Evaluating the Quantum Confinement Effect of Isolated ZnO Nanorod Single-Quantum-Well Structures Using Near-Field Ultraviolet Photoluminescence Spectroscopy Takashi YATSUI 1 , Motoich OHTSU 1;2 , Sung Jin AN 3 , Jinkyoung YOO 3 and Gyu-Chul YI 3 1 Solution-Oriented Research for Science and Technology (SORST), Japan Science and Technology Agency, 687-1 Tsuruma, Machida, Tokyo 194-0004, Japan 2 School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan 3 National CRI Center for Semiconductor Nanorods and Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), San 31 Hyoja-dong, Pohang, Gyeongbuk 790-784, Korea (Received November 15, 2005; Accepted January 20, 2006) Using low-temperature near-field spectroscopy, we obtained spatially and spectrally resolved photoluminescence (PL) images of individual ZnO nanorod single-quantum-well structures (SQWs) with a spatial resolution of 20 nm. We observed the dependence of the quantum confinement effect of the PL peak on the well width (L aw ), from which the linewidths of near-field PL spectra of ZnO nanorod SQWs (L aw ¼ 2:5 and 3.75 nm) were determined to be as narrow as 3 meV. However, near-field PL spectra of individual SQWs with L aw ¼ 5:0 nm exhibited two PL peaks, presumably due to strains or defects in the ZnMgO in the nanorod SQWs. 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. # 2006 The Optical Society of Japan Key words: ZnO, nanorod, single-quantum-well structures, quantum confinement effect 1. Introduction Future optical transmission systems will require nano- photonic integrated circuits 1) composed of nanometer-scale dots to increase data transmission rates and capacity. ZnO nanocrystallite is a promising material for realizing room- temperature nanophotonic devices, owing to its exciton binding energy, which is as large as 110 meV in quantum structures, 2) and large oscillator strength. 3) Furthermore, the recent demonstration of a semiconductor nanorod quantum- well (QW) structure enabled us to fabricate nanometer-scale electronic and photonic devices on single nanorods. 4–6) Recently, ZnO/ZnMgO nanorod multiple-QW structures (MQWs) exhibiting the quantum confinement effect have also been fabricated. 7) Further improvement in the fabrica- tion of nanorod heterostructures has resulted in the obser- vation of significantly enhanced photoluminescence (PL) intensity, even from ZnO/ZnMgO nanorod single-QW structures (SQWs). 8) Furthermore, spatially resolved near- field PL spectra of individual nanorod ZnO QWs have been measured to realize nanophotonic devices using low-temper- ature near-field optical microscopy (NOM). 9) 2. Experimental ZnO/ZnMgO SQWs were fabricated on the ends of ZnO nanorods with a mean diameter of 40 nm using catalyst-free metalorganic vapor phase epitaxy. 10) The average concen- tration of Mg in the ZnMgO layers used in this study was determined to be 20 at. %. The average ZnO well layer thicknesses in the substrate, L aw , investigated in this study were 2.5, 3.75, and 5.0 nm, while the thicknesses of the ZnMgO bottom and top barrier layers in the SQWs were fixed at 60 and 18 nm, respectively. These thicknesses were determined by the transmission electron microscopy (TEM) measurement. After growing ZnO nanorod SQWs on sapphire (0001) substrate, they were dispersed on a flat sapphire substrate to measure the near-field PL of isolated nanorod QWs [Fig. 1(a)]. Far-field PL spectra were obtained using a He–Cd laser ( ¼ 325 nm) before dispersing the ZnO/ZnMgO nanorod SQWs. The emission signal was collected with an achro- matic lens ( f ¼ 50 mm). The optical properties of individual ZnO SQWs were investigated by collection-mode NOM at 15 K, using a He–Cd laser ( ¼ 325 nm) for excitation and an UV fiber probe with an aperture diameter of 30 nm to detect the PL signals [Fig. 1(b)]. In contrast to the naturally formed quantum dot (QD) structure formed in a two- dimensional (2D) narrow QW, 11–13) the barrier and cap layers laid on the substrate allowed the probe tip to access the PL source, which reduced carrier diffusion in the ZnO SQWs and the subsequent linewidth broadening, thereby resulting in high spatial and spectral resolution. The excitation source, which had a spot size of approximately 100 mm in diameter was focused on a nanorod sample laid on the substrate. 3. Results and Discussion The solid curves in Fig. 2 show the near-field PL spectra of the isolated ZnO SQW nanorods with well-layer widths (L aw ) of 5.0 (NF a ), 3.75 (NF b ), and 2.5 (NF c ) nm. In these spectra, the emission peaks around 3.365 (I ZnO 2 ) and 3.555 (I ZnMgO ) 7,8) eV are associated with neutral–donor bound excitons in the ZnO stem, and the excitons in the Zn 0:8 Mg 0:2 O layers, respectively, which correspond to the peaks in the respective far-field spectra [dashed curves FF a , FF b , and FF c in Fig. 2(a)]. Blue-shifted PL emission peaks  E-mail address: yatsui@ohtsu.jst.go.jp OPTICAL REVIEW Vol. 13, No. 4 (2006) 218–221 218