Growth striations are a major problem in growth crystal, which limits the application of semiconductor single crystals in nanoelectronics. External fields such as microgravity, magnetic fields, and ultrasound can create crystal growth conditions for homogeneous component distribution in semiconductor single crystals. Our group has been studying the effect of high frequency ultrasound on the growth of semiconductor crystals by the Czochralski method for the past 28 years and demonstrated the potential of this method for the striation decrease. Experimental results of GaAs, InSb, Bi_xSb_1-x, and Ga_xIn_1-xSb solid solution growth are unclear, depending on their physical and chemical properties.

The direction of ultrasonic waves was parallel to the pulling axis in the first series of our experiments. InSb, Bi_xSb_1-x, and Ga_xIn_1-xSb single crystals with 10 mm diameter were grown with rotation rates of < 10 rpm from 150 g melt, without crucible rotation. The crystals were pulled in the atmosphere of high purity Ar at pressures 0.2÷2 atm. The ultrasound was introduced into the melt from a piezotransducer through a fused silica waveguide with 10 mm diameter and 300 mm length, fused to the bottom of the silica crucible. Each single crystal had regions, which were pulled with and without ultrasound. The pulled single crystals were cut parallel to the growth direction, ground, polished and etched in order to reveal the growth striations under an optical microscope and an electronic microscope “Nanolab-2100”.

Sb striations in Bi_xSb_1-x single crystals (x = 0.03÷0.15) with separations of 7 to 400 μm were located mainly parallel to or at an angle of 60° with the solid-liquid (S/L) interface, and were observed in each region grown without ultrasound. The introduction of ultrasound at frequencies of 0.65, 2.5, or 5 MHz into the melt eliminated Sb striations in Bi_xSb_1-x single crystals grown with constant diameter and in the central part of the crystals with changing diameter. It was found that for constant diameter growth, after “processing” the melt with ultrasound, the striations did not reappear until 2 hours have passed.

InSb:Te single crystals were grown in the <111> direction. The effect of ultrasonic field at frequencies 0.25, 0.6, 1.2, 2.5, 5, or 10 MHz eliminated the striations with width of 5 to 15 μm in the facet growth region. However, striations with smaller periodicity remained. A similar result of incomplete striation removal was observed in Ga_0.03In_0.97Sb single crystals grown with ultrasound at frequencies 0.72 and 1.44 MHz.

GaAs single crystals with 50 mm diameter were pulled from 1.5 kg of the melt containing Cr impurity at a concentration of 10¹⁶ cm^-3 using B₂O₃ as an encapsulant. The seed rotation rate was 4–5 rpm, and the crucible rotation rate was 16–18 rpm, in opposite directions. An investigation of As distribution was carried out in GaAs single crystals pulled in the presence of ultrasound at a frequency of 0.15 MHz. Striations with separations of 25 to 140 μm were located parallel to the (S/L) interface. In the region where the diameter of the crystal was changed, the ultrasound did not decrease the As inhomogeneity. There was a decrease of As inhomogeneity in the region of the crystal grown with constant diameter. However, striations reappeared again when the ultrasound was interrupted for a short duration. Additionally, it was found that the ultrasound did not affect striations in 3 kg melt, when the melt depth increased from 30 mm to 60 mm in a 90 mm diameter crucible.

We also studied the influence of ultrasound in two orthogonal directions, and the effect of the S/L interface shape to eliminate striations with smaller periodicity in InSb single crystals, pulled by the Czochralski method. The ultrasound at frequencies from 1.25 to 2 MHz introduced into the melt in a direction perpendicular to the pulling axis did not eliminate striations in the grown crystals. However, the introduction of ultrasound simultaneously in two orthogonal directions at frequencies different by a factor of 2 totally eliminated striations in grown crystals with convex S/L interface. This was a result of complex oscillations of melt particles in ultrasonic standing waves, which damped convection under the S/L interface.

We also investigated the influence of ultrasound at a frequency of 3 MHz on the growth of GaAs layers by liquid-phase epitaxy (LPE). The introduction of ultrasonic waves in Ga solution promoted the conditions for LPE growth with flat shape of the S/L interface. Ultrasonic waves in Ga solution changed the macrostep morphology within a few minutes after switching on the ultrasound. In these experiments, we observed LPE growth with one macrostep. However, the macrostep growth converted into an ordinary growth with a few macrosteps within 2 minutes after switching off the ultrasound.

We modeled the growth conditions and the ultrasound effect using the light cut method. It was found that a standing wave channel formed between the S/L interface and a waveguide. The standing waves damped the convective flow under the S/L interface, and as expected, the component inhomogeneity in grown single crystals decreased. The ultrasonic standing waves induced a stationary convection at a Raleigh number less than Ra < 10⁵ in studied melts and solutions.

However, the melt depth continuously decreases during the growth by the Czochralski method. Maintaining a stable standing wave channel in the melt during the growth process is an important circumstance for the method application. Therefore, we investigated the effect of changing melt level on the behavior of ultrasonic standing waves. We saw that the standing wave channel did not disappear during a decrease or increase of the liquid depth. We found that ultrasonic standing waves can form and stay in a liquid even when the distance between the ultrasonic transducer and reflector is not equal to a multiple of the total number of half wavelengths. The standing wave channel remained in the liquid above the ultrasonic transducer, and any change of the liquid level leap-changed its height by the multiple of one antinode.

From these experimental results, we can say that it is possible to solve the striation problem in Czochralski crystal growth using the ultrasound. The potential virtue of ultrasound is that it can form standing waves to dump convective flow and eliminate striations as a result. Ultrasonic fields can be implemented at a reasonable cost in industrial crystal growth of semiconductor single crystals to improve growth conditions of striation-free crystals. Therefore, we recommend this technique for commercial application. 

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