Metalorganic chemical vapor deposition (MOCVD) is a semiconductor growth technique that proceeds by exposing the surface of a heated substrate to vapors of different organic molecules. Atomic layer epitaxy (ALE) is a modification of MOCVD which alternates the exposure to vapors producing atoms of different columns in the periodical table in order to grow the crystal one atomic layer at a time.

The ALE growth of GaAs and InAs has been studied in situ by an optical technique called reflectance difference spectroscopy (RDS). For GaAs it is demonstrated that when trimethylgallium (TMGa) first arrives at the surface there is more than 1 monolayer (ML) of As terminating the crystal. The As lying in the topmost layer does not produce any extra Ga incorporation but plays an important role in the preservation of the 1 ML/cycle growth regime by inhibiting the desorption of As from the underlying layer.

The desorption of As from the topmost layer is facilitated by the presence of Ga on the surface. This is attributed to the chemical bonding of the As with methyl radicals produced by the decomposition of TMGa at the growth surface. The first 0.5 ML of incorporated Ga does not form dimers and remains invisible to the RDS measurements. The combined role of step edges and the redistribution of first layer As maintains the As termination of the surface until that point.

The presence of methyl radicals attached to the surface permits the deposition of a full ML of Ga and preserves the stoichiometry of the process. The methyl radicals are also observed to inhibit the formation of Ga droplets on the surface during exposure to the group III precursor.

A model using gas phase reactions exclusively is developed and shows that the experimental measurements of the incorporation of Ga during both the ALE and MOCVD processes can be reproduced, indicating that gas phase reactions have to be included in any complete picture of the ALE growth process.

The growth of ultrathin InAs/GaAs heterostructures by ALE has been characterized in situ for the first time using RDS. Significant In segregation is observed when InAs is buried with GaAs and none is detected for the inverse structure. This asymmetry points to thermodynamic effects during growth favoring the exchange between the top and buried layers and bringing the In atoms to the surface.

The incorporation of In on GaAs proceeds without saturation for up to 4 ML coverage as opposed to the InAs surface where In stops entering the crystal after the surface is completely covered. The presence of strain stimulates the formation of islands on the surface during In exposure which disrupts the conditions for self-limiting growth.