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© 2007 IOP Publishing Ltd. The study was partially supported by the local government (Junta de Castilla y León) under Grant No VA018A06 and by the Spanish TEC2005 under Grant No 05101/MIC. The authors acknowledge CAI de Técnicas Físicas, CAI de Espectroscopía y Espectrometría and CAI de Microscopía y Citometría of the Universidad Complutense de Madrid for technical support. This work was made possible thanks to the AP2003-4434-FPU grant of the SpanishMEC and the contract TEC 2004-1237/MIC of the Spanish MCYT., Thin films of hafnium oxide have been deposited by the high-pressure reactive sputtering (HPRS) system. In this growth system the deposition pressure is around 1 mbar, three orders of magnitude higher than in the conventional ones, assuring that both reflected and sputtered particles reach the substrate with a low energy. The amorphous or polycrystalline structure is modified by adjusting the ratio of oxygen to argon of the sputtering gas. The electrical characteristics of both polycrystalline and amorphous films are compared. In all cases, the leakage current can be fitted to Poole-Frenkel emission. Amorphous films show the best characteristics in terms of capacitance-voltage behaviour, leakage current and interfacial state density, with conductance and flat-band voltage transients almost negligible., Spanish MEC, Spanish MCYT, Depto. de Estructura de la Materia, Física Térmica y Electrónica, Fac. de Ciencias Físicas, TRUE, pub