Effects of Miscut Substrates on Electrical Conductivity Across InP and GaAs Wafer-Bonded Structures

Direct wafer bonding of III-V materials is of great interest for the fabrication of next generation high efficiency solar cells. Traditional wafer bonding methods of III-V materials lead to highly unstable oxide interfaces and poor electrical properties. To resolve these issues, elevated annealing temperatures (>600 °C), long times, and large compressive forces are employed to break up this interfacial oxide layer, so that there is better contact between the III-V bonding interfaces. These harsh processing conditions ultimately limit the achievable efficiencies, due to dopant diffusion and residual thermal strains in processed devices. Sulfur surface treatment prior to bonding was found to reduce the inclusion unfavorable oxides at the interface, reducing the post-bonding thermal and compressive force requirements, and promoting the ability to bond large (100 mm diameter) wafers..

                Various combinations of sulfur treated n-GaAs and n-InP directed bonded structures were measured to investigate the effects of offcut angles on the electrical conductivity. The impact of the relative offcut angle on the electrical properties is an  new unexplored area which is of important issuece, as misorientated substrates are typically used for the growth of III-V epitaxial layers in the solar industry to avoid anti-phase boundaries. It is observed in both the GaAs and InP bonded structures, that out-of-plane relative surface orientations between the (001) planes that are greater than 4° exhibit non-ohmic behavior. A theoretical model that describes the electron tunneling across a grain boundary between semiconductor bicrystals is used to represent the bonded interface and estimate the barrier conduction height. Fitting the zero-bias conductance over a range of temperatures reveals an increase in barrier heights for samples with greater than 4° misoriented bonded pairs, correspondingly the interface resistance at room temperature increases.  These results demonstrate that the out-of-plane relative surface misorientation is the critical parameter to be monitored in order to achieve superior electrical conductivity in direct-bonded multijunction solar applications.  In addition, differences between GaAs-GaAs bonded structures and InP-InP bonded structures indicate that the barrier for GaAs-GaAs is significantly greater than for InP-InP.  The impact of the miscut itself, however, is similar in both cases, showing that both the chemical nature and the structural interface properties play roles in trans-interface electrical conduction.