Emission Mechanisms in UV Emitting GaN/AlN Multiple Quantum Well Structures
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Emission Mechanisms in UV Emitting GaN/AlN Multiple Quantum Well Structures Madalina Furis, Alexander N. Cartwright, Hong Wu1 and William J. Schaff1, Department of Electrical Engineering, University at Buffalo, Buffalo, NY, 14260, USA 1 Department of Electrical Engineering, Cornell University, Ithaca, NY, 14853 USA ABSTRACT The need for efficient UV emitting semiconductor sources has prompted the study of a number of heterostructures of III-N materials. In this work, the temperature dependence of the photoluminescence (PL) properties of UV-emitting GaN/AlN multiple quantum well (MQW) heterostructures were investigated in detail. In all samples studied, the structure consisted of 20 GaN quantum wells, with well widths varying between 7 and 15 Å, clad by 6nm AlN barriers, grown on top of a thick AlN buffer that was deposited on sapphire by molecular beam epitaxy. The observed energy corresponding to the peak of the emission spectrum is in agreement with a model that includes the strong confinement present in these structures and the existence of the large built-in piezoelectric field and spontaneous polarization present inside the wells. The observed emission varies from 3.5 eV (15 Å well) to 4.4 eV (7 Å well). Two activation energies associated with the photoluminescence quenching are extracted from the temperature dependence of the time-integrated PL intensity. These activation energies are consistent with donor and acceptor binding energies and the PL is dominated by recombination involving carriers localized on donor and/or acceptor states. Moreover, the temperature dependence of the full width at half-maximum (FWHM) of the PL feature indicates that inhomogeneous broadening dominates the spectrum at all temperatures. For the 15 and 13 Å wells, we estimate that the electron-phonon interaction is responsible for less than 30% of the broadening at room temperature. This broadening is negligible in the 9 Å wells over the entire temperature range studied. Well width fluctuations are primarily responsible for the inhomogeneous broadening, estimated to be of the order of 250meV for one monolayer fluctuation in well width. INTRODUCTION Achieving emission wavelengths shorter than 350nm in nitride semiconductors and heterostructures is essential for applications such as detection of chemical and biological agents. Recently, we proposed another solution for a UV emitter: GaN/AlN multiple quantum wells (MQWs) heterostructures [1] grown by molecular beam epitaxy (MBE). Despite the large lattice mismatch between wells and barriers (approx. 2.6%), such structures exhibit monolayer-abrupt interfaces and the 2D-3D growth mode transition can be avoided if the growth parameters are finely tuned [1]. Temperature-dependent time-resolved photoluminescence (TRPL) studies performed on such quantum wells have revealed that the structures exhibit strong photoluminescence even at room temperature [2,3]. In the present work, we report on a detailed study of the inhomogeneous broadening of the PL spectra as a function of temperature.
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