GaAs crystal Growth techniques

Dr. Shahzad Hussain

GaAss advantages over silicon in semiconductor usey High electron mobility (~8x Silicon) y Significant reduction in signal noise y High power transmission y High breakdown voltages y Direct bandgap

Crystal structure

http://jas.eng.buffalo.edu/education/solid/unitCell /home.html

Galliumy Physical properties

Gallium is a soft, silvery metal with a shiny surface. In some ways, however, it is very un-metal-like. It is so soft that it can melt when held in the hand, be cut with a knife. It has a very low melting point of only 29.7C (85.5F). Gallium is a liquid at 30C Gallium's boiling point is about 2,400C (4,400F) and its density is 5.9037 grams per cubic centimeter.y Chemical properties

Gallium is a fairly reactive element. It combines with most non-metals at high temperatures, and it reacts with both acids and alkalis. An alkali is a chemical with properties opposite those of an acid. Sodium hydroxide (common lye, such as Drano) and bleach are examples of alkalis. y Occurrence in natureGallium is a moderately abundant element in the Earth's crust. Its abundance has been estimated to be about 5 parts per million. It is found primarily in combination with zinc and aluminum ores. It is also found in germanite, an ore of copper sulfide (CuS). The United States produces no gallium. The largest producers are Australia, Russia, France, and Germany. Read more: http://www.chemistryexplained.com/elements/CK/Gallium.html#ixzz0z70DfFuV

Arsenicy y

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Physical properties Arsenic occurs in two allotropic forms. Allotropes are forms of an element with different physical and chemical properties. The more common form of arsenic is a shiny, gray, brittle, metallic-looking solid. The less common form is a yellow crystalline solid. It is produced when vapors of arsenic are cooled suddenly. When heated, arsenic does not melt, as most solids do. Instead, it changes directly into a vapor (gas). This process is known as sublimation. However, under high pressure, arsenic can be forced to melt at about 814C (1,500F). Arsenic has a density of 5.72 grams per cubic centimeter. Chemical properties Arsenic is a metalloid. A metalloid is an element that has properties of both metals and non-metals. Metalloids occur in the periodic table on either side of the staircase line that starts between boron and aluminum. When heated in air, arsenic combines with oxygen to form arsenic oxide (As 2 O 3 ). A blue flame is produced, and arsenic oxide can be identified by its distinctive garlic-like odor. Arsenic combines with oxygen more slowly at room temperatures. The thin coating of arsenic oxide that forms on the element prevents it from reacting further. Arsenic does not dissolve in water or most cold acids. It does react with some hot acids to form arsenous acid (H 3 AsO 3 ) or arsenic acid (H 3 AsO 4 ). Occurrence in nature Arsenic rarely occurs as a pure element. It is usually found as a compound. The most common ores of arsenic are arsenopyrite (FeAsS), orpiment (As 2 S 3 ), and realgar (As 4 S 4 ). These compounds are obtained as a byproduct of the mining and purification of silver metal. The abundance of arsenic in the Earth's crust is thought to be about 5 parts per million. That places it among the bottom third of the elements in abundance in the Earth's crust. The world's largest producers of arsenic are China, Chile, Mexico, Belgium, Namibia, and the Philippines. The United States does not produce any arsenic. Read more: http://www.chemistryexplained.com/elements/A-C/Arsenic.html#ixzz0z73zgVQN

Synthesis and Purification of Gallium Arsenide (GaAs)

Synthesis and Purification of GaAsy GaAs can be prepared by directly reaction of the elements

Schematic representation of a sealed (evacuated) tube synthesis of GaAs.

At correct temperature, the Arsenic vapor is transported to Gallium and they react to form polycrystalline GaAs

Impurity concentration found in Polycrystalline GaAsElement boron carbon nitrogen oxygen fluorine magnesium aluminum Concentration (ppm) 0.1 0.7 0.1 0.5 0.2 0.02 0.02 Element silicon phosphorus sulfur chlorine nickel copper zinc Concentration (ppm) 0.02 0.1 0.01 0.08 0.04 0.01 0.05

Growth of GaAs Crystals1. Bridgman Growth 2. Czochralski Method

Bridgman technique for growing single crystal GaAs