The Effect of Starting Silicon Crystal Structure on Photoluminescence Intensity of Porous Silicon
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K. L. Kavanaghb
Naval Command, Control, and Ocean Surveillance Center, RDT&E Division (NRaD), Code 553, San Diego, CA 92152-7633 b Department of Electrical and Computer Engineering, University of California, San Diego, La Jolla, CA 92093-0407 a
ABSTRACT In previous work we reported that porous silicon (PS) films formed using a dilute HF:HNO 3 chemical etch on polycrystalline, implant damaged single crystal, or amorphous starting material have 1 luminescent characteristics that differ from PS fabricated on single crystal silicon . Polycrystalline and implant damaged porous silicon exhibits brighter luminescence compared to single crystal silicon etched under identical conditions. No photoluminescence is detected from the porous amorphous silicon. In this work these effects are examined using HF:NaNO 2 solutions with freely available NO2. The accelerated etching effects from work damage are reduced, and the PS from polycrystalline and implant damaged silicon luminesce with the same intensity as the PS from single crystal silicon. Again, etched amorphous silicon does not luminesce. TEM and EDX porosity measurements are used to determine the differences in structure and etching characteristics between the luminescent and non-luminescent materials. INTRODUCTION 2 Since the discovery that porous silicon photoluminesces various theories have been advanced to explain the source of the light. Chief among these is the idea that the etching mechanism causes the creation of quantum particles or wires with a direct band structure which allows the material to 3 luminesce efficiently in the visible region . This theory requires that the silicon remain in a single crystal form in order to luminesce. Other theories note that other silicon materials such as amorphous also luminesce in the region observed and propose these materials silicon, silicon oxides, and siloxenes 4
as a source of the luminescence ,". Recent work proposes a more complex model than the simple quantum wire model which includes surface interactions forming radiative centers on the single crystal
6 structures which assist in luminescence. Porous silicon photoluminescence studies have been based primarily on porous silicon formed through electrochemical etching in a HF:ethanol solution. This technique has the advantage of providing thick films for analysis as well as good control over the etching parameters. However, this method cannot easily be applied to thin silicon films on non-conducting substrates such as silicon on sapphire (SOS) and can be difficult to pattern using photolithography. Some workers in the field have 7 These films are looked at photoluminescent porous silicon fabricated using chemical etches ',. considerably thinner than electrochemically etched films, since the nature of the corrosion process is limited by carrier transport which is not assisted by an external electrical field in the case of the chemical etch9 . With increasing etch time, the reaction reaches an equilibrium between film dissolution and formation. The final porous silicon
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