The origin of non-uniformity of MBE-grown InGaAs ridge quantum wires (QWRs) in sub 10-nm wire width range was investigated in detail by SEM, in situ XPS, TEM and PL measurements. InAlAs/InGaAs/InAlAs wires were selectively grown on InGaAs ridge structures prepared also by MBE on [(1) over bar 10] stripe patterned (001) InP substrates. The main source of non-uniformity was thermal cleaning of InP in As, done prior to InGaAs ridge formation which produced ridges having irregular sized InGaAs islands due to an initial As-P exchange reaction. Low temperature atomic hydrogen cleaning removed this problem, and led to successful formation of sub 10-nm QWRs.