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Quantification of layer composition in compound semiconductors


Compound semiconductors are the key underpinning technology in optoelectronics, and also used in electronic applications with specialist requirements (e.g. power). The ability to engineer the electronic and optical properties of compound semiconductor alloys, for example in terms of their alloy composition, which may be binary, ternary, quaternary or quinary, and grow multiple layers of different semiconductor alloys on top of each other (heterostructures), is a key part of their success.

Excellent and extreme examples of this are devices that contain distributed Bragg reflectors: alternating layers of high- and low refractive-index material (typically GaAs/AlxGa1-xAs) to create a stop-band where a very particular set of wavelengths are almost fully reflected (ideally over 99.9%). For example, vertical cavity surface emitting lasers (VCSELs) are tiny (low-cost) semiconductor lasers that use a pair of DBRs to form the mirrors of the lasing cavity. In VCSELs, the quality and consistency of the DBRs is important, as a VCSEL has a gain length on average 105 times smaller than an edge-emitting laser, and therefore needs ultra-high reflectivity mirrors to achieve a reasonable threshold current. Examples of other, emerging, devices that use DBRs are single photon LEDs (SPLEDs); these are needed for quantum key distribution in quantum cryptography networks. This study focuses on these DBRs, and methods to accurately characterise their structure, including determining whether the semiconductor layer growth has proceeded as desired. X-ray photoelectron spectroscopy (XPS) depth profiles are taken to measure the chemical composition of the DBR layers to further characterise the growth. Even a small change in Al composition affects the refractive index, thus changing the optical path length of the layer, with consequences for everything from mirror characteristics to laser output wavelength. XPS yields quantitative information regarding Al content for the DBR structure, which directly relates to device performance.



Analysis of drug coated polymer stents studied by XPS

Cardiovascular interventional therapy with stents has emerged as the most effective method for coronary heart disease. However, thrombosis and hyperplasia are the usual pathological responses to the implantation of foreign devices. To suppress this immune response and that of overgrowth and subsequent restenosis anti-inflammatory drugs are now loaded onto the surface of stent implants. Here we investigate the surface of drug loaded polymer stents. The stents are made of polylactic acid (PLA) dosed with an anti-inflammatory drug with a molecular structure of C51HxNO13. XPS yields quantitative information regarding drug distribution and using Argon cluster sputtering we can see the distribution of the drug into the stent structure. Analysis is also performed on stents submerged in buffer solution (PBS) to see the effects on ageing and the propensity for the drug to migrate into the solution with time.

XPS analysis of frozen 1,4-dibromobenzene

The molecule 1,4-dibromobenzene is of interest as a precursor for the synthesis of conjugated polymers with applications in nanoelectronic and electronic devices. 1,4-dibromobenzene has a vapour pressure of 0.0575 mmHg at 25˚C such that it will volatilise in vacuum at room temperature. To achieve XPS characterisation this organic material requires cooling to <100˚C before pumping and introduction to the analysis chamber.

Combined Argon cluster UPS-XPS depth profile of OLED thin-film

Low-energy argon cluster ions were used to depth profile through an organoelectronic (TCTA) thin-film deposited on ITO. XPS and UPS spectra were acquired after each etching cycle. Compositional changes were seen through the film and at the interface with the substrate. Changes in UPS spectra were directly compared to changes in compositional change.

Investigating the surface and interface of Li ion thin-film battery materials

Here we use conventional surface analysis techniques of XPS and sputter depth
profiling to understand the surface and bulk chemistry of LiPON films formed
via atomic-layer deposition (ALD).  Comparisons are made between the results obtained from conventional monatomic depth profiling and cluster depth profiling.

Spherical mirror analyser (SMA)

This technical note outlines the properties and uses of the spherical mirror analyser (SMA) for XPS imaging and it's integration into a modern photoelectron spectrometer.  It details the electron optical properties of the analyser for high energy and spatial resolution imaging.

Getting more from XPS imaging : multivariate analysis

Here we review the properties of the SMA including spatial and energy resolution and provide examples of the capabilities of such an imaging analyser.  In the last few years the combination of the SMA with a two-dimensional, pulse counting electron detector has again increased the level of information available for surface characterisation.  Generating such information requires the acquisition of multi-spectral datasets comprising a series of images incremented in energy so that each pixel contains photoelectron intensity as a function of energy. The datasets generated by this method contain >65,500 spectra and are therefore ideally suited to multivariate analysis to analyse the information content of the dataset and as a tool for noise reduction in individual images or spectra.

Surface chemistry of plasma coated textiles studied by XPS

The performance of fluorinated textile treatment methods, in terms of wash resistance, was evaluated by XPS. Two different fluorination methods, solution based treatment and plasma polymer deposition, and four different textiles were studied with a view to producing long lasting, high performance clothing systems for outdoor activities. An important attribute of such textiles is the ability of the surface of the material to retain or regain its liquid repellent nature following conventional washing cycles.
XPS analysis revealed the degree of surface fluorination before and after wash cycles enabling the durability of the thin film coatings to be evaluated. Results indicated that for most materials the plasma polymer coated textiles exhibited better performance in terms of fluorine coverage and retention than the solution based fluorination metho

Sample cleaning using Ar-GCIS

Typical samples are often presented for analysis following transportation in a less than ideal environment. This results in an analysis and measured surface composition that is not representative of the true surface of the original material. There are a limited number of methodologies available that can effectively clean the sample and restore the original surface without inducing some additional chemical changes and thereby changing the very surface that is being investigated. The development of Ar-gas cluster ion source has changed this.