High purity isotopically enriched 70 Ge and 74 Ge single crystals: Isotope separation, growth, and properties
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W. L. Hansen Lawrence Berkeley Laboratory, Berkeley, California 94720
E.E. Haller University of California at Berkeley and Lawrence Berkeley Laboratory, Berkeley, California 94720
J. W. Farmer University of Missouri, Columbia, Missouri 65211
V. I. Ozhogin, A. Rudnev, and A. Tikhomirov Russian Science Center, Kurchatov Institute, 123182 Moscow, Russia (Received 30 November 1992; accepted 16 February 1993) 70 Ge
and 74 Ge isotopes were successfully separated from natural Ge and zone purified. Several highly enriched, high purity 70 Ge and 74Ge single crystals were grown by the vertical Bridgman method. The growth system was designed for reliable growth of low dislocation density, high purity Ge single crystals of very small weight (~4 g). A 70 Ge and a 74 Ge crystal were selected for complete characterization. In spite of the large surface to volume ratio of these ingots, both 70Ge and 74Ge crystals contain low electrically active chemical net-impurity concentrations of ~ 2 X 1012 cm" 3 , which is two orders of magnitude better than that of 74Ge crystals previously grown by two different groups.1'2 Isotopic enrichment of the 70Ge and the 74 Ge crystals is 96.3% and 96.8%, respectively. The residual donors and acceptors present in both crystals were identified as phosphorus and copper, respectively. In addition, less than 10 n cm" 3 gallium, aluminum, and indium were found in the 70 Ge crystal.
I. INTRODUCTION Along with the substantial progress in semiconductor science and technology, the control of the isotopic composition of semiconductor materials ("Isotope Engineering") is starting to attract attention. Several physical properties which are influenced by isotopic composition can be studied in isotopically controlled crystals.1"9 Development of isotope engineered materials may become important in the future for the semiconductor industry. Application of the neutron transmutation doping technique10 to isotopically controlled multilayer structures11 or to isotopically controlled Si-Ge alloys may lead to superior doping control. Because many elements in the periodic table consist of a number of stable isotopes present in certain fractions, the isotopic composition of all standard semiconductor materials is predetermined and fixed unless a special isotope separation process is applied. To date only a few different types of isotopically enriched semiconductor crystals have been produced: 74 Ge single crystals,1"3 76Ge single crystals,4-5 and 12C and 13C diamond single crystals.6"9 High purity and low dislocation density isotopically
enriched 70 Ge, 73 Ge, and 74 Ge single crystals of larger size (~800 g each) have been grown most recently by the Czochralski method in our group. In this paper we will describe the isotope separation, purification, growth, and characterization of chemically pure, isotopically enriched 70 Ge and 74 Ge single crystals of ~ 1 cm3 in volume. The development of a growth system that produces high quality Ge crystals in small size was necessary for our future goal of growing Ge crystals co
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