number H94003-10-2-1003 and the National Science Foundation (NSF) DMR-0807232. Authors acknowledge the use of FEI quanta 3D FEG dual-beam instrument and CM-20 TEM in Calit2 microscopy Center and Materials Characterization Center at UC Irvine for cross-sectional TEM sample preparation and diffraction contrast imaging.

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