Si2p Core Level Absorption and Photoemission Spectra of Porous Si
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2 Si p CORE LEVEL ABSORPTION AND PHOTOEMISSION SPECTRA OF POROUS Si
HISAO NAKASHIMA, KOICHI INOUE AND KENZO MAEHASHI The Institube of Scientiffc and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567, Japan ABSTRACT Si2p core level absorption and photoemission spectra are taken for different porous Si layers using synchrotron radiation, to know the electronic structures of porous Si. The core level absorption spectra show the high energy shift of the conduction band which correlates with the photoluminescence blue shift. The oxidation states of porous Si are clarified from the photoemission spectra. INTRODUCTION Since the recent observation of intense visible photoluminescence from porous Si 1 , many experiments have been carried out to clarity the origin of the visible luminescence. There are two completely different interpretation. The one is that the luminescence comes from the columnar structure of porous Si and the blue-shift of the photoluminescence is due to the quantum size effectI. The other is that the luminescence comes from Si compounds such as siloxene 2 , which are expected to exist on large surfaces of porous Si. Since there are some controversies in experimental results, the origin of the visible luminescence has not been clarified. From micro-photoluminescence and micro-Raman scattering measurements, we have found that these controversies are due to the inhomogeneous porous structures, particularly due to the depth inhomogeneity 3 . Because of this inhomogeneity, no clear picture of the electronic structure of porous Si has not been obtained from the optical measurements. In this paper, we report Si2p core level absorption and photoemission spectra of porous Si which exhibit the electronic structure of porous Si. We discuss the correlation with the photoluminescence spectra and the oxidized states of porous Si. EXPERIMENTAL Three different porous Si layers were prepared by anodizing n- (5-10 Qcm) or p+ (0.01 Qcm) Si (100) wafers in the solution of hydro fluoride acid with (for n-Si) or without (for p+Si) light exposure from a 300 W tungsten lump. Sample preparation conditions are shown in Table I. These samples were held in air for a few days before the measurements with synchrotron radiation. An n- bulk Si crystal, which was treated in the solution of hydro fluoride acid immediately before the measurements, was
Table I
Preparation conditions of porous Si layers
sample
anodizing time (min.)
n- porous Si #1 n- porous Si #2 p+ porous Si
current density 2 (mA/cm )
light exposure
2
20
o
0.5
80
0
20
x
10
Mat. Res. Soc. Symp. Proc. Vol. 283. 01993 Materials Research Society
140
also measured for comparison. After the photoemission measurements, the p+ porous sample was exposed to air for about a day and again introduced in the vacuum chamber to examine the influence of the vacuum-ultra-violet (VUV) irradiation. In photoemission experiments, synchrotron radiation from SOR-RING, the 0.4 GeV electron-storage ring at the Institute for Solid State Physics of the Univ
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