The regrowth technique of highly doped n-type GaN layers is reliable and effective for lowering the ohmic contact resistance. The interface between metal contacts with Ti/Al/Ti/Au and regrown n+-GaN/GaN layers were analyzed in detail with transmission electron microscopy. During the annealing process, Ti metals and N atoms diffusing from GaN layers formed TiN epitaxial layers between metal alloys and n+-GaN layers. The orientational relationship between GaN and TiN was [1 0 0]GaN//[−1 1 0]TiN verified by nano-beam diffraction. Al atoms diffused through the GaN layers and formed thin AlGaN phase. Al content was confirmed as 60% by high-resolution transmission electron microscopy images. Electron energy loss spectroscopy showed that Si dopants were confined within n+-GaN layers. These results show that in regrowth technique both TiN layers and Si dopants affect the contact properties because the formation of TiN layers can induce nitrogen vacancies from GaN, while Si-doped GaN layers can enhance the tunneling effect through the metal contacts resulting in reduced contact resistance. Copyright © 2011 John Wiley & Sons, Ltd.