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In this work, we present the investigation of InN/GaN multiple-quantum-well (MQW) growth by plasma-assisted molecular beam epitaxy using in-situ reflection high-energy electron diffraction (RHEED) to monitorthe growth process. The analysis of the RHEED intensity and pattern transitions identified an indium surface ac-cumulation even with a nominal thickness of InN as small as 0.5 monolayer (ML). This result explicitly showsthat, even at low growth temperaturesof ~550 °C, not all of the supplied indium isincorporated into the quantumwell (QW). Moreover, the residual indium can become incorporated into the GaN matrix on either side of theQW. Both QW thickness and the photoluminescence (PL) emission energy showed a self-regulating behavior.The apparent thickness did not exceed 2 MLs even when the deposited InN thickness is as large as 5 MLs. ThePL emission shows a continuous redshift with the deposited InN from ~370 nm for 0.5 ML until it saturates at~423 nm forN2 ML. Based on the observed growth phenomena, a qualitative growth model was developed to ex-plain the self-limited epitaxial growth of ultrathin In(Ga)N/GaN QWs