Schottky barrier contacts to n-lnP

This paper reviews various models of Schottky barrier formation, presents work of others in increasing barrier height (φ_b) on n-type InP and introduces a cryogenic process for significantly increasing φ_b. Structural analysis of the interface shows the differences between Pd and Au metallizations even though both give a high φ_b to n-InP. InP and alloys thereof are important for optical, high-speed and field effect transistor applications Ohmic or Schottky contacts are required in most cases Low values of barrier height (φ_b) in Schottky contacts are an especially important problem in n-type materials, and particularly in InGaAs Chemical reactivity and an atomically clean surface are critical factors in Schottky barrier height formation. Metal adatoms form electrically active defects from which surface states arise. These surface defects lead to a degradation in performance of these Schottky devices Many models, including metal-induced gap states, have been used to explain lower than desired values of barrier height A more recent one is that of the inhomogeneous metallization model Methods to increase φ_b have included an interfacial insulator, altering of surface chemistry, and use of sulfides, phosphides or nitrides. A cryogenic or low-temperature process (LT), involving depositing the metal in a vacuum onto a substrate cooled to 77K, has recently been very effective in greatly increasing φ_b. The focus upon n-InP has been extended to n-type GaAs, InGaAs, and ZnSSe The success in improving φ_b in these 4 materials indicates a systematic effect and not one which is particular to a certain semiconductor. An increased value of φ_b follows an increase in the metal work function, indicating an unpinning of the Fermi level Current transport for both LT and conventional processing is principally by thermionic emission but shifts to thermionic field emission for LT processing on higher doped substrates. Analysis by high resolution transmission electron microscopy and energy dispersive spectroscopy have revealed much detail about the interface. Pd deposited at room temperature (RT) produces a thick amorphous interface whereas LT deposition gives a much thinner amorphous region. Au deposition does not give such an amorphous region in either case For both metals, RT deposition gives a coarse polycrystalline (poly) metal structure whereas LT deposition gives a fine poly structure. For both metals, with conventional processing, In tries to diffuse into the metal and metal diffuses into the InP. LT deposition greatly reduces these effects, leading to much lesser chemical interactions. Thus, the Fermi level is unpinned and φ_b greatly increased. A more uniform metallization also results which avoids the inhomogeneous metallization problem and leads to a higher barrier height.