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By combining transport measurements and optical second harmonic generation, we have investigated heterostructures formed between crystalline thin films of LaAlO3, with varyingstoichiometry and TiO2-terminated SrTiO3(001) substrates. Optical second harmonic generationdirectly probes the polarity of these heterostructures, thus complementing the transport data. Thestoichiometry and the growth temperature are found to be critical parameters for controlling boththe interfacial conductivity and the heterostructure polarity. In agreement with the previous work,all of the samples display an insulator-to-metal transition in the Al-reach region, with the conductivityfirst increasing and then saturating at the highest Al/La ratios. The second harmonic signalalso increases as a function of the Al/La ratio, but, at the highest growth temperature, it does notsaturate. This unusual behavior is consistent with the formation of an ordered structure of defectdipoles in the LaAlO3 film caused by the off-centering of excess Al atoms in agreement with thetheory.