Electrically Pumped Rare Earth Doped Semiconductor Lasers
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ELECTRICALLY PUMPED RARE EARTH DOPED SEMICONDUCTOR LASERS JACQUES I. PANKOVE AND ROBERT J. FEUERSTEIN Optoelectronic Computing Systems Center Department of Electrical and Computer Engineering University of Colorado, Boulder, CO 80309-0425 ABSTRACT The interest in rare earth (RE) elements as luminescent centers is due to the narrowness and stability of the luminescent transitions. In this paper we review the mechanisms that can be used to electrically excite rare earth impurities in semiconductors: pair recombination energy exchange and impact excitation. The different means of providing energetic electrons for impact excitation are also discussed. We also propose a possible explanation for the temperature dependence of photoluminescence in silicon. INTRODUCTION The interest in rare earth (RE) elements as luminescent centers is due to the narrowness and stability of the luminescent transitions. These transitions are sharp because they involve atomic levels, unlike the bands of levels involved in the luminescence of semiconductors. A great number of luminescent lines from IR through visible can be generated by RE atoms. Rare earths of the lanthanide series have three outer electrons that are used in bonding to neighboring atoms. These outer electrons residing near the neighboring atoms give an ionic character to the RE, essentially putting it in the 3+ state. The electrons of the next or inner shell are the ones involved in the luminescence process. These inner electrons are decoupled from the adjacent atoms. Their transitions are localized processes that are not influenced by the host atoms. These transitions are characteristic of the RE element only; however, they can be perturbed slightly by local fields. Hence, one obtains nearly the same emission wavelength (1.54 gIm) from Er regardless of whether it is inside glass, Si, GaAs, or any other host. Furthermore, the energy levels of the rare earth form a system independent of the energy levels of the host material. One cannot say that the ground state is in the energy gap of the host crystal or in its valence or conduction bands. In this paper we shall consider only the rare earth erbium, since it is the most widely used RE element, the main practical application being optical communication via optical fibers, where the minimum absorption coincides with the most efficient emission at 1.54 gim. In addition to the low loss, optical fibers also exhibit a low spectral dispersion at this wavelength. First the scheme of energy levels will be described and the transitions discussed, then the excitation mechanisms will be considered. We shall compare the merits and disadvantages of optical and electrical pumping. The various means for electrical excitation will be explored. The role of the host and of co-dopant atoms will be presented, leading to a new hypothesis about the excitation mechanism. Experimental results and calculations will be compared to test the model. Finally, our experimental approach to electrical pumping will be described. EXCITATION MECHANISMS Direct Opt
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