Matching Items (6)
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- All Subjects: Semiconductors
- Creators: Ponce, Fernando
Description
Group III-nitride semiconductors have been commercially used in the fabrication of light-emitting diodes and laser diodes, covering the ultraviolet-visible-infrared spectral range and exhibit unique properties suitable for modern optoelectronic applications. InGaN ternary alloys have energy band gaps ranging from 0.7 to 3.4 eV. It has a great potential in the application for high efficient solar cells. AlGaN ternary alloys have energy band gaps ranging from 3.4 to 6.2 eV. These alloys have a great potential in the application of deep ultra violet laser diodes. However, there are still many issues with these materials that remain to be solved. In this dissertation, several issues concerning structural, electronic, and optical properties of III-nitrides have been investigated using transmission electron microscopy. First, the microstructure of InxGa1-xN (x = 0.22, 0.46, 0.60, and 0.67) films grown by metal-modulated epitaxy on GaN buffer /sapphire substrates is studied. The effect of indium composition on the structure of InGaN films and strain relaxation is carefully analyzed. High luminescence intensity, low defect density, and uniform full misfit strain relaxation are observed for x = 0.67. Second, the properties of high-indium-content InGaN thin films using a new molecular beam epitaxy method have been studied for applications in solar cell technologies. This method uses a high quality AlN buffer with large lattice mismatch that results in a critical thickness below one lattice parameter. Finally, the effect of different substrates and number of gallium sources on the microstructure of AlGaN-based deep ultraviolet laser has been studied. It is found that defects in epitaxial layer are greatly reduced when the structure is deposited on a single crystal AlN substrate. Two gallium sources in the growth of multiple quantum wells active region are found to cause a significant improvement in the quality of quantum well structures.
ContributorsWei, Yong (Author) / Ponce, Fernando (Thesis advisor) / Chizmeshya, Andrew (Committee member) / McCartney, Martha (Committee member) / Menéndez, Jose (Committee member) / Yu, Hongbin (Committee member) / Arizona State University (Publisher)
Created2014
Raman spectroscopy characterization of anharmonicity and alloying effects in semiconductor materials
Description
The chemical sensitivity and spatial resolution of Raman spectroscopy, combined with the sensitivity of modern systems that can easily detect single atomic layers, have made this technique a preferred choice for the strain characterization of complex systems such as nanoscale complementary metal-oxide-semiconductor - CMOS - devices. A disadvantage of Raman spectroscopy, however, is that the shifts associated with strain are not related to the geometrical deformations in any obvious way, so that careful calibrations are needed to determine the anharmonic coefficients (p, q and r) that relate strain to Raman shifts. A new set of measurements of the Raman shift in strained Ge films grown on relaxed SiGe buffer layers deposited on Si substrates is presented, and thereby, a new consistent set of values for the parameters p and q for Ge has been proposed. In this dissertation the study of the vibrational properties of Ge1-xSnx alloys has also been reported. The temperature dependence of the Raman spectrum of Ge-rich Ge1-x Snx and Ge1-x-ySi xSny alloys has been determined in the 10 K - 450 K range. The Raman line shift and width changes as a function of temperature are found to be virtually identical to those observed in bulk Ge. This result shows that the anharmonic decay process responsible for the temperature dependence is extremely robust against the alloy perturbation.
ContributorsBagchi, Sampriti (Author) / Menéndez, Jose (Thesis advisor) / Treacy, Michael (Committee member) / Ponce, Fernando (Committee member) / Tsen, Kong-Thon (Committee member) / Rez, Peter (Committee member) / Arizona State University (Publisher)
Created2011
Description
ABSTRACT Group III-nitride semiconductor materials have been commercially used in fabrication of light-emitting diodes (LEDs) and laser diodes (LDs) covering the spectral range from UV to visible and infrared, and exhibit unique properties suitable for modern optoelectronic applications. Great advances have recently happened in the research and development in high-power and high-efficiency blue-green-white LEDs, blue LDs and other optoelectronic applications. However, there are still many unsolved challenges with these materials. In this dissertation, several issues concerning structural, electronic and optical properties of III-nitrides have been investigated using a combination of transmission electron microscopy (TEM), electron holography (EH) and cathodoluminescence (CL) techniques. First, a trend of indium chemical inhomogeneity has been found as the indium composition increases for the InGaN epitaxial layers grown by hydride vapor phase epitaxy. Second, different mechanisms contributing to the strain relaxation have been studied for non-polar InGaN epitaxial layers grown on zinc oxide (ZnO) substrate. Third, various structural morphologies of non-polar InGaN epitaxial layers grown on free-standing GaN substrate have been investigated. Fourth, the effect of the growth temperature on the performance of GaN lattice-matched InAlN electron blocking layers has been studied. Finally, the electronic and optical properties of GaN nanowires containing a AlN/GaN superlattice structure have been investigated showing relatively small internal electric field and superlattice- and defect-related emissions along the nanowires.
ContributorsSun, Kewei (Author) / Ponce, Fernando (Thesis advisor) / Smith, David (Committee member) / Treacy, Michael (Committee member) / Drucker, Jeffery (Committee member) / Schmidt, Kevin (Committee member) / Arizona State University (Publisher)
Created2011
Description
Group-IV semiconductor alloys are of interest for Si-integrated optoelectronic applications due to the band gap tunability and enhanced optical capabilities that can be achieved through compositional tuning. This work advances the field by presenting a systematic study of the optical and electronic properties of Ge1-ySny and analogous Ge1-x-ySixSny alloys.
The fundamental direct and indirect band gaps of Ge1-ySny materials are measured by room temperature photoluminescence in samples containing 0 ≤ y ≤ 0.11 and a transition to direct gap materials is found to occur at yc = 0.087. This result is enabled by the development of sample growth and processing protocols that produce high-quality materials epitaxially on Ge-buffered Si(100) substrates. Strategies to optimize the optical performance are explored by varying the film thickness, thermal and surface treatments, and n-type doping. The electrical and optical properties of diodes based on these materials are characterized by current-voltage, optical responsivity, and electroluminescence measurements. These show improved optical performance near yc with tunable emission out to 2500 nm. Measuring the carrier lifetimes in devices with strain relaxed and fully strained interfaces show significantly longer lifetimes in the fully strained case.
The direct and indirect band gaps of Sn-rich (y > x) Ge1-x-ySixSny materials are measured by room temperature photoluminescence on optimized samples. These data confirm a transition to direct gap materials occurs for the ternary alloy as well. Devices based on compositions 0.02 ≤ x ≤ 0.10 and 0.03 ≤ y ≤ 0.11 are characterized by current-voltage, optical responsivity, and electroluminescence measurements and show competitive performance with analogous devices based on Ge1-ySny materials. A detailed study of the direct gap in Ge1-xSix alloys gives parameters crucial en route to a global description of the Ge1-x-ySixSny fundamental band gaps.
Archetypal laser device designs on Si are explored by fabricating degenerate pn junction diodes and highly doped waveguide devices based on high-quality Ge1-ySny materials. The diodes showed tunnel-like current-voltage characteristics and tailored electroluminescence based on the doping profile. The waveguides demonstrate emission under optical stimulation.
The fundamental direct and indirect band gaps of Ge1-ySny materials are measured by room temperature photoluminescence in samples containing 0 ≤ y ≤ 0.11 and a transition to direct gap materials is found to occur at yc = 0.087. This result is enabled by the development of sample growth and processing protocols that produce high-quality materials epitaxially on Ge-buffered Si(100) substrates. Strategies to optimize the optical performance are explored by varying the film thickness, thermal and surface treatments, and n-type doping. The electrical and optical properties of diodes based on these materials are characterized by current-voltage, optical responsivity, and electroluminescence measurements. These show improved optical performance near yc with tunable emission out to 2500 nm. Measuring the carrier lifetimes in devices with strain relaxed and fully strained interfaces show significantly longer lifetimes in the fully strained case.
The direct and indirect band gaps of Sn-rich (y > x) Ge1-x-ySixSny materials are measured by room temperature photoluminescence on optimized samples. These data confirm a transition to direct gap materials occurs for the ternary alloy as well. Devices based on compositions 0.02 ≤ x ≤ 0.10 and 0.03 ≤ y ≤ 0.11 are characterized by current-voltage, optical responsivity, and electroluminescence measurements and show competitive performance with analogous devices based on Ge1-ySny materials. A detailed study of the direct gap in Ge1-xSix alloys gives parameters crucial en route to a global description of the Ge1-x-ySixSny fundamental band gaps.
Archetypal laser device designs on Si are explored by fabricating degenerate pn junction diodes and highly doped waveguide devices based on high-quality Ge1-ySny materials. The diodes showed tunnel-like current-voltage characteristics and tailored electroluminescence based on the doping profile. The waveguides demonstrate emission under optical stimulation.
ContributorsGallagher, James Dennis (Author) / Menéndez, Jose (Thesis advisor) / Kouvetakis, John (Thesis advisor) / Chizmeshya, Andrew (Committee member) / Ponce, Fernando (Committee member) / Arizona State University (Publisher)
Created2015
Description
Diamond and cubic boron nitride (c-BN) are ultra wide band gap semiconductors (Eg>3.4 eV) and share similar properties in various aspects, including being isoelectronic, a 1% lattice mismatch, large band gap, high thermal conductivity. Particularly, the negative electron affinity (NEA) of diamond and c-BN is an unusual property that has led to effects such as p-type surface conductivity, low temperature thermionic emission, and photon enhanced thermionic emission. In this dissertation, the interface chemistry and electronic structure of dielectrics on diamond and c-BN are investigated with X-ray and ultraviolet photoemission spectroscopy (XPS and UPS). The first study established that the surface conductive states could be established for thin Al2O3 on diamond using a post deposition H-plasma process. At each step of the atomic layer deposition (ALD) and plasma processing, the band alignment was characterized by in situ photoemission and related to interface charges. An interface layer between the diamond and dielectric layer was proposed to explain the surface conductivity. The second study further investigated the improvement of the hole mobility of surface conductive diamond. A thin layer of Al2O3 was employed as an interfacial layer between surface conductive hydrogen-terminated (H-terminated) diamond and MoO3 to increase the distance between the hole accumulation layer in diamond and negatively charged states in acceptor layer. With an interfacial layer, the ionic scattering, which was considered to limit the hole mobility, was reduced. By combining two oxides (Al2O3 and MoO3), the hole mobility and concentration were modulated by altering the thickness of the Al2O3 interfacial layer. The third study focused on the electronic structure of vanadium-oxide-terminated c-BN surfaces. The vanadium-oxide-termination was formed on c-BN by combining vanadium deposition using molecular beam deposition (MBD) and oxygen plasma treatment. After thermal annealing, a thermally stable NEA was achieved on c-BN. A model was proposed based on the deduced interface charge distribution to explain the establishment of an NEA.
ContributorsYang, Yu (Author) / Nemanich, Robert J (Thesis advisor) / McCartney, Martha (Committee member) / Ponce, Fernando (Committee member) / Qing, Quan (Committee member) / Arizona State University (Publisher)
Created2018
Description
Nanoscale semiconductors with their unique properties and potential applications have been a focus of extensive research in recent years. There are many ways in which semiconductors change the world with computers, cell phones, and solar panels, and nanoscale semiconductors having a promising potential to expand the efficiency, reduce the cost, and improve the flexibility and durability of their design. In this study, theoretical quantum mechanical simulations were performed on several different nanoscale semiconductor materials, including graphene/phosphorene nanoribbons and group III-V nanowires. First principles density functional theory (DFT) was used to study the electronic and structural properties of these nanomaterials in their fully relaxed and strained states. The electronic band gap, effective masses of charge carriers, electronic orbitals, and density of states were most commonly examined with strain, both from intrinsic and external sources. For example, armchair graphene nanoribbons (AGNR) were found to have unprecedented band gap-strain dependence. Phosphorene nanoribbons (PNRs) demonstrate a different behavior, including a chemical scissors effect, and studies revealed a strong relationship between passivation species and band gap tunability. Unlike the super mechanical flexibility of AGNRs and PNRs which can sustain incredible strain, modest yet large strain was applied to group III-V nanowires such as GaAs/InAs. The calculations showed that a direct and indirect band gap transition occurs at some critical strains and the origination of these gap transitions were explored in detail. In addition to the pure nanowires, GaAs/InAs core/shell heterostructure nanowires were also studied. Due to the lattice mismatch between GaAs and InAs, the intrinsic strain in the core/shell nanowires demonstrates an interesting behavior on tuning the electronic properties. This interesting behavior suggests a mechanical way to exert compressive strain on nanowires experimentally, and can create a finite quantum confinement effect on the core.
ContributorsCopple, Andrew (Author) / Peng, Xihong (Thesis advisor) / Chan, Candace (Committee member) / Chizmeshya, Andrew (Committee member) / Ponce, Fernando (Committee member) / Arizona State University (Publisher)
Created2016