Ga-vacancy activation under low energy electron irradiation in GaN-based materials
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Ga-vacancy activation under low energy electron irradiation in GaN-based materials Henri Nykänen1, Sami Suihkonen1, Lucasz Kilanski2,3, Markku Sopanen1 and Filip Tuomisto2 1 Department of Micro- and Nanosciences, Aalto University, P.O. Box 13500, FI-00076 Aalto, Finland 2
Department of Applied Physics, Aalto University, P.O. Box 11100, FI-00076 Aalto, Finland 3 Institute of Physics, Polish Academy of Sciences, Al. Lotnikow 32/65, 02-668 Warsaw, Poland
ABSTRACT We present results on optical degradation of gallium nitride based materials under low energy electron beam irradiation (LEEBI). GaN thin film and GaN/InGaN quantum well samples, grown by metal-organic vapor phase epitaxy (MOVPE), were exposed to a tightly focused (ø = 2 nm, J = 0-130 kA/cm2), rapidly scanning electron beam (e-beam) with energy of 5-20 keV and dose of 0-500 μC/cm2. The irradiation severely reduced the band-to-band photoluminescence of the exposed sample areas. Performing positron annihilation spectroscopy measurements on the irradiated films revealed an important increase of Ga-vacancy concentration as a function of the irradiation dose. Based on the measurements we propose that in-grown passive VGa-Hn complexes are present in MOVPE grown GaN (and its alloys), and are activated by LEEBI. INTRODUCTION Gallium nitride is a widely used material for blue/UV LEDs and laser diodes. It is mechanically and chemically hard and considered as a suitable material for rough environments. Although GaN is in general well resistant to radiation, generation of Ga and N vacancies by MeV electron irradiation has been reported. [1, 2] Minimum electron beam energies for VN-Ni and VGa-Gai Frenkel pair generation are estimated to be 0.42 MeV and 2 MeV, respectively. Defect formation has also been associated with low energy electron beam irradiation (LEEBI). [3, 4, 5, 6] In this case the defect formation is thought to involve activation of vacancies, defect migration and clustering of point defects. In this paper, we present LEEBI induced optical degradation of GaN thin films and GaN/InGaN near surface single quantum wells (SQW), grown by metal-organic vapor phase epitaxy (MOVPE). The decrease of the band-to-band photoluminescence (PL) emission is associated with activation of in-grown Ga-vacancies during LEEBI. Based on the results we propose that in-grown passive VGa-Hn complexes are present in MOVPE grown GaN (and its alloys) and can be activated by the LEEBI treatment.
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EXPERIMENTAL All GaN-based samples were grown on c-plane sapphire (Al2O3) by MOVPE. The precursors for N, In and Ga were ammonia, trimethylindium and trimethylgallium, respectively. First, a 3-μm-thick undoped c-plane GaN layer was grown on sapphire, with the usual two-step method. [7] On two samples 3-nm-thick InxGa1-xN SQWs were grown on the GaN buffer layer, together with an undoped 20 nm GaN capping layer. The emission wavelengths of the quantum wells were (x = 0.12) 470 nm and (x = 0.20) 525 nm. A Zeiss Supra 40 scanning electron microscope (SEM) was used as a source of the ebeam irra
