Reduction of Neutral Dangling Bond Density by Light Soaking in Nanocrystalline Silicon
- PDF / 250,382 Bytes
- 4 Pages / 414.72 x 648 pts Page_size
- 93 Downloads / 170 Views
ABSTRACT The effects of light exposure on neutral defect density at the surface of nanocrystalline Si are investigated by electron-spin resonance (ESR) experiments. A decrease of the neutral dangling bond density by light soaking was observed in this nanostructure. The reduction rate of ESR signal intensity becomes large with increasing light exposure intensity, and the reduction occurs from the excitation energy higher than 2 eV in vacuum. The reduction of the defect density can be explained in terms of the conversion of neutral states to charged states by carrier trapping. INTRODUCTION The generation of neutral defects by light soaking in hydrogenated amorphous silicon (aSi:H), which is called Staebler-Wronski effect [1], has drawn broad attention not only from the point of physical understanding but also from the point of the stability of various optoelectronic devices such as solar cells. Porous Si fabricated from electrochemical anodization, which has received considerable attention as a luminescent material [2], is a Si-nanostructure terminated with a huge number of hydrogen [3]. This material also shows strong degradation of photoluminescnece (PL) by exposing laser light in oxidizing ambient [4]. Therefore, from the similarity between a-Si:H and Porous Si in terms of the large hydrogen content, one might consider the increase of the dangling bonds as the origin of the degradation. However, in this paper, we present an opposite result-the neutral dangling bond density decreases by light soaking in porous Si. This is in contrast to the Staebler-Wronski effect observed in a-Si:H, where the dangling bonds act as nonradiative recombination centers and their density is increased by light soaking. Both the intensity and the energy dependencies of light soaking for electron spin resonance (ESR) measurements show that the reduction of the neutral defect density (DO) can be explained from the conversion of neutral states to charged states by carrier trapping. EXPERIMENTAL The porous Si layers were formed by anodization of p-type Si wafers with 3-5 i)cm resistivity in a solution of HF (49 %) : ethanol = 1:1 at a current density of 20 mA/cm 2 for 30-60 min. After anodization, the porous Si layer was removed by increasing the current density abruptly to 700-1000 mA/cm 2 . The porosity of the sample is estimated to be about 80 % from the mass of the free-standing layer. Light soaking of the sample was performed in air or in vacuum using Ar ion laser (514. 5 nm) with an intensity of the order of 10-1000 mW/cm 2 . ESR measurements were performed using a X-band spectrometer with a microwave power of 1 mW. The spin density was calibrated using a standard sample. RESULTS AND DISCUSSION Figure 1 shows the ESR signal of as-prepared- (dotted line) and of Ar-ion-laser-lightsoaked- (solid line) porous Si layers in vacuum. The light soaking was performed for 30 min with the intensity of 200 mW/cm 2. It is interesting that in spite of 30-min light soaking performed for the porous layer, the dangling bond density of this layer was alm
Data Loading...
