Synthesis and memory effect study of Ge nanocrystals embedded in LaAlO3 high-k dielectrics

Abstract

A floating gate memory structure utilizing Ge nanocrystals embedded in LaAlO 3 ͑LAO͒ high-k dielectric films has been fabricated by pulsed-laser deposition. A cross-sectional high-resolution transmission electron microscopy study revealed that the floating gate structure contains 5-nm-diam spherelike Ge nanocrystals embedded in amorphous LAO. A significant memory effect with a very high density of charge storage up to 2 ϫ 10 13 /cm 2 in the Ge nanocrystals and a maximum flat band voltage shift of 3.2 V have been achieved for the trilayer structure of LAO͑8 nm͒ / Ge/ LAO͑3 nm͒ / Si. The memory structure utilizing the Ge nanocrystals grown in 1 min showed excellent charge retention characteristics, whereas the decay in memory capacitance after 10 4 s of stress under a flat band voltage was only 8%. These results suggest that this memory structure utilizing Ge nanocrystals embedded in a LAO dielectric offers a high potential for the further scaling of floating gate memory devices. In addition, the effects of Ge growth time, and thus the size and density of the Ge nanocrystals, to the charge storage and charge retention characteristics were also studied. Metal-oxide-semiconductor ͑MOS͒ memory structures based on Si quantum dots or nanocrystals have recently attracted a great deal of interest due to the physical phenomena and potential applications in scaled flash memory. [1][2][3][4] In this kind of memory structure, Si nanocrystals are embedded as charge-storage nodes in an oxide layer between the control gate and the tunneling layer, which can reduce the problem of charge loss encountered in conventional flash memories. It allows thinner injection oxides and, hence, smaller operating voltages, better endurance, and faster write/erase speeds. 1,2 The memory function of these devices has been attributed to the charge exchange between the nanocrystals and inversion layer. Most quantum dot flash memories use Si nanocrystals to replace the continuous floating gate layer. However, research results in several groups have demonstrated the superior properties of Ge-based nanocrystal memories over those based on Si. [4][5][6] The Ge nanocrystals offer improved nonvolatile charge retention time due to their smaller band gap. The smaller band gap provides both a higher confinement barrier for the retention mode and a smaller barrier for the program and erase modes. Various methods of fabrication have been reported to form the Ge dot, such as implantation of Ge ions in SiO 2 , 7 the sputter deposition of Ge on the Si substrate, 8 and the rapid thermal annealing of chemical-vapordeposited Ge layers. 9 Pulsed-laser deposition ͑PLD͒ has also proven to be very successful in the fabrication of complex oxide materials, and is also effective in the growth of Ge nanocrystals.High-k gate dielectrics have been extensively investigated for replacing SiO 2 in future MOS field effect transistors ͑MOSFETs͒. [10][11][12] In recent studies, high-k gate dielectrics were also used as the control and tunneling oxide layers in floating gate memor...

Experimental investigation of tunnel oxide thickness on charge transport through Si nanocrystal dot floating gate memories

“…A previous study showed that dot size of Ge nanocrystals affected charge loss rate. 9 Our preliminary observation of the effects of Si nanocrystal dot size and density on memory characteristics shows that variation in deposition temperature results in different nanocrystal dot morphologies, and the memory characteristics of nanocrystal dots differ depending on deposition temperature. 10 These factors, therefore, should be carefully investigated to enhance memory quality.…”

Electrical Characterization of Ge Nanocrystals in Oxide Matrix

“…Memory devices based on Ge QDs are very promising due to a long charge retention time in the Ge QDs, faster writing/erasing times, and low operating voltages [1,2] Therefore, numerous investigations have been performed on this system, where the influence of different properties on charge trapping and retention were investigated [1][2][3][4][5]. However, the main mechanism of the charge storage is still not completely clear.…”

Epitaxy of Si nanocrystals by molecular beam epitaxy on a crystalline insulator LaAlO3(001)

“…Non-volatile flash memories embedding nanocrystals (NC) are promising devices useful in computers, mobile phones or USB sticks [1,2]. The insertion of semiconducting (SC) NC in an insulating matrix requires the elaboration of complex ''oxide/SC/ oxide/Si(0 0 1)'' heterostructures and the control of the associated successive growth steps.…”