Effects of AsH3 surface treatment just prior to regrowth on ultrashallow (49 nm) p(+)n(+) GaAs sidewall tunnel junction characteristics were investigated. Fabricated tunnel junctions have shown extremely high record peak current density of 37,000 A cm(-2), and the surface treatment just prior to regrowth has shown great reduction of the valley current by a factor of 4 and the improvement in peak-to-valley current ratio has been achieved by a factor of 2.6. Mechanism of the AsH3 surface treatment effects are discussed in view of the control of surface stoichiometry and related interface phenomena. (C) 2004 The Electrochemical Society.