500 nm is formed by keeping the temperature at 850 u00b0 c and by introducing n 2 at a flow rate of 20 l min nh 3 at a flow rate of 10 l min tmg at a flow rate of 1 53 u00d7 10 u2212 4 mol min and tmi at a flow rate of 0 02 u00d7 10 u2212 4 mol min then the second clad layer 5 made of magnesium doped gan and having a film thickness of about 1000 nm is formed by keeping the temperature at 850 u00b0 c and by introducing n 2 at a flow rate of 20 l min nh 3 at a flow rate of 10 l min tmg at a flow rate of 1 12 u00d7 10 u2212 4 mol min and cp 2 mg at a flow rate of 2 u00d7 10 u2212 4 mol min the concentration of magnesium in the second clad layer 5 is 1 u00d7 10 20 cm 3 the second clad layer 5 is a high resistance semi insulator electron beams are thereafter irradiated uniformly onto the second clad layer 5 using an electron beam irradiation system the electron beam irradiating conditions are an acceleration voltage of about 10 kv a sample current of 1 u03bca a beam moving velocity of 0 2 mm sec a beam diameter of 60 u03bcm and a vacuum degree of 5 0 u00d7 10 u2212 5 torr the second clad layer 5 is transformed into a.