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.
Gibb's free energies due to hole transitions between the C level of Si:V and the valence band have been determined for eleven different temperatures within the range 190 KрTр270 K on the basis of simultaneous measurements of corresponding thermal hole capture coefficients and emission rates. Zero-phonon binding energies have been determined from the low-energy part of photoionization cross-section spectra at five different temperatures within the range 75 KрTр170 K. The temperature dependence of these binding energies can be described by a Varshni-type analytical formula with a zero-temperature level position of E v ϩ0.361 eV ͑Ϯ0.003 eV͒. The associated ratio of the temperature-induced change of this level position with respect to the one of the band-gap energy is about 0.80 ͑Ϯ0.10͒. The inherent correlation between electrical and optical level position parameters was used to calculate the temperature dependence of the zero-phonon binding energy, J p (T), the Gibb's free energy, G p (T), and the enthalpy, H p (T), from 0 K to room temperature. Using two photoionization cross-section spectra, the associated Franck-Condon shift was estimated to be about 0.04 eV.
A processing scheme for the fabrication of embedded W–GaAs contacts has been established and the resulting contact characteristics have been evaluated. The main advantage of these contacts is that they are stable during high-temperature epitaxial overgrowth. The fabrication scheme is based on a liftoff process with electron beam evaporation of tungsten and subsequent epitaxial overgrowth using metalorganic vapor phase epitaxy. Various methods were used to characterize the buried contacts. First, the structural properties of GaAs surrounding embedded W features, with widths down to 50 nm, were characterized by high-resolution transmission electron microscopy. Measurements of the conductivity in individual, buried wires were performed in order to study the influence of the overgrowth process on the properties of the tungsten. We also evaluated the current–voltage characteristics for macroscopic contacts, which revealed a clear dependence on processing parameters. Optimized processing conditions could thus be established under which limited contact degradation occurred during overgrowth. Finally, we used the overgrowth technique to perform a detailed investigation of the electrical and optical properties of floating-potential embedded nano-Schottky contacts by space-charge spectroscopy.
A theoretical treatment of the photoelectric yield of metal‐semiconductor (MS‐) structures is developed, predicting that the classical Fowler interpretation of yield spectra is valid mainly for thin metal layers. For thick metals, the spectra are influenced by the photon energy dependence of the absorption coefficient of light and by the charge carrier attenuation length in the metal. The terms “thin” and “thick” refer to the values of these latter parameters. Measurements are performed on MS‐structures with gold, aluminium, palladium, and palladium silicide as metal layers on p‐type silicon. The form of the yield spectra is found to follow the theory in the cases of gold and aluminium. Certain deviations for palladium are suggested to originate from lateral variations of the energy barrier height in these types of structures.
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