Seminar

Two-dimensional Electron Gases in Ultra-pure GaAs/AlGaAs Heterostructures

Speaker

Stefan Schmult
Max-Planck-Institute for Solid State Research, Stuttgart, Germany

Date & Time

Friday, November 13, 2009
Time: Noon-1:00pm
Location: ESB 1001

The high electron mobility of a two-dimensional electron gas (2DEG) is a prerequisite for investigations of electron correlation phenomena in semiconductors, such as the integer and fractional quantum Hall effect [1]. Among numerous factors limiting the electron mobility of a 2DEG confined in a GaAs/AlGaAs heterostructure grown by molecular beam epitaxy (MBE) is the purity of the source materials. In particular different gallium batches with nominally identical quality can yield varying saturation mobilities within a growth campaign due to different levels of residual charged impurities in the GaAs hosting the 2DEG.

Therefore, we developed a method to evaluate the composition of gallium loads prior to and independent of the later use in the MBE system. It is found, that at the beginning of a growth campaign gallium-oxygen and gallium-hydrogen compounds represent the sole contaminants resulting from above-average MBE-grade gallium [2].

Utilizing the aforementioned gallium, low-temperature 2DEG mobilities exceeding 1000 m2/Vs were obtained in single-interface heterostructures. 2DEG-density-dependent transport measurements indicate that charged residual impurities do not limit the electron mobility. Its value is rather limited by the simple architecture of the heterostructure. As a matter of fact, employing alternative doping schemes, which efficiently separate the 2DEG from the remote ionized donors, the mobilities can be almost doubled, leading to our current record value of 1800 m2/Vs.

[1] L.N. Pfeiffer and K.W. West., Physica E 20, 57 (2003)
[2] S. Schmult, S. Taylor and W. Dietsche, J. Cryst. Growth 311, 1655 (2009)