Analysis of the Deep Sub-Micron a-Si:H Thin Film Transistors

The recent developments of high resolution flat panel imagers have prompted interests in fabricating smaller on-pixel transistors to obtain higher fill factor and faster speed. This thesis presents fabrication and modeling of short channel amorphous silicon (a-Si:H) vertical thin film transistors (VTFT). A variety of a-Si:H VTFTs with different channel lengths, from 100 nm to 1 μm, are successfully fabricated using the discussed processing steps. Different structural and electrical characteristics of the fabricated device are measured. The results of I-V and C-V characteristics are comprehensively discussed. The 100 nm channel length transistor performance is diverged from regular long channel TFT characteristics, as the short channel effects become dominant in the device, giving rise to necessity of having a physical model to explain such effects. An above threshold model for a-Si:H VTFT current characteristics is extracted. The transport mechanisms are explained and simulated for amorphous silicon material to be used in the device model. The final model shows good agreement with experimental results. However, we used numerical simulation, run in Medici, to further verify the model validity. Simulation allows us to vary different device and material parameters in order to optimize fabrication process for VTFT. The capacitance behavior of the device is extensively studied alongside with a TFT breakdown discussion

Terahertz Quantum Cascade Lasers: towards high performance operation

Terahertz (THz) frequency range (wavelength of 300-30 μm, frequency of 1-10 THz and photon energy of ~4-40meV), the gap between infrared and microwave electromagnetic waves, have remained relatively unexplored for a long time, due to lack of a high power, coherent, and compact source, as well as the lack of an appropriate detector and the transmission devices. THz wave has recently received considerable attention for potential applications in non-invasive medical imaging, detecting trace of gases in the environment, sensing of organic and biological molecules, security controls, local oscillators for heterodyne receiver systems, free space communication, etc. THz quantum cascade laser (QCL), as the relatively high power and coherent THz radiation source, was demonstrated in 2002. After near a decade of intense research, THz QCLs operate only up to 186K in pulse mode with maximum power of 250 mW at 10 K. This thesis discusses many aspects of theoretical and experimental design considerations for THz QCLs. The objective is to obtain a laser device that emits high powers and works towards the temperatures achievable by thermoelectric coolers. This work includes designing the active gain medium, and the engineering of the waveguide and heat removal structures. A density matrix based model is developed to explain the charge transport and gain mechanism in the intersubband devices, particularly for three well resonant phonon based THz QCLs. The model allows for designing of the optimum and novel active gain mediums that work at higher temperatures. The designed active gain mediums are fabricated using discussed low loss waveguide and efficient heat removal structures. The maximum operating temperatures as high as ~176 K is achieved. Finally a promising lasing scheme based on phonon-photon-phonon emissions is proposed that improves the population inversion and offers high gain peak

Electrically switching transverse modes in high power THz quantum cascade lasers.

The design and fabrication of a high power THz quantum cascade laser (QCL), with electrically controllable transverse mode is presented. The switching of the beam pattern results in dynamic beam switching using a symmetric side current injection scheme. The angular-resolved L-I curves measurements, near-field and far-field patterns and angular-resolved lasing spectra are presented. The measurement results confirm that the quasi-TM(01) transverse mode lases first and dominates the lasing operation at lower current injection, while the quasi-TM(00) mode lases at a higher threshold current density and becomes dominant at high current injection. The near-field and far-field measurements confirm that the lasing THz beam is maneuvered by 25 degrees in emission angle, when the current density changes from 1.9 kA/cm(2) to 2.3 kA/cm(2). A two-dimension (2D) current and mode calculation provides a simple model to explain the behavior of each mode under different bias conditions

Analysis of the Deep Sub-Micron a-Si:H Thin Film Transistors

I hereby declare that I am the sole author of this thesis. This is a true copy of the thesis, including any required final revisions, as accepted by my examiners. I understand that my thesis may be made electronically available to the public. ii The recent developments of high resolution flat panel imagers have prompted interests in fabricating smaller on-pixel transistors to obtain higher fill factor and faster speed. This thesis presents fabrication and modeling of short channel amorphous silicon (a-Si:H) vertical thin film transistors (VTFT). A variety of a-Si:H VTFTs with different channel lengths, from 100 nm to 1 µm, are successfully fabricated using the discussed processing steps. Different structural and electrical characteristics of the fabricated device are measured. The results of I-V and C-V characteristics are comprehensively discussed. The 100 nm channel length transistor performance is diverged from regular long channel TFT characteristics, as the short channel effects become dominant in the device, giving rise to necessity of having a physical model to explain such effects. An above threshold model for a-Si:H VTFT current characteristics is extracted. The transport mechanisms are explained and simulated for amorphous silicon material to be used in the device model. The final model shows good agreement with experimental results. However, we used numerical simulation, run in Medici, to further verify the model validity. Simulation allows us to vary different device and material parameters in order to optimize fabrication process for VTFT. The capacitance behavior of the device is extensively studied alongside with a TFT breakdown discussion. iii Acknowledgements I would like to thank Professor Arokia Nathan for his kind supervision and inspiration. Without his continuous support and invaluable guidance, accomplishment of this project would have been impossible. Also, I would like to express my appreciation to my parents, my brother and his lovely wife for their unconditional support

On metal contacts of terahertz quantum cascade lasers with a metal-metal waveguide

This paper reports an experimental study of the effects of different metal claddings on the performance of terahertz quantum cascade lasers. The experimental results show that by using a metal cladding made of Ta/Cu/Au to replace that of Pd/Ge/Ti/Pt/Au, the maximum lasing temperature of the devices is increased from 132 to 172 K, and the threshold current density of the devices at 10 K can be reduced from 0.74 to 0.68 kA cm 122. The improvement of the device performance is attributed to lower optical losses associated with the metal cladding layers. The different effects of the metal contacts on device optical properties and electrical properties are also discussed.Peer reviewed: YesNRC publication: Ye

On the efficiency droop of top-down etched InGaN/GaN nanorod light emitting diodes under optical pumping

The optical performance of top-down etched InGaN/GaN nanorod light emitting diodes (LEDs) was studied using temperature variable photoluminescence spectroscopy with a 405 nm pump laser. Efficiency droop is measured from such nanorod structures, which is further enhanced with decreasing temperature. Through detailed rate equation analysis of the temperature-dependent carrier distribution and modeling of the quantum efficiency, this unique phenomenon can be largely explained by the interplay and dynamics between carrier radiative recombination in localized states and nonradiative recombination via surface states/defects

Large-Scale Cubic InN Nanocrystals by a Combined Solution- and Vapor-Phase Method under Silica Confinement

Large-scale cubic InN nanocrystals were synthesized by a combined solution- and vapor-phase method under silica confinement. Nearly monodisperse cubic InN nanocrystals with uniform spherical shape were dispersed stably in various organic solvents after removal of the silica shells. The average size of InN nanocrystals is 5.7 ± 0.6 nm. Powder X-ray diffraction results indicate that the InN nanocrystals are of high crystallinity with a cubic phase. X-ray photoelectron spectroscopy and energy-dispersive spectroscopy confirm that the nanocrystals are composed of In and N elements. The InN nanocrystals exhibit infrared photoluminescence at room temperature, with a peak energy of ∼0.62 eV, which is smaller than that of high-quality wurtzite InN (∼0.65–0.7 eV) and is in agreement with theoretical calculations. The small emission peak energy of InN nanocrystals, as compared to other low-cost solution or vapor methods, reveals the superior crystalline quality of our samples, with low or negligible defect density. This work will significantly promote InN-based applications in IR optoelectronic device and biology