The right heat treatment can stabilize and improve InAs sub-monolayer quantum dots
Quantum dots (QDs) made of indium arsenide (InAs) semiconducting material may soon give us power-efficient lasers that can be used in high-speed optical communications. For them to light up brightly at a precise wavelength, it is crucial to get uniformly sized dots with minimal defects. To this end, Prof. S. Chakrabarti’s group fabricated nano devices consisting of InAs QDs and subjected them to high temperatures. The energy and precision of light emission from the QD nanostructures increased with temperature treatment.
Every nano device in this study consisted of multiple QD layers, with each layer comprising of 4 to 10 stacks of active QDs. They formed active QDs by sandwiching 0.5 monolayers (ML) of InAs between 2 monolayers of In0.15Ga0.85As. This method uses less than one monolayer of the active material (in this case, InAs) to form a QD – hence, the resultant dots are known as sub-monolayer quantum dots. Sub-monolayer QDs are known for being precise and tunable in their emission wavelength, which is a desirable quality for creating precise lasers. However, the challenge lies in getting multiple layers of uniform sized quantum dots with minimal defects.
Heating these QD nanostructures tackles the above challenge. In this work, A. Manohar and his colleagues heated their InAs sub-monolayer QDs to temperatures upto 800°C. Through photoluminescence (PL) experiments, they found that heat treatments improved the emission accuracy and energy of these nanostructures, possibly due to removal of inherent defects. However, improvement in activation energy was most pronounced only at 650°C, after which it dropped in value. They deduce that higher temperatures not only remove defects but also lead to diffusion of Ga atoms into InAs QDs. Hence, finding the right temperature to iron-out defects is critical for these structures.
Work funded/ supported by: Department of Information and Technology (DeIT) and Department of Science and Technology (DST), Government of India; Indian Space Research Organization (ISRO) and Riber Systems, France.
Published paper: A.Manohar, S. Sengupta, H. Ghadi, and S. Chakrabarti, “A detailed study of the effects of rapid thermal annealing on the luminescence properties of InAs sub-monolayer quantum dots,” Journal of Luminescence, vol. 158, 149 (2015).