Surfaces Under Growth Conditions

Depending on the specific growth environment, the chemical background of a growing surface can be very different. If one compares, for example, the conditions under MOVPE or chemical beam epitaxy (CBE) with those in an MBE setup, very 

The main difference between these growth methods is the chemical composition of the sources. In the case of MOVPE and CBE, the anions are usually provided as hydrides and the anions (or metals) as organic compounds, both are called precursors. In Table 13.2, a few examples of such precursors are listed that are used in MOVPE or CBE growth environments. Additionally, carrier gases such as H2 or N2 are used in MOVPE. 

During these simplified chemical reactions, the intermediate processes produce reactive radicals such as CH, and H as transitional structures. In Refs. [147, 148], more than 15 such reactions were reported, demonstrating the rather comphcated reaction chains in the gas phase. Many of these reactions produce H radicals independent of the used carrier gas in MOVPE. The next formula gives an example 

Here a decomposed C4H9 radical forms C4H8 and atomic hydrogen (H ). 

The generation of such intermediate products does not take place in the case of MBE. Here the sources are either purely elemental or at least do not contain hydro-organic compounds. This means that reactive CHj or H radicals cannot be produced during or before MBE growth. The formation of such chemical byproducts in the form of carbon hydrides or hydrogen occurs during the pyrolysis of aU the precursors mentioned in Table 13.2 and likewise influence the surface formation of aU III-V and II-VI compounds. This influence on the atomic structure of the growing surface can take place in two ways  

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At 175°C, t > 0.2 s for E, > 100 kJ.mok. For the value of 50 kJ.mol given by Welipitiya et al., the adsorption time is only 7 x 10 s. The extreme sensitivity on does not allow a precise estimation of the adsorption time in the present case. However, the value of 50 kJ.mof proposed for E is certainly underestimated, because it concerns an atomically smooth surface. On a surface under growth conditions that are far from the Frank-van der... 

Exposure of the clean stepped platinum surface to oxygen caused saturation of the step and kink sites (no adsorption occurred on a 111 surface under identical conditions). The oxygen atom-saturated surface was then exposed to varying amounts of carbon monoxide. Both carbide carbon and CO carbon C Is peaks formed, with a one-to-one correspondence between the growth of carbide and the decrease of surface oxygen atoms. These data are consistent with threee possible reaction schemes ... 

Furthermore, the radicals formed upon field-induced hydrogen abstraction can lead to polymerization products on the emitter surface. The mechanism of this field polymerization helped to elucidate the phenomenon of activation of field emitters, i.e., the growth of microneedles on the emitter surface under the conditions of field ionization of certain polar organic compounds. [59]... 

Nucleation of two-dimensional clusters mechanism. In cases where the crystal face is smooth, growth can occur by either a two-dimensional nucleation mechanism or by a spiral-growth mechanism. For two-dimensional nucleation, growth occurs by attachment of molecules to the edge of a nucleus on the surface. Under ideal conditions, the growing step on a crystal surface will advance across the crystal face until that particular layer is complete. Before another layer starts, a center of crystallization has to form via surface nucleation. The growth rate forthis mechanism is exponentially dependent on the driving force ... 

Electrochemical polymerisation produces films on an electrode surface.. Under controlled conditions uniform films up to a few mm thick, which carl be removed from the electrode for subsequent study, can be prepared. Physical properties can be modified by choice of the counterions (dopants) included in the film during growth. It is, however, more difficult to control chain structure and crosslinking than in chemical methods. Electrochemically produced polymers are, therefore, less well characterised than the best directly-synthesised polymers. While this is less satisfactory for fundamental investigations, it is of less concern for applications such as battery electrodes, artificial muscles and drug release agents. The two main approaches, direct-synthesis and electrochemical, are described in the following two sections. 

The difference between the desorption energy observed under growth conditions and in other measurements must be explained. If this is caused by differences in defect density, it should be possible to observe different structures by STM. This would confirm the idea that desorption from defects has a lower activation barrier than desorption from the ideal surface. How-... 

SIMS and secondary electron-emission examination revealed that the Cu layers deposited on the c(2 x 2)-O/Ni(100) substrate are always covered by an adsorbed layer of oxygen [2]. AES [3], ICISS [4], work function measurements [5, 6], and STM observations [7, 8] revealed that oxygen atom is always present on the surface of the grown Cu films deposited on an oxygen pre-covered Ru(OOOl) surface. Under certain conditions (6>o = 0.2-0.4 ML, T 400 K), the work function, monitored during film deposition, oscillates with a period of one monolayer of copper epitaxial growth. 

This brief summary of the CVD process illustrates the importance of surface physics in the synthesis of diamond. Because the growth process itself is not directly accessible so far, most of the microscopic growth steps are inferred from experiments on diamond surfaces under idealized conditions and an analysis of the gas phase during CVD by mass as well as optical spectroscopies. These observations combined with estabhshed reaction sequences for the gas phase chemistry yield data such as growth rate and crystal habit and how they change with CVD conditions that can be measured and serve as benchmarks for particular growth models. 

BEP), influencing also the chemical surface compositions. However, most of these reconstructions only exist under growth conditions and are not stable in thermal equilibrium. In fact, there are only three main reconstructions (indicated in Figure 13.14 as shaded areas), which are also energetically stable, namely, the As-rich c(4 x 4), the Ga-rich (2 x 4), and the Ga-rich (4 x 2). 

These results demonstrate that atomic hydrogen is able to stabilize the InP(001)(2 X 1) surface reconstraction under growth conditions (as well as in UHV). Similar surface stractures have also been reported for GaP(001)(2 x 1) [162-165]. 

Unfortunately the mass accommodation coefficient a is unknown for solid surfaces under stratospheric conditions. It contains information on crystal growth habit and virtually nothing is known about this. MacKenzie... 

In the large crystal model new growth steps nucleate before the entire substrate is covered, i.e. multinucleation takes place on a given surface. Under these conditions, the growth rate G depends on both N and g. Therefore, the number of nuclei formed on the crystal surface in the time interval t to t + dt, that nucleated at r = 0, is given by... 

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