The progress in blue laser diodes is still limited by the lack of high quality, large and cheap gallium nitride wafers. One of the main goal of nitride community is to have GaN single crystals in shape of real ingots, ready to slice them to form of two or three inches substrates about 300 mm thick. However due to extreme melting conditions of GaN (6 GPa of N₂ and 2497 K [1]), this nitride cannot be grown from its stoichiometric melt by the Czochralski or Bridgman methods commonly used for semiconductors “cousins” as Si or GaAs. Therefore, the GaN crystals have to be grown by ways allowing lower pressures and temperatures.

There are some promising methods to obtain GaN substrates. First of them is Hydride Vapor Phase Epitaxy (HVPE) where GaN is deposited on foreign substrates (sapphire, GaAs, SiC) at temperatures about 1323 K and at ambient pressure. The foreign substrate is removed from the sample by etching or other lift off technique and then a large diameter free-standing GaN wafer may be obtained. The big advantage of this method is relatively fast growth rate in c-direction, exceeding 100 mm/h. The best quality free standing GaN crystals have been grown in Sumitomo Electric Industries Ltd. using A-DEEP technology (Advanced Dislocation Elimination by the Epitaxial growth with inverse pyramidal Pits) by deposition of GaN on stripe or round shape patterned GaAs wafers [2]. High quality laser structures have been grown on these substrates by MOCVD and MBE methods at Sony, NEC and Sharp Corporations. The present Nichia’s lasers structures are grown exclusively on their HVPE free-standing substrates.

Gallium nitride can be also grown using the Na flux method at temperatures from 973 K to 1273 K and nitrogen pressure up to 5 MPa. This method is beeing developed in Tohoku University [3] and Osaka University [4] and yields bulk GaN single crystals with a size of few millimeters and defect density of order of 10⁴ cm^-2. Various flux composition like Ca-Na and Li-Na have been also studied. Recently, the liquid phase epitaxy (LPE) technique applied to the Ca-Na flux system has been reported. 1 mm GaN thick layer with the dislocation density of order of 10⁴ cm^-2 has been deposited on MOCVD sapphire/GaN template [5]. None device or other epitaxial structure grown on the crystals has been ever reported in the literature.

In this paper the present status of the GaN growth by High Pressure Solution (HPS) method and the combination of HPS and HVPE methods will be presented. The challenges and difficulties on the road to obtain 2 inch low defect density GaN substrates will be discussed. Up to now the spontaneous high pressure solution growth of GaN results only in crystals having habit of hexagonal platelets of surface area of 3 cm² or needles with length up to 1 cm. The platelets with dislocation density 10² cm^-2 have been used with success as substrates for lasers diodes. Recently, the platelets and needles have been used also as seeds for the HVPE growth. On the other hand the LPE technique under pressure with GaN/sapphire templates, patterned GaN/sapphire templates and free standing HVPE as seeds has been examined and developed. These results will be described in this paper in details.

[1] W. Utsumi et al., Nature Materials, Vol. 2, November (2003), 735

[2] K. Motoki, reported on 4^th International Workshop on Bulk Nitride Semiconductors, Makino, Japan, 2006

[3] T. Yamada et al., J. Cryst. Growth 286 (2006) 494

[4] F. Kawamura et al., Jpn. J. Appl. Phys. 42 (2003) L729

[5] Y. Mori, reported on 4^th International Workshop on Bulk Nitride Semiconductors, Makino, Japan, 2006

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