Lamellar GeP thin films: the missing link between bulk GeP and 2D-GeP
Résumé
We investigate the growth, the microstructure and the thermal stability of Ge:P thin films prepared by co-evaporation of Ge and P in ultra-high vacuum. Ex-situ annealing of Ge:P thin films containing up to 45 at.% of P at temperatures close to 500°C leads to the formation of GeP, which crystallizes in a monoclinic structure with space group C2/m. Microstructural investigations show that GeP thin films are polycrystalline with typical grain sizes in the micrometer range. High resolution imaging gives clear evidence that the GeP films are lamellar with an atomic structure in good agreement with a monoclinic structure. When annealing at temperatures larger than 530°C, only pure crystalline Ge remains in the films. Thermal desorption spectroscopy measurements show that P desorption occurs for annealing temperatures above 500°C. For annealing temperatures larger than 530°C, GeP is decomposed and P atoms diffuse towards the surface prior to effusion. A detailed analysis based on the theory of Eyring allows us to determine both the activation energy for P diffusion as well as the P diffusion coefficient in Ge:P thin films