Films zinc oxide

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 experiment was carried out in a reaction cell shown in Fig. 3.3 with inner walls covered by a zinc oxide film having thickness 10 pm [20]. The surface area of the measuring film on the quartz plate was about 1/445 of the total film area on the wall of the vessel. The results of direct experimental measurements obtained when the adsorbent temperature was -196 C and temperature of pyrolysis filament (emitter of H-atoms) 1000°C and 1100°C, are shown on Fig. 3.4. One can see a satisfactory linear dependence between parameters A r (the change in film conductivity) and APh2 (reduction of hydrogen pressure due to adsorption of H-atoms), i.e. relations... 

Over large periods of time, the conductivity of oxide film grows linearly with time during chemisorbtion of atoms of oxygen and nitrogen at ultra low concentrations on the zinc oxide film over wide time interval at moderate and elevated temperatures, i. e. the following simple kinetic equation is valid ... 

Figure 3.15 shows the validity of above simplest equation for adsorption of O-atoms provided that there are different concentrations of interstitial zinc atoms on the zinc oxide surface. In case of oxygen atoms the experiment has been carried out in absence of molecular oxygen so that effect of its adsorption on change in conductivity was ruled out. O-atoms were produced by means of pyrolysis of carbon dioxide. From this figure we notice that zinc atoms (superstoichiometric) applied onto the surface of the zinc oxide film are the active centres of adsorption of... 

Our comments on adsorption of oxygen and nitrogen atoms lead to conclusion that practically under all conditions the initial rate of variation of conductivity of zinc oxide film due to adsorption of acceptor particles discussed in this section is proportional to the concentration of particles in the space adjacent to the surface of oxide film. This is similar to the case of donor particles. This means that the following equation is applicable ... 

Fig. 3.17. Kinetics of conductivity of a zinc oxide film in the process of adsorption of CH2-biradicals at 100 C. The radicals have been produced by means of photol)rsis ketene vapour at pressure P = 0,5 Torr. a - after adsorption of Zn-atoms b - prior (/) and after adsorption of (2) Zn-atoms.

Further investigations of the above discussed effects show that, at fixed temperature of the oxide film (catalyst), the jump in the electric conductivity first increases in amplitude, as the portion of alcohol vapor admitted into the vessel increases. On further increase of the admitted portion of alcohol, the jump amplitude reduces (starting with the pressure of 3.6-10 2 Torr). At the pressure of 3.2-10 Torr, the jump in the electric conductivity of the zinc oxide film is less pronounced. Finally, at still higher pressures, it disappears (Fig.4.9). This effect is not unexpected. On our mind, it is associated with the fact that, as the concentration of alcohol vapor increases, the sum of the rate of interaction of the vapor with adsorbed hydrogen atoms and the rate of surface recombination of hydrogen atoms at the time instant of production becomes higher than the chemisorption rate of these atoms. The latter is responsible for the increase of the electric conductivity of the semiconductor oxide film via the reaction... 

It was shown in a number of works [29] that impurity conductivity of thin zinc oxide films are extremely sensitive to adsorption of atoms of various metals (see Chapters 2 and 3). Using this feature of oxide films, we first employed the sensor method to study evaporation of superstechiometric atoms of metals from metal oxide surfaces, zinc oxide in particular [30]. 

The above rate equations confirm the suggested explanation of dynamics of silver particles on the surface of zinc oxide. They account for their relatively fast migration and recombination, as well as formation of larger particles (clusters) not interacting with electronic subsystem of the semiconductor. Note, however, that at longer time intervals, the appearance of a new phase (formation of silver crystals on the surface) results in phase interactions, which are accompanied by the appearance of potential jumps influencing the electronic subsystem of a zinc oxide film. Such an interaction also modifies the adsorption capability of the areas of zinc oxide surface in the vicinity of electrodes [43]. 

The above results put forward the question, whether or not one can directly compare in experiment the influence produced by adsorbed atomic and molecular silver particles on electric conductivity of a zinc oxide film. 

The above results once again make clear the reason for relaxation of impurity conductivity of a zinc oxide film alter termination of a beam of metal particles used for doping the surface of these films. Additionally, these results contain earlier suppositions of molecular particles of metals being electrically inactive. 

The method of semiconductor sensors allows one to determine the flux of atoms, to which the sensor was exposed, from electric conductivity measurements (provided coefficients of ionization and reflection of oxygen atoms from zinc oxide films are known). In other words, the sensor technique can be used in this case as an absolute method [21]. Indeed, variation of electric conductivity of a semiconductor film Acrpi due to adsorption is known to be caused by variation of carrier concentration An in the film, rather than by variation of their mobility / [21] ... 

If coefficients of ionization y and reflection of of atoms for zinc oxide film are known, we have... 

The problem of measuring small concentration of oxygen in a buffer gas can be solved by using the semiconductor sensor with a sensitive element consisting of a zinc oxide film immersed in a polar or, better, a protodonor liquid (see Section 3.4). 

From the above consideration we suppose that the increase of electric conductivity a of the zinc oxide film is proportional to [Hj], as is the case when hydrogen atoms produced by pyrolysis or discharge in the gas phase are adsorbed on the zinc oxide film. Reverse (competitive) changes of electric conductivity are proportional to the a value of the semiconductor film and to [ ]. Thus, taking into account both chemi-... 

For this purpose, the authors used a special vacuum cell with a controlled focused electron beam incident on a zinc oxide film target. In these experiments, the role of the film was twofold. It served as an adsorbent and as a high-sensitivity detector of hydrogen atoms (10 at/cm ). Hydrogein atoms were produced due to surface dissociation of adsorbed molecular hydrogen. This process was induced by heating or bombardment of the adsorbed layer by an electron beam. 

This supposition is experimentally substantiated by Kupriyanov et al. [160], In this work they investigated the influence of RGMAs upon the electrical conductivity of pure zinc oxide films and films activated by microcrystals of gold. The gold was chosen as the activator because of its chemical inactivity and high lateral mobility. This makes it possible to obtain islet films on a ZnO surface at room temperature, thus avoiding probable metallurgical processes. 

Fig. 6.7. The change in electroconductivity of a zinc oxide film due to hydrogen atoms emitted from the surface of the formed layers of platinum on the surface of fused silica during introduction of molecular hydrogen The arrow head indicates the beginning of evacuation of hydrogen...

The procedures of experiments were the following [15, 26]. After deposition of a specific quantity of silver on substrate the heating of a tray with silver was turned off, the shutter 7 was opened and the sensor was positioned opposite to the substrate in such a manner that the surface of the sensor was parallel to the surface of substrate. In these experiments we detected an irreversible donor signal of the sensor which can be related to adsorption silver atoms on the sensor made of a zinc oxide film. It is known [27] that silver atoms are donors of electrons. Note that the signals of the sensor were observed only when the sensor was positioned in front of a substrate. There were no signals detected in any other arrangement between sensor and substrate. 

K.R. Zhang, F.R. Zhu, C.H.A. Huan, A.T.S. Wee, and T. Osipowicz, Indium-doped zinc oxide films prepared by simultaneous rf and dc magnetron sputtering, Surf. Interface Anal., 28 271-274, 1999. 

S. Major and K.L. Chopra, Indium-doped zinc-oxide films as transparent electrodes for solar-cells, Sol. Energy Mater., 17 319-327, 1988. 

X. Jiang, F.L. Wong, M.K. Fung, and S.T. Lee, Aluminum-doped zinc oxide films as transparent conductive electrode for organic light-emitting devices, Appl. Phys. Lett., 83 1875-1877, 2003. 

A number of works are devoted to the electrochemical preparation of ZnO, which may have application in photocatalysis, ceramics, piezoelectric transducers, chemical sensors, photovoltaics, and others. ZnO has the same band-gap energy as Ti02, and the oxygenation capacities for both compounds should be similar. Ya-maguchi et al. [155] prepared photoactive zinc oxide films by anodizing a zinc plate. Such films could decompose gaseous acetaldehyde with the aid of black lights. 

Fig. 1. Time variation of electrical conductivity of zinc oxide film at 46O C. in vacuum (data replotted from ref. 23).

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