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(1 - 9 of 9)
- Title
- NEW LASER TECHNOLOGIES: ANALYSIS OF QUANTUM DOT ANDLITHOGRAPHIC LASER DIODES.
- Creator
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Demir, Abdullah, Deppe, Dennis, University of Central Florida
- Abstract / Description
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The first part of this dissertation presents a comprehensive study of quantum dot (QD) lasers threshold characteristics. The threshold temperature dependence of a QD laser diode is studied in different limits of p-doping, hole level spacing and inhomogeneous broadening. Theoretical analysis shows that the threshold current of a QD laser in the limit of uniform QDs is not temperature independent and actually more temperature sensitive than the quantum well laser. The results also explain the...
Show moreThe first part of this dissertation presents a comprehensive study of quantum dot (QD) lasers threshold characteristics. The threshold temperature dependence of a QD laser diode is studied in different limits of p-doping, hole level spacing and inhomogeneous broadening. Theoretical analysis shows that the threshold current of a QD laser in the limit of uniform QDs is not temperature independent and actually more temperature sensitive than the quantum well laser. The results also explain the experimental trends of negative characteristic temperature observed in QD lasers and clarify how the carrier distribution mechanisms inside and among the QDs affect the threshold temperature dependence of a QD laser diode. The second part is on the experimental demonstration of lithographic lasers. Today's vertical-cavity surface-emitting lasers (VCSELs) based on oxide-aperture suffer from serious problems such as heat dissipation, internal strain, reliability, uniformity and size scaling. The lithographic laser provides solutions to all these problems. The transverse mode and cavity are defined using only lithography and epitaxial crystal growth providing simultaneous mode- and current-confinement. Eliminating the oxide aperture is shown to reduce the thermal resistance of the device and leading to increased power density in smaller lasers. When it is combined with better mode matching to gain for smaller devices, high output power density of 58 kW/cm2 is possible for a 3 micron VCSEL with threshold current of 260 microamperes. These VCSELs also have grating-free single-mode single-polarization emission. The demonstration of lithographic laser diodes with good scaling properties is therefore an important step toward producing ultra-small size laser diodes with high output power density, high speed, high manufacturability and high reliability. Lithographic VCSELs ability to control size lithographically in a strain-free, high efficiency device is a major milestone in VCSEL technology.
Show less - Date Issued
- 2010
- Identifier
- CFE0003304, ucf:48494
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0003304
- Title
- LONG CAVITY QUANTUM DOT LASER DIODE AND MONOLITHIC PASSIVELY MODE-LOCKED OPERATION.
- Creator
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Shavitranuruk, K, Deppe, Dennis, University of Central Florida
- Abstract / Description
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Advantage of the single QD active layer is its potential for very low threshold current density, which in turn can produce low internal optical loss. The low threshold current density and low internal loss thus enable a significant increase in laser diode cavity length. Because of the importance of the threshold current density in heatsinking, future technology of broad-area monolithic laser diodes can be implemented. The dissertation describes the development and the unique characteristics...
Show moreAdvantage of the single QD active layer is its potential for very low threshold current density, which in turn can produce low internal optical loss. The low threshold current density and low internal loss thus enable a significant increase in laser diode cavity length. Because of the importance of the threshold current density in heatsinking, future technology of broad-area monolithic laser diodes can be implemented. The dissertation describes the development and the unique characteristics of single QD active layer laser with long cavity. The data are presented on single layer QD laser diodes that reach threshold current densities values of 11.7 A/cm2 in a p-up mounted 2 cm long cavity and as low as 10 A/cm2, with CW output power of 2 W in a p-down mounted 1.6 cm long cavity. The 8.8 A/cm2 in a p-down mounted 2 cm long cavity is reported. To our knowledge the value 8.8 A/cm2 is the lowest threshold current density ever reported for a room temperature laser diode. These single layer QD laser diodes reach an internal loss of ~0.25 cm-1, which is also the lowest ever reported for a room temperature laser diode. These unique characteristics of single layer QD and laser diode size are potentially promising for the monolithic mode-locked laser because of relatively high peak power with a low repetition rate that is on the order of a few GHz, which can be the novel device for external clocking in the optical interconnect applications. In this dissertation, the stable optical pulse train in a 40 µm wide stripe with a repetition rate of 3.75 GHz with 1.1 cm cavity length through the passive mode-locked onto the monolithic two-section device fabricated from this single layer QD laser is observed.
Show less - Date Issued
- 2010
- Identifier
- CFE0003145, ucf:48646
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0003145
- Title
- Cryogenic performance projections for ultra-small oxide-free vertical-cavity surface-emitting lasers.
- Creator
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Bayat, Mina, Deppe, Dennis, Li, Guifang, Schoenfeld, Winston, Lyakh, Arkadiy, University of Central Florida
- Abstract / Description
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Small-sized vertical-cavity surface-emitting laser (VCSEL) may offer very low power consumption along with high reliability for cryogenic data transfer. Cryogenic data transfer has application in supercomputers and superconducting for efficient computing and also focal plane array cameras operating at 77 K, and at the lower temperature of 4 K for data extraction from superconducting circuits. A theoretical analysis is presented for 77 K and 4 K operation based on small cavity, oxide-free...
Show moreSmall-sized vertical-cavity surface-emitting laser (VCSEL) may offer very low power consumption along with high reliability for cryogenic data transfer. Cryogenic data transfer has application in supercomputers and superconducting for efficient computing and also focal plane array cameras operating at 77 K, and at the lower temperature of 4 K for data extraction from superconducting circuits. A theoretical analysis is presented for 77 K and 4 K operation based on small cavity, oxide-free VCSEL sizes of 2 to 6 (&)#181;m, that have been shown to operate efficiently at room temperature. Temperature dependent operation for optimally-designed VCSELs are studied by calculating the response of the laser at 77 K and 4 K to estimate their bias conditions needed to reach modulation speed for cryogenic optical links. The temperature influence is to decrease threshold for reducing temperature, and to increase differential gain for reducing temperature. The two effects predict very low bias currents for small cavity VCSELs to reach needed data speed for cryogenic optical data links. Projections are made for different cavity structures (half-wave cavity and full-wave cavity) shown that half-wave cavity structure has better performance. Changing the number of top-mirror pairs has also been studied to determine how cavity design impacts speed and bit energy. Our design and performance predictions paves the way for realizing highly efficient, ultra-small VCSEL arrays with applications in optical interconnects.
Show less - Date Issued
- 2019
- Identifier
- CFE0007782, ucf:52330
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0007782
- Title
- Intrinsic Modulation Response Modeling and Analysis for Lithographic Vertical-Cavity Surface-Emitting Lasers.
- Creator
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Li, Mingxin, Deppe, Dennis, Fathpour, Sasan, Wu, Shintson, Malocha, Donald, University of Central Florida
- Abstract / Description
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Vertical-cavity surface-emitting lasers (VCSELs) have been greatly improved and successfully commercialized over the past few decades owing to their ability to provide both mode and current confinement that enables low energy consumption, high efficiency and high modulation speed. However, further improvement of oxide VCSELs is limited by the nature of the oxide aperture because of self-heating, internal strain and difficulties in precise size control. In this dissertation, VCSELs using...
Show moreVertical-cavity surface-emitting lasers (VCSELs) have been greatly improved and successfully commercialized over the past few decades owing to their ability to provide both mode and current confinement that enables low energy consumption, high efficiency and high modulation speed. However, further improvement of oxide VCSELs is limited by the nature of the oxide aperture because of self-heating, internal strain and difficulties in precise size control. In this dissertation, VCSELs using lithographic approach are demonstrated to overcome the limitations of oxide VCSELs, in which an intra-cavity phase shifting mesa is applied to define the device size and provide optical mode and electrical current confinement instead of an oxide aperture. A newly developed model of intrinsic modulation response is proposed and analyzed to focus on the thermal limit of the modulation speed of VCSELs. The results show that both the temperature dependent differential gain and stimulated emission rate impact laser speed and the stimulated emission rate dominates the speed limit. Thermal limits of modulation response are compared for oxide and lithographic VCSELs for various sizes. The results predict that the intrinsic modulation response can be significantly increased by using lithographic VCSELs due to low thermal resistance and reduced mode volume while maintaining high efficiency. The intrinsic bandwidth could exceed 100 GHz for a 2-?m-diameter lithographic VCSEL. Combined with low electrical parasitics, it is expected to produce over 100 Gb/s data rate from a single directly modulated laser. VCSELs designed for high speed are discussed and their characteristics are demonstrated.
Show less - Date Issued
- 2016
- Identifier
- CFE0006346, ucf:51556
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0006346
- Title
- Electrical Parasitic Bandwidth Limitations of Oxide-Free Lithographic Vertical-Cavity Surface-Emitting Lasers.
- Creator
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Yang, Xu, Deppe, Dennis, Fathpour, Sasan, Wu, Shintson, Gong, Xun, University of Central Florida
- Abstract / Description
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Nowadays, Vertical-Cavity Surface-Emitting Lasers (VCSELs) are the most popular optical sources in short-reach data communications. In the commercial oxide VCSEL technology, an oxide aperture is created inside resonant cavity in realizing good mode and current confinement, however, high electrical resistance comes along with forming the oxide aperture and the electrical parasitic bandwidth becomes the main limitation in modulation speed. In this report, electrical bandwidths of oxide-free...
Show moreNowadays, Vertical-Cavity Surface-Emitting Lasers (VCSELs) are the most popular optical sources in short-reach data communications. In the commercial oxide VCSEL technology, an oxide aperture is created inside resonant cavity in realizing good mode and current confinement, however, high electrical resistance comes along with forming the oxide aperture and the electrical parasitic bandwidth becomes the main limitation in modulation speed. In this report, electrical bandwidths of oxide-free lithographic VCSELs have been studied along with their general lasing properties. Due to the new ways of fabricating the aperture, record low resistances have been achieved in oxide-free lithographic VCSELs with various sizes, while high slope efficiencies and high output powers have been maintained. High speed simulation has been performed showing the very low differential resistances will benefit much to the electrical parasitic bandwidths, and are expected to produce higher modulation speed. A bottom emitting structure has been proposed and analyzed, showing reduction in both mirror resistance and capacitance will further improve the modulation speed. The total 3-dB modulation bandwidth is expected to be 50-80 GHz, much higher than the bandwidth reached in existing oxide VCSELs. Lithographic VCSELs also show superior lasing characteristics, including record low thermal resistance and record high output power. The maximum power exceeds 19 mW in a 6 (&)#181;m device and over 50 % power conversion efficiency has been achieved. A maximum single mode operation power of 5 mW has been observed from a 1 (&)#181;m diameter VCSEL. High temperature stress testing has been performed showing lithographic VCSELs can operate more reliably than oxide VCSELs under extreme operating conditions. Lithographic VCSEL with low electrical resistance, single-mode operation, high efficiency, and high power will be a strong candidate as the optical source in high speed data communications, as well as other applications such as high power VCSEL arrays and optical sensing.
Show less - Date Issued
- 2016
- Identifier
- CFE0006425, ucf:51491
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0006425
- Title
- Semiconductor Laser Based on Thermoelectrophotonics.
- Creator
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Liu, Xiaohang, Deppe, Dennis, Vanstryland, Eric, Dogariu, Aristide, Bass, Michael, University of Central Florida
- Abstract / Description
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This dissertation presents to our knowledge the first demonstration of a quantum well (QW) laser monolithically integrated with internal optical pump based on a light emitting diode (LED). The LED with high efficiency is operated in a thermoelectrophotonic (TEP) regime for which it can absorb both its own emitted light and heat. The LED optical pump can reduce internal optical loss in the QW laser, and enables monolithically integrated TEP heat pumps to the semiconductor laser. The design,...
Show moreThis dissertation presents to our knowledge the first demonstration of a quantum well (QW) laser monolithically integrated with internal optical pump based on a light emitting diode (LED). The LED with high efficiency is operated in a thermoelectrophotonic (TEP) regime for which it can absorb both its own emitted light and heat. The LED optical pump can reduce internal optical loss in the QW laser, and enables monolithically integrated TEP heat pumps to the semiconductor laser. The design, growth and fabrication processes of the laser chip are discussed, and its experimental data is presented. In order to further increase the TEP laser efficiency the development of QDs as the active region for TEP edge emitting laser (EEL) is studied. The usage of QD as TEP laser's active region is significant in terms of its low threshold current density, low internal optical loss and high reliability, which are mainly due to low transparency in QD laser. The crystal growth of self-organized QDs in molecular beam epitaxial (MBE) system and characterization of QDs are mentioned. The design, growth, processing and fabrication of a QD laser structure are detailed. The characteristics of laser devices with different cavity length are reported. QD active regions with different amount of material are grown to improve the active region performance. Theoretical calculations based on material parameters and semiconductor physics indicate that with proper design, the combination of high efficiency LED in TEP regime with a QD laser can result in the integrated laser chip power conversion efficiency exceeding unity.
Show less - Date Issued
- 2014
- Identifier
- CFE0005369, ucf:50477
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0005369
- Title
- Self-heating control of edge emitting and vertical cavity surface emitting lasers.
- Creator
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Zhang, Yu, Deppe, Dennis, Fathpour, Sasan, Likamwa, Patrick, Wu, Thomas, University of Central Florida
- Abstract / Description
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Self-heating leads to temperature rise of laser diode and limits the output power, efficiency and modulation bandwidth due to increased loss and decreased differential gain. The main heat sources in laser diode during continuous wave operation are Joule heating and free carrier absorption loss. To control device self-heating, the epi structure needs to be designed with low electrical resistance and low absorption loss, while the heat flux must spread out of the device efficiently. This...
Show moreSelf-heating leads to temperature rise of laser diode and limits the output power, efficiency and modulation bandwidth due to increased loss and decreased differential gain. The main heat sources in laser diode during continuous wave operation are Joule heating and free carrier absorption loss. To control device self-heating, the epi structure needs to be designed with low electrical resistance and low absorption loss, while the heat flux must spread out of the device efficiently. This dissertation presents the control of self-heating of both edge emitting laser diodes and vertical cavity surface emitting lasers (VCSELs). For the 980nm high power edge emitting laser, asymmetric waveguide is used for low free carrier absorption loss. The waveguide and cladding materials are optimized for high injection efficiency. BeO heatsink is applied to spread the heat efficiently. Injection efficiency of 71% and internal loss of 0.3 cm-1 have been achieved. A total output power of 9.3 W is measured from 0.5cm long device at 14.5A injection current. To further reduce the internal loss, the development of 980nm quantum dot active region is studied. Threshold current density as low as 59A/cm2 is reached. For the VCSELs, oxide-free structure is used to solve the self-heating problem of oxide VCSELs. Removing the oxide layer and using AlAs in the DBRs leads to record low thermal resistance. Optimization of the DBRs leads to low resistance and low free carrier absorption. Power conversion efficiency higher than 50% is achieved. To further reduce device voltage and heat generation, the development of intracavity contacts devices is introduced.
Show less - Date Issued
- 2014
- Identifier
- CFE0005749, ucf:50076
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0005749
- Title
- Lithographic Vertical-Cavity Surface-Emitting Lasers.
- Creator
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Zhao, Guowei, Deppe, Dennis, Likamwa, Patrick, Fathpour, Sasan, Sundaram, Kalpathy, University of Central Florida
- Abstract / Description
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Remarkable improvements in vertical-cavity surface-emitting lasers (VCSELs) have been made by the introduction of mode- and current-confining oxide optical aperture now used commercially. However, the oxide aperture blocks heat flow inside the device, causing a larger thermal resistance, and the internal strain caused by the oxide can degrade device reliability, also the diffusion process used for the oxide formation can limit device uniformity and scalability.Oxide-free lithographic VCSELs...
Show moreRemarkable improvements in vertical-cavity surface-emitting lasers (VCSELs) have been made by the introduction of mode- and current-confining oxide optical aperture now used commercially. However, the oxide aperture blocks heat flow inside the device, causing a larger thermal resistance, and the internal strain caused by the oxide can degrade device reliability, also the diffusion process used for the oxide formation can limit device uniformity and scalability.Oxide-free lithographic VCSELs are introduced to overcome these device limitations, with both the mode and current confined within the lithographically defined intracavity mesa, scaling and mass production of small size device could be possible. The 3 ?m diameter lithographic VCSEL shows a threshold current of 260 ?A, differential quantum efficiency of 60% and maximum output power density of 65 kW/cm2, and shows single-mode single-polarization operation with side-mode-suppression-ratio over 25 dB at output power up to 1 mW. The device also shows reliable operation during 1000 hours stress test with high injection current density of 142 kA/cm2. The lithographic VCSELs have much lower thermal resistance than oxide-confined VCSELs due to elimination of the oxide aperture. The improved thermal property allows the device to have wide operating temperature range of up to 190 (&)deg;C heat sink temperature, high output power density especially in small device, high rollover current density and high rollover cavity temperature. Research is still underway to reduce the operating voltage of lithographic VCSELs for high wall plug efficiency, and the voltage of 6 (&)#181;m device at injection current density of 10 kA/cm2 is reduces to 1.83 V with optimized mesa and DBR mirror structure. The lithographic VCSELS are promising to become the next generation VCSEL technology.
Show less - Date Issued
- 2012
- Identifier
- CFE0004634, ucf:49912
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0004634
- Title
- On-Chip Electro-Static Discharge (ESD) Protection for Radio-Frequency Integrated Circuits.
- Creator
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Cui, Qiang, Liou, Juin, Yuan, Jiann-Shiun, Wu, Xinzhang, Haralambous, Michael, Shen, Zheng, Deppe, Dennis, University of Central Florida
- Abstract / Description
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Electrostatic Discharge (ESD) phenomenon is a common phenomenon in daily life and it could damage the integrated circuit throughout the whole cycle of product from the manufacturing. Several ESD stress models and test methods have been used to reproduce ESD events and characterize ESD protection device's performance. The basic ESD stress models are: Human Body Model (HBM), Machine Model (MM), and Charged Device Model (CDM). On-chip ESD protection devices are widely used to discharge ESD...
Show moreElectrostatic Discharge (ESD) phenomenon is a common phenomenon in daily life and it could damage the integrated circuit throughout the whole cycle of product from the manufacturing. Several ESD stress models and test methods have been used to reproduce ESD events and characterize ESD protection device's performance. The basic ESD stress models are: Human Body Model (HBM), Machine Model (MM), and Charged Device Model (CDM). On-chip ESD protection devices are widely used to discharge ESD current and limit the overstress voltage under different ESD events. Some effective ESD protection devices were reported for low speed circuit applications such as analog ICs or digital ICs in CMOS process. On the contrast, only a few ESD protection devices available for radio frequency integrated circuits (RF ICs). ESD protection for RF ICs is more challenging than traditional low speed CMOS ESD protection design because of the facts that: (1) Process limitation: High-performance RF ICs are typically fabricated in compound semiconductor process such as GaAs pHEMT and SiGe HBT process. And some proved effective ESD devices (e.g. SCR) are not able to be fabricated in those processes due to process limitation. Moreover, compound semiconductor process has lower thermal conductivity which will worsen its ESD damage immunity. (2) Parasitic capacitance limitation: Even for RF CMOS process, the inherent parasitic capacitance of ESD protection devices is a big concern. Therefore, this dissertation will contribute on ESD protection designs for RF ICs in all the major processes including GaAs pHEMT, SiGe BiCMOS and standard CMOS.The ESD protection for RF ICs in GaAs pHEMT process is very difficult, and the typical HBM protection level is below 1-kV HBM level. The first part of our work is to analyze pHEMT's snapback, post-snapback saturation and thermal failure under ESD stress using TLP-like Sentaurus TCAD simulation. The snapback is caused by virtual bipolar transistor due to large electron-hole pairs impacted near drain region. Post-snapback saturation is caused by temperature-induced mobility degradation due to III-V compound semiconductor materials' poor thermal conductivity. And thermal failure is found to be caused by hot spot located in pHEMT's InGaAs layer. Understanding of these physical mechanisms is critical to design effective ESD protection device in GaAs pHEMT process. Several novel ESD protection devices were designed in 0.5um GaAs pHEMT process. The multi-gate pHEMT based ESD protection devices in both enhancement-mode and depletion-mode were reported and characterized then. Due to the multiple current paths available in the multi-gate pHEMT, the new ESD protection clamp showed significantly improved ESD performances over the conventional single-gate pHEMT ESD clamp, including higher current discharge capability, lower on-state resistance, and smaller voltage transient. We proposed another further enhanced ESD protection clamp based on a novel drain-less, multi-gate pHEMT in a 0.5um GaAs pHEMT technology. Based on Barth 4002 TLP measurement results, the ESD protection devices proposed in this chapter can improve the ESD level from 1-kV (0.6 A It2) to up to 8-kV ((>) 5.2 A It2) under HBM. Then we optimized SiGe-based silicon controlled rectifiers (SiGe SCR) in SiGe BiCMOS process. SiGe SCR is considered a good candidate ESD protection device in this process. But the possible slow turn-on issue under CDM ESD events is the major concern. In order to optimize the turn-on performance of SiGe SCR against CDM ESD, the Barth 4012 very fast TLP (vfTLP) and vfTLP-like TCAD simulation were used for characterization and analysis. It was demonstrated that a SiGe SCR implemented with a P PLUG layer and minimal PNP base width can supply the smallest peak voltage and fastest response time which is resulted from the fact that the impact ionization region and effective base width in the SiGe SCR were reduced due to the presence of the P PLUG layer. This work demonstrated a practical approach for designing optimum ESD protection solutions for the low-voltage/radio frequency integrated circuits in SiGe BiCMOS process.In the end, we optimized SCRs in standard silicon-based CMOS process to supply protection for high speed/radio-frequency ICs. SCR is again considered the best for its excellent current handling ability. But the parasitic capacitance of SCRs needs to be reduced to limit SCR's impact to RF performance. We proposed a novel SCR-based ESD structure and characterize it experimentally for the design of effective ESD protection in high-frequency CMOS based integrated circuits. The proposed SCR-based ESD protection device showed a much lower parasitic capacitance and better ESD performance than the conventional SCR and a low-capacitance SCR reported in the literature. The physics underlying the low capacitance was explained by measurements using HP 4284 capacitance meter.Throughout the dissertation work, all the measurements are mainly conducted using Barth 4002 transimission line pulsing (TLP) and Barth 4012 very fast transmission line pulsing (vfTLP) testers. All the simulation was performed using Sentaurus TCAD tool from Synopsys.
Show less - Date Issued
- 2013
- Identifier
- CFE0004668, ucf:49848
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0004668
