DIFFERENCES IN EPITAXIAL-LAYER SUBSTRATE INTERFACE PROPERTIES OF HETERO-JUNCTION FIELD-EFFECT TRANSISTORS FABRICATED BY MOLECULAR-BEAM EPITAXY AND METAL-ORGANIC CHEMICAL-VAPOR-DEPOSITION

Electrical characteristics of epitaxial layers which are supplied by three manufacturers for molecular beam epitaxy (MBE) grown wafers and two manufacturers for metal organic chemical vapor deposition (MOCVD) grown wafers are analyzed. There are remarkable differences in the electrical characteristics between the MBE grown epitaxial-layers and the MOCVD grown ones. The heterojunction field effect transistors (HJFETs) fabricated by MBE show a large side-gating effect, while those fabricated by MOCVD have high threshold voltages for side-gating effect. The HJFETs fabricated by both methods have frequency dispersion in drain conductance (G(ds)). The activation energies of the associated deep levels are estimated at 0.34 eV for MBE and 0.73 eV for MOCVD. Diodes with n/n layers, in which the growth is interrupted and the interface is air contaminated, show electron depletion and Fermi level pinning at the interface for the MBE wafers, while almost no peculiar effect at around the interface for the MOCVD wafers. Secondary ion-mass spectroscopy (SIMS) analysis indicate a large amount of oxygen and carbon contamination in the MBE wafers, while these impurities are under the detection limit in the MOCVD wafers. These results indicate that the interfaces formed by MBE contain deep levels of oxygen and shallow carbon accepters, which cause G(ds) frequency dispersion and the side-gating effect, respectively. On the other hand, there is no substantial contamination in the MOCVD wafers, while G(ds) dispersion appearing in the MOCVD FETs can be attributed to the EL2 substrate deep level. These results indicate that there exist a large difference in the susceptibility to the surface cleaning between two technologies.