Ultrahigh vacuum evaporation

Thurner, G. and Abermann, R. (1990), Internal stress and structure of ultrahigh vacuum evaporated chromium and iron films and their dependence on substrate temperature and oxygen partial pressure. Thin Solid Films 192, 277-285. 

Fig. 4. Schematic of an ultrahigh vacuum molecular beam epitaxy (MBE) growth chamber, showing the source ovens from which the Group 111—V elements are evaporated the shutters corresponding to the required elements, such as that ia front of Source 1, which control the composition of the grown layer an electron gun which produces a beam for reflection high energy electron diffraction (rheed) and monitors the crystal stmcture of the growing layer and the substrate holder which rotates to provide more uniformity ia the deposited film. After Ref. 14, see text.

PVD reactors may use a solid, liquid, or vapor raw material in a variety of source configurations. The energy required to evaporate liquid or solid sources can be supplied in various ways. Resistive heating is common, induction heating of the source bottle is sometimes used, and electron beams are also employed. Molecular-beam-epitaxy (MBE) systems are PVD-type reactors that operate at ultrahigh vacuum. Very low growth rates are used ( 1 xm/h), and considerable attention is devoted to in situ material characterization to obtain high-purity epitaxial layers (2). 

For the study of crystalline surfaces, ultrahigh vacuum (UHV) is required. The preparation of clean crystalline surfaces is usually carried out within the UHV system by cleavage, sputtering, evaporation, thermal treatment, or molecular beam epitaxy. 

Distillation is a method in which a fulleride film with a nominal starting composition M C is heated for a prolonged period in ultrahigh vacuum. Depending on the starting composition and the stable phases of the system, either fullerene molecules or metal intercalant atoms evaporate from the film, driving the composition towards a stable phase [see Poirier DM, Weaver JH (1993) Phys Rev B 47 10959]. This method enables phase-pure films of intercalated fullerenes to be produced for spectroscopic studies. The low quantities in... 

Back migration can also be caused by improper cooling around the orifice of the pump. With this problem, pump oil that had condensed begins to re-evaporate. This problem is more significant with ultrahigh-vacuum systems. 

Upon heating, the Ge02 reacts with germanium to form GeO, which then evaporates. Thus, a reduced surface can be obtained by heating under vacuum, but an ultrahigh vacuum is, of course, required if the bare surface is to be maintained. At lO-6 mm. of Hg, extrapolated data indicate (5) that about one layer per minute would be lost at 400°, and about one tenth of a layer per minute at 350°. However, it is doubtful whether the first adsorbed layer of oxygen is removed under these conditions. At 200°, the loss is estimated to be extremely slow or negligible. 

For Dewar flasks, metal evaporation apparatus, and most research apparatus, a vacuum of 10 ° to 10 ° Torr is sufficient this is in the high vacuum range, while 10 ° Torr would be termed ultrahigh vacuum. However, for many routine purposes a utility vacuum or forepump vacuum of about 10 ° Torr will suffice, and for vacuum distillations only a partial vacuum of the order of 1 to 50 Torr is needed. 

Molecular beam epitaxy (MBE) is really evaporation followed by condensation and reaction on a substrate, done under very clean, ultrahigh vacuum conditions at... 

Torr of CO, the doublet structure was not observed. This result was also confirmed by Harrod et al. (102) on rhodium films evaporated under ultrahigh vacuum conditions. The difference in these results obtained using different methods of sample preparation can be accounted for either in terms of crystallite size or differences in electronic environment at the metal surface. Blyholder (98) believes that crystallite size also accounts for the differences observed in the infrared spectra of CO adsorbed on the nickel-titania system compared with the nickel-silica and nickel-alumina systems (111) as outlined in Table VI (107). 

Studies carried out using metal films evaporated under ultrahigh vacuum conditions, and observed without removing the sample from the system offer one method of eliminating the problems of contamination... 

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