Origins and suppressions of parasitic emissions in ultraviolet light-emitting diode structures
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The AlGaN-based ultraviolet (UV) light-emitting diode (LED) structures with AlN as buffer were grown on sapphire substrate by metalorganic vapor-phase epitaxy (MOVPE). A series of cathodoluminescence (CL) spectra were measured from the cross section of the UV-LED structure using point-by-point sampling to investigate the origins of the broad parasitic emissions between 300 and 400 nm, and they were found to come from the n-type AlGaN and AlN layers rather than p-type AlGaN. The parasitic emissions were effectively suppressed by adding an n-type AlN as the hole-blocking layer. Electroluminescence (EL) and atomic force microscopy (AFM) measurements have revealed that the interface abruptness and crystalline quality of the UV-LED structure are essential for the achievement of the EL emissions from the multiple quantum wells (MQWs). I. INTRODUCTION
AlGaN has a direct transition band gap between 3.4 and 6.2 eV, covering the spectrum obtained with conventional gas and solid-state ultraviolet (UV) lasers. Currently, high-brightness AlGaN deep UV light-emitting diodes (LEDs) with emission wavelength below 300 nm have become the focus of intense research because of a wide range of potential applications, such as in water purification, sterilization, medicine and biochemistry, solid-state white lighting via phosphor excitation, and light sources for high-density optical recording.1–3 So far the external quantum efficiency (EQE) of UV LEDs with wavelengths shorter than 340 nm is relatively lower than that of InGaN-based visible LEDs, and this is thought to be caused by the difficulty in growing highquality Al-rich AlGaN, low carrier confinement, poor p-type conductivity in AlGaN alloy, and so on.2,3 Common electroluminescence (EL) spectra from UV LEDs were reported to be accompanied by parasitic emissions between 300 and 400 nm.4–9 These unintended radiative emissions indicate a loss of carriers to regions other than the active layers. Much work has been done to improve the quantum efficiency by growing the higher crystalline quality AlGaN, and suppression of the parasitic emissions is expected to be another step in state-of-the-art AlGaN growth technique.10,11 However, the origin of the parasitic emissions is still under debate. Some researchers determined the long wavelength emissions a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0135 J. Mater. Res., Vol. 25, No. 6, Jun 2010
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to be a recombination of overflowing electrons and holes in the p-AlGaN layer, others ascribed them to the deep-level transitions in the multiple quantum wells (MQWs).11 The indistinct origin of the parasitic emissions is unfavorable for improving the quantum efficiency of UV LEDs. In this work, typical deep UV-LED structures were investigated using point-by-point sampling along the cross section using cathodoluminescence (CL) to clarify the origin of the parasitic emissions. The suppression of the parasitic emissions was achieved by addition of n-ty
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