Solid State Physics and NanoLund, Lund University , P.O. Box 118, SE-221 00 Lund, Sweden.
Laboratory of Mathematics, Physics and Electrical Engineering, Halmstad University , P.O. Box 823, SE-301 18 Halmstad, Sweden.
Nano Lett. 2017 Jun 14;17(6):3599-3606. doi: 10.1021/acs.nanolett.7b00759. Epub 2017 May 31.
Core-shell nanowires offer great potential to enhance the efficiency of light-emitting diodes (LEDs) and expand the attainable wavelength range of LEDs over the whole visible spectrum. Additionally, nanowire (NW) LEDs can offer both improved light extraction and emission enhancement if the diameter of the wires is not larger than half the emission wavelength (λ/2). However, AlGaInP nanowire LEDs have so far failed to match the high efficiencies of traditional planar technologies, and the parameters limiting the efficiency remain unidentified. In this work, we show by experimental and theoretical studies that the small nanowire dimensions required for efficient light extraction and emission enhancement facilitate significant loss currents, which result in a low efficiency in radial NW LEDs in particular. To this end, we fabricate AlGaInP core-shell nanowire LEDs where the nanowire diameter is roughly equal to λ/2, and we find that both a large loss current and a large contact resistance are present in the samples. To investigate the significant loss current observed in the experiments in more detail, we carry out device simulations accounting for the full 3D nanowire geometry. According to the simulations, the low efficiency of radial AlGaInP nanowire LEDs can be explained by a substantial hole leakage to the outer barrier layer due to the small layer thicknesses and the close proximity of the shell contact. Using further simulations, we propose modifications to the epitaxial structure to eliminate such leakage currents and to increase the efficiency to near unity without sacrificing the λ/2 upper limit of the nanowire diameter. To gain a better insight of the device physics, we introduce an optical output measurement technique to estimate an ideality factor that is only dependent on the quasi-Fermi level separation in the LED. The results show ideality factors in the range of 1-2 around the maximum LED efficiency even in the presence of a very large voltage loss, indicating that the technique is especially attractive for measuring nanowire LEDs at an early stage of development before electrical contacts have been optimized. The presented results and characterization techniques form a basis of how to simultaneously optimize the electrical and optical efficiency of core-shell nanowire LEDs, paving the way to nanowire light emitters that make true use of larger-than-unity Purcell factors and the consequently enhanced spontaneous emission.
核壳纳米线在提高发光二极管（LED）的效率和扩展整个可见光谱范围内的 LED 可达波长范围方面具有巨大的潜力。此外，如果纳米线的直径不超过发射波长（λ/2）的一半，则纳米线（NW）LED 还可以提供更好的光提取和发射增强效果。然而，到目前为止，AlGaInP 纳米线 LED 的效率还无法与传统的平面技术相媲美，限制其效率的参数仍然不清楚。在这项工作中，我们通过实验和理论研究表明，对于高效的光提取和发射增强，所需的小纳米线尺寸会导致显著的损耗电流，这会导致特别是在径向 NW LED 中效率低下。为此，我们制造了 AlGaInP 核壳纳米线 LED，其中纳米线的直径大致等于λ/2，我们发现样品中存在大的损耗电流和大的接触电阻。为了更详细地研究实验中观察到的大损耗电流，我们进行了考虑全 3D 纳米线几何形状的器件模拟。根据模拟，由于层厚度小和外壳接触紧密，径向 AlGaInP 纳米线 LED 的低效率可以用大量空穴漏到外势垒层来解释。使用进一步的模拟，我们提出了对外延结构的修改，以消除这种漏电流，并在不牺牲纳米线直径的λ/2 上限的情况下将效率提高到接近 1。为了更好地了解器件物理，我们引入了一种光学输出测量技术来估计理想因子，该因子仅取决于 LED 中的准费米能级分离。结果表明，即使在存在非常大的电压损耗的情况下，在 LED 的最大效率附近，理想因子的范围为 1-2，这表明该技术特别适合在电气接触尚未优化之前，用于测量处于早期开发阶段的纳米线 LED。所提出的结果和表征技术为如何同时优化核壳纳米线 LED 的光电效率奠定了基础，为真正利用大于 1 的 Purcell 因子和由此增强的自发发射的纳米线发光器铺平了道路。
