Multi-element compounds have been used ubiquitously in various applications, including electronics, optics, opto-electronics, thermoelectrics, superconductivity, and the recently developed application of spintronics. Besides being the main components of some of these devices, the bulk form of these compounds is needed as a standard for fundamental property characterizations as well as the starting materials for thin-film deposition. Hence, the chemical purity and crystalline quality of these bulk compounds are critical for the applications.
The typical process of synthesizing high-purity bulk multi-element compounds involves sealing a fused quartz/silica ampoule loaded with the elements under vacuum condition, heating the sealed ampoule to above the melting point of the compound, cooling the ampoule in a controlled manner, and finally, retrieving the synthesized material. In many instances, the resultant sample reacts with and adheres to the inner wall of the ampoule, also known as wetting. Since the samples usually have larger thermal expansion coefficients than that of the ampoule, the samples shrink more than the ampoule during cooling. Consequently, the wetting, or the adhesion of samples to the ampoule inner surface, causes interior cracks and the resultant inferior crystalline quality and low yield of the sample. This problem is especially prevalent when the more reactive elements, i.e., group I to IV in the periodic table, are involved. Certain methods, such as coating the internal surface of the ampoule with carbon, have been adopted to reduce this interaction between samples and the ampoule wall. The coating is usually not uniform, thick enough, or reproducible such that local wettings cannot be completely eliminated. It also causes the undesired contamination of the sample by the introduction of carbon.
In this invention, because the wetting is caused by the chemical reactions of the sample elements and the impurities in the ampoule — specifically free oxygen at elevated temperatures — the crystalline quality of the resultant materials can be vastly improved by the following two steps: 1) specifically preparing the fused quartz/silica ampoule, and 2) carefully processing the heating procedure of the alloying to minimize the wetting. The chemical purity will be maintained near the level of that of the starting elements since no extraneous chemicals, such as carbon, will be introduced during the synthesis.
The purpose of the procedure in step 1 is to reduce the OH species content in the synthesis ampoule by continuously subliming and evacuating them from ampoule interior and exterior surfaces as they are diffusing from the inside of the quartz/silica tubing toward the surfaces. The procedures for the preparation of the synthesis ampoule are:
- Procure fused quartz/silica tubing with lower OH content.
- Make the synthesized ampoule from the selected tubing by glass blowing with a hydrogen/oxygen torch.
- After cleaning the empty synthesis ampoule, place it inside a larger fused quartz/silica ampoule.
- Connect the whole ampoule assembly to a vacuum pump and evacuate it to a vacuum level lower than 10-4 Torr.
- Slide a tubular furnace to cover the majority length of the ampoule assembly.
- Heat the furnace to the baked temperature (at least 1180 ºC) and bake for the planned time — at least 16 hours for an ampoule with a 2-mm-thick wall (longer time for a thicker wall) — before cooling to room temperature by turning the power to the furnace.