Mechanisms for N-Type Impurity-Induced Disordering of AlGaAs/GaAs Superlattices
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MECHANISMS FOR N-TYPE IMPURITY-INDUCED DISORDERING OF AlGaAs/GaAs SUPERLATTICES B.L. OLMSTED,* S.N. HOUDE-WALTER,* and R. ENRIQUE VITURRO** *The Institute of Optics, University of Rochester, Rochester, NY 14627 **Xerox Webster Research Center, 114-41D, Webster, NY 14580 Abstract We report on a study of disordering by the in-diffusion of a variety of Group IV and Group VI n-type impurities. Secondary ion mass spectroscopy, electrochemical C-V profiling, photoluminescence spectroscopy, and cathodoluminescence spectroscopy were used to determine the extent of interdiffusion, and the spatial distribution of impurities and native defects for impurity-induced disordering. In all cases, the n-type dopants enhance the Al-Ga interdiffusion coefficient over that due to an As overpressure of 2 atm alone. The Si-induced enhancement has been previously attributed to the change in Fermi level with doping and therefore should account for disordering using other n-type impurities. However, we observe important differences in the interdiffusion characteristics (diffusion rate, dopant profiles, energy and intensity of the deep-level emission) induced by Si or Ge, and that by S or Se. Whereas a strong correlation exists between the carrier concentration profile and the disordered regions in the Si-, Ge-, and Se-doped crystals, little direct correspondence is found for crystals doped with S. Instead, the disordering seems to be determined primarily by the presence of Group III vacancies, as is also the case in undoped crystals disordered by an As ambient alone. In addition, the deep level emission at 1.15 and 1.3 eV, which are associated with vacancy defects, correlates well with the extent of the Al-Ga interdiffusion. Introduction Al-Ga interdiffusion in undoped AIGaAs/GaAs multiple quantum wells (MQW's) is mediated predominantly by Group III vacancies from the Ga-rich to the As-rich solidus limits [1]. By doping the crystal with n-type impurities, the concentration of Group III vacancy acceptors can be increased by shifting the Fermi level towards the conduction band [2]. Siand Te-induced enhancement of Al-Ga interdiffusion have been attributed to the "Fermi-level effect" [3,4]. We have studied Al-Ga interdiffusion due to the in-diffusion of the n-type impurities Si and Ge, and, S and Se, from Groups IV and VI respectively, by correlating the results of secondary ion mass spectroscopy (SIMS), electrochemical C-V profiling, photoluminescence spectroscopy (PLS), and cathodoluminescence spectroscopy (CLS). All anneals were done in an As-rich ambient, so, an enhancement of the Group III vacancy concentration at the crystal surface was expected. The extent of Al-Ga interdiffusion in MQW's was used as a probe of the Group III vacancy profile after n-type impurity in-diffusion. Experimental The MQW's used in this work were grown by molecular beam epitaxy on n-type (Sidoped) GaAs substrates with n-10 1 8 cm-3 , and were unintentionally doped p-type with p=6x10' 5 cm- 3 . MQW structure I consisted of 50 periods of 110 A GaAs layers alternati
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