Theory of Porous Silicon Injection Electroluminescence
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THEORY OF POROUS SILICON INJECTION ELECTROLUMINESCENCE H. PAUL MARUSKA, F. NAMAVAR, and N.M. KALKHORAN Spire Corporation, One Patriots Park, Bedford, MA 01730-2396
ABSTRACT: We discuss the operation of porous silicon light-emitting diodes prepared as heterojunctions between n-type In.0 3:Sn (ITO) and p-type silicon nanostructures, exhibiting quantum confinement effects. The transparent ITO affords light emission through the top surface of the device, as well as providing passivation and hence long term stability. We describe a model for the injection of minority carrier electrons into the porous silicon regions, which results in the emission of yellow-orange DC electroluminescence. A detailed study of the forward bias current-voltage characteristics of the devices will be given, which allows calculations of the densities of interface states. A tendency to pin the hole fermi energy near the neutral level, 00, is shown to control the extraction of majority carriers. Methods for improving LED efficiency by alleviating a parasitic shunt current path through interface states will be addressed. INTRODUCTION There is increasing emphasis on the development of new materials which can be applied to the improvement of the speed and efficiency of information transfer in computer systems, whether from chip to chip, board to board, or machine to operator. Basically, it is desirable to supplant copper or aluminum wiring, as well as vacuum tube displays, with monolithically integrated all solid state opto-electronic devices. Therefore, the possibility of developing optical interconnects relying solely on silicon technology represents a much desired breakthrough in technology. True wafer scale integration will become a reality if the light"ri.'ssion and light detection functions can be implemented directly in silicon wafers. Present investigations involving the attachment of Ill-V compounds onto silicon chips, with the electronic functions carried by the silicon and light emission dependent on GaAs or InP would definitely be superseded by the new materials technology. Silicon nanostructures featuring quantum size effects may allow this goal to be attained. As a major contribution to this domain of materials research, we have recently reported the fabrication of visible np heterojunction light-emitting diodes (LEDs) based on porous silicon (PS), which emit yellow-orange light under forward bias.`~ Our electrochemically produced p-type porous silicon samples manifested the necessary quantum confined structures, and the transparent n-type semiconductor indium tin oxide (ITO: 91% In.0 3 and 9% SnO 2 ), deposited by RF sputtering, formed the heterojunction. A shadow mask was used to define square mesas of ITO on the porous silicon surface. For purposes of comparison, some portions of the silicon wafer were not etched, and ITO heterojunction diodes were also produced on this bulk silicon. ITO makes an excellent top contact, because it is transparent to all wavelengths of visible light. Photoluminescence spectra of porous silicon samples
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