Epitaxial wafer materials are widely used as starting materials in semiconductor device fabrication. The presence of defects in such wafer materials can seriously affect the subsequent device performance. For example, GaN and its related compounds InGaN and AlGaN are widely used in the fabrication of short-wavelength semiconductor laser diodes. The performance of such laser diodes is seriously degraded by the presence of threading dislocations, which thread vertically through the epitaxial layers. Similar defects are found in other material systems, for example, when GaAs is grown on SiGe/Si. A reduced dislocation density on the epitaxial wafer materials is therefore desired. It shall be understood in the following descriptions that GaN shall also refer to its compounds (In)(Al)(Ga)N, and may be p-type, n-type or undoped.
A previous approach to reducing the defect density of epitaxial wafer materials is Epitaxial Layer Over-Growth (ELOG) described in US patent application US 2002/0022290. In this approach, narrow stripes of silicon dioxide are patterned on a GaN buffer layer. GaN growth is then restarted until the SiO2 stripes are covered and a planar surface is achieved. The defects under the stripes are blanked out and epitaxial material above the stripes apparently has a lower defect density than the material grown between the stripes. The material above the SiO2 is found to be of high quality, but the material between the stripes is unchanged, and so it appears that multiple steps of ELOG need to be made to create large areas of good quality material. Defect density in efficient ELOG growth is reduced from 1010 cm−2 in standard GaN/Sapphire growth, to 108 cm−2 in single step ELOG, or to 5×105 cm−2 after multiple steps of ELOG. A defect density of 5×105 cm−2 corresponds to 1 defect per 14 μm×14 μm square area. Therefore, the size of a defect-free area is still small in comparison to the 50 mm diameter wafer area available for device fabrication. Another problem is that this approach requires considerable additional effort in processing and regrowth, requiring over 100 μm of epitaxial growth for best results.
A second approach, described in US patent application US 2002/0005593 is to grow standard GaN epitaxial layers at high temperature (1000° C.), then deposit a thin layer of GaN at a lower temperature (700-900° C.), then resume growth at the high temperature (1000° C.). It is claimed that this prevents defects from propagating vertically, and reduces the defect density from >1010 cm−2 to 4×107 cm−2. This approach suffers from insufficient removal of defects.
A third approach is the direct production of GaN substrates from liquid gallium, and nitrogen at very high pressure (45,000 bar) (by Unipress in Poland). This approach suffers from the use of very highly specialized and expensive equipment, and the production of rather small (˜1 cm2) GaN crystals.
The present invention provides systems and methods that, at least in part, address these and other issues.