Evaporation, sources sublimation

To control the properties of the fabricated thin films, it is necessary to operate under vacnnm to minimize the interaction between residnal gases and the surface of growing films. The thin films can be obtained in the crystalline or amorphous state by controlling the concentration of the vapor. The process of the film formation by this method involves the following three stages (1) evaporation or sublimation of the charge to form vapor, (2) transfer of atoms or molecnles from evaporation source to the substrate, and (3) condensation of vapor on the snbstrate. 

For evaporative or sublimation process, correct selection of evaporation method, the evaporation source, and the evaporation temperatnre is required to surmount the attractive intermolecular forces existing within the starting material. The parameters depend primarily on the materials nsed and the film purity required. Indirect resistance heating, flash evaporation, and electron beam heating techniques are used for this purpose. 

This method is most common. The evaporation material is placed in a container made of Mo, , W or C which can be in the form of a boat, crucible, coil or strip. In some cases ceramic crucibles or inserts are also employed, made of AI2O3 and BeO, BN or BN/TiB2. The container is heated by current flow and the material is evaporated or sublimed from this. Various types of resistance-heated evaporation sources are shown in Fig. 53. Undesired but possible chemical reactions can cause film contamination [274]. The formation and evaporation of vapourizable compounds of the boat material upon contact of the hot boat wall, with reactive gases, as well as with some reactive and/or decomposable film material, is not always low [275], but can, however, often be avoided by appropriate choice of the evaporation source material and by special pre-treatment. A further evaporation method uses radiation heating. The radiationheated vapour source consists generally of a resistance-heated spiral radiator of tungsten wire which is mounted above the evaporant surface in an open crucible. It can be used for the evaporation of easy volatilised materials. 

For many materials (including organic sources, which typically sublime at relatively low temperatures), the evaporation source may be used an unlimited number of times. Upon cooling, materials which melt may experience a thermal expansion mismatch which can cause flaking or delamination from the boat. When using evaporants which melt refilling pre-conditioned sources with molten evaporants while there is a pool of solid material in contact with the boat can lead to greater process uniformity. 

Fig. 4.4. A schematic cross-section of two evaporation boats showing the popcorn effect. In (a), the boat is open, and the evaporant material sublimes first at the hottest point of the assembly, which is the bottom of the material granules. This creates a vapor jet which can eject solid particles of evaporant from the boat. In a baffled arrangement (b), the source is constructed so as to prevent line of sight contact between the source and the substrate. Solid particles which are propelled are trapped, whereas material vapor can escape and deposit.

A direct insertion probe may be used for reasonably pure volatile solids. The sample is loaded into a quartz tube on the tip of a rod that is inserted into the evacuated source region. The sample is then evaporated or sublimed into the gas phase, usually by heating. The gaseous molecules are then ionized (often with accompanying fragmentation) and the ions are mass analyzed. In some techniques, volatilization and ionization occur at the same time. 

Vacuum evaporation (including sublimation) is a PVD process where material from a thermal vaporization source reaches the substrate without collision with gas molecules in the space between the source and substrate. The vacuum is required to allow the molecules to evaporate freely in the chamber and then subsequently condense on all surfaces... 

Fig. 4. Evaporation source for deposition of sublimating materials at high rates /7/.

Temperature and humidity conditions in the test chamber should be kept at a level to ensure that the effects of changes such as sublimation, evaporation, or melting are minimized in the physical state of the samples. Rate constants for photochemical reactions depend on the temperature because of secondary thermal reactions of the parent compound or primary products. Thermal stability of the material should independently be determined through accelerated stability testing. The ambient temperature and the temperature of the samples during irradiation are related to the photon source used for testing and the intensity and distance of the sample from the photon source. 

The rubrene source material was purchased from Aldrich (purity > 98%) and purified by threefold sublimation under dynamic vacuum conditions. The rubrene layers were evaporated in a standard HWE reactor on freshly cleaved 2Mi muscovite mica. Two different growth rates were used by employing different... 

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