Electron flow captured in time-resolved images

TSUKUBA--Visualizing the flow of electrons within semiconductors can provide vital clues to better design of chip nanostructures and insights into related electronic processes. Researchers at the Electro technical Laboratory (ETL; Tsukuba, Japan) have developed a visualization method that records the shape of time-resolved microphotoluminescence (PL) images shot with an ultrafast, high-repetition-rate shutter camera. The novel laser-based method contrasts with current methods that either measure

Electron flow captured in time-resolved images

Paul Mortensen

TSUKUBA--Visualizing the flow of electrons within semiconductors can provide vital clues to better design of chip nanostructures and insights into related electronic processes. Researchers at the Electro technical Laboratory (ETL; Tsukuba, Japan) have developed a visualization method that records the shape of time-resolved microphotoluminescence (PL) images shot with an ultrafast, high-repetition-rate shutter camera. The novel laser-based method contrasts with current methods that either measure current or voltage changes through a semiconductor or visualize possible flows of electrons with computer simulations.

In their experiments the ETL researchers captured time-resolved PL images of electron flow in selectively doped gallium arsenide/aluminum gallium arsenide (GaAs/AlGaAs) quantum nanostructures that were grown on GaAs substrates by selective molecular beam epitaxy. The imaged structures included channels, quantum wells, and edge wires.

To capture PL images the researchers focused the output of a pulsed Ti:sapphire laser through the objective lens of an optical microscope to a spot diameter of 1 µm onto the sample. Photo luminescent spectra from the sample were transferred to a conventional charge-coupled-device (CCD) camera using the same objective lens. For time-and-spatial-resolved PL measurements, the researchers installed a high-repetition-rate shutter camera supplied by Hamamatsu Photonics (Hamamatsu City, Japan) onto the PL measurement setup; the shutter speed of the camera was 90 ps with a repetition rate of 80 MHz.

By applying a bias voltage, the researchers induced electron flow in a GaAs channel 40 µm wide. The researchers made a composite of the channel image and several vector-indication-like PL images after applying laser light to various positions along the channel. Hence, by comparing three PL images across the corner of the channel, electron flow near the inner corner could be seen to be stronger than that at the outer corner--similar to how water flows in a curved river channel--with electrons (like water) tending to slow at the outer corner (see Fig. 1 on p. 38).

The researchers also shot three time-resolved two-dimensional PL images in the channel at 85 K. After applying a laser beam of 200-fs pulse width and 80-MHp repetition rate, three PL images were taken at delays of 0.2, 1.2, and 2.2 ns with the high-repetition-rate shutter camera. The images show acceleration of minority carriers (excitons), which are dragged with the flow of the majority electrons after application of a bias voltage between two terminals (see Fig. 2 on p. 40).

In another experiment, time-and-spatially resolved PL images of quantum well and edge wire clarified processes within these structures. The PL lifetime and drag length for the edge wire were seen to be much larger than those for the well; however, drag velocity of the wire was smaller than that of the well. These results indicate that the scattering by majority electrons to minority electron-hole pairs is weaker in the edge wire, and, therefore, mobility in the edge wire is higher in comparison to the well. The researchers expect that more-detailed images will be captured with newly developed femtosecond streak cameras.

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