RU2483130C1 - Method of producing isotopically-enriched germanium - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: proposed method comprises plasma chemical decomposition of isotopically-enriched germanium in the mix with hydrogen in nonequilibrium plasma of HF discharge and depositing germanium on substrate. Note here that said deposition is performed outside the discharge fire zone at the pressure of 200-300 mTorr, GeF₄-to-H₂ ratio making at least 1:4 and their total flow rate of 100-150 cm³/min. Output of this method makes at least 5 g/h of polycrystalline germanium with yield of finished product making 90-95% which allows growing monocrystals of isotopically-enriched germanium of good quality in amount of, at least, several tens of grams.

EFFECT: higher efficiency and yield.

1 ex, 1 tbl

Description

The invention relates to a technology for producing isotope-enriched germanium. Germanium isotopes are used for the production of certain classes of semiconductor devices with unique properties, nuclear-physical transformation detectors, and biomedical research.

In the technology of separation and enrichment of germanium isotopes, the main working substance is isotope-rich germanium tetrafluoride.

A known method of producing isotope-enriched germanium, based on the hydrolysis of its tetrafluoride with hydrogen (see RF patent No. 2280616, MKI C01G 17/02, C22B 41/00, publ. July 27, 2006).

The method consists in the fact that the isotope-enriched fraction of germanium tetrafluoride is dissolved in a mixture of ethyl alcohol and carbon tetrachloride in the presence of a complexing agent, for example, citric acid, a solution of hydrogen peroxide, nitric acid is added to the resulting solution, and it is evaporated to dryness, and the dry residue calcined and sent to recovery with hydrogen.

The output of the germanium isotope - 76 with an enrichment of at least 99.0% in the known method is at least 97%, chemical purity - 99.9%. The disadvantage of this method is its multi-stage, the use of a large number of additional chemical reagents, requiring, as a rule, additional purification. Therefore, the hydrolysis process may be accompanied by uncontrolled contamination of the target product and does not provide high performance. In addition, the source in question does not report on the form and purity of poly- and single-crystal germanium oxide obtained by reduction.

Another known method for producing isotope-enriched germanium is based on direct plasmochemical reduction of germanium tetrafluoride with hydrogen in a low-temperature nonequilibrium plasma (see P.G. Sennikov, S.V. Golubev, V.I. Shashkin, D.A. Pryakhin, N.V. .Abrosimov. Promising materials. Special issue. 2011. No. 10. S.160-166).

The method involves the deposition of germanium by decomposition of germanium tetrafluoride enriched in the germanium isotope 74 (83%), in a mixture with hydrogen in an HF (13.56 MHz) inductively coupled plasma. The deposition of germanium in the form of polycrystals with an average linear size of several mm and a thickness of up to 100 μm was carried out on the inner surface of the quartz tube directly in the combustion zone of the discharge at a pressure of 50-100 mTorr and a general ratio of GeF ₄ and H ₂ fluxes of 1:10. A single crystal of germanium - 74 is grown from the collected powder by the Czochralski method, specially developed for small loads (NVAbrosimov, H.Riemann, W.Schroder, H.-J. Pohl, AKKaliteevski, ONGodisov, VAKorolyov, A.Ju.Zhilnikov, ²⁹ Si and ³⁰ Si single crystal growth by mini-Czochralski technique, Cryst. Res. Technol. 38 (2003) p. 654-658).

An advantage of the known method is that, to activate the reaction, it is possible to introduce energy directly into the germanium tetrafluoride molecule, which allows localizing the reaction zone and eliminating or minimizing the ingress of impurities from the reactor material (See, e.g., F. Jansen. Plasma-Enhanced Chemical Vapor Deposition. In .: Handbook of Vacuum Science and Technology. Elsevier. 1998. Ch. 5.5. P.711-713).

The disadvantage of this method is the low productivity of about 0.5 g / hour. Due to the fact that germanium is deposited directly in the combustion zone of the discharge, this leads to screening of the discharge zone by polycrystalline germanium growing on the walls of the reactor and discharge attenuation. To resume deposition, it is necessary to first stop the process, open the reaction chamber and remove the overgrown layer of germanium. In the described method, the conversion of germanium tetrafluoride to germanium is at least 85%. The Ge-74 single crystal grown from the obtained powder (85.4%) weighing about 10 g has an n-type conductivity, n = 9 · 10 ¹⁴ cm ³ , and a specific resistance of 1.9 Ω · cm. The content of gas-forming impurities (Н, О, С, F) in the obtained crystal is at the level of 10 ¹⁵ -10 ¹⁶ cm ^-3 , the content of electroactive impurities (B, P, As) is less than 5 · 10 ¹⁵ -10 ¹⁶ cm ^-3 .

The mentioned solution is taken as a prototype.

The problem to which the invention is directed is the development of a continuous technology for the direct production of isotope-enriched germanium by plasma-chemical vapor deposition, aimed at increasing the yield and productivity of the method by increasing the energy input into the discharge using a more powerful RF generator, optimized pressure value in the reactor and the ratio of gas flow rates.

This problem is solved due to the fact that in the known method for producing isotope-enriched germanium by plasma-chemical decomposition of the corresponding isotope-enriched germanium tetrafluoride mixed with hydrogen in a nonequilibrium plasma of an RF discharge and deposition of germanium on a substrate according to the claimed solution, germanium is deposited outside the discharge combustion zone at a pressure of 200-300 mTorr, a ratio of GeF ₄ and H ₂ flows of at least 1: 4 and their total speed of 100-150 cm ³ / min.

The essence of the invention lies in the fact that the deposition of germanium is carried out outside the combustion zone of the discharge, while the conditions for the deposition of germanium tetrafluoride are developed, namely, the method is carried out at a pressure of 200-300 mTorr, the ratio of GeF ₄ and H ₂ flows is not less than 1: 4 and the total flow rate GeF ₄ and H ₂ 100-150 cm ³ / min. These conditions provide, in comparison with the prototype, an increase in the yield of germanium by 10% and an increase in the productivity of the method — not less than 10 times, while the method is carried out under conditions of continuous deposition.

It was experimentally established that at a pressure of less than 200 mTorr, a decrease in the productivity of germanium deposition occurs by about half. At pressures above 300 mTorr, the discharge goes out.

It was also experimentally established that carrying out the reaction with a flow ratio of H ₂ and GeF _{4 of} at least 4 is optimal for the maximum possible yield of germanium (up to 95%) and the maximum possible productivity of the method (5 g / h). With a ratio of H ₂ and GeF ₄ streams _of less than 4, the germanium yield decreases by about 5 times, the formation of polyfluorogermans also sharply increases, and with a ratio of H ₂ and GeF ₄ streams _of more than 4, the productivity of the process is reduced by half.

It was experimentally established that at a total flow rate of GeF ₄ and H ₂ less than 100 cm ³ / min, the discharge zone is rapidly shielded by a layer of polycrystalline germanium growing on the walls of the reactor and plasma attenuation. At a total flow rate of GeF ₄ and H _{2 of} more than 150 cm ³ / min, the contact time of the gas mixture with the plasma decreases, which leads to a sharp decrease in the yield of germanium (up to 50%). The selected flow rate of the reagents ensures the deposition of germanium in the region starting from half of the discharge zone, while the other half, on which no deposition occurs, ensures the penetration of the electromagnetic field into the reactor, which contributes to a long continuous deposition process.

All the mentioned features are essential, since each of them is necessary, and together they are sufficient to solve the problem - to increase the productivity of the method for producing isotope-enriched germanium and carrying it out under continuous production conditions.

The performance of the proposed method is at least 5 g / h of polycrystalline germanium, while in the prototype the productivity is 0.5 g / h. The yield was 90-95% against 85% in the prototype. The mentioned amount of germanium obtained by the developed technology is already sufficient for growing single crystals of isotope-enriched germanium of semiconductor quality weighing at least several tens of grams. In connection with the foregoing, the claimed technology can be recommended for the organization of the corresponding production.

Example. Obtaining a powder of a polycrystalline isotope germanium-72 (enrichment of 52%) and growing a single crystal from it. A substrate in the form of a quartz tube is placed in a quartz glass reactor, on the inner surface of which germanium is deposited. Then the reactor is pumped to a residual pressure of 10 ^-5 Torr and filled with an inert gas and turn on the discharge. Input power is 835 watts. The resulting low-temperature plasma cleans the inside of the reactor from moisture and other adsorbed impurities. Then the isotopically enriched germanium tetrafluoride - 72 in a mixture with hydrogen in a ratio of 1: 4 is fed into the reactor. The hydrogen consumption in this case is 100 cm ³ / min, the consumption of germanium tetrafluoride 25 cm ³ / min. After the decomposition of germanium tetrafluoride is completed, an inert gas is fed into the reactor and the discharge is turned off. The system is purged with an inert gas. After that, the reactor chamber is opened and the substrate with precipitated powdered polycrystalline germanium is transferred to a specially prepared box with an inert atmosphere, where the precipitated germanium - 72 is unloaded, which is then placed in a container with an inert atmosphere. The deposition rate of germanium is 5 g / h, the yield is 90%.

The content of electroactive and gas-forming impurities in the obtained germanium according to mass spectrometry of secondary ions is shown in table 1.

Table 1 The content of electroactive and gas-forming impurities in polycrystalline germanium-72 Impurity AT R As N 0 from F at / cm ³ <1.5 · 10 ¹⁶ <1 · 10 ¹⁷ 4,510 ¹⁶ 10 ¹⁷ ÷ 10 ¹⁸ 10 ¹⁹ ÷ 10 ²⁰ 5 · 10 ¹⁷ ÷ 5 · 10 ¹⁸ 2 · 10 ¹⁸ ÷ 2 · 10 ¹⁹

From the obtained germanium - 72, a single crystal with a (100) orientation of 50 g mass was grown by the Czochralski method. The concentration of charge carriers is n = 1 · 10 ¹⁵ at / cm ³ , the resistivity is ρ = 1.5 Ohm · cm. The oxygen concentration is 1 · 10 ¹⁶ at / cm ³ . The concentration of carbon and fluorine was approximately at the same level. Thus, the main electrophysical parameters of the obtained crystal approximately correspond to those for a germanium crystal - 74, obtained according to the prototype, while an order of magnitude with a higher deposition rate and a large yield. Then, from the obtained germanium single crystal - 72, as a result of three subsequent growth processes, a single crystal was obtained with a mass of 26 g with n = 3 · 10 ¹³ at / cm ³ , ρ = 42 Ohm · cm and an oxygen concentration of less than 1 · 10 ¹⁵ at / cm ³ , which corresponds to semiconductor quality.

Along with isotope-enriched germanium - 72, which is mentioned in the example, in this way, other germanium isotopes can be obtained under continuous deposition conditions.

The proposed method allows to obtain continuously isotopic modifications of germanium in an amount sufficient to grow single crystals of semiconductor quality, weighing at least several tens of grams.

Claims (1)

A method for producing isotope-enriched germanium, including plasma-chemical decomposition of isotope-enriched germanium tetrafluoride mixed with hydrogen in a non-equilibrium RF discharge plasma and deposition of germanium on a substrate, characterized in that the germanium is deposited outside the discharge combustion zone at a pressure of 200-300 mTorr, flow ratio GeF ₄ and H ₂ not less than 1: 4 and their total flow rate of 100-150 cm ³ / min.