Combined Optical, Surface and Nuclear Microscopic Assessment of Porous Silicon Formed in HF-Acetonitrile
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ABSTRACT A new type of HF solution, HF-acetonitrile (MeCN), has been employed to produce 10-30 pm thick porous silicon (P-Si) layers by photoelectrochemical etching of different types of Si wafers, Si(100), Si(1 11) and polycrystalline Si, with different resistivities. A combined optical, surface and nuclear microscopic assessment of these P-Si layers was performed using photoluminescence (PL), Raman scattering, X-ray photoelectron spectroscopy (XPS) and Rutherford backscattering spectroscopy (RBS). The PL emission intensities, Raman line shapes and structural features are strongly dependent on the properties of the substrates such as the crystallinity and resistivity of the Si wafers used for forming P-Si. With increasing resisitivity of the Si(100) wafers, the resulting P-Si layers show a slight blue-shift of their visible light emission peak energy, an up-shift of the peak position and a narrowing of the band width of the dominant Raman band, and a decrease inthe amount of residual elemental Si on the surface. Those Si(1 11) wafers, etched in HF-MeCN, showed no porous structures and no visible light emission. INTRODUCTION
The observations of the blue-shift of the absorption edge [1] and the visible photoluminescence (PL) at room temperature (RT) [2] from porous silicon (P-Si) in the 1990s have attracted considerable attention to P-Si and related materials [3-6]. These developments have led to the publication of a large number of reports and papers in various journals and conference proceedings [3-5] as well as a recent review book [6]. Traditionally, Si is the dominant material for modem electronics and computers. But, its indirect band gap, which lies in the near infrared (NIR), and low light emission quantum efficiency had limited its use in visible optoelectronics. The recent developments in porous Si may however, fortunately result in the use of the material in visible optoelectronics by merger of its optoelectronic properties and Si-based integrated processing techniques. This may open a new field of processed optoelectronic Si microelectronic devices. Porous Si can be formed easily by electrochemical etching of single crystalline Si in HF solutions containing electrolytes. The porous Si structures generate visible photoluminescent or electroluminescent light. Many authors [1,2] believe that quantum size effects, based upon the free standing quantum wire model, is the major reason for visible light emission, although several other mechanisms have also been proposed to explain the behavior of RT visible light emissions from P-Si, such as siloxene [7], oxidized Si [8], hydride complexes [9], and an amorphous phase of Si or its complexes [10]. We have used a combination of several sophisticated techniques to understand and distinguish different luminescent mechanisms [11-13] 345
Mat. Res. Soc. Symp. Proc. Vol. 358 01995 Materials Research Society
The ordinary electrochemical etching of Si in aqueous electrolytes leads to the formation and dissolution of oxide intermediates, which causes dissolution limited
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