For many years, NPSC – a joint team of PHELIQS (UGA, CEA, Grenoble INP) and Institut Néel (CNRS) – has been studying wide band gap semiconductors for their optical properties in the ultraviolet (UV) range. Recently we started to investigate optically active layers emitting at even shorter wavelengths, typically in the 200-250 nm spectral region. In order to study their photoluminescence properties under optical excitation, we installed a new laser source able to emit light pulses down to 195 nm: such a laser is still quite rare in optical spectroscopy labs (NPSC is the second laboratory in France with such an installation for semiconductor UV spectroscopy) ! This new light source complements our previous UV light source emitting at 244 nm. We hereafter detail the possibilities offered by this new instrument and present the first results obtained shortly after its installation.
Performing optical experiments in the ultraviolet range is more challenging that in the visible or near-infrared ranges. Optical elements are more expensive and overall less performant. Most importantly, few laser sources exist in the UV range. Still, UV optoelectronics is a growing topic, notably for applications where UV light is used for its virucide and bactericide properties. The new light source we installed consists of a titanium-sapphire pulsed laser emitting in the red/infrared range (680 to 1080 nm) followed by an « harmonic generator » relying on non-linear crystals that can divide by two, three and four the wavelength of the initial beam. The equipment was acquired in 2024 through a successful equipment funding application submitted by PHELIQS to UGA and Grenoble INP-UGA, and complemented with funds from IRIG. This light source has the potential to emit at all wavelengths between 195 nm and 1080 nm except for the 540-680 nm band. In practice, we use only the quadrupled beam at wavelengths between 195 nm and 230 nm. The output beam is not strictly speaking a laser beam as it stems from non-linear conversion processes, but still behaves similarly with a collimated and monochromatic beam. It is a pulsed laser with pulses of duration around 150 fs with a repetition frequency of 12.5 ns. This means that 99.999 % of the time, no light is emitted from this light source ! This also means that with a proper detection apparatus, time-resolved photoluminescence can be performed with such a source, giving access to the recombination dynamics of the electron-hole pairs in the samples that are studied.
The laser was installed in September 2024 and a few weeks after the installation, first experiments could be performed. One of them consisted in studying aluminum nitride (AlN) grown under various conditions in the form of nanowires. While AlN usually grows in the stable form of wurtzite crystals, it turns out that in the case of nanowires and under specific growth conditions, it can also form heterostructures of zinc-blende and wurtzite crystals. The bandgap of zinc-blende AlN being smaller than the bandgap of wurtzite AlN, it results in quantum-well like structures called stacking faults. By shining a 195 nm beam on the sample, we could analyze the photoluminescence stemming from these stacking faults which is around 210 nm (5.9 eV). This study adds new insights on the growth of AlN as nanostructures and its fundamental optical properties.