Symmetry of Electrons and Holes in Lightly Photo-Excited InGaN LEDs
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Cite this article as: MRS Internet J. Nitride Semicond. Res. 4S1, G7.4(1999) ABSTRACT The symmetry of the recombining electrons and holes in lightly photo-excited InGaN LEDs revealed through ODMR is related to the physical structure, band structure and defects present. Calculations of the electron-g within the kep formalism give the average shift from the freeelectron value for GaN but are not fully reconciled with the anisotropy. This theory is also extended to InGaN alloys for both pseudomorphic and relaxed layers. The average shift is close to the experimental values for the green LED. The strongly reduced hole anisotropies seen experimentally are explained by a recently published theory for acceptors in GaN. INTRODUCTION Advances in nitride materials have led to the development of state-of-the-art LEDs in the visible range and lasers in the near uv [Il]. While the success of these nitride devices has been remarkable, there is a need for an improved understanding of the recombination process. Interest at the moment is focused on the influence of the spontaneous polarization, the piezoelectric effect arising mismatch-induced strain and the potential fluctuations associated with the tendency in InGaN alloys toward phase separation. Numerous studies of these effects have been published. Among the optical characterizations are studies of photoluminescence, photoluminescence excitation and photoluminescence decay [1-5]. These studies reveal the effects of charge separation of the electron and hole in the growth direction and localization of the carriers or excitons by potential fluctuations in the growth plane. Optically detected magnetic resonance (ODMR) has recently been observed in green and extrablue InGaN single-quantum well diode structures [6]. The distinct electron and hole resonances denote significant charge separation in the lightly photo-excited structures. Analysis of the frequency dependence of the ODMR for magnetic-dipole microwave transitions gives long lifetimes [7]. The ODMR technique is particularly valuable since the g-factors directly reflect the symmetry of the recombining electron and hole. This symmetry reveals the detailed structure determining the electron- and hole-states. This paper explores the link between the symmetry as revealed by ODMR and the vertical and lateral structure of the InGaN LED. Theories for the g-factor of electrons and localized holes are presented, extended and compared with experiment. For the electron, the present extensions of the theory are still inadequate to achieving a good understanding of the data. For the hole, new spectral data is presented and discussed using a recent theory for acceptors in GaN [8]. The analysis shows that the hole states are affected by strong in-plane strain. G 7.4 Mat. Res. Soc. Symp. Proc. Vol. 537 ©1999 Materials Research Society
SYMMETRY OF THE ELECTRON The g-factor for a conduction electron in a direct-gap semiconductor can be calculated using kep theory [9]. The g-values for GaN were calculated within a cubic appro
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