Quantum Information Group, Ultra-High Resolution Cathodoluminescence
Cathodoluminescence is the process in which high energy electrons impacting with atoms in a solid lose energy, part of which is then re-emitted as photons of light.
It is precisely the process that most of us observe every day when viewing images on a standard vacuum tube television or computer screen. The energy and number of the emitted photons yields information about the physical and electronic properties of the target material.
This sort of information is vital for designers and manufacturers of all manner of optoelectronic devices, and can be obtained quickly, with the bare minimum of specimen preparation. Recent refinement of the cathodoluminescence technique at the Cambridge Research Labs (see photo) has produced a world record in the field of electron microscopy.
When the highly focused beam of a scanning electron microscope is used as the electron probe, very small volumes of material can be investigated. To minimise this volume, the energy of the electron beam must be kept as low as possible, since lower energy electrons penetrate a lower distance into any given material. Working with a beam energy of just 1.5keV, Toshiba has been able to demonstrate a ground-breaking 20nm resolution. This is fully a factor of three times better than has been previously reported.
For such a result to be possible, virtually every single aspect of the equipment had to be optimised or upgraded. This included the high vacuum system and the isolation from both electrical and mechanical noise. In order to confirm the high resolution thus achieved, the emission from individual quantum wells within a multi-well stack was examined. A sample containing seven sets of three quantum wells, each of known thickness and separated by a known distance, was purpose grown by molecular beam epitaxy.
Secondary electron image (left) of the multiple quantum wells in cross section, together with a higher-resolution pseudocoloured cathodoluminescence image (right) showing the narrowest wells. The well widths and separations are (left to right) 40nm, 30nm, 20nm and 10nm.
Cathodoluminescence imaging shows that it is possible to resolve the individual quantum wells, down to separations of tens of nanometres. Quantitative cathodoluminescence linescans across the cleaved ends of the quantum wells demonstrate unequivocally that wells as narrow as 20nm can be resolved, despite being separated by barriers only 20nm thick. The same images and linescans showed that the photon emission from the 10nm quantum well, which was deposited first during the growth sequence, is reduced by about 50% compared to the other two wells of each set. This phenomenon is attributable to the growth conditions used during epitaxy, and would previously have gone unnoticed. Identifying this problem using ultra-high resolution cathodoluminescence thus allowed the growth process to be optimised, thereby producing brighter, more efficient, quantum structures.
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