Sublimation method

Scientific research in the field of phosphors is almost 140 years old. In 1866, the young French chemist Theodore Sidot prepared, by a sublimation method, tiny ZnS crystals that manifested phosphorescence in the dark.4 After the experiment was repeated and confirmed it was presented in a note to the Academy of Sciences of Paris the note was then published by Becquerel.5 From present knowledge of phosphors it seems likely that Sidot s ZnS contained a small quantity of copper as an impurity, and was the precursor for ZnS-type phosphors. 

Property measurements of fullerenes are made either on powder samples, films or single crystals. Microcrystalline C6o powder containing small amounts of residual solvent is obtained by vacuum evaporation of the solvent from the solution used in the extraction and separation steps. Pristine Cgo films used for property measurements are typically deposited onto a variety of substrates (< . , a clean silicon (100) surface to achieve lattice matching between the crystalline C60 and the substrate) by sublimation of the Cr,o powder in an inert atmosphere (e.g., Ar) or in vacuum. Single crystals can be grown either from solution using solvents such as CS and toluene, or by vacuum sublimation [16, 17, 18], The sublimation method yields solvent-free crystals, and is the method of choice. 

Figure 11.3 H MAS NMR spectra of the reaction products of [TiNp4] with (a) fully deuterated SiO2-(S00), prepared by the sublimation method (b-d) SiO2-(500), SiO2-(700), and SiO2-(200), respectively, prepared by the impregnation method and (e and f) MCM-41 (500) and MCM-41 (200),...

The chemical vapour deposition (CVD) technique can be regarded as an extension of the sublimation method used for the preparation of single crystals discussed before, but adapted to thin him growth implying deposihon on substrates. A schematic diagram of a horizontal hot-wall CVD apparatus is shown in Fig. 3.15. 

This form of selenium can also be produced from amorphous selenium by heating with quinoline, pyridine, aniline or other basic organic solvent,7 and also by sublimation. In this latter case the sublimate also contains the amorphous form.8 Atmospheric oxidation of solutions of alkali selenides gives this variety of selenium as a granular deposit. When produced by these last three methods, leafy crystals may be obtained, and Muthmann (1890) discovered that crystals which he obtained by the sublimation method were of the trigonal (rhombohedral) system and isomorphous with those of tellurium. 

The technical-grade molybdenum(VI) oxide obtained as described previously can be further purified by solvent extraction as an alternative to the conventional sublimation method,216-217 the molybdenum being brought into solution by leaching of the oxide with sodium hydroxide ... 

One of the methods of growing bulk GaN is a sublimation method [1-3], In this method, the source GaN powder and a seed crystal or a substrate facing the source are loaded in a crucible. Upon heating the crucible, the source powder sublimates and recrystallises onto the seed crystal or on the substrate. When a system needs a reaction gas such as NH3 for growth, the system is not a sublimation but a VPE system. Now, a 4-inch SiC wafer grown by a sublimation method is commercially available [4],... 

In this Datareview, bulk growth of GaN and AIN by a sublimation method and of GaN by a sublimation sandwich method is described. The source powder was analysed. The bulk GaN obtained was characterised by XRD (X-ray diffraction), TEM (transmission electron microscopy), and so on. 

FIGURE 2 Bulk GaN obtained by the sublimation method, (a) Bulk GaN grown on sapphire substrate, (b) Free-standing bulk. 

The sublimation sandwich method (SSM) is similar to the sublimation method, except for the small distance between source powder and substrate, 2-5 mm [10-12], Ga or GaN was used as a source and the substrate was sapphire or 6H-SiC. The optimal growth temperature was around 1200°C as temperatures lower than 1150°C caused the formation of structural defects such as voids and micropipes. Polycrystalline GaN was grown under 1050°C. At higher temperatures, GaN was thermally decomposed. The growth rate is much higher, up to 300 pm/hr [10] or 0.2 - 1.1 mm/hr [12], and crystalline GaN with the maximum thickness of 500 pm could be obtained. 

Bulk growth of GaN and AIN has been achieved by a sublimation method and a sublimation sandwich method. Bulk GaN and AIN bulk crystals were proved to have high crystallinity. It will improve the quality of nitride-based optoelectronic devices, if these bulk crystals are used as substrates for homoepitaxial growth. The size of the bulk GaN, however, is not large enough at this moment, and enlargement of bulk GaN may be necessary. 

In this kind of separation, the target must be very thin to get good yields of the recoiled atoms. Thus the preparation technique for thin targets is very Important. For such purposes electrodeposition or vacuum sublimation methods can often be used to advantage. 

S. Kuarai, K. Nishino and S. Sakai, Nucleation control in the growth of bulk GaN by sublimation method , Jpn J. Appl. Pbys., 36, LI84 (1997). 

In addition to showing that solvent-free melt and sublimation crystallization conditions offer an attractive route to new polymorphs, a CSD survey of these methods of crystallization and the frequency of Z was performed (Table 3-5). There is a dramatic increase in the occurrence of Z > 3 crystal structures when melt or sublimation crystallization conditions are used [20]. The occurrence of high Z in melt crystallization and sublimation methods is ascribed to the rapid cooling of the hot liquid or vapor (100-300° C) in the open flask or on the cold finger (kinetic phase), conditions under which hydrogen-bonded clusters are likely to condense in a pseudo-symmetric crystalline arrangement. On the other hand, the slower nucleation process of solution crystallization gives the frequent situation of Z < 1 (88% hits). 

The sublimation method is used for mass transfer measurements in air flows. For measurement in some liquids, the electrochemical technique can be used. 

In the 6H polytype, with the stacking sequence ABCACB, the (0001) carbon face and the (0001) silicon face etch differently because the SiC bond is slightly ionic. Many researchers use this fact to determine the difference between the two faces. Koga et al [9] used KOH to study three kinds of etch pits in 6H-SiC single crystal grown by the sublimation method. They observed that by increasing the pressure they could reduce one type of etch pit. 

C-SiC bulk material is required for device applications. However, usable sizes of 3C-SiC bulk are not produced. In 1987, Yoo et al [50] obtained 3C-SiC films 100 pm thick on 3C-SiC CVD grown substrates (20 pm thick) by the sublimation method. In 1993, Furukawa [51] reported 10 mm diameter, 4 mm long 3C-SiC crystals grown by a sublimation method. Several attempts followed, but reproducible bulk crystals like 6H-SiC polytype have not been reported as yet [52-55]. 

Epitaxial growth was carried out by modifying the sublimation method, the so-called sandwich method [8,56-60]. In this method, source and substrate are separated by a small gap of about 1 mm. The growth is carried out close to equilibrium in this arrangement. In this method, the substrate is thermally etched before sublimation begins. This makes it possible to grow, at low rates, thin homo-epitaxial layers on 6H and 4H a-SiC with residual impurity concentration. This method is used for device fabrication. 

The high efficiency of the method is evidenced by the fact that the first sublimation method of growing the crystalline material, the Acherson method, was proposed at the beginning of the 20th century and it is used today with only small variations. The Acherson process yields material for abrasive use and the rate of production is really very high, more than half a million tons per year [1]. No other technique of growing silicon carbide can be compared with sublimation in its productivity and efficiency. 

Sublimation method for indirect separation of enantiomers is very similar, in principle to the above mentioned distillation-based procedure. In the next example the diastereoisomeric molecular complexes could be separated using the significant difference between their thermal stability. 

Two basic principles are commonly used for the preparation of OLEDs the sublimation method, in which the organic layers are prepared by vapor deposition results in well-defined layers of excellent purity but tolerates only low molecular mass molecules with high temperature stability [16]. The less expensive preparation out of solution, requires soluble substances or precursors [17] and is therefore widely used in combination... 

Sublimation method with spontaneous nucleation (mass crystallization). Crystal growth is carried out in cylindrical graphite crucibles (Fig. 14). Powder silicon carbide is used as a source material. Crucibles are heated in cylindrical furnaces with resistive or inductive heating. Growth proceeds in an inert atmosphere (argon, helium) at a temperature of 2500-... 

---