Disordering and Characterization Studies of 69 GaAs/ 71 GaAs Isotope Superlattice Structures: The Effect of Outdiffusion
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DISORDERING AND CHARACTERIZATION STUDIES OF 6 9 GaAs/ 7 1GaAs ISOTOPE SUPERLATTICE STRUCTURES: THE EFFECT OF OUTDIFFUSION OF THE SUBSTRATE DOPANT SI T. Y. Tan%, H. M. You%, S. Yu*, U. M. G6esele*, W. Jager", F. Zypman+, R. Tsu+,
and S.-T. Lee++ Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC 27706 Institute for Solid State Physics, Research Center D-5170 JOlich, Germany +Department of Electrical Engineering, University of North Carolina at Charlotte, Charlotte, NC 28213 ++Corporate Research Laboratories, Eastman Kodak Company, Rochester, NY 14650 ABSTRACT
Undoped 69 GaAs/ 71 GaAs isotope superlattice structures grown by MBE on n-type GaAs substrates, doped by Si to -3x1018 cm" 3 , have been used to study Ga self-diffusion in GaAs by disordering reactions. In the temperature range of 850-960 0C, the SIMS measured Ga self-diffusivity values showed an activation enthalpy of 4 eV, and are larger than previously compiled Ga self-diffusivity and AI-Ga interdiffusivity values obtained under thermal equilibrium and intrinsic conditions, which are characterized by a 6 eV activation enthalpy. SIMS, CV, and TEM characterizations showed that the as-grown superlattice layers were intrinsic which became p-type with hole concentrations up to -2x10 17 cm-3 after annealing, because the layers contain carbon. Dislocations of a density of ~106_107 cm" 2 were also present. However, the factor responsible for the presently observed larger Ga self-diffusivity values appears to be Si outdiffusion from the substrate, which was determined using CV measurements. Outdiffusion of Si decreases the n value in the substrate which causes the release of excess Ga vacancies into the superlattice layers where the supersaturated Ga vacancies enhance Ga self-diffusion. INTRODUCTION The effect of n-doping on enhancing GaAs/AIGaAs superlattice (SL) disordering reactions has been determined to be that of the Fermi-level [1,2] which increases the concentration of the triply-negatively-charged group III sublattice vacancies, designated as V&, which govern AI-Ga interdiffusion and Ga self-diffusion under Intrinsic and n-doping conditions [1]. Under intrinsic and thermal equilibrium conditions with an As4 vapor pressure of -1 atm, the compiled data of Ga self-diffusion above 1025 0C and of AI-Ga interdiffusion from 825 to 1100oC showed that the V contribution to the group IIIatom diffusivity is given by [1]
D,0n,)
=
2.9x10 8 exp(-
) cm 2 s-,
(1)
where kB is Boltzmann's constant and T is the absolute temperature. Since in Eq. (1) values obtained from AI-Ga interdiffusion have been included, we thought it should be interesting to compare these values to that obtained from Ga self-diffusion in the same temperature range. To this end, 69 GaAs/ 71 GaAs isotope SL structures have been grown on n-type GaAs substrates doped by Si to -3x10 18 cm" 3 . The structure was disordered to study the Ga diffuMat. Res. Soc. Symp. Proc. Vol. 262. @1992 Materials Research Society
874
sivity in the temperature range of 850-960 0C
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