Master thesis defense by Magnus Oddershede

Optimizing and characterizing growth of GaAs/AlGaAs heterostructures using molecular beam epitaxy

In the present work, growth of bulks and heterostructure devices using GaAs and AlGaAs has been investigated with the purpose of providing an efficient way to determine suitable growth parameters and a reliable, reproducible characterization method for the surface structure and purity of grown materials.

The surface roughness of grown samples was chosen as the metric of interest used to optimize process control parameters. The substrate temperature and the effective V:III ratio were identified as the two main process control parameters that are critical to the quality of the surface measured via the surface roughness. A method of determining a suitable value of the effective V:III ratio with little to no trial-and-error based work has been suggested and demonstrated based on calibrations of the beam equivalent pressure at which MBE growth of GaAs or AlGaAs transitions from being group III limited to being As limited.

Using these established suitable growth conditions, two samples of two-dimensional electron gas systems have been grown: one using a finite square well, and the other using a heterojunction well to form the two-dimensional electron gas, with carrier mobilities greater than 600 × 103 cm2/Vs and 300 × 103 cm2/Vs, respectively. A method of benchmarking relative background impurity levels in the material comprising the 2DEG was established using a gated Hall Bar. This includes operating the Hall Bar at carrier densities low enough that scattering due to background impurities is the main scattering mechanism and reporting the mobility at this set carrier density rather than the peak mobility.