Electrical and Computer Engineering

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Browsing Electrical and Computer Engineering by Subject "2D materials"

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    Effect of Layer Transfer and Plasma Etching on The Behavior of Transition-Metal Dichalcogenide Field-Effect Transistor Arrays
    (University of Waterloo, 2023-05-30) Nouri, Mohammad
    The application of two-dimensional (2D) layered transition metal dichalcogenide (TMDC) for high-performance large-area memory applications requires establishing long-term electrical stability through an understanding of the carrier transport and the effect of the materials processing on the device behavior. In this Ph.D. dissertation, a novel two-step approach for creating arrays of thin-film and few-layer molybdenum disulfide (MoS2)-based field-effect transistors is developed through mechanical exfoliation and a dry etching process. Few-layer structures (~ 3 monolayers) were fabricated using a dry etching process to thin multilayer (~ 60-90 nm thick) TMDC structures. Then, the effect of plasma etching of the TFT backchannel surface and bulk defects in the layers on the electrical performance and stability of the n-channel depletion-mode TFTs were investigated. The etching improved the threshold voltage of the TFTs, resulting in a positive threshold voltage shift of +40 Volts after etching the back channel, correlating to a bulk trap density of approximately 1×1016 cm-3eV-1 per monolayer. Etching the MoS2 surface resulted in a threshold voltage shift of 0.2 V per nanometer of MoS2 removed (for MoS2 thicknesses >15nm). For etched MoS2 layers reduced to < 15 nm, a threshold voltage change of ~1.8 V per nanometer was measured. The backchannel surface was also found to be doped and roughen due to the dry etching process but did not significantly affect the device performance until the MoS2 thickness was below 15 nm. An observed degradation of the carrier transport and electrical stability of these samples were found to be due to the proximity of the etched surface approaching the active channel region of the device. The results reveal the performance tradeoffs of fabricating large-area arrays of few-layer TMDC TFTs using a mechanical exfoliation and dry etching approach. Hydrogen-contained passivation layers were then used to mitigate the impact of the surface defects on the TFT's electrical performance. It is discovered that the diffusion of the hydrogen into the active region of the TMDC devices after the passivation process causes n-type doping, leading to a degradation in the electrical performance and stability of the devices and a negative threshold voltage shift. Furthermore, it is shown that hydrogen diffusion has a higher impact on the electrical performance of TMDC devices with thicknesses less than 15 nm due to the hydrogen diffusion length. Therefore, a hydrogen barrier layer was used to reduce the adverse effect of hydrogen-containing backchannel passivation layers and processes. This approach might be used to make hydrogen-contained passivation layers more compatible with the TMDC semiconductors for developing the next generation of TMDC-based memory devices. Finally, dual-gate TMDC-based TFT arrays were fabricated using a bilayer dielectric on pristine and backchannel etched TMDC films. The electrical characterization shows that the dual-gate TFTs suffered due to trap states on the backchannel surface of the etched TMDC-based TFTs. The research also revealed that the effectiveness of the top-gate electric field in regulating the electrical performance and stability of dual-gate TFTs is affected by both the presence of backchannel surface traps and the distance between the top gate and the active channel region. The findings of this study demonstrate that using a dual-gate structure is a feasible method for managing and adjusting the electrical performance and stability of TMDC-based TFTs affected by backchannel surface states.
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    Modeling and Simulations of Molybdenum Diselenide (MoSe₂) Phototransistors
    (University of Waterloo, 2020-01-10) Han, Gyu Chull
    Since the first demonstration of graphene, two-dimensional (2D) materials have attracted gigantic attentions from the electronic device community. Various 2D materials showing metallic, semiconducting, and insulating properties have been reported. Among them, transition metal dichalcogenide (TMDs) is one of the most latent 2D materials, having excellent semiconducting properties with electronical, optical, and mechanical advantages. Due to their intriguing properties, TMD devices have been investigated for various applications such as switching devices or sensing applications. In particular, 2D TMD phototransistors exhibit high optical device performance, demonstrating their potential for future wearable devices and health monitoring systems. In this thesis, to investigate optical properties of the 2D TMDs phototransistors, comprehensive theoretical studies are performed based on analytical modeling and numerical simulation. I introduce overall simulation schemes showing self-consistent iterative calculations between transport and electrostatics. Non-equilibrium Green’s function (NEGF) transport method is mainly used for device simulations in this study. I discuss two major methods to calculate electronic states of channel materials in the devices: effective mass and tight-binding approximations. I develop new method, modified effective mass approximation, for maintaining rigorous calculation and having low computational cost. Following that, the Poisson’s equation solver is presented to calculate electrostatics. I first analyze unique device performance of fabricated 2D TMDs phototransistors. To confirm origin of uncommon p-type behavior and large photoresponsivity of chemical vapor deposition (CVD) grown multilayer MoSe₂ phototransistors, density-of-states in the band gap is extracted by temperature-dependent analysis. Next, physical models for optical behaviors of MoSe₂ phototransistors under illumination are developed, considering two key mechanisms: photoconductive (PC) and photogating (PG) effects. The models are used to analyze large photoresponsivity of multilayer MoSe₂ phototransistors fabricated by CVD under Mo-rich condition, where it turns out that observed Mo interstitials on the materials are critical factor of large optical response. Then, the MoSe₂ phototransistor is computationally simulated based on NEGF method with trap models. PG effect is deeply examined from the microscopic perspectives. I also simulate optical device performance such as photoresponsivity and detectivity. After that, for further improvement of the device performance, dependence of photoresponsivity on material properties is investigated. The simulation methods can be extended to other phototransistors based on new materials to strengthen the photoresponsivity or PG effect. A phototransistor with hexagonal nano-patterned multilayer MoS₂ as channel layer is simulated to scrutinize high photoresponsivity of it. Material and device simulations are carried on to specify energy level of a trap state mainly contributing to the large photogain. We study dependence of photocurrent on energy level of trap states with realistic capture cross sections. Overall, MoSe₂ phototransistors are physically modeled and numerically simulated. Those trends of models and simulation results successfully reproduce those by experiments, providing in-depth understanding of underlying photoillumination effects. The proposed modeling and simulators can not only accurately predict the photoelectric performances of the future 2D phototransistors but also deeply quantify and analyze the internal physical mechanisms, providing a useful framework for the improvement and optimal design of popular 2D phototransistors for a wide range of applications.
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    Polymeric semiconductor and transition-metal dichalcogenide nanocomposites for inkjet-printed thin-film transistor devices
    (University of Waterloo, 2023-10-20) Choi, Hyunwoo
    Patterned using subtractive processes, conventional thin-film deposition techniques inevitably require high-vacuum deposition and photolithography to define functional layers to create a device structure. Inkjet printing technology has received considerable attention to realize low-cost and potential mass production of large-area electronics at low temperatures using an additive process approach. However, the materials used in the printing process are based on solution-based electronic inks formulated with organic electronic materials. Among them, conjugated polymers are widely used as a semiconductor for thin-film transistor (TFT) applications, but they possess poor charge transport properties compared to other single or polycrystalline inorganic semiconductors. Moreover, the inkjet printing method has a weakness for depositing polymeric solution that form thin films having a highly ordered molecular structure. To overcome this limitation when using printed polymers, a hybrid organic/inorganic semiconductor ink was explored. The hybrid semiconductor ink was prepared by mixing two different materials, molybdenum disulfide (MoS₂) nanosheets and solution-based poly(3-hexylthiopene-2,5-diyl) (P3HT), the former is a two-dimensional semiconductor and the latter a conjugated polymer. To enhance the level of exfoliation and stability of MoS₂ nanosheets in P3HT, the surfactant trichloro(dodecyl)silane (DDTS), was used to functionalize the MoS₂ surface. Printed TFTs using the nanosheet suspension were found to enhance the field-effect mobility by approximately 3× compared to TFTs without the suspension. The introduced single-crystalline MoS₂ nanosheets in the P3HT matrix improved the electrical and structural properties of the inkjet-printed thin-film polymer. Based on these findings and insights, the observed effects can be extended to second-generation polymeric semiconductors, specifically the donor-acceptor (D-A) co-polymers. These materials are renowned for exhibiting the highest mobilities among printable polymers while maintaining ambipolarity, a desirable trait for configuring complementary metal-oxide-semiconductor (CMOS) circuits. In light of this, novel nanocomposite semiconductor inks were developed to demonstrate the influence of 2D nanoparticles on the electronic properties of D-A copolymers, diketopyrrolopyrrole-thieno[3,2-b]thiophene (DPPT-TT). Printed TFTs using this new hybrid semiconductor showed that the field-effect mobility of the devices increased by 33 % and 140 % in both hole (p-type) and electron (n-type) transports, respectively. Atomic force microscopy (AFM) results of the printed hybrid thin film revealed that strongly aggregated polymer domains were observed in films containing the MoS₂ nanosheets. In ultraviolet–visible–near infrared spectroscopy (UV-vis-NIR) measurement, increased intensity of 0-0 and 0-1 peaks from hybrid film indicates improved charge transport was due to enhanced intermolecular charge transfer in the microstructure of the polymer film. Furthermore, the incorporation of hybrid nanocomposites proved particularly beneficial for inkjet-printed TFTs utilizing metal electrodes, as the latter had a tendency to augment contact resistance and thereby compromise device performance. However, the introduction of hybrid nanocomposites effectively counteracted the performance degradation arising from the printed metal electrodes by enhancing the crystallinity of the polymeric film. Moreover, these findings also highlight the feasibility of employing lower sintering temperatures for inkjet-printed metal electrodes. This is attributed to the fact that the result of increased contact resistance associated with lower sintering temperatures can be effectively mitigated by the nanocomposite semiconductor. Consequently, an overall enhancement in device performance was achieved by applying the hybrid nanocomposite ink. This study elucidated the advantageous influence of solution-processed MoS₂ nanosheets on the crystallinity and electrical properties of polymeric thin films, consequently leading to significant improvements in the performance parameters of inkjet-printed TFTs.
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    PtSe2 and HfSe2: New Transition Metal Dischalcogenides for Switching Device Applications
    (University of Waterloo, 2018-08-03) AlMutairi, AbdulAziz
    Recently, silicon-based complementary metal-oxide-semiconductor (CMOS) technology has been struggling in keeping the continuous improvement predicted by Moore’s Law. Hence, significant efforts have been made to find alternatives to the conventional silicon technology. Nanoelectronics based on two-dimensional (2D) materials, such as black phosphorus (BP) and molybdenum disulfide (MoS2), have demonstrated great potential for electronic devices due to their intriguing mechanical, optical and electrical properties. In this thesis, two of novel 2D materials among the transition metal dichalcogenides (TMDs) family have been explored for the use in nanoelectronic devices – platinum diselenide (PtSe2) and hafnium diselenide (HfSe2). It was reported earlier that PtSe2 and HfSe2 exhibit higher carrier mobilities among PtX2 and HfX2 families. First-principle simulations and atomistic quantum transport simulations based on the non-equilibrium Green’s function (NEGF) method within a tight-binding (TB) approximation are used to study PtSe2 and HfSe2 and their device applications. Despite the fact that PtSe2 has a relatively small electron effective mass (0.21m0), its six conduction valleys in the first Brillouin zone give rise to relativity large density of states (DOS). As a result, compared to its molybdenum diselenide (MoSe2) counterpart, PtSe2 field-effect transistors (FETs) exhibit better on-states characteristics (>30%) while maintaining a near-ideal subthreshold swing (SS) of ~64 mV/dec. The scaling study of the channel length (Lch) and the equivalent oxide thickness (EOT) show that a near ideal SS can be persevered with channel lengths longer than 15 nm or through aggressive scaling of the gate oxide (e.g., EOT = 0.4 nm). On the other hand, HfSe2 FETs exhibit nearly identical symmetric transfer characteristics for n-type (NMOS) and p-type transistors (PMOS) despite its asymmetrical effective mass and DOS in the conduction and the valence band. Both exhibits steep switching (<70 mV/dec) with exceptional on- current (~1 mA/μm). Through the scaling study, it was revealed that HfSe2 FETs exhibit great immunity to short-channel effects (SCE) at Lch ≥ 15 nm, but show significant degradations in subthreshold swing and drain-induced barrier lowering at sub-10 nm channel even with a thin gate dielectric. Finally, both NMOS and PMOS HfSe2 devices exhibited excellent intrinsic device performance, making them promising candidates for future logic device applications.
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    Rod Coating MoS2 Films and Silver Nanowire Electrodes for 2D Material-based Devices
    (University of Waterloo, 2023-09-19) Yang, Zhiqiao
    Two-dimensional materials have garnered significant attention in the research community due to their unique physical and electrical properties. Among these materials, molybdenum disulfide (MoS2) is a promising candidate for the development of next-generation electronic and optoelectronic devices, which is attributed to its layer-dependent bandgap, a strong light-matter interaction despite its atomically thin nature and potentially high in-plane carrier mobility. Mayer rod coating emerges as a scalable solution processing method to directly deposit dispersions of MoS2 flakes on a variety of substrates, thus paving the way for the fabrication of flexible electronics in the future. In this study, to the best of the author’s knowledge, rod coating was employed first time to deposit a MoS2 dispersion in ethanol onto SiO2/Si substrates. The optimal MoS2 film estimated several hundreds of nanometers thick on a SiO2/Si substrate pretreated with piranha solution and oxygen plasma using a 100 mg/ml MoS2 solution for 16 coats was thermally annealed at 400 °C in argon to achieve a sheet resistance on the order of kiloohms per square (KΩ/□). Thin film transistors (TFTs) were subsequently fabricated based on the rod-coated MoS2 film, and electrical characterizations demonstrated s-shaped non-linear Isd -Vsd characteristics but no significant gate modulation effect was observed. Rod coating was also applied to deposit silver nanowires on a glass substrate, showcasing the versatility of the method. The silver nanowire electrode was integrated into light emitting devices (LEDs) based on tungsten disulfide (WS2), enabling bidirectional emission as transparent LEDs. This study indicated rod coating as a viable method for the deposition of 2D and other low-dimensional materials, providing an alternative approach for the assembly of 2D materials other than commonly reported solution processing methods in the literature.