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First step to atomic-scale memory?
RESEARCH NEWS
Diamond chips? MICROELECTRONICS
Plasma-deposited diamond. (Courtesy DeBeers Industrial Diamonds.)
New results hint that there could be a bright future ahead for diamond in microelectronics. In theory, diamond electronic devices could out-perform existing power diodes and high-frequency field-effect transistors because of the material’s excellent intrinsic properties, such as high carrier mobilities and high breakdown fields. In practice, the difficulties associated with fabricating diamond free from defects and impurities have prevented diamond electronics from being realized. The presence of grain boundaries in polycrystalline diamond is a particular impedance to good electronic performance. However, by growing diamond structures using two of the best techniques available, researchers from ABB, De Beers, and Uppsala University in Sweden have reported phenomenally high carrier mobilities [Science (2002) 297, 1670-1672]. Jan Isberg and coworkers report room-temperature drift velocities of 4500 cm2 V-1s-1
for electrons and 3800 cm2 V-1s-1 for holes – a factor of two higher than typically quoted for natural single-crystal diamond. The high-purity, single-crystal diamond was produced by growing homoepitaxial layers on a high-pressure, high-temperature (HPHT) synthetic diamond substrate using microwave-plasma assisted chemical vapor deposition (MPCVD). The exceptional purity represents a “significant improvement” and a “major step toward the realization of viable diamond electronic devices,” say the researchers. The results “could be a watershed for carbon electronics,” according to Gehan A. J. Amaratunga of Cambridge University in an accompanying Perspectives article [Science (2002), 297, 1657-1658]. Despite the hope that this work provides for the future of diamond electronics, many problems with the fabrication of actual devices remain to solved, say the authors. The ability to dope diamond is also crucial if electronic applications are to be realized. Researchers from the University of ElectroCommunications in Japan have devised a new way of impurity doping using the bias method [Saito, D., et al., Diamond and Related Materials (2002) 11, 1804-1807]. Based on a conventional MPCVD apparatus, the researchers negatively bias the impurity source, effectively to ion-sputter impurities into the plasma. By applying a positive bias to the substrate, the electron shower effect is induced and impurities are doped into diamond films. The researchers report doping boron, nitrogen, and titanium uniformly into highly crystalline diamond films and resulting Hall mobility levels of 1000 cm2 V-1s-1 for B-doped films. For other applications of diamond, such as anvils used in high-pressure research, growth rates become important. One of the disadvantages of MPCVD is its slow rate and the concentration of the plasma around tips and edges. But researchers from the Carnegie Institution of Washington and the University of Alabama have redesigned the cavity and substrate stage of an MPCVD chamber, which enhances growth rates by a factor of ten [Yan, C., et al., PNAS (2002), in press, 192464799].
First step to atomic-scale memory? INFORMATION STORAGE It may not be replacing the CD-ROM anytime soon, but researchers at the University of Wisconsin-Madison have created an atomic-scale memory using single atoms of Si [Bennewitz, R., et al., Nanotechnology (2002) 13, 499-502]. The memory consists of a Si wafer coated with 0.4 monolayers of Au, which forms a track-like structure on the surface. Controlled deposition of Si onto the vacant sites creates a preformatted memory with a ‘1’ everywhere. Writing is extremely
6
November 2002
difficult, especially at room temperature, but the researchers used a scanning tunneling microscopy (STM) tip to remove Si atoms from the surface to create a ‘0’. Reading is easier, as a STM tip can be scanned across the surface following the surface tracks. The storage density of the Si memory is potentially much higher than conventional media (250 Tbit per square inch compared with 100 Gbit). But reliability is a problem at such small scales and the researchers are now working on correlations between
adjacent bits (which is a limiting factor to increasing storage density) and error correction, filtering, and coding. While the technology is far too rudimentary to be commercialized for now, important lessons are being learned about the limits of data storage. “A key message,” says Franz Himpsel, “is that speed will have to be sacrificed when increasing the storage density. That will require parallel storage architectures, such as large arrays of reading heads or arrays of cheap disks.”
An atomic-scale memory chip, made by removing individual atoms from the surface of a Si wafer, has been created by scientists at the University of WisconsinMadison led by Franz Himpsel.
Diamond chips? MICROELECTRONICS
Plasma-deposited diamond. (Courtesy DeBeers Industrial Diamonds.)
New results hint that there could be a bright future ahead for diamond in microelectronics. In theory, diamond electronic devices could out-perform existing power diodes and high-frequency field-effect transistors because of the material’s excellent intrinsic properties, such as high carrier mobilities and high breakdown fields. In practice, the difficulties associated with fabricating diamond free from defects and impurities have prevented diamond electronics from being realized. The presence of grain boundaries in polycrystalline diamond is a particular impedance to good electronic performance. However, by growing diamond structures using two of the best techniques available, researchers from ABB, De Beers, and Uppsala University in Sweden have reported phenomenally high carrier mobilities [Science (2002) 297, 1670-1672]. Jan Isberg and coworkers report room-temperature drift velocities of 4500 cm2 V-1s-1
for electrons and 3800 cm2 V-1s-1 for holes – a factor of two higher than typically quoted for natural single-crystal diamond. The high-purity, single-crystal diamond was produced by growing homoepitaxial layers on a high-pressure, high-temperature (HPHT) synthetic diamond substrate using microwave-plasma assisted chemical vapor deposition (MPCVD). The exceptional purity represents a “significant improvement” and a “major step toward the realization of viable diamond electronic devices,” say the researchers. The results “could be a watershed for carbon electronics,” according to Gehan A. J. Amaratunga of Cambridge University in an accompanying Perspectives article [Science (2002), 297, 1657-1658]. Despite the hope that this work provides for the future of diamond electronics, many problems with the fabrication of actual devices remain to solved, say the authors. The ability to dope diamond is also crucial if electronic applications are to be realized. Researchers from the University of ElectroCommunications in Japan have devised a new way of impurity doping using the bias method [Saito, D., et al., Diamond and Related Materials (2002) 11, 1804-1807]. Based on a conventional MPCVD apparatus, the researchers negatively bias the impurity source, effectively to ion-sputter impurities into the plasma. By applying a positive bias to the substrate, the electron shower effect is induced and impurities are doped into diamond films. The researchers report doping boron, nitrogen, and titanium uniformly into highly crystalline diamond films and resulting Hall mobility levels of 1000 cm2 V-1s-1 for B-doped films. For other applications of diamond, such as anvils used in high-pressure research, growth rates become important. One of the disadvantages of MPCVD is its slow rate and the concentration of the plasma around tips and edges. But researchers from the Carnegie Institution of Washington and the University of Alabama have redesigned the cavity and substrate stage of an MPCVD chamber, which enhances growth rates by a factor of ten [Yan, C., et al., PNAS (2002), in press, 192464799].
First step to atomic-scale memory? INFORMATION STORAGE It may not be replacing the CD-ROM anytime soon, but researchers at the University of Wisconsin-Madison have created an atomic-scale memory using single atoms of Si [Bennewitz, R., et al., Nanotechnology (2002) 13, 499-502]. The memory consists of a Si wafer coated with 0.4 monolayers of Au, which forms a track-like structure on the surface. Controlled deposition of Si onto the vacant sites creates a preformatted memory with a ‘1’ everywhere. Writing is extremely
6
November 2002
difficult, especially at room temperature, but the researchers used a scanning tunneling microscopy (STM) tip to remove Si atoms from the surface to create a ‘0’. Reading is easier, as a STM tip can be scanned across the surface following the surface tracks. The storage density of the Si memory is potentially much higher than conventional media (250 Tbit per square inch compared with 100 Gbit). But reliability is a problem at such small scales and the researchers are now working on correlations between
adjacent bits (which is a limiting factor to increasing storage density) and error correction, filtering, and coding. While the technology is far too rudimentary to be commercialized for now, important lessons are being learned about the limits of data storage. “A key message,” says Franz Himpsel, “is that speed will have to be sacrificed when increasing the storage density. That will require parallel storage architectures, such as large arrays of reading heads or arrays of cheap disks.”
An atomic-scale memory chip, made by removing individual atoms from the surface of a Si wafer, has been created by scientists at the University of WisconsinMadison led by Franz Himpsel.