We present a novel technique for reliable electrical injection into single semiconductor nanowires for light-emitting diodes and lasers. The method makes use of a high-resolution negative electron-beam resist and direct electron-beam patterning for the precise fabrication of a metallic top contact along the length of the nanowire, while a planar substrate is used as a bottom contact. It can be applied to any nanowire structure with an arbitrary cross section. We demonstrate this technique by constructing the first zinc oxide single-nanowire light-emitting diode. The device exhibits broad sub-bandgap emission at room temperature.Semiconductor devices are found in a myriad of technological applications. These devices were made possible with the development of planar fabrication techniques, which allowed for the accurate control of their physical dimensions and, more importantly, their reproducibility. Semiconductor nanowire structures are now emerging as promising candidates for even further miniaturization, opening the door to interesting new functionalities. 1,2 A powerful hybrid approach for engineering their optical properties by means of lithographic techniques has been reported recently. 3 However, electrical injection into nanowires still remains a technical challenge. 4 In this Letter, we address this issue by demonstrating a new method for achieving reliable electrical injection into a semiconductor nanowire. We show the potential of this technique by constructing the first zinc oxide (ZnO) singlenanowire light-emitting diode (LED). Previous studies on ZnO nanowire LEDs were carried out on large numbers of nanowires simultaneously by defining a metallic contact on a thin film of nanowires. 5,6 ZnO is a large band-gap (E g ) 3.35 eV at 300 K) 7 semiconductor, with several desirable properties for nanowire laser diodes and LEDs, among many other applications. 5,6,[8][9][10] In particular, the high exciton binding energy (60 meV), which is the result of a strong Coulomb interaction between electrons and holes, causes an enhancement of the radiative transition rate in the ultraviolet (UV) part of the spectrum. 9,10 Furthermore, the electronic and optical properties of ZnO nanowires can be tailored by altering the growth conditions, as well as by appropriate post-growth treatment. 11 For example, as-made ZnO nanowires grown on sapphire substrates exhibit a prominent near band-edge UV peak, whereas nanowires grown on graphite flakes exhibit broad subbandgap luminescence. 11 In addition, a single ZnO nanowire can form a resonant cavity with two naturally faceted hexagonal end faces acting as reflecting mirrors. 9 The ZnO nanowires used in this study were grown by vaporizing and condensing a mixture of ZnO and graphite powder on carbon cloth. 11,19,20 The resulting nanowires are n-type, similar to ZnO in bulk or thin films. [15][16][17] Our technique (Figure 1) relies on the use of poly(methyl methacrylate) (PMMA) 22 as a negative resist 12,13 and electrically insulating layer. PMMA is commonly used as a ...

Direct evidence of the transition from amplified spontaneous emission to laser action in optically pumped zinc oxide (ZnO) nanowires, at room temperature, is presented. The optical power evolves from a superlinear to a linear regime as the pump power exceeds threshold, concomitant with a transition to directional emission along the nanowire and the emergence of well defined cavity Fabry–Pérot modes around a wavelength of ≈385 nm, the intensity of which exceeds the spontaneous emission background by orders of magnitude. The laser oscillation threshold is found to be strongly dependent on nanowire diameter, with no laser oscillation observed for diameters smaller than ∼150 nm. Finally, we use an alternative “head on” detection geometry to measure the output power of a single nanowire laser.

This paper reviews several topics related to optically pumped ZnO nanowire lasers. A systematic study of the various properties of a device as it evolves from the regime of amplified spontaneous emission to laser oscillation above threshold is presented. The key dependence of the laser threshold on nanowire diameter is demonstrated and explained by means of a thorough study of guided modes in semiconducting nanowires for a nanowire-on-substrate geometry. A 'head on' detection geometry is used to measure the far-field profile of a nanowire laser and thus identify the modes responsible for lasing. Finally, the temperature behavior of a nanowire laser is reported, and possible mechanisms that may be responsible for gain are suggested.

We present a method which can be used for the mass-fabrication of nanowire photonic and electronic devices based on spin-on glass technology and on the photolithographic definition of independent electrical contacts to the top and the bottom of a nanowire. This method allows for the fabrication of nanowire devices in a reliable, fast, and low cost way, and it can be applied to nanowires with arbitrary cross section and doping type (p and n). We demonstrate this technique by fabricating single-nanowire p-Si(substrate)-n-ZnO(nanowire) heterojunction diodes, which show good rectification properties and, furthermore, which function as ultraviolet light-emitting diodes.