Ga-terminated GaN(0001) surface, covered with gallium metal is studied by density functional calculations within PBE version of GGA approximation, projector augmented waves method and periodic boundary conditions. Slab-vacuum model, rarely used for metal-semiconductor interface simulations, allowed us to vary electric field present in the semiconductor part by introduction of artificial acceptor state on the slab termination surface. We studied the system properties in function of the electric field, Fermi level or equivalently, the doping of the semiconductor bulk. The configurations used are 1x1 and 2x2 slabs with vacuum up to 30 Å to cancel the effect of periodicity. Metallic part of the structure was deposited and relaxed one by one Ga layers on top of GaN slab. The system is classical metal-semiconductor Schottky junction, but due to the low melting temperature of gallium, this work could be treated as the first step towards modelling of GaN and liquid phase of Ga interface. The work of Rosa and Nuegebauer [Phys. Rev. B 73, 205346 (2006)] where 1-2 gallium monolayer coverage of GaN was considered, is extended for several metal layers. The following structural and electronic parameters, customary used to describe metal-semiconductor interface, are calculated: structural parameters of Ga phase imposed by GaN (hexagonal ABAB with possible stacking faults) as compared to orthorhombic α-Ga phase, the most stable in the room temperature (few kelvins below melting point), interlayer distances of fully relaxed Ga bulk showing pronounced deviations (about 6.5%), Schottky barrier height (SBH, ~2.6 V for p-type or, adequately, ~0.8 V for n-type GaN), electric potential irregularities of Ga part (3-layer range order), projected density of states across the structure confirming fast fade-out of metallic states (two layers) inside the semiconductor, band edges of the semiconductor part, band structure projected onto interface plane (Γ-K-M). The possibility of emergence of the localized midgap states is discussed. The consequences of the obtained results are discussed, e.g., metallicity of the system (bandgap closure), linear dependence of SBH and energy levels on electric field. (The research supported by the EU within European Regional Development Fund, grant Innovative Economy POIG.01.01.02-00-008/08, and Polish National Science Center, Grant No. 2012/05/B/ST3/02516.)

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