Role of offcut and spall direction in surface morphology of spalled (110) GaAs, The

Abstract

III-V materials boast some of the highest solar cell conversion efficiencies, but their entry into the terrestrial PV market is hindered by high substrate cost. Controlled spalling of substrates provides a path towards substrate reuse with potential to reduce manufacturing cost. Controlled spalling works by propagating a sub-surface crack, parallel to the substrate surface, by applying an external force to a sub-critically stressed overlayer deposited on the surface. One of the limiting factors of controlled spalling is the surface quality of the spalled substrate, which impacts reusability, regrowth, and device performance. (100)-oriented GaAs spalling results in large facets due to cleavage that is restricted to {110} and other low-energy planes. Spalling of cleavage-plane-oriented substrates may produce smooth spalled surfaces. This work develops a method for controlled spalling of 50 mm-diameter, (110)-oriented GaAs wafers by first evaluating alternative crack initiation methods on (100) Ge and then porting the lessons learned to (110) GaAs. Methods are evaluated for spall reliability, process compatibility, and spalled surface quality. A variety of methods work for Ge. A photoresist edge-adhesion demoter was chosen for (110) GaAs, enabling reproducible controlled spalling at wafer scale. Once controlled spalling was established, the influence of substrate offcut and spalling direction were varied and spall morphology was characterized. Spalled (110) GaAs results in a sub-micrometer step-terrace morphology linked to the restricted cleavage system. The size and shape of steps and terraces can be partially controlled by changing substrate orientation and spall direction. Terraces have sub-nanometer roughness, suggesting single-plane cleavage on (110). The richness of surface morphologies with sub-micrometer steps suggests there is ample room for optimization of spalling and subsequent epitaxial growth to enable (110) GaAs as a lower-cost III-V substrate.