Characterization of electrostatic potential of compound semiconductors using off-axis electron holography.

摘要

The research of this dissertation involved the use of off-axis electron holography to investigate electrostatic potentials within AlGaAs/GaAs heterostructures. The inelastic mean-free-paths of high-energy electrons (200 keV) for AlAs and GaAs have been determined to be 77 nm and 67 nm, respectively, based on comparing thicknesses by electron holography and convergent beam electron diffraction. The mean inner potentials of AlAs and GaAs were determined to be 12.1 V and 14.0 V, respectively. Accurate knowledge of these parameters was essential before commencing potential profiling across AlGaAs/GaAs heterojunction devices.;The electrostatic potential profiles across AlGaAs/AlAs/GaAs heterostructures containing n- and p-doped AlGaAs layers were measured. Potential drops of 0.5 V, caused by ionized carbon impurities, were observed at the epilayer/substrate interface of both heterostructures. The measured potential profiles in the region of the buffer/substrate interface were simulated by adding a sheet acceptor layer at the buffer/substrate interface. The additional interface impurities pull the conduction band up at the interface relative to the Fermi level.;The heavily doped p-n junctions in AlGaAs light-emitting diodes with triangular barriers were investigated with emphasis on the efficiency of dopant activation. Comparison of experimental and simulated p-n junction potential profiles enabled the Si and Be dopant activation to be determined. The low Si activation efficiency of 12 percent at 0.6 Al was attributed to the crossover of gamma and X conduction-band minima near the concentration of 0.43 Al. Simulated band-edge diagrams with nominal and activated dopants showed the effect of AlGaAs barrier shape on carrier transport. The lightly doped p-n junctions in InGaAs light-emitting diodes with linearly graded barriers were also studied, and the effect of unintentional oxygen impurities was investigated in terms of doping efficiency.;Overall, the success of these studies has verified the capability of off-axis electron holography to provide unique information about complex heterostructures of compound semiconductors with nanoscale resolution. The technique should become a useful tool for future device development and process optimization.