Ion-induced electron emission as a means of studying energy- and angle-dependent compositional changes of solids bombarded with reactive ions - II. Nitrogen bombardment of silicon

Target current measurements were used to determine ion-induced electron yie lds gamma of silicon bombarded with 2-10 keV N-2(+) ions at impact angles t heta between 0 degrees (normal incidence) and 84 degrees, both for sputter- cleaned samples and under conditions of full loading with nitrogen. At norm al incidence, the electron yields of nitrogen saturated samples gamma(0)(sa t) were much higher than the yields gamma(0)(0) of clean samples. The ratio s r(0)=gamma(0)(sat)/gamma(0)(0) decreased with increasing energy, from sim ilar to 9 at 2 keV to similar to 6 at 10 keV. This energy dependence of r(0 ) is attributed to preferential loss of nitrogen. With increasing impact an gle, the yields of nitrogen-saturated samples first increased and then decr eased rather rapidly at some critical angle theta(c). After having passed t hrough a minimum between 50 degrees (10 keV) and 70 degrees (2 keV) the sta tionary yields increased in a manner similar to the yields of clean silicon . Compared with O-2(+) bombardment, several important differences were obse rved with primary N-2(+) ions. (i) The fall-off in electron yield occurred at larger angles the higher the beam energy, 25-27 degrees at 2 keV and 30- 32 degrees at 10 keV, the half-width of the fall-off being independent of t he energy. (ii) Even at the most oblique impact angles the stationary yield s were significantly higher than with clean silicon (12-26% higher at 80 de grees). (iii) There was no correlation of theta(c) with the angles at which the sputtering yield equals the silicon-to-nitrogen ratio of Si3N4, i.e. Y = 3/4 Si atoms/N atom. (iv) At oblique N-2(+) incidence, i.e. between simi lar to 38 and similar to 75 degrees, ripples started to grow at rather smal l depths of erosion, as low as 30 nm at 5 keV (topography of crater bottoms investigated by atomic force microscopy). Ripple formation strongly enhanc es the electron yields. The results are discussed with reference to the ver y low mobility of nitrogen in Si3N4 which allows 'supersaturation' of the s ample within the range of the implanted ions. (C) 1999 Elsevier Science B.V . All rights reserved.