METHODS TO IMPROVE THE P AS COMPOSITIONAL UNIFORMITY OF INGAASP THIN-FILMS PREPARED BY MOCVD/

Preparation of large area InxGa1-xAs1-yPy thin films by metal-organic chemical vapor deposition (MOCVD) is limited by P/As compositional non -uniformity. A compositional uniformity model has been developed based on gas phase depletion of As and P which correlates well with experim ental results. The phosphorus content in the solid y is related to the ratio of phosphorus and arsenic in the gas f(P)/f(As) by y/(1 - y) = K(f(P)/S-As), where the distribution coefficient K is dependent on tem perature, pressure, and the chemical forms of arsenic and phosphorus i n the feed stream. When InGaAsP thin films are prepared on wafers held at constant temperature from AsH3 and PH3, K < 1 at typical growth te mperatures of 600-650 degrees C, and compositional variations in the G roup V sublattice with axial distance result from the preferential dep letion of arsenic from the gas stream due to its selective incorporati on into the solid. Compositional uniformity can be enhanced by using h igh V/III ratios, operating under conditions where only a small fracti on of the Group III reagents are incorporated into the film, or choosi ng reactants which yield a distribution coefficient near unity. When K deviates significantly from unity, compositional uniformity combined with high reactant utilization can still be obtained by introducing a small negative axial temperature gradient along the susceptor to decre ase K, which compensates for the increasing P/As ratio in the gas phas e. Optimal temperature gradients for growth from AsH3 and PH, are esti mated for a variety of growth conditions.