Aluminum nitride single-crystal growth
Aluminum nitride (AlN) is a promising material for many future applications, including blue-ultraviolet LEDs as well as surface acoustic wave (SAW) devices. SAW devices can act as resonators or filters integrated circuits. They can also be used in wireless sensors operating in harsh environment for instance. As such, they play an essential role in applications ranging from professional radar to mobile wireless systems. It goes without saying that these combined markets are worth well over several billion dollars, hence providing a strong incentive for R&D. SAW devices require a piezoelectric material such as quartz, which has been so far the material of choice.
However, ceaseless miniaturization makes the development of new SAW devices critical spurring a growing interest in developing new piezoelectric materials exhibiting novel or at least improved characteristics in terms of operating frequency and efficiency. Importantly, a growing trend is to fabricate SAW resonators as a whole using methods compatible with CMOS technologies making cheaper, lower-power consumption and smaller SAW devices a reality.
Over the past ten years, AlN has gradually gained ground in the semiconductor industry because of its unique combination of electrical, mechanical and piezoelectric properties. Indeed, AlN is a wide bandgap semiconductor featuring high thermal conductivity and resistivity, high SAW velocity, moderately high electromechanical coupling coefficient, high temperature and chemical stability to atmospheric gases up to 700 ºC combined wih mechanical hardness. Therefore, AlN-based SAW devices could be the next generation of high performance SAW products. Nonetheless, the availability of high structural quality AlN crystals is still a major concern.
Fabricating high-quality bulk and thin AlN layers and their epitaxial layers is challenging. Our crystals are grown by High Temperature Chemical Vapor Deposition (HT-CVD) above 1200°C. This versatile growth technique involves reactant gases leading to chemical surface reactions on a heated substrate leading to a thin or thick layer, depending on the growth time. AlN is epitaxially grown along its polar c-axis (0001) on c-plane sapphire substrates. Therefore, the structural quality of the substrate is also an important parameter as it strongly determines the quality of the grown layer which in return affects the performance of the future electronic devices. Another challenge is managing the stress caused by the different thermal expansion coefficients of the AlN layer and the sapphire substrate. It typically shows up during the cooling down stage resulting in cracked layers. This is issue is particularly acute for high temperature growth. Eventually, the required properties of the AlN layers are: no cracks, low porosity, high resistivity, low dislocation density as well as low C, Si and O contamination.
To accelerate the development of next generation SAW devices, Sil'Tronix ST is partnering with the SIMaP (featuring a long experience in AlN thin film growth) and femto-st laboratories (http://simap.grenoble-inp.fr/ and http://www.femto-st.fr/, Rakon (http://www.rakon.com/), and frecNsys (http://www.frecnsys.fr/) and aims at commercializing the next-generation high quality AlN thin layers.