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/Al_xGa_1-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.

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.

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.

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.