Determination of Nitrogen Concentration in GaP Epitaxial Layers by Two Independent Methods

The nitrogen concentration in GaP is determined by optical absorption in the A-line at T = 77 K. The concentration of the isolated nitrogen atoms is given for T = 77 K by the modified LIGHTOWLERS' relation N_A] cm⁻³ = 8.2 · 10¹⁴ phα_max, where α is measured in cm⁻³, h in meV. p is a dimensionless line shape factor. It is shown that at higher N concentrations considering only the A-line absorption the impurity density is underestimated because the nitrogen atoms included in NN_i pairs give no contribution to the absorption. The measurements have been made in the range from N] = 5 · 10¹⁶…︁ 10¹⁹ cm⁻³ in layers grown by vapour phase epitaxy. The results are compared with nitrogen concentrations obtained by precision lattice parameter measurements. The change of the lattice parameter is calculated using VEGARD's law. The good agreement between the nitrogen densities obtained by two different independent methods reveals (i) that the LIGHTOWLERS' calibration factor is valid also at higher N-concentrations and (ii) that the nitrogen atoms are predominantly incorporated into P-lattice sites.