Stable, Guest-Free Clathrate Form of Crystalline Silicon Synthesized
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ble to read the status of any given MTJ without including the contributions from the rest of them in the array. Wang solved this problem by introducing a “virtual ground,”a concept used in operational amplifiers, into a 2 × 2 bit memory “chiplet.” Because the input impedance of an operational amplifier is very high or infinite, no current can flow into or out of the inverting input terminals, creating a virtual ground at 0 V. MTJs were fabricated as a sandwich by deposition of a 100 nm layer of Co, followed by a 3–8 nm layer of Al 2O 3, topped with a 100 nm layer of NiFe, by sputtering with argon. The four MTJs in the 2 times 2 grid were designated Bit11, Bit12, Bit21, and Bit22, with each subscript referring to the corresponding word line and bit line, respectively, of the conducting paths at the nearest intersection. Setting word line 2 to an input excitation voltage while clamping bit line 2 to virtual ground through a read-mode switch caused the status of Bit22, and only Bit22, to appear at the output of the operational amplifier, thus proving the diodefree MRAM concept. According to Wang,
the greatly improved memory density this development promises could boost the use of MRAM devices over semiconductor memories or magnetic disks in the future. TIM PALUCKA
Stable, Guest-Free Clathrate Form of Crystalline Silicon Synthesized A team of researchers from Jacksonville State University in Alabama and Arizona State University in Tempe, Arizona has reported the synthesis of a guest-free clathrate form of crystalline silicon. Optical and electrical measurements indicate that the properties of these structures are in reasonable agreement with existing theories predicting such new forms of crystalline silicon, with a wide (1.9 eV) and indirect bandgap. Silicon clathrates were first obtained as metastable intermediate phases during thermal decomposition of alkali metal silicides. Previous forms of these silicon clathrates required the presence of clumps of alkali or alkaline earth metals as “guest” materials to prevent collapse of the struc-
MRS BULLETIN/OCTOBER 2000
RESEARCH/RESEARCHERS
ture. Aiming for a structure of the NaxSi136 type, this team has used a combination of successive vacuum “degassing” of Na from ionic Zintl phase sodium silicide, followed by density separation and centrifugation. As reported in the September 15 issue of Physical Review B, NaxSi136 samples with low sodium concentrations (
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