Behavior of Metastable Te Donor Concentrations in Q-Switched Ruby Laser Annealed GaAs
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BEHAVIOR OF METASTABLE Te DONOR CON(MIMTIONS IN Q-SWIT=CHE RUBY LASER ANNEALED GaAs P. PIANETTA, J. AMANO, G. W[X)IOUSE AND C. A. STOLTE Hewlett-Packard Laboratories, 1501 Page Mill Road, Palo Alto, California, USA ABSTRACT The thermal behavior of Te implanted, laser annealed GaAs vas investigated by helium backscattering spectroscopy and transmission electron microscopy in order to correlate structural changes with the two stage reduction in the carrier concentration due to post laser anneal heating above 2000C. The activation energy for stage one vhich occurs in the range 200-400 0 C was determined to be approximately 1.3 eV. Post laser anneal heating at 450 C caused no observable structural changes. On the other hand, 850 0C post laser anneal heating induced the formation of precipitates and dislocation loops as well as narrowing the channeling half-angle of Te by about 11%. INRDCTION It has been shown that pulsed laser and electron beam annealing are effective in achieving high carrier concentrations in ion irplanted GaAs [1-5]. However, significant decreases in the free electron concentration of these transient annealed samples have been observed as a result of heat treatnent above 2000C [6]. In particular, Te implanted samples exhibit a two stage reduction in carrier concentration6 The first stage occurs from 200 to 450 0C and the second occurs above 600 C. This problem is important since it may linit the applicability of pulsed laser annealing for obtaining high carrier concentrations in ion-implanted GaAs and may cause long term reliability problems in devices fabricated by this technology. In order to gain insight into the mechanisms for this reduction in carrier concentration, we have correlated electrical measurerents with helium backscattering spectroscopy [7] and transmission electron microscopy (TEM). EXPERIMENAL The samples were prepared by the implantation of Te ions into 3 micron thick, high purity LPE layers grcm-n on semi-insulating Cr doped fi00) _aAs substrates. The Te ions were implanted at 250 keV to a dose of 5 X 102 cm- at 10 to the surface normal to minimize channeling. Uncapped 8 X 8 mm samples were 9nnealed with a single 20 nsec, TIM 0O ruby laser pulse with a fluence of 0.8 J/cm [8]. The output energy of the ruby laser, as monitored by a beamsplitter and volume absorbing calorimeter, was repeatable to better than 5%. After laser annealing, the samples were cleaned in concentrated HCI and thje carrier concentration measured by the Van der Pauw method [9] using ohmic contacts of an InGa eutectic composition alloy applied at roam temperature. Prior to heating6 the ohmic contacts were removed with dilute HCI. For temperatures up to 550 C, uncapped samples were heated in a flowing nitrogen atmosphere and then cleaned in concentrated HCI before further measurements were performed. For temperatures above 650 0 C, the samples were capped with 0.15 microns of Si 3 N4 . After heat treatment, the cap was removed by plasma etching.
240 A General Ionex Tandetron (Model 4410A) was used for the
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