Electrical Characterization of Defects Introduced In n-GaN During High Energy Proton and He-Ion Irradiation
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d from https://www.cambridge.org/core. IP address: 141.101.132.34, on 02 Nov 2018 at 05:15:12, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/S1092578300003124
introduced during irradiation with 0.7 - 1.0 MeV electrons. [6] Subsequently, Fang et al observed, using deep level transient spectroscopy, that electron irradiation introduced a defect, which they labelled E, with a level at 0.18 eV below the conduction band. [7] In this paper we report the introduction rates and electronic properties of defects introduced in n-GaN by 5.4 MeV He-ion and 2.0 MeV proton irradiation. The dependence of the emission rate on electric field strength of defect ER3 is presented and discussed and we also present the true capture cross section of a deep lying defect, ER5. EXPERIMENTAL PROCEDURE For this study, 5 µm thick GaN epitaxial layers grown at 10800C on a 250 Å GaN buffer layer on sapphire by metal-organic vapor phase epitaxy (MOVPE) were either exposed to 5.4 MeV He-ions from a 241Am radio-nuclide source or 2.0 MeV protons from a Van de Graaff accelerator. The nominally undoped GaN epitaxial layer had a free carrier concentration of approximately 2 - 3 x 1016 cm-3. After boiling the samples in aqua-regia for ten minutes the samples were degreased [8]. Prior to ohmic contact fabrication the oxide layer was removed form the sample surface using a HCl : H2O (1 : 1) solution for 10 seconds [9]. The composite ohmic contact layer [10] was Ti/Al/Ni/Au (150 Å/2200 Å/400 Å/500 Å). The contact fabrication was followed by a five minute anneal at 500 oC in an inert gas atmosphere. Gold Schottky barrier diodes (SBDs), 0.5 mm in diameter and 3000 Å thick were resistively deposited, these diodes had reverse leakage currents of the order of 10-10 A at 1 V and ideality factors between 1.05 and 1.10. The samples were exposed to 5.4 MeV He-ions by placing them on an 241Am foil. The activity of the radionuclide being 192 µCi.cm-2 and the dose rate was 7.1 x 106 cm-2.s-1. Sampes exposed to 2.0 MeV protons in the Van de Graaff accelerator recieved a dose of (3 ± 1) x 1011 cm-2 at a dose rate of approximately 1 x 1010 cm-2s-1. A two-phase lock-in-amplifier-based (LIA) deep level transient spectroscopy (DLTS) system was used for the defect characterization in the asgrown material and the particle bombarded material. In order to simplify the determination of the emission kinetics of ER3 at different electrical field strengths in the space-charge region, isothermal DLTS was used.
RESULTS AND DISCUSSION Fig. 1 depicts the DLTS spectra of control (curve (a)), 5.4 MeV He-ion irradiated (curves (b & c)) and 2.0 MeV proton irradiated (curves (d & e)) epitaxial n-GaN. Consider the spectra for the as-grown material (curve (a)). In this defect labelling nomenclature, "E" implies electron trap and "O" that the material was grown by MOVPE. From the literature it appears that EO2 and EO5 are the same as the E1 and E2, respectively, observed by Hacke et al in n-GaN grown by hydride vapor-phas
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