Photon-triggered nanowire transistors.

Photon-triggered electronic circuits have been a long-standing goal of photonics. Recent demonstrations include either all-optical transistors in which photons control other photons or phototransistors with the gate response tuned or enhanced by photons. However, only a few studies report on devices in which electronic currents are optically switched and amplified without an electrical gate. Here we show photon-triggered nanowire (NW) transistors, photon-triggered NW logic gates and a single NW photodetection system. NWs are synthesized with long crystalline silicon (CSi) segments connected by short porous silicon (PSi) segments. In a fabricated device, the electrical contacts on both ends of the NW are connected to a single PSi segment in the middle. Exposing the PSi segment to light triggers a current in the NW with a high on/off ratio of >8 × 106. A device that contains two PSi segments along the NW can be triggered using two independent optical input signals. Using localized pump lasers, we demonstrate photon-triggered logic gates including AND, OR and NAND gates. A photon-triggered NW transistor of diameter 25 nm with a single 100 nm PSi segment requires less than 300 pW of power. Furthermore, we take advantage of the high photosensitivity and fabricate a submicrometre-resolution photodetection system. Photon-triggered transistors offer a new venue towards multifunctional device applications such as programmable logic elements and ultrasensitive photodetectors.

Nonlinear mixing in nanowire subwavelength waveguides.

The realization of nonlinear photonic circuits to achieve the control of light-by-light is contingent upon a strong nonlinear response that can be captured in a guided-wave geometry. There remains a need to further scale down waveguides while maintaining a strong nonlinear response. In this study, we report second-harmonic generation and optical parametric generation using the second-order nonlinear response in an 80 nm thick CdS nanowire subwavelength waveguide. Moreover, our three-dimensional finite-difference time-domain (FDTD) simulations demonstrate that it is possible to enhance the coherence length due to the very nature of the subwavelength geometry. Nonlinear mixing in a nanowire subwavelength waveguide represents an advance toward all-optical processing and all-optical switching in integrated photonic circuits.

Single p-type/intrinsic/n-type silicon nanowires as nanoscale avalanche photodetectors.

We report the controlled synthesis of axial modulation-doped p-type/intrinsic/n-type (p-i-n) silicon nanowires with uniform diameters and single-crystal structures. The p-i-n nanowires were grown in three sequential steps: in the presence of diborane for the p-type region, in the absence of chemical dopant sources for the middle segment, and in the presence of phosphine for the n-type region. The p-i-n nanowires were structurally characterized by transmission electron microscopy, and the spatially resolved electrical properties of individual nanowires were determined by electrostatic force and scanning gate microscopies. Temperature-dependent current-voltage measurements recorded from individual p-i-n devices show an increase in the breakdown voltage with temperature, characteristic of band-to-band impact ionization, or avalanche breakdown. Spatially resolved photocurrent measurements show that the largest photocurrent is generated at the intrinsic region located between the electrode contacts, with multiplication factors in excess of ca. 30, and demonstrate that single p-i-n nanowires function as avalanche photodiodes. Electron- and hole-initiated avalanche gain measurements performed by localized photoexcitation of the p-type and n-type regions yield multiplication factors of ca. 100 and 20, respectively. These results demonstrate the significant potential of single p-i-n nanowires as nanoscale avalanche photodetectors and open possible opportunities for studying impact ionization of electrons and holes within quasi-one-dimensional semiconductor systems.

Hybrid single-nanowire photonic crystal and microresonator structures.

We report a hybrid approach for photonic systems that combines chemically synthesized single nanowire emitters with lithographically defined photonic crystal and racetrack microresonator structures. Finite-difference time-domain calculations were used to design nanowire photonic crystal structures where the photonic band gap overlaps the electronic band gap of the nanowire. Photoluminescence (PL) images of cadmium sulfide (CdS) nanowire photonic crystal structures designed in this way demonstrate localized emission from engineered defects and light suppression in regions of the photonic crystal. PL spectroscopy studies of defect-free nanowire photonic crystal structures further demonstrate the photonic band gap or stop band that spans most of the CdS band edge emission spectrum. In addition, single CdS nanowire-racetrack microresonator structures were fabricated, and PL imaging and spectroscopy measurements show good coupling of the nanowire to the microcavity including efficient feedback and amplified spontaneous emission. These hybrid structures exploit unique strengths of bottom-up and top-down approaches and thereby open new opportunities in nanophotonics from efficient and localized light sources to integrated optical processing.

General synthesis of manganese-doped II-VI and III-V semiconductor nanowires.

A general approach for the synthesis of manganese-doped II-VI and III-V nanowires based on metal nanocluster-catalyzed chemical vapor deposition has been developed. High-resolution transmission electron microscopy and energy-dispersive X-ray spectroscopy studies of Mn-doped CdS, ZnS, and GaN nanowires demonstrate that the nanowires are single-crystal structures and homogeneously doped with controllable concentrations of manganese ions. Photoluminescence measurements of individual Mn-doped CdS and ZnS nanowires show characteristic pseudo-tetrahedral Mn2+ ((4)T1-->(6)A1) transitions that match the corresponding transitions in bulk single-crystal materials well. Photoluminescence studies of Mn-doped GaN nanowires suggest that manganese is incorporated as a neutral (Mn3+) dopant that partially quenches the GaN band-edge emission. The general and controlled synthesis of nanowires doped with magnetic metal ions opens up opportunities for fundamental physical studies and could lead to the development of nanoscale spintronic devices.

Lasing in single cadmium sulfide nanowire optical cavities.

The mechanism of lasing in single cadmium sulfide (CdS) nanowire cavities was elucidated by temperature-dependent and time-resolved photoluminescence (PL) measurements. Temperature-dependent PL studies reveal rich spectral features and show that an exciton-exciton interaction is critical to lasing up to 75 K, while an exciton-phonon process dominates at higher temperatures. These measurements together with temperature and intensity dependent lifetime and threshold studies show that lasing is due to formation of excitons and, moreover, have implications for the design of efficient, low threshold nanowire lasers.

Imaging and analysis of nanowires.

We used vapor-liquid-solid (VLS) methods to synthesize discrete single-element semiconductor nanowires and multicomposition nanowire heterostructures, and then characterized their structure and composition using high-resolution electron microscopy (HRTEM) and analytical electron microscopy techniques. Imaging nanowires requires the modification of the established HRTEM imaging procedures for bulk material to take into consideration the effects of finite nanowire width and thickness. We show that high-resolution atomic structure images of nanowires less than 6 nm in thickness have lattice "streaking" due to the finite crystal lattice in two dimensions of the nanowire structure. Diffraction pattern analysis of nanowires must also consider the effects of a finite structure producing a large reciprocal space function, and we demonstrate that the classically forbidden 1/3 [422] reflections are present in the [111] zone axis orientation of silicon nanowires due to the finite thickness and lattice plane edge effects that allow incomplete diffracted beam cancellation. If the operating conditions are not carefully considered, we found that HRTEM image delocalization becomes apparent when employing a field emission transmission electron microscope (TEM) to image nanowires and such effects have been shown to produce images of the silicon lattice structure outside of the nanowire itself. We show that pseudo low-dose imaging methods are effective in reducing nanowire structure degradation caused by electron beam irradiation. We also show that scanning TEM (STEM) with energy dispersive X-ray microanalysis (EDS) is critical in the examination of multicomponent nanowire heterostructures.

Synthesis of CdS and ZnS nanowires using single-source molecular precursors.

Single-source molecular precursors were used to synthesize II-VI compound semiconductor nanowires for the first time. Cadmium sulfide and zinc sulfide nanowires were prepared using cadmium diethyldithiocarbamate, Cd(S2CNEt2)2, and zinc diethyldithiocarbamate, Zn(S2CNEt2)2, respectively, as precursors in a gold nanocluster-catalyzed vapor-liquid-solid growth process. High-resolution transmission electron microscopy studies show that the CdS and ZnS nanowires are single-crystal wurtzite structures with stoichiometric compositions. In addition, photoluminescence measurements demonstrate that these nanowires exhibit high-quality optical properties. The applicability of our approach to the synthesis of other compound and alloy semiconductors nanowires as well as nanowire heterostructures of these materials is discussed.