Optical properties of Si nanowires on a Si {111} surface
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ABSTRACT We have investigated optical properties of straight silicon nanowires by means of in-situ cathodoluminescence spectroscopy in a transmission electron microscope. The nanowires, grown on a Si { Ill } surface via vapor-liquid-solid growth mechanism, have no structural defects such as kink, and the diameter and growth direction are controlled by varying the growth conditions. We have found that the nanowires emit intense light. These lines have not been observed in other kinds of Si nanostructures such as porous Si. 1. INTRODUCTION Nanostructural semiconductors, such as nanowires and nanodots, are expected to show distinctive optical and electronic properties from bulk semiconductors due to quantum confinement effects. Various kinds of nanocrystalline semiconductors have so far been fabricated and studied their properties. Especially, nanocrystalline silicon such as porous silicon has been extensively investigated, since they emit visible light [1]. Though a number of reports on the light emission from nanocrystalline silicon have been published, their optical property is still debatable. This is because the difficulties of fabrication of the nanostructure with a simple confinement structure. Recently, we have fabricated straight silicon nanowires in which no structural defects are introduced: the diameter and the growth direction of the nanowires are well controlled by varying the growth conditions [2]. In the present study, we have systematically investigated the optical properties of the silicon nanowires by means of in-situ cathodoluminescence (CL) spectroscopy combined with transmission electron microscopy (TEM), which enable us to obtain simultaneously the structural data in a higher spatial resolution and luminescence spectra at the same mesoscopic location. We have found novel light emission from the silicon nanowires. 2. EXPERIMENTS Sample preparation Figure 1 shows silicon nanowires grown on a H-terminated Sit 1I ) surface by the vapor-liquid-solid growth mechanism [3]. No bend and kink are observable in the nanowires. The diameter of the nanowires range from 2 to 9 nm (Fig. lb), and the average diameter is 4.5 nm. We confirmed that many of the nanowires grow on a substrate along the direction (0 = 1150) as well as the direction (0 = 900) [2].
99 Mat. Res. Soc. Symp. Proc. Vol. 588 ©2000 Materials Research Society
40 (b)
30 1V
20 110 -
0 1
15
3
7 5 9 diameter (nm)
11
(c)
10 E 5
0 20 40 60 80 100 120 140160 180 0 (degree) Fig. 1 (a) A TEM image of silicon nanowires on a H-terminated Si{ 1111 surface. (b, c) Historams of the diameter and the growth directions of the nanowires. 0 represents the angle between the growth direction of a nanowire and the substrate surface.
Figure 2 shows a high-resolution TEM image of a silicon nanowire, indicating that it consists of single crystalline silicon covered with an amorphous silicon oxide layer, and the growth direction is the direction.
100
vacuum a-Si Oxide layer
Si crystalline
a-Si Oxide layer vacuum Fig. 2 A high-resolution TEM image of a
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