Prove that Gamma function is divergent for $\Re(z)\leq 0$

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How can I prove by definition that the Gamma function $$ \Gamma(z):=\int_0^{\infty}e^{-t}t^{z-1} \ dt $$ is divergent for $\Re(z)\leq 0$. I can prove it for real $z\leq 0$ only. Thanks!