Radiative Recombination and Carrier Lifetimes in Surface-Free GaAs Homostructures
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RADIATIVE RECOMBINATION AND CARRIER LIFETIMES IN SURFACE-FREE GaAs HOMOSTRUCTURES L. M. Smith and D. J. Wolford IBM T. J. Watson Research Center Yorktown Heights, NY 10598 R. Venkatasubramanian and S.K. Ghandhi Rensselaer Polytechnic Institute Troy, NY 12180 ABSTRACT We show that the radiative efficiencies and lifetimes of photoexcited carriers in epitaxial GaAs may be enhanced by 3 to 4 orders-of-magnitude by the preparation of n÷, doped layers at surface and substrate interfaces. Samples were prepared by Organo-Metallic Vapor Phase Epitaxy (OMVPE), with n-region thicknesses of 3-10 /m, and narrow layers Si-doped to n÷ concentrations of 5x10' 8 cm- 3 . Time-resolved luminescence in such structures, under both surface and bulk (near-band-edge) excitation conditions, reveal near-edge-excitonic or band-to-band-dominated recombination spectra, with carrier lifetimes ranging from 1.5 nsec at 1.5 K to nearly I jisec at room temperature. This is in contrast to the sub-nanosecond lifetimes typical in conventionally prepared bulk GaAs, but is comparable to the best reported for high-purity LPE-prepared GaAs/AlxGa-,,As double heterostructures. The spatial distributions of photoexcited carriers in these structures are observed to expand by over an order of magnitude during their 1 1tsec room temperature lifetime. The expansion is diffusive, with a measured diffusion constant of 14 cm 2/sec at 300 K. This corresponds to a room temperature mobility of 525 cm 2 !/Vsec,comparable to previously measured hole mobilities in bulk p-type GaAs of similar purity. These results are clear evidence that the narrow, heavily doped layers effectively "shield" minority carriers from the interfaces, thereby reducing interface recombination. INTRODUCTION Direct-gap semiconductors, such as GaAs, have always been recognized as the material of choice for optoelectronic devices. Since the radiative band-to-band transitions are dipole allowed and non-radiative Auger processes are nearly negligible, the theoretical quantum efficiency for the production of light in GaAs approaches 100%. In "real" structures, however, this limit is rarely, if ever, attained, because of non-radiative recombination at defects both in the bulk and at surfaces and interfaces. While non-radiative bulk defects have been rendered virtually negligible by modern epitaxial growth techniques, robust solutions to the problems of surface and interface states have remained elusive. In fact, recombination of carriers in GaAs may, in practice, be completely dominated by nonradiative recombination at the crystal surface. The surface recombination velocity - generally greater than 101 cm/sec in GaAs - limits the minority carrier lifetime to less than a nanosecond. Many novel methods for passivation of these surface states have been attempted in recent years.[1-7] However, the standard against which all methods are compared are GaAs/AlGal,,As heterostructures,[8-16] where the larger band gap AlGa-_,As layers serve to confine the carriers to the GaAs layers inside the structure, and away
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