Blue Light Emission from Germanium Ultrafine Particles by the Gas Evaporation Technique
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SHINJI NOZAKI, S. SATO, A. DENDA, H. ONO AND H. MORISAKI The University of Electro-Communications , Department of Communications and Systems, 1-5-1 Chofugaoka, Chofu-shi, Tokyo 182, Japan
ABSTRACT The ultrafine particles with diameters in the order of 10 nim were deposited onto Si and SiO 2 substrates by evaporation of Ge in a pure hydrogen atmosphere. Although the as-deposited Ge ultrafine particles do not show any detectable luminescence, they emit blue light after being exposed to the UV light for a long time. The blue light is strong enough to be seen with the naked eye even under a room light. The photooxidation, unique to the Ge ultrafine particles, has been identified as a major factor contributing to the blue light emission.
INTRODUCTION There is an increasing interest in semiconductor nanostructures, which are expected to possess electrical and optical properties distinct from those of bulk semiconductors. The discovery of visible light luminescence from porous Si has triggered studies on various forms of Si nanostructures such as the Si nanocrystals embedded in Si0 2 [2] and the Si ultrafine particles deposited by the gas-evaporation technique [3,4]. We reported orange-red and blue light emission from the Si ultrafine particles by evaporation of Si powder in an argon and an oxygencontaining argon atmosphere, respectively. Through a series of studies on the Si nanostructures, we found that oxidation of Si plays an important role in the visible light luminescence and were not able to attribute the blue light emission directly to the three-dimensional quantum size effect [4]. It, however, continues to be a subject of debate whether a semiconductor material with an indirect bandgap which is smaller than the energies of photons in visible light can efficiently emit visible light when its size becomes small enough to manifest the three-dimensional quantum size effect. Physical interpretation of visible light luminescence from Si nanostructures is often hindered by presence of native oxide at the Si surface, which may influence a radiative recombination process. This time we have studied Ge ultrafine particles deposited by the gasevaporation technique. Since Ge is an indirect-gap material like Si but is less subject to natural oxidation, clear understanding of the luminescence mechanism in the Ge ultrafine particles may help to clarify visible light luminescence from Si nanostructures.
EXPERIMENTAL A gas-evaporation technique similar to the technique used to deposit Si ultrafine particles [3,4] was employed to deposit Ge ultrafine particles on Si and SiO 2 substrates. After evacuating a chamber, pure hydrogen gas was introduced into the chamber. When the gas pressure reached 5 torr, small pieces cut from Ge substrates in a boron-nitride (BN) boat was heated in a 133
Mat. Res. Soc. Symp. Proc. Vol. 358 01995 Materials Research Society
hydrogen atmosphere. A typical film thickness was 30-50 nm after 20 min evaporation. Transmission electron microscopy (TEM) was performed on the as-deposited Ge film. A sample
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