Matching Items (6)
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- All Subjects: Molecular beam epitaxy
- All Subjects: Optoelectronics
- Creators: Ning, Cun-Zheng
Description
Multiple quantum well (MQW) structures have been employed in a variety of solid state devices. The InGaAs/GaAs material system is of special interest for many optoelectronic applications. This study examines epitaxial growth and defect creation in InGaAs/GaAs MQWs at its initial stage. Correlations between physical properties, crystal perfection of epitaxial structures, and growth conditions under which desired properties are achieved appear as highly important for the realization and final performance of semiconductor based devices.
Molecular beam epitaxy was utilized to grow InGaAs/GaAs MQW structures with a variation in deposition temperature Tdep among the samples to change crystalline and physical properties. High resolution x-ray diffraction and transmission electron microscopy were utilized to probe crystal properties, whereas photoluminescence spectroscopy evaluated optical response. An optimal growth temperature Tdep=505°C was found for 20% In composition. The density of 60° primary and secondary dislocation loops increased continuously at lower growth temperatures and reduced crystal perfection, as evaluated by lateral and vertical coherence lengths and diffuse scattering in reciprocal space maps. Likewise, the strength of non-radiative Shockley-Read-Hall recombination increased as deposition temperature was reduced. Elevated deposition temperature led to InGaAs decay in the structures and manifested in different crystalline defects with a rather isotropic distribution and no lateral ordering. High available thermal energy increased atomic surface diffusivity and resulted in growth surface instability against perturbations, manifesting in lateral layer thickness undulations. Carriers in structures grown at elevated temperature experience localization in local energy minima.InGaAs/GaAs MQW structures reveal correlation between their crystal quality and optical properties. It can be suggested that there is an optimal growth temperature range for each In composition with high crystal perfection and best physical response.
Molecular beam epitaxy was utilized to grow InGaAs/GaAs MQW structures with a variation in deposition temperature Tdep among the samples to change crystalline and physical properties. High resolution x-ray diffraction and transmission electron microscopy were utilized to probe crystal properties, whereas photoluminescence spectroscopy evaluated optical response. An optimal growth temperature Tdep=505°C was found for 20% In composition. The density of 60° primary and secondary dislocation loops increased continuously at lower growth temperatures and reduced crystal perfection, as evaluated by lateral and vertical coherence lengths and diffuse scattering in reciprocal space maps. Likewise, the strength of non-radiative Shockley-Read-Hall recombination increased as deposition temperature was reduced. Elevated deposition temperature led to InGaAs decay in the structures and manifested in different crystalline defects with a rather isotropic distribution and no lateral ordering. High available thermal energy increased atomic surface diffusivity and resulted in growth surface instability against perturbations, manifesting in lateral layer thickness undulations. Carriers in structures grown at elevated temperature experience localization in local energy minima.InGaAs/GaAs MQW structures reveal correlation between their crystal quality and optical properties. It can be suggested that there is an optimal growth temperature range for each In composition with high crystal perfection and best physical response.
ContributorsKarow, Matthias (Author) / Honsberg, C. (Christiana B.) (Thesis advisor) / Faleev, Nikolai N (Committee member) / Ning, Cun-Zheng (Committee member) / Arizona State University (Publisher)
Created2014
Description
Dual-wavelength laser sources have various existing and potential applications in wavelength division multiplexing, differential techniques in spectroscopy for chemical sensing, multiple-wavelength interferometry, terahertz-wave generation, microelectromechanical systems, and microfluidic lab-on-chip systems. In the drive for ever smaller and increasingly mobile electronic devices, dual-wavelength coherent light output from a single semiconductor laser diode would enable further advances and deployment of these technologies. The output of conventional laser diodes is however limited to a single wavelength band with a few subsequent lasing modes depending on the device design. This thesis investigates a novel semiconductor laser device design with a single cavity waveguide capable of dual-wavelength laser output with large spectral separation. The novel dual-wavelength semiconductor laser diode uses two shorter- and longer-wavelength active regions that have separate electron and hole quasi-Fermi energy levels and carrier distributions. The shorter-wavelength active region is based on electrical injection as in conventional laser diodes, and the longer-wavelength active region is then pumped optically by the internal optical field of the shorter-wavelength laser mode, resulting in stable dual-wavelength laser emission at two different wavelengths quite far apart. Different designs of the device are studied using a theoretical model developed in this work to describe the internal optical pumping scheme. The carrier transport and separation of the quasi-Fermi distributions are then modeled using a software package that solves Poisson's equation and the continuity equations to simulate semiconductor devices. Three different designs are grown using molecular beam epitaxy, and broad-area-contact laser diodes are processed using conventional methods. The modeling and experimental results of the first generation design indicate that the optical confinement factor of the longer-wavelength active region is a critical element in realizing dual-wavelength laser output. The modeling predicts lower laser thresholds for the second and third generation designs; however, the experimental results of the second and third generation devices confirm challenges related to the epitaxial growth of the structures in eventually demonstrating dual-wavelength laser output.
ContributorsGreen, Benjamin C (Author) / Zhang, Yong-Hang (Thesis advisor) / Ning, Cun-Zheng (Committee member) / Tao, Nongjian (Committee member) / Roedel, Ronald J (Committee member) / Arizona State University (Publisher)
Created2011
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 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.
ContributorsLi, Jingjing (Author) / Zhang, Yong-Hang (Thesis advisor) / Tao, Meng (Committee member) / Schroder, Dieter (Committee member) / Vasileska, Dragica (Committee member) / Arizona State University (Publisher)
Created2012
Description
The larger tolerance to lattice mismatch in growth of semiconductor nanowires (NWs) offers much more flexibility for achieving a wide range of compositions and bandgaps via alloying within a single substrate. The bandgap of III-V InGaAsP alloy NWs can be tuned to cover a wide range of (0.4, 2.25) eV, appealing for various optoelectronic applications such as photodetectors, solar cells, Light Emitting Diodes (LEDs), lasers, etc., given the existing rich knowledge in device fabrication based on these materials.
This dissertation explores the growth of InGaAsP alloys using a low-cost method that could be potentially important especially for III-V NW-based solar cells. The NWs were grown by Vapor-Liquid-Solid (VLS) and Vapor-Solid (VS) mechanisms using a Low-Pressure Chemical Vapor Deposition (LPCVD) technique. The concept of supersaturation was employed to control the morphology of NWs through the interplay between VLS and VS growth mechanisms. Comprehensive optical and material characterizations were carried out to evaluate the quality of the grown materials.
The growth of exceptionally high quality III-V phosphide NWs of InP and GaP was studied with an emphasis on the effects of vastly different sublimation rates of the associated III and V elements. The incorporation of defects exerted by deviation from stoichiometry was examined for GaP NWs, with an aim towards maximization of bandedge-to-defect emission ratio. In addition, a VLS-VS assisted growth of highly stoichiometric InP thin films and nano-networks with a wide temperature window from 560◦C to 720◦C was demonstrated. Such growth is shown to be insensitive to the type of substrates such as silicon, InP, and fused quartz. The dual gradient method was exploited to grow composition-graded ternary alloy NWs of InGaP, InGaAs, and GaAsP with different bandgaps ranging from 0.6 eV to 2.2 eV, to be used for making laterally-arrayed multiple bandgap (LAMB) solar cells. Furthermore, a template-based growth of the NWs was attempted based on the Si/SiO2 substrate. Such platform can be used to grow a wide range of alloy nanopillar materials, without being limited by typical lattice mismatch, providing a low cost universal platform for future PV solar cells.
This dissertation explores the growth of InGaAsP alloys using a low-cost method that could be potentially important especially for III-V NW-based solar cells. The NWs were grown by Vapor-Liquid-Solid (VLS) and Vapor-Solid (VS) mechanisms using a Low-Pressure Chemical Vapor Deposition (LPCVD) technique. The concept of supersaturation was employed to control the morphology of NWs through the interplay between VLS and VS growth mechanisms. Comprehensive optical and material characterizations were carried out to evaluate the quality of the grown materials.
The growth of exceptionally high quality III-V phosphide NWs of InP and GaP was studied with an emphasis on the effects of vastly different sublimation rates of the associated III and V elements. The incorporation of defects exerted by deviation from stoichiometry was examined for GaP NWs, with an aim towards maximization of bandedge-to-defect emission ratio. In addition, a VLS-VS assisted growth of highly stoichiometric InP thin films and nano-networks with a wide temperature window from 560◦C to 720◦C was demonstrated. Such growth is shown to be insensitive to the type of substrates such as silicon, InP, and fused quartz. The dual gradient method was exploited to grow composition-graded ternary alloy NWs of InGaP, InGaAs, and GaAsP with different bandgaps ranging from 0.6 eV to 2.2 eV, to be used for making laterally-arrayed multiple bandgap (LAMB) solar cells. Furthermore, a template-based growth of the NWs was attempted based on the Si/SiO2 substrate. Such platform can be used to grow a wide range of alloy nanopillar materials, without being limited by typical lattice mismatch, providing a low cost universal platform for future PV solar cells.
ContributorsHashemi Amiri, Seyed Ebrahim (Author) / Ning, Cun-Zheng (Thesis advisor) / Petuskey, William (Committee member) / Yu, Hongbin (Committee member) / Arizona State University (Publisher)
Created2018
Description
Light Emitting Diodes even with their longer life, robust build and low power consumption, they are still plagued by some problems the most significant of which are the current droop and thermal droop. Current droop causes a lowering in the Internal Quantum Efficiency with increased current injection while thermal droop lowers the whole Internal Quantum Efficiency curve with increase in temperature. The focus here was understanding effects of thermal droop and develop a method to control it.
Shockley Read Hall recombination plays a dominant role in the thermal droop effect when the current injection is low. Since the blue light emitting diode is based on Gallium Nitride, we need to take into consideration the effect of piezoelectric polarization in the quantum wells. The effects of the piezoelectric fields were studied based on the Gallium Nitride plane orientations. It was found in a Gallium Nitride light emitting diodes simulation study that more the number of quantum wells, lower would be the Radiative recombination rate. The problem of exacerbated spatial separation of electron hole wavefunctions in a thick single quantum well structure lead to the development of a dual well structure where one well assisted the other during high temperature operations. The Electron Blocking Layer was reduced in thickness and was made only 10 nm thick with a 5 nm Gallium Nitride buffer between it and the active region wells. The main reason for reducing the electron blocking layer thickness was to reduce the valance band offset and improve hole transport into the active region. Three different dual well designs were simulated of 3nm, 6nm and 9nm wide wells. The output parameters like the Power Spectral Density, Electron bound density, Light Output Power and Electron-Hole wavefunction overlaps were calculated. It was found that one of the wells acted as an assisting well where it had very little radiative recombination activity in it at room temperature.
As the temperature increased, it was observed that the electrons in the main well started to overflow out of it and into the assisting well where the radiative recombination rate increased significantly. This lead to a boost in Internal Quantum Efficiency.
Shockley Read Hall recombination plays a dominant role in the thermal droop effect when the current injection is low. Since the blue light emitting diode is based on Gallium Nitride, we need to take into consideration the effect of piezoelectric polarization in the quantum wells. The effects of the piezoelectric fields were studied based on the Gallium Nitride plane orientations. It was found in a Gallium Nitride light emitting diodes simulation study that more the number of quantum wells, lower would be the Radiative recombination rate. The problem of exacerbated spatial separation of electron hole wavefunctions in a thick single quantum well structure lead to the development of a dual well structure where one well assisted the other during high temperature operations. The Electron Blocking Layer was reduced in thickness and was made only 10 nm thick with a 5 nm Gallium Nitride buffer between it and the active region wells. The main reason for reducing the electron blocking layer thickness was to reduce the valance band offset and improve hole transport into the active region. Three different dual well designs were simulated of 3nm, 6nm and 9nm wide wells. The output parameters like the Power Spectral Density, Electron bound density, Light Output Power and Electron-Hole wavefunction overlaps were calculated. It was found that one of the wells acted as an assisting well where it had very little radiative recombination activity in it at room temperature.
As the temperature increased, it was observed that the electrons in the main well started to overflow out of it and into the assisting well where the radiative recombination rate increased significantly. This lead to a boost in Internal Quantum Efficiency.
ContributorsDas, Shiladitya (Author) / Zhao, Yuji (Thesis advisor) / Vasileska, Dragica (Committee member) / Ning, Cun-Zheng (Committee member) / Arizona State University (Publisher)
Created2017
Metasurface-Based Optoelectronic Devices for Polarization Detection and Ultrafast Optical Modulation
Description
Optical metasurfaces, i.e. artificially engineered arrays of subwavelength building blocks supporting abrupt and substantial light confinement, was employed to demonstrate a novel generation of devices for circularly polarized detection, full-Stokes polarimetry and all-optical modulation with ultra-compact footprint and chip-integrability.
Optical chirality is essential for generation, manipulation and detection of circularly polarized light (CPL), thus finds many applications in quantum computing, communication, spectroscopy, biomedical diagnosis, imaging and sensing. Compared to natural chiral materials, chiral metamaterials and metasurfaces enable much stronger chirality on subwavelength scale; therefore, they are ideal for device miniaturization and system integration. However, they are usually associated with low performance due to limited fabrication tolerance and high dissipation mainly caused by plasmonic materials. Here, a bio-inspired submicron-thick chiral metamaterial structure was designed and demonstrated experimentally with high contrast (extinction ratio >35) detection of CPL with different handedness and high efficiency (>80%) of the overall device. Furthermore, integration of left- and right-handed CPL detection units with nanograting linear polarization filters enabled full-Stokes polarimetry of arbitrarily input polarization states with high accuracy and very low insertion loss, all on a submillimeter single chip. These unprecedented highly efficient and high extinction ratio devices pave the way for on-chip polarimetric measurements.
All-optical modulation is widely used for optical interconnects, communication, information processing, and ultrafast spectroscopy. Yet, there’s deficiency of ultrafast, compact and energy-efficient solutions all in one device. Here, all-optical modulation of light in the near- and mid-infrared regimes were experimentally demonstrated based on a graphene-integrated plasmonic nanoantenna array. The remarkable feature of the device design is its simultaneous near-field enhancement for pump and probe (signal) beams, owing to the localized surface plasmon resonance excitation, while preserving the ultrafast photocarrier relaxation in graphene. Hence, a distinct modulation at 1560nm with record-low pump fluence (<8μJ/cm^2) was reported with ~1ps response time. Besides, relying on broadband interaction of graphene with incident light, a first-time demonstration of graphene-based all-optical modulation in mid-infrared spectral region (6-7μm) was reported based on the above double-enhancement design concept. Relying on the tunability of metasurface design, the proposed device can be used for ultrafast optical modulation from near-infrared to terahertz regime.
Optical chirality is essential for generation, manipulation and detection of circularly polarized light (CPL), thus finds many applications in quantum computing, communication, spectroscopy, biomedical diagnosis, imaging and sensing. Compared to natural chiral materials, chiral metamaterials and metasurfaces enable much stronger chirality on subwavelength scale; therefore, they are ideal for device miniaturization and system integration. However, they are usually associated with low performance due to limited fabrication tolerance and high dissipation mainly caused by plasmonic materials. Here, a bio-inspired submicron-thick chiral metamaterial structure was designed and demonstrated experimentally with high contrast (extinction ratio >35) detection of CPL with different handedness and high efficiency (>80%) of the overall device. Furthermore, integration of left- and right-handed CPL detection units with nanograting linear polarization filters enabled full-Stokes polarimetry of arbitrarily input polarization states with high accuracy and very low insertion loss, all on a submillimeter single chip. These unprecedented highly efficient and high extinction ratio devices pave the way for on-chip polarimetric measurements.
All-optical modulation is widely used for optical interconnects, communication, information processing, and ultrafast spectroscopy. Yet, there’s deficiency of ultrafast, compact and energy-efficient solutions all in one device. Here, all-optical modulation of light in the near- and mid-infrared regimes were experimentally demonstrated based on a graphene-integrated plasmonic nanoantenna array. The remarkable feature of the device design is its simultaneous near-field enhancement for pump and probe (signal) beams, owing to the localized surface plasmon resonance excitation, while preserving the ultrafast photocarrier relaxation in graphene. Hence, a distinct modulation at 1560nm with record-low pump fluence (<8μJ/cm^2) was reported with ~1ps response time. Besides, relying on broadband interaction of graphene with incident light, a first-time demonstration of graphene-based all-optical modulation in mid-infrared spectral region (6-7μm) was reported based on the above double-enhancement design concept. Relying on the tunability of metasurface design, the proposed device can be used for ultrafast optical modulation from near-infrared to terahertz regime.
ContributorsBasiri, Ali (Author) / Yao, Yu (Thesis advisor) / Ning, Cun-Zheng (Committee member) / Palais, Joseph (Committee member) / Zhang, Yong-Hang (Committee member) / Arizona State University (Publisher)
Created2020