Baseline material qualification

When maintenance is performed on the MBE machines, numerous opportunities for unwanted contamination can occur. Contamination can result in unacceptably large unwanted impurity concentrations in the epitaxial III-V semiconductors.

Contamination is avoided through:

  • minimum handling of machine components
  • gloves to avoid getting skin contaminants on MBE components
  • baking of crucibles prior to loading to outgas contamination
  • baking of the MBE machine including:


    After baking the machine, furnaces are operated approximately 10% above their highest anticipated operating point for 18-24 hours to remove the last traces of impurities that might be likely to outgas and contaminate epitaxial wafers. The background impurity density in GaAs is then inferred from 77K Hall mobility measurements made on a wide spacer 2 dimensional electron gas (2DEG) structure. An undoped AlGaAs spacer 150 Angstroms thick is used to create a 2DEG which will have a mobility that is predominantly limited by scattering due to unintentional impurities. The structure is shown below.


    This structure has been in use for many years to detect the presence of unanticipated contamination of the MBE machine. This baseline characterization is performed each time the machine is baked. The results which have been obtained following bakeouts over a several year period are shown above. There are 2 points omitted where the mobility was found to be lower than 100,000 as a result of contamination, or leaks, which then had to identified and removed.

    It can be seen that there has been a long term increase in the mobilities measured just after bakeout. This is likely to be the result of:

  • gradual outgassing / burial of impurities from original machine parts
  • marginal improvements in machine maintenance procedures
  • marginal improvements in manufacture of MBE components
  • marginal improvements in source materials

    A jump in material purity occured in 1988 when the substrate heater was baked during bakeout for the first time. Previously, it was not baked because it was not originally wired with bakeable heater and thermocouple cables. Approximately 2,500 wafers were grown prior to this change in procedure. Since then, more than 2,500 have been grown including modulation doped field effect transistors (MODFET)s and quantum well lasers which have demonstrated record high frequency performance. Both classes of devices require low unintentional impurity concentrations.

    The same baseline structure above has been grown following bakeout procedures at two other labs. One lab measured a mobility of 145,000 in a turbomolecular pumped system, while the other lab measured only 90,000-110,000. The second lab did not follow the bakeout procedure outlined above.

    The mobility that is measured 30-60 days after the initial background check will be almost 200,000 at 77K. When the machine exhibits this level of purity, a 300 Angstrom spacer structure with half as much doping has been shown to exhibit mobility of 2,000,000 at 50mK.




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    February 20, 1977

    Please send your comments and suggestions to: schaff@iiiv.tn.cornell.edu.