THERMODYNAMIC INTERPRETATION OF QUATERNARY (INGAASP) LAYER UNIFORMITY GROWN BY LOW-PRESSURE METALORGANIC VAPOR-PHASE EPITAXY

According to a recent study, the compositional nonuniformity for group V sites of quaternary (Q) In0.68Ga0.32AS0.7P0.3 layers (lambda = 1. 3 5 mum) grown by low-pressure metalorganic vapor phase epitaxy (LP-MOVPE) on 50 mm diam InP wafers was reduced from 3% to 1% when tertiary butyl phosphine (TBP) + AsH3 instead of PH3 + AsH3 or TBP + tertiary butyl arsine (TBAs) were employed. In this letter we offer a thermodynamic interpretation of these observations. Under LP-MOVPE conditions at a total pressure of 0.08 atm for fully decomposed group V precursors diluted with H-2, We show that while the dominant species are the tetramers, the concentration of the dimers is significant and becomes more prominent with rising temperature (T). Examining the [As/P] ratio in the gas phase as a function of reciprocal T, we demonstrate that in the MOVPE range (900-950 K) the slope of these curves changes from steep to moderate and then nearly flat for the sources AsH3 + PH3, TBAs + TBP, and AsH3 + TBP, respectively. The insensitivity of the [As/P] ratio to the approximately 30 K gradient over the susceptor when combining AsH3 and TBP suggests a very effective method to assure compositional uniformity in Q layers prepared by LP-MOVPE. Uniformity results for atmospheric pressure MOVPE employing AsH3 and PH3 are also consistent with the thermodynamic argument.