Comparison of the Band Gap of Porous Silicon as Measured by Photoelectron Spectroscopy and Photoluminescence
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Department of Physics and *Department of Electrical Engineering University of British Columbia, Vancouver, B.C. V6T-1Z1 YUAN GAO Department of Physics Simon Fraser University, Burnaby, B.C, V5A-1S6. ABSTRACT
The peak energy of the room temperature photoluminescence of porous silicon is compared with the bandgap determined from photoelectron spectroscopy measurements for a series of porous silicon samples prepared under different conditions. The photoluminescence bandgap is found to be smaller than the photoelectron spectroscopy bandgap, but exhibits the same trend with preparation conditions. The width of both the photoluminescence spectrum and the L-absorption edge increases when the current density during the preparation is increased or the sample is allowed to soak in HF after preparation. INTRODUCTION
Photoelectron spectroscopy experiments including total electron yield absorption measurements and photoemission spectroscopy have shown that the energy of the conduction and valence band edges of porous silicon (PS) depend on the conditions used in the synthesis of the material and on treatments after preparation.1- 3 The experimental results were found to be consistent with a quantum confinement model. 1-3 Since the photoelectron spectroscopies involve collection of low energy electrons emitted from the material they are inherently surface sensitive. Consequently in an inhomogeneous material, such as porous silicon, the results of the photoelectron spectroscopies may not be directly comparable with more bulk sensitive optical techniques such as photoluminescence (PL). However both the PL and the photoelectron spectroscopy measurements are sensitive to inhomogeneities in the sample. In this paper we explore the relationship between the PL spectrum of PS and the photoelectron spectroscopy measurements of the bandgap and the Si L-edge absorption spectra for a series of PS samples prepared under different conditions. EXPERIMENT
The PS samples were prepared by electrochemical anodization of p and n-type silicon at constant current in a 2:1:1, ethanol:HF:H 20 solution. The current density in the n-type material was controlled by varying the intensity of illumination from a halogen lamp. The electronic properties of the porous silicon made from n-type material were modified by changing the current density during anodization. A p-type sample, made at lmA/cm 2, was modified after preparation by soaking it in HF for varying lengths of time. After preparation the PS samples were exposed to air for a few minutes to an hour before the photoluminescence measurements were made. Photoluminescence spectra were obtained at room temperature with the sample held in a flowing nitrogen ambient. The nitrogen ambient was needed to maintain a stable PL emission intensity during the time period of the measurement. The PL was excited with the 457 nm line from an Ar ion laser and the emission was detected with a Bomem interferometer equipped with a room temperature silicon photodiode detector. PL spectra for a series of PS samples prepar
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