Irradiation-induced defects in InN and GaN studied with positron annihilation

Floris Reurings 1Filip Tuomisto 1Werner Egger 2Benjamin Löwe 2Luca Ravelli 3Stanislav Sojak 4Zuzanna Lilental-Weber 5Rebecca E. Jones 5Kin M. Yu 5Wladek Walukiewicz 5William J. Schaff 6

1. Helsinki University of Technology, Department of Applied Physics (TKK), POB 1100, Otakaari 1 M, Espoo 02015-TKK, Finland
2. Institut für Angewandte Physik und Messtechnik, Universität der Bundeswehr München, Neubiberg 87755, Germany
3. Dipartimento di Fisica, Università degli studi di Trento, via Sommarive 14, Povo 38123, Italy
4. Department of Nuclear Physics and Technology, Slovak University of Technology in Bratislava, Ilkovicova 3, Bratislava 81219, Slovakia (Slovak Rep.)
5. Lawrence Berkeley National Laboratory (LBNL), 1 Cyclotron Road, Berkeley, CA 94720, United States
6. Cornell University, 425 Philips Hall, Ithaca, NY 14853, United States

Abstract

Positron annihilation is an effective method for the investigation of vacancy-type defects in semiconductors. Positrons can get trapped at vacancies, resulting in observable changes in the characteristics of the annihilation radiation. The annihilation data can be used to determine vacancy concentrations as well as to distinguish between different vacancy types. In this work, we apply positron annihilation to investigate radiation-induced defects in InN and GaN grown by molecular-beam epitaxy. InN and GaN samples were irradiated with 2-MeV 4He+ ions to fluences in the range 5×1014–2×1016 cm-2, and subsequently subjected to rapid thermal-annealing below 500°C. This has previously been found to result in increased electron concentrations and mobilities in the InN films.

Our results show that in InN, negative In vacancies are introduced in the irradiation at a significantly low rate of 100 cm-1, with their concentration saturating in the mid-1017 cm-3 at an irradiation fluence of 2×1015 cm-2. The annealing is found to result in efficient clustering of the In vacancies near the film-substrate interface, in good agreement with results on void formation obtained by transmission electron microscopy. This is completely opposite to the behaviour observed in GaN, where Ga vacancies are introduced in the irradiation at a much higher rate of 3600 cm-1 showing no saturation, but where no clustering or recovery is seen after annealing. The cluster formation suggests that a large number of N vacancies need to be present in the InN films after irradiation, possibly being the cause of the increased n-type conductivity. Also, it is likely that the introduction rate of In vacancies is much higher than observed, which could be evidence of a similar acceptor-to-donor transition as the VGa ↔ (VAs + AsGa) transition in GaAs.

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Presentation: Poster at E-MRS Fall Meeting 2009, Symposium A, by Floris Reurings
See On-line Journal of E-MRS Fall Meeting 2009

Submitted: 2009-05-07 12:58
Revised:   2009-09-08 10:38
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