Amorphous Thin Films
Amorphous materials are solids lacking in long range orders as is found in crystalline materials. This lack of long range order makes the study of such materials difficult, and so historically they have recieved less attention than ordered materials. However, amorphous solids can have a number of attractive properties lacking in crystalline materials. An example is a complete lack of grain boundaries, which can lead to excellent mechanical properties. A wide range of compositions is also possible in amorphous solids beyond those allowed for crystals where incompatible crystal structures may inhibit the formation of long range order. This is espeically true in thin films where the composition can be carefully controlled through deposition conditions. However, there are few approaches for the study of such materials, especially as regards the local and intermediate range structural ordering. Total scattering of x-rays or neutrons has been used for pair distribution function analysis (PDF) in bulk materials. In collaboration with Apurva Mehta and Badri Shyam, we are developing tools to extend this approach to thin films and surfaces using a grazing incidence measurement geometry.
Crystallization of Amorphous VO2 Films:
Vanadia films deposited at low temperature on glass substrates form amorphous thin films. Using grazing incidence PDF measurments, we were able to observe subtle structural differences between films deposited with different laser pulse repetition rates.
When these films are annealed, they crystallize into different polymorphs of VO2. The films crystallized into different phases, indicative that the local structure has an influence on polymorph formation. This may be a novel approach to forming otherwise difficult to synthesize crystalline polymorphs in thin films.