Control of Zn diffusion in InP/InGaAs heterojunction bipolar transistor structures grown by metalorganic vapor phase epitaxy

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Abstract
It is known that high n+ doping in the cap layers of heterojunction bipolar transistor structures induces anomalous Zn diffusion in the base region during metalorganic vapor phase epitaxial growth. This phenomenon has been explained in terms of nonequilibrium group III interstitials generated in the n+ cap layer, which create highly diffusive Zn interstitials via the kick-out mechanism. In this article, we show that low-temperature growth (550 °C) is effective in alleviating the influence of the n+ cap layer. Due to a large time constant for the recovery of thermal point-defect equilibrium, the last-to-grow n+ cap layer cannot inject the excessive group III interstitials into the base region within a growth sequence. Under the low-temperature growth, however, the first-to-grow n+ subcollector produces group III interstitials during the whole growth sequence and thereby causes anomalous Zn diffusion. To prevent this effect, we propose interrupting the growth for a long time period (30 min) before growing the base layer, and growing the n+ subcollector at a higher temperature (600 °C). These growth techniques are shown to be effective in purging the subcollector of the undesirable group III interstitials before base-layer formation.