Compositionally Graded Indium Gallium Nitride Solar Cells

For the past several decades, methods to harvest solar energy have been investigated intensively. A majority of the work done in this field has been on solar cells made with silicon – the most mature semiconductor material. Recent developments in material fabrication and processing techniques have enabled other semiconductor materials to attract practical interest and research effort as well. Indium gallium nitride (InGaN) is one such material. The material properties of InGaN indicate that solar cells made with it have the potential to achieve much higher power density than a standard silicon solar cell. High power density InGaN solar cells could replace silicon cells in applications where size and weight are critical, or in environments where silicon devices cannot survive. This is especially true of space, and most InGaN development has been done with that in mind. However, at high enough power densities, InGaN solar cells could begin to compete with silicon devices in commercial applications. The goal of this research is to investigate the effect a novel growth technique for InGaN – graded layer deposition – has on the power density of an InGaN solar cell. In this research, first a baseline InGaN solar cell was grown, fabricated, and characterized. A standard PiN (P: p-type, i: intrinsic, N: n-type) structure was used for this baseline device. The reference alloy composition was chosen to be 20% indium and 80% gallium (In­0.2Ga0.8N). This sample was grown using molecular beam epitaxy (MBE) under standard conditions for the material. Once the reference crystal was fabricated it was optically and electrically characterized. The material composition was verified through a combination of x-ray diffraction (XRD), photoluminescence (PL), and transmittance/reflectance measurements. The quality of the surface of the crystal was examined using atomic force microscopy (AFM). Once the optical characterization of the material was complete, the crystal was processed for electrical characterization. Individual devices were constructed by etching away much of the p-type and intrinsic layer, leaving behind circular mesas. Each mesa was then given a top and bottom contact, so that it could be connected to test equipment electrically. After the crystal was processed into a solar cell in this way, each device was connected to a test source electrically, and the current-voltage (I-V) curves were taken. This information was used to find the current and power densities of each device. The second step in this work was fabricating and characterizing a graded layer device that was similar to the reference cell. To this end, the graded layer device was chosen to have a starting composition of 25% indium and 75% gallium, with an ending composition of 15% indium and 85% gallium in place of the intrinsic layer. This new crystal was grown under identical conditions as the baseline cell, except for the graded layer, which required a slightly different approach. The graded layer crystal was then characterized and processed consistent with the reference in an attempt to get as accurate of a comparison between the two as possible. The results of this research could significantly affect the field of III-nitride solar cells

Study of Thin GaN/InGaN/GaN double graded structures for Future photovoltaic application

Indium gallium nitride (In_x Ga_(1-x) N) materials have displayed great potential for photovoltaic and optoelectronic devices due to their optical and electrical properties. Properties such as direct bandgap, strong bandgap absorption, thermal stability and high radiation resistance qualify them as great materials for photovoltaic devices. The tunable bandgap which absorbs the whole solar spectrum is the most significant feature which became attractive for scientists. The bandgap for these materials varies from 0.7 eV for InN to 3.4 eV for GaN covering from infrared to ultraviolet. In_x Ga_(1-x) N wurtzite crystal is grown on GaN buffer layer by Molecular Beam Epitaxy (MBE). Epitaxial growth of high quality In_x Ga_(1-x) N material, however, creates great challenges due to lattice mismatch between InN and GaN (up to 11%). This might be the actual reason of partially and fully strain at the interface relating to growth condition which affect optical properties of the materials. Therefore, studying solar cell parameters for different indium compositions (low to high) in the material is significant. In this work, graded composition In_x Ga_(1-x) N (44 nm ramping up followed by 44 nm ramping down) were grown on GaN/sapphire template. The growth was done at different indium compositions (low to high) in plasma-assisted MBE. Additionally, optical and structural characterizations of the materials were done. The results showed that by increasing indium composition, the composition was not linearly graded as expected and was accompanied by strain relaxation along the growth direction. In other words, for low indium composition, the results showed fully strained. However, for high indium composition partially strain relaxation was seen. The optical respond of three samples was studied with photoluminescence. For the first: to study the source of each peak in aspect of either exciton or different kinds of defect states. Second, peaks related to ground state transition. Furthermore, nextnano3 and nextnano+ software were used to simulate optical properties of 100 nm graded structures such as the band structure, ground state wave-function position as well as determine the optical transition probabilities among ground state hole and electrons as well as solar cell parameters for different structures under different strained conditions. Simulation continued for higher alloys (20% to 90%) under strain and (20%-100%) under relaxed condition. An equation like Vegard’s law was created to predict the energy bandgap under strain for different indium compositions. The simulation was performed for 100 nm -graded structure to find the optimum xmax for both conditions for maximum solar efficiency. In addition, the performance of graded structure in a Flat Base Graded (FBG) was studied to compare with Square well and Homojunction structure

Molecular beam epitaxy growth of indium nitride and indium gallium nitride materials for photovoltaic applications

The objective of the proposed research is to establish the technology for material growth by molecular beam epitaxy (MBE) and fabrication of indium gallium nitride/gallium nitride (InxGa1-xN/GaN) heterojunction solar cells. InxGa1-xN solar cell have the potential to span 90% of the solar spectrum, however there has been no success with high indium (In) incorporation and only limited success with low In incorporation InxGa1-xN. Therefore, this present work focuses on 15 - 30% In incorporation leading to a bandgap value of 2.3 - 2.8 eV. This work will exploit the revision of the indium nitride (InN) bandgap value of 0.68 eV, which expands the range of the optical emission of nitride-based devices from ultraviolet to near infrared regions, by developing transparent InxGa1-xN solar cells outside the visible spectrum. Photovoltaic devices with a bandgap greater than 2.0 eV are attractive because over half the available power in the solar spectrum is above the photon energy of 2.0 eV. The ability of InxGa1-xN materials to optimally span the solar spectrum offers a tantalizing solution for high-efficiency photovoltaics. Using the metal modulated epitaxy (MME) technique in a new, ultra-clean refurbished MBE system, an innovative growth regime is established where In and Ga phase separation is diminished by increasing the growth rate for InxGa1-xN. The MME technique modulates the metal shutters with a fixed duty cycle while maintaining a constant nitrogen flux and proves effective for improving crystal quality and p-type doping. We demonstrate the ability to repeatedly grow high hole concentration Mg-doped GaN films using the MME technique. The highest hole concentration obtained is equal to 4.26 e19 cm-3, resistivity of 0.5 Ω-cm, and mobility of 0.28 cm2/V-s. We have achieved hole concentrations significantly higher than recorded in the literature, proving that our growth parameters and the MME technique is feasible, repeatable, and beneficial. The high hole concentration p-GaN is used as the emitter in our InxGa1-xN solar cell devices.Ph.D.Committee Chair: Doolittle, W. Alan; Committee Member: Ferguson, Ian; Committee Member: Graham, Samuel; Committee Member: Rohatgi, Ajeet; Committee Member: Shen, Shyh-Chian

Calibration of Polarization Fields and Electro-Optical Response of Group-III Nitride Based c-Plane Quantum-Well Heterostructures by Application of Electro-Modulation Techniques

The polarization fields and electro-optical response of PIN-diodes based on nearly lattice-matched InGaN/GaN and InAlN/GaN double heterostructure quantum wells grown on (0001) sapphire substrates by metalorganic vapor phase epitaxy were experimentally quantified. Dependent on the indium content and the applied voltage, an intense near ultra-violet emission was observed from GaN (with fundamental energy gap Eg = 3.4 eV) in the electroluminescence (EL) spectra of the InGaN/GaN and InAlN/GaN PIN-diodes. In addition, in the electroreflectance (ER) spectra of the GaN barrier structure of InAlN/GaN diodes, the three valence-split bands, Γ9, Γ7+, and Γ7−, could selectively be excited by varying the applied AC voltage, which opens new possibilities for the fine adjustment of UV emission components in deep well/shallow barrier DHS. The internal polarization field Epol = 5.4 ± 1.6 MV/cm extracted from the ER spectra of the In0.21Al0.79N/GaN DHS is in excellent agreement with the literature value of capacitance-voltage measurements (CVM) Epol = 5.1 ± 0.8 MV/cm. The strength and direction of the polarization field Epol = −2.3 ± 0.3 MV/cm of the (0001) In0.055Ga0.945N/GaN DHS determined, under flat-barrier conditions, from the Franz-Keldysh oscillations (FKOs) of the electro-optically modulated field are also in agreement with the CVM results Epol = −1.2 ± 0.4 MV/cm. The (absolute) field strength is accordingly significantly higher than the Epol strength quantified in published literature by FKOs on a semipolar (112¯2) oriented In0.12Ga0.88N quantum well

Novel transparent conductive materials: understanding and prediction

Transparent conductive oxides (TCOs) such as doped In₂O₃, ZnO, SnO₂ or CdO are highly attractive due to their special properties, which are electrical conductivity and optical transparency. The lack of complete understanding of the fundamentals behind this unique phenomena and rapidly increasing commercial demand draw a lot of interest in investigating this kind of materials. More efficient, environmentally friendly and less expensive transparent conductive materials are needed for a variety of applications, and a general understanding of the origins of the unusual behavior would help further search for potential transparent conductive (TC) candidates. Understanding the basic properties of conventional TCOs theory and predicting new TC candidates are the two main goals of this study. To achieve these goals, several classes of materials including conventional TCOs and other metal oxides, nitrides, selenides, sulfides, fluorides and a few others were investigated by using first-principles electronic band structure simulations. Compounds with same-group next-period cations or anions are studied. For example, vertical chains of compounds Al₂O₃ -\u3e Ga₂O₃ -\u3e In₂O₃ -\u3e Tl₂O₃, and In₂O₃ -\u3e In₂S₃ -\u3e In₂Se₃ -\u3e In₂Te₃, were systematically considered. The results comprise electronic band structure, band gap, density of states, electron effective mass and holes effective mass. The calculated materials were separated into different groups based on the location of the cation or anion in the periodic table, i.e., on its electronic configuration or atomic weight, and comparisons were carried out within the same group. The differences among the compounds in the same series are discussed in details based on the calculated results --Abstract, page iii

Molecular beam epitaxial growth of rare-earth compounds for semimetal/semiconductor heterostructure optical devices

textHeterostructures of materials with dramatically different properties are exciting for a variety of devices. In particular, the epitaxial integration of metals with semiconductors is promising for low-loss tunnel junctions, embedded Ohmic contacts, high-conductivity spreading layers, as well as optical devices based on the surface plasmons at metal/semiconductor interfaces. This thesis investigates the structural, electrical, and optical properties of compound (III-V) semiconductors employing rare-earth monopnictide (RE-V) nanostructures. Tunnel junctions employing RE-V nanoparticles are developed to enhance current optical devices, and the epitaxial incorporation of RE-V films is discussed for embedded electrical and plasmonic devices. Leveraging the favorable band alignments of RE-V materials in GaAs and GaSb semiconductors, nanoparticle-enhanced tunnel junctions are investigated for applications of wide-bandgap tunnel junctions and lightly-doped tunnel junctions in optical devices. Through optimization of the growth space, ErAs nanoparticle-enhanced GaAs tunnel junctions exhibit conductivity similar to the best reports on the material system. Additionally, GaSb-based tunnel junctions are developed with low p-type doping that could reduce optical loss in the cladding of a 4 μm laser by ~75%. These tunnel junctions have several advantages over competing approaches, including improved thermal stability, precise control over nanoparticle location, and incorporation of a manifold of states at the tunnel junction interface. Investigating the integration of RE-V nanostructures into optical devices revealed important details of the RE-V growth, allowing for quantum wells to be grown within 15nm of an ErAs nanoparticle layer with minimal degradation (i.e. 95% of the peak photoluminescence intensity). This investigation into the MBE growth of ErAs provides the foundation for enhancing optical devices with RE-V nanostructures. Additionally, the improved understanding of ErAs growth leads to development of a method to grow full films of RE-V embedded in III-V materials. The growth method overcomes the mismatch in rotational symmetry of RE-V and III-V materials by seeding film growth with epitaxial nanoparticles, and growing the film through a thin III-V spacer. The growth of RE-V films is promising for both embedded electrical devices as well as a potential path towards realization of plasmonic devices with epitaxially integrated metallic films.Electrical and Computer Engineerin

First Workshop on Identification of Future Emerging Technologies for Low Carbon Energy Supply

As part of the European Commission's internal Low Carbon Energy Observatory project, the Joint Research Centre is developing an inventory of future emerging technologies relevant to energy supply. A key part of this initiative is consultation with external experts. This workshop is the first step in this process. It targets two main energy research areas: electricity from electromagnetic irradiation (principally photovoltaics, but also thermo-electric concepts) and fuels (comprising fuel cells, hydrogen and biofuels). Issues of general interest are also considered. The goal is to identify innovative technologies and processes for energy supply, possibly not sufficiently considered in current research funding programs.JRC.C.4-Sustainable Transpor

Polarization Doped Nanowire Devices as an Alternative to Impurity Doping

Engineering: 2nd Place (The Ohio State University Edward F. Hayes Graduate Research Forum)It is difficult to control electrical conductivity in wide band gap semiconductors using impurity doping because of large ionization energies in these materials. Polarization-induced doping in compositionally graded heterostructures is a possible alternative to impurity doping in certain wide band gap materials. Here we present compositionally graded AlGaN nanowires grown on Si(111) substrates by plasma-assisted molecular beam epitaxy that utilize polarization-induced doping to form p-n junctions. GaN quantum wells are inserted into the nanowires to create polarization-induced nanowire light emitting diodes (PINLEDs) that emit ultraviolet light. Variable temperature electrical measurements show that dopants in the structure are ionized by polarization-induced charge and therefore do not freeze-out at cryogenic temperatures. Furthermore, electroluminescence measurements show that polarization-induced charge alone (i.e. with no intentionally added dopants) can be used to form working LEDs.No embarg

Epitaxial growth of iii-nitride nanostructures and their optoelectronic applications

Light-emitting diodes (LEDs) using III-nitride nanowire heterostructures have been intensively studied as promising candidates for future phosphor-free solid-state lighting and full-color displays. Compared to conventional GaN-based planar LEDs, III-nitride nanowire LEDs exhibit numerous advantages including greatly reduced dislocation densities, polarization fields, and quantum-confined Stark effect due to the effective lateral stress relaxation, promising high efficiency full-color LEDs. Beside these advantages, however, several factors have been identified as the limiting factors for further enhancing the nanowire LED quantum efficiency and light output power. Some of the most probable causes have been identified as due to the lack of carrier confinement in the active region, non-uniform carrier distribution, and electron overflow. Moreover, the presence of large surface states and defects contribute significantly to the carrier loss in nanowire LEDs. In this dissertation, a unique core-shell nanowire heterostructure is reported, that could overcome some of the aforementioned-problems of nanowire LEDs. The device performance of such core-shell nanowire LEDs is significantly enhanced by employing several effective approaches. For instance, electron overflow and surface states/defects issues can be significantly improved by the usage of electron blocking layer and by passivating the nanowire surface with either dielectric material / large bandgap energy semiconductors, respectively. Such core-shell nanowire structures exhibit significantly increased carrier lifetime and massively enhanced photoluminescence intensity compared to conventional InGaN/GaN nanowire LEDs. Furthermore, AlGaN based ultraviolet LEDs are studied and demonstrated in this dissertation. The simulation studies using Finite-Difference Time-Domain method (FDTD) substantiate the design modifications such as flip-chip nanowire LED introduced in this work. High performance nanowire LEDs on metal substrates (copper) were fabricated via substrate-transfer process. These LEDs display higher output power in comparison to typical nanowire LEDs grown on Si substrates. By engineering the device active region, high brightness phosphor-free LEDs on Cu with highly stable white light emission and high color rendering index of \u3e 95 are realized. High performance nickel?zinc oxide (Ni-ZnO) and zinc oxide-graphene (ZnO-G) particles have been fabricated through a modified polyol route at 250?C. Such materials exhibit great potential for dye-sensitized solar cell (DSSC) applications on account of the enhanced short-circuit current density values and improved efficiency that stems from the enhanced absorption and large surface area of the composite. The enhanced absorption of Ni-ZnO composites can be explained by the reduction in grain boundaries of the composite structure as well as to scattering at the grain boundaries. The impregnation of graphene into ZnO structures results in a significant increase in photocurrent consequently due to graphene\u27s unique attributes including high surface area and ultra-high electron mobility. Future research directions will involve the development of such wide-bandgap devices such as solar cells, full color LEDs, phosphor free white-LEDs, UV LEDs and laser diodes for several applications including general lighting, wearable flexible electronics, water purification, as well as high speed LEDs for visible light communications

Simulation of High Temperature InGaN Photovoltaic Devices

abstract: In recent years, there has been increased interest in the Indium Gallium Nitride (InGaN) material system for photovoltaic (PV) applications. The InGaN alloy system has demonstrated high performance for high frequency power devices, as well as for optical light emitters. This material system is also promising for photovoltaic applications due to broad range of bandgaps of InxGa1-xN alloys from 0.65 eV (InN) to 3.42 eV (GaN), which covers most of the electromagnetic spectrum from ultraviolet to infrared wavelengths. InGaN’s high absorption coefficient, radiation resistance and thermal stability (operating with temperature > 450 ℃) makes it a suitable PV candidate for hybrid concentrating solar thermal systems as well as other high temperature applications. This work proposed a high efficiency InGaN-based 2J tandem cell for high temperature (450 ℃) and concentration (200 X) hybrid concentrated solar thermal (CSP) application via numerical simulation. In order to address the polarization and band-offset issues for GaN/InGaN hetero-solar cells, band-engineering techniques are adopted and a simple interlayer is proposed at the hetero-interface rather than an Indium composition grading layer which is not practical in fabrication. The base absorber thickness and doping has been optimized for 1J cell performance and current matching has been achieved for 2J tandem cell design. The simulations also suggest that the issue of crystalline quality (i.e. short SRH lifetime) of the nitride material system to date is a crucial factor limiting the performance of the designed 2J cell at high temperature. Three pathways to achieve ~25% efficiency have been proposed under 450 ℃ and 200 X. An anti-reflection coating (ARC) for the InGaN solar cell optical management has been designed. Finally, effective mobility model for quantum well solar cells has been developed for efficient quasi-bulk simulation.Dissertation/ThesisDoctoral Dissertation Physics 201