Coalescence of GaP on V-Groove Si Substrates

With an increase of control over crystalline defects, metallorganic vapor phase epitaxy (MOVPE)-grown III-V-on-Si multijunction solar cells have seen rapid increases in efficiency in recent years [1, 2], pointing to a promising path to lower cost III-V solar cells. However, the cost of chemo-mechanical polishing the Si wafers to prepare them for epitaxy is high. The use of V-groove nanopatterns enables similar defect reduction to that achieved on planar wafers [3–5], but the nanopatterns can be fabricated with a low-cost process [6]. While V-grooves offer advantages over planar Si, they add complexity to the growth process. In particular, coalescence can cause the formation of threading dislocations [7], and the highly-directional growth conditions required for coalescence are unusual for MOVPE.<br/><br/>We observed that for optimized growth conditions (V/III=5,000 and T=800 C) two growth modes were possible, and the resulting morphology depended on the exact geometry of the SiNx cap used to cover the (001)-oriented Si at the tops of the grooves. For caps with a width &gt;100 nm, non-coalescing, nano ridge-like growth terminating in {111} facets was observed. For thinner caps, coalescence with an RMS roughness of 0.2 nm as measured by atomic force microscopy was observed. We will discuss possible mechanisms for this phenomenon, including the role of Si from the substrate surface.<br/>The dislocation dynamics of this system were studied with electron chan- neling contrast imaging (ECCI) and transmission electron microscopy (TEM). V-grooves do not block dislocation glide; ECCI measurements show misfit dis- locations greater than 10 μm long observed to continue perpendicularly across neighboring V-grooves. In addition, the TEM micrographs did not show evi- dence of sessile dislocations expected to result from coalescence-related dislo- cation formation [7]; instead, the elevated threading dislocation density (TDD) of 5 × 10⁷ cm^-2 likely is driven inefficient strain relaxation in the GaP due to non-optimized growth conditions rather than coalescence-related problems. The dislocation dynamics and morphological evolution of the coalescence of GaP on Si, possible mechanisms behind the observed phenomenons, and further dislo- cation mitigation strategies for these materials will be presented.<br/>References<br/>(1) Feifel, M. et al. Sol. RRL 2021, 5, 2000763.<br/>(2) Lepkowski, D. L. et al. Sol. Energy Mater. Sol. Cells 2021, 230, 111299.<br/>(3) Kunert, B. et al. Semicond. Sci. Technol. 2018, 33, 093002.<br/>(4) Li, Q.; Lau, K. M. Prog. Crys. Growth Char. Mater. 2017, 63, 105–120.<br/>(5) Saenz, T. E. et al. Crys. Growth Des. 2020, 20, 67456751.<br/>(6) Warren, E. L. et al. In Proc. WCPEC-7, 2018.<br/>(7) McMahon, W. E. et al. APL Mater. 2018, 6, 120903.