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Ingot Growing

Introduction

In the Gallium Arsenide ingot and wafer growth process, elemental forms of gallium (Ga) and arsenic (As), plus small quantities of dopant material (silicon, tellurium, or chromium) react at elevated temperatures to form ingots of doped single crystal GaAs. Three generalized methods of ingot production are used:

  1. Horizontal Bridgeman (HB)
  2. Gradient Freeze (GF)
  3. Liquid Encapsulated Czochralski (LEC Cz)

The reaction of As vapor with Ga metal at elevated temperatures, in sealed quartz ampoules, form the bulk polycrystalline GaAs compound. Typically, an As reservoir contained at one end of the ampoule is heated to approximately 600ºC. This generates approximately 1 atm of As vapor pressure in the system, a prerequisite for obtaining stoichiometric GaAs. The As vapor reacts with the Ga metal which is heated to approximately 1260ºC and located at the other end of the ampoule in a quartz board. After the reaction is complete, single-crystal growth begins by programmed cooling (Gradient Freeze) or by physically moving either the ampoule or furnace to provide proper temperature gradients for growth (Horizontal Bridgeman). This indirect approach (Arsenic transport) for compounding and growth of GaAs is used because of the high vapor pressure of As at its melting point and at the melting point of GaAs, about 20 atm at 812ºC, and 60 atm at 1238ºC, respectively.

Another approach to the commercial production of bulk single-crystal GaAs that is gaining favor is the liquid encapsulated Czochralski technique (LEC Cz). A Cz crystal puller is loaded with chunk GaAs in a quartz ampoule with an outer graphite receptor. The bulk GaAs melts at temperatures close to 1238ºC, and the crystal is pulled in a pressurized atmosphere of approximately 100 atm. A viscous glass, B2O3, completely encapsulates the melt, which prevents melt dislocation when the dissociation vapor (As) is less than or equal to the pressure of an inert gas (argon, typically) applied in the puller chamber.

The following description delineates processing steps in use in LED production facilities. The Horizontal Bridgeman (HB) and Gradient Freeze (GF) methods of single-crystal gallium arsenide ingot growth are the dominant techniques in use in LED production.

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