CONTROL OF SIDEGATING EFFECTS IN ALGAAS/GAAS HETEROSTRUCTURE FIELD-EFFECT TRANSISTORS BY MODIFICATION OF GAAS WAFER SURFACES

Sidegating characteristics of AlGaAs/GaAs heterostructure field-effect transistors, fabricated on molecular-beam epitaxially grown layers, were investigated with emphasis on the material properties. A systematic analyses of the epitaxial layers concluded with the identification of the substrate-superlattice-buffer-layer interface as the predominant cause of the sidegating effect. Remnant carbon contamination on the GaAs surface was found to produce a p-type, conducting interfacial region. Controlled oxidation of the carbon on the wafers was accomplished using ultraviolet radiation. This oxide was desorbed in situ before epitaxial growth. Secondary-ion-mass spectroscopy was employed to estimate the carbon concentration at the substrate-epitaxial- layer interface for standard cleaned and ultraviolet-ozone-treated wafers. The carbon concentration of the interfacial region decreased by two orders of magnitude for the wafers exposed to the ultraviolet radiation. Hall-effect measurements of standard cleaned and ultraviolet-ozone-treated heterostructure wafers, prepared with various buffer layer thicknesses, demonstrated the dominant influence of the interfacial p-type region on the electronic properties of the material. A comparison of sidegating characteristics for devices fabricated on the two types of wafers is presented and discussed. A dramatic improvement in sidegating was observed for the wafers subjected to the ultraviolet-ozone cleaning procedure.