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In this paper, we have investigated a pathway to mitigate the arsenic (As) cross-contamination on a back side Si wafer during GaAs growth by metal-organic chemical vapor deposition (MOCVD). Without a proper protocol doing a III-V on Si heterogeneous epitaxy, we have observed high levels of the As concentration on the back side Si wafer, easily in excess of 1 × 10 20 atoms/cm 3 by secondary ion mass spectrometry (SIMS) analysis and 10 15 atoms/cm 2 by total reflection X-ray fluorescence (TXRF) analysis, after GaAs growth on Si. This known level of contamination on wafers would disqualify them for fabrication in existing Si VLSI fabs. In order to mitigate the As cross-contamination, we have proposed a SiO 2 protection layer on the back side of the Si wafer. From both SIMS and TXRF analysis, the proposed scheme has dramatically lowered the back side as concentration to 1.5 × 10 16 atoms/cm 3 by SIMS and 1.0 × 10 10 atoms/cm 2 by TXRF. III-V compound semiconductors have recently been explored as the most promising n-channel material for next-generation CMOS logic transistors. Especially, an indium-gallium-arsenide (InGaAs) material system has the spotlight due to its outstanding electron transport properties, relative maturity, and demonstrated reliability, compared with other candidates such as carbon nanotube transistors (CNTs) and semiconductor nanowires. [1][2][3][4][5] In fact, many research groups have paid attention to the InGaAs material system, and demonstrated excellent electrical characteristics of InGaAs metal-oxidesemiconductor field-effect transistors (MOSFETs).1-5 Even with such notable improvements, what is critical at the end is to co-integrate those InGaAs materials with mainstream Si wafers and VLSI infrastructures in a heterogeneous integration fashion. Other approaches for integration of InGaAs materials, including wafer-bonding, are not as attractive economically. This strongly emphasizes that the InGaAs materials must be defined only in a certain area of the Si wafer for highperformance and low-power logic operations. Naturally, this calls for a selective epitaxy of the InGaAs material on the 300-mm Si wafer. In this regard, metal-organic chemical vapor deposition (MOCVD) is highly appropriate for the selective growth of the InGaAs onto the Si, as opposed to molecular-beam-epitaxy (MBE). 6,7 However, the use of MOCVD is not the complete solution for those Si wafers with selective growth of the InGaAs material to be successfully introduced in the existing 300-mm Si Fab. In addition, it is of critical importance to understand what kinds of new materials are present on both sides of the 300-mm Si wafers after the selective growth of the InGaAs layer is completed with MOCVD, to safely preserve the existing Si infra-structures.Unfortunately, during the selective growth of the InGaAs material on the front-side of the 300-mm Si wafer, unwanted metallic cross-contaminations, such as As and Ga, occur on the back-side of the 300-mm Si wafer, which then seriously contaminates other infra-structures as th...