• PDF / 269,977 Bytes
• 5 Pages / 414.72 x 648 pts Page_size
• 51 Downloads / 237 Views

DOWNLOAD

REPORT

---

ABSTRACT Influence of different adsorbates on recovery of porous Si luminescence was in situ investigated for samples preheated under ultra-high vacuum conditions. Exposure to simple adsorbates (02, H 2, N2 up to a pressure of 10-3 to 10-1 Torr), long exposure to air at the atmosphere pressure, or immersion into distilled water fail to recover the red luminescence. On the other hand, we found that the red luminescence can be recovered by adsorption of water vapour onto the sample surface. The red luminescence of as-prepared porous Si can be caused by water molecules (together with possible impurities) adsorbed in the pores. The immense surface of the pores provides a large number of emitting sites. The porous Si layer can filter for the emitted light, cutting off the wave region shorter than yellow. The combination of these two factors (emission and filtering) can result in the visible red-orange luminescence of porous Si. INTRODUCTION Obviously for light-emitting from porous silicon the role of the physical and chemical state of its surface should be very important, if not critical [1]. For example, the surface (or interface) electronic states in the silicon bandgap may act as centers of non-radiative recombination suppressing the luminescence; on the other hand, according to the "smart quantum confinement" model [2], these states may also participate in the luminescence as centers of radiative recombination. In this work, we study under high vacuum conditions the influence of various adsorbates on photo-, electro- and cathodoluminescence of porous silicon. EXPERIMENT AND RESULTS Sample preparation and characterization Samples of porous silicon under study were prepared by the conventional electrochemical technique (p-Si, 0.02 Ohm.cm). Also, we have fabricated silicon nanoparticles (islands) by the technique used earlier for preparation of island metal films [3, 4]. For this purpose, a thin (about 100 nm) crystalline film of n-Si (10 Ohm.cm) in a 50 ptm wide gap between two silicon contacts on a sapphire substrate was subjected to the forming procedure (by passing the electric current). The sample was placed in an evacuated tube with a CaF 2 window. Photoluminescence (PL) was exited with a pulse N 2 laser (X = 337.1 nm, pulse duration 10 ns, pulse frequency 100 Hz). The experimental set-up is schematically shown in fig. 1. 197

Mat. Res. Soc. Symp. Proc. Vol. 405 01996 Materials Research Society

Fig. 1. Block diagram of the experimental set-up: I -

porous silicon sample; 2-

N2 -laser (X = 337.1 nm, P = 200 kWt/cm2 , 10 nsec pulse duration, 100 Hz pulse frequency); 3 - diffiraction spectrometer; 4 - sample heater; 5 - H 2 , 02 and N2 sources; 6 - getter pump; 7 - CaF 2 window of the vacuum chamber; 8 - ionization vacuum gauge; 9 - vacuum valve. The morphology and structure of the as prepared porous Si layers have been examined by the conventional transmission electron microscopy using self-supported films several tens of nanometers thick. The films were found consists of two phases: (1) crystallites with a perfec