SOLID-LIQUID REACTION-MECHANISMS IN THE FORMATION OF HIGH-QUALITY CUINSE2 BY THE STACKED ELEMENTAL LAYER (SEL) TECHNIQUE

The formation of CuInSe2 thin films from Cu-In-Se stacked elemental layers was performed to evaluate the reaction mechanism involved using the Stacked Elemental Layers (SEL) technique. The nature of the deposited single layer of Cu, In and Se, and the reactions involved in the binary compounds of Cu-Se, In-Se, and Cu-In after annealing at different temperatures was used to provide evidence for the formation of CuInSe2 from the ternary compound of Cu-In-Se using XRD, EDX and SEM. Studies of the time progressive reaction at a temperature of 450 degrees C showed that CuInSe2 formation starts immediately and a single phase chalcopyrite film was formed after a reaction time of less than 60 s. By varying the temperature for a constant annealing time of 60 s, the formation of CuInSe2 was seen to begin at 200 degrees C with a film orientation in the 112 plane. An exothermic reaction was found to take place in the films at this temperature due to the reaction of beta-In2Se3 with liquid Se, resulting in a large amount of Se loss from the film. This is the cause of the film delamination which has been a major problem identified in SEL films produced from multistack structures. Film delamination was eliminated by using an alternative processing method involving annealing the initial stack at low temperature of about 200 degrees C before deposition of the next stacks, followed by a final annealing at a higher temperature of 450 degrees C. This initial annealing is important in order to suppress the energetic reaction species and to form thermodynamically stable beta-In2Se3 and Cu2Se before forming a homogeneous layer of CuInSe2 suitable for device fabrication.