Facilitating automatic analysis of thin-film structures
Advanced analysis of GaN-on-Si high electron mobility transistors (HEMTs)
GaN-on-Si technology has evolved from extensive research activities and is now entering global large-volume production by a broad range of suppliers. Automated metrology solutions for controlling and optimizing the production of this kind of epitaxial wafer are now in high demand.
Gallium nitride (GaN) and related III/V compound semiconductors are now widely used in the production of RF modules, power conversion devices, and high brightness light-emitting diodes (LEDs). GaN-based high electron mobility transistors (HEMT) is the most promising technology for high-power, high-efficiency devices in many applications. Here, GaN is epitaxially grown on a variety of substrates; for example, GaN-on-Si, which is formed by thin-layered GaN-based stacks on silicon (Si) substrates which are relatively cheap and are available in diameters up to 300 mm.
One application of GaN-on-Si is in power devices such as battery chargers, smartphones, computers, servers, automotive, lighting systems and photovoltaics. Such devices based on GaN have just entered the market playing a key role in the emerging power conversion market.
Production of GaN-on-Si is based on the epitaxial growth of multilayer GaN heterostructures on Si substrates using metal-organic chemical vapor deposition (MOCVD). These devices contain epitaxial layered structures of increasing complexity.
The quality of the AlGaN barrier and the underlying GaN layer which forms the 2DEG is crucial for the device’s performance and reliability. These depend on the quality of the thick GaN layer grown on the buffer structure. A thorough analysis of the individual layers of these multilayer structures is a challenge for existing characterization methods.
AMASS with comprehensive functionality can be used to determine the uniformity of all parameters on a wafer. This is essential for optimizing the production of epitaxial wafers.
Lars Grieger – Business Development Manager Semiconductor Metrology at Malvern Panalytical
X-ray metrology
High-resolution X-ray diffraction is an established and powerful tool for the non-destructive characterization of heteroepitaxial structures. It is routinely employed for both materials research and quality control in production. X-ray diffraction patterns provide information about composition and uniformity, layer thickness, strain relaxation, crystalline perfection, and much more.
Maps around reciprocal lattice spots (RSM) can reveal additional information beyond that provided by single line scans such as high-resolution rocking curves. RSMs are typically used to aid the interpretation of peak displacement, peak broadening of peak overlap. They have, however, not often been used because they are considered too time-consuming to collect and analyze.
However, recent advances in X-ray area detector technology combined with smart positioning algorithms and data processing now allow recording of RSMs at faster timescales as rocking curves. These high-speed measurements find applications in the characterization of all types of crystalline advanced materials. In the example below, we can see how RSMs can be used to characterize GaN-on-Si HEMTs.
Analysis of HEMT structures
Determination of the composition and thickness of the barrier layer of a GaN-based HEMT is a two-step process: Evaluation of the RSM around the (-1 -1 4) GaN reflection reveals the composition and relaxation values of the buffer layers and of the AlGaN barrier. Our recently introduced AMASS (Advanced Materials Analysis and Simulation Software) automatically searches and labels peak positions and determines Al composition and relaxation of the barrier layer. The same applies to the buffer layer peaks.
While the RSM cannot provide information about the thickness of the thin topmost layers, this information While the RSM cannot provide information about the thickness of the thin topmost layers, this information is accessible from the full-pattern fitting of rocking curves of the symmetrical (00l) reflections. Due to heavy overlap with the buffer peak positions, the thin barrier layer cannot be resolved, but its thickness fringes are present. AMASS’s fitting option keeps the previously determined composition and relaxation values fixed while fitting the thickness values of a corresponding sample model. The described two-step workflow can be fully automated in AMASS.
AMASS is our thin-film analysis toolbox. It covers and expands the functionality of our former Epitaxy and X’Pert Reflectivity software. Additionally, it comes with comprehensive functionality to display, analyze, simulate and fit X-ray data from thin-film layered structures,” says Lars Grieger, Malvern Panalytical Business Development Manager Semiconductor Metrology. “Results of multiple data sets collected on different X, Y wafer positions can be used to determine the uniformity of all parameters on a wafer. AMASS takes care of the necessary steps and can be remotely controlled via the command line or from any higher language environment. This enables a whole new level of integration into existing lab data streams.
Free XRD Software licenses for Academics
Why not check out what this software offers for your analytical problems with layered structures! For more information about the applications of AMASS, go to the Malvern Panalytical website. And there’s more! To help researchers and students continue their studies, we offer a personal copy of XRD analysis software AMASS or HighSore Plus, free of charge. Applying online is quick and easy. Learn more: www.malvernpanalytical.com/freelicense
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Glossary
GaN gallium nitride
AlGaN aluminum gallium nitride
GaN-on-Si gallium nitride on silicon
2DEG ‘two-dimensional electron gas’
HEMT high electron mobility transistors
RSM reciprocal space map
Further reading