  We have performed local tunneling spectroscopy on high quality
Mg$_{1-x}$Al$_x$B$_2$ single crystals by means of Variable Temperature Scanning
Tunneling Spectroscopy (STS) in magnetic field up to 3 Tesla. Single gap
conductance spectra due to c-axis tunneling were extensively measured, probing
different amplitudes of the three-dimensional $\Delta_\pi$ as a function of Al
content. Temperature and magnetic field dependences of the conductance spectra
were studied in S-I-N configuration: the effect of the doping resulted in a
monotonous reduction of the locally measured $T_C$ down to 24K for x=0.2. On
the other hand, we have found that the gap amplitude shows a maximum value
$\Delta_\pi= 2.3$ meV for x=0.1, while the $\Delta_\pi / T_C$ ratio increases
monotonously with doping. The locally measured upper critical field was found
to be strongly related to the gap amplitude, showing the maximum value
$H_{c2}\simeq3T$ for x=0.1 substituted samples. For this Al concentration the
data revealed some spatial inhomogeneity in the distribution of $\Delta_\pi$ on
nanometer scale.
