Localized Charge Storage in CeO 2 /Si(111) By Electrostatic Force Microscopy
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have been written. The dependence of dot size and total stored charge on various writing parameters such as tip writing bias, tip to sample separation, and write time is examined. The total stored charge is found to be Q = 5 - 200 e per charge dot. These dots of charge are shown to be stable over periods of time greater than 24 hrs, with an initial charge decay time constant of -r - 9.5 hrs followed by a period of much slower decay with r > 24 hrs. Charge decay time constants are found to be dependent on the thickness of the lower CeO2 tunneling barrier. INTRODUCTION Cerium oxide (CeO 2 ) is an insulating material with a lattice mismatch of only 0.35 % to silicon (Si) and an energy bandgap of - 5.5 eV. This attractive set of properties has the potential to lead to a fully functional silicon heterojunction technology. A significant amount of work has been done examining the growth and characterization of CeO 2 crystals on Si [1, 2, 3, 4, 5], and the growth of single crystal Si on to CeO 2 /Si heterostuctures [6] has been recently reported. Based on these promising results, a silicon resonant tunneling diode, an improved silicon-on-insulator (SOI) technology, and stacked silicon electronics have all been proposed. A valuable and interesting addition to this array of technologies would be the capacity for electrostatic patterning and charge storage. EXPERIMENTAL DETAILS The sample preparation has been previously reported [7], and is included here for completeness. Samples were produced from commercially available 3" Si(111) wafers, n-type with 3.0-4.3 0?-cm resistivity. After being subjected to a standard acetone, isopropyl alcohol, de-ionized water degrease in ultrasound, the wafer was etched in 50:1 HF solution until hydrophobic, rinsed in de-ionized water, and immediately introduced into vacuum. Electron beam evaporation was used to deposit material from an undoped Si charge and a 99.99 % CeO 2 charge to grow the structures. Initially, a 200 A Si buffer layer was grown and examined by RHEED to assure the characteristic (7 x 7) reconstruction was apparent, indicative of a clean Si surface ready for further growth. Cerium oxide thin films were grown at a wafer temperature of 550 'C, with chamber pressures ranging from 1 X 10-7 - 2 x 10-6 Torr due primarily to outgassing from the CeO 2 charge. Silicon thin films were also grown at a wafer temperature of 550 'C, with chamber pressures of 5 x 10-8 - 2 x 10-7 Torr. A double barrier structure, CeO 2/Si/CeO 2/Si(lll), was produced with symmetric 35 A CeO 2 barriers and an intermediate 25 A Si film. In situ RHEED was again used to monitor film growth 331
Mat. Res. Soc. Symp. Proc. Vol. 584,©2000 Materials Research Society
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Figure 1: A 5 pm x 5 pm AFM image (A) and EFM image (B) of a CeO 2 /Si/CeO
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after applying 10 V to the tip for 60 s at a separation of 3 nm to the sample. Corresponding linescans exhibit the mesas to be topographically flat (A) but with a distinct, electrostatic feature (B) corresponding to the written charge dot. A simi
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