Insertion of CdSe quantum dots in ZnSe nanowires: MBE growth and microstructure analysis

Abstract

a b s t r a c tZnSe nanowire growth has been successfully achieved on ZnSe (1 0 0) and (1 1 1)B buffer layers deposited on GaAs substrates. Cubic [1 0 0] oriented ZnSe nanowires or [0 0 0 1] oriented hexagonal NWs are obtained on (1 0 0) substrates while [1 1 1] oriented cubic mixed with [0 0 0 1] oriented hexagonal regions are obtained on (1 1 1)B substrates. Most of the NWs are perpendicular to the surface in the last case. CdSe quantum dots were successfully incorporated in the ZnSe NWs as demonstrated by transmission electron microscopy, energy filtered TEM and high angle annular dark field scanning TEM measurements. & 2010 Elsevier B.V. All rights reserved.ZnSe nanowires (NWs) with CdSe quantum dot (QD) insertions are promising objects for opto-electronic applications, for example due to the large exciton binding energy and strong carrier confinement. ZnSe NWs can be prepared by vapor phase growth [1], metalorganic chemical vapor deposition [2], molecular beam epitaxy (MBE) [3,4] or thermo-chemical processes [5]. With the ability to precisely control growth parameters and to accurately monitor the growth process, MBE is an ideal tool to grow nano-structured materials.We have recently shown that a single CdSe quantum dot (QD) embedded in a ZnSe NW is an efficient single photon source operating at temperatures as high as 220 K [6]. However, when grown on an oxidized Si (1 0 0) substrate in the VLS growth mode catalyzed by gold particles, the NWs present a random distribution of orientations. Ohno et al. [7] reported MBE growth of ZnSe NWs on ZnSe/GaAs templates using Fe as catalyst but no clear epitaxial relationship between the template and the NWs was observed.In this contribution we report on the epitaxial growth of ZnSe and ZnSe-CdSe-ZnSe QD embedded heterostructured NWs deposited on 2D ZnSe (1 0 0) or ZnSe (1 1 1)B buffer layer epitaxially grown on GaAs (1 0 0) or GaAs (1 1 1)B substrates, respectively.The NWs are grown by MBE using gold as a catalyst. Samples used in this study were all grown in a Riber 32P solid source MBE system. The source materials for the MBE system were elemental Zn, Se and Cd. GaAs (1 0 0) and (1 1 1)B commercial wafers were used as substrates. The GaAs substrates are deoxidized under As flow. Then a GaAs buffer layer is grown by MBE in a connected III-V MBE chamber. In order to avoid Ga incorporation in NWs and to improve the expitaxial relation between NW and substrate, a thin ZnSe buffer layer (about 30 nm thick) is grown at 280 1C on the GaAs epi-layer. Gold is then evaporated on the ZnSe buffer layer at room temperature in a dedicated metal deposition chamber connected to the II-VI and III-V growth chambers by UHV path. Dewetting of the gold film was done at 500-530 1C for several minutes. The ZnSe NWs are grown at different temperatures between 300 and 450 1C under excess of Se flux. The beam equivalent pressure ratio Zn:Se is 1:4 and the pressure is in the 10 À 7 Torr range. The CdSe insertions are grown during the NW epitaxial growth process by switching the Zn flux to a C...