Effects of Substrate Temperature on Nanomechanical Properties of Pulsed Laser Deposited Bi₂Te₃ Films

The correlations among microstructure, surface morphology, hardness, and elastic modulus of Bi₂Te₃ thin films deposited on c-plane sapphire substrates by pulsed laser deposition are investigated. X-ray diffraction (XRD) and transmission electron microscopy are used to characterize the microstructures of the Bi₂Te₃ thin films. The XRD analyses revealed that the Bi₂Te₃ thin films were highly (00l)-oriented and exhibited progressively improved crystallinity when the substrate temperature (T_S) increased. The hardness and elastic modulus of the Bi₂Te₃ thin films determined by nanoindentation operated with the continuous contact stiffness measurement (CSM) mode are both substantially larger than those reported for bulk samples, albeit both decrease monotonically with increasing crystallite size and follow the Hall—Petch relation closely. Moreover, the Berkovich nanoindentation-induced crack exhibited trans-granular cracking behaviors for all films investigated. The fracture toughness was significantly higher for films deposited at the lower T_S; meanwhile, the fracture energy was almost the same when the crystallite size was suppressed, which indicated a prominent role of grain boundary in governing the deformation characteristics of the present Bi₂Te₃ films.