Characterization of InGaAsN solar-cell structures on Ge substrates

A GaAs/InGaAsN p-i-n solar-cell structure on a Ge substrate has been fabricated intending incorporation in high-efficiency InGaP/InGaAs/InGaAsN/Ge four-junction-structure solar cells. An improved InGaAsN MOVPE growth process is adopted in which the Ge(001) vicinal substrates and a two-step-growth GaAs buffer layer combined with post-growth rapid thermal annealing (RTA) are the keys. With a prototype p-i-n photo-cell, the photovoltaic effect and a photo-current edge shift to the longer wavelength of InGaAsN with 1 eV bandgap are demonstrated. After RTA, however, the I-V characteristics are degraded with a significant reverse-bias current leakage. Electrical measurements showed a conduction-type conversion from n-type to p-type and significant increase of the hole concentration. The N-H-VGa complexes acting as shallow acceptors formed through RTA may be the most feasible cause of the drastic change of the electrical property of the InGaAsN film after RTA. (copyright 2014 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)