Optical Investigations of Semiconductor Quantum Dots with High Spatial Resolution

Two important material systems are under investigation in this work: III-V dilute nitride alloys and Ge/Si self-assembled quantum dot (SAQDs) layers. The theme of this research is the study of these structures using high spatial resolution(HSR). Two alternate techniques were utilized to break through the optical diffraction limit in order to obtain high spatial resolution: near-field scanning optical microscopy (NSOM), and nano-fabrication.

First, NSOM was used to 'see' individual quantum dots (QDs), and to study the uniformity of semiconductor epitaxial layers to get information on compositional fluctuations and the orientation of separated phases. The pressure produced by the NSOM fiber tip on the surface of the samples has been developed in our laboratory to 'tune' the photoluminescence (PL) emission energy of QDs. Extensive calculations were carried out to determine the stress distribution produced during these experiments. Second, e-beam lithography was used to fabricate metallic nano-apertures or nano-mesas on the sample surface, allowing the measurement of the optical properties from these structures with high spatial resolution using a simple micro-PL measurement. Temperature dependent PL measurements have also been done to study carrier localization and nonradiative-centers in dilute nitrides. High spatial resolution PL and temperature-dependent PL study reveal the important role of quantum dot-like compositional fluctuations in suppressing the lateral carrier transport to nonradiative recombination centers and achieving high emission intensity at room temperature in the dilute nitrides. A temperature-dependent and power-dependent PL study implies our Ge/Si SAQDs structures grown by MBE have type-II band-alignment. .

Finally, we strived to develop a novel system which can achieve high spatial resolution beyond that currently available for far infrared measurements. Typically, far infrared measurements are carried out utilizing a Fourier transform infrared spectrometer (FTIR), and the high spatial resolution can be achieved using scanning probe techniques. Preliminary experiments have been done to investigate the intersubband absorption of Ge/Si QDs and III-V QDs, using an FTIR to couple light into waveguide structures having 45-degree entrance and exit faces to enhance the optical absorption. Broad absorption peaks around 3000 cm-1 of a p-doped Ge/Si QD sample were found. Future work involves the study of undoped Ge/Si QD samples by shining a laser beam to generate holes inside the Ge dot layers. These intersubband absorption experiments could also be done on p-doped and n-doped InAs/GaAs QDs.