Near-field measurement of spectral anisotropy and optical absorption of isolated ZnO nanorod single-quantum-well structures

Abstract

We report low-temperature near-field spectroscopy of isolated ZnO / ZnMgO single-quantum-well structures ͑SQWs͒ on the end of ZnO nanorod to define their potential for nanophotonics. First, absorption spectra of isolated ZnO / ZnMgO nanorod SQWs with the Stokes shift as small as 3 meV and very sharp photoluminescent peaks indicate that the nanorod SQWs are of very high optical quality. Furthermore, we performed polarization spectroscopy of isolated ZnO SQWs, and observed valence-band anisotropy of ZnO SQWs in photoluminescence spectra directly. Since the exciton in a quantum structure is an ideal two-level system with long coherence times, our results provide criteria for designing nanophotonic devices. © 2005 American Institute of Physics. ͓DOI: 10.1063/1.1990247͔ ZnO nanocrystallites are a promising material for realizing nanometer-scale photonic devices, 1 i.e., nanophotonic devices, at room temperature, owing to their large exciton binding energy 2-4 and large oscillator strength. 5 Furthermore, the recent demonstration of semiconductor nanorod quantumwell structures enables us to fabricate nanometer-scale electronic and photonic devices on single nanorods. [6][7][8][9] Recently, ZnO / ZnMgO nanorod heterostructures were fabricated and the quantum confinement effect even from the singlequantum-well structures ͑SQWs͒ was observed. 10 Near-field spectroscopy has made a remarkable contribution to investigations of the optical properties in nanocrystallite, 11 and has resulted in the observation of nanometer-scale optical images, such as the local density of exciton states. 12 However, reports on semiconductor quantum structure are limited to naturally formed quantum dots ͑QDs͒. 12-14 Here we report low-temperature near-field spectroscopy of artificially fabricated ZnO SQWs on the end of a ZnO nanorod.ZnO / ZnMgO SQWs were fabricated on the ends of ZnO stems with a mean diameter of 40 nm and a length of 1 m using catalyst-free metalorganic vapor phase epitxy, in which the ZnO nanorods were grown vertically from a sapphire ͑0001͒ substrate in the c orientation. 10,15 The Mg concentration in the ZnMgO layers averaged 20 at. %. Two samples were prepared for this study: their ZnO well layer thickness L w , were 2.5 and 3.75 nm, while the thicknesses of the ZnMgO bottom and top barrier layers in the SQWs were fixed at 60 and 18 nm, respectively. After growing the ZnO / ZnMgO nanorod SQWs, they were dispersed so that they were laid down on a flat sapphire substrate to isolate them from each other ͓Fig. 1͑a͔͒.The far-field photoluminescence ͑PL͒ spectra were obtained using a He-Cd laser ͑ = 325 nm͒ before dispersion of the ZnO / ZnMgO nanorod SQWs. The emission signal was collected with the acromatic lens ͑f =50 mm͒. To confirm that the optical qualities of individual ZnO / ZnMgO SQWs were sufficiently high, we used a collection-mode near-field optical microscope ͑NOM͒ using a He-Cd laser ͑ = 325 nm͒ for excitation, and a UV fiber probe with an aperture diameter of 30 nm. The excitation source was focused on a nanorod ...

Heterointerfaces in Semiconductor Nanowires

“…The absorption spectra of isolated quantum‐well structures displayed a small Stokes shift of 3 meV and sharp PL peaks. Furthermore, polarization spectroscopy of isolated nanorod quantum wells revealed valence‐band anisotropy in PL spectra 157. Since the exciton in a quantum structure is an ideal two‐level system with long coherence times, their results provided criteria for designing nanophotonic switches, which was indeed reported in a more recent publication 158.…”

“…Even though it is an interesting observation, more detailed experiments and theoretical modeling are required to make such claims in complex heterostructured systems at length scales that are smaller than the exciton Bohr radius. Yatsui et al157 reported low‐temperature near‐field spectroscopy of isolated ZnOZnMgO single‐quantum‐well structures grown on the end of ZnO nanorods. The absorption spectra of isolated quantum‐well structures displayed a small Stokes shift of 3 meV and sharp PL peaks.…”

Time-resolved spectroscopy of excitonic transitions in ZnO/(Zn, Mg)O quantum wells

“…Five transitions are observed in reflectivity spectra of ZnO bulk layers [16,17], corresponding to the 3Γ 5 excitons (A, B, C) -coupled to light emitted parallel to the growth axis in our QWs -and the 2Γ 1 excitons (B, C) -coupled to light emitted perpendicular to the growth axis in our QWs. Γ 5 excitons are therefore accessible in our PL experimental conditions, whereas Γ 1 excitons can only be observed from the edges of the samples [18]. The energy dispersion of these excitons is governed by the crystal field, the spin-orbit interaction and the electron-hole exchange interaction [19,20].…”

Self‐organized growth of ZnO‐based nano‐ and microstructures

“…Recently, indications for embedded ZnO QDs were found in axial ZnO/MgZnO multiple QW structures, showing an additional peak in low-temperature photoluminescence spectroscopy [44]. In single ZnO/MgZnO QWs on top of isolated nanowires, the usually broad QW emission band consisted of two sharp and polarization dependent peaks in near-field PL [45] with line widths between 3 and 12 meV.…”