Unintentional calcium incorporation in Ga(Al, In, N)As

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Abstract
Unintentional calcium incorporation into GaInNAs causes an acceptor-type impurity, which limits the ability of ∼1eV GaInNAs-based solar cells to collect photogenerated current. Here, the authors focus on better understanding the conditions by which Ca is incorporated into GaInNAs. Various material combinations were grown including GaAs, InGaAs, GaInNAs, and Al(Ga)As. The materials were primarily grown by solid-source molecular-beam epitaxy (MBE) at ∼400 and 580–620°C, with comparisons made to metal-organic chemical vapor deposition (MOCVD)-grown materials where appropriate. Calcium incorporation was measured through secondary ion mass spectrometry. There was no measurable Ca incorporation into MBE-grown GaAs at 580°C, but Ca incorporates into GaAs at low MBE growth temperatures (∼400°C) that are comparable to those typically used for GaInNAs. This suggests that the N species is not solely responsible for the observed Ca incorporation into MBE-grown GaInNAs; but rather, defects associated with the low temperature growth may also be a factor. The effects of defect-related Ca incorporation were further studied at interfaces between Al(Ga)As and GaAs, and substrates pretreated with an O2 plasma and NH4OH etch. Ca incorporation was observed at the interface between Al0.3Ga0.7As∕GaAs, AlAs∕GaAs, and at the epilayer/substrate interface for the pretreated samples. No Ca was observed in InGaAs samples grown by either MBE or MOCVD, suggesting the Ca incorporation into GaInNAs has something to do with the addition of nitrogen. Therefore, the authors propose that the likely causes of Ca incorporation into GaInNAs are a Ca–N affinity, defects generated by low temperature growth, defects generated by the incorporation of N into GaAs, or some combination thereof.

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