Matching Items (9)

Filtering by

Clear all filters

151142-Thumbnail Image.png

Novel materials, grid design rule, and characterization methods for multi-junction solar cells

Description

This dissertation addresses challenges pertaining to multi-junction (MJ) solar cells from material development to device design and characterization. Firstly, among the various methods to improve the energy conversion efficiency of MJ solar cells using, a novel approach proposed recently is

This dissertation addresses challenges pertaining to multi-junction (MJ) solar cells from material development to device design and characterization. Firstly, among the various methods to improve the energy conversion efficiency of MJ solar cells using, a novel approach proposed recently is to use II-VI (MgZnCd)(SeTe) and III-V (AlGaIn)(AsSb) semiconductors lattice-matched on GaSb or InAs substrates for current-matched subcells with minimal defect densities. CdSe/CdTe superlattices are proposed as a potential candidate for a subcell in the MJ solar cell designs using this material system, and therefore the material properties of the superlattices are studied. The high structural qualities of the superlattices are obtained from high resolution X-ray diffraction measurements and cross-sectional transmission electron microscopy images. The effective bandgap energies of the superlattices obtained from the photoluminescence (PL) measurements vary with the layer thicknesses, and are smaller than the bandgap energies of either the constituent material. Furthermore, The PL peak position measured at the steady state exhibits a blue shift that increases with the excess carrier concentration. These results confirm a strong type-II band edge alignment between CdSe and CdTe. The valence band offset between unstrained CdSe and CdTe is determined as 0.63 eV±0.06 eV by fitting the measured PL peak positions using the Kronig-Penney model. The blue shift in PL peak position is found to be primarily caused by the band bending effect based on self-consistent solutions of the Schrödinger and Poisson equations. Secondly, the design of the contact grid layout is studied to maximize the power output and energy conversion efficiency for concentrator solar cells. Because the conventional minimum power loss method used for the contact design is not accurate in determining the series resistance loss, a method of using a distributed series resistance model to maximize the power output is proposed for the contact design. It is found that the junction recombination loss in addition to the series resistance loss and shadowing loss can significantly affect the contact layout. The optimal finger spacing and maximum efficiency calculated by the two methods are close, and the differences are dependent on the series resistance and saturation currents of solar cells. Lastly, the accurate measurements of external quantum efficiency (EQE) are important for the design and development of MJ solar cells. However, the electrical and optical couplings between the subcells have caused EQE measurement artifacts. In order to interpret the measurement artifacts, DC and small signal models are built for the bias condition and the scan of chopped monochromatic light in the EQE measurements. Characterization methods are developed for the device parameters used in the models. The EQE measurement artifacts are found to be caused by the shunt and luminescence coupling effects, and can be minimized using proper voltage and light biases. Novel measurement methods using a pulse voltage bias or a pulse light bias are invented to eliminate the EQE measurement artifacts. These measurement methods are nondestructive and easy to implement. The pulse voltage bias or pulse light bias is superimposed on the conventional DC voltage and light biases, in order to control the operating points of the subcells and counterbalance the effects of shunt and luminescence coupling. The methods are demonstrated for the first time to effectively eliminate the measurement artifacts.

Contributors

Agent

Created

Date Created
2012

151648-Thumbnail Image.png

Modeling of self-heating effects in 25nm SOI devices

Description

Since its inception about three decades ago, silicon on insulator (SOI) technology has come a long way to be included in the microelectronics roadmap. Earlier, scientists and engineers focused on ways to increase the microprocessor clock frequency and speed. Today,

Since its inception about three decades ago, silicon on insulator (SOI) technology has come a long way to be included in the microelectronics roadmap. Earlier, scientists and engineers focused on ways to increase the microprocessor clock frequency and speed. Today, with smart phones and tablets gaining popularity, power consumption has become a major factor. In this thesis, self-heating effects in a 25nm fully depleted (FD) SOI device are studied by implementing a 2-D particle based device simulator coupled self-consistently with the energy balance equations for both acoustic and optical phonons. Semi-analytical expressions for acoustic and optical phonon scattering rates (all modes) are derived and evaluated using quadratic dispersion relationships. Moreover, probability distribution functions for the final polar angle after scattering is also computed and the rejection technique is implemented for its selection. Since the temperature profile varies throughout the device, temperature dependent scattering tables are used for the electron transport kernel. The phonon energy balance equations are also modified to account for inelasticity in acoustic phonon scattering for all branches. Results obtained from this simulation help in understanding self-heating and the effects it has on the device characteristics. The temperature profiles in the device show a decreasing trend which can be attributed to the inelastic interaction between the electrons and the acoustic phonons. This is further proven by comparing the temperature plots with the simulation results that assume the elastic and equipartition approximation for acoustic and the Einstein model for optical phonons. Thus, acoustic phonon inelasticity and the quadratic phonon dispersion relationships play a crucial role in studying self-heating effects.

Contributors

Agent

Created

Date Created
2013

150443-Thumbnail Image.png

Monte Carlo studies of electron transport in semiconductor nanostructures

Description

ABSTRACT An Ensemble Monte Carlo (EMC) computer code has been developed to simulate, semi-classically, spin-dependent electron transport in quasi two-dimensional (2D) III-V semiconductors. The code accounts for both three-dimensional (3D) and quasi-2D transport, utilizing either 3D or 2D scattering mechanisms,

ABSTRACT An Ensemble Monte Carlo (EMC) computer code has been developed to simulate, semi-classically, spin-dependent electron transport in quasi two-dimensional (2D) III-V semiconductors. The code accounts for both three-dimensional (3D) and quasi-2D transport, utilizing either 3D or 2D scattering mechanisms, as appropriate. Phonon, alloy, interface roughness, and impurity scattering mechanisms are included, accounting for the Pauli Exclusion Principle via a rejection algorithm. The 2D carrier states are calculated via a self-consistent 1D Schrödinger-3D-Poisson solution in which the charge distribution of the 2D carriers in the quantization direction is taken as the spatial distribution of the squared envelope functions within the Hartree approximation. The wavefunctions, subband energies, and 2D scattering rates are updated periodically by solving a series of 1D Schrödinger wave equations (SWE) over the real-space domain of the device at fixed time intervals. The electrostatic potential is updated by periodically solving the 3D Poisson equation. Spin-polarized transport is modeled via a spin density-matrix formalism that accounts for D'yakanov-Perel (DP) scattering. Also, the code allows for the easy inclusion of additional scattering mechanisms and structural modifications to devices. As an application of the simulator, the current voltage characteristics of an InGaAs/InAlAs HEMT are simulated, corresponding to nanoscale III-V HEMTs currently being fabricated by Intel Corporation. The comparative effects of various scattering parameters, material properties and structural attributes are investigated and compared with experiments where reasonable agreement is obtained. The spatial evolution of spin-polarized carriers in prototypical Spin Field Effect Transistor (SpinFET) devices is then simulated. Studies of the spin coherence times in quasi-2D structures is first investigated and compared to experimental results. It is found that the simulated spin coherence times for GaAs structures are in reasonable agreement with experiment. The SpinFET structure studied is a scaled-down version of the InGaAs/InAlAs HEMT discussed in this work, in which spin-polarized carriers are injected at the source, and the coherence length is studied as a function of gate voltage via the Rashba effect.

Contributors

Agent

Created

Date Created
2011

150588-Thumbnail Image.png

Design, modeling and simulation of nanoscale optoelectronic devices: semiconductor nano-lasers and plasmonic waveguides

Description

This thesis summarizes the research work carried out on design, modeling and simulation of semiconductor nanophotonic devices. The research includes design of nanowire (NW) lasers, modeling of active plasmonic waveguides, design of plasmonic nano-lasers, and design of all-semiconductor plasmonic systems.

This thesis summarizes the research work carried out on design, modeling and simulation of semiconductor nanophotonic devices. The research includes design of nanowire (NW) lasers, modeling of active plasmonic waveguides, design of plasmonic nano-lasers, and design of all-semiconductor plasmonic systems. For the NW part, a comparative study of electrical injection in the longitudinal p-i-n and coaxial p-n core-shell NWs was performed. It is found that high density carriers can be efficiently injected into and confined in the core-shell structure. The required bias voltage and doping concentrations in the core-shell structure are smaller than those in the longitudinal p-i-n structure. A new device structure with core-shell configuration at the p and n contact regions for electrically driven single NW laser was proposed. Through a comprehensive design trade-off between threshold gain and threshold voltage, room temperature lasing has been proved in the laser with low threshold current and large output efficiency. For the plasmonic part, the propagation of surface plasmon polariton (SPP) in a metal-semiconductor-metal structure where semiconductor is highly excited to have an optical gain was investigated. It is shown that near the resonance the SPP mode experiences an unexpected giant modal gain that is 1000 times of the material gain in the semiconductor and the corresponding confinement factor is as high as 105. The physical origin of the giant modal gain is the slowing down of the average energy propagation in the structure. Secondly, SPP modes lasing in a metal-insulator-semiconductor multi-layer structure was investigated. It is shown that the lasing threshold can be reduced by structural optimization. A specific design example was optimized using AlGaAs/GaAs/AlGaAs single quantum well sandwiched between silver layers. This cavity has a physical volume of 1.5×10-4 λ03 which is the smallest nanolaser reported so far. Finally, the all-semiconductor based plasmonics was studied. It is found that InAs is superior to other common semiconductors for plasmonic application in mid-infrared range. A plasmonic system made of InAs, GaSb and AlSb layers, consisting of a plasmonic source, waveguide and detector was proposed. This on-chip integrated system is realizable in a single epitaxial growth process.

Contributors

Agent

Created

Date Created
2012

154989-Thumbnail Image.png

Full band Monte Carlo simulation of nanowires and nanowire field effect transistors

Description

In this work, transport in nanowire materials and nanowire field effect transistors is studied using a full band Monte Carlo simulator within the tight binding basis. Chapter 1 is dedicated to the importance of nanowires and nanoscale devices in present

In this work, transport in nanowire materials and nanowire field effect transistors is studied using a full band Monte Carlo simulator within the tight binding basis. Chapter 1 is dedicated to the importance of nanowires and nanoscale devices in present day electronics and the necessity to use a computationally efficient tool to simulate transport in these devices. Chapter 2 discusses the calculation of the full band structure of nanowires based on an atomistic tight binding approach, particularly noting the use of the exact same tight binding parameters for bulk band structures as well as the nanowire band structures. Chapter 3 contains the scattering rate formula for deformation potential, polar optical phonon, ionized impurity and impact ionization scattering in nanowires using Fermi’s golden rule and the tight binding basis to describe the wave functions. A method to calculate the dielectric screening in 1D systems within the tight binding basis is also described. Importantly, the scattering rates of nanowires tends to the bulk scattering rates at high energies, enabling the use of the same parameter set that were fitted to bulk experimental data to be used in the simulation of nanowire transport. A robust and efficient method to model interband tunneling is discussed in chapter 4 and its importance in nanowire transport is highlighted. In chapter 5, energy relaxation of excited electrons is studied for free standing nanowires and cladded nanowires. Finally, in chapter 6, a full band Monte Carlo particle based solver is created which treats confinement in a full quantum way and the current voltage characteristics as well as the subthreshold swing and percentage of ballistic transport is analyzed for an In0.7Ga0.3As junctionless nanowire field effect transistor.

Contributors

Agent

Created

Date Created
2016

156549-Thumbnail Image.png

RPCVD Growth of Epitaxial Si-Ge-Sn Alloys for Optoelectronics Applications

Description

Ge1-xSnx and SiyGe1-x-ySnx materials are being researched intensively for applications in infra-red optoelectronic devices. Due to their direct band gap these materials may in-fact be the enabling factor in the commercial realization of silicon photonics/group IV photonics and the integration

Ge1-xSnx and SiyGe1-x-ySnx materials are being researched intensively for applications in infra-red optoelectronic devices. Due to their direct band gap these materials may in-fact be the enabling factor in the commercial realization of silicon photonics/group IV photonics and the integration of nanophotonics with nanoelectronics. However the synthesis of these meta-stable semiconductor alloys, with a range of Sn-compositions, remains the primary technical challenge. Highly specialized epitaxial growth methods must be employed to produce single crystal layers which have sufficient quality for optoelectronic device applications. Up to this point these methods have been unfavorable from a semiconductor manufacturing perspective. In this work the growth of high-quality Si-Ge-Sn epitaxial alloys on Ge-buffered Si (100) using an industry-standard reduced pressure chemical vapor deposition reactor and a cost-effective chemistry is demonstrated. The growth kinetics are studied in detail in-order to understand the factors influencing layer composition, morphology, and defectivity. In doing so breakthrough GeSn materials and device results are achieved including methods to overcome the limits of Sn-incorporation and the realization of low-defect and strain-relaxed epitaxial layers with up to 20% Sn.

P and n-type doping methods are presented in addition to the production of SiGeSn ternary alloys. Finally optically stimulated lasing in thick GeSn layers and SiGeSn/GeSn multiple quantum wells is demonstrated. Lasing wavelengths ranging from 2-3 µm at temperatures up to 180K are realized in thick layers. Whereas SiGeSn/GeSn multiple quantum wells on a strain-relaxed GeSn buffers have enabled the first reported SiGeSn/GeSn multiple quantum well laser operating up to 80K with threshold power densities as low as 33 kW/cm2.

Contributors

Agent

Created

Date Created
2018

158089-Thumbnail Image.png

Design and Development of High Performance III-Nitrides Photovoltaics

Description

Wurtzite (In, Ga, Al) N semiconductors, especially InGaN material systems, demonstrate immense promises for the high efficiency thin film photovoltaic (PV) applications for future generation. Their unique and intriguing merits include continuously tunable wide band gap from 0.70 eV to

Wurtzite (In, Ga, Al) N semiconductors, especially InGaN material systems, demonstrate immense promises for the high efficiency thin film photovoltaic (PV) applications for future generation. Their unique and intriguing merits include continuously tunable wide band gap from 0.70 eV to 3.4 eV, strong absorption coefficient on the order of ∼105 cm−1, superior radiation resistance under harsh environment, and high saturation velocities and high mobility. Calculation from the detailed balance model also revealed that in multi-junction (MJ) solar cell device, materials with band gaps higher than 2.4 eV are required to achieve PV efficiencies greater than 50%, which is practically and easily feasible for InGaN materials. Other state-of-art modeling on InGaN solar cells also demonstrate great potential for applications of III-nitride solar cells in four-junction solar cell devices as well as in the integration with a non-III-nitride junction in multi-junction devices.

This dissertation first theoretically analyzed loss mechanisms and studied the theoretical limit of PV performance of InGaN solar cells with a semi-analytical model. Then three device design strategies are proposed to study and improve PV performance: band polarization engineering, structural design and band engineering. Moreover, three physical mechanisms related to high temperature performance of InGaN solar cells have been thoroughly investigated: thermal reliability issue, enhanced external quantum efficiency (EQE) and conversion efficiency with rising temperatures and carrier dynamics and localization effects inside nonpolar m-plane InGaN quantum wells (QWs) at high temperatures. In the end several future work will also be proposed.

Although still in its infancy, past and projected future progress of device design will ultimately achieve this very goal that III-nitride based solar cells will be indispensable for today and future’s society, technologies and society.

Contributors

Agent

Created

Date Created
2020

157046-Thumbnail Image.png

Wide Bandgap Semiconductors Based Energy-Efficient Optoelectronics and Power Electronics

Description

Wide bandgap (WBG) semiconductors GaN (3.4 eV), Ga2O3 (4.8 eV) and AlN (6.2 eV), have gained considerable interests for energy-efficient optoelectronic and electronic applications in solid-state lighting, photovoltaics, power conversion, and so on. They can offer unique device performance compared

Wide bandgap (WBG) semiconductors GaN (3.4 eV), Ga2O3 (4.8 eV) and AlN (6.2 eV), have gained considerable interests for energy-efficient optoelectronic and electronic applications in solid-state lighting, photovoltaics, power conversion, and so on. They can offer unique device performance compared with traditional semiconductors such as Si. Efficient GaN based light-emitting diodes (LEDs) have increasingly displaced incandescent and fluorescent bulbs as the new major light sources for lighting and display. In addition, due to their large bandgap and high critical electrical field, WBG semiconductors are also ideal candidates for efficient power conversion.

In this dissertation, two types of devices are demonstrated: optoelectronic and electronic devices. Commercial polar c-plane LEDs suffer from reduced efficiency with increasing current densities, knowns as “efficiency droop”, while nonpolar/semipolar LEDs exhibit a very low efficiency droop. A modified ABC model with weak phase space filling effects is proposed to explain the low droop performance, providing insights for designing droop-free LEDs. The other emerging optoelectronics is nonpolar/semipolar III-nitride intersubband transition (ISBT) based photodetectors in terahertz and far infrared regime due to the large optical phonon energy and band offset, and the potential of room-temperature operation. ISBT properties are systematically studied for devices with different structures parameters.

In terms of electronic devices, vertical GaN p-n diodes and Schottky barrier diodes (SBDs) with high breakdown voltages are homoepitaxially grown on GaN bulk substrates with much reduced defect densities and improved device performance. The advantages of the vertical structure over the lateral structure are multifold: smaller chip area, larger current, less sensitivity to surface states, better scalability, and smaller current dispersion. Three methods are proposed to boost the device performances: thick buffer layer design, hydrogen-plasma based edge termination technique, and multiple drift layer design. In addition, newly emerged Ga2O3 and AlN power electronics may outperform GaN devices. Because of the highly anisotropic crystal structure of Ga2O3, anisotropic electrical properties have been observed in Ga2O3 electronics. The first 1-kV-class AlN SBDs are demonstrated on cost-effective sapphire substrates. Several future topics are also proposed including selective-area doping in GaN power devices, vertical AlN power devices, and (Al,Ga,In)2O3 materials and devices.

Contributors

Agent

Created

Date Created
2019

168405-Thumbnail Image.png

Chip-integrated Plasmonic Optics for Polarization Control and Detection

Description

Polarization detection and control techniques play essential roles in various applications, including optical communication, polarization imaging, chemical analysis, target detection, and biomedical diagnosis. Conventional methods for polarization detection and polarization control require bulky optical systems. Flat optics opens a new

Polarization detection and control techniques play essential roles in various applications, including optical communication, polarization imaging, chemical analysis, target detection, and biomedical diagnosis. Conventional methods for polarization detection and polarization control require bulky optical systems. Flat optics opens a new way for ultra-compact, lower-cost devices and systems for polarization detection and control. However, polarization measurement and manipulating devices with high efficiency and accuracy in the mid-infrared (MIR) range remain elusive. This dissertation presented design concepts and experimental demonstrations of full-Stokes parameters detection and polarization generation devices based on chip-integrated plasmonic metasurfaces with high performance and record efficiency. One of the significant challenges for full-Stokes polarization detection is to achieve high-performance circular polarization (CP) filters. The first design presented in this dissertation is based on the direct integration of plasmonic quarter-wave plate (QWP) onto gold nanowire gratings. It is featured with the subwavelength thickness (~500nm) and extinction ratio around 16. The second design is based on the anisotropic thin-film interference between two vertically integrated anisotropic plasmonic metasurfaces. It provides record high efficiency (around 90%) and extinction ratio (>180). These plasmonic CP filters can be used for circular, elliptical, and linear polarization generation at different wavelengths. The maximum degree of circular polarization (DOCP) measured from the sample achieves 0.99998.
The proposed CP filters were integrated with nanograting-based linear polarization (LP) filters on the same chip for single-shot polarization detection. Full-Stokes measurements were experimentally demonstrated with high accuracy at the single wavelength using the direct subtraction method and over a broad wavelength range from 3.5 to 4.5mm using the Mueller matrix method. This design concept was later expanded to a pixelized array of polarization filters. A full-Stokes imaging system was experimentally demonstrated based on integrating a metasurface with pixelized polarization filters arrays and an MIR camera.

Contributors

Agent

Created

Date Created
2021