Novel materials are the bedrock of today’s technology innovations, whether for better ways to harvest and store energy or for extending the lifetime of critical mechanical components. But creating the perfect structure for any particular application requires a precise and detailed understanding of various candidate materials – both in the bulk and when fabricated into layered structures.
One of the fastest techniques in the material scientist’s toolkit is glow-discharge optical electrical spectroscopy (GDOES), which takes just a few minutes to deliver a high-resolution depth profile of the elements present in a sample – which may be up to a few hundred microns thick. Initially developed in the 1970s for use in the steel industry, recent advances have extended the capabilities of the technique to reveal the elemental composition of many different materials and structures, including insulating and conducting layers, fragile and flexible samples, and organic and carbon-based materials. Unlike many other analysis techniques, GDOES delivers nanometre resolution without the need for an ultrahigh vacuum.
The speed and ease-of-use provided by GDOES has made it a popular technique for optimizing the performance of thin-film solar cells, including the latest generation of perovskite and cadmium-telluride devices. “For thin-film photovoltaics it is important to analyse multiple samples to refine the manufacturing process and enhance the overall performance,” says Patrick Chapon, product manager for elemental analysis at HORIBA, which has become the market leader for GDOES analysis. “Depth-profile chemical analysis is crucial to match the bandgaps and optimize the process.”
Chapon is the driving force behind SurfaceFest, a biannual gathering of scientists who rely on GDOES and other characterization techniques for their research. A quick glance at the line-up for the 2018 event, held in June in Bordeaux, France, reveals that the technique is now yielding valuable insights in such diverse applications as organic electronics, micro-LED displays, and corrosion-resistant coatings. In battery research, too, GDOES is uniquely positioned to reveal the movement of lithium ions from one electrode to the other.
“GDOES can measure lithium and other elements that are too light to be detected by other common spectrometry techniques,” says Chapon. “In addition, the layers involved in lithium-ion migration must be thick to generate enough current, so it is important to have a technique that is capable of probing thick layers.”
Indeed, in a single experiment GDOES can provide detailed information about both the uppermost atomic layers and much deeper interfaces – revealing, for example, whether any diffusion or contamination might have occurred between different layers. As well as lithium, the technique can detect the presence of hydrogen, oxygen, and other light elements at trace concentrations of parts-per-million or even less.
What has enabled these enhanced capabilities, says Chapon, is the use of a radio-frequency pulsed source to control the glow discharge. “GDOES is a destructive analysis technique that uses a plasma to sputter material from the surface of the sample,” he explains. “The atoms removed from the surface become excited inside the plasma, and an optical spectrometer can then be used to detect the light emitted as these excited species relax into their ground state.”
Pulsed discharge enhances the signal
A pulsed RF discharge makes it possible to reduce the thermal load on the sample, which improves the signal-to-noise ratio because higher sputtering powers can be used without damaging the sample. Time-resolved measurements using a fast-acquisition spectrometer also allow researchers to obtain information from measurements taken during the different phases of each pulse.
“The benefits of an increased signal-to-noise ratio is particularly important for nanometre-thick multilayer structures,” says Chapon. “But we have only been able to routinely implement pulsed RF operation in all our GD spectrometers since 2011, when we developed a patented technique to auto-match the RF source in both pulsed and non-pulsed modes.” HORIBA has also recently added differential interferometry profiling to directly measure the erosion rate and sputtering depth in real time, rather than inferring this information from complex calculations.
For organic and carbon-based materials, Chapon and his colleagues have found a way to replace the usual argon plasma gas with a combination of argon and oxygen. In this patented operation mode, known as UFS, the plasma becomes reactive and causes material to be removed from the surface more quickly – further improving the signal-to-noise ratio and speeding up the analysis time. Scientists at the Université Paris-Saclay in France have used this UFS mode to study a complex perovskite photovoltaic structure composed of an ITO-coated glass substrate covered with several organic and inorganic layers. “This made it possible for the first time to reveal the diffusion of some elements within the perovskite layer under a bias voltage,” says Chapon, who was a co-author of the study.
Strength in numbers
Despite the many advantages of GDOES, Chapon believes that some of the most interesting results can be achieved by combining it with other analysis techniques. While GDOES provides detailed chemical data through a depth of a sample, it has no lateral resolution – which means that surface techniques such as X-ray photoelectron spectroscopy (XPS) can provide complementary information.
“The strength of XPS is in its capability to look at chemical shifts and, for instance, to reveal the oxidation levels of a component or the binding between elements,” Chapon explains. “For depth profiling the practical limit of XPS is limited to less than 1 µm, but over the last few years we have shown how XPS can be used within the GD crater to provide a powerful tool to probe the chemistry of deep interfaces.”
Many of these advances were on display at SurfaceFest, which has grown rapidly since the first meeting for the GD user community was organized by HORIBA back in 2002. Chapon also hints that further improvements are in the pipeline, most likely by combining GDOES with other techniques. He has already co-ordinated an EU-funded project called EMDPA that combined a GD source with a time-of-flight mass analyser, with the aim of obtaining the entire mass spectrum at any depth. “We made an instrument after this project that has already sold a few units,” he says. “We have in HORIBA various technologies that could lead to new generation of instruments.”
More information about GDOES is available on the HORIBA website.