A study on the physical properties of quantum dot structures for infrared photodetection

Abstract

This thesis is part of a project where the overall goal is to master the technology of infrared photodetectors based on self-organized semiconductor quantum dots, the Quantum Dot Infrared Photodetectors (QDIPs) for the wavelength range from 2 to 20 ìm. The thesis focuses on the physical properties of quantum dots and QDIPs structures, especially on the intraband transitions and extraction mechanisms involvedin the photocurrent generation. We studied original and innovative structures based on self-organized InAs quantum dots grown on InP substrates. The main results presented in this thesis are based on photocurrent measurements as a function of temperature andexternal applied bias voltages, using a Fourier Transform Infrared spectrometer. The experimental techniques of photoluminescence, atomic force microscopy, transmission electron microscopy and current-voltage curves were also performed to achieve a betterunderstanding of the physical mechanisms involved. To explain the results and assign each photocurrent peak to a particular transition, fully three dimensional theoretical calculations were done. The main results presented in this thesis are: i- It is shown that the intraband Auger effect can be an important process for the photocurrent generation in QDIPs. Intraband photocurrent and absorption measurements, together with a full three-dimensional theoretical modeling revealed that a bound-to-bound optical transition, where the final state is about 200 meV deep below the conduction band continuum, is responsible for the photogenerated current in the particular QDIP structure investigated. Photoluminescence and interband photocurrent spectra further support this conclusion. ii- We studied the influence of different structures in theneighborhood of the quantum dot on the photocurrent response of quantum dot infrared photodetectors. We measured a photocurrent with positive and a negative sign for the same external electric field in some QDIP structures. The dual sign photocurrent signal is attributed to asymmetries on the structures which can privilege the extraction of the carriers from the dots for one of the two possible senses of the current. This process is seen for small external applied bias voltages or when no bias is applied. For high external fields the photoexcited electrons go in the same sense of the applied field, as expected. iii- We present a very highly selective QDIP, which combines InAs quantum dots and InGaAs wells, operating at 12 ìm. The transition responsible for the exceptionally narrow photocurrent is attributed to photon absorption between quantum dot bound states, followed by a carrier extraction mechanism where the coupling of the final state of the transition to the adjacent quantum well is highlighted