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The Z 1/2 defect, a deep acceptor level in high purity semi-insulating (HPSI) 4H-SiC, plays an important role in optoelectronic properties, particularly in below bandgap photon absorption and carrier recombination processes. Its concentration is highly dependent upon high temperature annealing. To study the effect of high temperature annealing on the properties of HPSI 4H-SiC, vertical-type photoconductive semiconductor switches were fabricated on two types of substrates, where one was non-annealed (nPCSS) and the other was annealed at 1500°C for 15 min (aPCSS). A 532-nm pulsed laser was utilized to excite the PCSSs from the side facet. The high temperature annealing reduced the Z 1/2 defect concentration in the aPCSS, which led to fewer photo-generated carriers in the aPCSS compared with nPCSS. The optoelectronic properties of the aPCSS exhibited much stronger dependence on the excitation optical energy and bias voltage than those of the nPCSS. The falling time in the aPCSS increases as the optical energy goes higher than 2 mJ. At the constant optical excitation energy of 2 mJ, the falling time in the aPCSS increases as the bias voltage increases, whereas that in the nPCSS stays almost constant.