At present several methods are used to obtain GaN substrates for optoelectronic. The high nitrogen pressure solution growth (HNPSG) is characterized by its ability to provide very high crystallographic quality crystal substrates. The typical dislocation density is of order of 10² cm^-2, which fulfills industrial standards.

A considerable drawback of the method is the limited size of the GaN crystals. The routinely obtained GaN crystal are about 1 cm in linear dimensions and about 100-00 micron in thickness. Therefore further studies are necessary to develop the method to increase the size and also the efficiency of the HNPSG method.

The investigation of the method requires reliable knowledge of the equilibrium and transport properties of nitrogen at very high pressure and temperature. Recent works reported on ab initio and MD studies of nitrogen properties such as nitrogen intermolecular potential and equation of state [1,2]. In this paper we report on the preliminary results on nitrogen viscosity at the pressures up to 20 kbar and the temperatures up to 2000K.

[1] Krukowski S, Strak P, Journal-of-Chemical-Physics. 7 April 2006; 124(13): 134501-1-9

[2] Strak P, Krukowski S, Journal-of-Chemical-Physics. - accepted (2007)