Liquid-encapsulated Czochralski Growth of Compound Semiconductor Crystals with Steady and Rotating Magnetic Fields

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Title: Liquid-encapsulated Czochralski Growth of Compound Semiconductor Crystals with Steady and Rotating Magnetic Fields
Author: Yang, Mei
Advisors: Dr. Nancy Ma, Committee Chair
Dr. Kevin Lyons, Committee Member
Dr. Zhilin Li, Committee Member
Dr. Tarek Echekki, Committee Member
Abstract: Integrated circuits and optoelectronic devices are produced on surfaces of thin wafers sliced from a photonic or compound semiconductor crystal. The growth of compound semiconductor crystals is critically important because viable substrates which are compositionally uniform both within a wafer and from wafer to wafer are needed. A dopant is an element that is added to the melt during growth to give the semiconductor crystal specific electrical and/or optical properties. More and better compound semiconductor crystals are needed for advanced optoelectronic devices. This investigation is focused on developing mathematical and numerical models to understand transport phenomena during bulk growth of compound semiconductor crystals. Since molten semiconductors are electrical conductors, magnetic fields can be used to control the melt motion in order to control the crystal's dopant distribution. Compound semiconductor crystals can be grown from the melt by the liquid-encapsulated Czochralski (LEC) process with a steady magnetic field. During this process, the molten semiconductor (melt) is covered with a layer of liquid encapsulant in order to prevent the escape of the volatile component. In this dissertation, we treat several different problems. We investigate the coupling of free convections in the melt and liquid encapsulant in a rectangular enclosure with steady vertical and horizontal magnetic fields, and find that these flows are coupled and the competition between these flows determines the direction of the horizontal velocity of the encapsulant-melt interface. We also investigate the dopant transport during the LEC process with a steady axial magnetic field, and find that both the radial and axial homogeneity of the crystal improves as the magnetic field strength decreases. With magnetic stabilization alone, however, the radially-inward flow below the crystal-melt interface does not become large enough to produce acceptable levels of segregation. A transverse magnetic field which rotates around the centerline of the melt can provide an electromagnetic stirring of the melt, and may represent a promising means to produce a crystal with good homogeneity. We investigate LEC growth with a combination of steady and rotating magnetic fields, and find that a rotating field can increase the magnitude of the radially-inward flow below the crystal-melt interface.
Date: 2006-07-13
Degree: PhD
Discipline: Mechanical Engineering
URI: http://www.lib.ncsu.edu/resolver/1840.16/3860


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