Technology

In a nutshell, Crystal IS substrates:

• are larger in diameter than any other AlN single crystal in the world,
• use innovative, crucible technology that can withstand hundreds of hours of continuous crystal growth,
• have the highest thermal conductivity ever measured for AlN,
• have higher resistivity than the best SiC (and more consistently uniform),
• have much lower defect densities,
• use cost-effective “polishing” techniques in preparation for epitaxial growth,
• are grown with excellent crystal growth rates exceeding all previous nitride semiconductor records (close to 1mm/h), and
• use high purity AlN starting material to deliver good crystal growth.

The Details

Semiconductor nitrides are being used or considered for making blue-light emitting diodes (LEDs), high-density optical data storage, high power, and high temperature devices.

Currently single-crystal substrates of sapphire are used to grow epitaxial layers of GaN and AlxGa1-xN alloys. Many manufacturers have chosen sapphire because high-quality, inexpensive substrates are not commercially viable. However sapphire is not ideal to satisfy commercial applications. The following is an explanation of the weaknesses of sapphire substrates.

Nitride layers grown on sapphire suffer from:

• high defect density (due to the large lattice mismatch between sapphire and GaN),
• large thermal expansion mismatch (which causes cracking of the device layers), and
• poor thermal conductivity.

Single-crystal substrates are used to provide a template on which the appropriate layers of nitride semiconductors can be grown to fabricate devices.

A good substrate needs to:

• have mechanical strength,
• closely match the crystal structure and lattice parameter,
• be chemically compatible with the device layers that are to be grown on it,
• have high thermal conductivity (to carry away waste heat produced by the active device), and
• have a close thermal expansion match.

For many currently proposed device structures (which are Ga-rich), gallium nitride (GaN) would be an ideal substrate. This is because the chemical compatibility, lattice match, and thermal expansion would all be exactly matched to the epitaxial layers. The problem is: GaN substrates are very expensive to produce.

Substrates of AlN, however, are more attractive because:

• the crystal structure and chemical compatibility of AlN with GaN and AlxGa1-xN alloys is ideal, and
• while the thermal conductivity of AlN is a factor 10 higher than sapphire and almost double that of GaN, the difference in thermal expansion between AlN and GaN between 1000°C and room temperature is almost negligible.

Additionally:

• the lattice mismatch between GaN and AlN is much smaller than that between GaN and sapphire and gets better with higher Al concentration in the device layer, and
• bulk crystals of AlN allow the possibility to cut other surfaces for epitaxial growth.

Accordingly, Crystal IS is concentrating on developing the commercialization of AlN substrates.

Future markets for AlN substrates include:

• High temperature electronic and opto-electronic devices
• High power microwave devices
• High power RF
• UV optical detectors
• UV light emitting diodes
• Blue / UV solid-state lasers. These solid-state lasers have projected applications in:
  • High-density optical data storage (ex. Compact Disc (CD), Digital Versatile Disc (DVD)
  • Medical applications
  • Projection display applications
  • Surface acoustic wave (SAW) devices for wireless communication

The current production of material for the above applications is still beyond the cost-sensitive buyer. Crystal IS’ innovative technology makes the applications affordable and within reach of mass commercialization.

Success in new substrates could mean this technology is extended commercially to bioterrorism detection, satellite communication, data storage and analytical biotechnology devices.

Read more about the applications of AlN substrates.