Sapphire growth techniques

However, such comparison is a meaningless exercise. A comparison of silicon, sapphire, GaAs, or other established materials, rather than other SiC growth techniques, should occur instead. The blue and white LEDs can, for instance, be made on either sapphire or SiC substrates. From a performance point of view, there appear to be advantages with the SiC substrate but the substrate cost is lower on sapphire, therefore making it the most-used substrate. Granted, once larger substrates of SiC are made available, it is doubtful that the LED cost will be lower on sapphire. 

Although first deposition experiments of nonpolar GaN films on r-plane sapphire were performed as early as in 1987 [5], the defect structure in wurtzite GaN and related compounds has been mainly studied in the past focusing on films oriented along the hexagonal [0001] axis (c-axis). Growth techniques such as molecular beam epitaxy (MBE) [1], metal organic vapor phase epitaxy (MOVPE) [6], and hydride vapor phase epitaxy (HVPE) [7,8] are established for polar as well as nonpolar GaN growth. The structural quality of a-plane GaN... 

Fig. 1. The Verneuil technique, or flame-fusion growth, as used for synthetic mby and sapphire.

A continuous monocrystalline sapphire (A1203) fibre has been prepared as single-crystal fibres by LaBelle and Mlavsky using a modified czochralski puller and radio frequency heating. The technique adopted in this method is called edge-defined film-fed growth (EFG) [18-22], Figure 3.4 shows a schematic of the EFG method. 

There are reports [24,25] that it is possible to decrease the MOVPE growth temperature to 550°C by using the nitridated surface of sapphire without the subsequent buffer layer. Tokuda et al [24] reported the GaN 00.2 FWHM dependence on the nitridation time of sapphire at 940°C in a plasma-excited N2 atmosphere. Using this technique, the best MOVPE GaN layers of FWHM less than 20 arc min were grown on a surface nitridated for 10 - 20 min. In a similar experiment, Kim et al [25] obtained GaN of the best 00.2 FWHM of 0.4 degrees for 10 min nitridation time. 

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