Inverse Fourier transform of unilateral exponential function - ifourier MATLAB

Hi everyone. How can I get the inverse Fourier tranform of unilateral exponential? I tried with the following code:

    close all
    clear all
    syms t f
    a=sym('a','positive');
    y=ifourier(1/(a + pi*f*2i),t)

the result is Y=(exp(-(a*t)/(2*pi))*(sign(t) + 1))/(4*pi). Instead I'd like to get Y=exp(-a*t)*heaviside(t).

Thank you for your time.