Atomic adsorption, semiconductor

G.V. Malinova, Investigation of Adsorption of Oxygen Atoms on Semiconductor Oxides of Metals, Doctorate thesis (Chemistry), Moscow, 1971. 

This chapter is organized as follows. First, in sect. 2, we consider the surfaces of metals. In sect. 2.1 we describe the structure of unreconstructed clean metal surfaces and then proceed, in sect. 2.2, to consider the reconstructed surfaces. The surface structure of ordered and disordered metallic alloys is described in sect. 2.3. In sect. 2.4 we describe the surface structures associated with atomic adsorption on metals and in sect. 2.5 we consider molecular adsorption on metals. The structure of semiconductor surfaces is... 

Atomic adsorption on elemental semiconductors 3.1.2.1. A tomic adsorption on (100) surfaces... 

Structural parameters for atomic adsorption on the (110) and (1010) surfaces of zincblende and wurtzile structure compound semiconductors. The bond length is that of the anion-cation dimer in the first layer. D is the tilt angle in the top layer. The other parameters arc defined by fig. 16. 

The same Chapter contains results of studies of effects of adsorption of atom particles as well as simplest free radicals on electric conductivity of semiconductor zinc oxide films. 

The combined use of the method of semiconductor sensors and that of molecular beams enabled us to investigate adsorption of atom, molecular and cluster particles of metals on metal oxides. 

We should note that adsorption of acceptor particles on oxide semiconductors of p-type influences their electric conductivity and work function in the opposite way. As for donor particles such as atmns of H, Na, K, Zn, Cd, Pb, Ag, Fe, Ti, Pt, Pd and many others, their adsorption at medium and low temperatures (when there is no notable diffusion of atoms proper into the crystal and, consequently, there is no substitution of atoms created, the latter obeying the Vervey rule) is always accompanied by increase in electric conductivity and decrease in the work function for semiconductor adsorbent of -type, the opposite being valid in case of p-type adsorbent. 

So far, we have focused our attention on adsorption of donor particles on semiconductor oxides. As for the effect of adsorption of acceptor particles on electrophysical characteristics, in concurrence with conclusions made none of adsorption phenomenon involving such characteristic acceptor particles as molecular and atom oxygen on -semiconductor, atoms of nitrogen and simplest alkyl and amine radicals brought about a non-monotonous change in characteristics of adsorbents, despite the fact that experiments had been conducted at various conditions. 

In above sections the main attention has been paid to adsorption-caused change in electrophysical characteristics of semiconductor adsorbent caused by surface charging effects. However, as it was mentioned in section 1.6, the change in electrophysical characteristics of such adsorbents can be caused by other mechanisms, e.g. by direct interaction of absorbate with the surface defects provided (as in the case of oxide adsorbents) by superstoichiometric atoms of metals and oxygen... 

The conclusion of the study [89] concerning non-dissociated character of hydrogen adsorption on semiconductors of ZnO type at low temperatures (lower than 70 C) was based on experimental facts indicating that hydrogen atoms which independently arrive at the surface of semi-... 

Thus, we have considered in detail various theoretical models of effect of adsorption of molecular, atom and radical particles on electric conductivity of semiconductor adsorbents of various crystalline types. Special attention has been paid to sintered and partially reduced oxide adsorbents characterized by the bridge type of intercrystalline contacts with the dominant content of bridges of open type because of wide domain of application of this very type of adsorbents as sensitive elements used in our physical and chemical studies. 

If the above comparison of the properties of metal atoms with those of hydrogen deposited on the surface of a solid body (semiconductor) is correct, the effect of their adsorption on electric properties of semiconductor oxide films will be similar to features accompanying adsorption of hydrogen atoms. The atoms of hydrogen are very mobile and, in contrast to metal atoms, are capable of surface recombination resulting in formation of saturated molecules with strong covalent bond. 

To resolve the problem applying methods of collimated atom beams, equilibrium vapour as well as radioactive isotopes, the Hall effect and measurement of conductivity in thin layers of semiconductor-adsorbents using adsorption of atoms of silver and sodium as an example the relationship between the number of Ag-atoms adsorbed on a film of zinc oxide and the increase in concentration of current carriers in the film caused by a partial ionization of atoms in adsorbed layer were examined. 

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