Implanting protons in quantum dots makes them better radiation detectors

An array of InGaAs quantum dot islands on GaAs substrates. The QD are crystals of nanometer scale with special properties.

Quantum dots (QD) are special nanocrystals which have discrete energy levels like individual atoms instead of energy bands as found in solids. This restricts the wavelengths of photons they can absorb or emit, making them useful as radiation detectors. Researchers led by Prof. S. Chakrabarti have for the first time reported an enhancement in the optical and electronic properties for InGaAs QD by injecting high energy protons in the QD structure.

QD based detectors fabricated by students in Prof. Chakrabarti’s group consist of several layers of alternating InGaAs QD, and GaAs film -known as capping layer. The QDs are only 5-7 Monolayer (atomic layer) thick. Quality of overgrown dots were improved by adding a combinational capping of 30 Angstrom In0.21Al0.21Ga0.58As and 50nm GaAs layer on the InGaAs QD. However, some defects inevitably occur in the QD, resulting in some non-idealities one among which is additional energy levels known as trap/ defect states.

The team inserted protons into the structure using high energy implantation. This treatment increased the intensity of photoluminiscence spectra of the QD, which represents an increase in emissions from QD as a response to incoming radiation. It also supressed the “dark current” in QD, ie. inherent current without stimulation by radiation, equivalent to reducing “noise” in detectors.

Introducing protons fills trap levels and decimates defects in QD, either filling or eliminating any unwanted energy levels. The elimination of defect states means that all charge carriers will transit only between the exact energy levels of ideal QD material. This leads to emission of particular wavelengths with high intensity.

-Kalyani Patrikar

Published Paper: S. Upadhyay, A. Mandal, H. Ghadi, D. Pal, A. Basu, A. Agarwal, N. B. V. Subrahmanyam, P. Singh and S. Chakrabarti, “Effects of high energy proton implantation on the material and electrical properties of In(Ga)As/GaAs QD heterostructures with a variation in capping layer,” Journal of Luminescence

Work Supported by: Department of Science and Technology, Government of India, and Ministry of Communications & Information Technology, Government of India