Dynamics of Anomalous Temperature-Induced Emission Shift in MOCVD-grown (Al, In)GaN Thin Films
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ABSTRACT We present a comprehensive study of the optical characteristics of (Al, In)GaN epilayers measured by photoluminescence (PL), integrated PL intensity, and timeresolved PL spectroscopy. For not only InGaN, but also AlGaN epilayers with large Al content, we observed an anomalous PL temperature dependence: (i) an “S-shaped” PL peak energy shift (decrease-increase-decrease) and (ii) an “inverted S-shaped” full width at half maximum (FWHM) change (increase-decrease-increase) with increasing temperature. Based on time-resolved PL, the S shape (inverted S shape) of the PL peak position (FWHM) as a function of temperature, and the much smaller PL intensity decrease in the temperature range showing the anomalous emission behavior, we conclude that strong localization of carriers occurs in InGaN and even in AlGaN with rather high Al content. We observed that the following increase with increasing Al content in AlGaN epilayers: (i) a Stokes shift between the PL peak energy and the absorption edge, (ii) a redshift of the emission with decay time, (iii) the deviations of the PL peak energy, FWHM, and PL intensity from their typical temperature dependence, and (iv) the corresponding temperature range of the anomalous emission behavior. This indicates that the band-gap fluctuation responsible for these characteristics is due to energy tail states caused by non-random inhomogeneous alloy potential variations enhanced with increasing Al content.
INTRODUCTION Much interest has been focused on (Al, In)GaN alloys and their heterostructures, because their band gap energy varies between 6.2 and 1.9 eV at room temperature, and because of their potential applications such as red-ultraviolet (UV) light emitting devices [1,2], solar-blind ultraviolet detectors [3], and high power and high temperature devices [4,5]. It has been demonstrated that InGaN-based light emitting devices are highly efficient and have very low thresholds, and it is believed that their recent success is deeply related to the role of carriers localized in the InGaN active region. For the InGaNbased light emitting device structures, In alloy inhomogeneity and/or quantum-dot-like In phase separation have been proposed as the origin of the localized states [6-10], and an anomalous temperature dependence of the InGaN emission peak energy due to band-tail states was observed [11-13]. However, according to recent thermodynamical calculations, ternary AlGaN alloys are predicted to not have an unstable mixing region, and hence, no phase separation is
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expected, in contrast to InGaN and InAlN alloys [14]. Although understanding the emission mechanism and the role of the energy tail states in (Al, In)GaN alloys is very important for shorter wavelength light-emitting devices, the detailed emission properties of these materials have not been fully clarified. In this work, we report optical properties of AlxGa1-xN epilayers (x ≤ 0.6) in compare with GaN and In0.18Ga0.82N, as a function of temperature using photoluminescence (PL), integrated PL intensity, and
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