RADIATION-INDUCED CARRIER ENHANCEMENT AND INTRINSIC DEFECT TRANSFORMATION IN N-GAAS

Gallium arsenide grown by the metallorganic chemical vapor deposition method and n doped with silicon to a concentration of 10(15) cm-3 was exposed to reactor neutron irradiation in the 10(12) to 3 X 10(14) cm-2 1 MeV equivalent fluence range. Studies of the defects through deep level transient spectroscopy (DLTS), photoluminescence (PL), and transport measurements on this material indicate correlation between the nature and density of defects, and some of the transport parameters. Contrary to the general perception of degradation of electronic properties of semiconductors on nuclear irradiation, we observe enhancement in some of the electrical/optical properties of GaAs on irradiation at lower fluence levels. These properties degrade on irradiation at higher fluences. The PL intensity of irradiated GaAs increases over the 1 X 10(12) to 1 X 10(13) cm-2 fluence range. At 1 X 10(13) cm-2, the increase in the signal from different PL peaks ranges from 25% to 200%. Similarly, the carrier density of irradiated GaAs, as determined by transport measurements, increases by about 25% on irradiation at 3 X 10(12) cm-2, before decreasing at and above 3 X 10(13) cm-2 fluence. The density of the EL2 trap as determined through the DLTS technique, remains constant with fluence and that of the EL12 trap at 780 meV below the conduction band decreases at an injection ratio of -0.5+/-0.3 cm-1 up to congruent-to 1 X 10(14) cm-2 fluence. The EL6 trap at 0.38 meV below the conduction band is introduced at a rate of 0.30 +/- 0.04 cm-1. Thus, neutron irradiation restructures the deep levels, with the consequence that the carrier density goes through a maximum at low fluence levels. Through PL measurements, the maximum in carrier density corresponds to a fluence level of (1.0+/-0.2) X 10(13) cm-2. At fluences above 10(14) cm-2, we observe the introduction of a deep level at 260 +/- 85 meV below the conduction band (EL 14) at a rate of 1.7+/-0.4 cm-1. Above 10(14) cm-2, we observe an increase in carrier depletion in GaAs through PL and transport measurements.