Optical Absorption of Deep Defects in Neutron Irradiated Semi-Insulating GaAs .
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OPTICAL ABSORPTION OF DEEP DEFECTS IN NEUTRON IRRADIATED SEMI-INSULATING GaAs. M. 0. MANASREH* and P. J. PEARAH** *Electronic Technology Laboratory (WRDC/ELRA), Wright Research and Development Center, Wright-Patterson Air Force Base, Ohio 45433-6543, **Spectrum Technology Incorporated, 6 October Hill Drive, Hollison, MA 01746.
ABSTRACT Two defects were observed in thermal neutron irradiated semi-insulating liquidencapsulated Czochralski (LEC) GaAs materials by using the infrared absorption technique. The first defect was observed before thermal annealing and it has a broad peak at -0.83 eV. The second defect was observed after annealing the sample at 250 °C for 15 min. The latter defect is an EL2 - like defect, but is thermally unstable at 400 °C with a concentration of about an order of magnitude larger than [EL2] observed in as grown LEC materials. Both defects were found to photoquench with white light or 1.1 eV monochromatic light at 9 K and thermally recovered at 150 K. The concentration of the EL2 - like defect is reduced from 8.5xl0 16cm- 3 after annealing the sample at 300 "C for 15 min to 2xl016cm"3 after annealing at 400 "C for 15 min. This defect becomes unquenchable after the latter annealing conditions. An explanation for this behavior is offered. INTRODUCTION Recent reports1 -3 show that high-dose neutron irradiation decomposes completely the atomic structure of the EL2 defect in semi-insulating GaAs while an EL2-like defect is
generated after thermal annealing at certain temperatures. The disappearance of the EL2 defect in irradiated samples may rule out the speculation of identifying EL2 with the isolated arsenic antisite (AsGa). The excemption of Ascja as being the candidate for the EL2 atomic structure is supported by the the absence of the metastability of AsGa in neutron irradiated materials 4- 7 in which the concentration of AsGa is increased as a function irradiation dose as detected by the electron paramagnetic resonance technique. In addition, metastability was not observed 8 for AsGa present in a large concentration in As-rich GaAs grown by molecular beam epitaxy. In this article we report the optical absorption spectra of two deep defects in thermal neutron irradiated GaAs material. It was found that these defects have similar photoquenching and thermal recovery properties to those of EL2, but are different from EL2 in many other aspects.
EXPERIMENTAL TECHNIQUE Several samples were cut from a LEC semi-insulating GaAs boule. The thermal neutron irradiation was performed at Chalk River Nuclear Laboratory, Ontario, Canada in a flux of -2x1013 neutron/cm 2 -s. The thermal/fast neutron ration is -5xl0 3 . The resistivity 7 7 and free carrier concentration of the irradiated samples were 8.83x10 0-cm and 9.29x10 3 cm- , respectively, prior to thermal annealing. After thermal annealing one sample at 850 "C for 30 min, the resistivity and free carrier concentration become 3.16xl0-3f2-cm and 8.22x10 1 7cm- 3 , respectively. The free carrier concentration of 8.22x10 1 7cm"3 was taken to indicate t
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