Confinement Of Nanocrystals And Possible Charge Storage Mechanism For MIS Memory Devices With Ge Nanocrystals Embedded I
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Confinement Of Nanocrystals And Possible Charge Storage Mechanism For MIS Memory Devices With Ge Nanocrystals Embedded In SiO2 V. Ho,1 W.K. Choi,1,2 W.K. Chim,1,2 L.W. Teo,2 A. Y. Du 3 and C. H. Tung 3 1 Department of Electrical &Computer Engineering, National University of Singapore, 4 Engineering Drive 3, Singapore 117576 2 Singapore-MIT Alliance, 4 Engineering Drive 3, Singapore 117576 3 Institute of Microelectronics, 11 Science Park Road, Singapore Science Park II, Singapore 117685
ABSTRACT Memory effect in a metal-insulator-silicon (MIS) structure with the insulator layer consisting of a sputtered capping SiO2 / Ge nanocrystals embedded in SiO2 / rapid thermal oxide structure has been observed. For the devices with a co-sputtered SiO2+Ge middle layer, larger nanocrystals were formed devices with a higher Ge concentration. It was noted that for such devices, the RTO and the capping oxide layers were able to confine the nanocrystals within the middle layer to some extent. However, in devices with pure sputtered Ge middle layer, the RTO and capping oxide layers were not as effective in confining the Ge nanocrystals. In addition, we have consistently observed memory effect from devices annealed in Ar and the absence of memory effect from devices annealed in forming gas. However, Ge nanocrystals were found in devices annealed in both ambient. This implies that having nanocrystals does not necessarily imply the presence of charge storage or memory effect.
INTRODUCTION Nano-particles based on group IV elements have been studied extensively with regards to their applications in electronic and optoelectronic devices and their compatibility with the current silicon (Si) based technology. Tiwari et al [1] have shown that Si nanocrystals embedded in a silicon dioxide matrix can provide charge storage sites that were isolated electrically, and the charge leakage through localized oxide defects reduced that resulted in superior charge retention characteristics. It was also suggested that a thinner tunneling oxide layer can be used in nanocrystal based memory as compared to conventional flash memory cells. This will give rise to faster write and erasing speeds. Other potential advantages of nanocrystal memory devices include a lower voltage programming, a higher cyclability and lower power consumption as compared to conventional memory devices. As a group IV element, Ge can be readily incorporated into silicon technologies. Ge nanocrystals have several attractive properties such as a higher dielectric constant and a smaller effective mass, which makes the Ge band structure more sensitive to size effects as compared to Si [2]. In addition, diffusion of Ge is not significant at temperatures below 500°C [3]. As such, the device will not be adversely affected by subsequent low temperature processing steps. Last but not least, Ge nanocrystals can be more easily differentiated from Si in a SiO2 matrix. King et al [4,5] have demonstrated a Ge nanocrystal memory device that can be programmed at low
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