The Electrical Transport Properties Of Cr-SiO₂ And W-SiO₂ Nanogranular Films

Granular metal films are a new class of artificial materials,in which the metallic granules with size ranging from a few to hundreds of nanometers are embedded in insulating matrix.Many quantum effects,such as disorder,electronic correlations and quantum confinement effect,determine the electrical transport properties of the granular metals.At present,there are still many new physical phenomena and unsolved issues in granular metals.In this dissertation,a series of three-dimensional Crx(SiO₂)1-x and Wx(SiO₂)1-x nanogranular films were fabricated by the co-sputtering method,the microstructure and electrical transport properties for these films are studied systematically.Firstly,we study the microstructure and electrical transport properties of metallic Crx(SiO₂)l-x nanogranular films(0.67 ? x ? 0.90).The Cr-SiO₂ films are quite inhomogeneous:some Cr granules directly connect to others while some Cr granules with size?1-3 nm disperse in the SiO₂ dielectric matrix.For each film,the Hall coefficient RH varies linearly with the natural logarithm of temperature,i.e.,ARh ? InT,above?60 K,saturates at?60 K,and retains the saturating value below?60 K.The temperature dependence of RH can be explained by the electron-electron interaction theory in granular metals and originates from virtual diffusion of electrons through the metallic granules.For the conductivity ?,a robust law???(?)is observed from?50 down to 2 K.The behavior of the conductivity stems from the "Altshuler-Aronov" correction,whose influence on the Hall coefficient is not present in the Cr-SiO₂ films.Secondly,we study the microstructure and transport properties of Wx(SiO₂)1-x nanogranular films with x lying in the dielectric regime(0.28?x?0.38).For the insulating W-SiO₂ films,the size of the metallic granule varies from 1 to 2 nm,and the distribution is relatively homogeneous.It was found that the conductivity satisfies?(T)?exp[-(T₀/T)^1/2]in a wide temperature range,where To is the characteristic temperature.The upper bound temperature for the above expression holding decreases with decreasing x.The electrical transport properties of the W-SiO₂ films were compared and discussed in detail by using the variable range hopping conduction mechanism proposed by Efros and Shklovskii and the co-tunneling model proposed by Beloborodov.