Enhanced cathodoluminescence from nano-indentations in MgO

Cathodoluminescence (CL) imaging in the wavelength range of 380 to 800 nm of nano-indents in MgO crystals made with a Berkovich diamond triangular pyramid has been carried out using a scanning electron microscope. It is shown that the centres of nano-indents of estimated residual depths in the range of about 570 to 325 nm do luminesce fairly strongly and the spatial resolution of the image appears to be better than 1 mu m as revealed by the structure within the CL image. Centres of larger indents of residual depths greater than about 1300 nm do not show luminescence. It is suggested that most of the CL from the indentation centres is due to F and F+ colour centres produced by plastic flow, with emission peaks at 525 and 375 nm respectively. Furthermore, it is also suggested that relative number density of F+ centres increases with increasing plastic strain, which in turn increases with increasing indentation size. Therefore, beyond some critical indentation size, most of the CL from the indentation centre will be due to the F+ centres and. as its luminescence is beyond the detection sensitivity of the CL system, the Indentation centre shows an absence of CL.