Nanowire Heterostructures Form Tunneling Diodes
Daniel S. Burgess
Continuing their research into nanoscale semiconductor heterostructures known as nanowires, a team at Lund University in Sweden has produced one-dimensional resonant tunneling diodes that display nearly ideal current-to-voltage characteristics. The devices, which electrically address a single quantum dot, herald nano-optoelectronics of improved performance.
Lund University researchers have constructed resonant tunneling diodes using one-dimensional semiconductor heterostructures. A micrograph of the nanowire device (a) illustrates the double-barrier geometry (scale bar is 30 nm). The thickness of the InAs quantum dot region and the diameter of the N-type nanowire determine the longitudinal and the lateral confinement, respectively, and thus the energy of the respective quantized states (b). Measured at 4.2 K, the current-to-voltage data (c) display a sharp peak at a bias voltage of approximately 80 mV with a half width of 5 mV, or an energy sharpness of 1 to 2 meV, indicating resonant tunneling into the ground state. A magnification of the peak (inset) at increasing and decreasing bias voltage demonstrates that the device experiences minimal hysteresis. Courtesy of Lars Samuelson.
Semiconductor heterostructures -- investigated independently by Herbert Kroemer and Zhores I. Alferov, who shared the 2000 Nobel Prize in physics in recognition of the impact of their efforts on information technology -- are crucial to photonics and the basis of various types of photodetectors, LEDs and laser diodes. "All these important inventions are based on the development of techniques allowing the growth of ultrathin layers and abrupt transitions between semiconductor layers of different compositions," said Lars Samuelson, a professor of solid-state physics and head of the nanometer consortium at the university.
However, he added, the fabrication of compound semiconductor devices is top-down, in which planar heterostructures are grown and then patterned, etched and shaped, which leads to reduced performance by processing-induced damage. In contrast, the Lund researchers employ a bottom-up, self-assembly method.
To produce the heterostructures, they deposit uniformly sized particles of gold on a growth substrate and place this target in an ultrahigh-vacuum chemical beam epitaxy chamber. They heat the chamber to melt the particles and introduce group III and V materials. The gold particles act as catalytic seeds, causing the semiconductors to precipitate at the melt sites and to form segmented, whiskerlike structures, with the size of the melt site controlling the diameter of the structure.
In the recent demonstration, the researchers employed InAs to form an emitter, collector and quantum dot in the nanowire tunneling diodes and InP to serve as the barrier regions. They selected gold aerosol particles that would yield nanowires 40 to 50 nm in diameter and introduced the semiconductors to create 5-nm-thick barrier segments on either side of a 15-nm-thick quantum dot. They transferred the nanowires to a silicon wafer coated with SiO2 and used electron-beam lithography to attach metallic contacts. The final devices, measured at 4.2 K, displayed an energy sharpness of 1 to 2 meV at a bias of 80 mV and a peak-to-valley current ratio of 50:1.
Samuelson said that controlling doping of the nanowires and inducing carriers in the structures remain challenges, but he is confident that photonic devices based on the technology will appear soon. For example, by P-doping one of the injection regions, it should be possible to fabricate single-quantum-dot nanowire LEDs, and one-dimensional superlattices of quantum dots might enable quantum cascade lasers operating in the terahertz regime.
"We expect single-quantum-dot LEDs to become available very soon in the lab," he said, "and, if the commercial need is there, such unique light sources could be commercially available within a few years." He predicts that viable nanowire-based detectors and photovoltaic cells may be only five years away.
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