Nanowire-based structures for infrared to ultraviolet emitters studied by cathodoluminescence

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Nanowires are structures with features on the nanoscale, and it is therefore essential to study their properties on that scale. We present optical data from a variety of nanowire-based structures using cathodoluminescence imaging and spectroscopy. One important feature of nanowires is the stacking sequence of the crystal, either zincblede, wurtzite or a mix of the two. We show that this has an impact on the optical properties. In radial quantum wells, the thickness can be controlled on a monolayer level, in the case of flat side facets of the nanowires. With rough side facets, the quantum well collapses into quantum dots, as revealed by cathodoluminescence imaging. In order to extend the emission wavelength of light-emitting diodes into the ultraviolet or to cover the whole visible range, we use nanowire-seeded truncated pyramids as bases for these devices, based on either GaInN (visible) and AlGaN (ultraviolet).

Lay description

Semiconductors have attracted much attention in the search for more energy efficient lighting sources. They have been used in light emitting diodes (LEDs) for decades. Recently, they have entered our homes as white-light sources for lighting as they are more energy efficient and have significantly longer life times than incandescent light bulbs. The LEDs of today are based on thin layers of mixtures of semiconductor materials in a sandwich-like structure. One complication with this technique is to get a homogeneous cover of the various layers. They may not always want to cover the surface in a flat layer, but behave rather like the tomato-slices that are in chunks in the sandwich. To avoid this we use an array of nanowires. A nanowire is a very thin semiconductor needle of, with a typical diameter of a tenth of a micrometre, and a length of a few micrometres. A bundle of around a quarter of a million nanowires is needed to equal the diameter of one strand of human hair, which is typically 50 micrometres. Using many identical nanowires reduces the need for material in a LED device.

Lay description

To study and improve the nanowires, we need analysis methods to match the size of the nanowires. The technique must be able to distinguish individual nanowires in a field of nanowires. With our eyes, we can observe a single stand of hair, but not an individual nanowire. This is not possible even with a good optical microscope. A microscope using an electron beam instead of light is needed. Here we can study the light emitted from a semiconductor when hit by the electrons. This phenomenon is used in old “fat” TVs. An electron beam is scanned over the screen, generating the red blue and green colours we see as light. The colours are generated using three different semiconductors. If a beam is scanned over a small area, we can compare the colour of individual nanowires and if they all have the same colour along their lengths. Ideally, they should be equally bright and have the same colour. We have investigated nanowires to be used in infrared emitters for fibre-optical communication; visible light for illumination; and ultraviolet for water purification. This gives important feedback to the quality of these structures in order to improve the performance of the devices.

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