Ultra-high-quality GaAs and AlAs two-dimensional electron systems via molecular beam epitaxy

Abstract

This work summarizes how to prepare high-quality GaAs and AlAs two-dimensional electron systems (2DESs) via molecular beam epitaxy (MBE). At the time of writing this thesis samples grown using the methods provided here hold world-record results for mobility in both GaAs and AlAs 2DESs. This was achieved by optimization of sample design and systematic reduction of impurities in the structure. In the first few chapters, the working principles of electron transfer in the process of forming a 2DES in AlAs and GaAs quantum wells (QWs) is established. We show that AlAs 2DESs can be prepared in a fashion analogous to that of modulation-doped GaAs 2DESs. Moreover, we elaborate on how the more sophisticated doping-well structure, commonly used for ultra-high-mobility samples, works. Several experimental parameters are tuned in the doped region and their impact on the resultant 2DES density is discussed. Finally, we also demonstrate a heterostructure design that allows the preparation of high-quality, high-density GaAs 2DESs at elevated hydrostatic pressures. The new scheme suppresses the reduction of electron density as a function of pressure by more than a factor of 3.

The latter chapters discuss impurities incorporated during the MBE growth of GaAs and AlAs. We start off by devising a strategy to evaluate the cleanliness of our source material. Because the sensitivity of conventional analysis tools such as secondary ion mass spectrometry is too low to probe the amount of impurities in ultra-high-quality GaAs samples, we use the mobility of a specially designed GaAs 2DES as a metric for cleanliness. The main idea here is to exploit the surface segregation of impurities on the growth front, explained in finer detail in the main text. With the cleanliness being quantifiable, we systematically purified our source material until no significant improvement could be observed by offline bakes. This soft limit on sample cleanliness can be lifted by improving the vacuum in our MBE chamber, which we achieve by implementing additional cryogenic cold plates in the growth space. Under the best conditions of source purity and vacuum, we were able to grow samples that demonstrate the highest electron mobilities in the world.