Method of growing germanium crystals

Abstract

In accordance with the present invention, taught is a high purity germanium crystal growth method utilizing a quartz shield inside a steel furnace. The quartz shield is adapted for not only guiding the flow of an inert gas but also preventing the germanium melt from contamination by insulation materials, graphite crucible, induction coil and stainless steel chamber. A load cell provides automatic control of crystal diameter and helps to ensure exhaustion of the germanium melt. The method is both convenient and effective at producing high purity germanium crystals by relatively low skilled operators.

Claims

1. A method of growing high purity germanium crystals, comprising the steps of: (a) isolating a first crucible containing molten germanium inside a melt container comprising a quartz shield having a removable lid; (b) locating said melt container inside a furnace for controlling the temperature of said germanium; (c) passing an inert gas through said melt container; (d) controlling the diameter of a crystal of germanium drawn from said first crucible at a rate controlled by a load scale that monitors the weight of the crystal; and (f) removing the lid from the quartz shield to permit withdrawl of the germanium crystal from the melt container.

2. The method of claim 1, wherein said molten germanium has a net carrier concentration between about 10.sup.9 and about 10.sup.10 cm.sup.3.

3. The method of claim 1, wherein said quartz shield includes a quartz lid and a quartz tube.

4. The method of claim 3, further comprising a graphite crucible and wherein the quartz tube is located between the graphite crucible and said first crucible.

5. The method of claim 1, wherein said inert gas is hydrogen and the flow rate is between about 50 and about 250 L/h.

6. The method of claim 1, wherein a weigh signal from the load scale is used to modify the temperature of the molten germanium to control the crystal diameter and the exhaustion of all the melt.

7. The method of claim 1, wherein said crystal of germanium has a dislocation density of between about 10.sup.2 and about 10.sup.4 cm.sup.2.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 is schematic figure of a crystal growing furnace of the present invention.

(2) FIG. 2 is schematic figure of a quartz lid and quartz tube of the present invention.

(3) FIGS. 3(a) and (b) are graphical representations of the changes in the relationship between weight and power over time for a 3 crystal and a 9 crystal, respectively.

(4) FIGS. 4(a) and (b) are graphical representations of the crystal diameter and length for obtained 3 crystal and obtained 9 crystal, respectively.

DISCLOSURE OF INVENTION

(5) It is an object of the present invention to use a stainless steel furnace and provide a quartz shield designed to isolate the germanium melt from contaminations that are a common problem in the art, such as phosphorus (P), arsenic (As), boron (B), and the like, which may come from the stainless steel chamber and insulations.

(6) In a preferred embodiment, a stainless steel furnace is modified to practice the method. Located inside a stainless steel chamber 1 of the furnace is a quartz shield consisting of a quartz tube 5 topped by a quartz lid 4 (FIG. 1). Inside the quartz shield is a silica crucible 7 which contains molten germanium from which a germanium crystal 6 is drawn by a seed shaft 2 that extends downwardly through an aperture in the quartz lid 4. An inert gas, preferably hydrogen, flows into the quartz shield from an inlet pipe 3, out into the steel chamber 1 thorough aperture 4 and is exhausted through outlet pipe 12. The bottom section of the quartz shield is enclosed by a graphite crucible 8 which in turn is surrounded by insulation 10. A radio frequency (RF) coil 9 partially surrounds the graphite crucible 8 to adjustably heat the germanium. The RF coil 9, insulation 10, graphite crucible 8, quartz shield, silica crucible 7 and molten germanium are all supported on a supporter 11. A load scale (not shown) monitors the weight of the growing crystal.

(7) In operation, the steel chamber 1 is evacuated down to 10.sup.6 Torr. Highly pure H.sub.2 is then injected through inlet pipe 3 at a pressure between 26 PSI and a flow rate of between 50250 l/h. The hydrogen flow can prevent the contaminations going into the quartz tube 5. The contaminations such as P, As, B, etc., from the stainless steel chamber 1 and insulations will be isolated from the germanium melt by the quartz shield and H.sub.2 flow. Therefore, the purity of germanium crystal will be preserved. The graphite crucible 8 is heated by the RF induction coil 9 to melt the germanium ingot and the crystal growth process is started. A pulling rate of between 50 to120 mm/h and any value within such range is used. A rotation rate of between 5 to 30 rpm and any value within such range is used. An axial thermal gradient of between 10 to 40 K/cm and any value within such range is used. The load cell is used to control the crystal growth process according to a software control. Preferably, the molten germanium has a net carrier concentration between about 10.sup.9 and about 10.sup.13 cm.sup.3.

(8) In accordance with the present invention, we provided a HP-Ge crystal growth method, comprising a quartz shield for HP-Ge crystal growth by in a load cell controlled stainless steel furnace and automatic diameter control of between about 3 cm and about 15 cm in diameter HP-Ge crystal growth.

(9) In a preferred embodiment, a load cell with sensitivity 0.01 g is placed in line with the seed shaft 2 to which the seed crystal 6 is attached. As the crystal 6 is pulled from the melt, the change in weight is used to generate a control signal that modifies the RF generator output power to the crucible, thereby controlling the diameter through small changes in the melt temperature. FIG. 3 shows the typical relationship between crystal weight and RF generator power for 3 and 9 cm crystals, respectively. The balance or load cell shows the weight of grown crystal and makes it easy to exhaust the melt to avoid the left-over melt breaking the silica crucible. When the last melt drop is solidified, this solidified drop will touch the crucible. Before this happens, very fast pulling speed (typically 2 cm/min) should be applied, otherwise, the seed will be broken and the crystal will drop down.

(10) The crystal diameter is calculated from the formula:

(11) $\begin{array}{cc}d=\sqrt{\frac{4.Math.\mathrm{dw}/\mathrm{dt}}{\frac{\mathrm{dw}/\mathrm{dt}.Math.{}_s}{R^2.Math.{}_l}+p.Math..Math.{}_s}}& \left(1\right)\end{array}$
where dw/dt is the rate of change of weight , .sub.s the solid density, .sub.L the liquid density, R the crucible radius and P the pull speed. FIG. 4 shows the calculated crystal diameter for 3 and 9 cm crystals basing on the change in weight of the crystal growth process. After the crystals taken out from the furnace, the crystal's diameter was measured by calipers (as shown in FIG. 4). The results of experiments and measurements are in good agreement, such that Equation (1) can be used to monitor the crystal diameter in crystal growth process. This makes it possible for the automatic control in high purity germanium crystal growth.

(12) After crystal growth, the supporter 11 will move down and the seed shaft 2 with crystal 6 will be pulled up. When the crystal 6 with the quartz lid 4 exits the quartz tube 5, the crystal 6 is held by hand and the neck is cut. Then the crystal 6 is taken out of the furnace.

(13) The present invention will be explained in more detail with reference to examples and comparative examples, but the present invention is not restricted thereto.

EXAMPLE 1

(14) The pulling apparatus shown in FIG. 1 was filled with a germanium raw material of 900 g into a quartz crucible 7 having an inner diameter 70 mm. The diameter and length of quartz tube 5 is 90 mm and 450 mm, respectively. The hydrogen flow rate was 80 L/h. The raw materials were melted by radio frequency (20 KHz) power. Then the crystal growth was controlled as shown in FIG. 3(a). The crystal weight was used to control the crystal diameter and the exhaustion of melt. The measured diameter and the calculated diameter from Equation (1) of the grown crystal are shown in FIG. 4(a).

EXAMPLE 2

(15) The pulling apparatus shown in FIG. 1 was filled with a germanium raw material of 4000 g into a quartz crucible having an inner diameter 180 mm The diameter and length of quartz tube 5 was 220 mm and 450 mm, respectively. The hydrogen flow rate was 100 L/h. The raw materials were melted by radio frequency (20 KHz) power. Then the crystal growth was controlled as shown in FIG. 3(b). The crystal weight was used to control the crystal diameter and the exhaustion of melt. The measured diameter and the calculated diameter from Equation (1) of the grown crystal were shown in FIG. 4(b).

(16) The foregoing description and drawings comprise illustrative embodiments of the present inventions. The foregoing embodiments and the methods described herein may vary based on the ability, experience, and preference of those skilled in the art. Merely listing the steps of the method in a certain order does not constitute any limitation on the order of the steps of the method. The foregoing description and drawings merely explain and illustrate the invention, and the invention is not limited thereto, except insofar as the claims are so limited. Those skilled in the art that have the disclosure before them will be able to make modifications and variations therein without departing from the scope of the invention.

REFERENCES

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