Calcium impurity as a source of non-radiative recombination in (In,Ga)N layers grown by molecular beam epitaxy

Ca as an unintentional impurity has been investigated in III-nitride layersgrown bymolecular beam epitaxy(MBE). It is found that Ca originates from the substratesurface, even if carefulcleaning and rinsing procedures are applied. The initial Casurface coverage is ∼1012  cm−2, which is consistent with previous reports onGaAs and silicon wafers. At the onset ofgrowth, the Ca species segregates at thegrowth front while incorporating at low levels. The incorporation rate is strongly temperature dependent. It is about 0.03% at 820  °C and increases by two orders of magnitude when the temperature is reduced to 600 °C, which is the typicalgrowth temperature forInGaN alloy. Consequently, [Ca] is as high as 1018  cm−3 in InGaN/GaN quantum well structures. Such a huge concentration might be detrimental for the efficiency oflight emitting diodes(LEDs) if one considers that Ca is potentially a source of Shockley-Read-Hall (SRH) defects. We thus developed a specificgrowth strategy to reduce [Ca] in theMBEgrownLEDs, which consisted of burying Ca in a low temperature InGaN/GaN superlattice (SL) before thegrowth of the active region. Finally, twoLED samples with and without an SL were fabricated. An increase in the output power by one order of magnitude was achieved when Ca was reduced in theLED active region, providing evidence for the role of Ca in the SRH recombination.

Source: Applied Physics Letters - November 23, 2016 Category: Physics Authors: E. C. Young, N. Grandjean, T. E. Mates and J. S. Speck Source Type: research