Quantum-Confined Excitons Produce Luminescence in Silicon Nanowires

Lauren I. Rugani

Light emission from silicon nanowires has been demonstrated previously, but there has not been conclusive evidence that the emissions can be obtained from quantum-confined excitons within the nanowire core. Furthermore, studies have neglected to distinguish potential quantum-confined exciton emission from other sources of luminescence within the silicon structure.

A scanning electron microscopy image reveals an array of as-grown nanowires before oxidation.

Researchers from Stanford University in California have collaborated with Hewlett-Packard Laboratories in Palo Alto, Calif., to study TiSi₂-catalyzed silicon nanowires. They combined transmission electron microscopy and time-resolved photoluminescence to examine the size-dependent optical properties of oxidized nanowires.

After each oxidation, the scientists employed high-resolution and dark-field transmission electron microscopy using a Philips field-emission microscope. Both techniques provided highly accurate images of the nanowire diameters, which exhibited a progressive decrease, from about 16 nm to about 3.5 nm, as a function of oxidation time.

Photoluminescence spectra of TiSi₂-catalyzed silicon nanowires, taken after 10 and 120 minutes of thermal oxidation, demonstrate the blueshift of the photoluminescence band peaks with increasing oxidation time.

Photoluminescence measurements were performed by exciting the nanowire samples with a 488-nm beam from a Coherent Ar⁺ laser. The spectra were recorded with a silicon CCD camera from Princeton Instruments coupled to an Acton grating spectrometer. After an oxidation period of 10 minutes, the researchers observed a broad luminescence band that peaked at 1.55 eV. As the oxidation time increased, the band experienced a blueshift, and the peak energy saturated around 1.66 eV. The photoluminescence decay lifetime was found to decline with increasing energy.

Oxidation declines

The investigators found that the oxidation rate decreased for longer periods. Eventually, oxidation stopped as the nanowire core reached a self-limited diameter. This enabled careful control of the diameter of a nanowire for specific size-dependent studies of its optical properties. After 120 minutes, the longest oxidation time, the nanowire diameter was smaller than the 4.9-nm Bohr exciton radius of bulk silicon. This resulted in quantum-mechanical effects on the band structure and optical properties of the wire.

A high-resolution transmission electron microscopy image of a silicon nanowire taken after 10 minutes of thermal oxidation reveals the nanowire core, which progressively decreases as oxidation time increases.

The researchers found all the observed effects of a decreasing core diameter — greater photoluminescence peak energy and shorter decay lifetimes — to be consistent with quantum-containment theory. They attribute the photoluminescence to the recombination of quantum-confined excitons.

In the future, these nanowires could be applied in CMOS-compatible photonic devices and enable low-cost on-chip light emitters, large-area displays or electrically pumped silicon based lasers.

Nano Letters, published online Aug. 9, 2006.