Self-organized Ge nanocrystals embedded in HfAlO fabricated by pulsed-laser deposition and application to floating gate memory

Abstract

A trilayer metal-oxide-semiconductor structure containing a HfAlO tunnel layer, isolated Ge nanocrystals, and a HfAlO control layer, was obtained using pulsed-laser deposition (PLD). Self-organized Ge nanocrystals were formed by PLD at 600°C, suggesting a useful low-temperature process for fabricating Ge nanocrystals embedded in dielectric materials. The self-organized Ge nanocrystals so formed were uniform in size and distribution with a density approaching 10 12 cm −2 . The effects of deposition temperature and growth rate in forming Ge nanocrystals were investigated and it was revealed that a relatively low temperature and growth rate are favorable for the formation of Ge nanocrystals. The memory effect of the Ge nanocrystals with storage charge density of up to 10 The nanocrystal floating gate embedded in dielectrics has attracted a great deal of attention recently because it can potentially be applied in nonvolatile, high-speed, highdensity, and low-power consuming memory. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] Compared to conventional floating gate memories such as flash, a device composed of nanocrystals isolated by dielectrics benefits from a relatively low operating voltage, high endurance, fast write-erase speeds, and better immunity to soft errors. 1 For the first nano-floating-gate memory, the materials of nanocrystals and dielectrics were Si and SiO 2 , respectively. 1 In recent years, several new memory nodes such as Ge, SiGe, Au, TiN, and Ag have been investigated, and some of them were incorporated with high-k dielectrics. [3][4][5][6][7][8][9][10][11][12][13][14] The Ge nanocrystal is considered to be an ideal memory node due to its relatively small band gap compared to Si and compatibility with current complementary metal-oxide-semiconductor (CMOS) technology. However, it is still difficult to fabricate uniform and self-organized Ge nanocrystals. Most of the methods, including the thermal annealing of Ge and dielectric mixture layer, 13-15 Ge ion implantation, 5 and the oxidation of SiGe, 6,7 require annealing at high temperatures. Baron et al. 16 has fabricated Ge nanodots on the SiO 2 matrix by low-pressure chemical vapor deposition. However, silicon nuclei are needed in this method. Pulsed-laser deposition (PLD) is a good method for growing high-quality structures at low temperatures. Hassan et al. 17 employed it to fabricate Ge nanocrystals embedded in an AlN matrix for optical application. In this Letter, we report on the formation of selforganized Ge nanocrystals at relatively low substrate temperatures using PLD, and the memory effect of the metaloxide-semiconductor (MOS) structure composed of Pt electrode, a HfAlO control gate, Ge nanocrystals, and a HfAlO tunnel layer on Si substrate. The motivation for integrating the Ge nanocrystals with HfAlO is its potential for implementation in the new generation CMOS technology. In fact, recent studies have shown that high-k dielectrics, instead of SiO 2 , for the tunnel layer in nanocrystal floating gate memory can also improve ...

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