Nitrogen doping profiles in gallium phosphide grown by liquid phase epitaxy

The nitrogen doping process used in liquid phase epitaxial growth of gallium phosphide has been examined by investigating the rate of reaction of ammonia with gallium as a function of temperature and by measuring the nitrogen concentration in epitaxial layers using optical absorption. These investigations have shown that nitrogen doping is a kinetic process which is strongly dependent upon the diffusion rate of nitrogen through the growth melt. This gives rise to nitrogen doping gradients at fast cooling rates. It is shown that nitrogen doping at high levels is limited by (a) the rapidly increasing rate of reaction of gallium with ammonia with increasing temperature and (b) the decreasing solubility of nitrogen in solid gallium phosphide with reducing temperature. Growth outside a range of conditions established by these limiting factors gives rise to break-up of the epitaxial growth. A knowledge of these conditions has enabled smooth nitrogen doped epitaxial layers to be grown on a routine basis and these have been used to make monolithic diode structures having typical light outputs of 2500 nits from a visible area of 5.2 × 10^-4 cm² measured at a current at 20 mA.