Vertical HVPE tool produces 2 inch GaN

Defects in GaN optoelectronic devices, such as 405 nm high-power laser diodes, and ultraviolet and blue LEDs, cut light output and lead to failure at high current densities. To combat this, manufacturers employ elaborate buffer schemes for growth on foreign substrates that minimize the defect densities in the epilayers. However, even the most effective technique – epitaxial lateral overgrowth – pays significant penalties for the reduced defect density. Process steps are more expensive and complex, and produce a smaller usable wafer area, which impairs production yield.

These weaknesses highlight the need for a free-standing, affordable, low-dislocation-density GaN substrate. A great deal of research is being carried out in this area, using techniques such as HVPE, high-pressure solution growth, ammonothermal growth, physical vapor transport and sublimation growth.

Today, HVPE is the most popular technique for manufacturing free-standing substrates, thanks to its process maturity, controllability and reasonably high growth rate. Several companies around the world are currently marketing GaN substrates made with this technique, which are often grown on single or multiwafer tools on foreign templates, such as sapphire.

It is logical to try to extend this growth and to produce a boule that is several centimeters thick, which is an approach that is already being pursued by various companies in the US and Asia. Wire saws can then cut several substrates from this boule before they are polished. In addition, one of these substrates can also be used as a seed for the subsequent growth, leading to a steady fall in the dislocations in the material, and allowing growth on lattice-matched material.

Growth by HVPE is clearly an attractive option, but it poses several challenges, including a by-product of GaN growth – ammonium chloride – which forms a caustic dust below 340 °C. This dust must be kept away from the growth area, where it could contaminate the GaN boule, and the exhaust system, which it can clog up, therefore delaying what are already lengthy growth processes.

At Aixtron, in Aachen, Germany, we have developed a vertical HVPE chamber that addresses this issue. The new tool is capable of producing 2 inch diameter boules up to 7 cm thick using growth rates of several hundreds of microns per hour (figure 1). This reactor, which we launched commercially this year, features a boule hanging face-down from the top of the reactor. This set-up prevents any ammonium chloride from falling onto the growth surface. The rotated boule can also be retracted at a rate that maintains a constant distance between the gas inlet and the growth surface.

Before launching this mass-production system, we built two similar prototype systems with comparable reactor geometry. These tools have been installed at the University of Linköping, Sweden, and the Ferdinand Braun Institute for High-Frequency Technology (FBH) in Berlin, Germany.

Researchers at the University of Linköping have assessed the quality of the material grown by HVPE. They used atomic force microscopy to investigate the surface of 250 µm thick and 2 mm thick layers of GaN that are grown on GaN-on-sapphire templates under identical growth conditions (figure 2). Increasing the layer thickness produces a change in growth morphology but does not affect the film s roughness. This maintains a root-mean-square value of 3 nm for a 10 × 10 µm scan size and a peak-to-valley height of 10–15 nm. This lack of variation in the surface s roughness indicates that its quality is unaffected by growth. As expected, additional GaN growth cuts the epilayer s defect density, with the etch pit density decreasing from 2 × 106 cm–2 for the 250 µm thick layer, to 5 × 105 cm–2 for the 2 mm thick layer. This implies that thicker boules will not only yield more wafers but also produce material with lower defect densities.

We are continuing to work with various research and development labs to improve the GaN HVPE tool and the growth process. This effort, which should ultimately improve the performance of GaN devices, includes the installation of a next-generation mass-production system later this year at FBH. This tool, which is similar to our commercial HVPE reactor but features a modified gas inlet configuration and substrate holder, will be used to improve boule crystalline quality. Once that has been done, efforts will turn to the growth of larger-diameter material, followed by the growth of ternary bulk AlGaN.

Further reading
C Hemmingson et al. 2006 Superlattices and Microstructures 40 205.
A Kasic et al. 2005 J. Appl. Phys. 98 073525.
T Paskova et al. 1999 Phys. Stat. Sol. (a) 176 415.