Compositional variation of microstructure in ion-implanted Al x Ga 1-x As
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L.E. Rehn and P.M. Baldo Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439
J.J. Coleman Department of Electrical and Computer Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801
T.S. Yeoh Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801 (Received 10 May 2000; accepted 7 July 2000)
The ion damage produced in alloys of AlxGa1−x As (x ⳱ 0.6, 0.7, 0.8, and 0.85) by implantation at 77 K with Kr ions (500, 700, and 1500 keV) was studied by using Rutherford backscattering channeling and transmission electron microscopy. In addition, the accumulation of ion damage at 50 K was studied by performing the ion implantations in situ in the transmission electron microscope. In Al0.8Ga0.2As, damage accumulation at 77 K was independent of dose rate, indicating that dynamic annealing is not occurring at 77 K. The in situ studies demonstrated that planar defects are produced on warm-up from 50 K to room temperature, indicating that they are not the nucleation site for amorphization. The lower energy implantations revealed that amorphization initiated within the AlxGa1−x As layer, showing that heterointerfaces are not required for amorphization. These results, along with the similarity of the room-temperature microstructures in the different alloys, imply that the amorphization mechanism is independent of Al content. It is proposed that the observed dependence of the amorphization dose on Al content is related to an increase in the number of cascade overlaps needed to initiate and to produce a continuous amorphous layer. A mechanism explaining the microstructural changes with composition, based on the thermal and physical properties of the alloy and on the distribution of energetic cascade events, is presented.
I. INTRODUCTION
The accumulation of ion-induced damage in AlxGa1−x As has been the subject of considerable study during the past decade.1–13 Although the general trends of damage accumulation are agreed upon, there is still debate about underlying mechanisms, especially the amorphization proces. It has been shown for a number of different implantation conditions, the ion dose to produce amorphous material increases with aluminum content. To explain this dependence of the amorphization dose on Al content, different amorphization mechanisms have been proposed to operate in different compositional regimes.
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e-mail: [email protected] J. Mater. Res., Vol. 15, No. 9, Sep 2000
In low Al content alloys (x < 0.3),14 amorphization occurs by the direct impact mechanism, in alloys with 0.3 < x < 0.85, it occurs by the superposition of energetic cascades on a high density of point defects;7,11,13 and in alloys with x > 0.85, it occurs by nucleation at the interface between AlxGa1−xAs and GaAs1,15 or on planar defects8 created by dynamic annealing at 77 K. One of the problems in determining the amorphization mechanism is that in many experiments the ion implantation temperature is 77 K while the analysis, Rutherfo
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