Direct probing of local-density-of-states in semiconductor nanostructures

The electronic features of semiconductor nanostructures, such as zero-dimensional states, are usually inferred from macroscopic optical and transport experiments. Although, direct probing of electrical features in semiconductor nanostructures looks very attractive, it is very difficult for a conventional semiconductor structure. However, direct probing becomes possible through a combination of low-temperature scanning tunneling microscopy and InAs(111)A surface in an ultra-high vacuum, where conductive electrons automatically accumulate near the clean surface. The clear observation of a Friedel oscillation pattern around a dislocation demonstrates successful mapping of the local-density-of-states (LDOS) of the conductive electrons. Inverted pyramidal defects are naturally formed during molecular beam epitaxial growth of InAs thin films on GaAs(111)A substrates and they operate as well-defined quantum dots. The measured LDOS pattern inside the quantum dots clearly changes as a function of energy, i.e. a sample bias, reflecting the LDOS pattern of each zero-dimensional state. A resonant concentration of the LDOS to the zero-dimensional energy levels is also demonstrated in these experiments. The LDOS measurements of a series of inverted pyramidal quantum dots with different side lengths and their comparison with theoretical calculations suggest a unique feature of the quantum dot system examined in this study.