<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 dots exhibit strongly size-dependent optical and electrical properties. The ability to join the dots into complex assemblies creates many opportunities for scientific discovery. This motivated our present research work on QDIPs, DWELLs, and graphene like QDs. The intention of this research was to study the size dependent achievements of QDIPs, DWELLs, and graphene like QDs with those of competitive technologies, with the emphasis on the material properties, device structure, and their impact on the device performance. In this dissertation four research studies pertaining to optical properties of quantum dot and dot-in-a-well infrared photodetectors, I-V characteristics of graphene quantum dots, and energy and spin texture of germanene quantum dots are presented. Improving self-assembled QD is a key issue in the increasing the absorption and improving the performance. In the present research work, an ideal self-assembled QD structure is analyzed theoretically with twenty-hole levels (Intraband optical transitions within the valence band) and twenty-electron energy levels (DWELL). Continuing the efforts to study self-assembled QDs we extended our work to graphene like quantum dots (graphene and germanene) to study the electronic transport properties. We study numerically the intraband optical transitions within the valence band of InxGa1-xAs/GaAs pyramidal quantum dots. We analyze the possibility of tuning of corresponding absorption spectra by varying the size and composition of the dots. Both ‘x ’ and the size of the quantum dot base are varied. We have found that the absorption spectra of such quantum dots are more sensitive to the in-plane incident light. We present numerically obtained absorption optical spectra of n-doped InAs/In0.15Ga0.85As/GaAs quantum dot-in-a-well systems. The absorption spectra are mainly determined by the size of the quantum dot and have weak dependence on the thickness of the quantum well and position of the dot in a well. The dot-in-a-well system is sensitive to both in-plane and out-of-plane polarizations of the incident light with much stronger absorption intensities for the in-plane-polarized light. We also present theoretically obtained I-V characteristics of graphene quantum dots, which are realized as a small piece of monolayer graphene. We describe graphene within the nearest-neighbor tight-binding model. The current versus the bias voltage has typical step-like shape, which is due to discrete energy spectrum of the quantum dot. The current through the dot system also depends on the position of the electrodes relative to the quantum dot. In relation to graphene quantum dots, we present our study of buckled graphene-like materials, like germanene and silicene. We consider theoretically germanene quantum dot, consisting of 13, 27, and 35 germanium atoms. Due to strong spin-orbit interaction and buckled structure of the germanene layer, the direction of the spin of an electron in the quantum dot depends on both the electron energy and external perpendicular electric field. With variation of energy, the direction of spin changes by approximately 4.50. Application of external electric field results in rotation of electron spin by approximately 0.50, where the direction of rotation depends on the electron energy.