Anomalous Point Defect Injection During Pulsed Laser Melting Processes: Direct Evidence of Ga i and As i Profiles in GaA
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ANOMALOUS POINT DEFECT INJECTION DURING PULSED LASER MELTING PROCESSES: DIRECT EVIDENCE OF Gai and Asi PROFILES IN GaAs
YIH CHANG AND THOMAS W. SIGMON Solid State Laboratory and Department of Electrical Engineering, Stanford University, Stanford, CA 94305
ABSTRACT Significant point defect injection during a pulsed laser melt process is reported for the first time. Heteroepitaxial InxGalixAs/GaAs layers fabricated by a pulsed laser induced epitaxy technique are used in this study. Transmission electron microscopy (TEM), energy-dispersive X-ray (EDX) and secondary ion mass spectrometry (SIMS) are employed to study the redistribution behavior of each species on the atomic scale. It is found that both the Si dopant species and the Ga, As, and In host atoms are injected into the underlying GaAs substrate. These species are then significantly redistributed, forming near spherical As-rich regions. Direct evidence of Asi and Gai (Ga and As interstitialcies) profiles in the GaAs substrate are also obtained for the first time. A hypothesis, based upon the combined effects of concentration impulse and large temperature gradients across the liquid-solid interface, is proposed to explain the significant solid phase diffusion observed during the pulsed laser melting process. We estimate the temperature gradient induced electric field during the process to be on the order of 104 V/cm. INTRODUCTION The interaction of pulsed UV-lasers with materials is receiving extensive study due to potential applications. These include, for example, gas immersion laser doping, laser assisted redistribution of implanted dopant atoms, and the fabrication of superconductors by UV-laser ablation [1,2]. A new growth technique, known as pulsed laser induced epitaxy (PLIE), has been developed to fabricate heteroepitaxial layers using a pulsed UV-laser melt-recrystallization process. This growth mechanism, based on planar epitaxial regrowth from the underlying substrate following pulsed UV-laser melting of the overlayers, has successfully fabricated heteroepitaxial InxGal-xAs/GaAs and patterned GexSil-x/Si layers [3-6]. Since the electrical properties of the pulsed UV-laser melted layers depend strongly on the concentration profiles of various species in the melted/unmelted region, we must fully understand the redistribution behavior of each species during the rapid melt-solidification processes. For example, Si, In, Ga and As profiles control the properties in the heteroepitaxial InxGal-xAs/n+-GaAs(100) layers since the In content determines the energy bandgap and the Si dopant controls the Fermi level in the GaAs matrix. We have characterized the Si and In profiles in the melted region for the InxGalxAs/GaAs layers fabricated by PLIE [5]. This presentation especially focuses on a detailed study of the redistribution profiles of each atomic species (Si, In, Ga, As) in the unmelted GaAs substrate. We find that both the dopant and host atoms are redistributed significantly in the underlying GaAs substrate. This leads us to discover the point defect inj
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