Optical Studies of Electroluminescent Structures from Porous Silicon
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OPTICAL STUDIES OF ELECTROLUMINESCENT STRUCTURES FROM POROUS SILICON
Harvey,*a R.A. Lux,* D.C. Morton,* G.F. McLane,* and R. Tsu**b J.F. U.S. Army Research Laboratory, FL Monmouth, New Jersey 07703 **University of North Carolina, Charlotte, North Carolina 28223 ABSTRACT
Two components of the electroluminescence (EL) from porous silicon light emitting diode (LED) devices have been observed. A slower component and a faster component have been identified. The slower component has a spectral peak shifted to the red from the corresponding photoluminescence (PL) spectrum. The faster component has a spectral peak well in the infrared (IR). Optical and electrical measurements of these two components are discussed. The temperature dependence of the two EL components are presented and contrasted. Our measurements demonstrate that the two EL components and the PL result from recombination in different parts of the porous silicon structure. As the temperature is reduced below room temperature the slower EL exhibits a decrease in intensity at relatively high temperatures, suggesting a freeze out of electrical carriers due to quantum confinement, resulting in a much reduced electrical excitation of the EL. INTRODUCTION
The first report of bright room temperature PL from porous siliconI generated strong interest in the scientific community because it opened the door for consideration of silicon as a potential optical material which could be directly integrated into VLSI circuits. Since that time a significant amount of research has been focused on the PL of porous silicon and the responsible physical mechanisms. However in order to be useful as an optoelectronic material, porous silicon must provide bright EL, not PL, at useful wavelengths and with reasonable quantum efficiencies. EL from porous silicon was reported using a liquid electrolytic solution as one of the electrical contacts. However applications to integrated circuits will require a solid state device EL.6 Several research groups have reported EL from solid state devices made from porous silicon.2Generally the EL reported so far is very weak compared with the bright PL. One potential application of porous silicon EL is to drive fiber-optic optical systems, which would require bright and very fast EL. A second potential application is in silicon EL displays, which would not require so much speed, but would require a very bright, visible EL source. In order to determine the optimum solid state structure for producing EL for a particular application, the physical mechanisms responsible for the EL and for the carrier transport in porous silicon must be well understood. In this paper electrical and optical characterization of porous silicon EL is examined to identify the principal physical mechanisms involved. For the first time two separate components of the EL, one faster and one slower, are observed. These two components clearly result from different physical and carrier transport mechanisms. Using the optical spectra, electrical measurements, and the temperature dep
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