RU2261295C1 - Germanium monocrystal growing method - Google Patents

Abstract

FIELD: crystal growing.

SUBSTANCE: method comprises growing germanium monocrystals from melt onto seed followed by heat treatment, the latter being effected without removing monocrystals from growing apparatus at temperature within 1140 and 1200 K during 60-100 h, temperature field being radially directed with temperature gradient 3.0 to 12.0 K/cm. Once heat treatment comes to end, monocrystals are cooled to 730-750°C at a rate of at most 60-80 K/h. Monocrystals are characterized by emission scattering at wavelength 10.6 μm not larger than 2.0-3.0% and extinction not higher than 0.02-0.03 cm^-1, which is appropriate for use of monocrystals in IR optics.

EFFECT: allowed growth of germanium monocrystals with high optical characteristics.

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Description

Technical field

The invention relates to methods for growing single crystals of germanium from a melt.

State of the art

A known method for producing semiconductors, including Germany (GB 755845 A (ASS ELECT IND), 08/29/1956), comprising melting the starting material in a crucible and then crystallizing the melt on a rotating seed. The method is used to create pn junctions by detaching an ingot from one melt and then placing another type of conductivity in the melt. To eliminate defects in the crystal structure that arise due to rapid cooling, the ingot is annealed for some time at a given temperature, thereby reducing the concentration of defects. The method cannot be directly used to obtain single crystals of germanium used in infrared optics; harsh temperature conditions for the formation of the ingot significantly reduce the structural perfection of the crystal lattice and degrade electrophysical, optical and other properties.

The closest in essence is a method for growing germanium single crystals from a melt by the Czochralski method and using additional heat treatment of a single crystal (Gafni G., Azoulay M., Shiloh C. et al. Large Diameter Germanium Single Crystals for Infrared Optics // Optical Engineering. 1989. V. 28. No. 9. P.1003-1007). Single crystals of germanium for optical use (infrared optics) with a diameter of 159-200 mm were grown by the Czochralski method and then heat treated. In germanium single crystals, the inhomogeneity of the refractive index and the values of the modulation transfer function were analyzed. The purpose of the heat treatment was to increase the optical homogeneity of the material and reduce optical loss. The goal was achieved by applying annealing of optical samples, which was carried out by heating to a temperature of 450 ° C, holding at this temperature for several days and further cooling to room temperature at a speed of 2 ° C per minute.

One of the important optical characteristics of materials used in optics — radiation scattering — was not considered and analyzed by the authors. It is known that the dislocation structure of germanium (dislocations and dislocation defects such as small-angle boundaries, slip lines) can lead to significant scattering of infrared radiation in optical germanium. High dislocation densities and especially the unevenness of their distribution do not allow one to obtain acceptable optical parameters — high transmission of radiation at a wavelength of 10.6 μm and sufficiently small radiation scattering. In some cases, with these parameters, single crystals of germanium are unsuitable for use in infrared optics. The low-temperature (450 ° С) annealing used by the authors cannot affect the dislocation structure; such an effect can be significant only in the region of germanium plasticity (at temperatures above 600 ° С); the maximum effect will be observed near the melting temperature (943 ° С).

SUMMARY OF THE INVENTION

The basis of the invention was the task of reducing radiation scattering at a wavelength of 10.6 μm while maintaining or even increasing the level of optical transmittance. The task was achieved by annealing, which is part of the entire process of obtaining a single crystal: from melting the load in the crucible to removing the single crystal from the installation. At the final stage, annealing of a single crystal grown from a melt was carried out without removing it from a growing unit. Annealing was carried out at a temperature close to the crystallization temperature (1210 K) for 60-100 hours with the obligatory presence of a temperature gradient directed along the radius of the single crystal. Thus, the temperatures of the central part of the single crystal and its periphery were significantly different.

Short description

The technological cycle of growing germanium single crystals with annealing is as follows: pre-chemically etched zone-purified germanium is loaded into the graphite crucible of the installation. A single crystal seed is mounted in the seed holder, oriented in the desired crystallographic direction (usually <111>). The growing unit is hermetically sealed. The supply of cooling water is turned on, the necessary vacuum is created in the installation. The resistance heater turns on. Germanium is melted, then its temperature is lowered to create the necessary supercooling of the melt. The seed is lowered into the melt, after which a single crystal is grown. After completion of the growth of the single crystal, the average temperature of the single crystal is set at 1140-1200 K. The average temperature is defined as the temperature difference at the periphery and in the center of the single crystal. The cooling of the installation, the seed holder and the power of the heater are controlled in such a way that the necessary temperature difference between the center of the single crystal and its periphery is created and maintained. The essence of this regulation is the creation of the required temperature gradient in the range of 3-12 K / cm. The magnitude of the gradient is selected depending on the diameter of the single crystal.

At this temperature, the single crystal is aged in the installation for 60-100 hours (depending on diameter). In general, the annealing mode of a single crystal depends on its diameter. An increase in the diameter of the grown ingots leads to an increase in the annealing time, a decrease in temperature gradients, and a decrease in the average temperature. The indicated gradients and annealing durations were established experimentally based on the results of determining the optical characteristics of single crystals, the main of which in this case was radiation scattering at a wavelength of 10.6 μm.

After completion of the exposure of the single crystal at the indicated temperature, the temperature decreases, and a decrease to 730-750 K occurs at a speed of not more than 60-80 K / h.

After complete cooling, the single crystal is removed, processed and transferred to determine the required technical characteristics.

Method implementation examples

Example 1

1.1. Single crystal without applying the invention:

length - 160 mm; diameter - 60 mm;

scattering by 10.6 μm in the central part of the single crystal - 8.5%;

absorption rate <0.02 cm ^-1 .

1.2. Single crystal using the invention (with annealing):

length - 150 mm; diameter - 58 mm;

annealing time - 60 hours;

temperature gradient - 11 K / cm;

average temperature - 1165 K;

scattering by 10.6 μm in the central part of the single crystal - 2.5%;

absorption rate <0.02 cm ^-1 .

Example 2

2.1. Single crystal without applying the invention:

length - 40 mm; diameter - 132 mm;

scattering by 10.6 μm: in the central part of the single crystal - 6.0 ± 1.0%,

on the periphery - 10.0 ± 2.0%;

the absorption rate at 10.6 μm <0.02 cm ^-1 .

2.2. Single crystal using the invention (with annealing):

length - 46 mm; diameter - 132 mm;

annealing time - 72 hours;

temperature gradient - 5 K / cm;

average temperature - 1180 K;

scattering by 10.6 μm - 2.0 ± 0.5%;

the level of this indicator is uniform;

the absorption rate at 10.6 μm <0.015 cm ^-1 .

Example 3

3.1. Single crystal without applying the invention:

length - 28 mm; diameter - 150 mm;

scattering by 10.6 μm: in the central part of the single crystal - 6.5 ± 0.5%,

at the periphery 10.0 ± 1.0%;

absorption rate <0.025 cm ^-1 .

3.2. Single crystal using the invention (with annealing):

length - 29 mm; diameter - 150 mm;

annealing time - 80 hours;

temperature gradient - 4 K / cm;

average temperature - 1190 K;

scattering by 10.6 μm - 1.5 ± 0.5%;

the level of this indicator is uniform;

the absorption rate at 10.6 μm <0.015 cm ^-1 .

Industrial applicability

The application of the method allowed to significantly improve the optical parameters of germanium single crystals used in IR devices. In essence, it was possible to use such germanium in those devices where unsatisfactory data on the scattering of radiation did not make it possible to create devices.

Single crystals obtained by the proposed method are used not only in new developments, but also in serial production in industry.

Claims (1)

A method of growing germanium single crystals from a melt to seed, followed by heat treatment of a single crystal, characterized in that the heat treatment is carried out without removing the single crystal from the growth apparatus at a temperature of 1140-1200 K for 60-100 h in the presence of a temperature gradient directed along the radius of the single crystal within 3.0 -12.0 K / cm, after which the single crystal is cooled to 730-750 K at a speed of not more than 60-80 K / h.