A new experimental approach is presented for studying the charge-transfer process involved in the chemisorption on polar semiconductors. This approach utilizes the surface piezoelectric effect, contact potential difference measurements, and surface photovoltage spectroscopy. From the study of oxygen adsorption on ZnO it was found that the rate of electron transfer varies exponentially with the surface barrier height and is proportional to the oxygen pressure (from 10−3 to 20 Torr). Furthermore, it was found that the charge transfer is characterized by a thermal activation energy of about 0.72 eV. At room temperature this activation energy constitutes the most significant rate-limiting factor and is largely responsible for the extremely slow rate of chemisorption. A model for chemisorption was developed in which the thermal activation is treated as an intermediate nonelectronic step involving metastable activated surface states. Upon capturing electrons from the bulk these states become stable surface states. A rate equation was derived through which the capture cross section of the activated surface states was calculated to be 10−16 cm2, in contrast to the unrealistically small value of 10−29 to 10−22 cm2 obtained with exclusively electronic models.
The normal mode of vibration of (111) GaAs wafers with a thickness below about 15 μm was found to depend strongly on the surface preparation and on the ambient atmosphere. This dependence was attributed to effects directly related to the surface stress σs. It was shown that σs can be evaluated from the natural frequency of vibration. The values of σs, in the 〈110〉 direction, for etched and unetched (111) GaAs wafers in room atmosphere were found to be 325 and 570 dyn/cm, respectively. It was further demonstrated that surface stress transients due to the adsorption processes (adsorption transients) can be determined by corresponding changes in the natural frequency of vibration.
Surface characteristics of the {111} crystallographic planes of the III-V intermetallic compounds (zinc-blende structure), and in particular those of InSb, are discussed. The polarity of these compounds along the <111> directions leads to pronounced physical chemical differences between the {111) surfaces terminating with group III atoms and those terminating with group V atoms. Differences in etching, dislocation etch pit formation, and electrode potential are presented. Dislocation etch pits form on the group III surfaces and not on the group V surfaces of the six compounds investigated (InSb, GaSb, A1Sb, InAs, GaAs, and I n P ) . A proposed interpretation is based on the relative reactivity of the group III and group V atoms as affected by their bond configuration and the polarity of the zinc-blende structure.
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