One of the main problems in the manufacture of GaN-based devices is the poor quality of device layers caused by the use of foreign substrates such as silicon carbide (SiC), sapphire (Al2O3) and silicon (Si). The substantial mismatch of the crystal lattices of the substrate and the epitaxial layer leads to the appearance in the active layers of the device structure of dislocations, the density of which reaches 108-109 cm-2. This leads to a deterioration in the parameters of the epitaxial layers:
- dislocations provoke the appearance of macrodefects and inhomogeneities in the composition of quantum-size layers;
- the thermal conductivity decreases, which makes it difficult to remove heat from the active structure;
- the mobility of carriers decreases, including in HEMT structures;
- the leakage currents of Schottky barriers, p-n junctions, and compensated semi-insulating layers increase;
- the breakdown voltage decreases;
- degradation of device structures is accelerated.
Also, the use of foreign substrates leads to bending and cracking of the wafers, due to the difference in the temperature coefficients of expansion of the materials of the epitaxial layer and the substrate material. The use of a bulk defect-free GaN substrate solves all these problems:
- the dislocation density in the epitaxial layer on the GaN substrate does not exceed the dislocation density in the substrate;
- there is no bending and cracking caused by the mismatch of the coefficients of thermal expansion;
- The thermal conductivity of GaN is comparable with the thermal conductivity of silicon carbide, and there is no interface layer between the epitaxial layer and the substrate, which hinders heat removal;
- The process of epitaxy of the device structure is simplified.
Together, this allows the creation of device structures, superior in their parameters to devices grown on foreign substrates.
Substrates from bulk GaN have not yet displaced foreign substrates from the market, mainly because of their high cost, due to the complexity of the processes of growth of bulk crystals of gallium nitride. It is possible to significantly reduce the cost price if we move from using bulk GaN substrates to thin GaN films made from bulk crystals and preserving their crystal perfection.
We implemented all the critical steps of the proposed technology and demonstrated thin-film separation from a bulk GaN crystal by Laser Stealth Slicing, and based on it a Substrate-Free Technology.