We report the first direct observation of phase decomposition in a luminescent alloy and show that this decomposition, allied to quantum confinement enhancements, accounts for the surprisingly high efficiency of InGaN-based diodes manufactured by Nichia Chemical Industries. Hence nanostructure, rather than composition, is responsible for the success of these devices. A common nanostructure, in the form of nearly pure InN quantum dots, occurs across a large range of average indium content in InGaN and leads to a universal scalability of the optical spectra. [S0031-9007(98)08055-7] PACS numbers: 85.60. Jb, 78.66.Fd, 85.30.Vw While Nichia Chemical Industries have recently commercialized blue and green light-emitting diodes (LEDs) based upon InGaN quantum wells [1,2], and demonstrated long-lived blue lasers at room temperature [3], rather little is known about the origin of the luminescence from such devices, or its apparent indifference to a huge density of structural defects [4]. Much of the discussion of the operational characteristics of Nichia diodes contradicts a theoretical demonstration [5] that In is, to a large extent, insoluble in GaN at typical growth temperatures. Hence the emission spectrum of, say, a "50% alloy of InGaN grown at 700 ± C" should actually contain two components at high and low energy, corresponding to the twocomponent phase decomposition demanded by thermodynamics. This decomposition of the emission spectrum has been observed for the first time in Nichia diode electroluminescence spectra. In addition, we observe a shift of the low-energy component due to quantum confinement in nanocrystalline inclusions of nearly pure InN, which leads to a dominant emission band in the blue or green spectral region.Although (partial) phase segregation has been shown to be present in InGaN alloys by a number of authors [6-8], we argue here that cation segregation takes an extreme form in Nichia diodes, and find that the origin of the luminescence in these devices is best described as emission from quantum dots of approximately constant composition-approaching InN-and having radii that increase monotonically with the average In incorporation.The intimate connection between exciton localization and luminescence in semiconductors is now well established. Luminescence occurs when the dwell time of an exciton in a localized site exceeds its radiative decay time. The process of luminescence therefore involves an exchange of energy between a massive stationary particle (the exciton) and a massless particle traveling out of the semiconductor at the speed of light (the photon). The interaction is well described in systems without disorder by the polariton model, but it should be noted that each luminescence event involves at least one defect, in the form of a surface. Since excitation, in contrast to luminescence, maps the joint density of states (JDOS), there is an energy shift (usually called the Stokes' shift) between peaks of luminescence and absorption in solid state systems with spatial energy disorder (s...

We report a comparative study of the emission and absorption spectra of a range of commercial InGaN light-emitting diodes and high-quality epilayers. A working definition of the form of the absorption edge for alloys is proposed, which allows a unique definition of the Stokes’ shift. A linear dependence of the Stokes’ shift on the emission peak energy is then demonstrated for InGaN using experimental spectra of both diode and epilayer samples, supplemented by data from the literature. In addition, the broadening of the absorption edge is shown to increase as the emission peak energy decreases. These results are discussed in terms of the localization of excitons at highly indium-rich quantum dots within a phase-segregated alloy.

A comparative study of the optical linewidths of photo- and electroluminescence from high-quality InGaN epilayers and commercial single quantum well light emitting diode structures was undertaken. Optical linewidths in both cases are temperature insensitive and increase systematically with increasing indium concentration. We assess the contribution of three mechanisms to the luminescence linewidth: alloy fluctuations, well width fluctuations, and strain effects. It is found that the broadening of the emission line is an intrinsic property of InGaN alloys. The piezoelectric effect in wurtzite semiconductor is proposed as a novel line-broadening mechanism.

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Continuous-wave photoluminescence (PL) and time-resolved photoluminescence of gallium nitride layers grown by pulsed laser deposition are compared. The temperature dependence of the photoluminescence decay time and the PL-integrated intensity allows a determination of radiative and nonradiative time constants of GaN. We find that luminescence peaks centered at 3.360 and 3.305 eV at low temperature can be attributed to recombination of excitons localized at extended defects. The photoluminescence radiative lifetime at room temperature is on the order of tens of ns.