The commercially available solar cells suffer from low conversion efficiency due to the thermalization and transmission losses arising from the mismatch between the band gap of the semiconductor materials and the solar spectrum. Advanced device architectures based on nanomaterial have been proposed and being successfully used to enhance the efficiency of the solar cells. Quantum dots (QDs) and nanowires (NWs) are the nanosclae structures that have been exploited for the development of the third generation solar cell devices and nanowire based solar cells, respectively. The optical and electrical properties of these materials can be tuned by their size and geometry; hence they have great potential for the production of highly efficient solar cell. Application of QDs and NWs with enhanced optoelectronic properties and development of low-cost fabrication processes render a new generation of economic highly efficient PV devices. The most significant contribution of this PhD study is the development of simple and cost effective methods for fabrication of nanowires and quantum dots for advanced solar cell architectures. In advanced silicon nanowires (SiNWs) array cell, SiNWs have been widely synthesised by the well-known vapor-liquid-solid method. Electron beam lithography and deep reactive ion etching have also been employed for fabrication of SiNWs. Due to the high price and complexity of these methods, simple and cost effective approaches are needed for the fabrication of SiNWs. In another approach, to enhance the cell efficiency, organic dyes and polymers have been widely used as luminescent centers and host mediums in the luminescent down shifting (LDS) layers. However, due to the narrow absorption band of the dyes and degradation of the polymers by moisture and heat, these materials are not promising candidates to use as LDS. Highly efficient luminescent materials and transparent host materials with stable mechanical properties are demanded for luminescent down shifting applications. In this project, simple fabrication processes were developed to produce SiNWs and QDs for application in advanced cell architectures. The SiNWs array were successfully fabricated, characterized and deployed in new cell architectures with radial p-n junction geometry. The luminescence down shifting of layers containing QDs in oxide and glass mediums was verified. The silica coated quantum dots which are suitable for luminescence down shifting, were also fabricated and characterized for deployment in new design architectures. Silicon nanowires were fabricated using two simplified methods. In the first approach, a maskless reactive ion etching process was developed to form upright ordered arrays of the SiNWs without relying on the complicated nano-scale lithography or masking methods. The fabricated structures were comprehensively characterized. Light trapping and photoluminescence properties of the medium were verified. In the second approach, combination of the nanosphere lithography and etching techniques were utilized for wire formation. This method provides a better control on the wire diameters and geometries in a very simple and cost effective way. The fabricated silicon nanowires were used for formation of the radial p-n junction array cells. The functionality of the new cell structures were confirmed through experimental and simulation results. Quantum dots are promising candidates as luminescent centers due to their tunable optical properties. Oxide/glass matrices are also preferred as the host medium for QDs because of their robust mechanical properties and their compatibility with standard silicon processing technology. Besides, the oxide layers are transparent mediums with good passivation and anti-reflection coating properties. They can also be used to encapsulate the cell. In this work, ordered arrays of QDs were incorporated in an oxide layer to form a luminescent down shifting layer. This design benefits from the enhanced absorption of a periodic QD structure in a transparent oxide. The down shifting properties of the layer after deployment on a crystalline silicon solar cell were examined. For this purpose, crystalline silicon solar cells were fabricated to use as test platform for down shifting. In order to examine the down-shifting effect, different approaches for formation of a luminescence down shifting layer were developed. The LDS layer consist of cadmium selenide- zinc sulfide (CdSe/ZnS) quantum dots in oxide and glass layers to act as luminescent centers and transparent host medium, respectively. The structural and optical properties of the fabricated layers were studied. The concept of spectral engineering was proved by the deployment of the layer on the solar cell. To further benefit from the LDS technique, quantum efficiency of the QDs and optical properties of the layer must be improved. Demand for the high quantum efficiency material with desired geometry leaded us to synthesis quantum dots coated with a layer of grown oxide. As the luminescence quantum efficiency of the QDs is correlated to the surface defects, one advantage of having oxide on the outer shell of the QDs, is to passivate the surface non-radiative recombination centers and produce QDs with high luminescent quantum yield. In addition, nanoparticles with desired size can be obtained only by changing the thickness of the oxide shell. This method also simplifies the fabrication of QD arrays for luminescence down shifting application, since it is easier to form ordered arrays from larger particles. QD superlattices in an oxide medium can be fabricated on a large area by a simple spin-coating or dip coating methods. The photonic crystal properties of the proposed structure can greatly increase the absorption in the QDs layer and enhance the effect of down shifting.

Semiconductor quantum dots and metal nanoparticles feature very strong light-matter interactions, which has led to their use in many photonic applications such as photodetectors, biosensors, components for telecommunications etc.Under illumination both structures exhibit collective electron-photon resonances, described in the frameworks of quasiparticles as exciton-polaritons for semiconductors and surface plasmon-polaritons for metals.To date these two approaches to controlling light interactions have usually been treated separately, with just a few simple attempts to consider exciton-plasmon interactions in a system consisting of both semiconductor and metal nanostructures.In this work, the exciton-polaritons and surface \\plasmon-polaritons are first considered separately, and then combined using the Finite Difference Time Domain numerical method coupled with a master equation for the exciton-polariton population dynamics.To better understand the properties of excitons and plasmons, each quasiparticle is used to investigate two open questions - the source of the Stokes shift between the absorption and luminescence peaks in quantum dots, and the source of the photocurrent increase in quantum dot infrared photodetectors coated by a thin metal film with holes. The combined numerical method is then used to study a system consisting of multiple metal nanoparticles close to a quantum dot, a system which has been predicted to exhibit quantum dot-induced transparency, but is demonstrated to just have a weak dip in the absorption. / <p>QC 20120417</p>

Herein, we report the first solution-processed broadband photodetectors to break the past compromise between sensitivity and speed of response. Specifically, we report photodiodes having normalized detectivity (D*) > 1012 Jones and a 3dB bandwidth of > 2.9 MHz. This finding represents a 170,000 fold improvement in response speed over the most sensitive colloidal quantum dot (CQD) photodetector reported1 and a 100,000 fold improvement in sensitivity over the fastest CQD photodetector reported2. At the outset of this study, sensitive, solution-processed IR photodetectors were severely limited by low response speeds1. Much faster response speeds had been demonstrated by solution-processed photodetectors operating in the visible3, but these devices offered no benefits for extending the spectral sensitivity of silicon. No available solution-processed photodetector combined high sensitivity, high operating speed, and response to illumination across the UV, visible and IR. We developed a fast, sensitive, solution-processed photodetector based on a photodiode formed by a Schottky barrier to a CQD film. Previous attempts to form sensitive photodetectors based on CQD photodiodes had demonstrated low quantum efficiencies that limited sensitivity4,5. Efficient, sensitive semiconductor photodiodes are based on two fundamental characteristics: a large built-in potential that separates photogenerated charge carriers and minimizes internal noise generation, and high semiconductor conductivity for efficient collection of photogenerated charge. Schottky barriers to CQD films were developed to provide high, uniform built-in potentials. A multi-step CQD ligand exchange procedure was developed to allow deposition of tightly packed films of CQDs with high mobility and sufficiently well-passivated surfaces to form high-quality metallurgical junctions. The temporal response of the CQD photodiodes showed separate drift and diffusion components. Combined with detailed measurements of the Schottky barrier, these characteristics provided the physical basis for a numerical model of device operation. Based on this understanding, devices that excluded the slow diffusive component were fabricated, exploiting only the sub-microsecond field-driven transient to achieve MHz response bandwidth. These devices are the first to combine megahertz-bandwidth, high sensitivity, and spectral-tunability in photodetectors based on semiconducting CQDs. Record performance is achieved through advances in materials and device architecture based on a detailed understanding the physical mechanisms underlying the operation of CQD photodiodes.

Herein, we report the first solution-processed broadband photodetectors to break the past compromise between sensitivity and speed of response. Specifically, we report photodiodes having normalized detectivity (D*) > 1012 Jones and a 3dB bandwidth of > 2.9 MHz. This finding represents a 170,000 fold improvement in response speed over the most sensitive colloidal quantum dot (CQD) photodetector reported1 and a 100,000 fold improvement in sensitivity over the fastest CQD photodetector reported2. At the outset of this study, sensitive, solution-processed IR photodetectors were severely limited by low response speeds1. Much faster response speeds had been demonstrated by solution-processed photodetectors operating in the visible3, but these devices offered no benefits for extending the spectral sensitivity of silicon. No available solution-processed photodetector combined high sensitivity, high operating speed, and response to illumination across the UV, visible and IR. We developed a fast, sensitive, solution-processed photodetector based on a photodiode formed by a Schottky barrier to a CQD film. Previous attempts to form sensitive photodetectors based on CQD photodiodes had demonstrated low quantum efficiencies that limited sensitivity4,5. Efficient, sensitive semiconductor photodiodes are based on two fundamental characteristics: a large built-in potential that separates photogenerated charge carriers and minimizes internal noise generation, and high semiconductor conductivity for efficient collection of photogenerated charge. Schottky barriers to CQD films were developed to provide high, uniform built-in potentials. A multi-step CQD ligand exchange procedure was developed to allow deposition of tightly packed films of CQDs with high mobility and sufficiently well-passivated surfaces to form high-quality metallurgical junctions. The temporal response of the CQD photodiodes showed separate drift and diffusion components. Combined with detailed measurements of the Schottky barrier, these characteristics provided the physical basis for a numerical model of device operation. Based on this understanding, devices that excluded the slow diffusive component were fabricated, exploiting only the sub-microsecond field-driven transient to achieve MHz response bandwidth. These devices are the first to combine megahertz-bandwidth, high sensitivity, and spectral-tunability in photodetectors based on semiconducting CQDs. Record performance is achieved through advances in materials and device architecture based on a detailed understanding the physical mechanisms underlying the operation of CQD photodiodes.

In this study silicon-germanium quantum dots grown on silicon have been investigated. The aim of the work was to find quantum dots suitable for use as a thermistor material. The quantum dots were produced at KTH, Stockholm, using a RPCVD reactor that is designed for industrial production. The techniques used to study the quantum dots were: HRSEM, AFM, HRXRD, FTPL, and Raman spectroscopy. Quantum dots have been produced in single and multilayer structures. As a result of this work a multilayer structure with 5 layers of quantum dots was produced with a theoretical temperature coefficient of resistance of 4.1 %/K.

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.

<p>Quantum Dot Infrared Photodetectors (QDIPs) are important alternatives to conventional infrared photodetectors with high potential to provide required detector performance, such as higher temperature operation and multispectral response, due to the 3-D quantum confinement of electrons, discrete energy levels, and intrinsic response to perpendicular incident light due to selection rules. However, excessive dark current density, which causes QDIPs to underperform theoretical predictions, is a limiting factor for the advancement of QDIP technologies. The purpose of this dissertation research is to achieve a better understanding of dopant incorporation into the active region of QDIPs, which is directly related to dark current control and spectral response. From this dissertation research, doping related dipole fields are found to be responsible for excessive dark current in QDIPs. </p><p>InAs/GaAs QDIPs were grown using solid source molecular beam epitaxy (MBE) with different doping conditions. The QDIPs were optically characterized using photoluminescence and Fourier transform infrared (FT-IR) spectroscopy. Devices were fabricated using standard cleanroom fabrication procedures. Dark current and capacitance measurements were performed under different temperature to reveal electronic properties of the materials and devices. A novel scanning capacitance microscopy (SCM) technique was used to study the band structure and carrier concentration on the cross section of a quantum dot (QD) heterostructure. In addition, dark current modeling and bandstructure calculations were performed to verify and better understand experimental results.</p><p>Two widely used QDIP doping methods with different doping concentrations have been studied in this dissertation research, namely direct doping in InAs QD layer, and modulation doping in the GaAs barrier above InAs QD layer. In the SCM experiment, electron redistribution has been observed due to band-bending in the modulation-doping region, while there is no band-bending observed in directly doped samples. A good agreement between the calculated bandstructure and experimental results leads to better understanding of doping in QD structures. The charge filling process in QDs has been observed by an innovative polarization-dependent FT-IR spectroscopy. The red-shift of QD absorbance peaks with increasing electron occupation supports a miniband electronic configuration for high-density QD ensembles. In addition, the FT-IR measurement indicates the existence of donor-complex (DX) defect centers in Si-doped QDIPs. The existence of DX centers and related dipole fields have been confirmed by dark current measurements to extract activation energies and by photocapacitance quenching measurements. </p><p>With the understanding achieved from experimental results, a further improved dark current model has been developed based on the previous model originally established by Ryzhii and improved by Stiff-Roberts. In the model described in this dissertation, two new factors have been considered. The inclusion of background drift current originating from Si shallow donors in the low bias region results in excellent agreement between calculated and measured dark currents at different temperatures, which has not been achieved by previous models. A very significant effect has been observed in that dark current leakage occurs due to the dipole field caused by doping induced charge distribution and impact-ionized DX centers. </p><p>Last but not least, QDIPs featuring the dipole interface doping (DID) method have been designed to reduce the dark current density without changing the activation energy (thus detection wavelength) of QDIPs. The DID samples involve an InAs QD layer directly-doped by Si, as well as Be doping in the GaAs barrier on both sides of the QD layer. The experimental result shows the dark current density has been significantly reduced by 104 times without any significant change to the corresponding activation energy. However, the high p-type doping in the GaAs barrier poses a challenge in that the Fermi level is reduced to be well below the QD energy states. High p-type doping is reported to reduce the dark current, photocurrent and the responsivity of the devices. </p><p>To conclude, it is significant to identify to effect of Si-induced defect centers on QDIP dark currents. The subsequent study reveals doping induced dipole fields can have significant effects on QDIP device performance, for example, causing charge leakage from QDs and reducing activation energy, thereby increasing dark current density. The DID approach developed in this work is a promising approach that could help address these issues by using controlled dipole fields to reduce dark current density without changing the minimum detectable energy of QDIPs.</p> / Dissertation

The purpose of our research is growing the coupled InGaAs quantum dots on the n-type substrate by molecular beam epitaxy in laboratory, and we choose 5,10 and 15 nanometers to be the thicknesses of GaAs spacer between the quantum dots layer. Due to the couple effect, we hope to realize the theorem of intermediate band proved by Luque and Marti. We measure the characteristic of samples by electroluminescence spectra, photoelectric current spectra, electrical absorption spectra and electro reflectance spectra in laboratory; moreover, we acquire the basic parameters of solar cell by AM1.5G for analyzing. From the basic parameters of solar cell, we know that the quantum dots can enhance the photocurrent by absorbing additional photons , however, the strain caused by quantum dots would decay the open voltage seriously, so that the efficiency always under the baseline. Each efficiency of 9-stack QDs are 4.3%(c494),5.1%(c519),5.3% (c520),and each efficiency of 9-stack Dwells are 3.9%(c524),4.2%(c525),4.7%(c526), and 10-stack QDs(5nm) is 2.9%(c514),and 12-stack QDs(10nm) is 4.48%(c538),and 12-stack QDs(15nm) is 5.89%. The break through of this paper is that the efficiency of c529¡]VOC=0.64V,JSC=11.97mA/cm2,FF=67%,£b=5.89%¡^is higher than GaAs¡]VOC =0.87 V, JSC =7.4 mA/cm2,FF=72.3%,£b=5.6%¡^,and we attribute this performance to its good quality of miniband, because the current can be enhanced a lot, and it will make up for the lose of open voltage and filling factor, so that the efficiency can be higher than GaAs baseline.

In this thesis, we have reported the MBE growth, design, and fabrication of the InGaAs quantum dots (QDs) laser/semiconductor optical amplifier, broadband QDs structure, coupled double cavity structure for terahertz emission on GaAs substrate. The emission wavelengths of the strain-induced S-K growth mode QDs structures are adjusted through the composition of QDs and strain-compensated capping layer. Also, the technique of growing high quality InGaAs QDs with solid source molecular beam epitaxy has been established and characterized by double crystal X-ray diffraction, transmission electron microscopy, photoluminescence, electroluminescence measurements. For 1.3£gm QDs laser samples, ridge waveguide lasers of the Fabry-Perot (FP) type are fabricated by wet-etching process. From the QDs laser L-I curve, the e2-hh2 transition at £f =1160nm have larger optical gain than e1-hh1 transition at £f =1220nm. The FP laser with 0.6£gm cavity length shows a lasing peak of 1160nm at threshold. As the cavity length increase to 2£gm, the lasing peak red shift to 1220nm (closed to ground state emission wavelength). This energy band gap transition phenomenon is obvious especially in the QDs laser with quantum well (QW) structure. When the injection current increase, two lasing peaks at £f= 1160 and 1175nm are observed sequentially. This unique lasing behavior is shown to be consistent with carriers localized in noninteracting dots. For the application of 1.3£gm light source, we optimum the growth condition for different needs in optical coherent tomography (OCT) light source, tandem solar cell, terahertz emission light source, etc. For the super luminescence diode (SLED) in OCT, we design multi-stacked asymmetric QDs structure (AMQD), QDs in the well structure (DWell), Dwell with p-doping in well structure to investigate the carrier recombination condition and bandwidth. Comparing with 5 structures in this study, the Dwell with p-doping in well structure has a maximum EL bandwidth exceed 198nm. The large bandwidth is attributed to the QW which increases the carrier capture rate and the p-doping which provide the efficient holes in valance band. This structure provides an excellent SLED light source solution to replace the existing program. For the tandem solar cell, we use the multi-stack QDs to compose broadband absorption in 1eV range. In order to avoid the degradation in the open circuit voltage, we use InGaAs QW to reduce the QDs strain. We observed the doping effect on the built in field through the photo-reflectance measurements. For the better photocurrent collection, we use p-doping in the QW to increase the built-in field intensity to obtain higher efficiency. For the terahertz emission, the QDs embedded in coupled double-cavity structures with an AlAs/GaAs intermediate distributed Bragg reflector (DBR) are grown on GaAs substrates. Two emission peaks at 1180, 1206 nm from the QDs corresponding to the coupled double-cavity resonant modes are observed in the high reflection band. The frequency differences for the two resonant coupled modes are of 5.5 terahertz, and have been successfully controlled by changing the pair numbers for the intermediate DBR. In addition, we have grown the InGa(Al)As nanostructures on InP substrate. The lattice constant difference between InGaAs and InP is relatively smaller compare with GaAs substrate, and it will be more challenge in epitaxial growth. After we investigate the strain, surface morphologies, optical properties for the nanostructures, we find the group III elements play an important role in the morphologies. Wire formation is attributed by the enhanced adatom diffusion length in the stepped surface front along [0-11] direction for the presence of Ga both in the nanostructure and buffer layer. Finally, we established QDs, Qwires database for the valuable new possibilities for designing new and original structures.

Quantum dots (QDs) are increasingly attracting attention in recent years due to their potential in biological imaging and drug delivery applications. Despite their significant advantages over organic dyes and fluorescent proteins, cytotoxicity is still a major problem in live-cell QD labeling. In this work, in-vitro characterization of a novel CdTe/2MPA quantum dot capped with CdS-DMSA was conducted on human cervical cancer (HeLa) and mouse fibroblast (NIH/3T3) cell lines. Biocompatibility of this novel particle was evaluated in comparison to a commercial quantum dot (Qdot 565) and various QDs with CdTe core. Cytotoxicity of quantum dots was investigated using XTT and proliferation assays. Cellular internalization and localization of particles were studied using confocal laser scanning microscopy. For quantitative determination of internalization and intracellular QD stability, we also performed uptake and cadmium release assays. Optimal cell imaging concentration with CdTe-CdS/2MPA-DMSA was determined as 10-50 ug/mL in HeLa cells. Localization of the internalized QD particles was observed in the perinuclear region of the cells. XTT and proliferation assays provided identical viability results for the tested QDs. CdS-DMSA capping increased cytocompatibility of CdTe/2MPA by 15% in NIH/3T3 cells. Biocompatibility of this capped particle was further increased by 3-folds with pegylation. For pegylated CdTe-CdS/2MPA-DMSA and commercial Qdot 565, we have not observed QD-related cytotoxicity on NIH/3T3 cells following 24-hr QD exposure at 50 ug/mL. Our in-vitro characterization studies indicate that CdTe-CdS/2MPA-DMSA is a promising live-cell imaging probe which can be effectively excited in the visible range of the electromagnetic spectrum.