Droplet epitaxy quantum dots for infrared detection applications

During the 20th century military applications in western countries (e.g. navigation, weapons detection, night vision) have spearheaded and dominated the requirement of infrared(IR) detectors. Nowadays, the market of IR technology has become wider and the applications currently utilizing IR detectors span commercial (e.g. communications, aerospace, medical imaging), public (e.g. atmospheric sounding, pollution control, meteorology, environmental monitoring), and academic (e.g. astronomy) domains, with new uses constantly arising as the various IR technologies become more established (e.g. automotive, smart-phone, building automation). Future employments and applications of IR sensors for imaging purposes are closely linked to the possibility of the development of a new generation of sensor: the “third generation”. The “third generation” of photo-detectors will provide enhanced capabilities such as larger number of pixels, higher frame rates, better thermal resolution and in particular multispectral functionality. In the IR regions of interest such as SWIR, MWIR and LWIR, four major technologies are developing multi-spectral capability: the HgCdTe photo-detector, the quantum well infrared photodetector (QWIP), the quantum dot infrared photodetectors (QDIP) and some antimonide based type II superlattices. In this PhD thesis in particular we have analysed the possibility of developing 1) a QDIP based on a quite new strain-free techniques called “droplet epitaxy” and 2) we have explored the possibility of monolithically integrating this device on silicon-germanium substrates employng different strategies developed in the L-ness laboratories in Como. In the first chapter of the thesis a brief but quite complete introduction on the IR radiation, IR sensors and their market, thier classification, architecture and functioning has been given. In the second chapter of the thesis the relationship between QDIP and QWIP are analyzed stressing on the advantages given by the 3D confinement. In particular the inhibition of incident absorption for the QWIP and the possibility of overcoming this issue with the QDIP is looked through the analysis of the quantum mechanical selection rules coming from the envelope funcion formalism. Furthermore, during this chapter, some state of the art QDIP design is presented and some of their feature is discussed. In the third chapter of the thesis an introduction to the used molecular beam epitaxy (MBE) system and in particular to “droplet epitaxy” techniques has been given. Major emphasis has been dedicated to the explanation of the versatility of a strain-free growth such as droplet epitaxy, especially for what concern the human capability of fine tailoriing quantum properties and energy levels in nanostrustures, and in particular in QDs. In the fourth chapter the interest and the issues related to monolithic integration of III/V materials on Si are briefly discussed. Three strategies developed in our laboratories are described as possible solutions. Two of this strategies are going to employ (111) orientated substrates. However in scientific literature the the growth of GaAs (111) is far to be optimized especially for what concern the (111)A orientation. For that reason in the central part of this chapter we have presented the morphlogical study we made and the model we have constructed to understand the prominent growth mechanisms that could lead to the growth to an highly flat surface (RMS<0.2 nm) and so to a right field for a fine tuned of nanostructures on this surface. At the end of the chapter a macro photoluminescence (PL) spectra of a 5.5 nm thick QW is presented. The spectra shows a very narrow broedening of 4.2 meV, demonstrating the high flatness achieved. The last chapter is the one dedicated to the QDIP manufacturing. In the first section a detailed description of the growth and of the design is given. In the second section all the processing procedures are described with particular stress on the MESA etching and metal deposition. These two steps have been particularly demanding for what concern the developing of the recipe of the wet etching solution and the right choice of the metallic-alloy for the fabrication of ohmic contacts. In the third section the electrical characterisation of the device is presented. Starting from the description of the method to evaluate the quality of our contacts we have measured the contact resistivity and so their “degree of ohmicity”. The proper characterization of the device has been conducted starting from the measurement of the dark current values under a bias range of -2V<bias<2V at room temperature. Different generation of sample with different intrinsic layer thickness have been characterized, looking at the dependence of dominant dark transport regime on the thickness. The temperature characterization has been conducted in a closed-loop He cryostat, on which a field-assisted transport is recognized as prominent effect on the dark current generation just over the |0.7| V, while under this value thermoionic regime is identified as prominent one. The verification of the IR absorption is conducted in two phase. In the first phase the absorption is verified in a setup for cryogenic photo-current measurement, in the second phase the spectral shape of the photocurrent is measured by the mean of a Fourier transform infrared spectrometer (FTIR) in collaboration with Patrick Rauter of the Johannes Kepler University of Linz. A qualitative comparison (different scales) with a commercial HgCdTe (MCT) detctor reveals that the region of detection is far from the expected one and even though the signal to noise ratio is strongly improved respected to previous generation of QDIP. Furthermore, the device needs long times for photo-current stabilization (about 30 min) suggesting the presence of unwanted capacitance. In the last section, a new design is tested, with an evolved setup configuration for photo-current measurement, trying to solve the puzzling observations of previous QDIP designs.