Electron Spin Resonance Investigations on Porous Silicon
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I. INTRODUCTION The electrolytically etched porous silicon (PS) has stimulated a considerable amount of interest and research in the last four years. The high quantum efficiency of the bright visible luminescence band at 1.7 eV gave impact to electroluminescence applications.' Very different spectroscopic techniques have been used to characterize the material and to gain insight in the formation and properties of porous Si. Still widely debated are two models for the explanation of the porous Si luminescence - the quantum size and siloxene model." ' In the first model crystalline Si particles of nanometer scale, formed during the etching process, are thought to give rise to quantum size effects. The band gap of Si is thus considerably increased and the bandstructure modified to allow emission in the visible spectral range. Disorder and size distribution of the particles account for the large linewidth of the emission band. In the second model siloxene type molecules (Si6 0 3+nH6 _,) formed during the etching process in diluted HF are made responsible for the light emission.3 Electron paramagnetic resonance (EPR) and other related resonance techniques such as spin dependent photoconductivity (SDPC) measurements as well as optically detected magnetic resonance (ODMR) have considerably contributed to the understanding of the nature of the defects involved in the luminescence process and made responsible for the degradation of the material.4 22 The degradation easily happens by ultra-violet (UV) and visible (VIS) light illumination4 , 24 beam. heating" , or exposure to an electron 453 Mat. Res. Soc. Symp. Proc. Vol. 358 0 1995 Materials Research Society
2.008 2.007
S2.006
cc$ 2.005 00 -2
Figure 1: ___
6) 2.004
Rotation pattern of the EPR signals observed in as etched as well as in thermally annealed (400'C, 30 min, Ar at-
2.002
mosphere) samples (full squares).
The
Sdrawn line is a simulation with the go
20
40
60
80
Angle (degrees)
values g.L = 2.0018 and gj, = 2.0080.
Inset: defect model of the Pb-center.
II. EXPERIMENTAL DETAILS The sample preparation was done by anodically etching (current density of 30 to 100 mA/cm 2) in an HF acid, the resistivity of the B-doped Si substrate material ranged from 0.01 to 1 acm, resulting in the formation of nano- and meso-porous Si. Some of the samples were annealed at 400°C for 30 min under a protecting Ar atmosphere. Oxidation was performed at various temperatures for a time interval of 30 s using a rapid thermal oxidation apparatus. For details see Ref. 19. The EPR measurements were performed on a BRUKER ESP 300 spectrometer. The nano-PS samples were not removed from the Si substrate. Due to the conducting substrate the EPR measurements were limited to temperatures below 80 K and performed at 6 K. Experimental conditions were chosen to avoid saturation effects of the EPR signal. Quantitative estimates of the defect densities were possible by a comparison with a Si:P standard. For the ODMR investigations we used a 36 GHz spectrometer with the microwaves sq
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