Synthesis and Electronic Application of Germanium Nanocrystals in Silicon Oxide Matrix
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Synthesis and Electronic Application of Germanium Nanocrystals in Silicon Oxide Matrix Wee Kiong Choi1,2, Wai Kin Chim1,2, and Han Guan Chew2 1 Electrical and Computer Engineering, National University of Singapore, 4 Engineering Drive 3, Singapore, 117576, Singapore 2 Singapore-MIT Alliance, National University of Singapore, 4 Engineering Drive 3, Singapore, 117576, Singapore
ABSTRACT The size of germanium (Ge) nanocrystals in a trilayer memory device structure was controlled by varying the thickness of the middle co-sputtered Ge plus silicon oxide layer. Such confinement of nanocrystals was not effective in a trilayer structure with a pure Ge middle layer. Significant diffusion of Ge atoms through the tunnel oxide or rapid thermal oxide (RTO) layer and into the silicon substrate was observed when the RTO layer thickness of the trilayer structure was reduced. This resulted in no (or very few) nanocrystals formed in the system. A higher charge storage capability was obtained from devices with a thinner RTO layer. INTRODUCTION The increasing use of portable electronics and embedded systems has resulted in a need for low-power high-density non-volatile memories. The current floating-gate flash memory cells use a relatively thick tunnel oxide to prevent direct tunneling current leakage to ensure good data retention capability and to reduce the off-state power consumption of the memory array. However, such a thick tunnel oxide means that the write and erase pulse durations during programming of the flash memory are relatively long and these compromise the programming speed of the device. Tiwari et al.[1] demonstrated the use of silicon nanocrystals to replace the floating gate of a conventional memory device with attractive properties. These include: lowvoltage programming at a faster programming speed (write and erase pulse durations of hundreds of nanoseconds) and improved retention times. Also by using such nanocrystal charge storage sites that are isolated electrically, charge leakage through localized oxide defects was reduced. This has generated great interest in quantum dot memory applications [2,3]. In this paper, we report results on the synthesis of Ge nanocrystals in silicon oxide matrix with particular emphasis on the Ge nanocrystal formation in thin silicon oxide matrix for memory application. EXPERIMENT The fabricated devices consist of a metal-insulator-semiconductor structure. The Si substrates were of orientation and are p-doped with resistivity of 4-8 Ωcm. The insulator region consists of a tunnel oxide layer, a thin sputtered oxide layer with Ge nanocrystals embedded and a sputtered oxide cap layer. Rapid thermal oxidation (RTO) was carried out in dry oxygen ambient at 1000°C to grow thin good quality thermal oxide of 2.5 to 10 nm which would serve as the tunnel layer. The sputtering target was a 4-inch SiO2 (99.999% pure) disk with pieces of undoped Ge attached. The Ge concentration in the co-sputtered layer was varied by
changing the number of Ge pieces attached to the target. Note th
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