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Abstract:
A novel method based on PLD has been formulated to produce nanocrystals embedded in high-k dielectrics. Application as nonvolatile memories gives advantages in good memory window and long data retention.

Nanocrystals formation for nonvolatile memory using PLD

Singapore | Posted on January 30th, 2007

Research in the field of nanoelectronics has been increasing in the recent years. This is true in particular for nonvolatile memories which have low power consumption with high density. Nonvolatile memories found their applications predominantly in many portable devices such as mobile phones, PDAs, etc. These devices gain advantage in using nonvolatile memories by exploiting the low power consumption and high density. As compared to the conventional floating gate memory, a structure having nanocrystals embedded in the dielectrics has exhibited the high potential to produce a memory with low operating voltage, high endurance, fast write-erase speeds, and better immunity to soft errors. The memory operation of these devices has been associated with the charge exchange between the nanocrystals and the inversion layer. Moreover, the use of high-k dielectric in place of the conventional SiO2 has enabled flash memory to obtain significantly improved programming efficiency on top of better data retention.

A group from Nanyang Technological University (NTU), School of Materials Science and Engineering, led by Asst. Prof. Lee Pooi See, has successfully developed a novel method based on pulsed laser deposition (PLD) to fabricate the memory structure of nanocrystals embedded in dielectric. By far, they have been successful in fabricating germanium (Ge), silicon dioxides (SiO2), strontium titanate (SrTiO3), and LaAlO3 embedded on a high-k dielectric. High-k dielectric used is lanthanide oxides, as they are potential candidates for gate insulator due to their desirable characteristics such as large band gap, high relative dielectric constant and low leakage current. In particular, lutetia (Lu2O3) is used because, despite the moderately high dielectric constant (~12), it is predicted to be thermodynamically stable on silicon, and it has a good conduction-band offset (CBO) with silicon. Among the lanthanide oxides, lutetia has the highest lattice energy and the largest bandgap. Therefore, it was expected to show better hygroscopic immunity as well as lower leakage current than other lanthanide oxides thin films.

The method based on PLD process, was done by placing the target in an ultrahigh vacuum chamber and then ablating it with a KrF pulsed laser. The target to be laser ablated was first prepared from a high-purity (99.999%) round lutetia target and one small nanocrystals square plate wafer. The nanocrystals plate was glued using a chemically nonreactive adhesive onto the surface of lutetia target, making a two-layer assembly with only physical, but not chemical, contact between them. During the PLD process, the center of the target assembly was set to spin slowly about its central axis and the laser beam vaporized the two component materials alternately. The plume of the particles would then be deposited on the p-type (100) Si substrate which has had its native oxide removed. After the deposition, the thin film was subjected to post-deposition annealing.

Upon examination of the film structure under high-resolution transmission electron microscope (HRTEM), it could clearly be seen that the nanocrystals has been successfully synthesized. Resulting nanocrystals with diameter of around 7 nm and the area density of around 8 x 1011 cm-2, depending on the type of the nanocrystals, are observed in the planar TEM image. A trilayer nanocrystals memory capacitor structure was observed consists of an amorphous lutetia tunneling layer, nanocrystals and amorphous lutetia control layers.

All nanocrystals embedded samples show large memory effect caused by the charges stored in the nanocrystals and/or at the nanocrystals interfaces. The resulting C-t curve obtained experimentally showed very good charge retention characteristics. Using this proposed method, the size and the density of nanocrystals to be deposited could also be controlled by modification of the target.

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About Nanyang Technological University, MSE
he School of Materials Science and Engineering offers engineering degree courses at the Bachelor, Master and PhD level. Research and development (R&D) carried out often goes hand-in-hand with the courses and programmes offered. Graduate students, academic staff and research staff all contribute to our R&D activities. Over the years, the School has set up state-of-the-art research facililties and infrastructure. Inter-disciplinary research is emphasized, involving faculty staff from other schools within NTU and overseas research institutions and universities.

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