High material-efficiency MOVPE of GaAs without degradation of photovoltaic performances

The upper limit of GaAs growth rate was explored for the metal-organic vapor phase epitaxy (MOVPE) of a GaAs pin photovoltaic cell, in order to increase the throughput and to reduce material wastage during MOVPE. The growth rate of the undoped GaAs (i-layer) alone was increased from 0.55 to 3.3 μm/h at a constant partial pressure of tertiarybutylarsine (TBAs), and the V/III ratio was reduced from 15 to 2.2 accordingly. The conversion efficiency of the pin cell with a 1-μm i-GaAs layer was almost independent of growth rate and reached 20%, although too thick an i-layer of 3 μm led to degraded efficiency. The crystal quality of that i-GaAs layer was investigated with Hall effect measurement and secondary ion mass spectroscopy (SIMS). With increase in growth rate, the hole concentration increased from 1×1016 to 8×1016 cm−3 due to increased carbon impurity concentration. We have estimated the upper limit of carbon impurity that does not degrade conversion efficiency of a GaAs pin cell with a 1-μm-thick i-layer: 1×1017 cm−3, corresponding to a growth rate of 6 μm/h.