Two approaches for metalorganic chemical vapor deposition (MOCVD)-grown compositionally graded metamorphic buffers on 6 degrees offcut bulk GaAs were investigated. The first approach consisted of tandem graded layers of InGaAs and InGaP with compositional grading of the In concentration. This tandem approach was found to be necessary because phase separation in the InGaAs alloys leads to surface roughening and high threading dislocation density when grading to lattice constants greater than that of In(0.30)Ga(0.70)As. An In(x)Ga(1-x)As graded buffer was grown at 700 degrees C for low In concentration (X(In)=0-0.10) and then the growth temperature was decreased to 450 degrees C for high In concentration (X(In)=0.10-0.30) to suppress the phase separation. The growth temperature was then increased to 650 degrees C and the graded In(y)Ga(1-y) P system was implemented to continue grading the lattice constant from In(0.30)Ga(0.70)As to InP, which allowed us to achieve InP on 6 offcut GaAs with a threading dislocation density of 7.9 x 10(6) cm(-2) and an RMS surface roughness of 33.0 nm on a 40 mu m x 40 mu m AFM scale. The second approach used GaAsSb alloys with compositional grading of the Sb concentration. Graded mixed-anion GaAsSb alloys grown at 575 degrees C did not exhibit phase separation, resulting in high quality InP lattice constant films on GaAs without the need to transition to another material system for compositional grading. We demonstrated a GaAsSb alloy on GaAs (with a grading rate of 1.06% strain/mu m) lattice-matched to InP with a threading dislocation density of 4.7 x 10(6) cm(-2) and a roughness of 7.4 nm on a 40 mu m x 40 mu m AFM scale. It was further demonstrated that the threading dislocation density of the GaAsSb graded buffer can be lowered to 2.7 x 10(6) cm(-2) with a slower grading rate (0.64% strain/mu m). (C) 2011 Elsevier B.V. All rights reserved.