Influence of boron on donor-acceptor pair recombination in type IIa HPHT diamonds

We report on the investigation of donor-acceptor pair (DAP) and free carrier recombination in HPHT IIa type diamonds and determination of boron concentration by differential transmittivity (DT) technique. Photoluminescence and photoluminescence excitation spectra were measured in 8-300 K temperature range and provided a broad (~ 0.67 eV) Gaussian DAP band which peaked at 2.2 eV at low temperatures, while above 200 K it sharply shifted to 2.5 eV and became more intense. Thermoluminescence measurements also demonstrated a similar tendency. This peculiarity was explained by DAP recombination between the nitrogen and the boron, the latter being in the ground and the excited states at low and high temperatures, respectively. A zero phonon line position coincided with the calculated one, when using nitrogen and boron activation energies. Scanning of DT across the sample at different delay times revealed the fast (200-500 ns) free carrier lifetime and the slow recovery time (of optically recharged boron to its initial state). The temperature dependence of the slow component decay time provided the boron activation energy of 360 meV. Saturation of the boron-related DT signal in the samples and the determined boron ionization cross section at 1064 nm (σ_B = 3.3 × 10 ^{- 17} cm²) provided the boron density in 10^14-16 cm^{- 3} range and revealed its strongly inhomogeneous distribution across the HPHT layers. The B density was found much lower than the density of nitrogen donors (~ 10¹⁷ cm^{- 3}), which were distributed in the layers much more homogeneously.