Towards electrically driven nanowire single-photon sources.

A vapor–liquid–solid (VLS) growth of microwhiskers was developed by Wagner et al. in the 1960s. The recent re-examination of this method by the groups of Lieber, Yang, Samuelson, Wang, and others has resulted in the flourishing research into nanowires (NWs). Nearly all the IV, II– VI, III–V, and IV–VI semiconductor NWs can be synthesized. Ternary compound semiconductors have also been demonstrated. NWs can be made with complex shapes and form hyperbranched networks. A unique characteristic of NWs is that they can transport electrons, photons, and ions by a macroscopic distance along their length while maintaining a nanoscale size effect (quantum confinement, large surface area, etc.) across the diameter dimension. NWs and their derivatives are promising in many applications such as transistors, biosensors, 12] nanogenerators, nanofluidic channels, and light-emission deACHTUNGTRENNUNGvices. In the area of light-emission devices, NWs provide new exciting advantages over embedded quantum wires. First, NWs can be grown epitaxially on Si due to the facile strain relaxation within the small cross section of nanowires, which allows optical functions directly integrated onto mature Si technology. Second, NWs can be assembled in the form of cross arrays, which offer the flexibility of combining different materials to realize integrated multicolor emission. Third, NWs are not embedded in high-refractiveindex substrates, which significantly increases their light-extraction efficiency. With the above advantages, a new exciting opportunity would be to engineer NWs as singlephoton sources (SPSs). A SPS is a critical component for quantum information processing. 23] A SPS requires photon antibunching, that is, emission of a single photon at a time. To realize a SPS in NWs, it is necessary to engineering a small-sized quantum dot within a single NW. The introduction of growth techACHTUNGTRENNUNGniques such as chemical beam epitaxy into the VLS mechanism allows precise control of the heterostructure down to the nanometer scale, 12] which opens up the opportunity to perform quantum engineering within single NWs. Recently Borgstrçm et al. have described photon antibunching in photoluminescence measurements of GaP–GaAsP–GaP linear heterostructured NWs, in which the 15 nm GaAsP segment functions as a quantum dot. Surrounded by low-refractive-index air, these NW dots show intense singlephoton emission with a brightness typically an order of magnitude larger than self-assembled quantum dots. To take this a step forward, it would be highly desirable to develop electrically driven single-photon sources that do not require expensive lasers as excitation sources. Here we highlight the work by Minot et al. on singlequantum-dot NW LEDs, which show great promise towards electrically driven SPSs. A NW consisting of a narrow InAsP section sandwiched between n-InP and p-InP sections was grown by a Au-colloid-catalyzed VLS method in a metal–organic vapor phase epitaxy (MOVPE) chamber (Figure 1). The central InAsP section has a smaller bandgap, thus forming a potential well or quantum dots in the NWs, while the long InP section functions as electrical wiring for selectively injecting electrons or holes into the quantum dots. The central quantum dots need to be small enough to manifest the quantum confinement effects. Central InAsP sections as small as 12 nm have been realized by the authors. To characterize these heterostructure NWs, the authors have taken a step back to first study p–n junction InP NWs without the central InAsP quantum dot. They used different contact metals for selectively contacting with the pand n-type InP sections, which is necessary for efficient and type-selective charge-carrier injection. They found that Ti/Al makes an Ohmic contact with n-InP, and Ti/Zn/ Au is used as a contact for p-InP, but here there is a high [*] J. Zhu Department of Electrical Engineering, Stanford University 476 Lomita Mall, Stanford, California 94305 (USA)