<p>This master’s thesis deals with studies of lateral and vertical carrier transport Dot-in- </p><p>a-Well (DWELL) Quantum Dot Infrared Photodetectors (QDIPs). During the pro ject, </p><p>devices have been developed and tested using a Fourier Transform Infrared (FTIR) spec- </p><p>trometer with the purpose to find the processes governing the flow of photocurrent in </p><p>the different kinds of detectors, the dark current magnitude in the vertical Quantum Dot </p><p>Infrared Photodetector (QDIP) and the Quantum Well Infrared Photodetector (QWIP) </p><p>and the light polarization dependences for the vertical QDIP and the QWIP. </p><p>The lateral carrier transport DWELL QDIP was found to have poor conduction </p><p>in the well mainly due to re-trapping of electrons in this region. The main process gov- </p><p>erning the flow of photocurrent for this type of device at 77K is photo-excitation from </p><p>the Quantum Dot (QD)s to the excited state in the Quantum Well (QW) and further </p><p>thermal excitation. If the electrons are mainly transported in the matrix or the well at </p><p>77K is presently not clear. </p><p>For the vertical carrier transport DWELL QDIP at 77K, the wavelength response </p><p>could be tuned by altering the applied voltage. At higher voltages, the dominant process </p><p>was found to be photo-excitation from the QDs to the excited state in the QW followed </p><p>by thermal assisted tunneling into the GaAs-matrix. At lower voltages, photo-excitation </p><p>from the QDs directly into the the GaAs-matrix was the predominant process. The dark </p><p>current level in the vertical QDIPs was found to be 1.5 to 5 orders of magnitude smaller </p><p>than for the QWIP measured at 77K. Furthermore, the QDIP was found to be close to </p><p>polarization independent. As expected the QWIP had a reduced sensitivity to normal </p><p>incident light. The existence of this signal was attributed to interface scattering of light </p><p>inside the device.</p>

This master’s thesis deals with studies of lateral and vertical carrier transport Dot-in- a-Well (DWELL) Quantum Dot Infrared Photodetectors (QDIPs). During the pro ject, devices have been developed and tested using a Fourier Transform Infrared (FTIR) spec- trometer with the purpose to find the processes governing the flow of photocurrent in the different kinds of detectors, the dark current magnitude in the vertical Quantum Dot Infrared Photodetector (QDIP) and the Quantum Well Infrared Photodetector (QWIP) and the light polarization dependences for the vertical QDIP and the QWIP. The lateral carrier transport DWELL QDIP was found to have poor conduction in the well mainly due to re-trapping of electrons in this region. The main process gov- erning the flow of photocurrent for this type of device at 77K is photo-excitation from the Quantum Dot (QD)s to the excited state in the Quantum Well (QW) and further thermal excitation. If the electrons are mainly transported in the matrix or the well at 77K is presently not clear. For the vertical carrier transport DWELL QDIP at 77K, the wavelength response could be tuned by altering the applied voltage. At higher voltages, the dominant process was found to be photo-excitation from the QDs to the excited state in the QW followed by thermal assisted tunneling into the GaAs-matrix. At lower voltages, photo-excitation from the QDs directly into the the GaAs-matrix was the predominant process. The dark current level in the vertical QDIPs was found to be 1.5 to 5 orders of magnitude smaller than for the QWIP measured at 77K. Furthermore, the QDIP was found to be close to polarization independent. As expected the QWIP had a reduced sensitivity to normal incident light. The existence of this signal was attributed to interface scattering of light inside the device.

Quantum dot infrared photodetectors (QDIPs) have emerged as a promising technology in the mid- and far-infrared (3-25 μm) for medical and environmental sensing that have a lot of advantages over current technologies based on Mercury Cadmium Telluride (MCT) and quantum well (QW) infrared photodetectors (QWIPs). In addition to the uniform and stable surface growth of III/V semiconductors suitable for large area focal plane applications and thermal imaging, the three dimension confinement in QDs allow sensitivity to normal incidence, high responsivity, low darkcurrent and high operating temperature. The growth, processing and characterizations of these detectors are costly and take a lot of time. So, developing theoretical models based on the physical operating principals will be so useful in characterizing and optimizing the device performance. Theoretical models based on non-equilibrium Green's functions have been developed to electrically and optically characterize different structures of QDIPs. The advantage of the model over the previous developed classical and semiclassical models is that it fairly describes quantum transport phenomenon playing a significant role in the performance of such nano-devices and considers the microscopic device structure including the shape and size of QDs, heterostructure device structure and doping density. The model calculates the density of states from which all possible energy transitions can be obtained and hence obtains the operating wavelengths for intersubband transitions. The responsivity due to intersubband transitions is calculated and the effect of having different sizes and different height-to-diameter ratio QDs can be obtained for optimization. The dark and photocurrent are calculated from the quantum transport equation provided by the model and their characteristics at different design parameter are studied. All the model results show good agreement with the available experimental results. The detectivity has been calculated from the dark and photocurrent characteristics at different design parameters. The results shows a trade off between the responsivity and detectivity and what determines the best performance is how much the rate of increase of the photocurrent and dark current is affected by changing the design parameters. Theoretical modeling developed in the thesis give good description to the QDIP different characteristics that will help in getting good estimation to their physical performance and hence allow for successful device design with optimized performance and creating new devices, thus saving both time and money. / Thesis / Doctor of Philosophy (PhD)