RESEARCH HIGHLIGHTS
Nanodevices Theory goes big
Computer simulations show how key properties of nanowires change as the diameter increases
Silicon nanowires are widely recognized as candidates for use in next-generation sensors, battery electrodes and solar cells, and first-principle calculations are an important tool in the develop- ment of these applications. Most of the calculations performed so far have only considered nanowires with diameters of less than 4 nanometers, although in practice, nanowire devices typi- cally have much larger diameters. Man-Fai Ng at the A*STAR Institute of High Performance Computing and co-workers in Singapore1
have now performed
first-principle calculations to simulate the properties of silicon nanowires with diameters of up to 7.3 nanometers (see image). The researchers have examined nanowires ranging from
atomic scales (~1 nanometer diameters) to the large-diameter limit, at which point they begin to resemble bulk silicon. The researchers studied the nanowire bandgap — a key parameter that affects both electrical and optical properties — and found that this decreased with increasing diameter. The simulation results were consistent with those obtained
from experiment, and the trend was more predictable at larger diameters. Ng and his co-workers also studied how the ‘direct’ and ‘indirect’ bandgaps change when the diameter of the silicon nanowire increases. Bulk silicon has an indirect bandgap, which means that the
excitation of a mobile charge carrier must be accompanied by a simultaneous change in its momentum. Because this is relatively unlikely, bulk silicon is a poor absorber and emitter of light. Semiconductors with direct bandgaps, on the other hand,
are optically active. The team of researchers found that silicon nanowire bandgaps assumed indirect characteristics above diameters of around 4 nanometers, and direct characteristics for smaller diameters. The researchers were also able to calculate the way in which
nanowire diameter affects the location of dopant atoms along the nanowire radius. “Foreign atoms like boron are used to increase the density of mobile charges, and their exact location can have a strong effect on nanowire behavior,” says Ng. “We showed that boron-dopant atoms are more likely to be found at both the nanowire core and
A*STAR RESEARCH OCTOBER 2011– MARCH 2012
The structure of a small-diameter (green) and large-diameter (blue) silicon nanowire, inside a bulk silicon (yellow) structure
surface in larger-diameter nanowires, and mainly at the surface for smaller diameters.” Ng and his co-workers envision that elucidating the relation- ship between bandgap and diameter will be useful for the devel- opment of nanoscale silicon devices. The work is also significant as a proof of principle. “As computational resources continue to improve and drop in price, the demand for first-principles simu- lations of large-scale problems will grow. Our work demonstrates the feasibility of addressing one such problem,” says Ng.
■
1. Ng, M.-F., Sullivan, M. B., Tong, S. W., & Wu, P. First-principles study of silicon nanowire approaching the bulk limit. Nano Letters 11, 4794–4799 (2011).
59
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96