Role of Localized Quantum Well Excitons in InGaN Quantum Well Structure Correlated with Microstructural Analysis
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ROLE OF LOCALIZED QUANTUM WELL EXCITONS IN InGaN QUANTUM WELL STRUCTURE CORRELATED WITH MICROSTRUCTURAL ANALYSIS S. F. Chichibu,*,a) T. Sota**, S. Nakamura***,b) *
Institue of Applied Physics, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan Department of Electrical, Electronics, and Computer Engineering, Waseda University, 3-4-1 Ohkubo, Shinjuku, Tokyo 169-8555, Japan *** Department of Research and Development, Nichia Chemical Industries Ltd., 491 Oka, Kaminaka, Anan, Tokushima 774-8601, Japan a) Contacting author, [email protected] b) Present address : Department of Materials Engineering, University of California, Santa Barbara, CA93106
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Abstract InxGa1-xN multiple-quantum-well laser diode structure, which lased at 405 nm, was shown to have atomically-flat interfaces between each layer. Nanometer-probe compositional analysis showed that InN mole fraction, x, in the wells and barriers are approximately 6 % and 2 %, respectively, which agreed with the result obtained from high-resolution x-ray diffraction measurement. The Stokes-like shift (SS) at 300 K was 49 meV, being approximately 65 % of the luminescence linewidth. The localization depth, E0, of qunatum-well (QW) excitons was estimated to be 35 meV at 300 K though the compositional fluctuation in the well was as small as 1 % or less (detection limit) within adjacent 20-30 nm lateral length scale. Since the well thickness fluctuation is insufficient to reproduce SS or E0, effective bandgap inhomogeneity is attributed to be due to large bandgap bowing in InGaN. The spontaneous emission was thus assigned as being due to the recombination of QW excitons weakly localized in exponential tail-type potential minima in the QW. The size of localization is smaller than the quantum-disk [M. Sugawara, Phys. Rev. B 51, 10743 (1995)]-size. Such small bandgap inhomogeneity can be leveled by injecting high density carriers under lasing conditions, which can explain the general experimental finding that the quantum efficiency decreases with increasing carrier density in InGaN QW devices due to free carrier trapping into threading dislocations. Introduction InxGa1-xN quantum-wells (QWs) are attracting special attention because they serve as an active region1-3) of UV to visible light-emitting-diodes (LEDs) and purple laser diodes (LDs). Since they exhibit an efficient emission with external quantum efficiency, ηext up to 20 % at 470 nm, in spite of large threading dislocation (TD) density up to 1010 cm-2,4) optical properties of InGaN QWs have been investigated intensively. Internal electric field, F, due to spontaneous and piezoelectric polarization5) has been shown to modify the QW energy states through quantum-confined Stark effect;6) redshift of the emission peak compared to unperturbed QW resonance energy7,8) and reduction of electron-hole wavefunction overlap (oscillator strength).5,9) Coulomb screening due to carrier injection7,8) or impurity doping9) partially recovers the overlap. A predominant reason for the efficient emission has bee
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