Accelerating HVPE of AlN
Quashing parasitic reactions enables HVPE to speed the growth of high-quality AlN
A collaboration between researchers in Japan and Poland is claiming to have made a significant breakthrough in the development of high-structural-quality AlN substrates with excellent transparency in the UV.
The team’s use of a new Taiyo Nippon Sanso HVPE reactor that supresses parasitic reactions has enabled the deposition of AlN at a growth rate of 150-170 µm/hr, The resulting AlN retains the structural quality of the seed grown by physical vapour transport (PVT), while reducing the density of impurities that inhibit transparency in the deep UV.
The team – a partnership between researchers at Tokyo University of Agriculture and Technology, Tokuyama Corporation, Fujitsu Limited, and the Institute for High Pressure Physics, Poland – hopes its advance will aid the productivity of freestanding AlN substrates. They are said to be an attractive foundation for the production of deep-UV LEDs and laser diodes that could be deployed for inactivation of viruses, sterilization, resin curing and processing.
Today, these deep-UV emitters are grown on either single-crystal bulk AlN substrates or engineered substrates, formed by growing a thin film of AlN on single-crystal sapphire. The latter ensures higher productivity, due to the availability of low-cost, large diameter sapphire, but dislocations are as high as mid-107 to 1010 cm-2, impairing device performance and reliability. Bulk AlN grown by PVT has a far lower dislocation density, typically 103 cm-2, but substrates are pricey, limited in availability, and have high concentrations of carbon, oxygen and silicon impurities that diminish transparency below 300nm.
Some of the researchers involved in the latest work have previous experience of using HVPE to grow AlN. For a previous study that employed a PVT-grown seed and selective generation of AlCl3, which does not react with quartz, the HVPE approach yielded crystalline AlN with a deep-UV transparency and a dislocation density of the order of 103 cm-2. However, parasitic reactions limited the growth rate to several tens of microns – that’s far slower than the growth rate for PVT, which can be 150-170 µm/hr at 2230 °C.
The team’s Taiyo Nippon Sanso HVPE_A111 quartz-based horizontal reactor supresses these parasitic reactions. It features an up-stream source zone heated by an electric furnace and a downstream growth zone heated by RF induction.
Using AlCl3 as the source of aluminium, supplied via a nozzle made of BN to prevent parasitic reactions, and NH3 for the source of nitrogen, the team grew films of AlN on 6 mm by 7 mm by 0.52 mm pieces of AlN(0001), cut from a 35 mm-diameter HexaTech wafer made by PVT. Those pieces provided the foundation for the growth of AlN at a range of growth rates up to 156 µm/hour
Inspecting films around 50 mm-thick with an optical microscope revealed that the growth rate influences morphology. A rate of 7.6 µm/hour produced a very smooth surface, while around 50 µm/hour or more induced a shift to a three-dimensional growth mode, resulting in hexagonal pyramidal hillocks. However, even at the fastest growth rate of 156 µm/hour, which led to hillocks with a typical width and height of 250 mm and 2 mm, respectively, no AlN microcrystals were found to fall on the surface.
According to X-ray diffraction rocking curves along a symmetric and skew symmetric plane, it is possible to grow homoepitaxial layers with a structural quality comparable to that of PVT-grown AlN, even when the growth rate exceeds 150 µm/hr.
Investigating the samples with secondary-ion mass spectrometry revealed that with increasing growth rate concentrations of oxygen and chlorine fell, while that of silicon went up. The researchers suggest that higher concentrations of silicon impurities at faster growth rates could be due to the appearance of facets on the surface, or an increase in AlCl relative to AlCl3. As quartz in the HVPE tool reacts with AlCl to generate the doping gas SiCl4, the team recommends removing quartz glass from the growth chamber to minimise the addition of silicon impurities.
To assess the optical characteristics of the HVPE-grown AlN, the researchers produced a 40 µm-thick free-standing substrate by the removal of PVT-AlN, followed by chemical-mechanical polishing. This sample has a steep optical absorption edge at 207 nm, and a high transmittance at longer wavelengths. There is an absence of an absorption band around 450 nm – it is observed in free-standing AlN substrates produced at lower growth rates, due to a higher level of oxygen impurities.
The team is now investigating growth of thicker homoepitaxial layers at rates higher than 150 mm/hr.
Y. Kumagai et al. Appl. Phys. Express 15 115501 (2022)