Efficient enlargement of bulk AlN
Introducing expansion angles in excess of 45 degrees can slash the time it takes to realise bulk AlN grown by physical vapour transport with industrially relevant crystal diameters.
BY CARSTEN HARTMANN AND THOMAS STRAUBINGER FROM THE LEIBNIZ INSTITUTE FOR CRYSTAL GROWTH
The ultra wide band gap semiconductor AlN, as well as its ternary cousin AlGaN, has a number of attributes that make this material system a promising one for electronic and optoelectronic devices. This class of nitrides can withstand harsh conditions and high temperatures, and thanks to the very wide bandgap, can be used to make LEDs that emit in the deep and far UVC. The largest markets for such devices, which span 220 nm to 280 nm, are the disinfection of drinking water and waste water treatment. However, emission in this spectral range can also be deployed for surface sterilisation and bio-chemical sensing. As well as these opportunities in deep-UV photonics, AlN-based devices are compelling candidates for next-generation high-frequency power-conversion. Already, AlN-based high-power transistors are outperforming those made from the two most common wide bandgap semiconductors, SiC and GaN.
Due to a lack of native substrates, researchers exploring the capability of AlN and its alloys began by using sapphire substrates for the development of epitaxial layers. However, even when they turned to complex processing steps, the threading dislocation densities in the AlGaN layers remained high – typically in excess of 1 x 108 cm-2. The high density of these threading dislocations is one of the primary reasons why the potential of the AlN material system is yet to be fully exploited.
The power of PVT
Offering a promising way forward is the growth of AlN crystals by Physical Vapour Transport (PVT). In recent years, this approach to the growth of AlN with a high degree of crystallinity has advanced significantly, laying the foundation for substantial improvements in device performance. Successes include the fabrication of electrically injected deep-UV laser diodes and LEDs with high output power over long lifetimes.
Pioneers of AlN substrates produced by PVT include Hexatech Inc., which has introduced AlN substrates with diameters of up to 2 inches and threading dislocation densities below 1 x 104 cm-2. Hexatech is offering this material to selected scientific partners.
Another provider is Crystal IS, which demonstrated 4-inch AlN substrates with an 80 percent useable area earlier this year, and makes 3-inch AlN substrates for internal use.
For both these trailblazers, progress has been slow and hard won. To increase the diameter of the AlN crystals to the size they are today, engineers at these companies have devoted more than a decade to producing generation after generation of crystals, each with a larger diameter. Increases in size have come from dynamic growth of unfaceted regions. This enlargement, occurring at the beginning of crystal growth (cone-shaped), is restricted to small expansion angles.
Our team at IKZ – that’s the common shorthand for the Leibniz Institute for Crystal Growth in Berlin, Germany – has developed an approach that overcomes this restriction. PVT is also used to grow our bulk AlN crystals, produced with an inductively heated graphite setup (see Figure 1).
Figure 1. (a) A sketch of the setup, with four identical PVT reactors built at IKZ. (b) one shown here with opened housing for a better view of the heated setup. For more details see C. Hartmann et al. Cryst. Eng. Comm. 18 3488 (2016).
A foundation for our latest success is our expertise acquired through the growth of SiC boules by PVT. When moving from SiC to AlN growth we retained the reactor, the thermal insulation felt (carbon-bonded carbon fibre), and large parts of the graphite components. The growth temperature for both materials is around 2200 °C.
One key difference between the growth of AlN and SiC is the ambient atmosphere. For SiC, an argon atmosphere below 50 mbar is employed, while for AlN its N2 at a pressure of more than 400 mbar. Crucibles also differ, with graphite used for the growth of SiC, and tungsten or TaC used for AlN. Note that these two options are the only refractory materials capable of withstanding the reactive aluminium vapour under growth conditions.
To produce a high-purity AlN source – it contains less than 150 ppm wt oxygen – we start with a commercially available AlN powder and apply multiple sublimation–recrystallisation steps. During crystal growth, this source, located in the lower part of the crucible, decomposes gradually into aluminium and N2 vapor species. The aluminium vapour pressure in the crucible depends on the temperature, and typically ranges from 50 mbar to 150 mbar – one may imagine an aluminium ‘fog’ inside the N2 ambient. During growth, gaseous aluminium species diffuses along the concentration and temperature gradient through the nitrogen ambient in the growth space, before re-condensing on the AlN seed.
Seeding the growth
Recently, we have developed a seed holder design that positions the seed on a TaC pedestal. With this arrangement, crystals grow freely without contact to parasitic grains (see Figure 2). This design allows high radial thermal gradients – the driving force for diameter expansion.
Figure 2. Simplified sketch of the seed holder design: TaC pedestal and fully faceted grown crystal with an expansion angle of 45°. For more details see C. Hartmann et al. Appl. Phys. Express 16 075502 (2023).
With this new configuration we have realised far higher lateral growth rates. At a seed temperature of 2230 °C growth rates are as fast as around 200 µm/h in both the N-polar and the prismatic m directions, resulting in huge expansion angles of around 45° along the entire crystal length.
The crystal habit consists of the (000.1) N-polar top facet, the (10.10) prismatic m facets, and (101x) rhombohedral r facets. The full diameter spans the entire crystal length, ensuring that all cut c-plane wafers have the same (final crystal) diameter.
AlN seeds with a diameter of 8 mm and a threading dislocation density below 1 x 103 cm-2 provided the starting point for this work. These first-generation seeds were prepared from spontaneous nucleated AlN crystals. Using our seeded process, growth on them produced crystals with a length of 5-7 mm and a diameter between 18-24 mm. With just two to three steps, we magnified dimensions, producing AlN crystals with diameters of more than 30 mm, which are very suitable for preparing 1-inch substrates (see Figure 3). Based on our current findings, we can’t foresee any obstacles to a fast scale-up to industrially relevant diameters of up to 4 inches or more.