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DIMENSIONAL SYSTEMS

Quantum Efficiency and Formation of the Emission Line in Light-Emitting Diodes Based on InGaN/GaN Quantum Well Structures N. I. Bochkareva^, D. V. Tarkhin, Yu. T. Rebane, R. I. Gorbunov, Yu. S. Lelikov, I. A. Martynov, and Yu. G. Shreter Ioffe Physicotechnical Institute, Russian Academy of Sciences, St. Petersburg, 194021 Russia ^e-mail: [email protected] Submitted May 30, 2006; accepted for publication June 19, 2006

Abstract—The spectra of electroluminescence, photoluminescence, and photocurrent for the In0.2Ga0.8N/GaN quantum-well structures are studied to clarify the causes for the reduction in quantum efficiency with increasing forward current. It is established that the quantum efficiency decreases as the emitting photon energy approaches the mobility edge in the In0.2Ga0.8N layer. The mobility edge determined from the photocurrent spectra is Eme = 2.89 eV. At the photon energies hν > 2.69 eV, the charge carriers can tunnel to nonradiative recombination centers with a certain probability, and therefore, the quantum efficiency decreases. The tunnel injection into deep localized states provides the maximum electroluminescence efficiency. This effect is responsible for the origin of the characteristic maximum in the quantum efficiency of the emitting diodes at current densities much lower than the operating densities. Occupation of the deep localized states in the density-ofstates “tails” in InGaN plays a crucial role in the formation of the emission line as well. It is shown that the increase in the quantum efficiency and the “red” shift of the photoluminescence spectra with the voltage correlate with the changes in the photocurrent and occur due to suppression of the separation of photogenerated carriers in the field of the space charge region and to their thermalization to deep local states. PACS numbers: 73.40.Kp, 73.63.Hs, 78.55.Cr, 78.60.Fi, 78.67.De, 85.60.Jb DOI: 10.1134/S1063782607010174

between the InGaN and GaN lattice and is directed oppositely relative to the field of the space charge region [10].

1. INTRODUCTION In the studies reported to date, the high quantum efficiency of emission from the InGaN/GaN quantum wells (QWs) is commonly related to localization of excitons at potential fluctuations in the InGaN active layer and at the InGaN/GaN interfaces [1–5]. The inhomogeneous potential fluctuations may be due to compositional fluctuations or phase separation in the InGaN layer and some other imperfections of the crystal, which give rise to local fluctuations of the band gap in the plane of the InGaN layers and to “tails” in the density of states [6–8]. Experimentally, both large-scale inhomogeneities (on the order of ~1 µm in size) and small-scale In-enriched inhomogeneities (smaller than 20–60 nm in size) were observed in the InGaN active layer [6, 7, 9]. It is believed that localization of charge carriers suppresses lateral diffusion to nonradiative recombination centers, including dislocations, grain boundaries, or interfaces between constituent phases, and ensures