(Invited) Dilute-Nitride-Antimonide Materials Grown by MOVPE for Multi-Junction Solar Cell Application

Multinary compounds, such as quaternary or quinternary materials, are attractive for solar cell implementation, since the fourth and fifth element in the alloy permits an additional degree of freedom for tailoring the energy band structure (i.e. bandgap and offsets), while fixing the lattice parameter. We have investigated the growth by metalorganic vapor phase epitaxy (MOVPE) of multinary (four- and five- element) dilute-nitride-antimonide materials on GaAs substrates. Bulk films of dilute-nitride-antimonide materials with a narrow energy bandgap (E_g ~ 1.0-1.2 eV), InGaAsN, GaAsSbN, and InGaAsSbN, are attractive for multi-junction solar cell applications owing to the ease of bandgap tuning and lattice-matching while requiring only a small amount of N.

Dilute-nitride materials grown by molecular beam epitaxy (MBE) generally have very low unintentional background carrier concentrations (<1x10¹⁵ cm^-3). By contrast, a high background carbon concentration, which correlates with poor luminescence properties and short minority carrier diffusion length, is a challenging issue for MOVPE-grown dilute-nitride-antimonide materials. In the MOVPE growth of dilute-nitride-antimonide materials, the background carbon concentration is highly dependent on the gas-phase growth conditions and selection of the specific metalorganic sources. Sb-containing materials are found to exhibit an order of magnitude higher background carbon concentration than dilute-nitride films without Sb. We have employed an unconventional antimony precursor, TrisSb, which results in a significant decrease in the carbon background levels in (In)GaAsSbN materials. Employing relatively high growth temperatures (> 600 ^oC) also leads to a further reduction of the background carbon impurity concentration. However, the presence of Sb is found to also significantly inhibit N incorporation, making it challenging to achieve (In)GaAsSbN grown at 600 ^oC which contains sufficient N for producing a 1 eV band gap energy. The lowest background carbon concentration (~ 5 × 10¹⁶ cm^-3) is observed in dilute-nitride materials grown at high temperature which do not contain Sb (i.e. InGaAsN). An increased depletion region width significantly improves the solar cell performance over that found from dilute-nitride cells grown at lower growth temperatures (~525^oC). The device performance of the solar cells with the low carbon background InGaAsN base region exhibit short-circuit current density, open-circuit voltage, fill factor, and efficiency values of 26.05 mA/cm², 0.67 V, 75.85 %, and 13.2 %, with anti-reflecting coating (ARC), respectively. This measured short-circuit current density value is sufficient for the current-matching condition (~ 15 mA/cm²) of a triple-junction solar cell on Ge substrate. These results demonstrate MOVPE-grown cells are viable with performance comparable to solar cell structures employing a similar band gap dilute-nitride material grown by MBE.