In Situ TEM Observations of Heavy Ion Damage in Gallium Arsenide
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IN S•rJ TEM OBSERVATI(CS OF HEAVY ION DAMAGE IN GAIUM ARSENIDE M.W. BEWN, *Dept. of Urbana, IL "Materials
* I.M. ROSBCON,* AND M.A. KIRK* Materials Science and Engineering, University of Illinois, 61801 Science Division, Aron National Laboratory, Argonne, IL 60439
ABSTRACr
Transmission electron microscopy experiments have been performed to investigate the lattice damage created by heavy-ion bombardments in GaAs. These experiments have been performed in situ by using the HVEN - Ion Accelerator Facility at Argonne National laboratory. The ion bcorbardments (50 keV Ar+ and Kr+) and the microscopy have been carried out at temperatures rangrin from 30 to 300 K. Ion fluences ranged from 2 x 1011 to 5 x 1013 iors cm-2. Direct-inpact amorphization is oserved to occur in both n-type and semi-insulating GaAs irradiated to low ion doses at 30 K and roam temperature. The probability of forming a visible defect is higher for low temperature irradiations than for room tAuperature irradiations. The amorphous zones formed at low temperature are stable to teiperatures above 250 K. Post implantation annealing is seen to occur at roam temperature for all samples irradiated to low doses until eventually all visible damage disappears. INTROWUCrICN The damage produced by ion implantation in GaAs has been the subject of extensive investigation for a number of years. Mudi of this research has focused on the structure of the amorphous layer created at high doses and on
the annealirg processes required to restore the crystalline lattice structure
[1]. Also, a considerable amount of work has focused on the structure of the primary damage and the medanism by which this damage builds up to form an amorphous layer (2]. T~o basic models for the mechanisms by which amorphization occurs in semiconductors have been proposed. In the first, amorphization oocurs by a build up of simple damage during irradiation until the defect density is so great that the region spontaneously collapses to an amorphous state [3].
In the second, small regions are directly amorphized
during individual collision cascades and complete amorphization occurs by the accumulation and overlap of these regions [4]. The first model has generally been used to explain the build up of damage for light ions and high temperature, whereas the second has been used to explain the damage created by heavy ions at low temperatures. A more realistic model would be one that allows both medanism to occur simultaneously and, as a result, a oamposite model of amorpous layer production has been developed (5] that includes both and also includes the effects of simultaneous damage annealing. While these models provide a good description of the general mechansis by which the crystalline-to-amorphous transformation occurs, the exact mechanisms remain controversial. Most investigations made on the build up of damage toward the production of an amorphous layer have been made by the Rubtherford backscatteringdcanneling technique [6,7,8]. Although much information has been gathered using this tech
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