Excitons and biexcitons in InGaN quantum dot like localization centers

Abstract

Indium segregation in a narrow InGaN single quantum well creates quantum dot (QD) like exciton localization centers. Cross-section transmission electron microscopy reveals varying shapes and lateral sizes in the range ∼1-5 nm of the QD-like features, while scanning near field optical microscopy demonstrates a highly inhomogeneous spatial distribution of optically active individual localization centers. Microphotoluminescence spectroscopy confirms the spectrally inhomogeneous distribution of localization centers, in which the exciton and the biexciton related emissions from single centers of varying geometry could be identified by means of excitation power dependencies. Interestingly, the biexciton binding energy (E(b)xx) was found to vary from center to center, between 3 to -22 meV, in correlation with the exciton emission energy. Negative binding energies are only justified by a three-dimensional quantum confinement, which confirms QD-like properties of the localization centers. The observed energy correlation is proposed to be understood as variations of the lateral extension of the confinement potential, which would yield smaller values of E(b)xx for reduced lateral extension and higher exciton emission energy. The proposed relation between lateral extension and E(b)xx is further supported by the exciton and the biexciton recombination lifetimes of a single QD, which suggest a lateral extension of merely ∼3 nm for a QD with strongly negative E(b)xx = -15.5 meV.

Dynamic control of the optical emission from GaN/InGaN nanowire quantum dots by surface acoustic waves

“…3(e)) are consistent with the effect of lateral size variation of the confinement potential on the biexciton binding energy, as reported for III-Nitride QDs. 27,34,35 The observed trend can thus be explained by the effect of the SAW strain field and some possible contribution of the SAW-induced electric field parallel to the c-axis on the QD confining potential, 30 affecting the Coulomb interactions within the four-particle biexciton complex. A similar difference in modulation amplitude has been observed (although not discussed) in neutral and charged excitons in AlGaAs-GaAs core-shell nanowires.…”

“…The latter is consistent with the random asymmetry of our nanowires QDs formed by InGaN alloy fluctuations. 27 The X and XX assignment is attested by measuring their integrated PL intensities as a function of the optical pump power (cf. Fig.…”

“…The negative binding energy, commonly observed in III-Nitride QDs with inherently large built-in electric fields, is indicative of strong confinement. 27,28 When the SAW is applied (lower trace in Fig. 2(a)), the emission spectrum undergoes an apparent splitting of the X and XX emission lines into two components.…”

“…Compression (tension) along the QD polar axis would result in an increase (decrease) of the electron-hole attraction J eh thus increasing (decreasing) E b XX . 27,34,35 The opposite holds for strain perpendicular to the polar axis, which would mainly affect the repulsive electron-electron (J ee ) and hole-hole (J hh ) interactions. Consequently, changes in E b XX with increasing SAW intensity (Fig.…”

Determination of critical diameters for intrinsic carrier diffusion-length of GaN nanorods with cryo-scanning near-field optical microscopy

“…As a novel route, scanning near-field optical microscopy (SNOM/NSOM) has been developed during the last decades. SNOM, with its ability operating with evanescent waves, opens a unique possibility to detect features smaller than the laser diffraction limit 14 15 16 . In this way, SNOM offers an exclusive approach to gain optical information characterized by a high spatial and spectral resolution, combined with a simultaneous high flexibility to guide the signal to different setups for various kinds of optical analysis.…”