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Dept. of Physics and Applied Physics, Strathclyde University, Glasgow, G4 0NG, U.K. Department of Information Technology, University of Ghent, Ghent 7500, Belgium. 3 Inst. Kern- en Stralingsfysica, Univ. of Leuven, B-3001 Leuven, Belgium. 4 CLRC, Daresbury Laboratories, Warrington WA4 4AD, England, U.K.. 2

ABSTRACT Commercial light emitting devices (LEDs) containing InGaN layers offer unrivalled performance in the violet (~400 nm), blue (~450 nm) and green (~520 nm) spectral regions. Nichia Chemicals Company has also produced amber InGaN LEDs with peak output near 590 nm. Here, we predict, on purely theoretical grounds, a surprisingly high limiting value of 1020 nm (peak) for InGaN intrinsic emission. We partly confirm this prediction by spectroscopic measurements of samples with photoluminescence (PL) peaks between 370 nm and 980 nm. In addition, we have measured the indium content of a range of light-emitting layers, using Rutherford Backscattering Spectrometry (RBS), Extended X-Ray Absorption Fine Structure (EXAFS) and Energy Dispersive X-Ray Analysis (EDX). The PL peak energy is found to depend linearly on the indium fraction: violet-emitting layers have an indium content of ~8%, blue layers ~16% and green layers ~25%. A linear extrapolation to the limit set by the Stokes’ shift prediction, mentioned earlier, yields a limiting indium concentration of only ~52%. The profound impact of these results on future extensions of nitride technology and current theoretical models of InGaN is briefly discussed. INTRODUCTION The extension of nitride technology to the full visible spectrum, 400 nm to 700 nm, is a technical challenge that depends upon the successful incorporation of increasing amounts of indium nitride into gallium nitride while maintaining high fluorescence efficiency in the solid solution, InxGa1-xN = x(InN) + (1-x)GaN. Of the commercial suppliers, only Nichia Chemicals Company has presented nitride LEDs that emit efficiently in the amber (~590 nm) and, recently, the red (~675 nm) spectral regions [1]. Some time ago, the Strathclyde group reported photoluminescence (PL) emission peaks up to 650 nm in epilayers grown at low temperatures [2]. It seemed reasonable to expect that the limit to the ‘redshifting’ of nitride technology would be set by the band gap of indium nitride (which, at 1.89 eV [3], is equivalent to a wavelength of 656 nm) but here we show, both theoretically and practically, that the limit, at 1.21 eV (1020 nm), is actually well below the InN band gap. On the basis of our Stokes’ shift model, the peak photon energy of the dominant luminescence band in InGaN samples is shown to be linearly related to the alloy band gap [4]. Denoting these optical energies by Ep and Eg, respectively, we find, from our own spectroscopic results and a survey of the relevant literature, that:

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Emission Energy (eV)

3.4 3.2 3.0 2.8 2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.8

T = 300K

2.0

2.2

2.4

2.6

2.8

3.0

3.2

3.4

Effective Band-gap Energy (eV)

Figure 1. The linear relation between optical energies